Ship Assist in Fully Exposed Conditions - Joint Industry Project SAFETUG

Transcription

1 Southampton UK Organised by the ABR Company Ltd Day 1 Paper No 2 Ship Assist in Fully Exposed Conditions - Joint Industry Project SAFETUG Johan H de Jong MARIN The Netherlands SYNOPSIS: SAFETUG Joint Industry Project (JIP) gathered almost 30 participants who jointly studied the performance and behaviour of tugs in waves while assisting at very slow speed (berthing) slow speed (escorting direct assist) or high speed (escorting indirect assist). The data on performance and behaviour is used to define the operability during berthing and escorting operations The operability determines the downtime of certain operations critical for the economic viability. The performance aspects in waves covered propulsion efficiency direct and indirect assist capabilities and pushing capabilities. Behavioural aspects focussed on dynamic towline forces required winch response extreme tug motions (primarily roll and yaw) and the identification of the operational criteria for the various modes of operation.. The extreme tug motions and criteria definition focussed on the human safety and workability issues of tug operation The project took as a reference a representative Azimuthing Stern Drive (ASD) design and a representative Voith Water Tractor (VWT) design The latter could be seen as comparable with Azimuthlng Water Tractor (AWT) The most important means to determine the performance and behaviour of the tugs Is done through model testing and limited model definition Operability methodologies were defined which used the collected data to settle the relevant model aspects 1 INTRODUCTION Tugs play an important role in the operation of terminals in ports and offshore. Terminals and offloading operations in general are more often located In exposed areas (wind waves and currents).. Large vessels or vessels with dangerous cargoes require tug assistance in the exposed approaches into ports. (Throughout this paper the word 'vessel' will be solely used to denote the assisted vessel.) The availability of tugs ultimately can determine the downtime of the operation and hence the economics involved The more effective and enduring tug operation in open seas will enable lower downtimes.. This requirement puts a larger demand on tugs and their crews as they have to assist vessels in exposed areas Operators in particular for LNG (both offshore and into ports) require low downtimes combined with a safe operation which can be achieved by proper tugs during approaches to offshore terminals and ports Equally important is the persistent operation of tugs in offshore circumstances during side to side tandem offloading operations and future offshore offloading concepts where tugs are needed throughout the (off)loading operation In and around ports escorting regimes are often put in place to cope with emergencies or to assist large vessels where speed control is important The SAFETUG Joint Industry Project (JIP) addresses the identification of the tug performance and behaviour in waves through the collection of data from model tests limited calculations and full-scale measurements the latter with the aim to also gather data on the human behaviour aspects of operations 1 2 OBJECTIVES The objective of the JIP is: to identify the relevant modes of tug-terminal operation in the various operational contexts; to quantify the operational effectiveness in waves; to quantify the appropriate operability envelopes while assisting in waves; to identify the relevant criteria per type of the operation; to ascertain safe and workable operations of tugs In the original project definition more attention was given to the possible improvements in the design of tugs for operation in waves.. However throughout the project this issue was replaced by the more important safety issue of extreme (roll) motions The attention for tug design improvements is now shifted to the likely emerging continuation of the project As the participants urged for direct application of the results in the berthing and escorting assist frameworks of operation issues not directly related to operation in waves were also addressed ie wake-hull interaction and tug propeller-vessel hull interaction 3 PARTICIPATION AND APPROACH The project was based on a high commitment from all parties involved in the tug-terminal operations and was essential for the success of this JIP The list of participants is shown in Table 1 The presence of the various parties with different interest and knowledge areas led to good talks and discussions both on a technical as well as on an operational level. The technical aspects were primarily covered by tug designers builders and equipment manufacturers The operational level was covered by the practical

2 experience of tug and port operators together with the oil majors aiming for the requirements for the tug operations to be extended in exposed areas. Oil Comnanies BP rotai SHELl ChevronTexaco ConocoPhillips Petrobras BHPBil1iton Neste Oil Tuoopcrnlors Adstcam Svitzcrwijsmullcr Larnnalco KoorcnTugs Smit Novarug Worldwisc Tidewater MODUK DesiO& Build. RobertAllanLtd SECengineering SBM SJPORT XXI DAMEN BbaratiShip Yards Otto Candies Port de Gijon Voith wartsila MARIN ' Table 1: Participants of the SAFETUG JIP The shown commitment is reflected in the realised project organisation which is given in Figure 1 The organisation also reflects the approach of the project. The project was split into three connected work packages (WP): WP 1 'Tug-Terminal operations' This WP focussed on the identification of the project area and type of operations. The Working Group 'Tug-Terminal operations' (see below) will be involved in this WP Full-scale tug performance and behaviour measurement was part of this WP. WP 2 'Offshore berthing and escorting model tests and analysis' This WP addresses the collection of the basic tug performance data on tug performance and behaviour through model testing and limited calculations WP 3 'Tug design parameters' This WP addressed the initial reference tug definitions the later tug definitions when small changes were proposed and the specification of details.. Additional attention was paid to the definition of winches.. The Working Group 'Tug Design' will be mainly involved in this WP L 1'1 TUG-TERMINAL OP'ERATIONS WORKING GROUP 4 SUMMARY OF MODEL TESTS 4.1 TESTED TUG DESIGNS The scope of work of done covers a large amount of systematic model testing. Model testing was performed on the two reference designs Their properties are given in Table 2 The VWT represents a fairly optimised design of which quite a few are around for generating high indirect escorting forces and being quite well designed for operating in exposed conditions See Figure 2 The ASD design is a MARIN design used earlier for generic testing See Figure 3 for its general appearance The design can be described as a robust but not specifically optimised for any particular operation (not least for operating in waves). Notwithstanding that fact it represents the designs that while not too modern are still in use The various design details of the tug such as weight weight distribution installed power and diesel engine controls were carefully determined and modelled Feedback from tug masters was used to tune the different settings of engine and thruster response Figure 4 shows the MARIN database of tugs and their common relation of MeR (kw) versus Bollard Pull (BP). It shows that the tugs are a good representation of the existing fleet The choice of Bollard Pull is a fairly arbitrary one although believed to be a good choice given the expected tasks to do in the exposed areas M onshore BERTHING MODEL TESTS AND ANALYSIS 1'1 TUG DESIGN PARAMETERS WORKING GROUl' 1'12 ESCORTING Moon TESTS ANDANALYSIS Figure 2: VWT Tug Model TUG ASSIST IMPROVEME1\'TS EJEJEJ Figure 1. Projec: organisation All working group participants were members of the project steering group which steered the overall project according to the above-proposed approach. The project started at the end of June 2005 and was completed two years later in June l\.1agnltude VWT Particulars SY.MBOL UNIT Length between perpendiculars L p p 3750 m Length On waterline L W l 3750 m Length ovemll submerged Los 3750 m Breadth on \Vl B 1382 m Breadth moulded B 1400 rn Draught moulded on FP T F 675 m Draught moulded On AP T A 675 m Draught ofhull I'H.'l1 375 m Displacement YoluITle moulded m' I ransyen;e ITletacentrk height GM rn Table 21: Main Particulars of VWT. 2

3 Wave thrust losses 2 Tests in waves to identify the thrust reduction of the propulsor due to the tug motions and wave motions Figure 3: ASD Tug model ASD Particulars SYMBOL MAGNITUDE UNII Length between perpendiculars Lpp 3600 m Length onwaterline L Wl 3384 m Length overall submerged Los 3384 m Breadth moulded onwl B 1294 m Draught moulded onfp IF 495 m Draught moulded onap I A 495 m Displacement volume moulded m J Iransverse metacentric height GM L81 m Table 2.2: Main Particulars of ASD. 000 W/Wi/C loads.tug Interactions 3 Calculations to identify the loss of effective BP due to wave loads. 4 Tests to identify loss of effective BP due to wake-hull interaction (influence of wake of tug on hull of vessel). CIlf=O.0116MC g ' 100 :g 10(1 l o U..' : :-- +- i. 5il '._.. +'.:.;... : L_ 5 Calculations to identify the wave sheltering effect of the tanker. ol---_------'- '_ u MeR bh Figure 4 Bollard Pull versus MeR..;..._-- ' 4..2 MODEL TEST SET-UPS The performed model tests represent the outstanding questions on tug performance In waves The tests/ calculations are done for both the tug concepts for different speeds and in various wave conditions. The following issues are identified for coverage by model tests or calculations (few yet): t!tffj! ;' i I /; / c/j 6 Tests to identify direct assist performance in waves 7 Tests to identify the indirect assist performance in waves. Ad 1 Model tests with ASD and VWT tugs are carried out in calm water without obstructions (x=oo) and at different positions (x) from a 'wall'. The effective thrust in calm water is determined (Fe).. The results are needed to determine the effects of the near vessel 3

4 _ hull in berthing conditions and the reference thrust in unrestricted conditions for further comparison with the restricted conditions Ad 2 Modei tests with ASD and vwr tugs in waves at zero speed are carried out with different wave heights wave periods and wave directions at zero speed. The effective thrust in waves is determined (F.) The results are needed to assess the deterioration of the effective thrusts due to the waves (tug motion and water particie motions due to the waves). It is assumed that for non-zero speed conditions the effects are comparable or less Ad 3 The effective load that the tug can apply on a towline or fender (F) is determined by subtracting the mean wave drift load current load and wind load from the available thrust. The results are needed to further assess the remaining effective BP whilst operating in waves wind and current. The current only applies to berthing conditions where the positioning in current conditions of the vessel is relevant Ad 4 The tug operating in close proximity to the vessel will exert forces on the vessel through its wake deflected by the hull of the vessel This level of influence is dependent on the distance (line length during pulling or pushing) to the vessel the tug's angle to the vessel and the position along the assisted vessel. All these variables were subsequently tested. See Figure 5 for the overview of tested positions. tests were all done at 10 knots vessel speed; the direct mode model tests were done at 5 knots. Careful attention was paid to the modelling of the engine thrusters azimuthing response and the Voith integral propulsion system. The performance of the bulk of the model tests in the model basin was preceded by tests including tug masters in order to define the laterused autopilot The latter was needed for repeatability and cost efficiency. The tug masters controlled the model very accurately and consistently. The model basin used for most model tests is shown in Figure 7 odeg 127Od 315 d :Y I Ode d 180 deg Figure 6 Designation of wave directions with respecl to assisted vessel. mam carnage sub c-..arriage bhrmalrml(]j1umrl B;'as foliowinglsre-nq S5'BS Figure 5. Overviewof tested tug positions relative to the vessel. Ad 5 Assist operations tend to use the sheltered areas around the vessel as much as possible although this is more likely in berthing operations than in escorting operations.. In the latter case the making fast of lines in waves was for this reason not seen as critical. However in berthing operations it can be highly effective to use the shelter of the vessel. Calculations were performed using diffraction software to determine the wave field around the vessel for the various wave conditions. The results can then be taken as input to determine the effective BP in these circumstances. Ad 6&7 The tug in the escorting assist mode either in the direct (5) or indirect (6) mode is tested in various relative incoming wave angle conditions. See Figure 6 A line angle and tug heading is chosen which gives maximum indirect towing forces. At the start of the project variations of these values are tested to identify the maximum towing force combination Indirect model Figure 7: View and plan of the Seakeeping and Manoeuvring Basin ofmarin 4.3 ENVIRONMENTAL CONDITIONS TESTED The test conditions for the escorting tests represented two typical wave periods (Wind Sea) with different wilve heights The realised conditions were: Sign Wave Height Wave peak period Hs [m] I p (s).._....._...._._._._..._._.._ _._._--_....._-_._._--_ _-_._.._-_._-_... _-_._-- ---_.._--_.._'_..---_.._--_ _-_._----_.-..._ !l ----_._-_._----_._ _

5 These wave conditions are compared to two typical scatter diagrams in Figures 8 and 9. > H2 North Sea source GWS Annual All Directions za a '006 ' '''' w es 118ft / 1()11 / '' t t E f s-a t t a. r-e t a t ' s 'e 2. ) t ta 0 58ft Q> s-e z t t.rs ss / / M 27 as as ' ' 23 t 17I3-tS3 4O@E 13 a. a 4S@B 121 roo to '. ' 0 1 ta as ss to a ' «a-s s.e t-e s.s >n Zero up-crossing period [s] a Figure 8 Selected wave conditions compared to typical climate in the North Sea > Northern Pacific s as Annual All Directions ' rss zs as s ' / / l1bf! '''' t z t s. t a. ' t n s ] a a zr ' / s-a 8 ' ts te tu s z sa ao ts s z tta -'&F / M sa an 7 JBft2 z-a 3-c 8 se 8S 3 E82 M rn $ ts s 1jI-A:'-.. ' o- te aa t a <'... s-a s-z t-e s-a 10_ >n Zero up-crossing period [5) Figure 9 Selected wave conditions compared to typical climate in the Northern Pacific. The chosen wave periods for the escorting tests represent two different types of sea states; the short crested more steep wind waves with adjoining wave heights (see scatter diagram) and a set of more developed seas or swell types of sea state with considerably higher waves though still covering a good percentage of occurring wave fields (see again scatter diagrams). The chosen combinations of wave period and wave height cover a good part of the feasible sea states worldwide.. However it has to be realised that shallow water conditions (Water depth «Wave length) will affect the shape and properties of the waves which certainly influences safety margins and to a lesser extent the tow performance The wave periods do not exactly correspond to the roll or pitch resonance period of the model. This roll resonance is influenced by the speed of the model and even more by the towline. Therefore it was decided to use common wave periods for a wind and swell wave of 6 sand 10 s respectively. In berthing conditions wave periods are chosen to be representative but more importantly (due to low speed operation and direct towing) also to reflect the roll response and/or pitch response values. Typical values are between four and seven seconds for the response values and between 11 and 18 seconds for the long swell offshore conditions 4.4 SOME MODEL TEST RESULTS THRUST REDUCTION DUE TO WAVES The wave drift forces lead to a significant reduction of the effective BP once working against the waves and to an increase when working with the waves In beam waves no reduction or increase of resultant force in thrust direction could be observed. However a smaller amount of thrust is necessary to compensate for moving slowly sidewards to keep on station. This amount might be relevant operating at large bollard pulls close to the maximum available bollard pull and is one of the reasons for a safety margin. It is noted that the drift forces increase when wake-wave interaction (see below) occurs The measurements done determining the differences between the push and pull conditions (without waves) quantified the already-known effect of the propeller wake flowing along the hull of the tug This difference between push and pull mode can be explained by thruster-hull interaction: the generated wake does not flow freely away from the tug but is directed towards the hull before ieaving it. This leads to a loss of effective thrust mainly due to friction and pressure effects (see sketch of ASD). The tug models show significant motions in irregular waves at the pitch response and roll resonance period. Apart from the aforementioned direct wave drift effects this also accounts for some indirect propulsion-related issues Thruster ventilation occurs when the tug motions become very large and leads to the emergence of one 5

6 or both of the thrusters and intakes air (see photo) Large relative wave motions increase the probability of thruster ventilation. It general it could be said that the Water Tractor concept due to the deeper and better position does show very little ventilation whereas the stern drive position is more sensitive (significant) to this effect in higher waves Thruster ventilation in high waves The propulsion of the tug generates a wake in the water. This wake can be considered as a local current encountering the waves on site. Thus the waves combine with the current where basically three different situations can be identified: The wake flows in the direction of wave propagation The wake is generated against the wave direction There is a right angle between wake and waves. Waves running against a current become steeper and shorter. They refract in a more unfavourable manner with respect to the tug as they focus on the vessel. The changed wave pattern continues to travel towards the tug and has a severe impact on the vessel: steep and high waves focus on the bow or stern of the tug (see Figure 10 and photo below) This leads to significantly increased tug motions and green water on deck. This effect of wave-wake interaction amplifies all other effects related to thrust degradation in waves such as relative wave motions and thruster ventilation. NO THRUST I 0- PULL \tj: '. ''''''''''''''...a'.'-..fl ' In the pull mode wake-wave interaction occurs and the pitch motions increase dramatically compared to the pitch motions in the push mode and without any thrust It could be argued that the situation of a tug operating in waves that are propagating against the wake is unrealistic. However there are several real operational contexts where this has to be considered ego The tug is pushing in a fender and the sea state is stern or stern-quartering; The tug is pulling in a line and encounters a locally head-on wave field reflected or radiated from the large vessel Summarising wake-wave interaction is considered as the effect of the wake acting against the incoming wave field with the results of shorter higher and steeper waves focusing on the bow or stern respectively. For an ASD tug this means head to head-quartering waves in the pull mode and stern to stern-quartering waves in the push mode. Wake-wave interaction increases tug motions wave drift forces and the amount of thruster ventilation. WAKE-HULL INTERACTION The results show that reductions can be as large as I5 per cent (and higher) for the most unfavourable positions obviously highly dependent on the position of the tug relevant to the vessel The phenomenon of reduction of effective force can be explained as follows: the tug is generating a wake by its propulsion units that results in the acceleration of water in the opposite direction of the thrust force Due to the law of conservation of momentum this force tends to move the tug in just opposite to the direction of water flow In the towline a mean force almost equal to the thrust force in calm water can be measured These considerations are valid in the 'near field' of the tug. At the vessel the wake is deflected by the hull and generates a pressure field with high values in the region where the wake impinges the hull and lower values where a part of the wake is directed alongside the hull. See photo. This can be referred to as the 'impinging jet problem'. The impinging jet induces a force on the Vessel in the direction of the wake. As a result the force on the vessel due to the impinged jet combines with the force induced by the towline. Figure 10 Breaking waves due to the wave-wake interaction Breaking wave due to the wake of the tug Photo counesy of Svitzer Wijsmuller. 6

7 The amount of force induced by the impinging wake is dependent on the iocal hull form SHELTERING The tug behaviour is strongly dependent on the local wave field and wave pattern around a vessel They vary significantly with the considered location ie with the position of the tug with respect to the vessel: the local waves are different depending on whether the tug is in a shielding position or not The incoming waves are amplified or decreased by the diffracted and radiated wave field This can also result in a change of local wave direction and in short-crested waves as compared to the incoming wave. Figure 11 below illustrates the described complex wave field around a large vessel to be towed by a tug Conclusions differ for different wave lengths wave height and towline angle combinations A separate set of model tests has been done to investigate the iinearity of the results with wave height The outcome of these tests enables the use of the results for other environmental conditions In this way the data can be used to define the operability over a large range of sea conditions. -:-t:::::;::::::::::::::= Figure 11 Complex wave field around a vessel ESCORTING IN WAVES A large set of model tests have been done in order to explore all the different incoming wave directions wave heights and wave lengths. Unlike the berthing assist it is assumed that the wake of the tanker generally doesn't give shelter. The used line length and the more likely required steering assist (vs pure braking) during most normal assist operations favours the unsheltered approach In total more than 150 tests have been done in the large Seakeeping basin at MARIN When operating in waves wave forces will be working on the tug. These forces could reduce or enhance the force in the tow line. Furthermore the waves can cause the tug to change heading or drift angle that causes a shift in the equilibrium and hence a change in tow line force. Below one example is given in figure 12 which shows the drift angle and the average tow line force This is done for one case with 75 degrees tow line angle and one wave period of 10 sec and for the ASD tug The polar diagram gives the performance for all wave directions for two wave heights for two initial calm water tow force levels (black lines). The blue and red lines show the performance in waves. In this case it can be concluded that the performance decreases significantly in particular when the incoming waves hit the tug perpendicular and at its lower side. The deterioration is obviously (partly) caused by the change in heading (drift) of the tug due to wave action as can be seen from the adjoining drift diagram Average <owline fo 1l=4ltm Tp=1O ><:0 (well driven os) 75degtl<)will'gle Avernge drin ogle 1l=4m Tp=IO'<e(W<lJdtiveaj 75degr <owngl< Figure 12 Average towline forces and drift angle for various wave directions and wave heights in a swell condition (2.5 and 4m wave height) TOWLINE FORCES Along with the average towline force performance in waves sets of polar diagrams were also generated which showed the towline force behaviour in time As expected the towline force maximum (exciuding the slack line events) can easily be triple the BP load even in relatively moderate sea states. The forces are used to determine the downtime of the assistance given set criteria. Table 3 gives an example for one wave direction of possible downtimes Downtime table: Name Criterium Downtime 600I braking force 6000 kn 4% 500t braking force 5000 kn 12% 400 t braking force 4000 kn 30% 300 t braking force 3000 kn 55% braking force 2 5*Bollard pull 1962 kn 94% braking force50* Bollard pull 3924 kn 30% braking force 3* Calmwatermaximum 3000 kn 55% braking force 5*Calm watermaximum 5000 kn 19% Table 3 Downtime based on tow line strength for a given seastate and relative wavedirection If the tug is operating on the brake the towline force data is used to identity the probability of exceeding a specified line load which ensures that the breaking of lines happens less than every once specified period of time. This data is together with the tug motions at the tow point position used for the assessment of the requirements of the winch based upon a winch model. 7

8 I 50 degrees 100Iioe 31lgle 75 degrees lml;ne angle The tug motions are also used to assess the consequences on green water water on the aft deck and possible course instability. The operations group within SAFETUG used part of the model tests to develop criteria for these issues which are added to the overall operational assessment of the various operations The used motion criteria from open literature (Nordforsk) are used as a starting point for the further refinement of the criteria for tugs. See Table 4. Figure 13 Vertical acceleration levels for two wave periods towline angles wave heights and towline forces TUG MOTIONS The motions of the tug are addressed in the SAFETUG JIP on two levels. Measurements are done during model tests and full-scale measurements are performed on board Anglegarth operated by SvitzerWijsmulier in Milford Haven. The model tests data is mainly used to determine the levels of vertical and horizontal acceleration Various locations on board can be selected for evaluation but the main focus is on the wheelhouse. Figure 13 shows the vertical accelerations on board the ASD at the wheeihouse location for different wave lengths wave heights iine loads and towline angles. The data is again used to define downtime due to exceeding set criteria on the vertical (and other) accelerations. Figure 14 shows the criteria plotted in the scatter diagram of the North Sea used previously. The total combination of all relevant motions (longitudinal roll and transverse accelerations) can be used to determine the overall downtime of the assist ooerations _13 11_ j[ 9-10 i.0 J.. 4-'<.. ' 2 0' q North Sea source GWS II '0 ' H ea.. 'W ' 0' ts e z Annual All Directions a Owm(2) 20/ ' < Zero up--crossin period lsi Figure 14.. Downtime based on vertical acceleration at the bridge. a Value Value Type of work (r.ni.') (Significant Double Ampl;tude) Veni] Lateral Venic.1 Laler:l] Roll Roll aoc. CO a. Simple lightwork ll.175g LlOg light manwl]wnrk s O.IOOg 0.80g OAOg Heavy mawliwnrk O.l50g g 4.0 O.60g 0.30g re Intellectual work O.IOOg g 3.0 OAOg O.20g 12 Pa.<.<'mg.<. on a fc...-y 0.050g g g O.l6g W Pas.<..gcro ona [1J;Sl: liu<... O020g 0.030g g O.l2g 8 Table 4. Limiting criteria for vertical accelerations lateral accelerations and roll motions depending on the type of work. The following criteria are used for tugs See Table 5 It is to be noted that the criteria for transverse and longitudinal acceleration are valid for a man standing upright The criteria that would be applicable to a man sitting in a chair are unknown at present Parameler Value Value Value (Significant (rrns) (rms) Double Amplitude] Vertical acelemlion O..20g 19 mls 2 77 mls 2 Transverse acceleration Ol3g 13 mig 5 I mls 2 Longitudinal acceleralion O13g 13 mls 5 I m/s Roll motion 6' 6' 24' Table 5 Motion criteria for tug operations.. The full-scale measurements on board Anglegarth are used to refine these criteria. More tests are needed to correlate these criteria to endurance variables and to finalise conclusions on this issue. 5 ULTIMATE STABILITY TESTS Halfway through the project it was decided by the steering group to focus more on ultimate stability tests for both tug types.. The operational group had pushed for attention to be paid to the behaviour of the tugs in extreme circumstances This is a mean to discover the margins during the less extreme wave conditions At the same time these tests were used to perform some variations on deck height metacentric heights and towing point positions (height and longitudinal position) The possible failure scenarios are summarised in Table 6 Phenomenon Wae direelion Procedure Nose diving 100_135 The 00 died in the walcr. hut due 10Ibe. forward speed model sheers under waler Broaching [)o 310 Due to loss of sreering power. the all cnd of '' ship 'ken wave crest 'overpasses' lhe fronl Throwing over ' In a very steep wave a large h.eling angle could occur Los.<; ofst.bilitydue 10waler on deck '50 Wae ate anaekiug on a side where the waler run como easily on deck Table 6.: Failure scenarios 8

9 Typical results are presented as in figure 15 showing the roll angles of the tug 6.2 BERTHING OPERABILITY METHODOLOGY It is concluded on the basis of many tests that it was only possible to capsize the vessel with too low GM high towing point connection and a high towline connection point This configuration is not desirable in calm water let alone in waves.. It can therefore also be concluded that when the workability of the tug is improved (acceptable accelerations winch capabilities etc) the fatal limits will be manageable The offshore berthing operability methodology is divided into six modules each of which considers one or more of the following aspects: Figure 15: Heel angles in Hs=1Om Tp=1 Os in various wave directions 6 BERTHING AND ESCORTING OPERABILITY METHODOLOGIES 6.1 INTRODUCTION The vast amount of data emerging from the SAFETug project encouraged MARIN to come up with a practical way to combine this data To this end an offshore berthing and escorting operability methodology for the evaluation of efficiency and feasibility of assist operations with tugs is introduced It incorporates the various performance items into one method which enables the design of the operations In the following two paragraphs the outline of the two different methodologies are presented Their concept is well defined now Within SAFETUG considerable attention is given to the mathematical modelling of the winch in order to use this model to define the requirements of future winches based upon the measured tug motions and towline force dynamics. The devised model had led to a reduction in the maximum peak line loads to only 25 per cent of the original loads through a more sophisticated (high power use) - albeit not yet very practical- specification of the constant tension winch Complex wave field around a large vessel to be (un)berthed (module 1); Thruster-hull interaction ie difference of bollard pull between push and pull mode (module 2); Thrust-degradation in waves due to thruster ventilation wake-wave interaction relative wave motions motions of the tug (module 3); Consideration of dynamics of towline winch and fender and tug motions which are crucial for safety and operability (module 4); Interaction of tug wake with Vessel (module 6); Combination of effects (module 6) The methodology has the following global inputs: Position of tug Global wave field Tug type Nominal bollard pull Line/winch/fender properties The outputs are: The effective tug force on the vessel to be towed Dynamic line/winch/fender loads Tug motions 6.3 ESCORTING OPERABILITY METHODOLOGY lityrmdel Further some relatively small parts need to be added to complete the tools which facilitate the application of the method. To determine the final downtime of the operation the resulting tow performance tug motions and towline behaviour need to be assessed on the basis of the defined operations required total tow force and moments motion criteria and towline/winch configuration. This is included in the escorting method 9 The escorting operability methodology contains in addition to the berthing method also the final tug

10 downtime assessment. For operators or engineers designing an operation (port approach) the last step is the determination of the operability of the assisted vessel by calculating the assist need based upon the envisaged operation and the operable wave environments for the tug (from item 7) and the effective mean tug forces (BP) in these environments (from item 13 through 7 12 and 11) Items 1 4 and 6 are the input of the tug (hardware) the various criteria (software) and the environment Items 13 and 7 are output effective mean tug forces per wave condition and a set of feasible of wave conditions in which the tug and its crew can operate Items 2 3 and 5 together determine the limiting conditions due to restrictions in motions based upon the set criteria (item 6) and are fed into the wave climate per type of operation Items 2 8 and 9 together determine the limiting conditions of the line dynamics based upon known winch and line characteristics and assumed towline breaking criteria The results are fed into the wave climate diagram per type of operation. Items 2 11 and 12 together determine the effective mean tug forces as being available based on the tug hardware and given the environment and again per type of operation. 6 REFERENCES 1 MARIN Joint Industry Project SAFE- Tug Ship Assist in Fully Exposed Conditions 2 Assessment ofship performance in a seaway Nordic Cooperation ofapplied Research November 1987 Not publicly available: 3 MARIN Report No 1791O-1-CPM SAFE-Tug Literature Survey For SAFE- Tug JIP May MARIN Report No MSCN SAFE-Tug Definition of Current Operations and Criteria February MARIN Report No SMB/BT SAFE-Tug Offshore Berthing. Model Tests with an Azimuth Stern Drive Tug Concept February MARIN Report No SMB SAFE-Tug Offshore Berthing Model Tests with a Voith Water Tractor Concept February MARIN Report No BT SAFE-Tug Offshore Berthing. Model Tests on the Interaction of an ASD Tug and an LNG Carrier Operating in Waves Concept February MARIN Report No 1791O-6-SMB SAFE-Tug ASD Tug Escorting Tests May MARIN Report No SMB SAFE-Tug VWT Tug Escorting Tests May MARIN Report No 1791O-8-CPM SAFE-Tug Tests on the Ultimate Stability ofan Azimuthing Stern Drive Tug Concept May MARIN Report No IO-PO SAFE-Tug Offshore Berthing Operability Concept February