Seakeeping considerations in ship design and operations

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Report No. 635001 - PAPER Seakeeping Considerations in Ship Design and Operations

by S.G. Tan

To be presented at Regional Maritime Conference, Indonesia 1995 October 1995

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Phone *31 317493911 Fax +31 317 493245 MARIN 2, Haagsteeg P.O. Box 28 6700 AA Wageningen The Netherlands

Seakeeping Considerations in Ship Design and Operations

by

S.G. Tan

Head Offshore Research Department

MARITIME RESEARCH INSTITUTE NETHERLANDS (MARIN)

ABSTRACT

This paper deals with the application of seakeeping research in designing ships which

can operate more effectively, safely and economically in rough seas. Seakeeping

research may be carried out using theoretical methods, model experiments, full scale trials or a combination of these tools. Important ship parameters and wave character-istics, which play a significant role in the ship performance at sea, are discussed.

With increasing acquired information on quantitative criteria various ship designs may be well evaluated and compared. As a result the ship design with the best seago-ing quantities may be assessed.

Furthermore, the seakeeping studies may be used by the ship master to avoid dangerous situations or to select a ship's route with respect to a minimum travelling time or fuel consumption.

In this paper some examples will be given to illustrate the advantages of taking into account the seakeeping considerations in ship design and operations.

INTRODUCTION

The success of a ship design depends ultimately on its performance in a seaway and not in calm water, since the sea is mostly not calm. Yet most ship designers still select their hull forms and ship dimensions mainly on the basis of calm water performance, without much direct consideration of the sea and weather conditions.

One of the main reasons is that in calm water the aim is simple and clear, for instance, to predict the smallest possible engine power required to attain a certain ship speed. Although the aim is simple, the solutions of the hydrodynamic problems are usually not simple and are, in many cases, even very complicated.

For the ship performance at sea many parameters play an important role such as, the sea conditions, the ship speed, the relationship of the ship length to significant wave length and the course of the ship with respect to the waves.

Another reason is the difficulty to describe quantitatively the hull form for optimum behaviour in a seaway. For instance, the optimum may be related to the smallest

possible accelerations or speed loss. In this respect, it may be stated that for many ship types the operational requirements are also insufficiently quantified to be applied for optimization purposes.

Furthermore, the interpretation of seakeeping behaviour is relatively difficult, since the sea in which the ship operates and hence the ship performance can only be

quantified in terms of statistics. The designer should, therefore, think ¡n probabilities that a certain critical level will be exceeded, which makes his task not easier.

However,

after more than 40 years

intensive and extensive seakeeping

research, both experimental and computational tools have been sufficiently developed to judge and optimize the seakeeping performance of ships, at least in the early stages of the design process.

This paper shows which steps should be taken and which information is required to design for seakeeping.

SEAKEEPING PERFORMANCE

In considering the performance of the ship at sea, the designer is primarily concerned with three qualities: habitability, operability and survivability.

Habitability deals with human comfort and performance on board of ships. The require-ments depend on the ship type and its mission. For example, a much higher degree of habitability is required for a passenger ship than for ordinary merchant vessels.

Operability is concerned with the ability of the ship, with all mechanical equipment and instrumentation systems on board, and its crew to carry out the assigned tasks at sea.

Survivability is concerned with the safety of the ship, its crew and cargo when sea

conditions become so rough that the ship, its crew and cargo, are in danger of damage or destruction.

Even though these three qualities are quite different in their nature, they may be affected by the following quantities or aspects:

Ship motions like heave, pitch and roll

Accelerations, in particular in vertical and transverse directions Course keeping including tendency to broaching

Increase in required power to attain the speed Global hull girder loads

Local sea loads

Deck wetness and water ingress Slamming (bow flare, bottom) Propeller emergence and racing

The importance of these quantities or aspects depend mainly on the type and mission of the ship. For example, Hadler and Sarchin [1] established that for destroyer-type

ships, ranged from 3400 to 5800 tons, the general conclusions of the operators in

order of priority for ship performance in a seaway were roll control, reduction of deck wetness, and slamming.

DESIGN FOR SEAKEEPING

In Figure 1 a simplified flow diagram ¡s given showing the necessary steps to be taken for designing a ship which should satisfy a certain prescribed seakeeping performance.

The results of the seakeeping performance analysis may also be used to supply the ship master and operators with information which may help them to take the necessary measures to avoid dangerous situations, see also [2].

The three major elements of the seakeeping performance analysis are [3], [4]: The ship with its given characteristics.

The specified sea environment in which the ship should perform its mission.

The operational criteria which should not be exceeded, so that the ship can

perform its mission effectively.

These and other elements of the flow diagram will be elaborated in the next sections.

SHIP CHARACTERISTICS

From seakeeping point of view the following ship characteristics are important:

Ship dimensions (like length, beam and draft) and their proportions Displacement and weight distribution

Longitudinal position of the centre of buoyancy (LOB) and of floatation (LO F) Shape of sections (U or V) below water

Freeboard and flare Ship speed

Bulbous bow

Anti-rolling devices such as bilge keels, anti-rolling tanks and fins Anti-pitching devices such as anti-pitching fins

The influence of these characteristics on seakeeping is reported, in general, sense, in the textbooks [5], [6], [7] and [8] and in various journals and conferences.

In the sixties, however, data on this influence were almost fully lacking. Therefore,

MARIN (at that time named NSMB) decided in 1958 to acquire knowledge from

sys-tematic model experiments with hull forms of the 'Series 60" models, with which

extensive resistance and propulsion tests in calm water were conducted. All experi-ments were conducted in regular waves of various lengths and directions at different

ship speeds. Besides the ship motions, the added thrust and torque were also

recorded. For some models amidship vertical and lateral bending moment measure-ments were also included.

Test data on the influence of principal ship proportions (length/beam, length/draft, block coefficient) were reported by Vossers, Swaan and Rijken [9], on the

influence of weight distribution by Swaan and Rijken [10], and on the influence of

forebody shape below and above water by Swaan and Vossers [11].

Perhaps one of the most important conclusions drawn from these extensive series of tests is that general trends should be interpreted with proper reserve; only a careful evaluation for a specific ship design can yield firm judgement of the ship behaviour and performance in a seaway. For an elaborate discussion of the general trends, derived from these systematic test series, reference is made to Van Sluijs and Tan [12].

Another systematic model test series of recent date concerned the high speed displacement hull forms. The aim of this program was to meet the demands for better

rounded bilge hull forms which may maintain high speed and small ship motions in high sea states.

To initialize the systematic series, a parent model has to be selected from the

data-base available at MARIN. From the trade-off between calm water resistance and seakeeping characteristics in head waves a parent hull form was selected having

'wide forebody and "wide' aftbody, see Figure 2.

This parent model has a

length/beam (LiB) ratio of 8.0, a beam/draft (B/I) ratio of 4.0 and a block coefficient (CB) of 0.4. A systematic series was constructed around this parent model varying LIB between 4 and 12, B/I between 2.5 and 5.5 and CB between 0.35 and 0.55.

All models indicated with a black dot in the cube, as shown in Figure 3, were tested in calm water for their resistance characteristics up to a speed corresponding to a Froude number Fn = 1.4. In some cases, e.g. for low LIB, the hull forni necessitated a lower maximum speed limit due to excessive trim, resulting in spray and wavemaking. The

experiments in waves were carried out in regular and irregular head waves at five

forward speeds, viz. Fn = 0.285, 0.430, 0.570, 0.855 and 1.140. During these tests

measurements were made of heave and pitch, vertical acceleration at Station 19,

relative motion with respect to the wave surface at Station 17 and resistance.

The experimental data provide the designer of high speed displacement vessels,

such as patrol boats, frigates, destroyers or cruisers, with a wealth of information.

Some data were reported by Blok and Beukelman [13], Van Oossanen and Pieffers [14] and Robson [15] indicating also the general trends with the prime variables.

SEA CONDITIONS

The second element in Figure 1 is concerned with the required information on the prevailing sea environment in which a ship is expected to operate such as:

Wind speed and direction Significant wave height Average wave period Wave spectra

Dominant wave direction

Angular spreading function (short-crestedness)

The oldest and simplest way is to characterize a sea condition with a Beaufort or sea

state number related to a certain wind speed. The area dependent "average wave

conditions are associated with these wind classes. Some common applied relationsare

summarized in Figure 4, [4]. This approach fails to recognize that one wind speedcan

generate a wide range of wave heights and periods depending strongly on the fetch

and duration of the wind. Since wind speed and direction are highly variable it _means

that the waves are never in equilibrium with the wind in practice. Figure 5 shows the scatter observed on the North Sea.

Furthermore, since the ship motions, accelerations and added resistance are

dependent on the wave period, using wave height statistics alone will not form a sound

scatter diagrams are available reflecting the joint statistics of wave height and period. The most comprehensive and applied data were collected by Hogben et al. [16].

Finally, for an accurate prediction of ship performance it is desirable to use real wave spectra whenever possible. A wave spectrum describes the wave energy dis-tribution over the wave frequency. In case no information is available, usually the wave spectra are applied which correspond to the Pierson-Moskowitz (P.M.) formulation [17]

for fully developed seas. In fetch limited sea areas preference

is given to the

JONSWAP type spectra [18]. In relating the spectra to the wave scatter diagrams, the average wave height observed is assumed to coincide with the significant wave height

w1/3 and the average period observed with the average zero-uperossing period T2. In Figure 6 a comparison is shown between a P.M. and JONSWAP spectrum for a significant wave height of 4.9 m and an average period of 8.4 s. This figure illustrates

that large differences in response predictions may be expected depending on the

spectral formulations applied, which statement is true for narrow-band peaked

response functions such as for roll.

SEAKEEPING TOOLS

To determine the seakeeping characteristics computational methods, model experi-ments or full scale trials may be used.

The computations give the designer an opportunity to investigate alternative designs

with a great flexibility and at relatively low costs. The computer programs should,

however, be well validated whereas the designer must be aware of the limitations of the various methods he has to his disposal.

Model experiments are used to provide insight in the development of computational tools and for validation. In the design process, model tests remain indispensable to check design calculations and to predict phenomena for which no reliable computation methods exist.

At MARIN, in many cases model testing is preceded by computer calculations. The computations are performed to optimize a design and/or to determine the most critical combination of operational and environmental parameters to define a cost effective model test program.

To provide the ultimate validation of both computational and physical models full scale measurements are needed. Full scale trials are also carried out to determine whether a ship fulfils its design specifications, to collect data on the actual sea

per-formance of the ship or to monitor continuously specific operations such as heavy

transport or offshore installations [19].

Table I summarizes the most frequently applied, in-house developed computer programs which are suitable to predict the behaviour of ships and offshore structures in waves, wind and current. All programs have been validated extensively. Within the scope of this paper, only a brief description of the programs SHIPMO and WASCO (see section Design Assessment) is given.

SHIPMO is based on the well-known strip theory, which is, generally speaking, still the most successful and applied theory for predicting wave-induced motions and related phenomena of ships sailing at moderate forward speed. Strip theory is based on many assumptions such as slenderness of the ship and the motions are small in relation to

the cross-sectional dimensions. Strip theory is also based on linear potential flow

theory which means that viscosity effects are neglected and the wave steepness is small. Strip theories account for the interaction with forward speed in a very simple way by neglecting the effect of the steady wave system.

Because of these assumptions strip theories should be validated extensively to judge their merits in predicting ship motions, added resistance and wave loads. Many validation studies have been conducted, of which the extensive studies of the Delft University of Technology are noted. For instance, Gerritsma et al. [20] investigated a systematic model series, derived from a Series 60 hull form with a block coefficient of 0.70. The models were oscillated in calm water at forward speeds corresponding to a Froude number of 0.2 and 0.3. The length/beam ratio of the ship models varied from 4 to 20. In addition, motion and added resistance experiments were carried out in regular head waves. The measured data confirm the applicability of the strip theory for a wide range of ships.

To investigate the applicability of the Delft strip theory to tankers, container ships and barges, Journée [21] carried out computer calculations and compared these results with model experiments. In Figures 7 and 8 the results of a comparative study of the 18th ITTC Seakeeping Commiftee on measured heave, pitch and acceleration are compared with two strip theory methods. In general, both strip theory methods yield fairly good predictions. For many other ship types much better agreement has been reported [21].

The studies mentioned above and numerous investigations from other institutes and universities demonstrate that present strip methods provide a sufficiently reliable basis for the motion optimization of ships, at least at an early design stage. The final design is usually tested in a laboratory.

At MARIN model tests related to seakeeping and offshore engineering are performed in four laboratories, see Table II, which are equipped with advanced measuring equip-ment and nstruequip-mentation. In two laboratories - the Seakeeping Basin and the Wave and Current Basin - waves can be generated from two sides to enable testing of the

scaled ship or offshore structure in regular or irregular waves from any direction

required. In the other two testing basins wave generators are located on one side. In the Shallow Water Basin the sea performance of ships in shallow water are studied,

whereas in the High Speed Basin the performance of high speed vessels, such as

catamarans, hydrofoils and surface effect ships, in head waves are investigated.

The hydrodynamic research in offshore engineering ¡s, among others, comprehensively described by Tan [22]. In the next section some arbitrarily selected examples of model test investigation on the performance of ships in a seaway are discussed.

SEAKEEPING CHARACTERISTICS

To evaluate a ship design many seakeeping characteristics should be considered such as ship motions heave, pitch and roll and surge, sway and yaw (in particular, important in moored conditions). From these basic motions the following quantities are derived: vertical and lateral motions, velocities and accelerations, relative motion with respect to the wave surface, wave loads, powering and ship course keeping in waves. From the

wetness, slamming and propeller racing.

As an example, the effect of ship length on the vertical accelerations at the fore and aft perpendiculars is shown in Figure 9. These data were derived from various model tests and apply to container ships having a block coefficient of about 0.60, a length/beam ratio of 7.5 and a length/draft ratio of about 25 when they sail at a Froude number of 0.25 in Beaufort 8 head seas on the North Atlantic.

The effect of ship length is much larger for the ship lengths smaller than 180 m. When the ship length is increased from 150 to 300 m, the accelerations are decreased by about 60%. Therefore, the behaviour of large ships, for instance the new generation container ships will be far superior compared to the first generation.

To gain more insight into the effect of ship and wave characteristics systematic computations or model experiments are required. The earlier mentioned systematic model experiments in section 'Ship Characteristics' may be used. For instance, Swaan [23] applied the results of the Series 60 experiments [9] to illustrate the influence of ship principal dimensions on the performance of the vessels having a constant dis-placement of 15,000 tons and sailing at a service speed of 15 knots.

In Figure 10 the heave motion is plotted on a base of the vessel's length. Two sea conditions were considered, namely a sea condition with a significant wave height of 2.0 m (this sea condition may be considered to be an average condition) and the other sea condition with a significant wave height of 4.5 m (this sea condition may be

considered as 'rather rough" since these wave heights are only exceeded in about

10% of the time). The predictions were made for two directions namely head seas Qi = 170°) and bow seas (ji = 130°). For the designs considered the block coefficient (CB) the length/draft (L/H) and length/beam (LiB) ratios were varied.

As can be seen, increasing the ship length by reducing the block coefficient leads

gen-erally to larger heave motions. If, however, the ship length increases because of

increased length/draft (LIH) ratio a reduction of heave is clearly obtained. The effect of the length/beam ratio is relatively small.

The same conclusions may be drawn for the pitch motions as shown in Figure 11. Comparing Figures 10 and 11 (on the motions) and Figure 12 (on the horse-power) the best design possibility in regard to power appears to be the design with a low length/draft ratio, whereas for the motions this lower length/draft ratio results into higher values. lt may be concluded that in this case the requirements of small motion amplitudes and low power absorption may be incompatible.

Besides heave, pitch and associated phenomena such as deck wetness and slamming, rolling is frequently mentioned as an important reason for course change either in combination with voluntary speed reduction or not. A method to reduce the roll motion considerably is the application of an anti-rolling tank. Figure 13 illustrates that for a 58 m long Tuna Seiner an installation of a well-designed, free-surface anti-rolling tank leads to a considerable reduction of the roll angle up to 80% at resonance beam waves. A cheaper, but less effective solution is the application of bilge keels.

The last example concerns the present study on the influence of bow form of a cargo ship on added resistance in short head waves. A total number of seven models

were designed and built. A part of this study was discussed by Blok [24J; for the

subseries the model was segmented and was fitted alternatively with three bow forms: a very fine slender bow with hollow waterline entrance (model 1), a wedge bow (model

4) and a blunt almost cylindrical bow (model 5). All bow forms had vertical sides, see Figure 14. The influence of bow form for the forward speed corresponding to a Froude number of 0.21 is shown in Figure 15. lt appeared that the added resistance of the blunt bow was larger than for the wedge and fine bow form.

OPERATIONAL CRITERIA

The seakeeping characteristics themselves are valuable for comparative purposes to determine the improvements in ship behaviour. To evaluate the seakeeping perform-ance of a ship more completely, a set of well-established operational criteria should be taken into account. These criteria must be related to a particular task or mission. The

responses selected for criteria assessment should be of actual concern to the mission being considered. Finally, numerical values of criteria should be determined by moni-toring the apparent performance of actual ships at sea [8]. Consequently, there can be no universal set of criteria applicable to all missions or ship types.

For instance, in a Nordic cooperative project criteria were established for various ship types [25], [26]. The criteria were formulated mostly on the probability of exceed-ing a certain level of motions, accelerations, deck wetness, slammexceed-ing and so on. They

are reviewed every three years by the ITTC Seakeeping Committee. Table ill lists

some criteria for operability in rough weather [27], whereas in Table IV a tentative

scale for vertical acceleration (root-mean square values) is shown which may be used for estimating the maximum acceptable magnitude for different activities on board and for the comfort of the crew and passengers [26].

In many cases, for which no operational criteria were established, the designers used published criteria of similar ships or ISO standards. More efforts should be made to establish mission-related seakeeping criteria for various type of ships especially for the high speed marine vehicles.

DESIGN ASSESSMENT

In previous sections it was described which principal ship and wave characteristics as well as operational criteria are required to carry out a seakeeping performance analy-sis. The available tools to predict seakeeping characteristics were also discussed.

To facilitate the seakeeping performance analysis MARIN developed a computer program, WASCO, as a post-processor for seakeeping calculations. WASCO stands for Workability Analysis of Ships and Constructions Offshore. WASCO can be used to quantify the performance of ship and offshore structures in terms of crew and passen-ger comfort and habitability, bow emergence and related impact loading, deck wetness, speed loss, seafastening loads and so on.

The theoretical framework of this program as well as some applications of it were pres-ented by Dallinga [4]. The basic result of the program is a 'downtime" or "operability figure. Downtime is defined as the annual time percentage when a specified criterion is exceeded for a ship sailing at a given speed in a given wave heading.

Figure 16 illustrates the results of such an analysis for a 36 m long trawler at a speed of 12 knots encountering bow quartering seas. The scatter diagram applies to the Central North Sea for which the figures indicate the number of occurrences of the

wave height/period combination out of thousand waves. The combinations above the ines will lead to conditions for which the specified criterion is exceeded leading to a downtime. It appears that the criterion of non-exceedance of a vertical acceleration of 4 m/s at the bridge results in the largest downtime of 41% in bow quartering seas.

A similar operability analysis was carried out by Keuning and Pinkster [28] in optimizing the seakeeping behaviour of an existing semi-planing patrol boat. This ship

was designed to perform tasks such as surveillance in coastal waters, fishery or ecological protection and pilot services. Use was made of the so-called enlarged concept. In this concept the hull lerqth was increased considerably, in particular

forward of the accommodation, but no changes were made to the payload functions and to the layout and interior of the ship. As a result the seakeeping performance was

improved due to the increased length, length to beam and length to displacement

ratios. The basic design and the two enlarged ships, with an increased length of 25%

and 50%, are shown in Figure 17. lt appeared that the larger ships were relatively lighter and required less power to attain a speed of 25 knots.

The study was restricted to head seas encountered on the southern part of the North Sea. As an operational criterion it was specified that the maximum significant vertical acceleration in the wheelhouse should be less than 0.35 g.

Keuning and Pinkster [28] reported that the operability increased from 44% for the original design to 51% for the 25% increased length and 74% for the 50% enlarged ship. They also carried out an economic evaluation of these alternatives:

Above results indicated that an increase of ship length of 50% led to a considerable improvement of operability by 68%. The building costs increased only by 6% provided that the vessel speed and payload capacity were kept unchanged.

OPERATOR GUIDANCE

Next to the design optimization the results of the seakeeping performance analysis may also be used by the ship officers to avoid dangerous situations or to select a ship route with respect to a minimum travelling time or fuel consumption.

An example is given in Figure 18 showing a polar plot of limiting roll motions for

Underway Replenishment Operations of a DDG2 frigate. De Kat et aI. [29] applied a limiting motion criterion of 5 degrees significant single roll amplitude for Connected Underway Replenishment. Operator guidance in this so-called TDA (Tactical Decision

Aid) is shown as the range of choices in heading and speeds where acceptable

motions are predicted in the non-shaded regions of this polar plot.

St. Patrol 2600 3300 4000

Building costs index _[-] 1 .00 1 .03 1 .06

Operational costs index _[-] 1 .00 0.94 0.93

Transport efficiency index [-J 1 .00 1 .54 1 .67

As to reduce the fuel consumption, the example in Figure 19 [30] illustrates that benefit is to be obtained when the course is changed from ahead to abeam, in particu-lar when rough seas are met.

CONCLUDING REMARKS

This paper deals step by step with the factors which play an important role in design-ing ships with a good seakeepdesign-ing performance. lt is briefly mentioned that the results of the seakeeping performance analysis may also be used for the development of the

operator guidance. The examples given are arbitrarily selected and are mainly

restricted to the research in the Netherlands.

lt is recommended that more mission-related operational criteria should be established

for various types of ships and that easy-to-use tools should be developed for the

designers and operators on board.

REFERENCES

Hadler, J.B. and Sarchin, T.H., Seakeeping Criteria and Specifications',

SNAME Seakeeping Symposium, 1973.

Conference on Operation of Ships in Rough Weather, "The Use of Onboard

Instrumentation", The Institute of Marine Engineers, London, February 1980.

Comstock, E,N. and Keane, R.G., "Seakeeping by Design, Naval Engineers

Journal, April 1 980.

Dallinga, R.P., Selection on Seakeeping', Proceedings of MARIN Workshop on:

Advanced Vessels, Stationkeepirig, Propeller-Hull Interaction, and Nautical

Simulators held at MARIN, Wageningen, May 1992.

Vossers, G.,

Behaviour of Ships in Waves', published by H. Stam N.y.,

Haarlem, The Netherlands, 1962.

Bhattacharyya, R., "Dynamics of Marine Vehicles", published by John Wiley & Sons, Inc., USA, 1978.

Lewis, E.V., Principles of Naval Architecture, Volume Ill. Motions in Waves and Controllability", published by SNAME, USA, 1989.

Lloyd, A.R.J.M., "Seakeeping: Ship Behaviour in Rough Weather', published by Ellis Horwood Ltd., Chichester, UK, 1989.

Vossers, G., Swaan, W.A. and Rijken, H., Experiments with Series 60 Models in Waves', Transactions of SNAME, Vol. 68, 1960.

Swaan, W.A. and Rijken, H., Speed Loss at Sea as a Function of Longitudinal Weight Distribution', International Shipbuilding Progress, Vol. 11, 1964.

Swaan, W.A. and Vossers, G., The Effect of the Forebody Section Shape on Ship Behaviour in Waves', International Shipbuilding Progress, Vol.8, 1961. Van Sluijs, M.F. and Tan S.G., 'Retrospection on 15 years NSMB Seakeeping Activities', International Jubilee Meeting on the Occasion of the 40th Anniversary of the Netherlands Ship Model Basin, 1972.

Blok, J.J. and Beukelman, W., "The High Speed Displacement Ship Systematic Series Hull Forms - Seakeeping Characteristics", Transactions of the SNAME, Vol. 92, 1984.

Van Oossanen, P. and Pieffers, J.B.M., NSMB - Systematic Series of High

Speed Displacement Hull Forms", Workshop on Developments in Hull Form Design, MARIN, Wageningen, October 1985.

Robson, B.L., Systematic Series of High Speed Displacement Hull Forms for

Naval Combatants", Transactions of the RINA, 1988.

Hogben, N., Dacunha, N.M.C. and 011iver, G.F., "Global Wave Statistics", BMT, London, 1986. Also available as "PC Global Statistics", 1987.

Pierson, W.J. and Moskowitz, L., "A Proposed Spectral Form for Fully Devel-oped Wind Seas based on Similarity Theory of S.A. Kitaigorodskii", Journal of Geophysical Research, Vol. 69, December 1964.

Hasselman, K. et al., "Measurement of Wind-Wave Growth and Swell Decay during the Joint North

Sea Wave

Project (JONSWAP)", Deutsches Hydrographisches Institut, Hamburg, 1973.

Van den Boom, H.J.J., "Reality in Modelling: Ideal Conditions in Maritime Soft-ware Engineering", MARIN Jubilee 1992, Special Jubilee Volume, published by

Elsevier, Amsterdam, 1 992.

Gerritsma, J., Beukelman, W. and Glansdorp, C.C., 'The Effects of Beam on the Hydrodynamic Characteristics of Ship Hulls", 10th Symposium on Naval Hydro-dynamics, Cambridge, Massachusetts, 1 974.

Journée, J.M.J., "Seaway Deift. User Manual of Release 4.00", DeIft University of Technology, Report No. 910, March 1992.

Tan, S.G., "Hydrodynamic Research in Ocean Engineering: Three Decades of Experience at MARIN", MARIN Jubilee 1992, Special Jubilee Volume, published by Elsevier, Amsterdam, 1992.

Swaan, W.A., "The Influence of Principal Dimensions on Ship Behaviour in

Irregular Waves", International Shipbuilding Progress, Vol. 8, 1961.

Blok, J.J., 'The Resistance Increase of a Ship in Waves", Doctor's Thesis, Delft University of Technology, 1993.

"Assessment of Ship Performance in a Seaway", NORDFORSK, 1987.

Karppinen, T., "Criteria for Seakeeping Performance Predictions", VU Technical Research Centre of Finland, 1987.

Report of the 19th IUC Seakeeping Committee, 1990.

Keuning, J.A. and Pinkster, J.A., "Optimisation of the Seakeeping Behaviour of a Fast Monohull", Proceedings of FAST '95, 1995.

De Kat, J.O., Brouwer, R., McTaggart, K.A. and Thomas, W.L., "Intact Ship

Survivability in Extreme Waves: New Criteria from a Research and Navy Per-spective", STAB '94, 1994.

Journée, J.M.J., "Reduction of Fuel Consumption by Guarding and Simulating the Ship Performance (in Dutch)", 'De Zee", Vol. 11, 1982.

Table I: Frequently applied computer programs Name Description SHIPMO DIFFRAC/DIFCUR SEMISUB WASCO TERMS IM Il DPSIM LIFSIM DYNFLX

Ship motion and seakeeping behaviour

2-D potential theory/strip theory with viscous roll damping

3-D potential theory including wave diffraction and current/forward speed for arbitrary floating structures (first and second order

loads and motions)

Motions of slender component structures 2-D potential theory and viscous effects

Workability analysis of ships and constructions offshore

Assessment of dynamic motions and mooring loads for tankers moored to a CALM buoy, multi-buoy mooring or a jetty system Time domain simulation of dynamically positioned vessels includ-ing propellers, thrusters and control system

Time domain simulation of heavy lift operations from lift-off to

set-down conditions

Dynamic analysis of mooring ines, flexible risers, umbilicals, pipe

Table Il: Seakeeping and offshore engineering facilities

Name Dimensions Type of tests

Seakeeping Basin 100 x 24.5 x 2.5 m

(pit depth 6.0 m)

Resistance and self-propulsion tests in

waves

Oscillation (PMM) tests and captive tests in waves

Wave load measurements

Sea transport, launching and instal-lation tests of jackets

Towing tests of gravity structures Mooring tests

High Speed Basin 220 x 4 x 4 m Resistance and self-propulsion tests of high speed ships

Oscillation (PMM) tests and captive tests in waves

Launching and installation tests of jackets

Shallow Water Basin 220 x 15.8 x 1.1 m

(variable water depth;

maximum pit depth 3.3 m)

Resistance and self-propulsion tests in shallow water and waves

Manoeuvring, zig-zag and stopping

tests

Oscillation (PMM) tests and captive tests in waves

Towing tests of gravity structures Mooring tests

Wave and Current Basin 60 x 40 x 1.1 m

(variable water depth) Partly deepened

20 x 35 x 2.1 m

(variable water depth;

maximum pit depth 6.4 m)

Testing of moored systems (SPM,

FPSO)

Dynamic positioning (DF) tests Installation tests of (liftable) jackets In-place tests of gravity structures Testing of TLP in waves, wind and

current

Table Ill: Criteria for survivability and operability in rough weather

Author Slam Wetness

Roll RMS Vertical acceleration RMS Survivability

Bales and 50/hr 50/hr 15.0 degrees 0.40 g on bridge

Cieslowski (1981) (USN combatant)

Operability

Hutchison and 20/hr 30/hr 5.0 degrees 0.20 g on bridge

Laible (1987) (research vessel)

McCreight (1987) 20/hr 30/hr 4.0 degrees 0.20 g on bridge (USN combatant

mobil-ity) Nordforsk (1987) (merchant) [25], [26] 5/100 6.0 degrees 0.15 g on bridge Nordforsk (1987) (naval) 3/100 5/100 4.0 degrees 0.20 g on bridge Pingree (1988) (warship personnel) - - 4.5 degrees 0.18 g

Table IV: Limiting criteria for vertical acceleration

Vertical acceleration

RMS

Description

0.275 g Simple light work. Most of the attention must be devoted to keeping balance. Tolerable only for short periods on high speed craft.

0.2 g Light manual work by people adapted to ship motions. Not tolerable for longer periods. Quickly causes fatigue.

0.15 g Heavy manual work by people adapted to ship motions: for instance on fishing vessels and supply ships.

0.1 g Intellectual work by people reasonably well adapted to ship motions (i.e. scientific personnel on ocean research vessels). Cognitive/manual work of a more demanding nature. Long-term tolerable for the crew. The International Standard ISO 2631/3 (1985) for half an hour exposure period for people

unused to ship motions.

0.05 g Passengers on a ferry. The International Standard for two hours exposure period for people unused to ship motions. Causes symptoms of motion sick-ness (vomiting) in approximately 10% of unacclimatized adults.

SHIP

CHARACTERISTICS

SEAKEEPING

TOOLS

SEAKEEPING

CHARACTERISTICS

DESIGN

ASSESSMENT

OPERATOR

GUIDANCE

SEA

CONDITIONS

Fig. 1: Seakeeping performance analysis.

OPERATIONAL

CRITERIA

Fig. 2: Body plan of parent hull. 0.55 0.50 0.45 0.40 0.35 4 ₁₂ L/B

>. 20 -z) a 10 0_ 20 10 0 o BEAUFORT NUMBER 2 3 1 S 6 7

Seo LLaLe nueber

Fig. 4: Beaufort number, sea state and wave height.

B I (il-15 kn) 663 cases r-CN B 6 (22-27 kn) 433 cases

LJEJ__.

C' Wave height H (m)

oD

Wave bejaht H

D

L" o

_uDL

(N (N i3 3 (16-21 kn) -30 cases Bf 8 (34-40 kn) 93 cases o

=

C' o 80 -20 -10

BHATTACHARYYA, FULLY ARISEN SEA, UNLiMITED FETCH FIJU.Y ARISEN SEA, UNLIMITED FETCH

NORTH SEA

NORTH ATLANTIC OCEAN

SPEED JANSSEN,

----e----

---t---+---

FETRLROLL WIND I/U / / II /

I

./_

'

/

., l'i

/

5 10 15 3 7 (28-33 kn) 234 cases -20 -10 Lb

10.0

7.5-

5.0- 2.5-0.0 THEORETICAL SPECTRUM P.M. = 4.9 m, T2 = 8.4 s - JONSWAP Ç113 = 4.9 m, T2 = 8.4 s, GAMMA = 3.3 I I / / / I / / I

/

/

\

_N N N N 0.0 0.5 1.0 15 FREQUENCY in radis

LOO 0.00 o 02 25 0 0.0 i. ' cJL/g

Ordinary strip theory method Modified strip theory method

0

K

0.00

Fig. 7: Comparison of calculated pitch motions with experimental data of the

_{1987 lITO}

comparative study.

Ordinary strip theory method Modified strip theory method

PITCH, Fn = 0.27S

52 o o b

cJL/g

Fig. 8: Comparison of calculated vertical accelerations forward with experimental data of

the 1987 ITTC cprnparative study.

0*0

_o o0 PITCH, Fn

ACCELER/[ION, Fn =

I. -0.200 :.

0275

ACCELERATION, Fn 25 0 L gÇa 00.0 25.0 0.0

0.75 0.50 0.25 o Fore perpendicular Aft perpendicular Beaufort 8 m

o

---T

_=8.4s

Fn = 0.25

N

NQ

o C

loo

150 200 250 300 350 Ship length in m

5 w1J3 45 m

»

130° 0 wl/3 = 4.5 m = 130° = 4.5 m 170°

00

=170° o 170° i EFFECT OF EFFECT OF L/H EFFECT OF LIB _______ w1/3 = 2.0 rn = 130° = 2.0 m - 130° O = 2.0 m Q =

/

130° p o 170°

LJH

= 170°

.

l.t =

L/B*

/

= W 170° 130 140 150 130 140 150 130 140 150

Length between perpendiculars in m

Fig. 10:

8 6 4 2 0 wl/3 45m p = 170° p = 130° Ç113 = 4.5 m Ç113 = 4,5 m

l

170°

-EFFECT OF wlI3 = 2.0 m EFFECT OF w1I3 = 2,0 m p=13O° EFFECT w1/3 = 2.0 m

-OF UB UH = 130°

-

°

-p = 130° C8 p = 130°

-p p=170° UB

p=170°

-o UH p = 170° 130 140 150 130 140 150 130 140 150

Length between perpendiculars in m

Fig. 11:

Significant pitch for the different design

20000 15000

Q-

'

10000 5000

o

Length between perpendiculars in m

Fig. 12: Shaft horsepower for different design possibHities.

150 EFFECT OF CB EFFECT OF UH EFFECT OF LIB p = 1 70° = 4.5 m p = 1300 = 4.5 m w1/3 = 4.5 m o

'

p = 130° = 130°

\.

wlI3 =2.0 m 70° = 2.0 m p = 130° ÇW113 = 2.0 m p = 1700 p = 130°

_luu.

o

170° p = 170° = 130° p = Still Still water Stilt water water

UH

UB.

130 140 150 130 140 150 130 140

40

30

10

O

Beam seas : Wave height li m

Water level = 0.85 m - Roll natural period = 89 sec

/ -'

//\

/1

_\\

\

1/

Ii

I.'

1/

¡f!

//

¡I

/

\\\

\\

¡ -.-.

i

\

/ /

/

\\

\speed

1

/

/

\

/

I

_/

\\

/ \

\\\

\\

o kn.

i

t / / I ¡ J

/

/

/

/

\

\

'

\

\\

\

\

\

\

speed 7 kn.

7

/

I

speed 14 - O kn

7kft

7 8 9 ₁₀

Wave period in sec

UNS TA BILIZED _STABILIZED

- Ship speed O kn. Ship speed O kn.

Ship speed 7 kn. Ship speed 7 kn.

Ship speed 14 kn. Ship speed 14 kn.

C,, C) a) C) -Q C) -Q

a

20 E C,

n

_:3 o -a o

Model

i

1.00

0.75

(\J E ()

cc 0.50

0.25

O Waterplane area Body plans 18

jJ

Model 4 16

Fig. 14: Body plans and waterlines forward for three cargo ship models.

\

Model i Model 4 5

---Model

\ \

--.

I/I

-0.1

0.35

0.6

0.85

18 À 6 Model 5

\

\

\ (I'

Ï

ii

3 i 9 7 18

i A

II9

6:75D26

I24B5i5i'-

16 85 8 2 2 1 2 o _{5 to} 15 Peak period (s)

Fig. 16: Example of a WASCO operability analysis for a trawler.

Wasco operabikty analysis 36.6 m. trawler

Chmate: Central Nooh Sea, annual 15

Shtp: 12 knots. bowquartenng as E 10 w (t C C Q 0, 5

Identification Criterion Average downtime

thousandth - - V. Acc Fwd 600 (iV5 2) 390 V Ace Bridge 400 (m/s) 410 V Ace Fwd 280 (iris 2> 156 V Acc Bridge 200 (rn/s ._2) 256 Flott 16 00(deg) 9 o

ENLARGED SHIP CONCEPT

1.00xL

(26.00 m.)

1.25xL

(33.00m.)

1.5OxL

(40.00 m.)

Main vessel design particulars for basc ship and alternatives St. Patrol 2600 3300 4000 length o a m 26.70 33.70 40 70 Length wi. m 24.85 31.85 38 85 Beam mid. m 5.80 5.80 5.80

Depth mid (1/2 L) m

3 35 3 35 3.35

Draught midships m

1.60 1.47 1.38

Draught aft approx. m 1.95 1 82 1.16

Displament

kN 970 1040 1110

Deadweight kN 170 170 170

GM m 1.62 1.93 2.19

W 270e U N REP WIND WAVES DIR = 000 DEG PER = O SEC HGHT = 0 FT DDG2 ROLL

0 N

150730 APR 88 PRIMARY SWELL DIR = loo DEC

PER = 12 SEC

HGHT = 7 FT

18O S

CONTOURS ARE IN DEGREES OF MOTION

HATCHING INDICATES AREAS OF RECOMMENDED CAUTION CROSS HATCHING INDICATES HAZARDOUS AREAS

90 E

0.3

0.2

0.1

0

Head seas _{Beam seas}

Fig. 19: Effect of course change on fue! consumption.

Following seas Container ship Speed: 1 9 knots (L = 200 m) Sea condition: w1/3 T 5.0 m 8.5 s 2.5 m 6.0 s Wind speed: 15 knots 10 knots Calm wate r