3rd International Proceedings of the 3rd ITTC Volume II 619 The Specialist Committee on Prediction of Extreme Ship Motions and Capsizing Final Report and Recommendations to the 3rd ITTC 1. INTRODUCTION 1.1. Membership, meetings and organisation Membership: The Committee appointed by the nd ITTC consisted of the following members: Professor D. Vassalos (Chairman) Universities of Glasgow and Strathclyde, UK Dr. M. Renilson (Secretary) Australian Maritime College, Australia, and QinetiQ, Haslar, UK Mr. A Damsgaard Danish Maritime Institute, Denmark Professor H.Q. Gao China Ship Scientific Research Centre, Mr. D. Molyneux Institute for Marine Dynamics, Canada Professor A. Papanikolaou National Technical University of Athens, Greece Professor N. Umeda Osaka University, Japan In addition, the following corresponding members contributed greatly to the work of the committee: Dr. J.O. De Kat MARIN, The Netherlands Professor A. Francescutto University of Trieste, Italy Professor J. Matusiak Helsinki University of Technology, Finland Meetings: Seven Committee meetings were held as follows: Shanghai, China, September 1999 Launceston, Australia, February Osaka, Japan, October Glasgow, Scotland, UK, May 1 Trieste, Italy, September 1 Heraklion, Greece, October 1 Glasgow, Scotland, UK, February (Editorial meeting) Organisation: The following working groups were established and chairmen appointed: Benchmark Testing for Intact Ship Stability (Umeda) Benchmark Testing for Damaged Ship Stability (Papanikolaou) Guidelines for Experimental Testing of Intact Ship Stability (de Kat) Guidelines for Experimental Testing of Damage Ship Stability (Damsgaard) Questionnaire (Molyneux) Symbols and Terminology (Francescutto)
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Liaisons: The following Committees and organisations have been contacted: Loads and Responses; Manoeuvring; Waves; IMO (Revision of 1966 ICLL, Intact Stability, Harmonisation Group); WEGEMT; CRN; SNAME Technical Panel; EU Thematic Network SAFER EURORO; SRA of Japan Panel RR71; COREDES. 1.. Tasks from the nd ITTC Coordinate a comparative study of mathematical models for the prediction of intact and damage stability in waves. The mathematical models will be compared to the results of benchmark tests for two test ships, Ships A and B, as specified in Section 7. of the report of the Stability Committee of the nd ITTC. Present the guidelines for experimental testing of intact and damage stability, as given in Appendix A of the report of the Stability Committee of the nd ITTC, in the format defined in the ITTC Quality Manual. Symbols and terminology should agree with those used in the 1999 version of the ITTC S&T List; if necessary, new symbols should be proposed. 1.3. Contents of the 3rd ITTC Report The following chapters detail the tasks undertaken by the Committee: Chapter : Benchmark Testing for Intact Ship Stability Chapter 3: Benchmark Testing for Damage Ship Stability Chapter : Guidelines for Model Testing of Intact and Damage Stability Chapter : Questionnaire Chapter 6: Symbols and Terminology Chapter 7: Conclusions and Recommendations Chapter 8: References and Nomenclature. BENCHMARK TESTING FOR INTACT SHIP STABILITY.1. Introduction This chapter describes results of the ITTC benchmark testing of intact stability. For these tests, a container ship and a fishing vessel were selected and their hull forms, captive test data and results of capsizing model experiments were provided in advance. On this basis, eight research organisations submitted numerical results. Comparisons between numerical and experimental results revealed that some numerical models are able to predict extreme motions qualitatively, including capsizing due to parametric resonance and due to broaching. Moreover, the importance of several factors necessary for capsize prediction is noted by mutual comparisons of the numerical studies. List of Participating Organisations Ship A-1: 1 Flensburger Schiffbau Gesellschaft 1 (Ms. Heike Cramer) Helsinki University of Technology (Prof. Jerzy Matusiak) Maritime Research Institute Netherlands (Dr. Jan O. de Kat) Osaka University (Prof. Naoya Umeda) Technical University of Malaysia (Dr. Adi Maimun) Universities of Glasgow and Strathclyde, The Ship Stability Research Centre (SSRC) (Prof. Dracos Vassalos) University of Tokyo (Prof. Masataka Fujino). The computer program at FSG was originally developed at Universitat Hamburg.
3rd International Proceedings of the 3rd ITTC Volume II 61 Ship A-: Helsinki University of Technology (Prof. Jerzy Matusiak) Memorial University of Newfoundland (Prof. Don Bass) Osaka University (Prof. Naoya Umeda) Universities of Glasgow and Strathclyde, The Ship Stability Research Centre (Prof. Dracos Vassalos) This order is not related to the code used in this report... Background The trend towards adopting performancebased criteria in favour of rules-based criteria aiming at safety improvement at sea continues unabated at the International Maritime Organisation (IMO), the rule making body of the United Nations. To facilitate this process, model experiments and numerical simulations tools need to be developed and validated. However, a standard numerical prediction technique for capsizing has not yet been established. Therefore, the nd ITTC (ITTC, 1999) organised a specialist committee for this purpose and planned benchmark testing of numerical predictions with selected data from free running model experiments. This chapter summarises the results of these benchmark tests and highlights the importance of a number of factors to the numerical prediction of ship capsizing..3. Framework of ITTC Benchmark Testing In the intact benchmark testing programme, two sets of free running model experiments were utilised. The first set was carried out with a 1/6 scaled model of a 1 gross tonnes container ship (Ship A-1) at the seakeeping and manoeuvring basin of the Ship Research Institute by Hamamoto et al. (1996). Here the ship model capsized mainly due to parametric resonance in the lower speed region. The second set was carried out with a 1/1 scaled model of a 13 gross tonnes purse seiner (Ship A-) at the seakeeping and manoeuvring basin of the National Research Institute of Fisheries Engineering (NRIFE) by Umeda et al. (1999). In these tests, the model capsized mainly due to broaching in the higher speed region. The principal particulars and body plans of these ships are shown in Table.1 and Figures.1 and.. In the experiments each ship model was self-propelled and free from any restraints, steered on a specified course by using an auto pilot in regular following and quartering waves. The angular velocities and angles were measured using an optical gyroscope, and were recorded on an onboard computer. The reference system used in this report is shown in Figure.3. Table.1 ships. Principal particulars of the test Items Ship A-1 Ship A- L PP (length) 1. m 3. m B (breadth) 7. m 7.6 m D (depth) 13. m 3.7 m T f (draught at FP) 8. m. m T (mean draught) 8. m.6 m T a (draught at AP) 8. m.8 m C b (block coefficient).667.97 k yy /L PP..3 (pitch radius of gyration) x CG (longitudinal position of centre of gravity from midships) 1.1 m aft 1.31 m aft GM (metacentric height).1 m 1. m T E (natural roll period) 3.3 s 7. s A R (rudder area) 8.11 m 3.9 m D P (propeller diameter). m.6 m T E (time constant of steering 1. s.63 s gear) K R (proportional gain) 1. 1. K R T D (differential gain) 3. s. s
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Among several hundreds of model runs, four runs were selected for each ship for the purpose of ITTC benchmark tests as described in Tables. and.3. Here the nominal Froude number, Fr, and the auto pilot course from the wave direction, χ c, are control parameters and the wave height, H, and wave length, λ, are the wave parameters. The initial values of ship motion were specified based on measured data except for the sway velocity, which was assumed to be zero because of measurements limitation. For ships A-1 and A-, the captive model experiments, e.g. resistance test, self- propulsion test, propeller open test, circular motion tests (CMT), roll decay test and so on, were carried out mainly in NRIFE s seakeeping and manoeuvring basin using an X-Y towing carriage. These data together with hull offset data and the above mentioned initial values were provided to the participating organisations prior to undertaking any numerical simulations... Results The ITTC benchmark test programme for intact stability commenced in March with numerical results submitted by March 1. Numerical prediction methods used by the participating organisations are outlined in Umeda (1) with numerical results shown in Figures. to.6 together with the experimental results. In agreement with the participating organisations the results have been presented anonymously throughout this benchmark programme. Figure. Body plan of Ship A-. RUDDER G Z YAW PITCH Figure.3 Reference system. ROLL The numerical predictions are firstly required to qualitatively agree with the corresponding model experiments. Thus, the qualitative nature of the results obtained from experiments and numerical calculations are overviewed in Tables. and.. This includes capsize, non-capsize, harmonic roll, sub-harmonic roll, surf-riding and broaching. Here as a judging criterion of broaching the proposal of Umeda (1999) is used. That is, broaching is a phenomenon in which both the yaw angle and yaw angular velocity increase despite the application of maximum opposite rudder angle. Lack of qualitative agreement between numerical and experimental results identified with shading. X Y Table. Calculated conditions for Ship A-1. Figure.1 Body plan of Ship A-1. H /λ λ/l PP Fr χ c degrees (a) 1/ 1.. (b) 1/ 1.. (c) 1/ 1..3 3 (d) 1/ 1.. 3
3rd International Proceedings of the 3rd ITTC Volume II 63 Table.3 Calculated conditions for Ship A-. H /λ λ/l PP Fr χ c degrees (a) 1/1 1.637.3-3 (b) 1/1 1.637.3-1 (c) 1/8.7 1.17.3-3 (d) 1/8.7 1.17.3-3.. Discussion (Tables.6-.7 & Figures.-.6) Ship A-1 For Ship A-1 all the participating organisations used 6 degrees of freedom (DOF) models. However, only Organisation-A submitted results that qualitatively agree with the experiments. Organisation-A calculated radiation and diffraction forces using a strip theory and dealt with manoeuvring forces by the MMG model, utilizing a body coordinate system. It evaluated the Froude-Krylov forces, including roll restoring moment in waves, by integrating incident wave pressure up to the instantaneous water surface. With this numerical model, capsizing with sub-harmonic rolling in case (a) and capsizing with harmonic rolling in case (d) were well predicted. Organisation-G also shows similar agreement but numerical results in case (a) predict capsizing with harmonic rolling, which was not observed in the corresponding experiment. The method used here is almost the same as that of Organisation-A except for radiation and diffraction modelling. Organisation-E has problems in the prediction of the heading angle. In some cases the ship course changes to bow sea and then a completely different situation occurs. This model is different from the above two organisations in a number of ways. The radiation forces were calculated using a a strip theory with hydrodynamic memory effects. The manoeuvring forces, roll damping moments, resistance and propulsion forces were estimated using available databases instead of the captive test data provided. Table. Overview of qualitative results for Ship A-1. Experiment A B C (a) cap (s) cap (s) cap (h) no roll (b) (s) (s) (s) N/A (c) (h) (h) N/A N/A (d) cap (h) cap (h) N/A N/A D E F G (a) (s) cap (s) cap (h) cap (h) (b) (s) (h) (h) (s) (c) (h) cap cap (h) (d) (h) cap cap cap (h) The method used by Organisation-B is based on a conventional seakeeping approach. That is, heave, pitch, sway and yaw are assumed to be linear around the averaged course. This organisation reported that this method is not able to deal with ship runs at Froude number greater than.3. Organisation-D proposed a method to avoid this limitation of the seakeeping model by a two-stage approach. Here the motions are assumed to be the sum of linear parts with hydrodynamic memory effects and nonlinear contributions. This means that linear motion was calculated around the instantaneous heading angle instead of the auto pilot course. However, agreement between predictions using this calculation and experiments is not satisfactory. This may be partly because the initial values used in order to take memory effects into account are different to those specified. Organisation-F is a unique example where diffraction forces were ignored, but the nu- Here (h) and (s) mean harmonic and subharmonic roll motions, respectively and cap indicates capsizing.
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International merical results do not agree well with the experimental results, particularly the case (b) concerning calculated pitch motion amplitudes. Following successful application to seakeeping predictions, Organisation-C attempted to apply CFD to the present problem. Here the Euler equation was solved by a finite difference method with fully nonlinear free surface and body surface conditions. However, it can provide a solution only for case (a) without lateral motions. If the specified initial values for lateral motions are input, even for case (a) the calculation process failed. In addition, it cannot deal with cases (b), (c) and (d), in which the desired heading angles are not zero. This fact demonstrates that the CFD approach is not yet appropriate for practical use in capsize prediction. Ship A- For Ship A-, only Organisation-A obtained qualitative agreement with experiments. Here a DOF model was used by assuming that heave and pitch motions trace their static equilibria, which are calculated as the limit of solution sets of a strip theory at zero encounter frequency. The manoeuvring forces were estimated using the MMG model and the wave-induced forces, including hydrodynamic lift due to wave fluid velocity, were calculated using Ohkusu s slender body theory. The wave effects on roll restoring moment and manoeuvring forces were ignored as higher order terms. As a result, this organisation succeeded in predicting capsizing due to broaching associated with surfriding as well as periodic motions. Organisation-C used a method that is almost the same as that of Organisation-A but the nonlinear terms in the manoeuvring models, deriving from the Froude-Krylov and radiation forces were added. As a result, for case (b) it predicted capsizing without surfriding and with a smaller rudder angle compared to the results from the experiment and those predicted by Organisation-A. Table. Overview of qualitative results for Ship A- 3. (a) (b) (c) exp. A B C D surf broach cap surf broach cap cap cap noncap noncap noncap noncap noncap surf noncap noncap noncap noncap noncap noncap (d) cap cap cap cap cap Organisation-B applies a 6 DOF model in which radiation and diffraction were calculated with a 3D Green function for zero forward velocity. Here the change of roll restoring moment due to waves was taken into account but the hydrodynamic lift due to wave fluid velocity was ignored. The hydrodynamic memory effect was included in this calculation, although the initial values were not exactly as specified. While the predictions of mean yaw angle for cases (a), (c) and (d) are better than those from the other organisations, the predicted rudder angle for case (b) is smaller than the corresponding experimental results. Organisation-D also takes memory effects into account but with a strip theory. Like organisation-b, the initial conditions are different from those specified. This organisation predicts stable surf-riding in the case (b). This may be a result of some shift of the stable equilibrium point towards a wave crest or inaccuracy of hydrodynamic lift due to wave. As a whole, the four participating organisations predicted the results relatively well compared to experiments for Ship A-, the obvious exception being broaching. 3 Here surf and broach mean surf-riding and broaching, respectively.
3rd International Proceedings of the 3rd ITTC Volume II 6.6. Factors Affecting Prediction Accuracy As mentioned above, the mathematical models for capsizing prediction involve a number of factors without clear guidance in place on which of these should be taken into account in which case. Mutual comparisons among the organisations do not easily clarify the importance of each particular factor because more than two factors are often different between the organisations. Therefore, this report reviews comparative studies of numerical simulations with and without each particular factor for Ships A-1, A- or indeed other ships. 6 DOF vs. DOF or 1 DOF Although all organisations submitted results with 6 DOF models for Ship A-1, many theoretical studies with 1 DOF models can be found for capsizing due to parametric rolling. Munif () estimated the capsizing boundaries for Ship A-1 with a 1 DOF model, a DOF model ignoring heave and pitch motions ( DOF A model), a DOF model with static equilibria of heave and pitch motions ( DOF B model) and a 6 DOF model. Here the first three models were obtained by simplifying the 6 DOF model. As a result, the following conclusions were drawn: The 1 DOF model overestimates capsizing danger. The difference between the DOF A model and the 6 DOF model can be significant. The results from the DOF B model are in reasonable agreement with those from the 6 DOF model and the experiment. The small difference between the DOF B model and the 6 DOF model derives from the fact that the natural frequency of heave and pitch motions is far from the encounter frequency with the ship running in following and quartering seas (Matsuda et al., 1997). This conclusion suggests also that coupling effects of heave and pitch on the extreme roll motion are not very important. Hydrodynamic memory effect It is well known that the linear transient motions of a ship with frequency-dependent hydrodynamic forces can be calculated using the convolution integral for hydrodynamic memory effect. However, it is not so clear for capsizing prediction whether the hydrodynamic memory effect should be taken into account or not. This is because an extreme motion leading to capsizing is nonlinear and the hydrodynamic forces acting on a ship running in following and quartering seas do not significantly depend on the encounter frequency. Hamamoto & Saito (199) carried out a comparative study for a container ship in following seas with and without the memory effect in heave and pitch motions. They concluded that no significant difference exists if the added mass and damping coefficients are calculated for the natural frequency of heave and pitch motions. Matusiak (1) investigated this problem and concluded that memory effects can improve agreement with experiments for Ship A-1. Here it is noteworthy that exact calculation with memory effects should be carried out from the start of the waves. Thus the present benchmark testing, which does not specify the initial conditions of fluid motions, is not appropriate for this purpose. Manoeuvring coefficients In following and quartering waves, prediction of manoeuvring coefficients is important because hydrodynamic lift is dominant. The first question here is whether the effect of nonlinear terms of manoeuvring forces on capsizing prediction is important or not. For Ship A-, Umeda et al. () produced time domain simulations with and without these non-linear terms and concluded that the effect of non-linear terms is negligibly small. This is because the sway velocity and yaw angular velocity non-dimensionalised with the higher forward velocity are not large even during the process of broaching.
66 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International The next problem is wave effect on the linear manoeuvring coefficients. This problem has been discussed for many years but its effect on capsizing prediction has not yet been fully investigated. Hashimoto & Umeda (1) tackled this problem with Ship A-. Their main conclusion is that the effect of waves on the derivatives of manoeuvring forces can be important with respect to sway velocity but it is not significant with respect to yaw angular velocity. Nonlinearity in yaw In seakeeping theory, ship motions, such as yaw, are often linearised around the inertial system moving with the averaged speed and course of a ship. In the field of manoeuvring on the other hand, ship motions are described with a body fixed coordinate system. Hamamoto & Kim (1993) introduced a horizontal body coordinate system, which is body-fixed but not allowed to roll. Cramer (1) reported the limitation of linearisation of yaw motion with an inertial coordinate system. Radiation and diffraction In following and quartering seas, the encounter frequency of a ship with forward speed is generally low and hence the wavemaking effect is not so significant. In this respect, it is difficult to predict pitch and heave motions near zero encounter frequency because of divergence of the D added mass. Matsuda et al. (1997) solved the problem by calculating the limit of the solution set of strip theory for the zero encounter frequency and confirmed that the new method explains the experimental results. In case of a 3D theory at very low encounter frequency, it is essential to use the Green function with both forward speed effect and frequency effect taken into account. This is because the wave related to frequency, the k wave, disappears at the zero encounter frequency and only the wave related to forward speed, the k 1 wave, remains. Hydrodynamic lift due to wave fluid velocity Very small effect of wave-making does not mean small effect of incident waves as a ship behaves like a lifting surface with a timevarying angle of attack due to wave fluid velocity and ship forward velocity. Within the assumption of small wave steepness, this hydrodynamic lift can be calculated as an end term of slender body theory or strip theory, which represents trailing vortices as a line doublet shed from the aft end (Umeda, 1988). For a 3D theory, it is necessary to include free vortex layers shed from the hull surface. Comparison between calculations with and without the hydrodynamic lift due to wave fluid velocity for Ship A- can be found in Umeda (). The results indicate that prediction of broaching is largely affected by this term. Roll damping moment Roll damping moment consists of wavemaking, eddy-making, lift and friction components, the main non-linearity deriving from the eddy-making component. However, as the experimental work of Umeda () showed, roll damping can be regarded as linear when the Froude number is greater than.. Since eddies are shed away at high speed, the eddy making component disappears. In addition, the wave-making component is not significant because of the low encounter frequency and the friction component is generally small. Therefore, roll damping relating to this benchmark testing scheme consists of mainly the lift component, which is linear and depends on forward velocity. A comparison of predictions of broaching boundary using empirical methods of calculating the lift component was presented by Ikeda et al. (1988) for the Ship A-, indicating that the predicted results depend on the selection of empirical methods. Because of this, roll decay tests with forward velocity were carried out for the Ship A- to obtain reliable results for use in the benchmark study.
3rd International Proceedings of the 3rd ITTC Volume II 67 Wave effect on roll restoring moment While all organisations took into account the wave effect on roll restoring moment to simulate parametric resonance, some organisations ignored this effect for broaching prediction. This is because the wave effect estimated on the basis of hydrostatics can be too large in case of high forward velocity. Results from captive model experiments (Umeda & Yamakoshi, 1986) for a small trawler indicate that the measured metacentric height in waves is smaller than the calculated value, derived by using hydrostatics. It is, therefore, necessary to develop theoretical or empirical methods for a more accurate prediction of the wave effect on roll restoring, particularly for higher forward speeds. Roll-yaw coupling When a ship runs in calm water with a constant heel angle, sway force, yaw moment and roll moment act on the hull in addition to conventional manoeuvring forces and moments. In this benchmarking scheme, these data from the captive tests for Ship A- at NRIFE were provided in advance. However, such data are not always available and empirical or theoretical methods have not yet been established. Renilson & Manwarring () reported a comparison in predictions of broaching boundary with and without the roll-yaw coupling for a trawler and the results indicate that the prediction without the roll-yaw coupling can underestimate the danger of broaching. Resistance and propulsion It is well accepted that predictions of hull resistance and propulsive performance have been the most crucial issue at ITTC. This is also the case in the prediction of capsizing of intact ships. This is because surf-riding, which can trigger off broaching and then capsizing, depends on hull resistance and propeller thrust in addition to wave-induced surge force. In the benchmarking study, data from calm-water model tests were provided in advance. However, since such data are not always available, an accurate prediction method is still desirable. Wave irregularity and short-crestedness The applicability of numerical models to realistic seaways, that is, short-crested irregular waves, should be examined in the future. Although capsizing model experiments for Ships A-1 and A- were carried out in both long-crested and short-crested irregular waves (Umeda et al., 199), the benchmark testing programme deals with only the case of regular waves. The experimental results indicate that capsizing danger is least in short-crested irregular waves, followed by long-crested and finally regular waves. However, numerical simulations in time domain for capsizing in short-crested irregular waves are very limited. Only recently Sera & Umeda (1) executed numerical calculation in both short-crested and long-crested irregular waves with a 1 DOF model, and confirmed the qualitative conclusion, from the experiments, that wave short-crestedness reduces capsizing danger..7. Concluding Remarks As a result of benchmark testing of intact stability, it was found that numerical models can qualitatively predict capsizing due to parametric resonance and due to broaching in the limited cases tested here. In the case of parametric resonance, a 6 DOF model, hydrostatics for the wave effect on roll restoring moment, strip theory for wave radiation and diffraction and experimental data of manoeuvring forces, hull resistance, propulsion force and roll damping are used. In the case of broaching, a DOF model, slender body theory for the added mass and hydrodynamic lift due to wave fluid velocity and experimental data of manoeuvring forces, hull resistance, propulsion force and roll damping are used. However, minimum requirements for accurate modelling of intact ship capsize have not yet been fully established.
68 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Table.6 Comparison of numerical prediction methods for Ship A-1. Organisations DOF Radiation A 6 strip theory B 6 C 6 D 6 (linear in sway, heave, pitch, yaw) (coupled with fluid motion) (two stage model) strip theory CFD (non-linear) strip theory Manoeuvring Damping exp. (non-linear) ignored ignored exp. (linear) E 6 strip theory empirical F 6 empirical empirical G 6 strip theory exp. (non-linear) Roll Restoring hydrostatics in waves hydrostatics in waves CFD (non-linear) hydrostatics in waves hydrostatics in wave hydrostatics in waves hydrostatics in waves Organisations Roll Damping Froude-Krylov Diffraction A experimental+ empirical forward speed Hydrodynamic Lift due to Wave Hull Resistance non-linear strip theory ignored experimental B empirical linear strip theory ignored experimental C no viscous effect non-linear (cfd) CFD (non-linear) ignored no viscous effect D empirical non-linear strip theory ignored experimental E empirical non-linear strip theory cross-flow model empirical F empirical non-linear ignored ignored empirical G Organisations experimental+ empirical forward speed Propeller Thrust non-linear Rudder Force slender body theory at ω= Incident Wave end term Hydrodynamic Memory Effect experimental Specified Initial Conditions A experimental experimental linear ignored yes B adjusted ignored linear ignored yes C adjusted ignored linear included no D experimental empirical linear included no E empirical empirical linear included no F empirical empirical linear ignored yes G experimental experimental linear ignored yes
3rd International Proceedings of the 3rd ITTC Volume II 69 Table.7 Comparison of numerical prediction methods for Ship A-. Organisations DOF Radiation Manoeuvring Damping Roll Damping Roll Restoring Froude- Krylov Diffraction A static heave & pitch slender body theory at ω= experimental (linear) exp.+ empirical forward speed effect hydrostatics in calm water linear slender body theory at ω= B 6 3D theory (Green function at Fr=) ignored empirical + tuned hydrostatics in waves nonlinear 3D theory (Green function at Fr=) C static heave & pitch strip theory experimental (non-linear) exp.+ empirical forward speed effect hydrostatics in calm water nonlinear slender body theory at ω= D 6 two stage model strip theory experimental (linear) empirical hydrostatics in waves nonlinear strip theory Organisations Hydrodynamic Lift due to Waves Hull Resistance Propeller Thrust Rudder Force Incident Wave Hydrodynamic Memory Effect Specified Initial Condition A end term exp. exp. exp. linear ignored yes B ignored empirical tuned empirical non-linear included no C ignored exp. exp. exp. linear ignored yes D ignored exp. exp. empirical linear included no To improve quantitative prediction accuracy, it is essential that the contribution of several factors should be further investigated through comparative studies of capsizing predictions with and without these factors being accounted for. These should include wave effect on manoeuvring coefficients and roll restoring moment, roll damping prediction, radiation and diffraction and resistance at higher speeds with use of captive model experiments. It is noteworthy that stability prediction should be based on accurate predictions of seakeeping, manoeuvring, resistance and propulsion, subjects which have been the focus of considerable effort by the relevant ITTC technical committees over many years. This alone offers ample justification of the problems encountered in attempting to accurately predict ship capsize. For wider validation studies, it is desirable to execute benchmark tests concerning the prediction of capsizing boundary curves as shown by Munif () for Ship A-1 and by Umeda & Hashimoto () for Ship A-. This is because capsizing boundaries can be complicated as a result of system nonlinearities. Furthermore, practical application to ship design, operation and regulation necessitates the extension of the current predictive capability to more realistic seaways. This, in turn, warrants the undertaking of a new benchmark testing of numerical codes with relevant experimental data from the most advanced model basins having multicomponent wave makers.
63 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Experiment Calculation Experiment Calculation 1 1 3 - -1 1 1 3 - -1 1 6 - -1 1 6 - -1 9 6 3-3 1 3-6 -9 9 6 3-3 1 3-6 -9 9 6 3-3 6-6 -9 9 6 3-3 6-6 -9 3 1-1 1 3-3 - 3 1-1 1 3-3 - 6-6 - -6 6-6 - -6 1 1-1 3-1 -1 1 1-1 3-1 -1 1 1-6 -1-1 1 1-6 -1-1 (a) (b) Experiment Calculation Experiment Calculation 1 1 3 - -1 1 1 3 - -1 1 1 1 - -1 1 1 1 - -1 9 6 3-3 1 3-6 -9 9 6 3-3 1 3-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 6-1 3 - -6 6-1 3 - -6 6-1 1 - -6 6-1 1 - -6 1 1-1 3-1 -1 1 1-1 3-1 -1 1 1-1 1-1 -1 1 1-1 1-1 -1 Organisation-A Organisation-E Organisation-G (c) Figure. Experimental results and numerical results for Ship A-1 from three organisations. (d)
3rd International Proceedings of the 3rd ITTC Volume II 631 Experiment Calculation Experiment Calculation 1 1 3 - -1 1 1 3 - -1 1 6 - -1 1 6 - -1 9 6 3-3 1 3-6 -9 9 6 3-3 1 3-6 -9 9 6 3-3 6-6 -9 9 6 3-3 6-6 -9 3 1-1 1 3-3 - 3 1-1 1 3-3 - 9 6 3-3 6-6 -9 9 6 3-3 6-6 -9 1 1-1 3-1 -1 1 1-1 3-1 -1 1 1-6 -1-1 1 1-6 -1-1 (a) (b) Experiment Calculation Experiment Calculation 1 1 3 - -1 1 1 3 - -1 1 1 1 - -1 1 1 1 - -1 9 6 3-3 1 3-6 -9 9 6 3-3 1 3-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 6-1 3 - -6 6-1 3 - -6 6-1 1 - -6 6-1 1 - -6 1 1-1 3-1 -1 1 1-1 3-1 -1 1 1-1 1-1 -1 1 1-1 1-1 -1 Organisation-B Organisation-C Organisation-D Organisation-F (c) Figure. Experimental results and numerical results for Ship A-1 from four organisations. (d)
63 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Experiment Calculation Experiment Calculation 1 1 1-1 - 1 1 1-1 - 1 1 1-1 - 1 1 1-1 - 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 6-1 1 - -6 6-1 1 - -6 6-1 1 - -6 6-1 1 - -6 3 1-1 1 1-3 - 3 1-1 1 1-3 - 3 1-1 1 1-3 - 3 1-1 1 1-3 - (a) (b) Experiment Calculation Experiment Calculation 1 1 1-1 - 1 1 1-1 - 1 1 1-1 - 1 1 1-1 - 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 9 6 3-3 1 1-6 -9 6-1 1 - -6 6-1 1 - -6 3 1-1 1 1-3 - 3 1-1 1 1-3 - 3 1-1 1 1-3 - 3 1-1 1 1-3 - Organisation-A Organisation-B Organisation-C Organisation-D (c) Figure.6 Experimental results and numerical results for Ship A- from four organisations. (d)
3rd International Proceedings of the 3rd ITTC Volume II 633 3. BENCHMARK TESTING FOR DAMAGE SHIP STABILITY number of incoming regular beam waves and the damage ship capsizing boundaries in irregular beam seas for varying KG values. 3.1. Introduction This chapter highlights the results of the benchmark testing on damage stability for Ship B- of the nd ITTC (ITTC, 1999). The benchmark test programme commenced in March, inviting ITTC member organisations and other qualified research institutions to express their interest in participating at the launched study. Based on this call, five organizations submitted numerical results. The selected ship for this investigation is a Ro-Ro Passenger vessel, which has been model tested in the Denny Tank (the test tank of the Ship Stability Research Centre) according to the Model Test Method of IMO SOLAS 9 Resolution 1 (Vassalos and Jasionowski, ). The condition addressed in the study concerns the midship damage as described in SOLAS (1997). Wave conditions were set in the benchmark guidelines (Umeda & Papanikolaou, ) as well as the range of tests in regular and irregular seas, as shown in Table 3.1. In particular it was requested to investigate the intact ship performance for a Lack of uniformity in the interpretation of the initial guidelines and specifications (Papanikolaou & Spanos, 1) led to specifications being revised (Vassalos, Umeda and Papanikolaou, 1) and resubmission of results (Papanikolaou, 1; Jasionowski, 1). Details of the experimental results and ship characteristics are reported by Jasionowski & Vassalos (1). Despite the fact that finally only a small number of organisations were able to participate in the benchmark study and that only one ship could be benchmark tested during, it is felt that the final outcome enabled the drawing of some important conclusions. Even if some numerical results cannot be considered satisfactory, compared to model experiment results, the study clearly identified advances and gaps in the present state of knowledge on the prediction of extreme ship motions and capsizing of damaged ships leading to recommendations on further studies concerning specifically identified problem areas. Table 3.1 Overview of benchmark test results. Participants results available; results not available P1 P P3 P P GZ curves (intact/damaged) Simulated free roll decay curves (intact) Simulated free roll decay curves (damaged) Simulated frequency roll response curves (intact/damaged RAOs), constant wave height Simulated roll response curves (intact), constant wave slope (1:) Simulated survivability boundaries (% capsize, 1% capsize for KG=1.89 m) Wave, roll, heave, water on deck time series for KG=1.89 m and Hs=. m,. m and. m ( runs)
63 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International List of Participants. The following five organisations participated in the ITTC ship B- damage stability benchmark study: Flensburger Schiffbau Gesellschaft, FSG (Ms. Heike Cramer) Maritime Research Institute Netherlands (Dr. Jan O. de Kat) National Technical University of Athens (Prof. Apostolos Papanikolaou) Osaka University (Prof. Naoya Umeda) Universities of Glasgow and Strathclyde, SSRC (Prof. Dracos Vassalos). 3.. Software employed All the software employed is non-linear time domain codes. Six degrees of freedom (DOF) models are used by all participants, except for Participant 3 where a three DOF model is used (sway, heave and roll). Table 3. Potential theory approaches to ship motions employed by participants. Participant P1 P P3 P P Table 3.3 Participant P1 P P3 P P Approach Strip theory, 6 DOF 3D source panel theory, 6 DOF Newly modified strip theory, 3 DOF 3D source panel theory, 6 DOF Strip theory, 6 DOF Modelling of damping forces. Approach Non-linear roll damping according to P. Blume Equivalent linear roll damping estimated from the available intact ship roll decay measurements Non-linear roll damping according to Ikeda Adaptive-linear roll damping according to Ikeda Non-linear (equivalent linear & quadratic) roll damping based on a modified Ikeda s approach Radiation and wave diffraction forces are calculated by a number of approaches, all in the framework of potential flow theory, as indicated in Table 3.. Additional information for the calculation of damping forces is shown in Table 3.3. The floodwater is generally considered as independent variable masses moving inside the flooded compartments and interacting with the ship. Modelling of floodwater motion can be generally described by the modelling of the floodwater free surface condition, as shown in Table 3.. Floodwater free-surface model- Table 3. ling. Participant P1 P P3 P P Approach Plane and free movable (when period away from natural); Glimm s equations (when period close to natural) Plane and free movable Plane and horizontal Plane and free movable Plane and horizontal Water ingress/egress through the damage opening is commonly based on hydraulic models following application of Bernoulli s dynamic pressure head equation. All participants used empirically determined coefficients to take into account the actual water ingress/egress flow through the specified damage opening. 3.3. The Test Ship The general particulars of the test ship in full and model scale are given in Table 3.. The model scale is 1:.
3rd International Proceedings of the 3rd ITTC Volume II 63 Table 3. Main ship particulars Full Scale Model Scale L OA 179. m,7. mm L BP 17. m, mm B 7.8 m 69. mm T 6. m 16.3 mm D CARDECK 9. m. mm Displacement (even keel) Intact KG (above BL) Intact Design GM 17,3 tonnes 7.3 kg 1.89 m 3. mm.63 m 6.8 mm The ship was studied in intact and damage condition. Some details of the model characteristics pertaining to these conditions are given next. Metacentric height (intact): GM = 6.76 mm. This is determined by inclining experiment. Roll radius of gyration, i xx /B=.3 (i xx = 163 mm). The roll radius of gyration i xx was estimated by analysis of the free roll decay measurements for the intact condition and given to the participants. This radius refers to the inertia of ship structural mass (derived from the relevant decay measurements by accounting for the hydrodynamic added inertia) and is a characteristic constant of the model, for the specific loading condition. Intact natural roll period: T ni =.6 seconds. This period was determined by analysis of records of free rolling tests in intact condition. Damaged natural roll period: T nd =.3 seconds. This period was determined by analysis of records of free rolling tests in damaged condition. Pitch radius of gyration: i yy /L PP =.17 (i yy = 87 mm). Radius i yy was estimated by analysis of free pitching experiments in air (Vassalos and Jasionowski, ). Yaw Radius of Gyration: i zz /L PP =.38 (i yy = 96 mm). Radius i zz was assumed to be 1% greater than pitch radius of gyration. This increase can be justified by the fact that for models in a damaged condition the mass of superstructures is normally absent and hence the lateral mass distribution of the model is expected to be greater than the vertical one. Figure 3.1 depicts the damaged case considered in the benchmark study with the GZ curves in intact and damaged cases given in Figures 3. and 3.3, respectively. 3.. GZ curves The accuracy of hydrostatic calculations of the numerical codes used in the benchmark depends on the discretisation of the ship s geometry. This explains the differences observed in Figures 3. and 3.3, which should be borne in mind when analysing the predicted ship responses by the participating organisations, as any inaccuracy in the geometry and ship hydrostatics will affect the estimated stiffness (restoring ability) of the ship and hence her natural frequencies. In the intact ship case the observed differences are minor. However, in the damaged case, the GZ curve by Participant 3 shows higher initial stiffness for the flooded ship, whereas the range of stability computed by Participants and is noticeably lower. Only Participants 1 and properly capture the hydrostatic properties of the benchmark ship over the entire stability range.
636 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Figure 3.1 Midship Damage Case (Vassalos & Jasionowski, ). Righting lever [m] 1.1 1.9.8.7.6. Participant 1 Participant. Participant 3.3 Participant. Participant.1 -.1 1 3 6 -. -.3 Heel angle [deg] Righting lever [m]..3..1 1 1 Participant 1 -.1 Participant Participant 3 -. Participant Participant -.3 Heel angle [deg] Figure 3. Computed GZ curves for the intact ship. Figure 3.3 Computed GZ curves for the damaged ship.
3rd International Proceedings of the 3rd ITTC Volume II 637 3.. Free Rolling Simulations The results of free rolling simulations are shown in Figure 3.. Simulation of free roll decay in the intact condition presents no difficulty, as generally good agreement with the experiments was achieved by all participants. However, similar attempts to simulate the free roll response in the damaged condition were less successful. Results presented by all participants show a distinctive overestimation of the natural roll frequency. In this respect, three possible sources for this discrepancy may be cited: (a) Lack of understanding of the complete hydrodynamics of the damaged ship (b) Inaccurate representation of the floodwater dynamics and its coupling with ship motion (c) Possible inconsistencies in the available experimental data (clarification of experimental conditions and way of analysis of data). As a first step towards improving this situation, it would seem necessary to undertake a new benchmark study in the future for at least another ship case and to perform additional experimental verifications of free roll tests in damaged conditions. PRR1, KG=1.89m, Free roll decay, Intact condition 8 6 EXPERIMENT Participant 1 Roll [deg] 6 8 1 1 - Not available - -6-8 Time [s] PRR1, KG=1.89m, Free roll decay, Intact condition PRR1, KG=1.89m, Free roll decay, Damage condition 8 6 EXPERIMENT Participant 6 EXPERIMENT Participant Roll [deg] 6 8 1 1 - Roll [deg] 6 8 1 1 - - - -6-6 -8-8 Time [s] -1 Time [s]
638 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International PRR1, KG=1.89m, Free roll decay, Intact condition PRR1, KG=1.89m, Free roll decay, Damage condition 8 6 6 EXPERIMENT Participant 3 EXPERIMENT Participant 3 Roll [deg] 6 8 1 1 - - -6 Roll [deg] 6 8 1 1 - - -6-8 Time [s] -8 Time [s] PRR1, KG=1.89m, Free roll decay, Intact condition PRR1, KG=1.89m, Free roll decay, Damage condition 8 6 Roll [deg] EXPERIMENT 6 Participant 6 8 1 1 - - Roll [deg] EXPERIMENT Participant 6 8 1 1 - - -6-6 -8 Time [s] -8 Time [s] PRR1, KG=1.89m, Free roll decay, Intact condition PRR1, KG=1.89m, Free roll decay, Damage condition 8 6 6 EXPERIMENT Participant EXPERIMENT Participant Roll [deg] 6 8 1 1 - - -6 Roll [deg] 6 8 1 1 - - -6-8 -8 Time [s] -1 Time [s] Figure 3. Free roll decay for the intact and damaged ship (measured and simulated).
3rd International Proceedings of the 3rd ITTC Volume II 639 3.6. Ship Performance in Regular Waves The results of the benchmark study for the intact and damaged ship roll response in regular beam waves, with constant wave height (H w = 1. m,. m and. m) and constant wave slope, namely constant wave height to wavelength ratio equal to 1/, are presented in this section. The purpose of this particular study was to gain insight into the modelling of roll motion by the participants in normal operational and extreme conditions (significant wave height up to m). Roll Response Amplitude Operators (RAOs) for Constant Wave Height Excitation. Based on the results presented in Figure 3., the following can be concluded : Simulation of intact ship roll RAO for constant wave height monochromatic wave excitation: All participants have generally accomplished the simulation of the basic intact ship roll RAO successfully. Some differences in the predicted peak values of ship response, occurring at the natural roll frequency (more pronounced in the simulation by Participant 1), are due to the differences in the modelling of roll damping, which is based on semiempirical coefficients and approaches that need further improvement. Simulation of damaged ship roll RAO for constant wave height monochromatic wave excitation: The simulation of the basic damaged ship roll RAO could not be predicted satisfactorily by the participants. The reasons given earlier concerning free rolling simulation apply equally here and become even more apparent in the simulation of the damaged ship roll RAO. In particular, none of the participants obtained the natural frequency of the damaged ship close to the value derived experimentally. In fact, the predicted natural frequency of the damaged ship is quite inconsistent among the participants: Participants, and predict a slight decrease of this frequency in relation to the natural frequency of the intact ship, a trend shown also in the damaged model experiments, but not to the extent measured in these experiments. Participant 1 is even predicting an increase of the intact natural frequency, whereas the predicted natural frequency by Participant 3 remains practically unchanged. The above results suggest that the predicted hydrodynamic added moment of inertia by all participants deviates significantly from the experimental value. Regarding the predicted peak values of the damaged ship roll response, practically all participants predict higher roll amplitudes indicating a clear underestimate of roll damping. The experimentally measured damaged ship roll response indicates the existence of a second resonance frequency at approximately twice the main roll resonance frequency. This phenomenon is predicted by Participants and but at much higher frequencies and for higher secondary resonance peak values. The above conclusions call for additional research in this area and a reassessment of the damaged ship roll RAO results in the future, when more experimental and numerical benchmark results become available. Note that experiments were performed in wave heights equal to 1. m and. m, whereas numerical simulations were undertaken in the range between 1. and. m.
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International PRR1, KG=1.89m, Roll RAO, Intact condition PRR1, KG=1.89m, Roll RAO, Damage condition 18 18 16 Experiment, (Hw=1.m) 16 Roll [deg/m] 1 1 1 8 6 Experiment, (Hw=.m) Participant 1 (Hw=.m) Roll [deg/m] g 1 1 1 8 6 Experiment, (Hw=1.m) Experiment, (Hw=.m) Participnt 1, (Hw=.m).3...6.7.8.9 1 1.1 1. ω [rad/s].1..3...6.7.8.9 1 1.1 1. ω [rad/s] PRR1, KG=1.89m, Roll RAO, Intact condition PRR1, KG=1.89m, Roll RAO, Damage condition 18 18 16 Experiment, (Hw=1.m) 16 Experiment, (Hw=1.m) 1 Experiment, (Hw=.m) 1 Experiment, (Hw=.m) Roll [deg/m] 1 1 8 6 Participant, (Hw=.m) Roll [deg/m] g 1 1 8 6 Participant, (Hw=.m).3...6.7.8.9 1 1.1 1. ω [rad/s].1..3...6.7.8.9 1 1.1 1. ω [rad/s] 18 PRR1, KG=1.89m, Roll RAO, Intact condition 18 PRR1, KG=1.89m, Roll RAO, Damage condition 16 Experiment, (Hw=1.m) 1 Experiment, (Hw=1.m) 1 Experiment, (Hw=.m) Experiment, (Hw=.m) 1 Participant 3, (Hw=1.m) 1 Participant 3, (Hw=1.m) Roll [deg/m] 1 8 6 Roll [deg/m] g 9 6 3.3...6.7.8.9 1 1.1 1. ω [rad/s].1..3...6.7.8.9 1 1.1 1. ω [rad/s]
3rd International Proceedings of the 3rd ITTC Volume II 61 PRR1, KG=1.89m, Roll RAO, Intact condition PRR1, KG=1.89m, Roll RAO, Damage condition 18 18 16 Experiment, (Hw=1.m) 16 Experiment, (Hw=1.m) 1 Experiment, (Hw=.m) 1 Experiment, (Hw=.m) Roll [deg/m] 1 1 8 6 Participant, (Hw=.m) Roll [deg/m] g 1 1 8 6 Participant, (Hw=.m).3...6.7.8.9 1 1.1 1. ω [rad/s].1..3...6.7.8.9 1 1.1 1. ω [rad/s] PRR1, KG=1.89m, Roll RAO, Intact condition PRR1, KG=1.89m, Roll RAO, Damage condition 18 18 16 Experiment, (Hw=1.m) 16 Experiment, (Hw=1.m) 1 Experiment, (Hw=.m) 1 Experiment, (Hw=.m) Roll [deg/m] 1 1 8 6 Participant, (Hw=1.m) Roll [deg/m] g 1 1 8 6 Participant, (Hw=1.m).3...6.7.8.9 1 1.1 1..1..3...6.7.8.9 1 1.1 1. ω [rad/s] ω [rad/s] Figure 3. Free roll decay for the intact and damaged ship (measured and simulated). Roll Response Amplitude Operators (RAOs) for Constant Wave Slope Excitation. This study was restricted to only the intact ship case and no experimental data were available to cross check the numerical predictions. The results are presented in Figures 3.6 and 3.7. PRR1, KG=1.89m, Roll RAO, Constant wave slope 1:, Intact condition PRR1, KG=1.89m, Roll RAO, Constant wave slope 1:, Intact condition. Participant 1 Participant Participant. Participant 1 Participant Participant 3. Participant 3. Participant Roll Ampl / ka [-] 3. 1. Roll Ampl / ka [-] 3. 1. 1 1... 1 1.. 3 λ/l [-]. 1 1.. 3 λ/l [-] Figure 3.6 Roll response in regular beam waves with constant wave slope over λ/l. Figure 3.7 Roll response in regular beam waves with constant wave slope over ω.
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Based on the derived results, the following can be stated: Overall, the results produced by the participating organisations differ significantly. Results for large wavelength to ship length ratios (λ/l) appear satisfactory but this is not the case for small λ/l ratios. Only results by Participants and appear to agree closely throughout the frequency range considered in the study. Predictions in the resonance region deviate substantially indicating clearly the differences of the semi-empirical damping models used by the various participants for the extreme motion amplitude predictions. In conclusion, it appears necessary that a more comprehensive study should be carried out in the future to investigate the relationship between the damping models used by the benchmark study participants. Unfortunately, experimental measurements were not available to enable a more thorough evaluation of the employed numerical procedures for the intact, large amplitude and large slope motion studies. The only apparent result from this comparison is that numerical modelling of highly nonlinear ship motion problems is not yet satisfactory. 3.7. Ship Performance in Irregular Waves The prediction of the damaged ship performance is assessed on the basis of analysis of the numerically simulated time series for the exciting wave, the ship motion response and the amount of floodwater, in comparison with the corresponding time series of model experiments, simulation of the damaged ship survival boundaries and finally identification of critical wave heights. Detailed results of this study (experimental and numerically simulated time series) are given by Jasionowski & Vassalos (1). In this section only sample results are presented (Figures 3.8 to 3.19) and discussed. Two representative runs per participant, one for survival and one for capsizal cases, are presented, all corresponding to a significant wave height excitation of. m. Wave elevation, heave and roll motions as well as the amount of water accumulated on the car deck are shown. Figure 3.8 Experimental measurements damaged ship model (survival case).
3rd International Proceedings of the 3rd ITTC Volume II 63 Figure 3.9 Experimental measurements damaged ship model (capsizal case). Figure 3.1 Numerical simulations by Participant 1 damaged ship model (survival case). Figure 3.11 Numerical simulations by Participant 1 damaged ship model (capsizal case).
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Figure 3.1 simulations by Participant damaged ship model (survival case). Figure 3.13 Numerical simulations by Participant damaged ship model (capsizal case). Figure 3.1 Numerical simulations by Participant 3 damaged ship model (survival case).
3rd International Proceedings of the 3rd ITTC Volume II 6 Figure 3.1 Numerical simulations by Participant 3 damaged ship model (capsizal case). Figure 3.16 Numerical simulations by Participant damaged ship model (survival case). Figure 3.17 Numerical simulations by Participant damaged ship model (capsizal case).
66 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Figure 3.18 Numerical simulations by Participant damaged ship model (survival case). Figure 3.19 Numerical simulations by Participant damaged ship model (capsizal case).
3rd International Proceedings of the 3rd ITTC Volume II 67 A visual comparison between the numerically predicted and experimentally measured time series shows a rather unsatisfactory level of agreement. Indeed, none of the numerical time series matches qualitatively the experimental values and only some of the numerical results agree qualitatively (Participant 1 and to some extent Participants and ). The roll response predicted by Participants and 3 displays again noticeably higher amplitudes, possibly due to inaccurate roll damping models. A Fourier spectral analysis of the calculated time series records enables a better understanding of the differences between the numerical simulations and the response characteristics derived by physical model tests (Figure 3.) as outlined next: Participant failed to reproduce exactly the exciting wave spectrum characteristics, and hence used a wave spectrum with its peak slightly shifted towards lower frequencies, closer to natural roll frequency of the ship. The predicted roll response spectra by Participants 1,, and indicate underestimation of roll damping, with resonant roll amplitudes significantly higher than the experimental values. Participants and 3 predict considerably higher roll spectral densities, partly as a consequence of the predicted higher peak roll values and partly due to the shift of the peak frequency of the exciting wave spectrum. A noticeable peak appears in the roll motion spectrum by Participant at a frequency of about 1. rad/s unlike the experimental results. The spectrum of heave response derived by Participant 3 shows a peculiar second lower peak around.6 rad/s.
68 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Figure 3. Spectral analysis of experimental and numerical ship responses.
3rd International Proceedings of the 3rd ITTC Volume II 69 3.8. Survivability Boundaries SURVIVE/CAPSIZE BOUNDARIES The reported survivability boundaries show consistently high accuracy by most the participants. Both, the lower and upper boundaries distinguishing between sea states leading to ship survival, marginal survivability or capsize were predicted with a spread of approximately ±. m in H s. Only Participant 3 fails to predict the capsizal boundary (no capsize predictions for the specified conditions). It must be mentioned here, that the simulation time is of importance in establishing the boundary consistently, as the longer the duration of the simulation, the lower the boundary tends to be. This, however, was clearly defined as a benchmark test constraint and some variation between the participants is noticeable. The demonstrated accuracy in predicting the critical sea states seems quite satisfactory from the point of view of application of such information to practical survivability assessment procedures. An overview of relevant results is shown In Table 3.6 and depicted in Figure 3.1. In compiling this table the ship survival boundary, for the particular damage case and sea state tested, is identified on the basis of zero capsize events occurring for five consecutive numerical simulations using different irregular wave realisations in each case. On the other hand a capsizal boundary is identified on the basis of all five runs leading to a capsize event. Table 3.6 Experimentally observed and numerically simulated capsize events. Experiment P1 P P3 P Hs=3.m - - - - - Hs=3.7m - - - 1 Hs=.m 1 3 Hs=.m 3 3 Hs=.m 1 3 Hs=.7m - - 1 3 - Hs=.m - - - - P Hs [m] 6 3 1 Figure 3.1 Comparison between predicted and measured survival/capsizal boundaries. 3.9. Concluding remarks Considering the relatively low number of the benchmark study participants and the complicate nature of the benchmark study problem being addressed it is felt that the main objectives of the present study have been met, though a similar study should be repeated in the future with the aim to alleviate the effects of some of the identified weaknesses. It has been ascertained that at the present state of knowledge, model experiments remain indispensable for assessing the survivability of damaged ships in waves, though theoreticalnumerical prediction methods can greatly contribute to the assessment of the survivability of damaged ships in waves.. GUIDELINES FOR MODEL TESTING OF INTACT AND DAMAGE STABILTY The purpose of these guidelines is to provide ITTC member organisations, intending to undertake intact and damage stability model tests in waves, with a sound basis for carrying out these tests. The derived guidelines are based on those presented to the nd ITTC, upgraded to reflect member organisation experience. The full recommended guidelines for model test on intact stability are included in the ITTC Quality Manual as Procedure 7.- -7-. and on damage stability as Procedure 7.--7-.6. Exp P1 P P3 P P
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International. QUESTIONNAIRE.1. Questionnaire on Model Experiments The questionnaire was formatted with the intention of validating the ITTC guidelines for Intact and Damaged model experiments, presented at the 1999 ITTC. The following organisations replied to the questionnaire: Institute for Marine Dynamics, National Research Council of Canada MARIN Naval Ship Research and Development Centre Osaka University, Department of Naval Architecture & Ocean Engineering QinetiQ The Universities of Glasgow and Strathclyde, SSRC A summary of the questionnaire is given in Table.1. The full questionnaire, including details of how to fill it out and submit it, can be found on the web site of the committee: www.ssrc.na-me.ac.uk/ittc/scexcap/ Table.1 Summary of questionnaire on model test procedures. 1. Past experience. Model design, construction and outfit Construction material Uppermost limit of accurate modelling Factors considered in scaling flooded compartments, deck, etc. Scaling flow through pipes and openings Modelling of flooded spaces Model scale Instrumentation and equipment Model Power supply systems Data recording Ballasting Control of models 3. Experiments Initial conditions Start of data acquisition Distance from wave maker Wave types used Model speed measurement. Data analysis Four of the organisations described experiments on intact and damaged ship models that fitted within the operating practice described by the 1999 ITTC Guidelines. Two of these organisations had experience with extreme motions, but not capsize. For experiments in oblique waves with forward speed, three organisations used battery based power systems onboard the model for intact extreme motions or capsize experiments and data was handled in one of the following ways: Collected and stored on the model Transferred to storage unit by telemetry Transferred to storage unit by cable. For capsize/extreme motions, all three organisations used a free running model, powered by a model propeller, with autopilot to keep the model on course, once the experiment had started. For the damaged case, all four organisations supplied power to the model and transferred the data to storage via an umbilical cable. The models were allowed to drift under the action of waves. Scaling of openings through which water would flow was based on geometry. No allowance was made for scale effects. Some notable exceptions to the guidelines were: Damaged stability model experiments at one organisation were carried out with length and scale requirements outside the guidelines. For one organisation, damaged stability experiments were on simplified hull forms and permeability of under deck spaces was 1%. Two organisations conducted damage stability tests using a much more sophisticated procedure than that recommended for the typical Ro-Ro ferry case, and this was to study damage stability of warships. For these tests, the experiment starts with an intact, self- propelled model. The preparation and procedures for this part of the experiment fit within those described in the ITTC guidelines
3rd International Proceedings of the 3rd ITTC Volume II 61 for intact model experiments. At some point in the experiment, the damage is simulated and the model floods. At this point the requirement is for the model to behave as a damaged ship and flooding is monitored with water depth sensors and video cameras. The internal structure of the model is much more complex than a Ro-Ro model, due to the more complex internal structure of the ship. Also, the requirement to have the model self-propelled at the start of the experiments brings in extra challenges for model construction. Model scale for these tests was 1: giving model lengths between and 6 metres. The preferred approach is to use a completely free running model, with telemetry to transfer data from the model to a shore station. For intact stability tests most organisations use models ranging in length from 3 to m, although there are cases of using models under m. In conclusion, the analysis of the small sample of questionnaires suggests that there are no major flaws in the ITTC guidelines for model tests as proposed in 1999. The committee was lucky in that much of the work done by researchers in this area is well documented in the literature and the performers have accepted the academic tradition of exchanging information freely on techniques and processes. Based on the results of the questionnaire, each organization has slightly different procedures, but all appear to be well founded on good experiment practice. The challenge for the future is to ensure that the guidelines apply to a wider range of ships than Ro-Ro ferries, and should also consider ships where portions of the ship are flooded in normal operations (cruise ships, landing platform docks, ships with moon pools, etc)... CFD Survey The following organisations replied to the questionnaire: MARIN Memorial University of Newfoundland, Department of Naval Architecture and Ocean Engineering National Technical University of Athens, Department of Naval Architecture and Marine Engineering W. S. Atkins Consultants Ltd. Universities of Glasgow and Strathclyde, Department of Naval Architecture and Marine Engineering. The survey is summarised in Table.. Table. CFD. Summary of survey on the use of 1. What was the original motivation for you incorporating CFD into ship motion prediction software?. Give a brief description of the ship motion prediction code used 3. Briefly describe the CFD code used. Give a brief description of how you integrated the CFD code with the ship motions code. Flow types modelled by CFD component 6. Linking Issues between ship motions code and CFD code 7. Post Processing 8. State of Development of Codes 9. Validation 1. Special Limitations 11. Future Developments 1. References The survey focused on combining codes to predict ship motions in waves with CFD, to model the combined motion of ship and fluid trapped on the deck or within a ship s hull. It should also be recognised that CFD has been used for other areas relevant to extreme motions including capsizing, such as modelling steep breaking waves, and also for seakeeping. Using CFD presents a means of overcoming the classical assumptions of linear theories, combining forward speed with motion in
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International waves and non-viscous flow. Beck & Reed (1) give a good overview of the development of numerical seakeeping predictions from their earliest beginnings to the latest unsteady RANS approaches. They also point out the huge computational requirements for using CFD based seakeeping predictions, and the level of uncertainty in the results. As such, CFD methods for full ship motion predictions are a long way from practical application in engineering situations. CFD can also be applied to the reduced problem of predicting viscous roll damping (Salui et al., ). This has always been a severe limitation for potential flow methods, which cannot realistically predict roll motion. Most work with CFD in this area has focused on forced rolling in calm water, to obtain predictions of the added mass and damping coefficients directly from the numerical procedure, hence eliminating the empirical assumptions. Three of the replies are related to the classical assumptions of CFD methods, and one was a simplified approach. For the three classical CFD approaches, the following observations were made. Three methods use a 6DOF time domain code for predicting ship motions, linked to a CFD code to predict the motion of water on the deck of the ship. Two ship motion codes have some linear and non-linear elements in the hydrodynamic coefficients and restoring moments. The third method uses a linear response time domain method as the basis for its motion code. All three CFD codes used variations of the Volume of Fluid technique, where the grid is fixed (body fitted) and the free surface is tracked either by grid elements that are partially full or by special free surface cells. In one case studying a flooded Ro-Ro, the method included fluid entering and leaving the deck area, based on the relative motion given by the seakeeping code and matching the internal fluid height with the external height at the damage opening. In another case studying a fishing vessel with antiroll tanks, the volume of water was fixed. Two methods kept the CFD and ship motions codes separate, but the methodology for linking them together varied. In one case, the exchange between the codes was by a data file. Another file controls which program runs, but each code runs completely separately on different computers. In the other case, the motion code was considered as an external subroutine by the CFD code. This was possible because the code was designed for analysing sloshing in spacecraft, and so it was possible to include externally computed body motions as input to the problem. The advantage of this approach is that the CFD code needs no modification. For one organisation, the CFD code was integrated into the ship motion code as a subroutine. Using linear motions to compute the ship motions is a simpler (and faster) process. However, it is limited to operational rather than extreme motions. Synchronising time steps between the two commercial CFD codes and the ship motion codes was an issue in both cases. In the Ro-Ro case, using files to transfer the data makes the coupling explicit, but the limitation of very small time steps within the CFD code makes this solution acceptable for the application considered, but this may not be so for other cases. In the fishing boat case, even though the motions code is treated as a subroutine by the CFD code, the two can function independently and the time step in each one can be varied internally to obtain solutions. The CFD code uses a non-inertial reference frame. The required gravity components are interpolated at whatever time step the CFD code needs to obtain a solution. The limitation of the non-inertial reference frame is reached when the fluid reaches the top of the boundary. This limitation is less critical for an enclosed tank than for water trapped on the deck, when at this point the water would spill out, which is not modelled by the CFD code. For one or-
3rd International Proceedings of the 3rd ITTC Volume II 63 ganisation, the CFD code adjusts the time steps internally to reach a solution, but since the two codes are combined the internal clock is consistent. All methods have been partially validated against experimental data (Bass & Cumming, ; van Daalen et al., ; Woodburn et al., 1). The two replies, also studying Ro-Ro ferries, took a different approach, which was outside the classical definition of CFD. In one case a 6DOF non-linear time domain motions code has been expanded to 9DOF by considering the fluid as a lump mass. Hydrodynamic coefficients related to ship-wave interaction are calculated externally with a frequency domain panel method. Calculated quantities are then transferred to the time domain by the Impulse Response Function technique. Flooding and draining of compartments can be modelled. The method has been validated against experiment data (Papanikolaou et al., ). In the other case, the underlying equations of the ship are derived from conservation of linear and angular momentum applied to rigid bodies, extended to include the internal fluid mass in six degrees of freedom. The Froude- Krylov and restoring forces and moments are integrated up to the instantaneous wave elevation, the radiation and diffraction forces and moments are derived from linear potential flow theory and expressed in time domain based on convolution and spectral techniques, respectively. The hull asymmetry due to ship flooding is taken into account by a database approach, whereby the hydrodynamic coefficients are predicted beforehand, and then interpolated during the simulation. The correction for viscous effects on roll and yaw modes of motion is applied based on empirical methods. The second order drift and current effects are also catered for, based on parametric formulations. Fluid sloshing has been modelled by a free mass point moving due to the acceleration field and restrained geometrically by predetermined potential surfaces of centre of buoyancy for given amount of floodwater. This model is derived from simple rigid body motion consideration. Finally, an artificial coefficient is introduced to represent damping of floodwater motion. An ad hoc value of.1 is adopted for this coefficient derived for simple box-shaped compartment from comparisons with experimental data. With the geometric information about the tank stored in a database, the model is complete. The method has been validated against experiment data (Jasionowski & Vassalos, 1). The major advantage of the lumped mass approach is that it is computationally more efficient than the traditional CFD methods, and provided that there is not a significant amount of sloshing of the fluid, the prediction of ship motions, including the dynamics of floodwater is of acceptable accuracy. Flooding and draining issues are handled with empirical methods. Using CFD for predicting water motion on the deck and its effect on ship motions was the focus of the survey. Other approaches that have been used in the past are to use either lumped mass (as one survey respondent did) or to use potential flow but for non-linear waves (Huang & Hsiung, 1996). In this case, Euler s equations were used for non-linear shallow water flow on the deck, with a flux split based on the characteristic directions of motion. The method was employed for two and three-dimensional decks. Whilst this is not CFD in the classical sense, it is a valid approach for tackling the problem, provided that the volume of water on the deck is constant or changing very slowly with time. An area that is similar to the extreme motions and capsize problem is sloshing of fluid within tanks on a ship. Related to this, Cariou & Cassela (1999) give a comparison of numerical results for 11 different CFD codes. This paper summarizes the time and space domains, viscosity, compressibility free surface and wall condition for each of the codes, and compares results for specified cases. The first was a simple two-dimensional problem, and compared the surface elevation of the fluid in
6 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International an oscillating rectangular tank for a range of amplitudes and periods. The second case was a 3D problem, consisting of an LNG cargo tank, excited for a combination of pitch and roll, again for a range of amplitudes and periods. An alternative to the grid based CFD approaches is Smoothed Particle Hydrodynamics (SPH). In this method the fluid is idealised as elemental particles, and the tracks of these particles are followed during the computation. The method was originally developed for astrophysics problems, but has since been applied to free surface problems. The method can give a good prediction of breaking waves, but according to Beck & Reed (1), detailed comparisons of pressures and fluid velocities for simplified problems are not available. The method has been used to obtain some interesting results for the sloshing problem and post-breaking behaviour of waves. Naito & Sueyoshi (1) presented results for an SPH prediction of water moving on the deck of a flooded Ro-Ro ferry. 6. SYMBOLS AND TERMINOLOGY A number of international organisations have been involved in ship stability for many years and have developed their own symbols and terminology for the field. As these organisations have been using their symbols and terminology for a number of years, it is felt that it is now not practical to develop a single set of symbols and terminology to be used by all working in this field. However, the need has been identified, following a comprehensive review by Francescutto (), for taking immediate action on some items that are directly linked to stability and recommendations to this effect are put forward. Moreover, a table comparing the symbols used by ITTC; ISO 117; HSC ; and IMO has also been prepared and submitted to the Symbols & Terminology Group (Francescutto, ). This report is also posted in the web site of the committee. 7. CONCLUSIONS AND RECOMMENDATIONS 7.1. General Technical Conclusions As the maritime industry progressively moves towards performance based criteria to address safety issues, there is wide scope and a major opportunity for member organisations to benefit from these developments. However, the severe limitations identified in the existing numerical models for predicting ship capsizing and extreme motions needs to be addressed as summarised below: 7.1.1 Prediction of Intact Ship Capsizing and Extreme Motions At this stage there is a limited number of numerical models for predicting intact ship capsizing and extreme motions with a range of different levels of sophistication and parameters. Only a few of these models consistently agree qualitatively with all the extreme motions and modes of capsize identified in free running model experiments. None of the models does so quantitatively. More work is required to improve the agreement between physical and numerical model tests results. 7.1. Prediction of Damaged Ship Capsizing At this stage there is a limited number of numerical models for predicting damaged ship capsizing. Unlike the case for intact ships, most models can consistently predict the capsize boundaries obtained from model experiments in realistic sea conditions, albeit for the specific damage scenario and mode of capsize considered in the benchmark tests. There are, however, fundamental differences in the way these models handle floodwater/ship dynamics, none of the models giving good agreement, either qualitatively or quantitatively, with results from physical model experiments.
3rd International Proceedings of the 3rd ITTC Volume II 6 Before confidence can be gained in these models, wider application to different damage scenarios and ship types is required. 7.1.3 Guidelines and Procedures Those member organisations involved with experimental testing of intact and damage stability generally follow the guidelines recommended by the nd ITTC as appropriate, justifying the adoption of procedures based on these guidelines as refined by the work undertaken by this committee. 7.1. Symbols and Terminology The rationalisation of symbols and terminology for stability has not been made possible, as international organisations directly dealing with the subject are too ingrained in the use of symbols different to those adopted by the ITTC. Notwithstanding this, it is recommended that steps are taken to communicate widely the use and encourage the adoption of the Greek symbol φ to denote heel/list/roll angle. 7.. Recommendations to the Adopt the Procedure Seakeeping Model Tests on Intact Stability 7.--7-.. Adopt the Procedure Seakeeping Model Tests on Damage Stability 7.--7-.6. 8. REFERENCES AND NOMENCLATURE 8.1. References Bass, D., and Cumming, D.,, Numerical and Experimental Investigation of Water Trapped on Deck on a Small Fishing Boat, STAB. Bass, D., and Cumming, D.,, Numerical and Experimental Investigation of Water Trapped on Deck on a Small Fishing Boat, STAB. Beck, R.F., and Reed, A.M., 1, Modern Computational Methods for Ships in a Seaway, SNAME Annual Meeting, 1. Cariou, A., and Cassela, G., 1999, Liquid Sloshing in Ship Tanks: A Comparative Study of Numerical Simulations, Marine Structures Vol. 1, No. 3, pp. 183-198. Cramer, H., 1, Effect of Non-Linearity in Yaw Motion on Capsizing Prediction Proceedings of the th International Workshop on Stability and Operational Safety of Ships, Trieste, Italy. van Daalen, E.F.G., Kleefsman, K.M.T., Gerrits, J., Luth, H.R. and Veldman, A.E.P.,, Anti-roll Tank Simulations with a Volume of fluid Based Navier-Stokes Solver, 3rd Symposium on Naval Hydrodynamics, Val de Reuil, France. Francescutto, A.,, A Critical Review of the Symbols and Terminology Relevant to Ship Stability, Department Naval Architecture, Ocean and Environmental Engineering, University of Trieste, Italy. Hamamoto, M., and Enomoto, T., 1996, Model Experiment of Ship Capsize in Astern Seas nd Report, J. Society of Naval Architects of Japan, Vol. 179, pp. 77-87. Hamamoto, M., and Kim, Y. S., 1993, A New Coordinate System and the Equations Describing Manoeuvring Motions of a Ship in Waves J. Society of Naval Architects of Japan, Vol. 173, pp. 9-, (in Japanese). Hamamoto, M., and Saito, K., 199, Time Domain Analysis of Ship Motions in Following Waves Proceeding of the 11th Australian Fluid Mechanics, Hobart, Vol. 1, pp. 3-38.
66 The Specialist Committee on Prediction of Exteme Ship Motions and Capsizing 3rd International Hashimoto, H., and Umeda, N., 1, Importance of Wave Effects on Manoeuvring Coefficients for Capsizing Prediction, Proceedings of the th International Workshop on Stability and Operational Safety of Ships, Trieste, Italy. Ikeda, Y., Umeda, N., and Tanaka, N., 1988, Effect of Forward Speed on Roll Damping of a High-Speed Craft Journal of Kansai Society of Naval Architects, Japan, Vol. 8, pp. 7-3 (in Japanese). ITTC, 1999, Specialist Committee on Ship Stability. Final Report and Recommendations to the nd ITTC Proceeding of the nd ITTC, Seoul, Korea and Shanghai, China, Vol., pp. 399-31. Jasionowski, A., 1, Detailed Analysis of the Revised Damage Benchmark Results. University of Strathclyde The Ship Stability Research Centre, October 1. Jasionowski, A., and Vassalos, D., 1, Detailed Analysis of the Final Revised Damage Benchmark Results, University of Strathclyde The Ship Stability Research Centre, December 1. Matsuda, A., Umeda, N., and Suzuki, S., 1997, Vertical Motions of a Ship Running in Following and Quartering Seas, J. Kansai Society of Naval Architects, 7 : 7-, (in Japanese). Matusiak, J., 1, Importance of Memory Effect for Capsizing Prediction Proceedings of the th International Workshop on Stability and Operational Safety of Ships, Trieste, Italy. Munif, A.,, Numerical Modelling on Extreme Motions and Capsizing of an Intact Ship in Following and Quartering Seas, Doctor Thesis, Osaka University, Japan. Naito, S., and Sueyoshi, M., 1, A Numerical Analysis of Violent Free Surface Flow on an Flooded Car Deck Using Particle Method, STAB1 th International Workshop on Stability and Operational Safety of Ships, Trieste,Italy,. Papanikolaou, A., Zaraphonitis, G., Spanos, D., Boulougouris, E., and Eliopoulou, E.,, Investigation into the Capsizing of Damaged RO-RO Passenger Ships in Waves, STAB. Papanikolaou, A., and Spanos, D., 1, Benchmark Study on the Capsizing of a Damaged Ro-Ro Passenger Ship in Waves Draft Final Report, May 1. Papanikolaou, A., 1, Benchmark Study on the Capsizing of a Damaged Ro-Ro Passenger Ship in Waves, Revised Final Report, October 1. Renilson, M.R., and Manwarring, T.,, An Investigation into Roll/Yaw Coupling and its Effect on Vessel Motions in Following and Quartering Seas STAB, Vol. A, pp. -9. Salui K.B., Sarkar, T., and Vassalos, D.,, An Improved Method for Determining Hydrodynamic coefficients in Roll Motion using CFD Techniques Ship Technology Research, Vol. 7, No.. Sera, W., and Umeda, N., 1, Effect of Short-Crestedness of Waves on Capsize of a Container Ship in Quartering Seas J Japan Institute of Navigation, Vol. 1, pp. 11-16, (in Japanese). SOLAS, 1997 Consolidated edition 1997- Annex : Resolutions of the 199 SOLAS. Model test method IMO, Resolutions of the of Contracting Governments to the International Convention for the Safety of Life at Sea, 197, adopted on 9 November 199. Umeda, N., and Yamakoshi, Y., 1986, Ex-
3rd International Proceedings of the 3rd ITTC Volume II 67 perimental Study on Pure Loss of Stability in Regular and Irregular Following Seas, STAB86. Umeda, N., 1988, Application of Slender Body Theory to Lateral Ship Motions with Both Free Surface and Free Vortex Layers, Bulletin of National Research Institute of Fisheries Engineering, No. 9, 18-3. Umeda, N., Hamamoto, M., and Takaishi, Y., 199, Model Experiments of Ship Capsize in Astern Seas J. Society of Naval Architects of Japan, Vol. 177, pp. 7-17. Umeda, N., Matsuda A., and Takagi M., 1999, Model Experiment on Anti-Broaching Steering System, J. Society of Naval Architects of Japan, Vol. 18, pp. 1-8. Umeda, N.,, Effects of Some Seakeeping/Manoeuvring Aspects on Broaching in Quartering Seas, Contemporary Ideas on Ship Stability, Elsevier Science Publications (Amsterdam), pp. 3-33. Umeda, N., Munif A. and Hashimoto H.,, Numerical Prediction of Extreme Motions and Capsizing for Intact Ships in Following/Quartering Seas, Proceeding of the th Osaka Colloquium on Seakeeping Performance of Ships, Osaka, Japan, 368-373. Umeda, N., and Papanikolaou, A.,, Revised Guidelines for ITTC committee on the prediction of Extreme Ship Motions and Capsizing Benchmark Tests. Umeda, N., Final Report on the Benchmark Tests for Intact Ships, ITTC SCEXCAP Progress Report, December 1. Umeda, N., and Hashimoto, H.,, Qualitative Aspects of Nonlinear Ship Motions in Following and Quartering Seas with High Forward Velocity Journal of Marine Science and Technology, in press. Umeda, N., and Hashimoto, H.,, Qualitative Aspects of Nonlinear Ship Motions in Following and Quartering Seas with High Forward Velocity Journal of Marine Science and Technology, in press. Woodburn, P., Galagher, P., and Letizia, L., 1, Fundamentals of Damaged Ship Survivability, RINA, Spring Meeting,. Vassalos, D., and Jasionowski, A.,, Stockholm Agreement Water on Deck Model Experiments for Passenger/Ro-Ro Vessel, Final Report, PSBG-RE--AY. University of Strathclyde The Ship Stability Research Centre, February. Vassalos, D., Umeda, N., and Papanikolaou, A., 1, nd Revised Guidelines for ITTC committee on the Prediction of Extreme Ship Motions and Capsizing Benchmark Tests, June 1. 8.. Nomenclature COREDES Committee on Research and Development in European Shipbuilding CRN Co-operative Research, Navy JSNAJ Journal of the Society of Naval Architects of Japan RINA Royal Institution of Naval Architects SNAME Society of Naval Architects and Marine Engineers STAB Int. Conf. on Stability of Ships and Ocean Vehicles WEGEMT European Association of Universities in Marine Technology and Related Sciences 9. ACKNOWLEDGEMENT The committee would like to thank wholeheartedly all the organisations and individuals who contributed to this work over the last three years.
3rd International Proceedings of the 3rd ITTC Volume III 7 The Specialist Committee on Prediction of Extreme Ship Motions and Capsizing Committee Chair: Prof. Dracos Vassalos (University of Strathclyde) Session Chair: Ir. George F.M. Remery (MARIN) I. DISCUSSIONS I.1. Discussion on the Report of the 3rd ITTC Specialist Committee on Prediction of Extreme Ship Motions and Capsizing: Deterministic analysis of extreme motions By: Günther F. Clauss, Technical University of Berlin, Germany Congratulations for this excellent report. International benchmark tests are extremely important, though they are very timeconsuming and work-intense for organizers and participants. Especially when regarding the development of rules and regulations towards direct analysis it is indispensable to evaluate the quality of available numerical tools. The current benchmark test reveals some deficiencies of the applied tools, however, it also shows its limitations in interpreting the results due to the selection of the reference ships and operating conditions. As presented in the Group Discussion B. IMO Standards and ITTC our contribution Evaluation of Capsizing Risk by Deterministic Analysis of Extreme Roll Motions (authors: G.F. Clauss, J. Hennig and H. Cramer) deals with recently built ships, and most investigations are carried out with speeds and stability characteristics which reflect real operating conditions. With deterministic seakeeping tests in longcrested regular and irregular seas the wave characterictics at the position of the cruising ship, i.e. wave elevation, pressure distribution as well as acceleration and velocity fields in space and time are known. Thus, these data can be related to the ship behaviour, and the physical phenomena, i.e. the mechanism of large roll motions with subsequent capsizing are investigated as cause-effect chain. Using these sophisticated data our non-linear numerical model has been validated. As an application of our numerical tool we evaluated the motion behaviour of selected ships in arbitrary seas, long-crested and shortcrested. Simulation results are presented in polar plots which give the limiting wave height for capsizing of a vessel depending on its speed and course. As shown in Figure I.1.1 the most critical regions of resonance motions as well as parametric resonance are identified. Only a change of trim by 1 m to stern reduces the capsizing risk considerably (comparing left hand and right hand diagram). Consequently, the assessment of the seakeeping behaviour of a floating structure requires a highly complex procedure combining nonlinear numerical simulation methods validated by deterministic seakeeping tests. As a result, safer ships can be designed and loading conditions optimized, improving ship operation and navigation significantly.
76 The Specialist Committee on Prediction of Extreme Ship Motions and Capsizing 3rd International Figure I.1.1 Polar plot with limiting wave heights for a RO-RO design-based on nonlinear calculation methods. I.. Discussion on the Report of the 3rd ITTC Specialist Committee on Prediction of Extreme Ship Motions and Capsizing: Characteristics of extreme waves in the model experiments and calculations By: Julian Wolfram, Heriot-Watt University, United Kingdom My compliments to the Committee on their report and the very extensive amount of work it represents. Extreme ship motions tend to be associated with extreme wave conditions, so I will preface my questions by a few comments drawn from observations of winddriven storm waves in the North Sea: Most big waves occur near the middle of a group of waves of above average height. These waves tend to be steeper than the preceding and following waves. Wave steepness increases with wave height, with large waves showing greater variability in steepness than smaller waves. Even in deep water storm waves are significantly non-linear with crests elevations much greater than trough depths. In shallow water the situation is more severe with increased steepness and asymmetry. Just of the coast near Venice a platform in 18 m of water has recorded a 1 m high wave. This platform is passed by fishing vessels regularly. My concern is that the characteristics of extreme waves are not captured in the physical model experiments and numerical calculations the Committee describes.