Aspects of model tests and computations
for ships and other structures in
waves
J.A. Pinkster
Report 1147-P
Project Code: 962
12th - 14th May 1998
PUblished in.. 11th WEMT International Conference
"The West European Maritime Industiy of the Global
Challenge of the Next Century, Rotterdam, 1998
Organised by NWS The Netherlands Society of Marine
Technologists
TU Deift
Faculty of Mechanical Engineering and Marine TechnologyShip Hydromechanics Laboratory
4,
Ç 8 ROTTERDA
Organised by NVTS
Tie Netherlands Sociéty
of Marine Technologists
Eleventh WEMT international Conference
THE WEST EUROPEAN MARITIME
INDUSTRY IN THE GLOBAL CHALLENGE
OF THE NEXT CENTURY
Rotterdam
12th
-
14th May 1998
PROGRAM OF THE CONFERENCE
LIST OF 1PARTICIPANTS
SESSION i
SESSION 2
SESSION 3
SESSION 4
SESSION 5
SESSION 6
SESSION 7
SESSION 8
SESSION 9
SES;SION lo
Program of the WEMT-98 Conference
Tuesday, May 12
08.15 - 09.15 Registration
09.15 -0930 Opening of the Conference
Prof Ir S.Hengst, Chairman of WEMT
09.30 - 10.30 Session 1: Communication networks, a new tool for shipping?
09.30 Opening by the chairman, Ir R.W.F.Kortenhorst, Managing Director of Bijisma
Shipyard, The Netherlands
09.40 MrMCremon, Project ManagerMRVELOUS & EDO
'
Fincantieri, Italy
10.00 Mr.J.W.Koutstaal, International Sàles Manager of PTT Telecom;
The Netherlands
10.20 Panel discussion
10.30 - 10.50 Coffee/tea Break
10.50 - 12.20 Sessión 2; Logistic approach in shipbuilding - Ae standard ships the
fùturefor shipyards?
10.50 Opening by the chairman, Ir R.W.F.Kortenhorst
11.00 TvItJ'.Kennemann, President of Kvaernçr Warnow Werfi, Germany
11 20 Mr J A Zarzosa, Manager Engineering Systems of Astilleros Español, Spain
11 40 Mr P Tang-Jensen, Vice President of Odense Steel Shipyard, Denmark
12.00 Panel discussion
12.20 - 13.40 Lunch
13.40 - 1510 Session 3: Do the international and the classification rules have a
positive or negative effect on develôpments in ship design?
13 40 Opening by the chairman, Dr U Bulgarelli, National Institute of Naval
Architecture, Italy
13.50 Prof Dr C.Arias, Head Technology of Astilleros Español, Spain
14.10 Ir WdeJong, Group Regional Manager for Europe of Lloyd's Register of
Shipping, London GB
14.3.0 Ing P.Maffioli, Product and Technology Development ofFincantieri, Italy
14.50 Panel discussion
15.10- 15.30 Coffee/teaBreak
15.30- i700 Session 4: Will computational fluid dynamics completely take the place of
model testing?
15.30 Opening by the chairman, Dr U.Bulgarelli
15.40 Prof D.Vassalös, University of Strathc1yde, Great Britain
16.00 Prof Dr Ir J.A.Pinkster, Deift University ofTechnolögy,
The Netherlands
16.20 Prof Dr L.Larsson, Chalmers University of Technology, Sweden
16.4.0 Panel discussion
17.30 - 18.30 ReceptiOn in the Town Hall
All participants and accompanying persons are cordially invited forthis
reception, hosted by the City of Rotterdam
Wednesday, May 13
08.30 - 09.00 Coffe/tea
09.00 - 10.30 Session 5: Is concentration of European knowledge and research
feasible?
09.00 Opening by the chairman; V.Cervera de Cóngora, representative of AINE,
former chairman of WEMT, Spain
09.. 10 Prof Thioukakis, National' Univerity of Technology, Greece
09.30 MrJCGonzález-Sama, Vice chairman of EUROYARDS (E.E.i.G.), Spain
09.50 Mr.P.Person, Chairman. COREDES, 'France
10.10 Panel discussion
10.30 - 1050 CofTee/tea Break
10.50 - 12.20 Session 6: How to disentangle the conflict triangle between fuel quality,
environment and diesel engines?
1050 Opening by the chairman, Mr.V.Cervera de Cóngora
11.00 Mr.N.Drafiin, Fuel brooker of E.A.Gibson, Education Coordinator IBIA, G.R
11.20 Ir A.P.Burgel, Còordinator 'Environment of the Ministry of Transport,
The Netherlands
11.40 Mr.I.Ahlqvist, Manager Technology Projects of Wärtsilä NSD, Finland
12.00 Panel discussion
12.20 - 13.40 Lunch
13.40 - 15.10 Session 7: Will a maintenance friendly design contribute to lower
life-time costs of a vessel?
13.40 Opening by the cháirman, Prof Ir J.Klein Woud, Delfi University of
Technology, The Netherlands
13.50 Ing G.Balzano, Italian Shipowners Research Consortium,. Itay
1.4.10 Mr.A.Vèldman, Managing Director of Wagenborg Shipping, The Netherlands
14.30 Mr.I.Garcia & E.Ibañez, Managers of Bazán Shipyards, Spain
14.50 Panel discussion
:15.110 - 15.30 Coffee/tea Break
15.30 - 17.00 Session 8: The unmanned vessel as an objective, the right way or not?
15.30 Opening by the chairman,Prof Ir J.Klein Woud
15.40 Dr Ing V.Bertram, Institute for Shipbuilding, Hamburg University, Germany
1.6.00 Prof A.Fiorentino, Naples University Frederico H, Italy
16.20 CaptT.Bouwman, Managing Director of the Royal Association of
Netherlands Shipowners, The Netherlands
16.40 Panel discussion
18.30 - 23.00 Boat tour and dinner
The conference dinner will be held on board the mv Riverstar,, moored at the
Westeckade at the West side of the Veerhaven. The Westerkade can be
reached by. tram number 5. Parking place is available on the Westerkade.
18.30 - 19.00 Embarkation
19.00 Departure for the tour
approx. 23.00 Disembarkation
Thursday, May 14
08.30 - 09.00 Coffee/tea
09.00 - 10;30 Session 9: High speed shipping
09.00 Opening by the chairman, Ir M.J.van der Wal, Managing Director of Verolme
Botlek Shipyard, Chairman of NVTS, The Netherlands
09.10 Dr.K.Branner, Research Engineer in Ship Tech Consultancy, Denmark
09.30 Mr.R.Svensson, Application manager KaMeWa water-jet division, Sweden
09.50 Mr.J.C.Lewthwaite, IMAA Maritime Transportation Consultancy, Great Britain
10.10 Panel discussion
10.30 - 10.50 Coffee/tea Break
10.50 - 12.10 Session 10: The European Approach
10.50 Opening by the chairman, Ir M.J.van der Wal
11.00 Mr.C.Andropoulos, Head of Unit Marine Industries of European Commission,
Directorate General ffl-D5, Brussels
11.20 Mr.P.G.Sulzer. Executive Director of Wärtsilä NSD, Switzerland
11.40 .Mr.J.E.Pérez, Director of AWES, Director General of UNINAVE, Spain
12.00 Panel discussion
12.10- 12.25 Concluding notes and closing of the conference
Prof Ir S.Hengst, Chairman of WEMT
Greece
Loukakis, Th. Technical University Athens Perdicaris, C. Assoc. of Naval Architects Theodoropoulos, A. Assoc. of Naval Architects
Italy
Balzano, G. CONSAR
Bulgarelli, U. INSEAN
Crernon, M. Fincantieri
Fiorentino, A. University of Naples
Lombardi, G. Association of Naval Engineers Maffioli, P. Fincantieri
Russian, F. Fincantieri
The Netherlands
Aalbers, A. Deift University of Technology Amtz, H.J.G.J. Arntz van Helden
Beek, M.R. Student
Beek, T. van Lips BV
List of Participants Belgium
Andropoulos, C European Commission
Denmark
Bakalus, I. Mrs Danish Maritime Authority
Brenner, K. Shiptech A/S
Hasholt, S. AarhusDockyard
Jensen, J.J. Technical University Denmark Kristensen, H.O. Danish Shipown Association
Moller, L.J. Lloyd's Register
Neergaard, C.H. Fundia Profiler AS
Pedersen, K. Odense Steel Shipyard
Tang Jensen, P. Odense Steel Shipyard
Vinther, L.H. Nay. Arch. & Mar. Eng.
Wittrop, S. Mrs. Journalist
Finland
Ählqvist, L. Wärtsilä :NSD
Osier, N. Wrtsiiä NSD
Vainio, P. Satakunta Pòlytechnik
France
Person, P. COREDES
Germany
Bertram, V. Technical University Hamburg Heynen, J. HM Consult GMBH
Kennemann, J. KvaernerWamow Shipyard Schwenker, E.H. Consultant
Great Britain
Akinyemi, A. University College London
Bunch, HM. Office of Naval Research Europe Draffin, N.A. Gibson Shipsbrokers
Lewthwaite, J.C. 1MM Ltd
Sloggett,J.E. Inst. of Marine Engineers
Tortike, W. RNN Ret OWMar SUpt
Biesheuvel, L.C. Pakhoed Shipping
Bijholt, H. Wijne & Barends
Bies, A.A. Vander Central Consulting Group Biok, J.K. Lips BV
Boekel1 P. Det Norske Ventas
Boon, B. Delfi University of Technology
Boonstra, l-1. Delft University of Technolog
Bouwman, T. Assoc.ofShipowners Burgel, A.P. Ministry of Transport
Busker, M.A. Assoc. of Netherlands Shipbuilders
Carpentier, D. Det Nonske Ventas
Daman, B Anthony Veder Damen, K. Damen Shipyards
Dirkse, C. Deift University of Technology Dool, J.P. v.d Student
Eekels, HIG. Croon Eiectrotechniek
Emens, R.G.M. Student
Feenstra TOG Det Norske Ventas
Feenstra, W.S. Student
Feith, J. Mrs. Dockwise
Gerritsen, k BOS Foundation
Gorter, A. Student
Haman, W.F.J. D. Touw Expertise
Heel, H.J.J. van P&O Nedlloyd
Heernskerk, R.J Ali SeasEngineening Heese, Y. Mrs. Min. of Economic Affairs Hencke, HH.. Germ. Lloyd Netherlands Hengst, S. Deift Universityof Technology Houtzager F. Diesel Mariheint.
Jong, W. de Lloyd's Register
Kamerman, G.K. Krupp Vosta
Keizer, E.W.H. NIM
Klein Woud, J Delft University of Technology Knecht, R. de Mateilaal Metingen
Kortenhorst, R.W.F. Bijlsma Shipyard
Koutstaal, J.W. P.TT. Telecom Kraaijenzang, K. Kamewa
Kruidenier, M.C. Student
Kunst, P.J.H.B. Min.of Transport
Kuyer,M.M.R. Centrai Consulting Group
Kuiper, D.H. Conoship
Lantau, T. Hogeschool Haarlem
Leenaars, C. Dockwise
Leenhouts; J. Kon. Scheide Groep
Mameren H.C. van Ha-Ce Marine B.V.
Molenaar, W.A. IHC Holland
Molenaar, A.E. Svanehoj mt. (retired)
Mourik, J. WC lJsseiwerf
Mulder, J.B Student
Nienhuis, N. NIM
Nolte, l.C. Student
Ovenhagen,J./Kik,A. IHC Holland
Perez Prat, J European Commission
Pont Maclame S.H. Student
Pronk, A. WC lJsselwerf
Pullen, Lloyds Bank Amsterdam
Ras, J; Student
Reezigt GD. Student
Schie, Ci-l.M. van Shipping Inspectorate
Schouten, R;J. Assoc. Netherlands Shipbuilders
Sliedregt, J. van IHC Holland
Splinter, J. Hogeschool van Amsterdam
Spuyman, W WEMT
Strijland, R.R. Shipdock Amsterdam Stuifbergen, J.P. Hogeschool van Amsterdam Swolfs, Ph. Conoship B.V.
Tan, S.G. MARIN Thiecke, H. Prlvate
Tienpont, A. Centr. Industry Group
Toonen, A; Wijne & Barends Vallianatos, A. Marine Survey B.V.
Vanagt P.J. Student Veitman, J.M. NVTS Visch, M. Prlvate \ñzee, K. Student Vliet, P.C. van Kahn Shipping
Waalewijn, J.F. Wärtsilä NSD Warbout, C. STÑ Atlas Ned. B.V.
Wal, M.J van der Verolme Botlek Wehe, GP. van WC lJsselwerf
Wert H.D. van der Wärtsilä NSD
WerffT.F. vander WC IJsselwerî
Wijkamp B.I.M. Wärtsilä NSD Wrt, J.W. de Krupp Vosta
Poland.
Dabrowski, F. Stocznia Szczecinska
Soyka, P. Gdansk Rernontowa
Wegrzyn, B. StoczniaSzczecinska Zochowski, C. PHZ "Navimor'
Spain
Alvarino, R. Arquaval Marine Arias-Rodrigo, C. Astilleros Espanoles
Carnevali, E. Association of Naval Engineers Cervera, V. Association of Naval Engineers
Garcia, l.
EN. BazanGonzalez-Sama, J; Astilleros Espanoles
lbanez-lglesias, E. E.N. Bazan
Moreno,A.Gutierrez Bilbao Plaza Mantima Perez, JLE. UNINAVE
Zarzosa- CebaIlosJ.A. Astilleros Espanoles
Sweden
Larsson, L. Chalmers University Technology Svensson, R. KarnewaAB
Switzerland
Aspects of model tests and computations for ships and other structures in
waves
J.A. Pinkster, Ship Hydromechanics Laboratory, Delfi University of Technology
Abstract
This paper is concerned with the role of model tests and computational methods for the prediction of loads and motions of ships. and other floating structures in waves A review is given of a number of the real.life situations for which. quantitative data on loads or motionsare required. Based on this review,, state-of-art methods in model tests and numerical computations are treated Finally, some general conclusions regarding the
conditions for further progress in the application of numerical prediction methods and.experimental techniques are presented.
Introduction
Knowledgeon the loads on, and the behaviour of ships and other floating structures in waves may be required for a variety of reasons related to the structural design or the operations of sUch structures Régarding structural loads, we are concerned with extreme loads which a vessel may have to withstand during its operational life time on the one hand and on theother hand thefatigue loads leading to the accumulatión offatigue damage.
The expected extreme loads arethe prime input tothe structuraldesign of a vessel, besides of coursethe input
based on the operational demandsplaced on the vessel. Extreme loads on ships are associated with high sea-states and often with large motion amplitudes. As a consequence non-linear effects, ice. that loads and motions are nota linear function ofthewave amplitudes and that the frequencies present in the wave loads contain super- and subharmonics of the wave frequencies, can become important. Expected fatigue lòads tend to have an impact on details of the structural design andnot dictate the overa! structural design to the same degree as the extreme loads Increasingly, however, information on fatigue loads is required as the stnictures tend to become lighter and less material is used. This requires that statistical data on the frequency distributiOn of load oscillations bedevelöped forshort and long term. Shortterm in this sense being the statistics related to a particular sea-condition and longterm being the statistics of the loads as related tothe life-span of thevessel. Besides information on the loads due to the continuous action of waves on thestructures of ships, information is requiredon the loads due to extreme events such as slamming See photo 1. The loads due to slamming are highly non-linear when related to the waves in that they are strongly non-linear when related to thewave amplitudes an that the frequencies associated with the load oscillátion after an initial wave impact are related more tO the virtual, mass of the vessel and the stiffness of the structure than to the wave frequencies.
Higher seastates and larger ship motions may lead to the occurrence of green water on deck. While this is also one of those undesirable occurrences which, as slamming, is avoided as much as possible, the effects of green
water on deck are potentially somuch more dangerous for the crew, th ship and its cargo that not only is the
probability ofoccurrence a focal pointbut increasingly attentión is being paidto theactual behaviourof water
on deck and to the effects iii terms ofwater heights, velOcities on deck and on impact pressures on supCrstructures. Research in this field is aimed at, among:others , more rational design ofwave breaking barriers on deck.
For ships at sea the influence ofwaves on theresistance and propulsion characteristics are ofimportance from the point of view of economy and the time taken to reach thedestination. The resistance characteiistis of a'
shiparetraditionally based on the stil waterresistance with correctiónsforthe in-service condition ofthehull
taking into account in an approximative way ,the resistance increase in waves. Nowadays, more detailed information on such effects. arerequired in the design stage in order to be able to asses influence of changes in hull fôrm, loading condition,course and speedetc. on the speed loss in waves. See figure 1.
Motions ofships in waves can be inflüenced significantly by non-linear effects. A well known aspect:in this
sense is the rolling ofships Primarily interestis focussedon the rolling motions as a result ofthedirect action
of wave coming from off-bow, off-stem or beam directions. Another effect which has become more important inrecent times is the occurrence of parametric rolling of ships which can occur in head or stem seas. This phenomenon is especially of importance for cruise vessels. See figure 2.
Broaching effects of ships in stem or stem-quartering seashave always been ofconcern todesignersand operators. The behaviour of a ship under such conditions is a complex combinatiòn of the effects of wave-induced motions and forces on the one hand and On the capacity of the vessels steering system to counteract these forces and the lateraistability of the vessel on the otherhand. The onset of broaching and the subsequent behaviour in which large yaw accelerations andcourse changes along with largeroll motions can occur are elements.ofhighly complex and non-linear flow phenomena. See, for instance de Kat (1994). Broaching becomes of greater importance as.the speed of ships increases. As such,the arrival of large,fast passenger ferries in sea-areas with 'significant wave action are a cause for extra concern. Recent experiences with such vessels has shown that beside broaching, a relatively new phenomenon,nose-diving, can occurwhen fast vessels of the catamaran type, which have relatively small waterplane areas, travel at high speeds in stem or stem-quartering seas.
Anumber Of fatal occurrences with Ro-Ro passenger ferries has made clearthat the behaviour of such vessels under the influence of uncontrolled ingress of water which may be due to either error on thepart ofcrew or due to a collision is an area of great concern. Recently rules and regulatiòns with respect to the;design and
operations of such vessels were adapted in order to reduce the probability of recurrence of such incidents. However, a substantial amount of infonnation, for instance on such phenomena as the behaviour of the water entering a vessel and thesubsequent instationary motion of a vessel stil need to be generated in orderto be able to make a sound evaluation of the safety of these ships in case of severe damage.
With respect tohigh-speed craft, planing or semi-planing craft form a groupon their own. Such crafi derive a large part of their load-carrying capacity from dynamic effects due to forward speed As is the case with conventional displacement vessels, the stil water resistance is the basis forthe prediction of the installed power. Very little factual information is known on the maneuvring properties of thistypeof vessel. In the future, due to changes in regulations instigated by the EU, even for the smaller craft, more factual data will need to be generated on this subject.
The behaviour n waves of these vessels isheavily influenced by non-linear effects. Athighspeeds, high
acceleration levels may be reached which can lead to structural damage or personal injury of crew and passengers; See photo 2. An indication of the'non-linear properties in the behaviour of these vessels isthefact that motions andaccelerations may reach peak valuesvery abruptly with very little warning being given by the behaviourj usi prior to the occurrence of such events See figure 3.
With the advent ofthe offshore industry in the late 60's, new activities were being developed at sea. The exploration for oil and gas deposits and the subsequent field developments which took place demanded new
equipment for the purpose ofcarrying out newandnovel tasks. Many ofthe aspects ofworking at sea made use
ofknowledge and experience gained from the operations ofships and other existing floating equipment. However, the scale and diversity of the activities also demanded equipment and methods tobe developed for which no precedents existed. This led to new procedures forthe désign of floating equipment which rely much more on the application of knowledge of fundamentals regarding hydrodynamics, strength of materials and structures and assesments of the fatigue life of structures than had previously been the case with ships. Much of the knowledge gained over the last thirty years is now being incorporated in design gùide lines and rules. FlOating offshore equipment is required to work in avariety of conditions and for awide range of applications. In the following a brief overview is given of a Oumber of examples which are related to the behaviour of such' structures in an open sea environment.
Early on in the life-cycleofan offshore oil or gas field, exploration drillingfollowed by early production may be carried out using a semi-submersible chilling rig which is moOred on location by means of anchor lines, or, in case of dóeper water, by means of dynamic positioning. The wave-induced motions will .be of interest from the point of view of the drilling operations. Atthedesign stage special attention is paid to minimisation of the wave-induced heave, roll and pitch motions of the platform in operational conditions and to the 'air-gap' in survival conditions. The mooring forces are a result of the wave, wind and' current forces acting on the structure. Wind and current forces contain both lift anddrag effects which, beside containing constant parts also contain sub- and superharmonic force fluctuations associated with flow instabilities. Wave forcesconsist of wave frequency force fluctuations leading to the well known Wave frequency motions and also contain mean an4 low frequencycomponents which contribute to themooring loads. The latter forcecomponents contain non-linear components which are both of potential as ofviscous origin. See figure 4. Thç lowfrequency motion
responses of such moored vesseLs areaffected by the. often non-linear restoring .and damping characteristics of the mooring system and by hydrodynainic daniping effects which are of mainly viscous origin. Due to the low natural roll and pitch periods of such structures, non-liñear low frequency wave forces can induce significant angular motiOns even at frequencies outside of the range ofthewave spectrum.
An item ofinterest in semi-submersibletype structures is theoccurrence owave impacting onthe undérside ofthedeck. Theprobability ofthis occurring is influenced by non-linearbéhaviour ofthewaves as seen in , for instance wave run-up against the columns of such structures. See photo 3.
Drilling operations may also be carried out using drilIships Such vessels can be mooredon location by means of conventional spread moorings, by means ofturret-type mooriuigs or, as is often the çasewith deep water exploration, by means of dynamic positioning. The operational efficiency ofsuch vessels is influenced by the roll, pitch and heave motións ofthe vessel at the location of the dEfiling tower and by the horizontal motionsas
dictated by the etivironmental loads andthemooring system. The etwironmental loads again are dueto
combined effect ofwaves, wind and current with the main dynamic part coming from thewaves with respect to the wave frequencymotions. In case of large shipsin high sea states; low frequencywave-induced non-linear horizontal motions and mooring forces may be governing factors for the desigii ofthe mooring systems. In case dynamic positionmg is used, the complex interaction ofthe thrusters acting under the vessel, the.vessel hull and the fluid motions around the ship as induced by waves and current need to be taken into account in the design process of the DP-system.
Offshore production platforms may beeither fixed of floating. Byfar the majority of theproduction platforms offshore are fixed, steel jacket type platforms. Of these there are probably more than 7000 in use world wide. Jackets with weights of up to approximately 18000 tons have been instal1ed Concrete gravity based; platforms are mainly found in the North Sea. Some 15 or morehave been placed with weights of up to about 1000000 tons. Installátion ofjacket platforms is most often carried out by transporting a jacket horizontally onto a
barge, towing the barge to the offshore location, launching thejacket from the barge at sea, upending the jacket by a controlled ballasting procedure and piling the jacket to thesea-floor. This operation involves structural
loads on the jacket which must beaccounted for in the design. After the jackethas been installed, the top-sides are placed on the platform by means of cranevessels which lift the top-side modules from barges andplace them on the jacket. For the design of the platform and the subsequent operations involved in installing the jacket and the top-sides thta isneeded with respect to motions, accelerations and forces on the structures
involved. In recent years much attention has been paidto the lifting and installatioin of large modules and the possibility of occurrence of impact loads during the lowering operation.
Large concrete platfonns have beeninstalled on location at sea after towing the structure from the construction site by means of a 6 to 8 or more ocean-going tugs. The tow-outofsuchhuge structures starts in a fjórd. Problemshave arisen in the past with respect tothe interaction effects between thetugs and the structure, especially in restricted channels. Items of interest during the tow-out are the towing forces and motions of the
towed platform with respect to the.horizontal:niotions in restricted channels and vertical motions andthe roll and pitch motions. In thetow-out phase such aspects ofthe behavióur are heavily iìffluenced bysuch
phenomena' as flow separation aiid vortex shedding. Lock-rn can occur between platform motions and vortex shedding which imply that non-linçar effects are important. The latter motions are of importance for the loads on the legs due to their large lengthand thehigh load ofthe already installed deck structrure on top of the platform. On location accurate placement ofthe platformis important. This operation involves concerted action
on the part ofthe masters ofthe 6 to 8 tugs. Special trainingis necessary in order to ensurethat the operation is succesfull first time around. In theinstalled condition extreme wave loads iii survival conditions and thedeck
c1earane are items ofcoñcem.
The number offloatingproduction platforms is oniy a fraction ofthe number of fixed platforms, however, from the beginning oftheoffshore, the movehas steadily been tOwards prodúction in deeperwaters. At present, designs are being developed for production ofoil and gas in waterdepths of around 2000-3000 m. In such
waterdepths, floatingproduction systen may be the only option. lithe location is near exitingpipe1ine
infrastructure, subsea completion is also an option. In such cases no floating production system is required.
Offshorefloating productionsystems have been based on semi-sübmersibles ifno storage was required.If
storage is required, tanker-based floating production systems are often selected. Nowadays, insteadof
converted existing tankers, new-build monohulls are also entering the market. After beingused formanyyears in locations with relatively mild sea-conditions, tanker-based floating production systemsare now being selected for applicatión in locations with harshconditions such as the Northern North Sea and theNorth Atlantic. The vessels and theniooring systems are required to. withstand the extremest condition (100 years survival condition) which dictate the ultimate strength of ship and mooring and also be able to stay on location for many years with a minimum of maintenance. The latter requirement has resulted in increased interest in the fatigue damage which the structures of such vessels can accumulate over longer periods of time and the measures which need to be taken to prolong the lifeof the construction. Besides interest inextreme loads and fatigue behaviour ofthevessel and the mooringsystem, the motiOns of the vessel are of interest from the point of view of theproduction process and the safety and.comfort of the crew. The occurrence of green water on deck under extreme wave conditions is an important item both from the point of view of the possibility of the deck-mounted production equipment sustaining damage as from the point of view, of crew safety. See
Buchner(1995) All of the named items aresignificantly influenced by non-linear effects due to large motions and' relative motions and also by viscous effects.
An important aspect of tanker based floating production systems iS the safety and efficiency of the offloading process whereby an export tanker is moored, either alongside the FPSO or as is probably more usual, in tandem aft of the FPSO. Such operations take place in relativelymild sea-conditions because of the potential hazards associated with accidental collision or sepaEatión of the tWo vessels. Items ofinteresthere are the relative motions of the ships and the loads in the mooring system between the vessels. The relative motions
between the vessels are influenced by interaction effects in the wiñd forces, current forces.and'wave forces acting. on both vessels and' the non-linearities in the mooring system.. Another aspect of interest here is the approach maneuver of the export tanker toward the FPSO. Even though this operation takes plâce on a regular basis, in order to ensurethe safety of this operation, it may be necessary to train' masters and crew specifically for this purpose.
A newtypeof flòating structure for whiçh iuiterestis gowing is'what is knownas the VeryLarge Floating
Structure. See VLFS(1996). These typesof.structuresare intended for use as floating airpôrtswhere thereis a
lack of suitable land, as Mobile Ofihore Base formilitary operations, or for supportirig power plants etc. The floating airports investigated in Japan have horizontal' dimensions of upto 5000mx 3000 m. The behaviour of
these sntictures represent a complex hydro-elasticproblem fòrwhich represent formidabel problems from both the pointof view of model testing and numerical analyses.
Model Tests
The above review was intended to givesome indication with respect to the data requfred in orderto design, transportorotherwise operate ships and offshore structures. in this section we will focus on the model' tests
which are being ued today forthe generation of some of the above-mentioneddata. Along with this
discussion attention will also be focussed on the experimental facilities which are needed 1h order to be able to carry out the respective tests.,
Model tests have been used for, many years as a means to generate quantitative data on many of the
aforementioned cases. For the great diversity'of real-life cases for which data is required it hasturned out that it is not possible to base these upon experiments carried out in standard towingtanks. From the fifties onward, new types of èxperimental fadiltities were designed and built which could be used to investigate the behaviour of shipsand other floating orfixed structures for such diverse aspects as manoeuvring,in open wäters,
sea-keeping behavioUr in open:seaconditions, maneuvring in harbour basins andthebehaviour of moored
structures in a wind; waveandcurrentenviromnent. In a numberof cases the facilities have ben built to
replicatecomplex sea'conditiòns including directionally spread seas, current and wind. The twentieth 1TTC
Sea-keeping committee, see HTC(1993), givesasurvey of the existingfacilities in which sea-keeping,
manoeuvring and/or offshore testing is carried out. From this review it appears that world-widè some 50 expethnentai facilitiesexist! Of these some 14 fall'into thecategory of maneuvering andseakeeping/ocean
engineering basinswith a length to widthratioofoneto about four. The resultant 36 basins are of the conventional towingtank type.
The types of model tests carried out in the experimental facilities has undergone a considerable expansion. We can categorize these as follows:
This type oftest is ainiedat giiing insightin basicphenomena reláted to specific aspects
ofthewater-structure interactiOn. Characteristic ofthese tests are that generally simplified modéls can be used (Wigley hull, for example) and thatgenerally only a small number ofquantities are measured. Often observations
play a large role in theanalysis ofthe results . See photo4. In some cases however large numbers of simultaneous measurements can be made. Based on results Of such tests, new impulses can be generated with respect to development of computational methods. See figure 5.
Correlation tests
When a computational method is availableto determine, for instance, wave loads or wave induced.motions model tests may be carried out with the purpose of validation In this case a systematic series ofmodel tests could becarrid out in regular waves or irregular waves. Measurements will mainly be restrictedto the quantities for which computeddata is available. seeAdegeest(1995).
Systematic tests
Such tests are aimed at the generation ofhydrodynamic data on thebasis of a systematic variation of the regulating parameters thus creating a 'data base' of experimental results which can used for a direct
comparison with computerpredictions., if these are available, but are primarily aimed at being incorporated in the form of empirical coefficients in software for the prediction of the behaviour ofships etc. by
numerical sitnultations. In this context we can mention systematic tests for thedetermination of maneuvering coefficients of ships, current forcemeasurements on ships, roll damping tests etc. Feasibility tests
New concepts of ships or floating structures are being developed and results of first estimates of the behaviourmay need to be confirmed before the concept is developed further. Sea conditions will generally consists of irregular waves, with or without wind orcurrent.This type of test was moreprevalent in the early days of the offshore developments than it is to-day. This is due to the increasing availability of numerical methods for the primary feasibility assessment.
Design tests
As the naine implies, these tests are aimed at producing data specifically relevant to the design of a particular vessel or operation .Emphasis is:placed on the behaviour and loads in thedesign condiiions See
photo 5. These will generally consists of tests in irregular waves for ships and for offshore structures, irregular waves, wind and current, if appropriate. Tests conditions will reflect survival conditions (100 years conditions for offshore structures) and milder operational conditiOns (10 years or 1 year conditions). The models will incorporate accurately the most important properties of the structure including
dimensions, wind area, stability, weight distribution etc. Test may include one ormore flOating or fixed structures which may be moored by means of fully modelled anchoring systems or positioned by means of fully operational dynamic positiOning systems consisting of multiple thrusters controlled bymeans of systems base4 on optimal control and thrust allocation algorithms. See photo 6. For model testing of
floatingproduction systems or of single point mooiing systems, modelling of risers or flexible hose systems may be required.
In some cases a large nuiuiber of quantities will be measured in order to validate the designvalues of a large number of critical forces or motions ofthe structure. In.such cases 30-60 measuring channels could be involved. In order to obtain statistically reliáble data, long duration model testing may be carried out. This places high demands on the testing facility with respect to thewave damping characteristicsof the beaches around the basin. Testdurations corresponding to anything from 0.5 hours to 48 hours durätion for the full scale structure have been applied. The shorter test dûration could be appliedto tests with ships in waves where wave frequency phenomena are dominant. In case of moored offshore structures, the desire for statistically reliable data on low frequency motions and forceswhich are inducedbymean and loW frequency non-linear wave and sometimes, current forces, can lead to the application of extremely long test durations.
Besides interest in extreme sea conditions and the associated extreme loads and motiOns of the structures,
nowadaysmore interest has arisen fordataon the fatigue loadingofships and structures This:has resulted in tests in relatively low sea-conditions which do not lead to high peak loadsbut which contribute
significantly to the accumulated fatigue damage over the lifetimeof the structure. Transportation and InstallatiOn tests for Offshore Structures
The design of an offshore structure is not always fully determined by the in-situ conditions but may also be inflüenced by the transportation. and installation phases Transportation can involve dry-towmethods with the structure being carried by a suitable towed or self-propelled barge. Items of interest in such cases are the extreme motions and accelerations of the transportattion barge and theoccurrence of extreme events
suchas slamming of overhanging parts ofthestructure. See photo 7. In case of deep floating concrete
gravity structures, a wet tow will be applied. In. such cases thestructure will be towed by 4 to 8 ocean going tugs. In such model tests the structure will be towed by means of the carriage of the towing tank. As the tow takes place at relatively low velocities of one or two knots, the carriage speed needs to be very accurate Speed variations of the carriage can setup unrealistic oscillations of the towedplatform. The behaviour of towed gravity platforms is oflen dominatedby flow:separation effects giving riseto the shedding of large vortices and accompanying force fluctuations On theplatform which in turn lead to oscillatOry motions mall six degrees of freedom.
The installation of jacket-type platformson location may involve at-sea launching of the structure froma
transportation barge, subsequent:upending by controlled ballastiñg and fmally the down-flooding operation to put the structure on the sea-floor. Modeltesting of these operations have been carried out involving intricate jacket models, ballasting systems, launching bargesand winches toposition the jacket inwave and current conditions. 1h recent years, the number of model tests on the launching and upending of jackets has reduced due, in part, to the gain in confidence in the applicability of computer codes.
Installation tests are sometimes carried out in order to validatea particularprocedure or sequenceof events
envisaged during, for instance,the hook-up operation of a rigid-arm-moored floating production vessel to
an articulated tower which isto serve as the mooringpoint. Model tests in such cases involve the use of winches, tie-back systems, modelling of details of hook-up mechanisms etc. The completeoperation which involves a transient winching-in procedure lastingperhaps one hourfull scale, is repeated several times under different (low)sea-conditions, wind and/or current direction in order to obtain an indication whether
the system will carry utunexpected motions or experience snap loadswhichmay jeopardize the
construction orthepersonnel. Launchingtests for ships
Ships or other structures built in a ship yard may be launched from the slipway either by longitudinal or transverse launching. Although this operation has been carried out for many years and in many shipyards, unusual circumstances may lead to model tests being carried out in order to verify the safety of this operation. This may bedue to the unusual dimensions of the structure or due to alterations made to the launch ways.
In the aforegoing a somewhat arbitrary classification of model test on ships and offshorestructures has been given along with' some indications of the items of interest, details of modelling etc. The purpose of the short review is to show that model testsare carried out on a routine basis on a great variety of real world cases involving simulation of complex sea-conditions, measuring of many quantities etc.
An aspect of importance with respect to model testing is that, not infrequently, feasibility tests or design tests yield unexpected behaviour of structures which in turn gives rise to more fundamentally oriented testing'and developmentof theory. The development of analysis andtestingtechniques with respect tosuch phenomena as
wave drift forces, wave damping, parametric rolling , to name just three cases, were greatly stimulated by the occurrence of unexpectedly large horizontal motions observed in tests in irregular waves of moored structures
in the lateseventies.
Throughout the years, besides developments in the model testing facilities with respect to size and depth, 'much effôrt has been put into improvement of the wave making equipment used to generate the sea conditions, systems to generate current and wind and the sensors used to measure motions, forces, pressures, velocities, etc. Parallel to these developments in the hardware, developments have been taking place in the methods and software used in the analysis of the measured data.
Numerical Methods
Computationsof the behaviour of structures in waves has for farthe greatest part concentrated on the behaviour of conventional ships underway. From themid fifties onwards linear seakeepingcomputational methods were developed. Most of these methods were based on the strip-theory method as first described by
Korvin-Kroukovsky (1955) Fundamentaito the linear methodsaretheassumptions ofinviscid fluid, small wave
steepness and small motions ofthe vessel ¡n relation to its draft. In the beginning onlyhead sea conditions and deep water were considered Using 2-dimensional potential theory methods, the wave frequency hydrodynainic added mass and damping of the crosseçtions of a vessel could be determined from the solution ofthe radiatiOn
potential. The use ofthe Haskind-Newmanrelationship made it possible to determinethe wave forcingbased ôn knowledgeofthe undisturbed wavepotential and theradiation potential without having to solvethe wave diffraction problem. Combining the added mass and dampingproperties forthecompletevessel with the rigid
body mass andhydrostatic spring properties on the one handand the wave fOrces on the other hand resulted in the frequency dOmain equations of motion. Solütion ofthese,frequency dependent, equations ofmotions yielded the rigid body motions of the vessel. Throtighout the last decades manyadrptations andversions of strip-theory ship motions predictors havebeen developedand have been applied successfully ¡n the evaluation
ofthebehaviour of shipsinwaves. Besides the rigid body motions, computations can yield such quantities as the bending moments and shear forces incrossections of the hull, relative motions etc. M item of special
interest is the added resistance in waves. Using strip theory , reasonably accurateprediction of this quantfty can
be made based on the method ofGerritsma,& Beukehnan (1972),see figure 6. Thestrip4heory methodhas
been extensively validated by comparisons with resùlts of model tests. Many extensions to the original method were put into effect such as the inclusion of oblique waves through which not only heave and pitch motions. but
also the sway, roll and yaw motiònscouldtbe predicted. In the case of the roll motions, however, empirical.data' on the roll damping of the vessel has to be included since potential flow theory does not include viscous effects which are of importance forthe roll motion& Such empirical data on the roll dampinginvariable is based on results of model tests since no accurate theoretical model is available to determine this effect. Strip-theory methodsas originally 'developed, is valid for high frequencies, i.e. it is suitable for predicting ship motions in head wave and' oblique waves as long as the frequency of encounter is not too low. Such methods are not capable of predicting. accurately the behaviour of a ship in stern seas when frequencies of encounter approach
zero. In. recent times, so-called"unified.strip-theorymethods" have beendevelopedby forinstanceKashiwagi
& Ohkusu( 1993) and Kashiwagi( i 995.) which also make it possible to predict the motions in stem seas. Stiip'theory methods arepopular due to the consistently accurate predictions of ship motions in waves, the relatively high availability of computer codes and ease of use The limitations to strip theory methods did however become more evident, as the number of comparisons with results of model tests increased, and as is general the case with extended application of such methods, accuracy demands increased, the attention became more focussed on the details of the flow around the ship. In recent years this has led to 'the development of computational methods based on linear 3-dimensional potential theory which included the effect of forward speed in a physically more correct way. See, for instance , Nakos k Sclavounos( 1991). These methods are based on the use of a source distribution on the hull of the vessel and on the free-surface near to thevessel. See figure 7. Each source is' homogeneously distributed over a triangular or rectangular flat panel. The radiation
and diffraction potentialsaresolved based on boundary conditions on the mean wetted hull of the vessel and the mean free-surfacewhich take into account the forward speed of the vessel through the solution of the double-body flow. In such codes the simple Rankine source which satisfies the Laplace equation butnot the linear homogeneous freesurface or a radiation condition at infinity, is used. It is for this reason that the source distribution is also applied to the free-surface. Three-dimensional methods, also known as Rankine Panel Methods, have made it possible to determine in greater detail the flow about the hull of the vessel .This has led
to improvements in the quality of the motion predicitions as is shown in figureS. Due to the computational load, the number of source panels on the free-surface near the vessel is limited. This means thatthe effect of the vessel on the wave field can only be computed close to the vessel. Rankine Panel Methods have been
developednot only for conventional ships, but also for spçcial: cases such as Surface Effect Ships in Which the code also takes into account the interaction between the'aircushion situated between the catamaran hulisand
the'waves. Seefor instance, Moulijn(l998) An examples of theresults of such computations are shownin
flgure 9 and figure 10.
Methods based on linear 3-dimensional potential theory have also been developed using a distribution
consisting of travelling sources which satisi' the linearized free-surface condition and the radiation condition.
See for instance Chang(l977). Such methods donotrequire panels on the freesurface However, due to
numerical problems such developments havenot: found wide application as yet.
For ship- or barge-shaped offshore structures which often operate in the moored condition, the same linear strip theory methOds are used to predict the motions at zero speed as are used forship underway. Journee(199l) has shown that liñear strip theory methods, which are based on the assumption of slenderness of the hull, can stil give reasonably accurate'results for small length/breadth ratios of moored barges. For the case of moored semi-submersibles a special type of strip theory method was developed by Hoofl(1972). In this method, theadded mass and damping and the wave forces on the structure is computed based on a simple summation of the corresponding quantity as computed on elements of the structure, without taking into account hy4rodynamic interaction effects. For the computations the structure is Subdivided in typical elements such as the columns and pontoons. The added mass anddamping of such elements is determined from potential theory results for cylinders, spheres etc. Wave forces on each element are computed based on the same data and' the application of the relative motion concept. Such methods yield consistently accurate results on the wave frequency motions of themore slender type of.semi-submersible.
In theearly seventies, new 3-dimensional frequency-domain potential' methods weredevelóped for the computation of wave frequency motions of arbitrarily shaped floating structures and' large volume fixed structures atzero speed. For this typeof structure, the slenderness assumption which applies to ship-shapes does not apply and 3-dimensional effects must be accounted for in a proper manner. Besides the work of
Garisson & Chow (1972) thework ofBoreel(1974)and ofvan Oortmerssen(1976)should be mentioned in this
respect.
The zero-speed three-dimensional diffractiOn method is based on linear potential: theory assuming invisid ,incompressible, irrotational flow. The meanwetted surface ofa fixed orfloating body is described by a number of flat rectangular or triangular panels representing a source distribution. An example ofthe panel model of a large structure in shown in figure i 1. The source formultation chosen (Green function) satisfies Laplaceequation, the linear free-surface condition,the no-leak.condition of the horizontal sea floor and the radiation condition at infinity. The unknown source strengths, or potential depending on the specific type of formulation chosen, aresolved taking hito account.the no-leak condition on the mean wettedhull of the structure. Radiation and wave diffraction potential solutiOns are found and yieldtheadded mass and danping and the wave forces. In caseof.a floating vessel,motions are found by solvmg the equations of motionthe
frequency domain. Subsequently, such quantities as local pressures, velocities, waveelevatións etc. can be computed Animportant feature of such codesis the possibility of computing mean and slowly varying;second orderwave drift forces. Mean drift forces can be computed by means of the far-field of Maruo(1960) or
Faltinen&Michelsen(1974).or the near field method of Pinkster(1980). Someresults of measured and
computed mean drift forces are shown in figure 12. Early on in thedevelopment of this very general method, extensions were made to include, among others, a multibody capability. This allows the simultaneous evaluation of the behaviour of two or more, different bodiesin waves, including,hydrodynamic interaction effects such as wave shielding etc.
This 'type of code has become a standard analysis tool fôr the behaviour of offshorestructures and many variants have been developed by different groups of researchers. Much ofthepioneering work was carriedout at MARIN in the seventies, especially with regard toe validation bymeans of model tests of both the wave frequency forces and behaviour as the mean and slowly vaiying drift forces and the associated low-frequency motion behaviour of moored.structures. The: most widely used computer program is the WAMIT code, developed by MIT (Newman( 1985)).
Following .the introduction of linçar strip-theoiy as a meansto computethe motions of ships in waves of small to moderate steepness,, the need soon arose forcomputational methods which were more capable of taking mto account large amplitude motions and waves in order to be able to improve assessments with regard to extreme behaviour. Since large 'amplitude motions involve, among others, largechanges in' the wetted shape of the hull through for-instance, bow emergence-or green water on dçck, tithe dômain, non-linear solùtions were
developed. Initially, many of these developments were an extension of the strip theory method. Also taking: into account the hull shape above water, the equations of motions of the' ship were integrated in time using the wave forces found from direct integration of the hydrostatic pressure and the undisturbed wave pressures over the instantaneous wetted' hull form (Froude-Kriloff component) and addition of dynamicterms basedon the
added mass and damping found from linear frequency domain computations. Such methods have shown to be very usefull in determining for instance, non-linear effects in wave loads in the ship hull. See figure 13. Keurnng(1994) has applied non-linear time domainmethods to the analysis of the behaviour of fast planing craft in waves. Results of simulations have confirmed that extÉeme vertical acellerations ofsuch vessels can be very much out of proportion to the significant values. This has led to the conclusion that for this type of vessel, motion analysis based on linearmethods could not be used for a proper evaluation of workability limits of such craft in waves. See figure 14.
In recent years, non-linear ship motion programs based on three-dimensional potential theory methods have been developed.. See for instance Lin, Meinhold & Salvesen (1994) or Beck& Magee (1991). Due to the compexity of the physical problem of large motions such.as the occurrence of slamming andgreen wateron
deck, somerestrictions have to be introduced inorderto make the problemanienable to computation. For
instance,no wave breaking is allowed in such computations (Súbramani,Beck & Schultz (1998)). The non-linearitiesshould be present not only in the motions butalso in the boundary conditions forthe potential flow asspecified on the free-surface and on the hull of the vessel. In some codes, only thenon-linearity on the hull surface may be included while the linearised boundary condition is retained for the free surface. With most non-linear codes the computational burden associated with determination of the motions for any length of time can be quite severe. For instance, in Lin,Meinhold & Salvesen(1994) it is stated that running the non-linear code LAMP-4 for aduration corresponding to 30 minutes full scale timewas "impractical" even though this
code is run on asupercomputer. It.should bestressedthat runnmg a non-linear time domaincode to determine
ship motions and loads implies that statisticalmodels for the quantities of interest cannotbe borrowed from linear theory. In order to determine, for instance, the statistics of the extremes of loads and motions, which is often what is needed for design evaluation, non-linearcodes have to be run fortünes corresponding to a considerable portion of the life of a ship. Clearly, at this stage, this is truly "impractical" and statistics will have to be obtained by other means. In this respect the work by Adegeest (1995) represents a different approach to this problem. In this work, a first step wastaken to translate non-linear behaviour from short timesimulations in regular waves todistribution functions of extremes in irregular waves.
At the present stage, resulta of non-Imear time domain simulation methods are generated for relatively short time periöds Such simulations serve to clari!' specific aspects related to the behaviour ofa ship or to the loads. See de Kat& Paulling( 1989) or de Kat(1 994) regarding research on the capsizing of ships in extreme seas. Photo 8 shows a snapshot of a computeranimation of the broaching behaviour of a frigate in extremewaves.
The aforegoing short overview cannot do justice to the many excellent developments that haveand are being carried out in the field of computational methods for the prediction of the behaviour of ships and other floating
structures in waves. The reader is referred to, for instance, Beck,Reed& Rood(1996) or Bertram & Yasukawa (1996) for an overview of numerical methods It does however, cover those types of computer programs which, at this point in time,, can be considered as being "state-of-art" either by the industry in general.or by the research community. The ship designers consider linearship motiöns prediction methods based on strip theory as such and forthe offshore industry also 3-dimensional diffraction codes represent state-of-art.
It should be realized that in all of the above mentioned methods, empirical data mostly obtained from model tests are necessary to account fòr significant physical effects which are not reproduced by potential theory methods. Specifically, non-linear viscous damping effects in the roll motions of ships, the heave, roll and pitch motions of semi-submersibles need to be accounted for if resonance frequencies are encountered. See for instance Dallinga, Blok & Luth (1998). FurthermOre, concerning the prediction methods for ships; considerable data has been generated to validatethe codes for behaviour in head seas. Motions in stern quartering or stem seas have not yielded consistently reliable results to date;
As indicated in the introduction to this paper, many real life situations involving ships or other floating equipment revolve around the behaviour of more than one vessel. Offshore loading operations or cargo transfer between ships are examples of such operations. For such specificcases, computational methods developed for single ships are used as a basis for computerprograms involving acomplex combination ofone ormore ships combined with, for instance, non-linear moonng systems or riser systems orboth. See for instance Wichers(l988) andHuijsmans(1996)..Such developments meet the needs of industry for analysis methods for complex situations in the design stage or as a means to evaluate the safety of new operations with existing equipment. Only limited information on thevalidation of such codes are available in theopen literature since development is often carried out within the context ofjoint industry research programs; With respect to theprediction of the behaviour of a single moored off hore.structurè, Herfjord&Nielsen( 1991) report on the comparison of computed motions of two types of structure, a deep draft floater and a monohull floater. Computations were carried out by a number of institutes using mostly linear 3-dimensional diffraction codes. It was shown that the wave frequency motions were generally welIpredicted. Lowfrequency motions due to second orderwave drift forces showed large differences. See figure 15; Thedifferences could mainly be contributed to low frequency viscous damping effects not accounted for in the computational methods. It can be concluded from such results that much remains to be done in this field before consistent results can be obtained entirely on the basis of computatons.
As indicated in the aforegoing, most computational methods in use to-day for the prediction of the behaviour of ships in waves are based on potential theory assuming inviscid, irrotational flow. Many of the shortcomings of such methods have, based on comparisons with results of models tests, been attributed to the neglect of viscous
effect& This has prompted.the development of viscous flow codes, initially for predicting the stil water resistanceand flowcharacteristics of ships withot1t;freesurfàce,effects,and morerecently also taking'into account the :free..surface. To date, no codes haveappeared by means of which it is possible to compute the motions of ships in waves by means of such codes Armenio('1994) has 'applied such codes totheprediction
of
the large amplitude motions of fluid in an anti-rolling tank.The computational methods are basedon the Reynolds-Averaged Navier Stokes'(RÂ1S) equations for the flow of a fluid. An iinportant aspect of such codes is the turbulence model used to describe the average behaviour of the flow at the smallest scale of the computational grid. Numerical computations require modelling of the complete fluid domàin around the vessel. This leads to very large systems' of equations which need .to be solved. Results obtained by such methods are encouraging but.cannotat this stage be considered as being state-of art in the sense that they are being applied generally.
Oneofthefutureareas of application of such codes will bethe field of maneuvering'ofships. Untilnow, mathematical models for maneuvering.ships arefor the most part based on experimentallyobtained dàta on the
hydrodynamic forces. Extensive 'model test programs need to becarried out in ordér toadequately describe maneuvering characteristics of a given ship. Consequently, such data has only been generated for a limited number Of vessels maneuvering:characteristics of aparticular.ship generally being determined from free-running model tests rather than fromsimulatión computations. Thedevelòpment of reliable andpractically usable viscous flow codes will.allow the preditión of many of the hydrodynamic maneuvering forces on a ship even though this is likely to be at the expense of huge computational loads.
DiscussiOn
In the previous sections a brief overviéw has been given of the 'state-of-art' in model testing and in the development of computational methods with respect to the behaviôur of.ships and.other floating structures in waves. In this section we will try to highlightsome of the conclusions which can be drawn with respect to these
developments.
Thenumerical' developments havemainlybeen concentrated on the behaviour of the single unit and more specifically on its motiOn behaviOur, waveloads and added.resistance.,Much attention has been paid to the comparison between results of computations and experiments, in deterministic seaconditions consisting of
regularwaves. In all these comparisons,.modeltestresults are considered to bethestandardby which
computations are judged. Comparisons with full scale experimental data is scarce and is generally considered to be unsuitable as a basis for judging the validity of numerical methods.
Non-linearpotential methods for the predicition of the behaviour of a ship arestill in their infancy.'Large amplitude motion programs have been developed which,yield valuable insight innon-linearities but theseare
all restricted to the case ofnon-breaking wave, no slamming, and no water on deck. Even with these restrictions, empirical data have to be used to account for important effects such as roll damping. Viscous flow computational methods, also known as CFD, are starting tè be developed and applied as a research tool. Some methods have reached a stage of mattirity which permits their use as a tool fôr preliminary evaluation of, specifically, viscous ship resistance and stem flow in stil water. Accuracy is stil, however, insufficient to replace experimental data.
Fewnumerical methods treating the interaction of aship with its surroundings in a consistent way have been developed. Some methods such as the linear three dimensional diffraction codes, allów the evaluation of the interaction of one or more floating or fixed bodies in waves. Non-linearities have been introduced in such aspects as mooring systems but no methods taking into account non-linearities in the flow have been developed to date
Even with these restrictions, it isclear that the interest in numerical methodsis increasing. This isnot only
based on the costaspectcomparedwithexperiments, butmainly from the fact that in the design stage, even
relatively simplenumerical methods, when used knowledgibly, can contributegreatly to therealisation of
effective designs for ships and floating structures in ashorter space of time andat less costs. This is the driving force with respect to the interest shown by designers for software developments. Designers can be interested in stand-alone software to evaluate, or instance, ship motions in the frequency domain. On the Other hand there is also interest in integrated software, able to evaluate in the timedomain complex operations such as the
behaviour'oftandem mooredtankers coupled to a turret mooriiigsystem and incorporatinga large heading
angle change possibility coupledwith modelling of dynamic effects in mooring chains. It is clear that in terms of design oriented software, different levels of sophistication and detail should be available for assessing a design or an operation
The success of codes such as WAMJT underlines the desireof industry to make use of generallyvalidatedand accredited software Validation in this context invariably involves comparison with results of model tests. With the development of CFD methods, this not only places demands on the numerical methods, but also on the experimental procedures which must be developed in order to be able to make comparisons at the detailed level at which the numerical results are generated.
Model tests of structures in waves have evolved from simple tests iii regular waves to determinemotión RAOs etc. into complex operations which can involvethe simultaneous modelling of more than one body in a wind, wave and current environment involving high sea states and allowing long duration tests aimed at developing statistics of measured datarelevant for direct appliCation to design. Significant advances have been made hi the development of, for instance, dynamic positioning systems for the control of models of DP-vessels These allow complete simulation of such vessels inclUding working thrusters systems controlled by realistic
DI'-algorithms. In this way all relevant hydrodynamic interaction effects between thrusters, the hull ofthe vessel, the effects:ofwave, wind andcurrents can be incorporated.
The design ofmodel basins hasalsoshown important advances iii the last decades. Conventional towing tanks which were the standard until the late fifties,.have dimensions which were dictated by the requirement of minimal blockage effects for the transverse dimensions and the model speeds for the length. They were generally equiped only with a towing carriage with the purpose of carrying out resistance and propulsion tests, and at a later stage, wave makers forcarrying out tests in regularheador;stem seas. The latest designs, being
the new Seakeepingand Offshore facilities ofMARIN, the contruction ofwhich wilItake place in thenext two
years, are: sophisticated, high-tech multi-functional facilities See figure 16 The Ofthhore Basin incorporates features as variable waterdepth by remotely operated rigid floor and an advanced, high capacity current
generation system. Both facilities areequiped with multi-directional wave makers and measuringsystems using high resolution, multi-body, optical motion tracking;systems, andhigh capacity data.transmissionand
acquisition systems based upon the use of, among others, fiber optics. Alsonew concepts of testing procedures and execution, data and analysis and reporting are beiñg incorporated thus leading to improved client service. At the high end of the experimental goals, such facilities will further extend the capability to carry out complex model tests involving multiple systems consisting of for instance, dynamically positioned floating vessels coupled with subsea equipment in realistic directional irregular wave, instationary current and instantionary wind conditions with the simultaneous measurement of large numbers of signals such as pressures, motions, forces etc. .The quality of the wave making equipment and the basin will allow exteiïded test duration under stationaryconditions thus making itpossibleto obtain reliable statistics of non-linear behaviOur on a.routine basis. There are still a numberof developments in thefield of experimental facilities and techniques which need to be adressed in the future. For example, at the lower end of the scale of experimental sphistication, in the area of systematic tests aiinedatgenerating basic hydrodynamic data, for instance for incorporation in numerical models for maneuvering of ships, developments in the field of process control need can be used to make :itpossible tocarry out systematictests làrgely in an automated environment allowing uninanned continuous operation of the basin. Coupled with automation in the field of data reductión, this will further
increase efficiency of operation of the experimental facilities, help to reduce costs of turn-around time and costs associated with such tests making them more attractive to prospective client&
It should be stressed that not only the capabilities of the experimental facilities or the numerical codeare of deciding importance with respect to their uses. At least as important is the realization that the personel making use of these capabilities are knowledgible with not only the operation of the facility or code, but also with respect to the fundamental assumptions, area of application and the limitations. With respect to experimental facilities this involves not only the knowledge and experience of the basin technicians but of the complete chain of personel from drafting officeto data analist including the scientific staf. With respect to the increasing
application of more sophisticated numerical methods in the design.phase by staf of engineering offices or by others on behalf of the design team, as also pointed out by Beck et al( 1996),, management willhave to take steps to ensure that staf have sufficient expertise, notnecessarily to run software, but at least to be able to communicate meaningfully with those who :do so on their behalf. This involves a concious effort, on behalf of management, to come to decisions with regard to the continuing maintenance ofup to date expertise of staf members and to decisions whether specialised software shall continue to be acquired as it becomes generally available or whether such capabilities are to be left to specialist suppliers as is thecase with model testing.
References
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Beck,R.F., Reed,A.M.,Rood, E.P.(1996):"Application of Modern Numerical Methods inMarine Hydrodynamics", SNAME Transactions, Vol. 104, 1996, pp. 519-537
Beck,R.F., Magee,A.R.(l991):"Timedomain Analysis for Predicting Ship Motions", Dynamicsof Marine Vehicles and Structures in Waves, W.GPriceet al. Elsevier Science Publishers B.V. 1991
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