CIF SHOPS IDAM~AG SIB]E WEGEMT WORKSHOP G)EM T

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1 BQok Ref. LY95001 WEGEMT WORKSHOP IDAM~AG SIB]E CIF SHOPS G)EM T Friday, 20 October, 1995 venue The Technical University of Denmark Lyngby, Denmark hosted by The Department of Ocean Engineering TU, Denmark

2 WEGEMT Workshop on Damage Stability of Ships Proceedings of a one-day Workshop held on Friday 20 October 1995 TU Denmark, Lyngby Hosted by the Department Ocean Engineering, TU Denmark Published by WEGENIT Publication reference number LY95001 C-

3 ABOUT WEGEMT WEGEMT is a European Association of 37 universities in 17 countries. It was formed in 1978 with the aim of increasing the knowledge base, and updating and extending the skills and competence of engineers and postgraduate students working at an advanced level in marine technology and related science. WEGEMT achieves this aim by encouraging universities to be associated with the Foundation, to operate as a network and therefore actively collaborate in initiatives relevant to this aim WEGEMT considers collaborative research, education and training activities at an advanced level and the exchange and dissemination of information, as activities which further the aim of the Association. NB For "marine technology and related science", this includes all aspects of offshore oil & gas exploration and production, shipping and shipbuilding. underwater technologies and other interdisciplinary areas concerned with the oceans and seas.,about THE PUBLICATION This publication represents a series of lecturers' notes which were presented at a oneday Workshop on Damage Stability of Ships first presented at TU Denmark, Lyngby on Friday 20 October ISBN Published by WEGEMT Copyright 1995 WEGEMT. All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or distributed in any form or by any means without the prior written consent of the publisher. This volume has been made available so that it contains the original authors' typescripts. The method may from time to time display txpographical limitations. It is hoped however, that they do not distract the attentions of the reader. Please note that the views expressed are those of the individual authoi(s) and the publishers cannot accept responsibility for any enors or omissions.

4 Programme for the Workshop 1. Collision damage statistics and Probabilistic damage stability calculations in preliminary ship design Dr J Juncher-Jensen and P Andersen, DTH Lyngby 2. Overview of the Joint Nordic Project Dr T Svensen, Project Manager DNV 3. Mathematical modelling of capsizal resistance of a damaged ship Dr D Vassalos, Strathclyde University 4. Experimental studies on the capsize safety of passenger/ro-ro ships S Velscho & M Schindler, Danish Maritime Institute 5. Recent developments, trends and proposals on damage stability criteria Professor M Pawlowski, Dansk University & V Aanesland,.AR'RINTEK 6. Panel discussion and recommendations -4

5 COLLISION DAMAGE STATISTICS AND PROBABILISTIC DAMAGE STABILITY CALCULATIONS IN PRELIMINARY SHIP DESIGN J. JUNCHER JENSEN, J. BAATRUP & P. ANDERSEN Department of Ocean Engineering Technical University of Denmark Building 101 E, DK-2800 Lyngby, DENMARK ABSTRACT The recent MO Resolution MSC 19(58) points towards a more rational way of obtaining subdivisions in ships to ensure a sufficient stability in damaged conditions. In the preliminary phase of ship design it is important to know how the attained subdivision index and the possible oil outflow in a collision are influenced by the actual positions of the watertight bulkheads. This information should be given in form of sensitivity factors yielding the change in attained index, subdivision and oil outflow for specified changes in the position of user-defined bulkheads. In the paper such a procedure will be described. The formulation is quite general implying that future improvements in the damage stability regulations can be easily implemented. Furthermore, information of oil outflow from damaged cargo tanks is included. By that, both the probability of zero outflow, average hypothetical oil outflow as well as mean local outflow are presented. The procedure can thus be used to compare the environmental damage to be expected from different type of oil tankers in a given collision scenario. INTRODUCTION Recently the assessment of damage stability of ships has received a great deal of attention due to the tragic losses of several Ro-Ro ferries: the "European Gateway' (1982), the "Herald of

6 Free Enterprise" (1987) and the "Estonia" (1994). Of course the international maritime society is seriously concerned and puts substantial effort in both deriving rational procedures for assessing the residual stability of a damaged ship and setting up reasonable minimum requirements. A significant development in the rules and regulations for damage stability assessment can be foreseen in the next few years, most certainly within the framework of a probabilistic -description of the damages a ship may suffer in a grounding or a collision accident. The probabilistic damage stability regulations for dry cargo ships of length greater than 100 mn issued by the International Maritime Organization (IMO) in 1990 [1] provide the most updated, generally accepted version of such a procedure. Whereas the probabilistic concept is simple and gives a single measure of the probability of surviving a collision accident its implementation in actual design poses a number of problems. The main problems are: (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) Establishment of realistic probability distributions for the location and extent of damages. Extension to cover damage statistics for grounding. Definition of the probability of damage of a given compartment or group of compartments. Realistic criteria for the probability of surviving a given damage. Definition of suitable loading conditions. Proper account of openings, downflooding points and crossflooding pipes. Specification of realistic permeabilities of flooded compartments. Definition of minimum requirements taking into account the number of people on board and the possible damage to the environment (oil outflow). Proper account of the water ingress process. Some of the problems are neglected in the present regulations [1] by simply demanding for example specific loading conditions and permneabilities. Other problems are covered by other regulations: grounding and oil outflow. The remaining items are treated in a more or less rigorous manner. allowing for some ambiguity in the application of the regulations. This is a very undesirable situation as the resulting safety measure, the attained subdivision index, should be the same for a given ship, irrespective of the design office or computer software

7 I I Figure 1: General arrangement of a Ro-Ro ship (student's exercise). The wireframe model of the outer hull is defined interactively in the programn package. It is possible to monitor important hydrostatic data such as displacement, center of buoyancy and center of flotation during definition and modification of the model. When the wireframe model is completed the topology of the hull can be established automatically and a surface model using N-sided patches is generated. Usually the surface model is only used for visual presentation of the hull while the hydrostatic properties are calculated using a longitudinal integration technique. However, a panel integration procedure is also available, [5]. The wireframe model can be transferred to any CAD/CAE package able to read points and B-splines from an IGES file. The drawing on Figure 1, showing the Ro-Ro ship used as an example in final section, is made by a transfer to AutoCAD. The internal subdivision of the hull is described by compartments, modelling all closed volumes e.g. cargo holds, engine room, water ballast tanks and void spaces. A compartment is a collection of volumes with each volume defined by two transverse bulkheads, a number of longitudinal surfaces and of course the outer hull. The longitudinal surfaces are defined by a number of transverse polygons representing the shape of the surface at given longitudinal positions. These transverse polygon curves form a skeleton which is skinned to give the exact shape of the surface. By this approach rather complicated volumes and hence compartments can be defined. For each compartment an intact and a damage volume permeability are given.

8 doing the calculations. Therefore IM40 has released an explanatory note, limo Resolution A.684(17), [2] which eliminates some of the ambiguity, but still accepts different modelling of the damaged compartments. In this paper an outline will be given of a computer program package for ship design including a probabilistic damage stability module in accordance with [I], but so versatile that improvements in the regulations can easily be accommodated. In addition the expected oil outflow in a collision is determined, using the procedure described in [3]. The computer software is especially intended for use in the preliminary design phase where the compartment layout can still be changed. Therefore, the procedure includes a sensitivity analysis for the attained subdivision index with respect to the vertical cefitre of gravity and on selected bulkhead and deck positions. The designer may from these results relatively easily determine a compartment layout satisfying the requirement to the attained subdivision index. In the next section a short description of the geometry definition of the hull and compartment layout in the present procedure is given. The following sections discuss some of the pertinent aspects in the probabilistic damage calculations and a possible extension to oil outflow estimation. The final section deals with an application of the procedure in preliminary ship design. GEOMETRY DEFINITON The first part in the description of the geometry is the definition of the outer hull. In the computer program package used in the present study the hull is defined by a number of twoand three dimensional curves. The available curves are station curves, contour curves, water lines, knuckle lines, buttocks and generic curves. A wireframe model of the outer hull surface can easily be defined using these curves. Both symmetrical and asymmetrical hulls can be represented. It is important to notice that the wireframe model is a connected wireframe, i.e. the curves are "glued " together at the intersection points. This forces e.,g. a water line intersecting a station to change shape if the shape of the station curve is modified. All the curves in the wireframe are represented by Ferguson splines [4] which is an interpolating spline containing the offset points.

9 xvj Y12 p= f c(y) f a(x) dxdy (4) 0 x 1 + y12 where J = x 2 - xi. Performing the integrations the results quoted in [1] are exactly obtained except for compartments bounded by either the forward or aft end of the ship, where p. is calculated slightly differently, [4]. For instance J../2 fl384j4 j-)y2 Po= f c~y) f a(x) dxdy + f c(y) f a(x) dxdy (5) a for the aft compartment. The first integral is seen to include damages aft of the aft end which seems strange, but could be argued with the need to include some damages to the stem not included in the damage location parameter x. Integration of Eq. (5) yields exactly the result quoted in [1]. A more rational calculation of pc would be to apply Eq. (4) also for the fore and aft compartments. Thereby, c(y) should be normalized such that yielding J, 1 -y/2 f c(y) f a(x) dxdy = 1 (6) 0 y/2 c&y)=c(1 -ty (7) where 2 1 = 08j 0O,12 (8) The fact that a more rational approach could be followed indicates that future changes in the damage statistics are very likely. Improvements in the damage statistics for the transverse penetration and for the vertical extent of damage are even more urgently needed as the present formulas in [1] can result in negative probabilities or ship designs which may suffer

10 This allows the designer to use correct permeabilities whenever applicable. It is assumed that the fluid contents in a compartment can move freely between all volumes used to define the compartment. It should be mentioned that the entire internal subdivision is semi parametric allowing the designer to make major changes with limited effort. The loading conditions are defined by a combination of fixed masses and liquid content in user-selected compartments. This ensures correct values for the center of gravity and free surface effects. PROBABILISTIC DAMAGE STABILITY CALCULATIONS The damage statistics applied in [I] is based on recorded data for 296 rammed ships. A quite extensive discussion of this damage statistics is given in [2], but the actual derivation of it is omitted. However, it may easily be shown, [6] that it is derived using a joint probability density function f(x,y) in the form ftx,y) = a(x)c(y) (1) where x and y are the dimensionless longitudinal position and extent, respectively of the damage. The functions a(x), c(y) are a(x) ={ x; < x : 1/2 (2) and c0,) = 1- ; J (3) and it is readily verified that Eq. (2) and Eq. (3) fit reasonably well with the damage statistics shown in [2] using Jm = The probability p, that only the compartment bounded by x, and x, (x 1 < x 2 ) is damaged in a collision becomes

11 a catastrophic accident if rammed by a ship with a bow height larger than the stipulated minimum bow height of the ramming ship. Therefore, the current work taking place in a Nordic Project on Safety of Passenger/RoRo Vessels should be followed with great attention. Some preliminary results indicate that using the theories outlined in [9], reasonable collision damage statistics may be obtained for ships sailing in specific routes. This is a very important development as it points toward a rational procedure for estimating the collision damage statistics without releying solely on actual recorded collision events. The procedure takes into account the ship traffic in the area, navigational aids and the structural layout of the ships and thus gives the ship designer and the maritime administrations a tool able to estimate the benefit of various changes in the ship structures and in the operational profile. For the complex compartment layout in actual ships, some simplifications must be done in order to determine the probability of damaging a single compartment or group of compartments. The explanatory notes [2] deal very extensively with this matter, but also demonstrate that various approaches, yielding different results, all are in accordance with the regulations [U]. From a computational point of view the most convenient of the acceptable procedures is to divide each compartment into fictitious, rectangular boxes. Thereby, the designer does not need to specify the transverse depth of a compartment which may be difficult, as shown in [2]. Furthermore, the use of fictitious compartments usually leads to the largest value of the attained subdivision index as beneficial effects of all kinds of recesses are taken into account. The computer module for probabilistic damage stability implemented in the present program therefore makes use of an automatic subdivision into fictitious rectangular boxes. The most time consuming part of the probabilistic damage stability calculation is determination of the GZ-curve for each damage configuration. As the same damage configuration can appear many times when fictitious compartments are used the GZ-cur-ve is of course stored when first calculated. In the preliminary design phase the vertical center of gravity is often not known precisely and therefore damage stability results for different values of the intact GM are useful. Such results are easily obtained without the need for further damage stability calculations as where 8 is the angle of heel and GG& is the change in vertical centre of gravity.

12 GZ(8) = G'Z(O) + GG' sin 8 (9) The sensitivity of the attained subdivision index to user-selected re-positions of bulkheads is calculated automatically. Thereby, the designer is guided towards bulkhead locations satisfying the requirements in [1]. Also some additional local requirements as discussed in [7] may be considered. Proper account of openings and pipe connections between compartments are extremely important and may be the most tedious point to deal with due to the complexity of the piping system in ships. As mentioned in [8], the bilge piping system effectively connects a large number of tanks in a damage condition, where a number of pipes are damaged. In the preliminary design phase the topology of the ship is seldom documented in such detail as to automatically extract information of these openings and the designer must therefore carefully either define such openings for a compartment or specify a probability of survival equal to zero for any damage conditions involving that compartment. Nearly as important as the openings are the permeabilities assigned to the flooded compartments. These values are fixed in the regulations [1], but as discussed in e.g-. [6], [8] more accurate values should be aimed at including both volumetric and surface permeabilities. Finally, the number of loading conditions may need some extension. Today only full loading and partial loading must be analysed. This seems to be too few and may lead to designs which are overly safe in one condition and disproportionately below the average requirement in the other condition. An extension to three loading conditions may be better. In this context one may also look at the specification of these loading conditions. It seems much more relevant to use actual loading conditions including liquid cargo content in tanks, rather than fictitious loading conditions based solely on the hydrostatics of the ship. PROBABILISTIC ASSESSMENT OF OIL OUTFLOW The present regulations concerning prevention of oil outflow are given in Regulation I13F of Annex I of MARPOL73/78. For purposes of comparison between different tanker designs three outflow parameters are defined: Probability of zero outflow, mean outflow and extreme

13 In the master's programme students are generally taught Ship Design and Construction in their fourth and fifth semester. They have prior to that learned the basics which include intact stability and to a very limited extent damage stability. But they have no or very limited experience with actual ship constructions. Some experience of this they will gain from the literature, mainly journals and periodicals, where they study general arrangements and descriptions of ships already built. This enables them to make a conceptual design of the ship of the type and size which they are assigned. In the conceptual phase of design the subdivision of the ship is carried out with consideration mainly for the use of various compartments, i.e. engine room, cargo space, tanks, etc.. The experienced naval architect will, however, already in this stage give some thoughts to the damage stability, keeping in mind that positions of bulkheads could be subject to minor modifications in the preliminary stage of design. When the designer has little experience, or is working with a novel and innovative design it may call for calculations for ]arge number of configurations to establish the optimum subdivision. It is therefore important that the procedure used will help the inexperienced as well as the innovative designer in obtaining an overview of the influence of the positions of watertight subdivisions on the attained subdivision index A thus giving guidelines towards the most favourable subdivision. As described previously the damage stability analysis is done for two loading cases (partial and full load) for a series of user-selected values of intact metacentric heights GM. For ships of the type where damage stability is crucial such calculations will put restrictions to the lowest acceptable value of GM in the intact condition and the designer should assess the most appropriate pair of restrictions on the basis of his knowledge regarding the operational profile of the ship. It is of course vital that in each of the two loading conditions a suitably safety level is chosen.

14 (local) outflow. A weighted single, so-called Pollution Index E, can be defined from these parameters and used for overall assessment of a given design relative to a reference double hull design. The damage statistics applied is specified separately for collision and grounding. For collision the damage statistics used in the MARPOL regulations differs quite significantly from the damage statistics applied in the damage stability regulations [1]. From a rational point of view such differences must be expected to diminish in future modifications of the regulations, although some correlation of the damage statistics with ship type may be included. The current damage statistics used in oil outflow calculations for collisions can yield a negative probability for damaging a group of compartments for certain compartment layouts. This can easily be demonstrated by examples. The same is true with the damnage statistics for damage stability calculations, but to a much lesser degree. In both cases this is due to approximations in the formulas for the damage statistics, and future modifications should remove this undesirable phenomena. A similar development of a rational procedure as mentioned for collision damage statistics may also be expected for grounding accidents and this may be the way to remove the inconsistencies in the current regulations. In the program package, the damage statistics specified in [1] for damage stability analysis is also applied for prediction of oil outflow. This procedure has previously been suggested, [3]. However, other damage statistics may quite easily be implemented as well. APPLICATION OF THE PROCEDURE Because the procedure has been developed by a University institute it has been implemented for two groups of users: Wi Students doing their first ship design in the institute's course on Ship Design and Construction. (ii) Experienced naval architects in shipyards and ship consultancies.

15 GMI (m] - original 1---: modified 21! A<R \ A, >R I' 0 -[ itact req 0 2 GMf [m] 3 Figure 3: Combinations of metacentric heights for full and partial loadings, GMf and GMP, satisfying A = R for both the original and modified design. REFERENCES [1] Resolution MSC 19(58), "On the Adoption of Amendments to SOLAS Convention, regarding Subdivision and Damage Stability of Cargo Ships", Report of the MSC on its 58th Session, MSC 58/25, Annex 2, ]IMO, London, UK [2] Resolution A.684(17), "Explanatory Notes to the SOLAS Regulations on Subdivision and Damage Stability of Cargo Ships of 100 Metres in Length and Over", W[O, London, UK

16 1.0 A :original modified 1// full GM:m] 3 Figure 2: Attained subdivision index A as function of intact metacentric height GM for the partial (AP) and full load (A,) conditions. The output of such an analysis for the two loading conditions is shown in Figure 2. It is now up to the designer to decide for the design conditions fulfilling A = 12 (Ap + A ) > R (10) where R is the required index, [1]. Such calculations must be done for many compartment layouts covering larger as well as smaller modifications in the internal subdivision of the ship. The computer program makes such modifications easily possible. If for instance the depth of a deck or the height of a double bottom is modified all compartments involved are automatically modified too. Such a situation is illustrated in the following, where a Ro-Ro ship design made by students as an exercise is modified by moving the main deck 0.5 m upwards. The general arrangement of the ship is shown in Figure 1. The attained subdivision indices (Af, Ap) as function of intact GM are shown in Figure 2 for the full and the partial load case both before and after the modification of the depth to the main deck. From these results the designer must select a pair of conditions which fulfil Eq. (10). These combinations are shown in Figure 3.

17 [3] Pawlowski, M., "Oil Spill Prevention with New Ship Types in the Light of the Probabilistic Concept", Proc. WEMT'95, Copenhagen, Denmark May, 1995, pp [4] Ferguson, J., "Multivariable Curve Interpolation", JA CM, 1112, pp , [5] Schalck, S. and Baatup, J. "Hydrostatic Stability Calculations by Pressure Integrations", Ocean Engineering, Vol. 17, No. 1-2, pp , [6] Jensen, J. Juncher, "Damage Stability Rules in Relation to Ship Design", Proc. WEMT'95, Copenhagen, Denmark, May, 1995, pp [7] Sen, P. and Gerigk, M.K., "Some Aspects of a Knowledge-Based Expert System for Preliminary Ship Subdivision Design for Safety", Proc. PRADS92, Vol. 2, pp Eds. Caldwell, J.B. and Ward, G., Elsevier Publ. Ltd. UK., [8] Koelman, H.J. "Freedom is just Another Word for Nothing Left to Loose", Proc. WEMT'95, Copenhagen, Denmark May, 1995, pp [9] Petersen, P. Temdrup. "Collision and Grounding Mechanics", Proc. WEMT'95, Copenhagen, Denmark May, 1995, pp

18 WEGEMT WORKSHOP: DAMAGE STABILITY OF SHIPS DTU, Lyngby, 20th October 1995 "A NEW SAFETY STANDARD FOR PASSENGER/RORO VESSELS" by Tor E. Svensen, Det Norske Veritas Classification AS ABSTRACT The paper presents the main objectives of the recently initiated joint North-West European project "Safety of Passenger/RoRo Vessels". A critical examination of existing damage stability standards is made and the principal risks not covered are discussed. Important aspects such as damage stability modelling methods, watertight integrity, intermediate stages of flooding and dynamic effects are discussed and some possible solutions outlined. Recent events have shown that passenger/roro vessels are vulnerable when subject to large scale flooding and that stability and survivability requirements must be improved. In particular the principle of creating a second barrier of defence against technical or human failure is discussed. Methods of performing a risk analysis on passenger/roro vessels are presented and the possible role of Formal Safety Assessment as part of vessel approval and certification procedures discussed. 1. INTRODUCTION Immediately following the "Estonia" disaster, the Nordic countries together with the United Kingdom and some major Classification Societies established a project to take a fundamental new look at the stability and survivability requirements for Passenger/RoRo vessels. The aim of the project is to come up with proposals for new design requirements leading to improved safety for new vessels. The project was set up to primarily address technical aspects relating to safety and survivability of RoRo vessels with particular reference to the damaged and flooded condition. However, it has been recognised by the project group that other risks should be considered in an overall assessment. In particular, it is considered important to ensure that other risks are not increased as a consequence of different design solutions that are intrinsically safer from a stability consideration. The project has been split into two phases with Phase I addressing the most urgent issue of improving the stability of Passenger/RoRo vessels when subject to large scale flooding. The project will specifically address the issue of identifying and testing a second line of defence against technical or human failure. In practice this means that a single failure or incident should not lead to catastrophic consequences. The results of the project will form the basis of proposals for new and extended Nordic and International rules

19 Phase 2 of the project will examine how safety assessment procedures can be applied to the passenger/roro type of design. The safety assessment will be applied to any new ruleframework to ensure that no future designs will be constructed and operated in such a way that a single failure may result in a major accident. Similarly, a minor accident should not be allowed to escalate into a major accident. The safety assessment study to be carried out in Phase 2 of the project will also describe a framework for how a safety assessment procedure should be carried out and documented on a new design. The project objectives and organisation is briefly described in the enclosed Appendix. 2. IMO STABILITY REQUIREMENTS - A BRIEF HISTORICAL REVIEW Historically, most changes in international regulations for ship design and operation have been introduced as a result of major disasters with a large loss of life. The first notable of such disasters was the well known sinking of the TITANIC. Probably the most important outcome of the international conference held after the TITANIC disaster was the new requirements for life saving appliances. A new conference held in 1929 resulted in requirements for subdivision in terms of floodable length calculations. It is important to note that the principal focus at the 1929 convention was on intact stability and floodable length requirements. The first damage stability requirements were introduced following the 1948 convention. Present damaged stability requirements for RoRo vessels are generally based upon the same deterministic principles, although some important improvements have been made. Most notably these improvements involve requirements to residual stability (range, height and area of GZ curve) after damage. These requirements were made effective from 1990, and for the first time in the history of the IMO, they were made retroactive to existing ships. The first probabilistic damage stability rules for passenger vessels were introduced in 1967 as an alternative to the deterministic requirements in SOLAS-60. For most of the passenger/roro vessels the requirements contained within this new probabilistic framework A.265 are more stringent than the deterministic requirements in SOLAS- 60 and therefore A.265 has generally not been much used on passenger/roro vessels. The next major step in the development of stability standards came in 1992 with the introduction of SOLAS part B-1 (in Chapter 11-1), containing a probabilistic standard for cargo vessels. The IMO has worked on the harmonisation of stability standards for several years. Despite this we are still faced with substantially different requirements for different ship types. When comparing the different standards for different ship types, the fact is that the present deterministic stability standard for passenger/roro vessels probably 1995-I 1-06

20 represents the lowest safety standard when compared with both A.265 and the new probabilistic standard for cargo vessels (SOLAS part B-1, 1992). Following ajoint research project, the Nordic countries presented to the 38th session of SLF a draft probabilistic damage stability regulation for passenger vessels. In this proposal the survival capability is solely based upon the GZ curve characteristics and the margin line and bulkhead deck are not considered. This proposal may represent the first step towards a new probabilistic standard for passenger/roro vessels. However, before this can become an acceptable standard, there are some important further problems that need to be addressed as outlined below. 3. PRESENT STABILITY REQUIREMENTS FOR RORO VESSELS - WHAT ARE THE PROBLEMS? In principle all existing RoRo vessels satisfying the SOLAS 2-compartment standard has an adequate stability margin for surviving a damage provided the weather is calm and there is no cargo shift. In practice it has been clearly demonstrated in the work carried out after the accident of the "Herald of Free Enterprise" that a modern passenger/roro vessel with a standard SOLAS side damage will rapidly be filled with water on the vehicle deck and capsize if the waveheight is above m. This has been somewhat improved with SOLAS-90, but is still considered inadequate. The number of recent major disasters with passenger RoRo vessels have clearly confirmed their extreme vulnerability when water is allowed to enter the vehicle deck. Combined with cargo shift the outcome can be rapid capsize without much time for passengers to evacuate the vessel. WHAT ARE THE M4hIV SHOR TCOMIVGS? The principal shortcomings of the present SOLAS standard for passenger/roro vessels in international unrestricted trade can be listed as follows: The possibility that the vehicle deck may be flooded is not included in the calculations. The type B freeboard definition used on RoRo vessels means that only compartments below the freeboard deck are considered in the damage stability calculations. Although recent tragedies have involved flooding through the bow doors it is well recognised that side collision with damage to compartments below the freeboard deck as well as opening to the freeboard deck itself represents one of the most likely accident scenarios. The extensive work carried out after the "Herald of Free Enterprise" accident clearly demonstrated that most existing designs will not survive a standard SOLAS side damage in waves above Im. Even vessels built to SOLAS-90 standard are unlikely to survive in waves much above 1.5-2m. Clearly this is inadequate for operation of large passenger/roro vessels in unrestricted waters

21 * Shift of cargo is not included as a risk. All RoRo cargo is assumed to remain safely in the original position and there is no additional heeling moment applied to the calculations due to shift of cargo. This is considered to be an unrealistic assumption for a vessel subject to unsymmetrical flooding after damage and rolling heavily in waves. * The present SOLAS-90 standard for Passenger/RoRo vessels is entirely deterministic. A new probabilistic standard should be developed. This will be more logical and will provide a more objective measure of the survival capability. Such a risk based method is consistent with current thinking on safety analysis and risk management. In addition, the fact is that most vessels operate with watertight doors open during the voyage and this is contrary to the assumptions made when damage stability calculations are approved. Some of the damage stability modelling methods in use make only static assumptions with respect to the internal waterline in flooded compartments. This is a very doubtful assumption. WHY PROBABIL IS TIC STABILITY CRITERIA? They allow the risk of a particular event such as collision and flooding to be combined with the probability of survival to give a resulting index describing a weighted survival capability. By combining the results of damage scenarios to one compartment or a group of compartments with a probability of the vessel surviving the damage, it is possible to calculate the attained subdivision index A. In practice this is a survival index for the complete design. The most important point about a probabilistic method is that it is less arbitrary and provides a more objective measure of the survival capability of the vessel in the case of damage compared with a standard deterministic method.. By using a risk-based method those events that have a likelihood of occurring carry a heavier weight and conversely those events with a very low probability of occurring have a small influence upon the final result. 4. REQUIREMENTS FOR A NEW STABILITY STANDARD. An important requirement for a new stability standard for passenger/roro vessels is that it should not destroy the basic principles behind the RoRo concept. It should be recognised that the RoRo design is part of a highway system. Any new regulations must recognise this basic principle and not result in rigid deterministic requirements making the RoRo concept totally unworkable. The basic requirements for a new stability standard for passenger/roro vessels can be listed as follows: 1) Should be based upon the probabilistic method

22 The basic requirements for a new stability standard for passcnger/roro vessels can be listed as follows: I) Should be based upon die probabilistic method. 2) Major risks such as flooding of the RoRo deck and cargo shift should be included. 3) A method for managing residual risk (i.e. preventing rapid capsize in those damage cases where the vessel does not survive) should be included The framework. The purpose of the framework is to control that the risk of sinking or capsizing as a result of damage to the vessel or malfunction of vessel's system or system components, whether due to technical or human failures, is brought down to an acceptable level. At the same time the residual risk should be managed in such a way that the number of fatalities are kept as low as practically possible. The risk may be expressed as: RISK = Prohbability - Consequence Consequence Max. tolerable consequence Uacpal risk iunacceptable Reducing probability Acceptable risk Reducing consequence Probability

23 If the risk is too large, it may be reduced by reducing the probability, reducing the consequence, or a combination of these. There will always be a limit of max. tolerable consequence, for example rapid capsize with loss of many lives. Above this limit, the risk can not be reduced by reducing the probability alone. In practice the proposed new stability framework for passenger/roro vessels will tentatively be based upon the following probabilistic calculation procedures: Calculation of attained subdivision index (A): A = 2(p * s) taken over all damage cases and combinations of damage cases where A = attained subdivision index p = probability of damage s = probability of survival with given damage A to be greater than a specified value Calculation otfcapsize index (C): C = * p c) taken over all damage cases and combinations of damage cases where C = attained capsize index p = probability of damage c = probability of capsizing with given damage (measure of residual risk) C to he less than a specified value This latter probabilistic index relatingt to a given capsize probability is introduced in order to ensure that future designs are constructed in such a way that they will sink in a controlled manner without capsizing after major damagce in those cases where the vessel will not survive. The indices may be illustrated in the fiollowing diagram: CONTROLLED SINKING - Attained capsize index "C" IControlled sinking (not capsize) A I Attained subdivision index "A"

24 MAIN ISSUES TO BE STUDIED: Issues that will be specifically studied in the project in order to address present shortcomings in the probabilistic methods as already introduced for other types of vessels are listed below. It is intended that the results will provide a reliable and well documented basis for a new ruleframework. Damage extent: Instead of using old damage statistics a new method will be developed based upon calculating the risk of collision on a given route. This will be combined with the distribution of ship types to calculate the risk of impact with different ship types. Using first principles methods the probability of size of damage in terms of vertical extent, damage length and penetration will be determined. The method will examine and, if possible, take into account the actual ship structural design of the RoRo, thus giving credit to a collision resistant structure. " Flooding and dynamic effects in waves: A critical parameter in a new probabilistic framework is how much water will enter the vehicle deck after a given collision damage and how large reserve is required on the GZ curve in order for the vessel to survive. Model tests are carried out to determine the time function of water entry as a function of waveheigth, GM, freeboard, damage size and other relevant parameters. Combined with the development of a theoretical model prediction of vessel motions and capsize it is expected that the project will arrive at clear criteria for survival to be used in a proposed new framework. Implicitly by introducing waveheigth as a parameter for survival will be the opportunity to introduce service restrictions operating in more protected waters. Damage stability calculation methods: Critical examination of the principles of damage stability modelling with particular reference to the basic assumptions and calculation methods employed, such as symmetrical flooding, intermediate stages of flooding, permeability. The key issue here will be to develop calculation methods that reflect the actual design solutions. " Cargo shift: Development of a deterministic requirement for reserve stability as a function of cargo type, number of lanes, deck layout etc. 5. SAFETY ASSESSMENT FOR PASSENGER/RoRo VESSELS Phase 2 of the project is devoted to the development of the safety assessment methodology to passenger/roro vessels. These methods and procedures have been used in the offshore industry, the nuclear power industry and the chemical process industries for many years. The number of applications in the shipping industry are to date very limited. Briefly the safety assessment procedure for a passenger/roro vessel will involve the following elements:

25 * System definition * Hazard identification * Frequency analysis and consequence modelling * Risk presentation " Evaluation using risk criteria * Selection of risk reduction measures A major difference between the offshore industry and the shipping industry is that individual ships within a class of ships are generically very similar. This is likely to result in procedures which are simpler to implement than what we have seen to date in the offshore industry. We are therefore likely to end up with a type of safety assessment where most of the requirements for redundancy, prevention of escalation etc. are covered by rules for design and constructions. Individual safety assessments for new designs will only be carried on a more limited scale and will concentrate on those items that are specific for the particular vessel design and operation. Phase 2 of the project will focus on the following items: * Qualitative risk analysis. Application of existing techniques such as Hazard Identification, FMECA and SWIFT to an existing vessel. Identification of shortcomings in present design practices and applicable rules. This work will focus on prevention of single failures resulting in major accidents and smaller accidents escalating into major accidents. " Quantitative risk analysis. Development of procedures for complete quantitative risk analysys on passenger/roro vessels. Data collection and application to an existing vessel and a new design on one or more routes. * Risk assessment for new stability framework. The purpose will be to document that the new framework has resulted in a significant reduction in the probability of capsize and sinking compared with the existing stability rules. * Development of procedures for safety assessment on individual vessels. The purpose is to develop recommendations for a rational procedure recognising that most safety aspects will be covered by prescriptive rules and concentrating on design aspects that are deviating from the rule basis. It is generally believed that by following the above described procedure this will result in the most cost effective solutions. A pre-study already carried out in the project has concluded that: I) Techniques for qualitative risk analysis on passenger/roro vessels are already available, have already been used and can be implemented immediately. Quantitative techniques require some further development and data sources need to be identified

26 However, it should be possible to have both techniques implemented within a timescale of a few months. 2) Future uses of risk assessment for passenger/roro vessels will be at two levels: " Assessment of the risks of passenger/roro vessels in general, to indicate the need for new initiatives in safety regulation, to target them cost effectively, or to estimate their benefit if they were adopted. " Assessment of individual vessels to indicate the need for safety measures in their design and operation, and to provide a basis for Safety Cases for them. 3) An initial risk analysis of a passenger/roro vessel has concluded that the risk to the individual passenger is no higher than for other means of public transport. However, the risk that many lives will be lost in a single accident is significantly higher than for other means of transport. 6. CONCLUSIONS The development of a new safety standard for new passenger/roro designs with particular focus on stability and survivability in the damaged and flooded condition is considered essential in the light of recent tragic accidents. The main aim will be to develop a second barrier of defence against technical or human failure. The main features of this new standard will be:, new stability framework based upon probabilistic methods, allowing a more objective assessment of survivability * criteria for managing residual risk to ensure that the probability of rapid capsize is as low as practically possible * new rule developments based upon safety assessment

27 REFERENCES: 1. SOLAS '29: International Conference on Safety of Life at Sea SOLAS '60: International Conference on Safety of Life at Sea SOLAS' 74: International Convention for the Safety of Life at Sea. London, IMO Resolution A.265 (VIII). Regulations on Subdivision and Stability of Passenger Ships as Equivalent to Part B of Chapter II of the International convention for the Safety of Life at Sea, IMO. London, SOLAS 90: Ch. 1I-1, Part B-i: Subdivision and damage stability of cargo ships. 6. LLOYD, C.J.: "Research into Enhancing the Stability and Survivability of RoRo Passenger Ferries - Overview Study", Joint RTNA/DTp International Symposium on the Safety of RoRo Passenger Ships, London, April Vassalos, D. Dr.: "Capsizal Resistance Prediction of a Damaged Ship in a Random Sea", Joint RINA/DTp International Symposium on RoRo Ships' Survivability, London, November Dand, I.W.: "Factors Affecting the Capsize of Damaged RoRo Vessels in Waves", Joint RINA/DTp International Symposium on RoRo Ships' Survivability., London, November Velschou, S. and Schindler, M.: "RoRo Passenger Ferry Damage Stability Studies - a Continuation of Model Tests for a Typical Ferry", Joint RINA/DTp International Symposium on RoRo Ships' Survivability, London, November I 1-06

28 APPENDIX Project Description for Joint Nordic Project "Safety of Passenger/RoRo Vessels" 1. OBJECTIVES The main objective of the project is to investigate technical aspects relating to safety and survivability of RoRo vessels with particular reference to the damaged and flooded condition. The project will specifically address the issue of identify'ing and testing a second line of defence against technical or human failure. This will form the basis of proposals for new and extended Nordic and international rules. 2. SCOPE OF WORK The scope of work is defined in principal tasks as follows: Phase 1 : Stability: Task 1: Damage stability modelling methods: Critical examination of the principles of damage stability modelling with particular reference to the basic assumptions and calculation methods employed, such as symmetrical flooding, intermediate stages of flooding and permeability. Recommendations with proposals for improvements in modelling methods including realistic conditions for treatment of compartment boundaries and penetrations. Task 2. 1: Damage Extent: Development of method to predict the size and extent of damage on passenger/roro vessels as a result of collisions with other vessels. The method will be based upon analytical techniques taking into account frequency estimates of collisions due to ship traffic in the area and rational models for consequences of given ship collisions. The method will be utilised in two ways: 1) to enhance the existing method based upon purely historical data, allowing the actual traffic and normal RoRo ship structures to be taken into account and 2) to determine estimates for the statistical distribution of ship damages to be used in proposed new rules. T ask 2. ' Large Scale Flooding: M,/odel test investigations to quantify' how water ingress on the RoRo deck depends upon damage size, freeboard. GM, deck layout and seastate. Progressive tests in which the flooding is free to develop will be performed for GM values close to capsize for variations in freeboard, size and location of damage,

29 different seastates and different deck layouts. The primary purpose of the tests is to determine the amount and distribution of water trapped on the RoRo deck as a function of time and to determine the relative water level at the damage opening. The test results will also be used to validate the theoretical model developed under Task 5. Task 3 : Dynamic Effects in Waves: Model tests will be performed in order to develop a method for calculating the dynamic effects acting upon a vessel in a seaway when subjected to flooding after damage. Alternative amounts of water on deck, different GM values, different seastates. vessel headings and speeds will be investigated. The results of the investigations will be used to verify the theoretical model developed under Task 5 and to develop a proposed simplified method for describing the amount of reserve stability required in order for the vessel to survive without capsizing in a given seastate. Task 4 : Cargo Securing and Cargo Shift: Existing rules and regulations for cargo stowage and securing will be reviewed. Maximum heeling moments caused by cargo shift will be calculated in a deterministic way as a function of cargo type, width and number of lanes etc. A method for calculation of accelerations and forces acting on the cargo onboard a damaged vessel moving in an open sea will be developed as a function of the relative heeling angle. Finally, a deterministic calculation method will be developed for predicting the heeling moment caused by cargo shift as a function of the relative heeling angle. Task 5: Development of M~athematical Model for Capsize Predictions: A mathematical model for assessing the capsize safety of passenger/roro, vessels will be finalised. The model will be used for a systematic parametric investigation to identify and quantify the effect of key influencing factors on vessel survivability. Calibration of the model will be undertaken using the results of the model tests performed in Tasks 2.2 and 3. The model will further be used towards the development of relationships between ship design and environmental parameters and stability related parameters to be used as basis for deciding on appropriate levels regarding new probabilistic criteria. Task 6 : Framework for New Damage Stability Standard: Requirements to damage stability assessment for RoRo ships will be developed and formulated, taking into account risk factors relevant for damage stability. The framework shall address all risks relevant to damage stability, like collisions. groundings, structural failures, etc. The prime goal is to provide a second line of defence against technical and human failures, such that adequate constructional features and technical/functional requirements may be provided. The framework shall describe procedures for formulation of requirements based on damaged GZ curve and other relevant damage stability parameters, together with assumed damage and damage statistics. Basis for the procedures shall be statistics on weather conditions and traffic density for a certain service area. All relevant effects, including large scale flooding, cargo shift, dynamic behaviour in waves, etc., shall be taken into account. Procedures for managing residual risks by

30 minimising the risk of capsize shall be included. Example on formulation of such regulations shall be given. Task 7: Example Design : One or more example design will be developed in close cooperation with a design consultant and/or shipyard in order to exemplify the proposed new rule framework and how this may be satisfied. Phase 2 : Safety Assessment Task 8: Risk assessment (stability): A risk study will be made based upon the new set of rule framework for existing designs, proposed new designs and other designs (dry cargo, passenger). Risk reduction factors for designs developed within the new framework will be documented. Task 9 & Task 10: Safety Assessment: " Development of procedures for safety assessment on individual vessels. The purpose is to develop a rational procedure recognising that most safety aspects will be covered by prescriptive rules and concentrating on design aspects that are deviating from the rule basis. * Safety assessment on specific parts of IMO rules for design ofpassenger/roro vessels. The purpose will be to identify shortcomings and propose new rules. This work will focus on prevention of single failures resulting in major accidents and smaller accidents escalating into major accidents. 3. PROJECT TIMEPLAN, ORGANISATION AND BUDGET Phase 1 of the project will be carried out over a period of approximately one year with the main task of developing a new rule framework completed by early Phase 2 is being carried out in parallel with Phase I. A pre-study on the application of safety assessment procedures to passenger/roro ships will be completed by April However, the main effort in Phase 2 is from mid-1995 and the work will be finalised by February The work on the project is split into tasks under the overall management of Det Norske Veritas Classification. The institutions and companies which are responsible for the tasks in Phase I are shown in the project organisation chart below

31 minimising the risk of capsize shall be included. Example on formulation of such regulations shall be given. Task 7 : Example Design : One or more example design will be developed in close cooperation with a design consultant and/or shipyard in order to exemplify the proposed new rule framework and how this may be satisfied. Phase 2 : Safety Assessment Task 8: Risk assessment (stability): A risk study will be made based upon the new set of rule framework for existing designs, proposed new designs and other designs (dry cargo, passenger). Risk reduction factors for designs developed within the new framework will be documented. Task 9 & Task 10: Safety Assessment: * Development ofprocedures for safety assessment on individual vessels. The purpose is to develop a rational procedure recognising that most safety aspects will be covered by prescriptive rules and concentrating on design aspects that are deviating from the rule basis. * Safety assessment on specific parts of IMO rules for design ofpassenger/roro vessels. The purpose will be to identify shortcomings and propose new rules. This work will focus on prevention of single failures resulting in major accidents and smaller accidents escalating into major accidents. 3. PROJECT TIMEPLAN, ORGANISATION AND BUDGET Phase 1 of the project will be carried out over a period of approximately one year with the main task of developing a new rule framework completed by early Phase 2 is being carried out in parallel with Phase 1. A pre-study on the application of safety assessment procedures to passenger/roro ships will be completed by April However, the main effort in Phase 2 is from mid-1995 and the work will be finalised by February The work on the project is split into tasks under the overall management of Det Norske Veritas Classification. The institutions and companies which are responsible for the tasks in Phase I are shown in the project organisation chart below: 1995-I 1-06

32 Main cornerstones of new probabilistic stability framework: SOLAS 1992, B-1) TASK 1 Stability TASK 2.1 Modelling Collision Methodsli - down flooding - damage size Damage Methods light structures -air pockets - damage water I - intenmed. stages distribution - cross flooding (p-factor) - asymetric flooding - time calculations Flooding Prediction NEW PROBABILISTIC STABILITY FRAMEWORK STASK 3 - residual Dynamic stability Effects TASK 4 capability TaSKe - heling mom. (s-factor) TASK 5 Cargo Shift IDevelopm. -securing stdcriteria C s Attained capsize index "C" A rn F Controlled sinking (not capsize) "S" Attained subdiv. -]index "A"

33 CAPSIZAL RESISTANCE OF DAMAGED RO-RO FERRIES: Modelling and Application DRACOS VASSALOS Ship Stability Research Group Department of Ship & Marine Technology University of Strathclyde, Glasgow ABSTRACT This paper presents a summary of the research work undertaken over the past few months in association with the Joint R&D Project between the Nordic countries, UK, France and Germany in so far as it outlines the fundamental thinking behind the approaches adopted and highlights some of the promising early findings. Representative results are also presented and discussed. INTRODUCTION Accident statistics clearly indicate that collision is the highest risk for passenger vessels, with 25% of accidents leading to water ingress of which more than 50% result in ship loss. Limiting understanding of the ensuing complex dynamics related to the dynamic behaviour of the vessel and the progression of flood water through the damaged ship in a random sea state resulted in approaches for assessing the damage survivability of ships that rely mainly on hydrostatic properties. Furthermore. in case of serious flooding of ships with large undivided deck spaces, such as Ro-Ro vessels, the loss could be catastrophic as a result of rapid capsize, rendering evacuation of passengers and crew impractical, with disastrous (unacceptable) consequences. The need for a methodology to reduce the risk ensuing from collision damage to-a level As Low As Reasonably Practicable cannot be overemphasised. The tragic accidents of the Herald of Free Enterprise and meire recently of Estonia were the strongest indicators yet of the existing gaps in assessing damage survivability concerning subdivision above the bulkhead deck of large

34 undivided deck spaces. They have also brought about a realisation that 'ship survival' might have to be addressed separately from 'passenger survival' in that the deterioration in the stability of such vessels when damaged could be 'catastrophic' rather than one of graceful degradation. It would appear, therefore, that the approach to assessing realistically the damage survivability of passenger ships and indeed any ships, must derive from a logical framework such as that offered by the probabilistic method and must, of necessity, offer the means of taking into consideration meaningfully both the operating environment and the hazards specific to the vessel in question. This, in turn, necessitates the development of suitable "tools" and procedures for dealing in a systematic manner with the main problems and uncertainties pertaining to serious flooding of passenger ships tollowing collision damage. Deriving from the above, one of the tasks should be to quantify the probability of damage with water ingress in a given service area and, the second, to quantify the consequences of damage by identifying and analysing all the important factors using probabilistic methods. However, even though it is self-evident that the risks involved can be reduced by reducing either the probability of damage or the consequences of damage or both, there is a level beyond which consequences cannot be tolerated. In this case, risk cannot be reduced by reducing the probability of damage alone. The need arises, therefore, for a methodology whereby key questions are addressed and answers sought concerning definition of acceptable risks, definition and nanagelment of maximum tolerable consequences and procedures for dealing with residual risks. The Joint R&D Project has been set to address this need urgently, by bringing together all the available expertise in the relevant areas. However, the wisdom of attempting to provide answers within what amounts to several months, forms in itself another interesting question. Details of the Project are provided by Dr Tor Svensen in paper 2 of these proceedings. This paper deals with Task 5 of the Joint R&D Project, pertaining to the development and validation of numerical tools for assessing the damage survivability of passenger/ Ro-Ro vessels, leading to the development of survival criteria. UK Ru-Ru Research Programme BACKGROUND In the wake of the Herald of Free Enterprise disaster, the need to evaluate the adequacy of the various standards in terms of providing sufficient residual stability to allow enough time for the orderly evacuation of passengers and crew in realistic sea states has prompted the Department of Transport to set up the Ro-Ro Research programme comprising txwo phases. Phase I addressed the residual stability of existing vessels and the key reasons behind capsizes. To this end theoretical studies were undertaken into the practical benefits and penalties of introducing a number of devices. [1], for improving the residual stability of existing Ro-Ro's. In addition, model experiments were carried out by the British Maritime Technology Ltd. [2] and the Danish Maritime Institute. [3] in order to gain an insight into the dvnamic behaviour of a damaged vessel in realistic environmental conditions and of the progression ot flood water through the ship.

35 Phase 11 was set up with the following objectives in mind: To confirm the findings of Phase I in respect of the ability of a damaged vessel to resist capsize in a given sea state. To carry out damaged model tests, in which the enhancing devices assessed in Phase I would be modelled to determine the improvements in survivability achieved in realistic sea-going conditions. To confirm that damage in the region amidships is likely to lead to the most onerous situation in respect of the probability of capsize. To undertake theoretical studies into the nature of the capsize phenomenon, with a view to extrapolating the model test results to Ro-Ro passenger ships of different sizes and proportions. The Department of Ship and Marine Technology at the University of Strathclyde was one of three organisations charged with the responsibility of developing and validating a theoretical capsize model which could predict the minimum stability needed by a damaged vessel to resist capsizing in a given sea state. This was subsequently to be used to establish limiting stability parameters that might form the basis tor developing realistic survival criteria. Full details are given in [4]. Joint R&D Project As the UK stood poised to share the findings from the Ro-Ro Research Programme with the rest of the world, the Estonia tragedy has once more shaken the foundations of shipping, forcing the profession to provide answers "immediately" and, in attempting to do so, to use the right expertise and experience to provide the right answers. The focus and effort in Ro-Ro and passenger ships capsize safety have suddenly reached the deserved and long overdue intensity. The Nordic countries responded quickly in undertaking this responsibility which led to a wider-based project within a very short period. Taking onboard the fact that. in addressing the probability of a ship surviving a given damage, the problem of damage survivability does not end with quantifying the probability of damage and the consequences of damage. As indicated above, the Estonia disaster was the strongest indicator vet of the urgent need to define acceptable risks and maximum tolerable consequences as well as to identifying procedures for managing such consequences and dealing with the residual risks. To this end. the Joint R&D Project adopted the following framework: A Framework for Rationalising the Probabilistic Approach The risk o" capsizing (or sinking) as a result of damage is given by Risk = P(damnage) x (I-A) AR where. P(daniage) = Probabilitv of damage with water ingress (per year) in a given service area

36 A = Probability of surviving the said damage (Attained Subdivision Index) AR = Acceptable risk When AR is defined and P(damage) is known, then A 2t 1 - AR /P(damage) - R where, R - Required Subdivision Index. The Attained Subdivision Index is currently calculated as A - p.s, taken over all damage cases and combination of damage cases. where. p Probability of damagze calculated from damage statistics on damage location on the ship; length, height and penetration. s Probability of survivingt a given damage depending on vessel condition before damage, permeability of damaged compartments and vessel residual stability. Currently the major defect in determining the factor -'p derives from the fact that the damage size is independent of the vessel's structural strength and the damage occurrence is also independent of the route of service. To rationalise the probabilistic approach, these two deficiencies ought to be rectified. This forms Task 2.1 in the Joint R&D Project. Deriving from the success demonstrated by the Strathclyde University Ship Stability (SUSS) Research Group during the UK Ro-Ro research and also during the two years following its completion, the Joint R&D Project decided to make full use of the mathematical model developed at Strathclyde. The intention is, following a process of further development and vigorous validation, to apply it to different vessel types, forms, sizes and compartmentation and to representative damage scenarios and environments to verify its general applicability to assessing the capsize safety of a damaged ship in a given sea state leading to the development of generalised expressions for the factor "s- to be used in the determination of A. This will facilitate the wav towards a Formal Safety Assessment methodology and help rationalise the probabilistic approach for assessing the damage survivability of ships. As mentioned above, this work forms Task 5 of the Project. The background philosophy and justification of the survival factor 's' are described in considerable detail by Professor Maciej Pawlowski (currently visiting Strathclyde) in paper 5 of these proceedings. together with a possible generalised procedure concerning its determination. The approach described in the foregoing will allow, in addition, the generation of knowledge for improving upon the design and operational practice of passenger ships.

37 Key aspects of this research are outlined in the following sections. STRATHCLYDE APPROACH General Remarks Since the dynamic behaviour of the damaged vessel and the progression of the flood water through the damaged ship in a random seaway are ever changing, rendering the dynamic system highly non-linear, the technique used, of necessity, is time simulation. The numerical experiment considered assumes a stationary ship, (forward speed could in principle be accommodated), beam on to the oncoming waves with progressive flooding taking place through the damage opening which could be of any shape, longitudinal and transverse extent and in any location throughout the vessel. As simulation begins with predefined initial conditions, the damaged ship starts moving under the action of random beam waves. Instantaneous water ingress is considered by taking into account the wave elevation and ship motions which are also estimated at each time step. For each case under investigation, simulations are carried out for different loading conditions while the sea state used in the calculations is progressively increased to a limit where the ship capsizes systematically, thus allowing for a definition of survival boundaries. Having said this, the complexity of the problem at hand dictates that several simplifications are adopted in both the mathematical formulation of the damaged vessel motions and of the water ingress in order to derive engineering solutions. These are explained next before considering some key research findings. Generalised Mathematical Models As is commonly known, the static and dynamic stability of a ship depend on its heeling or rolling motion. The heel or roll angle is itself a criterion which is taken into account by intact and damage ship stability assessment procedures. However. in a real environment other motions could significantly affect the ship's stability and roll motion directly or indirectly. In studying extreme vessel behaviour one should clearly aim bfor a model that represents reality meaningfully. The strong hydrodvnamic coupling of sway into roll and the non-linear hvdrostatic coupling of heave into both significantly change the underwater volume of the ship in roll. Heave motion is also clearly important in affecting the rate of flooding through the ship and in influencing the roll motion itself. In addition, a vessel in beam seas will drift and this gives rise to additional forces acting on the ship and so would the sloshing motion of the flood water. Therefore, the sway motion contribution to the ensuing vessel behaviour is expected to be significant. Furthermore, even in a beam seas situation, a vessel is generally expected to undergo a change in its heading relative to the waves, depending on the longitudinal distribution of the underwater volume and as a result of this will start pitching. To accom modate this situation and also the general situation of wave headings other than beam seas. a coupled six-degrees-of-freedom mathematical model of ship motions would he necessary. In the majority of cases considered so far, however, it appears that a coupled sway-heave-roll model with instantaneous sinkage and trim will normally suffice. Considering the above, two

38 generalised models are concurrently being pursued. The Ro-Ro Research Model This model was developed and used by the Stability Group during the UK Ro-Ro Damage Stability Research Programme and formed the basis for the Joint R&D Project. It is a non-linear three-degrees-of--freedom model, coupled in sway-heave-roll motions and comprises the following: {[M(t)] + [A]l {Qý + [B] {Q} + [C] {Q} = (F~wm + {F} WAVE + {F}WOD with, [M(t)] [A], [B] [C] {F}wixo {F}',,E Instantaneously varying mass and mass moment of inertia matrix. Generalised added mass and damping matrices, calculated once at the beginning of the simulation at the frequency corresponding to the peak frequency of the wave spectrum chosen to represent the random sea state. Instantaneous heave and roll restoring, taking into account ship motions, trim. sinkage and heel. Regular or random wind excitation vector Regular or random wave excitation vector, using 2D or 3D potential flow theory. {F wou Instantaneous heave force and trim and roll moments due to flood water. The latter is assumed to move in phase with the ship roll motion with an instantaneous free-surface parallel to the mean waterplane. This assumption is acceptable with large ferries since. owing to their low natural frequencies in roll. it is unlikely to excite the flood water in resonance and this is further spoiled as a result of progressive flooding. Indeed, when the water volume is sufficiently large to alter the vessel behaviour, small differences are expected between the flood water and ship roll motions. During simulation, the centre of gravity of the ship is assumed to be fixed and all subdivisions watertight. Current Research Model This model has been recently developed by the Stability Group and is currently undergoing validation. It allows for a vessel drifting with the centre of gravity updated instantaneously during progressive flooding. It is a non-linear, coupled six-degrees-of-freedom model comprising

39 the following': ([M]+[Mx (0t]+[AT]}{ Q }+{[Mw (t)+[b] vi..o) {() }+[K(t - 0 with, [M] Generalised mass matrix. [Mw (t)] Flood water moving independently of the vessel but with an instantaneous free surface parallel to the mean water-plane. [A 0 ] Generalised added mass matrix (asymptotic values) [M,(t) ] Rate of flood water matrix (acting as damping). [B] viso Non-linear damping matrix f [K(t -+){()d-, 0 {F}i (F)t Convolution integral, representing radiation damping Various generalised force vectors comprising wave (Ist and 2nd order), wind and current excitation as well as restoration and gravitational effects. All these are updated instantaneously as a function of the vessel attitude relative to the mean waterplane by using a database which spans the whole practical range of interest concerning heel, trim. sinkage, heading and frequency. The same applies to the hydrodynamic reaction forces Excitation from shifting of cargo can also be considered. This force vector is now comprised of dynamic effects of flood water in contrast to its counterpart in the previous model which involves only gravitational effects. The phase/a;nplitude difference between vessel roll and flood water motions will be determined again by building u comprehensive database through a systematic series of model experiments using a sway-h1ave-roll be nch test apparatus. This undertaking is currently under way at Inha University in South Korca through a collaborative research arrangement supported by the British Council. In the cases whlecn the dynamic behaviour of the flood water is considerable and could prove to be dominating or heavily influencing the vessel behaviour, the dynamic system of vessel-flood water must be treated as two separate worlds interacting, using CFD techniques to describe flood water sloshinig. Considerable effort along these lines has already been expended at the University of Trieste in It;ll with the Strathclyde Stability Group collaborating through vet another British Council supported rescarch link. Alodkllitri.' the Wateir Iirc..Is This is indeed a very difficult phenomenon to model as it involves very complex hydrodynamic flows. Some degree of approximnation is. therefore, expected in order to derive engineering solutions. In the approxirliate method adoptcd. water ingress is modelled as an intermittent probabilistic e'v'ent based on the ci Iculation of the relative position betweten wavve elevation and da mage loca tion. The miode of flow is affected airgely by the hydrostatic pressure head and the

40 area of the damage hole but this is influenced by dynamic effects, edge effect, shape of opening, wave direction and profile, water elevation on either side of the opening and damage location. Consider, for example. damage below the bulkhead deck depicted by the simplified picture shown in Figure I with the sea treated as a reservoir and the pressure distribution in the hold assumed hydrostatic. If Bernoulli's equation is applied at sections A and B, considering the total pressure head is maintained constant and the velocity is zero in the reservoir, the inflow velocity at point P can be found as follows: P ~+ Patnav hout +Patm + 0 = hvin + - v = /2g(hout - hil) Pg pg 2g and the flow rate through the horizontal layer around P: dq = K2g(hout - h )da The total flow rate can be found by integrating dq over the damage opening height. This expression reduces to the general form of those used for free-discharzing orifices and notches when either hout or hin is negative, if the follow ing limits are set: bin =0 if hin <0 hou t =0 if h -<0 This takes care of those situations in which water is present only on one side of the damage. Of course, when 11 is less than h,.. the flow becomes negative. and water is expected to flow out of the compa1rmencm 1t]d into the sea. To accommodate for this the pressure head equation is put into thle tb rii: dq= K.sign(h h in) 2gjhoul - hnida. with the same limits as above. Considering that Ihout - hin i represents the instantaneous downflooding distance which is relatively easy to compute, the whole problem of progressive flooding reduces to the evaluation of the coefficient K which is being done experimentally Validatioii/Calibuation (t' the Mathematical Model Ini addition to the work undertaken during the UK Ro-Ro research, considerable effort is being expended in die Joint R&D Project to ensure the validity of the mathemartical model on the whole range of possile applications. regarding vessel type innd partnlentation (above and below the bulkhead deck), loading condition and opetatinc environment is well as location and char;cteristics of daillc ice oe ning. Tile variorhus aspects involvedt are heing tackled on three friohts. involving two ship i modeis tested in randomn wave conditions. Details of the vessels and test conditions are civen in Tables I to 3 below.

41 Table 1: Principal Design Particulars of St Nicholas (UK Ro-Ro Research Vessel) Model Scale Length, L.p m Beam. B 25.50m Depth. D 8.35m draught, d 5.75m Displacement, A 12,000.0 tonnes Block Coefficient, C B K.M m KMd... (DL=24.05rn Fn,,,dj =1.50m) 13.46m K-M d... (DL=32.43m Fi,,ýI, =I.OOm) 14.48m KM dr... (DL=40.26m Fi,,-,,r =0.50m) 14.59m Table 2: Principal Design Particulars of NORA (Joint R&D Project Generic Vessel) Model Scale Length. 111., m Beam. B 26.00m Depth. D 7.80m draught. (I 6.12m Displacement. a 12,400.0 tonnes Block Coefficient. CB m KM,,...(DL= 13.5m : F,, -Jd =1.02m) 13.88m KIX, lr.. (DL=22.Srn : F,.,,d =0.55m) 14.34m K,'vl,... (DL=28.7n : Fm.u,,.d =0. 2 1rn) 14.74m

42 Table 3: Sea States (JONSWAP Spectrum with y=3.0) Sitmificant Wave Peak Period, T, Zero-Crossing Period, Height, H, seconds M metres seconds On the basis of the above, the following series of tests have been undertaken: DMVIJ Model Experiments (NORA model) The DMI experiments were designed to investigate the water ingress phenomena, comprehensively. To this end, the water level inside the deck as well as the water elevation outside the damage opening are measured using an array of wave gauges, together with roll and pitch motions including static heeling and trim. The analysis of these results will also be used to calibrate the developed numerical water ingress model in a range of sea states, conditions and compartmentation as indicated below: Open Ro-Ro deck Centre Casing Side Cisinus Size of damage opening (25%. 100% and 200% SOLAS) Location of damage (midship and forward) Freeboarirdx,,, (0.5m, I. Jm and 1.5m) Loading conditions (KG rangingi from 9.5m to 12.Om) Transverse Bulkheads (Partial and full height) Sea States (Hs=l.3m, 3.0m and 5.0m) Ro-Ro deck damage only In addition to information pertaining to water ingress. valuable information will also be obtained concerning the survivability of NORA in these conditions. Video records of all

43 the above were also obtained. Paper 4 of these proceedings describe these experiments in some detail. Work is still in progress concerning the processing of the DMI tests. MARINTEK Model Experiments (St Nicholas and NORA models) The MARINTEK tests were designed to test the capsizal resistance of both models in a range of loading conditions, sea states and compartmentation, whilst recording all relevant information pertaining to model motions and attitude, as described above, as well as the wave characteristics, including again video recordings. In this respect, the range of tests undertaken comprise the following: St Nicholas Centre Casing Freeboard,,,d (0.21m, 0.55m and 1.02m) Loading conditions (KG ranging from 10.Om to 12.0m) Sea States (Hs=1.Om to 5.0m) Forward Speed (5 and 10 knots in full scale) Wave Heading ('30 and 60 degrees) NORA Side Casings Transverse Bulkheads (Partial and full height) Freeboardn,,d, (0.5m, I.Om and 1.5m) Loading conditions (KG rancing from 11.5m to 13.0m) Sea States (Hs=I.Om to 8.Om) Preliminary comparisons between experimental and theoretical boundary survivability curves are shown in Figure 2. These experiments are also described in some detail in paper 4. Strathclvde Marine Technologv Centre Experiments (St Nicholas model) Following suggestions by the management of the Joint Nordic Project, the St Nicholas model was hrought to the University of Strathclyde for undertaking additional tests pertaining either to the recent recommendations by the IMO panel of experts or related to the project itself, particularly so tests relevant to the validation of the mathematical model. In relation to the above, the following modifications to the model were made: Decoupling of the car deck from the man hull and attaching on load-cells for continuous measurement of the water on deck. This is believed to be a more effective method for assessing inflow/outflow than the DMI method of using a number of capacitance probes inside the deck. Fittinu airangements allowing for the positioning of movable transverse (partial or full height) and longitudinal bulkheads/casings.

44 The suggested range of tests includes the following: Measurement of water accumulation in a range of sea states, loading conditions and freeboards Central Casing Partial height and full height bulkheads Varying number of transverse bulkheads Combinations of the above Testing damage survivability in a zero freeboard case (upright or inclined condition) Investigation of uncertainties that might arise through the correlation studies between experimental and numerical simulation results. Representative results demonstrating the effectiveness of measuring water accumulation on the Ro-Ro deck as well as comparisons between theoretical and experimental results are shown in Figures 3 and 4. Sensitivity study APPLICATION OF THE MATHEMATICAL MODEL In order to identify the most influential parameters for the stability and survivability of a damaged ship. a series of parametric studies have been carried out using the time simulation program. For this purpose a matrix which combines different damaged freeboards. vehicle deck subdivisions, loading conditions and sea states has been tested as shown in Table 4. Table 4: Sensitivity Study Test Matrix for St Nicholas OPEN CENTRAL SIDE OPEN DECK DECK CASING CASINGS -- TRANSVERSE BHDS (45m) KG(m) F1 F2 F3 Fl F2 F3 F1 F2 F3 FI F2 F3 9.0 X X X X X X X X X X X X X [ X X X X X X X X X X 10.0 x x X X X X X X X X X X liim xx x x x x x x x x x x 12.1 X X X X X X X X X X X X 13.0 X X X X X X X X X X X X

45 As can be seen there are 60 conditions and for each condition a minimum of four different sea states has been considered. The sea states were tested to 0.25m resolution (i.e. sea states were increased progressively by 0.25m intervals). Where necessary, several runs were carried out for the same conditions to ensure statistical consistency of the results. The damage conditions used and the corresponding details are as indicated in Table 1. Results and Discussion The results of the study are presented in the form of limiting boundary curves in the form of H, v GMf and are summarised in Figure 5. The damaged freeboards (F) and the corresponding metacentric heights (GMf) refer to the final equilibrium following flooding of the compartment below the bulkhead deck. Efkect of Damaraed Freeboard (F) on Survivabilirv The results clearly indicate that freeboard is one of the key parameters influencing stability and survivability of damaged ships. In this respect, it is interesting to note that the relationship between limiting sea states (Hs) and damaged freeboards (F) is not linear. Table 5, for example, shows the results corresponding to the open deck case and GM, of 3.0m. Table 5: Relationship Between Freeboard and Sea States F (m) Hs/F From this it is clear that the use of Hs/F ratios in boundary survivability curves, needs careful interpretation if one is not to be led to wrong conclusions. As shown in the table, a vessel with lower freeboard can survive at higher Hs/F but the actual sea state is in fact significantly smaller. It is also clear from Figure 5 that the open Ro-Ro deck and the central casing designs would need a damaged freeboard close to 1.5m to survive a sea state of 3-4 metres Hs. which is likely to be required by the tf-irthcoming regulations. However. the results relating to side casings show a marked improvement on the survivability of the vessel which appears now to be capable of surviving very high sea states. It is interesting also to note that. at very high GN'II, the effect of water on deck on damage survivability becomes less dominant as is the effect of freeboard, this particular ship rolling quite significantly due to the proximity of the spectral peak period to the natural roll perioed.

46 Effect ot Vehicle Deck Subdivision on Survivability The large open vehicle deck poses a great danger to the survivability of Ro-Ro type vessels if serious flooding of the vehicle deck takes place. Notwithstanding this, the majority of the existing designs have open deck or central casings as implementation of side tanks has been limited due to economical reasons. Thus, the clear and substantial benefit to be gained by a ship with side casings, as shown in Figure 6, has not been taken advantage of. In the example considered, the limiting boundary curves referring to the open deck and central casing are almost identical with the open deck showing a slight improvement which derives mainly from the fact that, under certain conditions, the open deck Ro-Ro vessel may incline to the lee side, thus enhancing her chance of survival. The beneficial effect of side casings on ship survivability derives mainly from the following: Due to their location away from the centre of rotation, side casings increase substantially the roll restoring ability of the damaged vessel in addition to improving significantly the reserve buoyancy. For the same reason, side casings decrease the heeling moment resulting from flooding of the vehicle deck, as the body of flood water moves closer to the centreline (roll centre). It is obvious that this beneficial effect increases as flooding progresses and the ship tends to return to the upright condition. This effect, however can be outweighed by low damaged freeboards and small GMf as shown in Figure 6. Effect of Transient Floodin y on Survivabiliiv Depending on the damaged freeboard, a vessel with small GMf may capsize due to transient heeling resulting from the flooding of the damaged compartment below the bulkhead deck. However, this depends critically on the direction of the initial heel. If this is to the lee side, asymmetric flooding of the compartment below the bulkhead deck will cause the vessel to incline to large angles, thus increasing her effective freeboard, water ingress on the vehicle deck is prevented and she survives. The ship may remain inclined or. due to the increasing amount of water in the compartment below the bulkhead deck she may return to the upright position. On the other hand, if the initial heel is to weather side, the asymmetric flooding of the compartment below the bulkhead deck will have the exact opposite effect on the survivability of the vessel. The above effects are demonstrated in Figures 7 and 8 which refer to the same vessel condition and sea state but to different wave realisations. The effect of transient flooding on survivahilitv diminishes with increasing damaged freeboard or GM, Sensirivitv of Si'vivabilitv on GM, and Other Residual Stabilirc Parameters If different subdivisions of the vehicle deck are contemplated, then clearly GMN cannot

47 be considered as a representative parameter to characterise the damage survivability of passenger/ro-ro vessels. This is demonstrated in Figure 6. This is not, however, the first time that GM has been dismissed as a parameter in assessing ship stability and as the "Rahola fans" might argue, GM in itself is the key opening the door to unarguably the most successful characteristic property to date of a vessel's ability to resist capsize in any condition and environment. Even if one does not support this view, any results that this route is likely to yield, offer two distinct advantages: simplicity and applicability. As explained in the foregoing, the objective is to express the survival factor "s" as a function of residual stability characteristics, judiciously chosen (e.g. systematic parametric investigations, regression analyses, experiential judgement, political *blessing" and so on) to enable such a factor to be generalised for application to all vessel types and compartmentation. Parameters to be considered in such an investigation include: GZ,.;,, at a certain angle Positive GZ range,a-ea under the GZ curve Area under the GZ curve up to a certain angle A first exploration in this direction met with a problem that needs careful thinking. Damage stability calculations for vessels damaged both above and below the bulkhead deck would require, according to 1MO, that the water level in each damaged compartment open to the sea must be at the same level as the sea i.e. final equilibrium be reached. However, the GZ curves derived on the basis of this approach, simply fail to offer any useful information. The principal reason lies on the wrong assumption that water is free-floodinu the deck in these calculations. Takingr heed from this and from the fact that water on deck is a dominant parameter affecting damage survivability, as earlier experience amply demonstrated, it was decided to attempt to quantify the critical amount of water on deck as a matter of top priority. It would appear that this effort is likely to bear fruits and this will explored in paper 5. Presently. an explanation will be provided in the following of what is meant by critical amount of water on deck. Critical Amouni of Water on Deck - "The Point of No-Return " The effect of random waves on the rolling motion of the damaged ship appears to be rather small and for capsize to occur in a "'pure" dynamic mode should be regarded as the exception rather than the rule. The main effect of the waves, therefore, is that they exacerbate flooding. In this respect. the effect of heave motion in reducing the damaged freeboard is as important as the roll motion. Model experiments and numerical simulations have clearly demonstrated that the dominant factor determining the behaviour of the vessel is the amount of flood water accutnulating on the vehicle deck. Observations of the mnode of capsize during progressive flooding of the vehicle deck show

48 the vessel motion to become subdued with the heel angle slowly increasing until a point is reached when heeling increases exponentially and the vessel capsizes very rapidly. This is the point of no-return. Put differently, the flood water on the vehicle deck increases slowly, depending on the vessel and environmental conditions, until the amount accumulated reaches a level that cannot be supported by the vessel/environment and the vessel capsizes very rapidly as a result. The amount of flood water when the point of no-return is reached is the critical amount of water on deck. In relation to this, two points deserve emphasis: This amount is substantially less than the amount of water just before the vessel actually capsizes but is considerably more than the amount required to statically capsize the ship. In this respect, the energy input on account of the waves help the vessel sustain a larger amount of water than what her static restoring characteristics appear to dictate. It is also worth mentioning here that the time taken for the vessel to capsize depends on a host of factors and is currently the focus of another debate. Because of the nature of the capsize mode when serious flooding of the vehicle deck takes place, it is not difficult to estimate the critical amount of water on deck at the point of no-return and this is demonstrated in Figures 9 and 10 using the generic vessel of the Joint R&D Project. NORA. The deck area is also shown in the Figures and it takes no hard calculations to compute that it is considerably less than the amount corresponding to 0.Sm water level. The stage has now been reached where meaningful investigation can be undertaken to provide much needed answers to the question of damage survivability of Ro-Ro vessels. This is further discussed and explored in paper 5. CONCLUDING REMLARKS As research on the damage survivability of passenger/ro-ro vessels gathers momentum and results from model experiments and numerical "tools" are being made available, the confidence is slowly build up that what was perceived to be an unapproachable problem, can in fact be tackled with sufficient engineering accuracy to yield solutions which by the nature of the problem are likely to have a profound effect on the way these vessels evolve. For this reason alone, the profession must take a step back and attempt to see the wider implications of the problem at hand, rather than jumping to premature conclusions and adopting unfounded solutions. Advocating caution is definite]\, a noveltv. coming from an academic who, in the past, has repeatedly condemned the inertia of our industry at large. But, it is time to exercise caution when hurried measures could threaten the very existence of an industry. The co mmercial success of Ro-Ro's lies principally on the provision of large unrestricted enclosed spaces for the stowage otf vehicles and cargco. When addressing the safety of Ro-Ro vessels, therefore, one has of necessity to focus on subdivision. In so doing, however. one is pointing a finger at the immediate problem rather than towards the

49 10' 7-U I ' 1 7 x %... x 7< X, FDI/i 4~~7 /..... x 7( xl. /0 / C*GP/B* --- No Casie X -X Casie FD -XM;A->mrn

50 required solution. One should not lose sight of this fact! ACKNOWLEDGEMENTS The financial support of the UK Department of Transport and of the Joint R&D Project is gratefully acknowledged. I should also like to record my appreciation to all my colleagues in the Project and in particular to Mr P. Paloyiannidis, Mr A. Graham, Mr S. Rusas and Dr. T. Svensen for useful suggestions and discussions. Special thanks are due to the Stability Research Group members, Dr. 0. Turan, Mr L. Letizia. Mr H. S. Kim, Mr M. Tsangaris and our visitors Professor M. Pawlowski and Dr. N. Umeda for their help, contribution and support in more ways than one. REFERENCES [1] "Research Into Enhancing the Stability and Survivability Standards of Ro-Ro Passenger Ferries: Overview Study", BMT Ltd., Report to the Department of Transport, March [2] Dand. l.w.. "'Experiments with a Floodable Model of a Ro-Ro Passenger Ferry", BMT Project Report to the Department of Transport. BNMT Fluid Mechanics Ltd., February [3] "Ro-Ro Passenger Ferry Studies, Model Tests for F10", Danish Maritime Institute, Final Report of Phase I to the Department of Transport, DMI 88116, February [4] Vassalos. D. and Turan, 0., "Development of Survival Criteria for Ro-Ro Passenger Ships - A Theoretical Approach"' Final Report on the Ro-Ro Damage Stability Programnme, Phase I. Marine Technology Centre. University of Strathclyde, December 1992.

51 WATER ON VEHICLE DECK St. Nicholas DAMAGE LENGTH 75m 40M UPRIGHT O.Qm FREEBOARD i Inra I I rsimulation A Expenment so COMPARTMENT LENGTH (in) * 4500 * : LIJ2000 ~ p EXPERIMENT i * 500I 0I So PO TIME (sect NUMERICAL SIMULATION Wil 1 917I"igI a 2500 ~1000 I 500 I -500 fl TIME (sec) Figure 3 :Water Ingress - Theory and Expeniment

52 WATER ON VEHICLE DECK St. Nicholas DAMAGE LENGTH =65m 3500 INCUINED(2.3deg) O.Om FREEBOARD I :B 6Numermici -J150 Isimulation 500I COMPARTMENT LENGTH (in) 2700 EXPERIMENT I TIME (sec) NUMERICAL SIMULATION 20MR TIME (sec) Figure 4: Water Ingress - Theory and Experiment

53 5 Open Deck 4 1~~ -... I o GMV (m) Centre Casing 4 ' GMt (in) 6 Side Casings 4 I~ F(m) GMf (Im) Figure 5 Effect of Damaged Freeboard on Survivability (St. Nicholas)

54 F=1.02m Centre Casing 1~ ---- Side I I I Casingsj 0 I GMtF (in) 6 F=0.55m 4-3 'open -.. Deck 2 - Centre I I Casing Side I Casings! GMf (in) F=O.21 m -:'-Open S Deck...- Centre 2 - Casing 1-~~~ -Side -~. - I o GMt (mn) Figure 6 Effect of Vehicle Deck Subdivision on Survivability (St. Nicholas)

55 Roll Motion 0 A I0C0 timne(secl -20 V -25, -30 Wave ~0.30- &0.20I 0.10 A L A= A~ 0.00 t-0.10 ' ! - -- Ee "3000 c Water inside Damaged Compartment (Below Bulkhead Deck) timejsec) e20 Water on Vehicle Deck Figure 7 Beneficial Effect of Transient Flooding on Survivability (St. Nicholas)

56 so 70J 60I 50 *~40 30 S Rail Motion tlmne(sec) 0.40 Wave 0.30I 0O.20 AI ~a0.10o 0 ~ ' in - n :1500 Water inside Damaged Compartment (Below Bulkhead Deck) in -n 'Intirne(sc) M Water an Vehicle Deck 4)4000I 3000 rt * D3 time(sec) Figure 8 Adverse Effect of Transient Flooding on Survivability (St. Nicholas)

57 o a gd 0 m 0 z 4* co D o I A' I I 01 ID C)toicN (Cca0)0 ap to 000 G )C z> Cm tm C w

58 DAMAGE STABILITY TESTS OF MODELS REPRESENTING RO-RO FERRIES PERFORMED AT DMI MICHAEL SCHINDLER, M.Sc. Danish Maritime Institute DK-2800 Lyngby, Denmark ABSTRACT This paper focuses on DMI's seven years experience in the field of model testing of damaged Ro-Ro ferry models exposed to rough seas. It outlines some of the design principles applied during construction of DMI models and finally it summarizes some of the most important trends in model behaviour. 1. INTRODUCTION Model testing of models representing damaged ships exposed to rough seas has become a very important tool for investigation of problems in the field of damage stability. The high degree of complexity caused by strong non-linear dynamic effects related to wave motions, the response of the damaged ship, and water ingess, makes these problems difficult to simulate by means of mathematical modelling. The same dynamic effects mean that the physical model testing makes heavy demands on the model construction and the test techniques. DMI has developed criteria for both of these. DMI has a leading position with regard to damage stability problems, drawing on more than 25 years of experience. In recent years, this position has been considerably strengthened due to the fact, that DMI has taken a very important part in damage stability projects initiated by the Department of Transport (UK) and a Nordic cooperation on the Safety of Passenger / Ro- Ro Vessels, later extended by a special test series for Nordic Shipowners.

59 The models specially designed for these investigations showed a very high degree of reliability, each spending several hundreds of hours in water. Our test technique, rigorously followed during every single test series, produced reliable results with excellent repeatability. 2. TASK SPECIFICATIONS DTp (UK) Investigation The extensive model tests performed at DMI were divided into two phases. objective of Phase I was to evaluate the importance of the four parameters: The main - KG - Sea State - Damaged Freeboard - Wave Orientation in respect to the model's ability to survive a midship damage. The results were published in /1/ and further discussed in /2/. The main objective of Phase II of this investigation was to examine the value of various realistic suggestions for improving the damage stability of a Ro-Ro ferry in rough seas. The devices systematically examined were mainly of the nature of reserve buoyancy or watertight bulkheads. The effects of internal devices such as: Side Casings at B/5 from the ship's side Side Casings at 2 m from the ship's side - Centre Casing - Transverse Full-Height Bulkheads on Ro-Ro Deck - Transverse Half-Height Bulkheads on Ro-Ro Deck were examined for mid and foreship damage cases. The effects of external devices, such as: - Sponsons - Flare - Buoyancy Air Bags were examined in case of midship damage, only.

60 3. THE MODELS The model used in the DTp investigation represents the lines of the existing British Ro-Ro Ferry "St. Nicolas". "Nora", the model used in the Nordic Investigation incorporates considerably higher freeboard than is typical for existing ferries and the lines correspond to the slightly transformed lines of a ferry now under construction for a Danish owner. Main Dimensions "St. Nicolas" "Nora" Dimensions Full- Model Full- Model Scale Scale Scale Length, L m 3117 mm m 3750 mm Breadth, B 26.0 m 619 mm 25.5 m 736 mm Draught 6.1 m 145 mm 5.75 m 166 mm Depth to bulkhead deck 7.8 m 186 mm 8.35 m 241 mm Height to top of superstructure 18.8 m 447 mm 17.5 m 505 mm Height to double bottom 1.6 m 38 mm 1.5 m 43 mm Displacement, intact t 164 kg t 282 kg Both models were, from the beginning, dedicated to research programmes, which in some aspects means more demanding requirements for design and construction. Compared with typical "commercial" investigations, the number and range of parameters investigated in research programmes are normally considerably higher. Related to the models, these are: - Length of Damaged Compartments. - Damaged Freeboards Positions of Damaged Zones - Size of Damage Openings - KG-Range Various Arrangements on Ro-Ro Deck. At the same time, all other demands like model weight, stiffness, and accuracy must not be reduced. Not only are the models open from above, leaving the whole Ro-Ro deck area visible, but

61 In addition, and among other subjects, the effects of wind forces, size of the damage opening, transient moments, simulated permeability below and above the Ro-Ro deck and under-deck space ventilation were examined. The results were published in /3/ and further discussed in /4/. The Nordic Investigation The main purpose of the investigation carried out by DMI (Task 2.2) is, by experiments with a model of a damaged ferry exposed to rough seas, to provide data which will be used for validation and calibration of theoretical models of water ingress on the Ro-Ro deck. These theoretical models are covered by Task 5. The test matrix includes the following subdivision on the Ro-Ro deck: Open Deck, i.e. no subdivison on the Ro-Ro deck Centre Casing Side Casings at 3.5 m from the ship's side all examined with and without full-height transverse bulkheads. The parameters examined are essentially the same as during the Phase I of the DTp Investigation, but extended by: - Size of the Damage Opening. Test Series for Nordic Shipowners The objective of this study is by model tests to investigate if the reduced height transverse bulkheads on Ro-Ro deck would provide sufficient protection of a Ro-Ro ferry against capsize in rough sea. Two different designs of these bulkheads are examined. The tests are performed as survivability tests for fixed KG-values typical for existing ferries of the same size as the model represents. The two parameters Sea State Damaged Freeboard were examined in respect to the ability to survive a midship damage with the model equipped with: - Centre Casing - Side Casings of the same design as under the Nordic Investigation.

62 DMI has put great efforts in making the majority of the space below the Ro-Ro deck visible as well. This is important for inspection of this space in the event of leaks. Both model hulls are primarily made of GRP with the model sides of aluminium plates, while the subdivision above and below the Ro-Ro deck is made of transparent material. For improving bending strength, stainless steel wires are cast into the GRP part of the hull. In connection with the weight lift arrangement above the Ro-Ro deck (for varying KG), the models have solid longitudinal strength elements linked with the model sides by aluminium angle bars. Upon completion of the construction work, check measurements of the models were carried out. 4. TEST PROCEDURE The tests were carried out in DMI's 240 m long towing tank, starting every time from a position 20 m from the wavemaker. At this position, the damaged model was placed in calm water. After the wavemaker was activated, the models were allowed to drift freely beam onto the oncoming waves, and data collection was started. All data collection lasted at least 60 minutes in ship scale, unless the models capsized. Safety lines were used to rescue the models in case of capsize. These light and flexible lines were slack during the measurements and did not effect the behaviour of the model. In this connection it is important, that all power supply and data communication with the computers on board was through only few cables in the umbilical cord attached to the models close to their natural roll centres. Before succesive runs, the models were drained for water on the Ro-Ro deck and inspected for water in dry compartments below the Ro-Ro deck. They were then again replaced at the test position, close to the wavemaker, and the tests were resumed. 5. MODEL BEHAVIOUR Although the determination of exact survival / capsize points by means of survival / capsize GM was not the objective of the Nordic Investigation, the observations made during this investigation confirmed the main conclusions of the DTp Investigation. Furthermore, the results of the tests for the Nordic Shipowners are also in full agreement with the conclusions reached in DTp Investigation. The most important trends in terms of survival / capsize GM as observed during testing of both models are stated below: Damaged Freeboards All four investigations agree that increasing damaged freeboard has positive effect on

63 survivability. Midship / Forward Damage Assuming the same damaged freeboard the survivability of the forward damage is much better than it is for the midship damage. Differences in lost buoyancy are not the only explanations. A more important factor is the forward slope of the Ro-Ro deck. After the water floods the Ro-Ro deck, the further increasing trim limits the amount of water on the Ro-Ro deck improving safety. Size of Damage Opening The DTp Investigation concluded, that a smaller damage opening would provide less safety against capsize than would the standard Solas-74 size opening, although in the cases where capsize occured, the rate of buoyancy / stability loss was considerably slower. These observations were confirmed during the Nordic Investigation. Ironically, the Nordic Investigation indicates, that extension of the damage opening considerably above the Solas-74 size will also worsen the survivability of the model. This is clearly an area for further research. Centre Casing / Side Casings / Open Deck In general, the ability of the models equipped with centre casing to survive high waves is much lower when compared to the models with side casings. There are two main reasons for this. The first is that the centre casing hinders the ingressing water in crossing over the centre line, thus depressing the damaged side. The second reason is, that the undamaged parts of the side casings represent considerably buoyancy. When compared with the side casings, and at least with respect to a midship damage, the results achieved during the DTp Investigation with the open deck, i.e. no buoyancy at all, were even better. The trends observed during the Nordic Investigation do not confirm this observation, but they still strongly indicate the superiority of an open-deck when compared with the centre casing. Transverse Bulkheads on Ro-Ro Deck The full height transverse bulkheads on the Ro-Ro deck proved their effectiveness in protecting against capsize. For the midship damage and centre casing combination, both models were tested in 5 m waves at GM-values as low as 2.2 m and never capsized. With side casings, "Nora" was tested down to the same GM-values and did not capsize either. In terms of GM required to prevent capsize, "St. Nicolas" showed even better results. In combination with the centre casing, transverse bulkheads of half height, when unprotected from above, provide only limited protection against capsize. Hanging car decks, although nonwatertight, will provide some protection from water passing over the half-height bulkheads due to a damping of the sloshing effects. This effect on capsize has not been investigated and is an important subject for further research. Equipped with side casings and bulkheads of a modified design, but same height, the tested model was much better protected against capsize.

64 6. RECOMMENDATION The observations of model behaviour made during the investigations described in this paper are of a general nature only. Any problem related to a specific Ro-Ro ferry in a specific condition should be examined by model testing individually dedicated to the particular vessel. 7. CONCLUSION DMI's understanding of the mechanism of capsize in rough seas is continually being improved. The very extensive model testing we have carried out so far, gives us a unique knowledge of the ability of various proposed external and internal devices for improving the chances of survival for a damaged ferry. 8. REFERENCES /1/ "DMI Ro-Ro Passenger Ferry, Safety Studies, Model Tests for F10 - Final Report of Phase 1", DMI /2/ Paper No. 7: "Ro-Ro Passenger Ferries Safety Studies - Model Test of Typical Ferry", by K.F. Pucill and S. Velschou, Danish Maritime Institute, International Symposium on the Safety of Ro-Ro Passenger Ships, The Royal Institution of Naval Architects and the UK Department of Transport, London 26 & 27 April, /3/ "DMI Ro-Ro Passenger Ferry Safety Studies, Phase 2, Model Test for F1O - Model 2 - Final Report", DMI October /4/ Paper No. 5 "Ro-Ro Passenger Ferry Damage Stability Study - A Continuation of Model Tests" by M. Schindler and S. Velschou, Danish Maritime Institute, Symposium on Ro- Ro Ship's Survivability, The Royal Institution of Naval Architects, London 25 November, MS/anbfini(DIV)

65 TECHNICAL UNIVERSITY OF DENMARK DEPARTMENT OF OCEAN ENGINEERING WEGEMT Workshop on Damage Stability of Ships Copenhagen, 20 October 1995 Recent Developments, Trends and Proposals on Damage Stability Criteria A CLOSED-FORM ASSESSMENT OF THE CAPSIZAL PROBABILITY -THE s i FACTOR Maciej Pawlowski ABSTRACT. A review is presented of the past efforts to develop an estimate for the factor sjthe most difficult and controversial part of the new subdivision regulations based on the probabilistic concept. This may facilitate our present efforts in developing a sound assessment, consistent with the results of damage stability model tests. A new method is put forward, based on the theory underlying numerical simulations of damaged ship behaviour in natural conditions, promising a high accuracy of prediction for the factor s i. INTRODUCTION The calculation of the factors, is definitely the weakest part of the new regulations, both for passenger ships [I] and dry cargo ships [2] alike. Though the methods for the s, factor in the two instruments are not identical, the differences are not substantial. The s, factor for dry cargo ships evolved in the late 1980's from the method developed about twenty years earlier for passenger ships. The knowledge on damage survivability of ships, however, did not increase duriiig that time - in fact - there was lack of knowledge for whatever progress. It was mainly decided at IMO to abandon the idea of the effective freeboard, not very handy in practical apphications and uncertain as to its correctness, and base the whole calculation on the GZ curve. Thus, in practice, the two methods are equally deficient and reflect the lack of relevant knowledge in the area. None the less, these new probabilistic regulations provide much higher standards of safety than the SOLAS Convention. The point is that none of the existing ro-ro passenger ships have been built according to these new regulations. THE ORIGINAL s i FACTOR The inadequacy of the original method is clearly illustrated in [I ] (in Fig on page 94), highlighting large and inconsistent discrepancies between the results of model experiments, carried out separately in the UK [3] and the USA [4], for two different ro-ro passenger ferries which indicates failure to generalise correctly the results of model experiments. The critical sea state, characterised by the significant wave height H,, was considered as a function of the damage stability parameter GMfFg B: H 5 = H,(GMI FI/B) which is simply not the case, where: G6A4 - the metacentric heigh flnoded

66 Fe B - the effective freeboard after damage [I] - breadth of the ship The above relationship was treated as determinate, even though some degree of randomness is undoubtedly present, especially in natural sea conditions and because of non-linear behaviour of a damaged ship in waves. The probability s that a ship with a given value of the damage stability parameter GMfFý/B will not capsize after damage is equal to the probability that the critical significant wave height related to this parameter is not exceeded. Therefore the probability s for a given particular case flooded can be derived from the sea state distribution at the moment of collision F = F(HI), with a combination of the above damage stability criterion (the boundary stability curve), as a composite function of GMfF/'B: s = F[H,(GA'f FIB)] A graph of this probability is shown in Fig. 1, obtained using data given in [1, 3]. A simple but adequate approximation of this function is s = 1.7(GMfF/B) = (xix,)" 6 (1) for se (0, 1), where x stands for the damage stability parameter in metres and x 1 ý m is a value of x yielding s = 1. [MO, however, approximates this probability with a large under-estimation, as shown in Fig. 1, using the equation: 4.9(GMf eb)" 2 = (xix 1 )"I 2 (2) for se (0, 1). Ifs is less than 0.6, which happens ifx (=- GAIfF,"B) is less than m, then LIMO requires in regulation 5 that s is taken as zero [I], reflecting thus the lack of confidence in the x=GM, i F, B o [M Figure 1. The probability of collision survival s: a - based on model tests and sea state distribution, b - based on a comparative method [6], c - approximation adopted by LMO

67 quality of prediction for small s factors, and admitting the poor adequacy of the current damage stability parameter for a measure of ship's ability to resist capsizing at a given sea state. In this range, the s factor could be equally well approximated using linear interpolation between s = 0.6 for x = 0.015m, and s = I for x = 0.04m. SOME MODILFICATIONS The damage stability parameter can be easily expressed in terms of parameters related to the GZ curve. It may be observed that GMfF, B - '/2 GZd, i.e. the damage stability parameter equals roughly half the righting arm at an angle of heel at which the deck edge becomes submerged. Hence, the following is obtained from Eq. 1: s = (GZdIGZ!)" 6 (3) for se (0, 1), where GZ m is a value of GZd yielding s = 1. According to the latest tests [5-7] the value of GZ, should be preferably 0.1 m. To facilitate applications in all cases of flooding, including end compartments, and to conform with the definition of the effective freeboard Fe, provided in [1], GZd should be taken as the maximum value of the GZ curve within the range extending from the angle of equilibrium 6 e up to: * (2/3)6,., taken as an assumed angle of deck edge immersion, where 6 r, is the angle at which GZ,,, occurs, or * 6,, - the angle at which weathertight openings immerse, or * (213)6f, where 6f is the angle of flooding, or * 22 degrees (_= arc tan 0.4), whichever is less. Such an estimation should be adopted for the s factor for dry cargo ships to adhere the method developed for passenger ships. Unfortunately, IMO has employed another estimation in [2], based on heuristic assumptions, using GZm, with a combination of other parameters associated with ship stability. It is worth noting that GZd by definition is less than GZ,,. NEW SURGE OF RESEARCH. Following the tragic capsize of the "Herald of Free Enterprise" in March 1987, realising the lack of a reliable damage stability criterion, the UK Department of Transport initiated an extensive Ro- Ro Passenger Ferry Safety Research Programme, comprising a number of studies into damage survivability of these ships, including model tests in waves [5-9] and numerical simulations [1 0-11]. The main objective of the above experiments was to determine the standard of residual stability needed to enable a ro-ro passenger ferry to survive flooding and avoid rapid capsize in realistic sea going conditions. Unfortunately, this primary objective has only been partly achieved. The results obtained, whether through model tests or numerical simulation, are fully valid only for the particular ships investigated. Whether they can be extended to other ships is a matter of speculation, although a new attempt to generalise them has been proposed in the form of a relationship between the ratio H/Fe and the non-dimensional metacentric height flooded GA4,, defined as follows: Gf =- GMf Af _ GMJ C B T () LGAp BP 3 B 2 3

68 where: Af - volume displacement of the vessel together with the flooded water C B - block coefficient of the vessel to the damaged waterline Tf - draught of the vessel in the flooded condition LBP - length between perpendiculars with the other parameters as defined earlier. The non-dimensional metacentric height flooded is derived from the equation: GMn = GM f where BA4fis the metacentric radius in the flooded condition, and this ratio should be used rather than Eq. 4. However, this type of boundary stability curve is still far from representing all vessels as GA4 and F, by nature do not reflect the effect of different structural arrangements on the vehicle deck and above that can be used for enhancing damage survivability of existing ro-ro passenger ships. For this purpose, the definition of the effective freeboard would have to be much more intricate. Damage stability is a complex problem involving a large number of variables and with results only from one or two given shapes it is difficult to identify the significant parameters governing the problem. The matter can be now much more extensively investigated using numerical simulations [10-I 1i, of the same accuracy as model tests. Full benefit from these results, however, will not be achieved until there is a consistent logical theory which will enable the results to be generalised to other ship forms, sizes, and subdivision arrangement. Such theory, at the moment, does not exist. The objective is then to define the unique boundary stability curve, valid for all vessels. Such a curve is badly needed by regulatory bodies, designers, and industry-to know how existing ships can be modified to make them safer for the public. However, it is worth telling loudly that not much can be done about existing ro-ro passenger ships -we cannot rebuild these ships extensively to meet the high standards of safety in the new regulations [1]. Many of them do not meet even the SOLAS 90 criteria which are rather lenient. These ships, as a rule, were poorly designed with no reserve of buoyancy above the subdivision (vehicle) deck and with excessively close bulkhead spacing below this deck, being misled by the illogical aspect of the SOLAS Convention. Subdivision of that kind results inevitably in very low indices of subdivision [12]. Such ships are simply unacceptable and, in practice, their safety can be upgraded only to a limited extent. On the other hand, new standards of safety can be easily implemented on new ro-ro designs [13-16]. The fact that ro-ro ships can be as safe, in terms of subdivision indices, as other ship types without destroying their operational features is not yet widely recognised. Fortunately, this fact has been eventually recolgnised by IMO who now make no distinction between ro-ro and conventional passenger ships, and the same applies to dry cargo ships. Nevertheless, establishing promptly the right boundary stability curve, valid for all ships and forms of subdivision, is of prime importance. Otherwise. we may allow for developing regulations that are unreasonable, more stringent than necessary which can even destroy in future the ro-ro concept. Another important aspect is that whatever we do now can be totally ignored or may be irrelevant to IMO activities for many years to come, including harmonisation of probabilistic subdivision regulations for all ships. Our efforts to solve these problems rationally can thus be wasted, with the detriment for the travelling public. 4

69 PROPERTIES OF BOUNDARY STABiLITY CURVE For any ship, with a given loading condition and compartment flooded, the critical sea state the ship can withstand, characterised by the significant wave height H,, cannot be determined uniquely, and this fact is widely acknowledged nowadays. This is not because of some inaccuracies of model experiments or numerical simulations, nor because of insufficient time of duration of test runs, but simply because of the random nature of the critical sea state, characterised by certain distribution. Hence, any boundary stability curve is a frizzy curve rather than distinct, indicating mean values of the critical sea states and surrounded by a confidence level. Therefore, to find out distribution of critical sea states (and its mean or median value first of al), you have to repeat many times the same case of flooding at the same sea state but with different initial conditions. To arrive at a boundary curve, you cannot do just one run. The random nature of critical sea states comes mainly from the non-linear roll motion in irregular waves. The probability s that a ship with a given loading condition and compartment flooded will not capsize a-fter damage is equal to the mean probability that the critical significant wave height related to this case is not exceeded: S = Ls1 F (Hs) f ( HS) dpi 5 (5) where: E(H 5 ) - cumulative distribution function of sea states at the moment of collision fc(i-4) - probability density function of critical sea states for the ship with a given loading condition and compartment flooded Because for moderate and higher critical sea states F(H 5 ) has a small rate of change, as can be seen in [I1] (in Fig on page 95), whereas for low critical sea states (when damaged stability is deficient) the range of variation of critical sea states is narrow, by virtue of the mean value theorem Eq. 5 yields: s = F( HSme (6) that is, in practice, the s factor can be calculated as if the critical sea states were of binary nature, cut off at the mean value. Averaging the s factor. The calculation of the probability s, -_5 would therefore be relatively simple if the mean critical sea state Hs m.n was determinate for each compartment group. However, this quantity is not determinate because it depends on such random quantities as the loading condition (draught T, trim t, metacentric height GM and permeability P) at the moment of collision, and vertical extent of flooding- H. Therefore, in order to obtain the composite probability s, for all possible combinations of p. H, T, t and GM, it is necessary to average s for each compartment group with respect to these random variables. This follows from the formula for the entire probability. Hence: S=E (s) = fu ft f G.1ISf(,u. T, t, GMI) dp dt dt dgm ý7) where the probability s = s~a, 7 t, GM)f is itself a function of the four random quantities, averaged previously, for ships with horizontal subdivision above the waterline, with respect to the vertical extent of flooding H that is of discrete character.

70 As can be seen, to find the s, factor for each compartment group it is necessary to know the joint distribution density functionf(p, T, t, GM) which can only be derived from statistical data, and which in practice is virtually impossible to obtain. Such a distribution might also be related to the ship type and possibly to the ship's route, but again the understandable lack of data would prevent these variables from being considered. Remembering that the method is aimed at arriving at an assumed rather than the actual probability of survival, the averaging procedure may be largely simplified by accepting draught and vertical extent of flooding as the only random variables and assuming the others to be determinate--either as constants or as functions of draught. Hence, Eq. 7 for the s, factor reduces then to the following: s= T s(t)f(t)dt (8) where s(t) is the probability s = s(u, 1, t GMv) as a function of the ship draught only, obtained by averaging the s factor (for each compartment group and draught) relative to different descrete vertical extents of flooding, if any, andf T) is the maroinal distribution density of draughts at the moment of collision. Hence, if several watertight decks are fitted above the waterline in question, with the heights Hk above the baseline (k = 1, 2,...), the factor s(t) for each compartment group and draught is given by: where: s(t)- Z s(n H)(vk - VA_-) (9) k s(hk) - s factor calculated for each compartment group and intact draught, assuming vertical extent of flooding up to a height HḄ vk - reduction factor which represents the probability that the spaces above the horizontal subdivision at a height Hk wvill not be flooded, with v. = 0, and vk = I for the last (heighest) deck. Obviously, if there is no horizontal subdivision within a given compartment group (k = I only), then there is only one vertical extent of flooding, usually from the base line or double bottom upwards without limit, and naturally there is no need for any averaging. In such a case, s(7) equals simply the one and only s factor calculated for the subject compartment group with a given intact ship draught and for the one extent of flooding, The reduction factor v is currently being developed at RMO. It will be based on bow heights statistics and will correspond to the cumulative distribution function of bow heights [17]. Having calculated the s(7) factor for each compartment group and intact draught, the s, factor is the mean value of s(7) relative to draught that can be obtained from Eq. 8 by applying numerical integration: a =t I nvjs(¾)(0 j=1 where: n - number of draughts used for calculating s. and {wj - weighting factors, depending on distribution density of draughts at the moment of collision and the number of draughts used. 6

71 The present calculation of the s, factor for dry cargo ships [2] is based on two draughts only. Because of the fact, however, that - flooding information for the master is supposed to be carried out for a wide range of draughts, and - there are problems arising with determination of the KGm,aX value if the calculation of the index is based on two draughts only a minimum of three draughts should be used for these calculations, It is noteworthy that three draughts of five are employed in [I ] for calculating the A index for passenger ships, although these ships have much smaller draught variation than cargo ships. Three draughts seem to be entirely sufficient for practical applications. The weighting factors for passenger ships were originally derived assuming a triangular distribution of draughts at a range between d. and d,, vanishing at the ends and assuming a maximum value at d,, as is explained in [ I]. These factors represent the relative frequency of a ship operating at a given draught at the moment of collision and can be derived from the damage statistics. They may depend on the type and category of ship. A NEW METHOD FOR THE s FACTOR As can be seen, of prime importance for the determination of the s, factor, and thus for the whole method, is the basic expression for the s factor, given by Eq. 5, that requires the critical sea state Hma to be known for each damage case, denoted further down simply by H,. However, to get this basic factor, we wish to avoid determination of H, with the aid of model tests or numerical simulations, as they are not suited and intended for routine applications. For this purpose, we have to find out a simple but meaningful procedure, verified by the previous results and preferably based on the theory underlyng numerical simulations [10]. We hope that in spite of the complexity of the problem, this goal can be achieved. Main Observations. The main observation which can be made from the previous tests results is that the damaged ship in waves behaves quasi-staticly when it reaches a point of no return-but reaching this point and the time to reach this point are determined by the dy'namnics of the ship. Quasi-static behaviour of the ship at the point of no return denotes lack of roll motion, which is marginal at this position of the ship, while the other ship motions such as sway, heave and possibly pitch (in case of flooded compartments other than those at mid-ships) remain in practice unaffected by the quasi-static heel of the ship. Heave is then particularly dominant. The three other motions in principle can be regarded as linear even though they can be of large amplitudes, in particular heave and pitch, and despite the large variation of the waterplane area with draught and trimn, typical for modem ferries. The linear part is dominant in these motions. What is more important in this case is that we are going to disregard here possible couplings between heave and roll, and heave and pitch. Above couplings can develop for asymmetrical floodings with a high position of the centre of gravity above the waterline, typical for short sea ferries, and for compartments flooded at the end pants of the ship. By doing so, we err on the side of safety and can then make use mainly of the results for symmetrical mid-ships floodings. Having analysed the theoretical model for the dynamic behaviour of a damaged ship in natural waves [10], and examined a number of the results from numerical simulations of the ship behaviour for different loading conditions, sea states and various internal configurations, the following observations can be made: 7

72 I. The point of no return occurs when the ship has reached the angle of heel 6m, at which a maximum of the GZ curve occurs. This angle, at a large majority of cases, is less then 10 degrees. Reference is made here to the GZ curve calculated traditionally, using the constant displacement method and allowing for free flooding of the vehicle deck when the deck edge is submerged. 2. The amount of water on deck at this point can be predicted from stability calculations for the ship at another flooding scenario, in which the ship is damaged only below the vehicle deck but with some amount of water on the (undamaged) deck inside the upper (intact) part of the ship. The critical amount of water on deck is such that the ship assumes the angle of loll (angle of equilibrium) 6, that equals the angle 6,., determined previously. 3. Of crucial importance for the seeking damaged stability criterion, capable of generalising damage stability model tests results, are the following quantities occurring at the point of no return: - the elevation of water on deck above the sea level, h, and - the depth of the deck edge below sea level, f measured at the centre of damage at the inner shell of wing spaces, if any. The space above the vehicle deck at the other scenario is enclosed by the undamaged deck and undamaged ship sides above the deck, with the damage extending from below the deck downwards. For the ship with a side casing or wing tanks, the space is enclosed by the outer shell beyond the flooded part of the double hull, and by the inner shell---in way of the flooded part of the wing spaces. Consequently, the depth of deck edge is understood then as draught of deck edge measured at the inner shell of the casing (wing tanks) in the middle of damage. Due to the dynamic action of waves, the flooded water accumulates on deck causing the ship to heel and, when the deck edge becomes submerged, the water continues to elevate above sea level until it reaches a height h, depending on sea state, at which inflow and outflow rate of water through the opening is balanced maintaining the amount of trapped water on deck constant over some period of time as if the upper part of the ship were intact. The ship assumes then a heel angle at which the heeling moment due to the accumulated water on deck is balanced by the restoring moment. This quasi-static heel angle determines in turn the mean roll angle about which the ship oscillates-it cannot, therefore, be greater than the angle 6,.. An immediate practical conclusion can be drawn from these observation-any measures increasing the GZ,, and/or decreasing the heeling moment due to water accumulated on deck are beneficial to ship safety. These are first of a: I. buoyant chambers made on the vehicle deck along the ship sides by sealing off the space between the flanges of side girders and the side itself. These chambers should be particularly effective and recommended as they do the both things and, in addition, do not restrict the stowage space. 2. down-flooding arrangements which counteract the accumulation of water on deck and, if properly designed. can largely reduce or even eiminate this phenomenon. There are many other solutions that are feasible for existing ro-ro ships such as sponsons, transverse removable bulkheads, partial bulkheads, to mention only few, but above two are ro-ro friendly and at the same time veryv effective. However, the use of passive down-flooding arrangements on existing ships is not an easy task and requires special attention--they have to be fitted in such a way to be put into operation exclusively within the extent of the compartment (group) flooded 8

73 Therefore, they have to be combined with other enhan- which can be known only after flooding. cing devices or be of a special design. A Rational Boundary Stability Curve. It can be expected that a generalised damage stability criterion (boundary stability curve), valid for any damage scenario and subdivision configuration, can be presented in a non-dimensional form: h/ihs = f (f/h 5 ) (11) which can be deduced from the considerations regarding water accumulation on deck [18], where H s is the mean critical sea state, characterised by significant wave height, found with the aid of physical or numerical experiments for any damage case, characterised by h andf as described above. This type of relationship can be influenced mainly by one parameter-the ratio TITZ, where T. is modal period of the critical sea state and T, is the natural heave period. The natural heave period T, is proportional to (TC,..)0-5 where Tand Cv, are ship draught and vertical prismatic coefficient for the ship in flooded condition. In practice, however, 7z is constant for a given ship, the same for intact and flooded condition. This is particularly true for ships with large B/T ratios and the V-type stations, typical for ferries and a majority of dry cargo ships. On the other hand, the modal period T. is itself a function of H, for the critical sea state, which in turn is a complex function of damaged stability and subdivision arrangement. Hence, for the end product to be simple, the boundary stability curve, in the form given by Eq. 11, should be necessarily obtained keeping constant values of the ratio T4/Tl, taken as a parameter. Such curves should be universal, valid for any ship of any size, any type of subdivision, loading condition and compartment flooded.. I Practical Applications. This type of boundary curves, although novel, would be very simple in practical applications. Having calculated for a given compartment group the elevation of flooded water on deck h and draught of deck edge at the openingf a straight line with a slope of hif can be drawn on a graph presenting the boundary curves, passing through the origin. The straight line intersects a boundary curve, with a given value of ratio TJ17',, at a certain point with given values of h/h 5 and flhs. The mean critical sea state is given then as HtI= h/(lh,)=f/(f/h 5 ) forgiven TJ1T (12) As T. is itself a function of sea state, the true value of H. is determined by a point of intersection between a curve H, = H 5 (To), defined by the above equation for different values of the parameter T., and a curve T. = To(H4), a characteristic curve known for the given sea spectrum. In natural sea conditions, the elevation of water above the sea level, h, is a random quantity, therefore the critical sea state is random as well, and what we use here is the mean critical sea state H,. Other ship types. The proposed procedure has so far focused on ro-ro vessels but the approach adopted could also be easily applied to conventional ships, if the effect of water shipping on the weather deck, unprotected by the ship's sides, is regarded as equivalent to the mechanism of water accumulating on the vehicle deck. In such a case, the method should be formally applied to the ship as if her sides were extended vertically above the upper deck, thus reducing this deck to a ro-ro type. 9

74 CONCLUSIONS The damaged stability criterion proposed here seems to be well founded as it is based on comparison of some real quantities derived from a physical model. There is, therefore, hope for a rational criterion that could be delivered to IMO in a short time scale. Equation II can be quickly established (and verified) if only the new quantities h andf are provided for the damage cases which have been previously investigated. For this purpose, existing software for damage stability calculation has to be modified slightly so as to be able to calculate them. Nobody before regarded them of interest for ship safety. ACKNOWLEDGEMENT The author is indebted to Dr. Dracos Vassalos who made the necessary arrangements for awarding him by Strathclyde University a six month Visiting Fellowship at the Department of Ship and Marine Technology. REFERENCES I- International Maritime Organisation: Regulation on subdivision and stability of passenger ships (as an equivalent to Part B of Chapter II of the 1974 SOLAS Convention), [MO, London, 1974, 114 pp. This publication contains IMO resolutions A.265 (VIII), A.266 (VIII), and explanatory notes. 2. Maritime Safety Committee: Adoption of amendments to the 1974 SOLAS Convention, regarding subdivision and damage stability of dry cargo ships, Resolution MSC 19(58), IMO, London, 1990, 13 pp. 3. Bird, H, and Browne, R. P.: Damage stability model experiments, Trans.RINA, Vol. 116, 1974, pp ; also in: The NavalArchitect, October 1974, ibid. 4. Middleton, E. H., and Numata, E.: Tests of a damaged stability model in waves, SNAME Spring Meeting, 1-3 April 1970, Washington DC, paper No. 7, 14 pp. 5. Graham, A.: The assessment of damaged stability criteria using model tests, Proc., 4th Int. Conf. on Stability of Ships and Ocean Vehicles - STAB '90, Naples, September 1990, Vol. II, pp ; also in: document STAB 35/INF. 2, IMO, London, 1991, ibid. 6. Pucill, K. F., and Velschow, S.: Ro-ro passenger ferries safety studies - model tests for a typical ferry, Proc., RINA and DoT Int. S)mnp. on the Safety of Ro-Ro Passenger Ships, RIINA, London, April 1990, paper No. 7, 14 pp. 7. Dand, I. W.: Experiments with a flooded model of a ro-ro passenger ferry, Proc., 2ndKummerman Int. Conf on Ro-Ro Safety and VuTnerability - The Way Ahead, RINA, London, April 1991, paper No. 11, 14 pp. 8. Velschow, S., and Schindler, M.: Ro-ro passenger ferry damage stability studies - a continuation of model tests for a typical ferry, Proc., The RINA Sy7np. on Ro-Ro Ship's Survivabiltv - Phase 2, RINA, London, 25 November 1994, paper No. 5, 15 pp. 9. Dand, I. W.: Factors affecting the capsize of damaged ro-ro vessels in waves, Proc., The RINA Symp. on Ro-Ro Ship's Survivability - Phase 2, RENA, London, 25 November 1994, paper No. 3, 20 pp. 10

75 10. Vassalos, D.: Capsizal resistance prediction of a damaged ship in a random sea, Proc., The RlNA Symp. on Ro-Ro Ship's Survivabilih, - Phase 2, RINA, London, 25 November 1994, paper No. 2, 15 pp., also in: Trans. RINA, Vol. 138, 1995, 20 pp. 11. Vassalos, D., and Turan, 0.: Damage survivability of passenger ships, Trans. SNAME, Vol- 102, 1994, 34 pp. 12. Grochowalski, S., and Pawlowski, M.: The safety of ro-ro vessels in the light if the probabilistic concept for standardising unsinkability, Inteniational Shipbuilding Progress, Vol. 28, No. 319, March 1981, pp Rusis, S.: Survival Capability Class: Increased safety, but does it destroy the ro-ro concept?, Proc., 9th Int. Coaf. on Through Transport using Roll-on. Roll-off Methods - RO-RO '88, BNL Ltd., Gothenburg, June 1988, pp Sen, P., and Wimalsiri, W. K.: Ro-ro cargo ship design and IMO subdivision regulations- Proc., 2nd Kummerman Int. Conf. on Ro-Ro Safery and Vulnerability - The Way Ahead, RNA, London, April 1991, paper No. 9, 7 pp. 15. Pawlowski, M., and Winkle, I. E.: Capsize resistance through flooding - a new approach to ro-ro safety, Proc., 9th Int. Conf. on Through Transport using roll-on/roll-off Methods - RO-RO '88, BML Ltd., Gothenburg, June 1988, pp Paw!owski, M.: A new method of subdvision of ro-ro ships for enhanced safety in the damaged condition, Proc., 12th Int. Conf. on Marine Transport using roll-on/roll-off Methods - RO-RO '94, BML Ltd., Gothenburg, April 1994, Vol. 2, pp. ; also: New forms of ro-ro ship subdivision, The NavalArchitect, April 1995, pp. E198, E20l-E USA: Statistical bow heights study and the probabilistic vertical extend of damage, document SLF 39/5/3, IMO, London, 1995, 5 pp. + Annex: Statistical bow heights study for determination of the probabilistic vertical extent of damage, 13 pp. 18. Hutchison, B. L.: Water on-deck accumulation studies by the SNAME ad hoc Ro-Ro Safety Panel, Workshop on tntmerical and physical simulation of ship capsize in heavy seas, Loch Lommond, July 1995, University of Strathclyde, Glasgow, 13 pp. 11

76 MODEL TESTS AT MARLNTEK WITH DAMAGED RO-RO VESSELS Presented at WEGEMT Workshop on Damage Stability of Ships, Friday 20 October 1995, at Department of Ocean Engineering, Technical University of Denmark, Lyngby, Denmark by Vidar Aanesland INTRODUCTION The disasters of several passenger/roro vessels have set focus on the behaviour of damaged RoRo vessels and in particular the survivability in a harsh environment. A number of theoretical, numerical and experimental research projects have been initiated and the present work has been carried out as a part of the research program "Safety of Passenger / RoRo Vessels". A set of model tests have been used partly to investigate the ship motions and capsize process as function of wave conditions and partly to validate and verify a new simulation program developed by University of Strathclyde. During the experiments new model test procedures were defined and a number of questions regarding the experimental methods arose which will be focused upon in the following. Some comments will be given both to centrecasing vessels, sidecasing vessels as well as introducing transverse bulkheads on the Ro-Ro deck to limit the water ingress. As new insight is obtained throughout the present and other investigations, new approaches and various model test techniques may evolve. The present work is by no means a work with final conclusions, but intended as a contribution to the present development of better understanding of the hydrodynamic mechanisms involved with damaged RoRo vessels and how to ensure that new designs will meet higher standards with respect to survivability. SHIP MODELS Two different models were used during the testing program. The first model has been tested extensively at the Danish Maritime Institute (DMI) on behalf of Department of Transport through their Ro-Ro Passenger Ferry Safety Studies. For a detailed description of the model it is referred to the reports, /1/, after the disaster of Herald of Free Enterprise. The second model, NORAL, was particularly designed for the present project and was first tested at DMI in order to investigate water ingress on the Ro-Ro deck and in particular the in and out flow through the damage opening. Only the main particulars are given herein in Table I, while details about the model can be found in the presentation from DMI.

77 Table 1. Main particulars Main Particulars Model I NORA Length Breadth D to main deck Draught Draug-ht to main deck tonnes tonnes Model scale 1: : It is very important that the models are geometrically correct both with respect to the hull form as well as with respect to the flooded compartments and Ro-Ro deck inside the model. The motion characteristics of the ship may change drastically within seconds and hence it is very important that this change is due to the correct flooding. In particular, the arrangement on the Ro-Ro deck has to be modelled with the main dimensions well represented due to the shipment of water from side to side or from bow area to stem area- The wall thickness in the ship side should be as low as possible which is one of the reasons for using thin metal plates from the Ro-Ro deck and upwards. In order to investigate the water motion and be able to make good video recordings, much of the interior is made Of transparent plastic. Both models have a midship damage condition where a 100 % SOLAS damage is covering both thie Ro-Ro deck and the compartment below. Hence, the damage freeboard is changed by the size of the damage length of the compartment below the water line. In the model tests the change of damnage length is changed by placing pieces of plastic foam in the compartment. A weight elevator is installed on the models which enables the centre of gravity, KG, to be changed rapidly by bringing weight elements up and down in the centre plane of the model. However, this means that the radius of gyration is different for the various KG values. INSTRUMENTATION The following channels have been measured: - Wave elevation in 2 reference points (1 fixed in the basin and 1 on the carriage) - Relative motion in 3 points (2 on damage side and 1 on intact side) - Vertical and lateral acceleration in 1 point on intact side at position of relative motion probe - Vessel motion in 6 DOF The OPTOPOS optical measuring system consists of 3 light emitting diodes mounted on the model. For the present model a modification of the standard system had to be made due to the necessity of heaving and lowering the model. Usually three masts are installed on the model, one in the bow region, one midship and one in the stem region. The wires and cables must not come between the diodes and the cameras on shore. A special rig had to be designed where all three diodes were mounted and the rig itself was placed in the stem region of the model.

78 In addition 2 video cameras were installed on the model showing the water motion on the Ro-Ro deck, and during the tests an additional camera was recording from the side of the basin. TEST FACILITY The experiments were carried out in the Ocean Basin at MIARRINTK, Trondheim, with dimensions 50 times 80 meters and a variable water depth of 0-10 meters. A double flap wave generator is mounted on one of the short sides in the basin, while a multi-flap wave generator system is mounted on one of the long sides. The waves are generated from either one of the two wave generators or both simultaneously. Effectively, any wave direction on the ship may be generated. In order to obtain the longest time recordings, it is preferable to run the double flap wave maker which is also preferable when long-crested waves are to be used. The Ocean Basin is equipped with a carriage system which follows the drifting model and it can easily be rescued when capsizing. The carriage also supports the cables from the measuring system and video cameras. For the main part of the model tests, the model is freely drifting, while 'in a number of supplementary tests the model is supported by a soft spring system. The purpose is to have a heading control system and enable the model to have a forward speed. The soft spring system was supported on the carriage which still was following the mean position of the model. TEST SET-UP AND TEST PROGRAM The main objective has been to investigate the behaviour of the damaged models as function of significant wave height for given combinations of KG values and damaged freeboard. For each selected configuration, the wave height has been increased or decreased according to the previous tests in such a manner that the limit between capsize and no-capsize is found. The no-capsize condition for the present tests are defined as surviving the given sea condition as long as the physical limitations of the Ocean Basin permits the model to drift. The drifting time is dependant on the wave heights due to the dependency on the mean wave drift forces. Higher wave heights will create higher drift forces and hence a higher drifting speed. Sea conditions Sea conditions have been calibrated for a Fis-range which covers the heights of interest, keeping the wave steepness constant, Hs / Ap = 0.04, where Hs is the significant wave height and Ap is the wave length defined from the peak period of the spectrum. A JONSWAP spectrum has been used with a y - factor of 3.0 and wave height range is from I to 8 meters. Three wave probes were mounted along the length of the basin during the wave calibration positioned at the centre of the tank and +/- 20 mn towards and away from the wave generator. One additional reference wave probe, fixed at the basin side, was present during the complete test program.

79 The tests were performed partly as a freely drifting model for the main test matrix and as tests with the model suspended in soft horizontal springs for the supplementary matrix. Model test matrix Somewhat different conditions were specified for the two models because the various aspects of the capsize process were to be investigated. The parameters of interest were different combinations of KG values and damage lengths. From the possible combinations a set was selected partly based on calculations performed prior to the experiments and partly based on the results obtained as the tests went on. For each selected condition a set of wave heights were run selected from already calibrated waves described earlier. In addition a number of model tests were carried out for a mean wave heading of 30 and 60 degrees relative to the incoming wave system, and in combination with forward ship speed. The procedure adopted for the tests was as follows: I ) Prepare the model for a new test 2) Start the video recordings 3) Start the wave generator and clock 4) Wait for 1 minute 5) Lower the model into the waves at a defined position in the basin 6) Release the cover 7) Follow the model with the gondola with the heaving wire slack 8) Continue until the model is capsizing and has to be rescued or the model is reaching the beach The initial conditions when starting the experiments have caused some concern when defining the test procedure. In any experiment it is important to have well defined starting conditions so that the tests are as repeatable as possible. In other words that two tests under the same conditions will have the same outcome. Any experiment will have to face this problem but the difficulty in obtaining "equal" starting conditions differ greatly depending on the type of model test. For experiments with intact models the model test procedures have been developed over decades and is normally reasonably easy to definie. The problems grow with the complexity of damaged models where water is supposed to enter and interact dynamically with the hull. As seen from the above list, the same procedure has been used throughout the test program. Hence the ship will meet the same realisation of the sea in all the cases with a specified wave height.

80 Preparation of the model for a new test After each test run the model is hoisted out of water by the heaving wires by which the model is connected to the carriage. The model is mounted in a crane system which consists of a tackle, a weighing-machine and a yoke by which it is possible to very fast check whether any water has been entering the model and to return to starting point of a new test. If any water has entered the model, it is removed by a vacuum cleaner system. Further, the model is checked visually and the cover to the damage is mounted. Starting the video recordings, wave generator and the clock The video cameras which are mounted on the model and the fixed camera onshore are started by a signal from the control room. Hence the tape recorders are running when the wave generator and the clock are started. The complete test sequence is then available on tape. Lowering the model into the water Thie model is hanging above the water surface for about one minute in order for the sea condition to develop. As the shorter waves travel more slowly than longer waves, it is common to wait for a certain rime instance before starting the tests. In the present case the tests are started as close as possible to the wave generators in order to get a long drifting distance and one minute is sufficient. When the minute has gone, the model is lowered into thie waves and after about 10 seconds the cover hiding the damage is removed. The model starts to take in water and sink to the level defined by the damage length. Controlling the model from the carriage The model is freely drifting and taking the heading and drifting speed as given by the waves. By this method a full blackout is simulated and the model is completely free in 6 degrees of freedom. The hoisting wires will be slack during the drifting period but available as soon as the model starts to capsize. It is also possible to control the behaviour of the model from the online measuring system and give the staff on the carriage a signal when the model is about to capsize. COMMENTS AND CONCLUSIONS As experimrents have been car-ied out with a model with centrecasing and a model with sidecasings it is interesting to note a significant improvement on the survivability for the latter configuration. It is then assuned that the sidecasings have watertight subdivision. This is a consequence of additional buoyancy placed at the ship sides as well as a reduction in flooded area on the Ro-Ro deck. The reduction in stability due to free surface effect is lower. In the centrecasing condition, the vessel was all the time capsizing towards the waves. Even though the Ro-Ro deck was open from side to side in the bow and stem region, the initial heel was towards

81 the damage and the water intgress only increased the heeling. In the sidecasing condition the model capsized to both sides depending on the initial heel and the specific wave trains that followed. In most cases the model capsized towards the side at which the model got the initial heeling. In some cases, however, the model started heeling towards the waves and suddenly got hit by some large waves which made the water flow to the opposite side of the deck. In such a case the model was not observed coming back towards the damage. By introducing transverse bulkheads the survivability is significantly increased in both the centrecasing and sidecasing design. The number and height of the transverse bulkheads needed for a given vessel will depend on the initial survivability without the transverse bulkheads. For lower significant wave heights half height bullkheads may be sufficient. In addition, if it is possible to lower and fasten ramps on top of these transverse bulkheads, it will further improve on the survivability. The violent water motion on the Ro-Ro deck, which can be compared to the sloshing in open water tanks, will be suppressed. At a certain significant wave height, however, the water motion inside the vessel will be so violent that only full height transverse bulkeads will be good enough. It should also be noted that in order to obtain a good effect of the ramps, any spacing between the ramps and the casings should be closed adjacent to the transverse bulkheads. In the case of a centre casing design, it only needs ramps covering the half ship breadth provided that the ramps are placed adjacent to the centre casin~g. Ramps at the ship sides do not seem to have any significant effect at all. In the sidecasing design, the ramps should be placed adjacent to the sidecasings. The positive damping effect due to the ramps can be compared to having tank tops in free surface tanks and water may not spill easily over the bulkheads. Large openings in the centrecasing do not seem to have large negative effects on the survivability of the vessel. In some cases the openings secure a shipping of water from the damaged side of the ship to the intact side and makes the vessel more stable. Damage about midship seems to be the worst case when it comes to water ingress and danger of capsize. In this case the vessel may stay on more or less even keel and the water may flow both forwards or aftwards and the outflow through the damage is much less than the inflow. A freely drifting ship tends to turn the side to the waves. The model tests show that by changing the ship heading so that the waves are hitting the vessel with an angle of 60 degrees or less, relative to the ship centre line, the vessel is able to take significantly higher waves. In practical terms it means that it is important to change the vessel heading in order to avoid beam sea waves. A ship with forward speed into the waves, however, can be even more dangerous that a freely drifting ship. The damaged freeboard is one of the major parameters defining the survivability of a vessel. Hence, a freeboard of 0.5 mn will be much better than 0. 1 m. The wave heights which the vessel can sustain is partly due to the freeboard itself because a smaller portion of the waves will be able to reach the Ro-Ro deck, and partly due to the fact that more water may be able to drain through the damage between the waves giving water ingress. REFERENCES /1/Danish Maritime Institute, RoRo Survivability Model, Tests - Phase 2, Final Reports 1-4.