Aspects of model tests and computations for ships and other structures in waves

Aspects of model tests and

computa-ti'ons for ships and other structures

in waves

J.A. Pinkster

(TUDeift)

S.G.Tan

(MARIN)

Report 1167-P

September 1998

Projectllr. 962

Plenaiy Session Lectures of the Seventh International

Symposium on Practical Design of Ships and Mobile

Units, PRADS'98, The Hague, The Netherlands,

Sep-tember '98. Edited by M. WC. Oosterveld & S. G. Tan

TU Deift

Faculty of Mechanical Engineeringand Marine Technology

Ship Hydromechanics Laboratory

il

Practical Design of Ships and

_Mobi1eUnits

-Plenary Session Lectures of the Seventh International Symposium on Practical Design of Ships and Mobile Units, The Hague, The Netherlands, September 1998

Edited

M.W.C. Oosterveld and S.G. Tan

MARIN - Maritime Research Institute Netherlands, Wageningen, The Netherlands

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PREFACE

The,7th:Intemational Symposium on Practical Design of Ships and Mobile Units, PRADS '98, is held from 20-25 September 1998 in The Netherlands Congress Centre in The Hague. Earlier PRADS

Conferences have been held in Tokyo, Seoul, Trondheim, Varna and Newcastle.,

The interest in the PRADS Conferences is growing and this time over 200 proposals for papers were received The final programme includes 126 papers of high quality which are published in the Proceedings of the Conference by Elsevier in the series Developments in Marine Technology'. 250. participants from

30 different countries will attend the Conference. The largest delegations come from The Netherlands, Japan Germany, Korea, Italy and UK.

Aspect of the Practical Design of Ships and Mobile Units considered in the selection of the papers were Design Synthesis, Production, Hydromechanics, Structures and Materials, and Offshore Engineering.

The organization of PRADS was supported by MARIN, Deift University of Technology, the Netherlands Organization for Applied Research, the Royal Institute of Engineers in The Netherlands, the Royal Netherlands Association of Maritime Engineers, and the Royal Netherlands Navy.

During the OPENING & PLENARY SESSION of the Cönference:

Ir J.Jf2.M. van Doorernalen, President Board of Directors, IHC Holland

will welcome the participants on behalf of the Netherlands Maritime Industry and the sUpporting

Organizations. Mr Van Dooremalen is.a member of the. Controlling Board of MARIN anda member of the Association of Shipyards in The Netherlands.

Representatives of the supporting organizations formed together the LOCAL ORGANIZING

COMMITTEE. They played a major role in the choice of the Technicál Themes and the Paper selection. In the Plenary Session the members of the LOCAL ORGANIZATION COMMiTTEE will give their vision about the main themes of the Conference. These Plenary léctures are included in this booklet.

PRADS Local Organizing Committee:

Dr M.W.C. Oosterveld, Chairman

Ir S.G. Tan, Secretary Prof. Ir A. Aalbers Ir G.T.M. Janssen li P.J. Keuning Prof. Dr J.A. Pinkster Mr J. Veltman Prof. Dr J.H. Vugts

M.C.W. Oosterveld and S.G. Tan Editors

- MARIN - MARIN

- Delft University of Technology

- Netherlands Organization for Applied Research - Royal Netherlands Navy

- Delft University of Technology Royal Institute of Engineers

- Royal Netherlands Association of Maritime Engineers

- Royal Institute ofEngineers

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CONTENTS

Page

Safety and Ship design A. Aalbers

Production of Complex Ships 10

A. Aalbers

Aspects of model testsand computations for ships and structures in waves I8

J.A. Pinkster and S.G. Tan

Some considerations on design of ship structures and materials 31

G.T.M Janssen

Offshore technology in perspective 37

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Aspects of model tests and computations for ships and other

structures ¡n waves

a b

J.A. Pinkster and S.G. Tan

a

Deift University of Technology, Ship Hydrodynamics Laboratory

Mekelweg 2, 2628 CD Delft,-TheNetherlands

b

Maritime Research Institute Netherlands, Research and Development Department

P.O. Box 28,6700 AA Wageningen, The Netherlands

1. INTRODUCTION

Knowledge on the loads on, and the behaviour

of ships and other floating equipment in Waves may be required fär a variety of reasons related to the structural design or the operations of such

structures; See Figure 1.

SHIP CHARACTERISTICS SEAKEEPING TOOLS SEAKEEPING CHARACTERISTICS J. DESIGN ASSESSMENT OPERATOR GUIDANCE SEA CONDITIONS OPERATIONAL CRITERIA

Figure 1. Seakeeping performance analysis

Regarding structurai 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 the other hand the fatigue loads leading to the accumulation. of fatigue damage.

The expected extreme loads are the prime input

to the structurai design of a vessel, besides of

course the input based on the operational demands

placed 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, i.e. that loads and motions are not a linear

function of the wave amplitudes and that

the

frequencies present in the wave lOads contain super-and subharmonics of the wave frequencies, can become important.

Expected fatigue loads tend to have an impact

on details of the structural design and will not dictate the overall structural design to the same

degree as the extreme loads. Increasingly, however,

information, on fatigue loads is required as the

structures tend to become lighter and less material

is used. This requires that statistical data on. the frequency distribution

of load

oscillations be

developed for short and long term. Short term in

this sense being the statistics related to a particular sea-condition and long term being the statistics of

the loads as' related to the lifetime of the vessel.

2. EXTREME BEHAVIOUR OF SHIPS

Besides information on' the loads due to the continuous action of waves on the structures of

ships, information is required on the loads due to extreme events such as slamming and green water. The loads due to slamming are highly non-linear when related

to the wave amplitudes and

the

frequencies associated with the load oscillation after

an initial wave impact are related more to the

frequencies of the vibratory modes than to thewave

frequencies.

Higher sea-states and larger ship motion 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 1.8

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are potentially so much more dangerous for the

crew the ship and its cargo Therefore riot only is

the probability of occurrence a focal point but increasingly attentión is being paid to the actual' behaviour of water on deck and to the effects in

terms of water- heights, velOcities--on- deck and impact pressures on superstructures. Research in this field is aimed at, among others, more rational

design of wave breaking barriers on deck.

For ships at sea the influence of waves on the

resistance and propulsion characteristics are of

importance from the point of view of economy and

the time taken to reach the destinatiOn The

resistance characteristics of a ship are traditionally based on the stil water resistance with corrections for the in-service condition of the hull taking into

account in an approximative way the resistance increase in waves. Nowadays, more detailed information on such effects are required in the

design stage in order to be able to asses influence of changes in hull form, loading condition, course and speed etc. on the speed loss in waves. See Figure 2,

Blok (1993).

Motions of ships in waves can be influenced significantly by non-linear effects. A well known aspect in this sense is the rolling of ships in waves

coming from off'bow, off-stern or beans directions. Another effect which has become more important in

recent times is the occurrence of parametric rolling of ships which can occur in head or stern seas. See

Dallinga et al. (1998). This phenomenon is

especially of importance for cruise vessels. See Figure 3.

Broaching effects of ships in stern or stern-quartering seas have always been of concern to

designers and operators. The behaviour of a ship under such conditions is a complex combination of the effects of wave-induced motions and forces on

the one hand and ori the capacity of the vessels

steering system to counteract these forces and the lateral stability of the vessel on the other hand The onset of broaching and the subsequent behaviour in which large yaw acceleratibns and course changes

along with large roll motions can occur are elements

of-highly complex and non-linear flow phenomena.

See, for

instance, De Kat

(l994). Broaching.

becomes of greater importance as the speed of ships increases. As such, the arrival of large, fast

passenger ferries in seaareas 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 occur when fast vessels, of the catamaran

type, which have relatively small waterplane areas,

travel ät high speeds in stern or stern-quartering seas.

With respect to high-speed craft, planing or

semi-planing craft form a group on their own. The

behaviour in waves of these vessels is heavily

influenced by non-linear effects. At high speeds, high acceleration levels may be reached which can lead to structural dàmage or personal injury of crew

and passengers.

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1.0

OC80.45 (Friqate) OAO

-ACßOSS (Ferries) 0.25<0 ir <0.50 a C8 0.85 (iTankers) :Au 0. SO<a/ 0.8' 8 0.6 D ₆ 0.4 o

-0.2

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WAVE 3 m PITcH dog pou. dog FIN ANGIE dog I5 1.50-15.00-. o 0UgO ANGLE

'5°L-Figure 3. Parametric rolling in stern waves. Dallinga et al (1998)

3. FLOATING OFFSHORE EQUIPMENT

The exploration for oil and gas deposits and: the subsequent field developments which took place

demanded new floating equipment for the purpose of carrying out new and novel tasks. Many of the aspects of working at sea made use of knowledge and experience gained from the operations of ships

and other existing floating equipment. However, the

scale and diversity of the activities also demanded equipment and methods to be developed for which no precedents existed. This led to new procedures

for the design of floating equipment which rely much more on the application of knowledge of

fundamentals regarding hydrodynamics, strength of

materials and structures and assessments 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 guide lines and rUles

Floating offshore equipment is required to

work in a variety of conditions and for a wide range

of applications.

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-Early on in the life-cycle of an offshore oil or

gas field, exploration drilling follòwed by early; production may be carried out using a semi-submersible drilling rig which is moored on lòcation by means of anchor lines, or, in case of deeper water, by means of dynamic positioning.

The wave-induced motions will be of interest from the point of view of the drilling operations. At the

design 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 and drag effects which, beside containing constant parts also contain sub- and superharmonic fòrce fluctuations associated with flow instabilities. Wave

forces consist of wave frequency force fluctuations leading to the well known wave frequency motions

and also cOntain mean and low frequency

components which contribute to the mooring loads.

The latter force components contain non-linear

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components which are both of potential as of

viscous origin as confirmed by Dey (1996). See also Figure 4. a

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_b,0 o0,0 'r Legend: e -Exp. data Fitted line Theory 0.. d. to i' o

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Figure 4. Measured mean drift forces -and

calculated values based on potential theory for a vertical cylinder.

Chakrabarti (A OR, Vol.6, Nr.2, 1984)

The low frequency motion responses of such moored vessels are affected by the often non-linear

restoring and damping characteristics of the mooring system and by hydrodynamic damping

effects- which are of mainly viscous origin. Due to

the low natural

roll and pitch periods of such

structures, non-linear low frequency wave forces

can induce significant angular motions even at frequencies

outside of the range of the wave

spectrum.

An item of interest in semi-submersible type structures is the occurrence of wave impacts on the

underside of the deck. The probability of this

occurring is influenced by non-linear behaviour of

the waves as seen in, for instance Wave run-up against the columns of such structures

Drilling operations may also be carried out using drillships. Such vessels can be moored on location by means of conventional spread moorings,

by means of turret-type moorings or, as is often the

case with deep Water explOration, by means of

dynamic positioning. The operational efficiency of

such vessels is influenced by the roll, pitch and

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heave motions of the vessel at the location of the

drilling tower and- by the horizontal motions as dictated by the environmental lOads and the

mooring system. The environmental loads again are

due to combined effect of waves, wind and current with the main dynamic part coming from the waves

with respect to thé wave frequency motions. In case of large ships in high sea states, low frequency

wave-induced non-linear horizontal motions and mooring forces may be governing factors for the

design of'the mooring systems.

In case dynamic positioning is used, the

complèx interaction of the thrusters acting under the

vessel, the vessel hull and the fluid motions around the ship as induced by waves and current needs to be taken into account in the- design process of the

DP-system.

After being used for -many years in locations with relatively mild sea-conditions, tanker-based floating production systems are now being selected for application in locations with- harsh conditions

such as the Northern North Sea and _the North

Atlantic. The vessels and the mooring 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 life of_the constrûction. Besides interest in extreme loads and fatigue

behaviour of the vessel and the mooring system, the

motions of the vessel are of interest from the point

of view of the production 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 are significantly influenced by

non-linear effects due to làrge absolute and -relative motions and also by viscous effects

A new type of floating structure for which

'interest is growing is What is known as the Very Large Floating Structure. See VLFS (1996). These

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types of structures are intended for use as floating

airports where there is a lack of suitable land, as

Mobile Offshore Base for military operations, or for

supporting power plants etc. The floating airports investigated in Japan have horizontal dimensions of

up to 5000 m x 3000 m. The behaviour of these structures represent a complex hydro-elastic problem which results into significant problems

from both the point of view of model testing and

numerical analyses.

4. MODEL 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 has

turned out that it is not possible to base these upon

experiments carried out in conventional towing

tanks. From the fifties onward, new types of

experimental facilities were designed and built

which could be used to investigate the behaviour of ships and other floating or fixed structures for such

diverse aspects as manoeuvring in open waters,

sea-keeping behaviour in open sea conditions, manoeuvring in harbour basins and the behaviour of moored structures in a wind, wave and current environment. In a number of cases the facilities have been built to reproduce complex sea

conditions including directionally spread seas,

current and wind.

The twentieth ITTC Seakeeping committee, see

ITTC (1993), gives a

survey of the

existing

Offshore Basin

Seakeeping Basin

Figure 5. New experimental facilities MARIN

facilities in which seakeeping, manoeuvring and/or offshore testing is carried out. From this review it

appears that world-wide some 50 experimental facilities exist. Of these some 14 fall into the category of manoeuvring and seakeeping/ocean

engineering basins with a length to width ratio of

one to about four. The other 36 basins are of the

conventional towing tank type. Recent examples of modern experimental facilities for seakeeping and

offshore research are the basins at present being constructed by MARIN. See Figure 5.

The types of model tests carried out in the experimental facilities has undergone a considerable

expansion. These range from basic tests aimed at

identifying fundamental physical effects to

elaborate sets of experiments involving one or more floating bodies in realistically simulated wind, waves and current conditions and the measurement of many signals. Time durations of model tests can range up to several hours when reliable statistical data on such effects as low-frequency motionsare

concerned.

An aspect of importance with respect to model testing is that, not infrequently, feasibility testsor design tests yield unexpected behaviour of

structures which in turn gives rise to more

fundamentally oriented model testing and

development of theory. The development of analysis and testing techniques with respect to such

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

late _seventies

I Towing carriage

2 Sub carriage

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Mufti directional wave generators

H sign = 0.40si (max)

Rapwklth= 0.40m Beaches

Current (maximum at surface 0.25 rn/sec)

'tMovabie bottom

i Towing carriage, maximum speed6rn/sec 2 Sub carnage4.5 rn/sec

Mufti - directional wave makers

H sign= 0.40m (max), segment flap width =0.60 rn

- Beaches

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5. NUMERICAL METHODS

Computations of the behaviour of structures in waves have for the greatest part concentrated on the behaviour of conventional ships underway. Most of

these- methods--were based on the strip-theory

method as first described by Korvin-Kroukovsky

(1955) and were applied successftlly in the

evaluation of the behaviour of ships in waves. Fundamental to the linear methods are the

assumptions of inviscid fluid, small wave steepness

and small motions of the vessel in relation to its draft.

Besides the rigid body motions computations can yield such quantities as the bending moments

and shear forces in cross sections of the

hull,

relative motions etc. An item of special interest is the added resistance in waves. Using strip theory, reasonably accurate prediction of this quantity can

be made based on the method' of Gerritsma &

Beukelman (1972), see Figure 6.

10.0 7.5 5:0 2.5 o 0 - Gemtsmalßeukelman!méuicd u Expenments o( Nakamura o Expenments ofFujii Cantainership = 180g Fn.= 0.15 s u e 0:5 1.0 1.5 2.0

Figure 6. Added resistance in waves

Gerriisma & Beukelinän (1972)

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The strip-theory method has been extensively

validated by comparisons with results of model tests. See, for instance, Journée (1991). 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 motions could be

predicted In the case of the rol! motions, however, empirical databasedon the results of model tests on

the roll damping of the vessel has to be included since potential flow theory does. not include viscous effects which are of importance for the roll motions. The limitations to strip theory methods' become

more evident as the number of comparisons with

results of model tests increases. Moreover, as is

general' the case with extended' application of such.

methods, as 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 computatiónal' 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 & Sclavounos ( 199 1 ). These methods are based on the use of a source distribution on the 'hull of the vessel and on the free-surface near to the vessel.

This has led to improvements in the quality 'of the

motion predictions as is shown in Figure 7. Such, so called Rankine Panel Methods have been developed

not only for conventiónal ships1 but also for special cases such as Sürface Effect Ships in which the code also takes into account the interaction between the air cushion situated between the catamaran hulls and the waves. See for instance, Moulijn (1998) and

van'.t Veer (1998). An examples of the results of

such computations are shown, in Figure 8.,

Following the development and application of

linear potential theory based methods as a meanS to

compute the motions of ships in waves of small to

modérate steepness, the need soon arose for

computational methods which were more capable of taking into 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, large

changes in the wetted shape of the 'hull through for instance, bow emergence or green water on deck,

time domain non-linear solutions were developed.

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0.10 0.00 -.10 -0.20 030 0h40 0.30 0.20 0.1 0 0.00 0.20 0.10 0.00 -0.10 010 0.00 -0.10 -0.20 -0.30

wave frequency (rad/si

0:12

08

0.04 0.00

Figure 7. Hydrodynamic coefficients of a Wigley hull according to Rankine Panel Methods and Strip Theory. Van 't Veer(1998)

Figure 8. Computer and measured heave of a SES for various wave steepness values (kA)

Moulijn (1998) 24 o > 1.2 1- 0.8- 0.6- 0.4- 0.2-I Ì kA =0.10 k.A =0.15 exoenment 0.05 -- I O o 0.2 0.4 06 0.8 I 1.2 1 4 / pVL2(g&)1

oO'''o

_o B./ p'7L(g/U"2 B /pVL(/t)"2 A./ pVL A/pVL 7.00 7.00 7.00 100 3.00 5.00 ci (Ug) 100 3.00 500 i 00 300 5.00 3:00 5.00 u, (L/g)1r2 7.00 7.00 loo 3.00 5.00 w (1./g) 3:00 5.00 w(ljg)"2 7.00 1 00 lOO

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Extension _{of strip-theory methods taking into} account large amplitude motions have shown to be

0 CS E

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>. o, C CS o 8M10, no bow flare Io 5 20 w (ad/sl 25 CS o1 E

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> 01 C CS o 3 - Ecperimeni -- - Simulation(ls( order) - - Simulation (2ndorder) Simulation (3rd!order)

very useful in determining for instance, non-linear

effects in wave loads in the ship hull. See Figure 9.

BMtS, no bow tiare

w ((ad/si

Figure 9. Computed and measured spectral density of bending moments at station 10 and F5 showing effect of bow flare..Adegeesz (1995)

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BM1O, bow fiare _{BM!15, bow Hare}

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Keuning (1994) has applied such non-linear time domain methods to the analysis

of the

behaviour of fast planing craft in waves. Results of simulations have confirmed that extreme vertical accelerations of such vessels can be very much out

5 10 Ï5 20 25 30 35

8 ..-deg.

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 (l99I). Due to the complexity of the physical problem of large motions such as the

occurrence of slamming and green water on deck, some restrictions have to be introduced in order to make the problem amenable to computation. For

instance, no wave breaking is allowed in such

computations (Subramani, Beck & Schultz (1998)).

The non-linearities should be present not only in the

motions but also in the boundary conditions for the potential flow as specified on the free-surfàce and on the hull of the vessel. In some codes, only the non-linearity on the hull surface may be included while the linearised boundary condition is retained

for the free surface.

In the early seventies; 3-dimensional linear

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of proportion to the significant Values This has led

to the conclusion that for this type of vessel, motion

analysis based on linear methods could not be used

for a proper evaluation of workability limits of such craft in waves. See Figure 10.

5 10 15 20 25 30 35

B deg.

Figure 10. _{Influence of non-linearities on extreme vertical accelerations of fast planing craft. Keuning (1994)}

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frequency-domain potentiali methods were

developed for the computation ofwave frequency motions of arbitrarily shaped floating structures and large volume fixed structures at zero speed. Fór this

type of 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) the work of Boreel (1974) and ofvan

Oortmerssen. (1976) should be mentioned in this respect. A well-known computer program based on

this method is the WAMIT code, developed by MIT (Newman (1985)).

With respect to the prediction of the behaviour of a single moored offshore structure basedon such

methods, Herfjord & Nielsen (F991) report on the comparison of computed motions of two types of

structüre,

_{a deep draft floater and a monohull}

floater. Computations were carried _{out by a number}

spectrun II

max.=4

measured significant spectrun LII

:

significant nea sured

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of institutes using mostly _{linear 3-dimensional}

diffraction codes. It _{was shown that the wave}

frequency motions were generally well _predicted. Low frequency motions due to second orderwave

drift forces showed large differences; See _Figure Il. The differences could mainly be contributed _to

low frequency viscous damping effects not

accounted for in the computational methods. Itcan

be concluded from such resûlts that much remains to be done in this field before consistent results_can

be obtained entirely on the basis of computations;

have and 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) foranoverview of numerical methods.

.

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 specific cases,

computational methods developed for single ships

are used as a basis for computer programs involving

a complex combination of one or more ships

combined with, for instance, non-linear mooring

systems or riser systems or both. See for 'instance Withers (1988) and Huijsmans (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 the validation of such codes

are available in the open literature since

development is often carried' out within the context ofjoint industry research programs.

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 effects. This has prompted the development

of viscous flow codes, initially for predicting the stil

water resistance and flow characteristics of ships without free-surface effects and more recently also

taking into account the free-surface.

The computational methods are based mainly on the Reynolds-Averaged Navier Stokes (RANS)

equations for the flow of a fluid. An important 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 domain around the vessel. 'This leads to very

large systems of equations which need to be soLved.

Results obtained by such methods are encouraging

27

03 04 0 06 ta 24 28

Institution, No:

Figure 1. Variability of predictions of wave frequency ('HF) and low frequency (LF) motions on a FPSO in irregular head seas. Hertfjord & Nielsen (1991) At the present stage, due to the high

computational load, results of non-linear time

domain simulation methods are generated for

relatively short time periods. This means that it is not feasible to generate long-duration time records in order to obtain reliable statistical data. Adegeest

(1995) has developed a method to generate

statistical data including non-linear effects basedon

the applicatiön of Volterra series and Short-time non-linear time-domain simulations

The afòregoing short overview based on a

somewhat arbitrary selection of references, cannot do justice to the many excellent developments that

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but cannot at this stage be considered_{as being}

state-of-art in the sense that they are being _applied

generally.

6. _CONCLUSIONS

In the previous sections a brief overview has

been given of some aspects of model testing and of computational methods with respect to the

behaviour of ships and other floating structures in

waves.

The numerical developments have mainly been concentrated on the behaviour of the Single unit and

more specifically on its motion behaviour, wave loads and added resistance. Much attention has been paid to the comparison between results of

compútatkns and experiments, in deterministicsea

conditions consisting of regular waves. In. all these comparisons, model test results are considered_{to be}

the standard by which computations _{are judged.}

Comparisons with full scale experimental: data are

scarce and are generallyconsidered tobe.unsuitable.

as a basis for judging the validity of numerical

methods.

Non-linear potential methods for the prediction

of the behaviour of a ship are stil in their infancy.

Large amplitude motion programs have been

developed which yield valuable insight in

non-linearities but these are all restricted to thecase of

non-breaking wave, no slamming and nowater. on.

decL Even with, these restrictiöns, 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 to be developed and applied as a research tool. Some methods have

reached a stage of maturity which permits their use

as a tool for preliminary evaluation of, specifically,

viscous ship resistance and stern flow in stil water.

Accuracy is stil, however1 insufficient to replace

experimental data.

Few numerical methods treating the interaction

of a ship with its surroundings in a consistent _way

have been developed. Some methods, such _{as the} linear three dimensional diffraction codes, allow the evalûation 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 as yet become state-of-art.

Even with these restrictions, it is clear that the interest in numerical methods is increasing. This is maìnly_duetothe:facLthatin. the design stage, even

relatively simplè numerical methods, when used

knowledgibly, can contribute greatly to the

realisation of effective designs for ships and

floating structures in a shorter space of time and at

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, for instance, ship motions in the frequency domain. On the other hand there is

also interest in integrated software, able to evaluate in the time domain complex operations such _{as the}

behaviour of tandem moored tankers coupled to a

turret mooring system and incorporating a large

heading angle change possibility coupled with

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 WAMIT

underlines the desire of industry to make use of

generally validàted and 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.

It should be stressed that not only the

capabilities of the experimental facilities _{or the}

numerical code are of deciding importance _with respect to their uses. At least as important is the realization that the personnel making use of these

capabilities are knowledgeable 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 technkians

but of the complete chain of personneF from

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drafting office to data analyst _including the

scientific staff. With respect to the increasing application of more sophisticated numerical

methods iii the design phase by staff of_engineering offices or by others on behalf of the design _{team, as} also pointed out by Beck et al. ( 1996), _management

will have to take steps to insure that staff have

sufficient expertise, not necessarily to rùn software, but at least to be able to communicate meaningfully with those who do so on their behalf. This involves.

a conscious effort, on behalf of management, to come to decisions with regard to the continuing

maintenance of up to

date expertise

of staff

members and to decisions whether specialised

software shall continue to be acquiredas it becomes

generally available or whether such capabilities are

to be left to specialist suppliers as is the case with

model testing.

REFERENCES

Adegeest, L.J.M.: "Nonlinear Hull Girder Loads in

Ships", Phd. Thesis, Delft University of

Technology, 1995

Beck, R.F., Reed, A.M. and Rood, E.P.:

"Application of Modern Numerical Methods in

Marine Hydrodynamics", SNAME Trans-actions, Vol. 104, ¡996, pp. 519-537

Beck, R.F. and Magee, A.R.: "Time-domain

Analysis for Predicting Ship Motions", Dynamics

of Marine Vehicles and Structures

in Waves,

W.G.Price et al., Elsevier Science Publishers B.V.

199.1

Bertram, V.. andi Yasukawa, R: "Rankine Source

Methods for Seakeeping Problems", Jahrbuch Schiffbautechnischen Gesellschaft, 1996

Blok J.J.: "The Resistance Increase of a Ship. in Waves", Phd., Thesis, Delft University of Tèchnology, 1993

Boreel, Li.: "Wave 'action on Large Offshore

Structures", Inst. Of Civil Engineers, London, F974

Buchner, B.: "The Impact of Green Water on FPSO

Design", Offshore Technology Conference, 1995

.Dallinga', R.P., Blok, J.J. and Luth, HR.:

"Excessive Rolling of Cruise Ships in Head and

Following Waves", mt. Conf. on Ship Motions and

Manoeuvring, RINA, Londbn, I 998

Dey, A.Kr.:. "Viscous Effects 'in Drift Forces on

SeniiSübmersjbl"PhJThsj

Deift University of Tedmolog.y, 1996

Garisson, C.L and Chow, P.Y: "Wave Forces on

Submerged Bodies", A.S.C.E. Waterways and

'Harbors, Di 1972

Gerritsma, J. and Beukelman,, W.:' "Analysis of the Resistance Iñcrease in Waves of a Fast Cárgo Ship", International 'Shipbuilding Progress, VoL 19, No. 2 17, 1972

Hertfjord, K. and Nielsen, FG. : "Motion Response

of Floating Production Units: Results from a

Comparative Study on Computer Programs", I 991

OMAE - Volume I-B,, Offshore Technology

'Huijsmans, R.H.M.: "Mathematical Modelling of

the Mean Wave Drift Force in Cürrent", Phd

Thesis, 'Delft University 'of Technology, 1996

ITFC: Report of the Seakeeping Comniittee, 20th

International Towing Tank Conference, 19-25

September, 1993', San Franciscó

Journée, J.M.J.: "Motions of R'ectangular Barges",

10th OMAE Conference, Bergen, 1991

Kat, J.O de : "Irregular Waves and their Influence

on Extreme Ship' Motiöns", 20th ONR Symposium. Santa Barbara, 1994

Keuning, '.A..:"The Nonlinear Behaviour of Fast

Monohulls in Head Waves", Phd. Thesis,' Delft University of Technology, 1994

Korvin-Kroukovsky, B,V.: "Investigation of Ship Motions in Regular Waves", Trans. SNAME, Vol.

63, 1955

Lin,W.M., Meinhold, M. and Salvesen, N.: _"Large

Amplitude Motions and' Wave' Loads for 'Ship Design", Proc. 20th ONR Symposium _{on Naval}

Hydrodynamics 1994

Moulijn, J.: "Added' Resistance of Surface _Effect Ships", International Workshop on Water Waves and Floating Bodies Aiphen aan den Rijrì, 1998

29

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30

i

Nakos, D.E. and Sciavounos, P.D.: "Ship Motions by a Three-dimensional Rankine Panel Method", Proc. of the I 8thSymp. on Naval Hydrodynamics,

Washington DC. 1991

Newman, J.N.: "Algorithms for the Free-surface Green Function", Journal Engineering Mathematics,

1985

Oortmerssen, G. van: "The motions of a Moored

Ship in Waves", Phd. Thesis, Delft University of

Technology, 1976

Sübramani, A.K., Beck, R.F. and Schultz, W.W.:

"Suppression of Wave Breaking in Nonlinear

Water Wave Computations", Proc 13th mt.

Workshop on Water Waves and Floating Bodies, Aiphen aan den Rijn, 1998

Veer, R. van't: "Experimental Validation of a Rankine Panel Method", 13th International

Workshop on Water Waves and Floating Bodies,

Aiphen aan den Rijn, 1998

YLFS: Proceedings of the Proceedings International Workshopon-- Very - Large Floating Structures, November 25-28, 1996, Hayarna3 Japan

Wichers, J.E.W.:'A Simulation Model for a Single

Point Moored Tanker", Phd. Thesis. Delft University of Technology, 1988