Semi-planing vessels in a seaway, comparative prediction of operability

SEMI-PLANING VESSELS IN A SEAWAY,

COMPARATIVE PREDICTION OF OPERABILITY

by.

A.M. van Wijngaarden and

W. Bèukelmán

Reportnr. 755-P

Workshop on developments in Hull

Form Design.

MARIN,, 22-24 October 1985

Deift University of Technology Ship Hydromechanics Laboratory Mekeiweg2

2628 CD Oath The Netherlands

HULL FIRM DESIGN

Volume 2

MARIN

22-24 OCTOBER 1985

WAGENINGEN THE NETHERLANDS

WORKSHOP ON DEVELOPMENTS IN HULL FORM DESIGN

October 22 - 24, 1985 Wageningen, The Netherlands

PROCEEDINGS

VOLUME II

Publication No. 785

Maritime Research Institute Netherlands Wageningen, The Netherlands

1. INTRODUCTION

The operability of ship and crew is reduced significantly long before the ship's motions become excessive. From this insight the desirability has. emerged in recent years to treat

seakeeping as an objective parameter in the preliminary design. The design information on seakeeping is needed already in the very first stage of the design, where (apart from boundary

limits) some freedom is still present in the selection of main dimensions and hull form.

The purpose of this investigation .has been the development of a calculation

SEMI-PLANING VESSELS IN A SE WAY, COMPARATIVE PREDICTION OF OPERAB,ILITY*

by A.M. van Wijngaarden** and W.. Beukelman'***

Abstract

The effects of key design variables on the seakeeping behaviour of small semi-planing vessels are investigated. Operability of. si.ip and crew is related to the vertical acceleration level when.

travelling in head waves. The calculation procedure is demonstrated for a North sea wave climate. For a ship with given main dimensions and speed, the operability percentage can be determined by

interpolation. The choice of the wave climate (wave periods, and heights), the acceleration criterion and its location of

application is left to the designer. The operability percentage, which is a relative measure of mertt, can be applied in the preliminary design stageof small,, fast vessels..

Sümm ry of sub-projects carried out by the Advanced Fast Vessels project team for the National Foundation for the Co-ordination of Maritime Research (CMO)

**) _{Maritime Research Institute Netherlands}

Del'f,t University of Technology

procedure for .the operability prediction

of semi-planing, vessels, in a realistic

wave climate. This prediction procedure should account for the effects of key design variables on the seakeeping

behaviour in. the chosen wa.ve environ-ment.

Ultimately the proposed calculation procedure will serve as the seákeeping module of a Concept Exploration Model for semi-planing vessels. The Concept Exploration Model (partly still to be

developed) will cover all design 'aspect's

in a global way. Among other things this

tool can be used to review, by

s.ys.te-matical variation of parameters., a range of designs and to test the feasibility with respect to the owners requirements,

2. PREDICTION OF OPERABILITY

The behaviour of a ship in a seaway is depending on a number of parameters. The

speed of the ship and the main dimen-sions (volume, length, beam and draft) are of prime interest. The weight distribution has a secondary influence.

In first instance. the underwater hull shape is charac-terised by hull form

parameters like C, C, LCB and

LCF. However, these parameters do not accurately represent the local geometry, which can also be important.

A detailed description of the wave climate to be- encountered is indispen-'sable. Generally it is represented by

wave spectra characterized by signif i-cant wave heights and wave periods'. Ship motions are very sensitive to the wave

direction. Moreover, a distinction should be made between long-crested and short-crested waves.

For relatively small, fast semi-planing vessels the vertical acceleration ampli-tudes when travelling in head waves are representative for the human performance degradation on board. TherefOre the acceleration level is decisive for the operability of ship and crew. The choice of an acceleration criterion value and its location of application is not' fixed in this calculation procedure.

The effects of the key design variables (speed and main

investigated by desire to cover

dimensions) have 'been stepwise. variations. The a broad area of

applica---tion together with -a practical limit for

the: number of ships -leads to rather

large variation increments-. The spread of main -dimensions has been pursued beyond realistic parameter combinations-to ensure the construction of an

inter-polation framework-.

For all ships and speeds the amplitudes

275

tions at selected locations along the ship's length. Limiting criteria were

imposed' on the calculated- acceleration

values. Subsequently the procedure has - been applied for a North Sea wave

cli-mate.

3. SHIPS AND SPEEDS

The parent model chosen for the mode-I series is a Dutch designed and built hard-chined patrol vessel whose hull form and main dimensions are shown in

Figure 1. TWO clusters of seven ships

were created by variation of. the main

dimensions. The-- small vessel series. (ship length around 20 metres.) was studied by the. Delft University of

-Technology; the- larger vessels '(ship

length around 35 metres) were covered by

MARIN.

I, Body p1.. s.d

ModsI U,. ii. -.1 both lb. p... p.tiol Ilod.1 -Ne. 3)

sod-The b-lock coefficient and the longi-

-tudinal location of the centre of

-buoyancy were kept constant

throughout

.the whole series. In each cluster one of the main dimensions CL, B or T) was varied twice while the other two ma-in dimensions remained constant. Under

th-ese -restrictions the displacement

104.6 9 I. B T C1 10

10. I.) - i.

1 44.0 2O6- i:.2 1:0P 0:34 .6.6

II 210.6 10.0

7l

0.16 - 0:10 06.1

of -the vertical acceleration have been varied proportionally to the variable

The two clusters of var,iaUons,in Figure 2 are connected along the length axis

(ship. Nos. 7 and 8:). The parent of the

second group (No. 11) was a re-scaled 3:5

rn Long version of the actual parent, form

(No. 3). The steps for the beam and

draft ',aria.tions in each cluster was

chosen in such a way as to obtain the. same volume as the length variations. This was done to enable comparison and

interpolation of the ships.

The ship speeds were chosen in coherence with the ship lengths. The small vessels

(L. = 15-25m) can be compared at 15, 20 and 25 knots,, for the longer vessels (L

= 25-4:5 rn) speeds of 20, 25 and 30 knots.

were selected. MØrever, for each ship the catculatjonsw,ere performed for a nunber of intermediate speeds. Their

choice enabled a comparison at two

constant F,roude number values, viz.

0,724 and 0.905,, for the entire ship

length ránge.

In total. the systematical series of .14

ships covers ship lengths from 15 to 45 metres, displacements from 33 up to 286 tons and a speed range from 15 to 35

knots.

4. THE ACCELERATION LEVEL

The responses of all ships in irregular long crested head: waves were computed by the strip .theory computer program named TRIAL of the Delft University. In a

recent investigation the application of strip theory calculations has been,

Index

Parameter. . Symbol Dimension

1 2 3 4 5 6' ' 7 Volume . V m3 32.0 44.0 48.2 53.3 154.1 215.7' 277.4 Length waterline ': L in 15.0 '20.6 25.0' 35.0 45.,0 'Beam waterline B in 4.42 4.85 5.36 7.51 9.65 Dra8 : p 0.92' 1.27 1.54 2.16 2.79 Model No. Parent model V5

Figure 2: Overview of. the variations in main, dimensions fOr the model series of

validated for very high Bpeed (round. bilge) displacement hulls1 see Blok' [2]. Up to high speeds a satisfactory agreement between experiment and calcu-lation was found for the vertical acce-lerations.. The correlation has been verified again.for the current parent

model.

The Bretschnejder formulation was chosen to describe the wave spectra. The sec-tion shapes were converted to the unit circle by a so-called Lewis transforma-tion. For the determination of the response amplitude operators a total of 33 wave length-ship length ratios were introduced in the program. The longi-tudinal radius of gyration was fixed at

0.25L.

The significant single vertical acce-leration amplitudes were calculated at unit signif leant wave height for the following wide range of modal wave periods: Tp = 3.2 4.8 6.3 7.5 -8.8 - 9.7 - 10.9 - 12.4 - 13.9 - 15.0 and 16.4 sec. The results were. collectei

in tabular form. In Table 1 an example

i given for ship 11. For each ship the

accelerations were calculated at the following iongitudinal locations: 0.00

LWL,

0.512

LWL and 0.939 LWL, see Fig.

1,.

4.41. I. l..11l..11 n.tlnI

- .1 4411 14. .1101.. II. _l,._... .414014.11 .4 4111 lllll&l0l 4010101.

277

5. DETERMiNATION OF OPERABILITY

Operability is defined here as the

annual time percentage in which on board a vessel1 travelling at a given speed in head waves, a specified acceleration criterion is not exceeded. The accelera-tion amplitudes were calculated for a 1 metre significant wave height. Assuming 'a linear relation between wave height

and responses,, the acceleration results

for each wave period were calculated at

wave heights of 0.5, 1 .5, 2 .5...9.5 metres by linear multiplication. Then

the calculated. responses were combined

with realistic wave data. The chosen North Sea area corresponds with Area 4

(see Figure 3) from the NATO wave and wind atlas, compiled by S.L.. Bales et al [3]. This is a collection of wave

climates in tables for the joint

proba-bility of wave period and wave height observation classes.

70 N

acceleratjon amplitude.. Forthose combinations of wave period and wave

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V13o.. 3. B.I..t100 .1 ..p 3 _{In th..IIorth *131.3W 04.3.. II).}

The acceleration values, were computed for each joint interval of wave period

(Tp ranging from 3.2 to 16.4 see) and wave height

H1!3 ranging from 0.5 to 9.5 rn).

Subsequently., a criterion wasimposed

height where the criterion stated was

not exceeded, the frequencies of

occurrence in the wave climate were added. The total sum of these

frequencies gave the probabilistic standard of operability:, expressed as a percentage. of the annual wave occurren-ces in the sea area under consideration.. In Table 2 an example of an operability calculation is presented. This calcula-tion has been performed for ship 11 at 30 knots,, the criterion applied is

a113 0.3 g at 0.512 LWL. in this table

.a thick line indicates up to which wave heights the vessel can. be operated per wave period. In other words, this is the. boundary between. operable and inoperable wave conditions.

Annoel ..v. c1l..t. .t.tt.tic. (ofAres 4 (North 0..) Unn,DTI(ORDC .pore aPP - 0919 01

Sobs. 2. ClompS. 01 on øp ctS*uIatjon (Or the North 8.0

N..,11. (or .hlp No. II 30 knot. .p..d and

o.St.rSao 1/35 0.3 .1 0.012

6.. OPERABILITY AS A FUNCTION OF MAIN

D'iMENS'I'ONS, SPEED, ACCELERATION

CRITERION AND LOCATION

For all sh.ips a number of operability percentages were calculated at several speeds and acceleration. limits. A rela-tion between operability and ship length is i11ustraedinFigUr.e_4._An increase

of ship 1gth.at.con8tant. speed (20.

-knots) causes a steady operability crease. Due to the sudden volume

in-4, V 7O. 0721. -Q8IpNo.

T0.1.11

S 4.47 ',T' 5.57 ).. 0 (rOq-.0 op.r.- , -(metre.) o;nry oIlily 0.5 -1.5 3.5 3.5 4.5 5.5 6.5 7.5 8S 9.5 RI,) I 3.3 2.4 5.4 2.4 3 4.8 3.5 3.8 5.8 17 1 6.2 55.0 . 50.8 3.4 7.4 0.3 4 7S 54.) 10.8 4.2 6.6 3.6 S e;8 . 13I 6.8 3.2 3.6' 0.6 0.9 6 9.7, :17.6 El 3.7 34' E3 46 05 - - - - . --7 10.9 50.5 4.) 5.1 3.21 5.6 1.8 2.0 0.4 - - - -8. 12.4' 0.8 4.3 3.1 3.2 5.5 0.9 5 2 5.6 0.6 0.5 - -'9 I38T 1.5 1.) 5.3 5.2 0.5 0.3 0I 0j ...3 0.1 -l0 :I50 .8.7 6.7 1.4' 3.1 1.2 0.2 0.2 03 O2 O3 0J 55 .594 J7 :2.8 0.8 08 0.7 ii10.3. 0.3 0.1 0.1 00S 0.5 Tot.I 99.6 62.8

rI8... 4. OporoblIlty .. ..fooctSon of .hlp.I.nOth. The .00.1

° 0.3 01

crease (between ships 7 and 8) a jump in the operability curves is observed at L

= 25 m. It must. be emphasized, however,

that ships 7 and 8 should not be compa-red directly, since they differ in all but the ship length. At a constant value of the Froude number, the dimensionless speed, the. mutual deviations diminish over the entire length range.

The operability as a function of main dimensions and ship speed can be read from Figure 5. in fact parallels are drawn along the "main axes" in the right hand cluster of Figure 2. The effects of draft changes are marginal. A beam

increase consistently resulted in. an operability improvement. However, this trend should not be pursued beyond realistic hull shape limits. Stretching the beam for instance decreases the deadrise angle and hence increases the sensitivity to bottom slamming.

The trend with length increase is generally beneficial for the smaller vessels. This applies only for these specific North Sea wave statistics. If a coastal wave-climate were chosen, the trend would have been changed by the predominant presence of short wave

periods.

SO 20'

111.1 155.7

80

60

V 20km.

V - 30 km.

Figure 5i Operability as a function of main dimensions and ship speed. The criterion is a113 ' O.3g at 0.512

There is a clear effect of speed in-crease. Acceleration amplitudes are

enlarged by the increase of. the

fre-quency of encounter in head waves. At the same acceleration criteriOn the operability is reduced consequently. In the current investigation three combina-tions of an acceleration criterion and its location of application (see also

Fig. 1.) have been studied:

- 0.3 g at 0.512

- a113 0.5 g at 0.512 LWL

a113 0.5 g at 0.939 _LWL

On small fast vessels the bridge and accommodation are often located near amidships. Therefore in our opinion the

first criterion/location combination yields the most realistic option for design comparison. Nevertheless, in.

future use of the method presented, here,

the 'chotheThf .tIó'n and location

will be left open to the designer

(be-9.66 Ship 12 23.14 277 279 80 70 60

sides the choice of key design

parame-ters like sea area, speed and mai'n di-' inensions).

7 APPLICATION

OF THE OPERABILITY

PREDICTION IN PRELIMINARY DESIGN

One of the first quantities to be

limi-ted' in ship design is the displacement.

In theory, the distribution of a fixed displacement volume over main dimensions and hull form is free. The, selection of main dimensions can now also be judged

from a seakeeping point of' view.

In Figure 6 the information from Figure 5 is presented on a base of volume. A series of' 3 ships can be compared twice at constant volume. These ships differ

'in two' dimensions, see Figure 2. From

Figure ,6 it could be concluded to maxi-mize both length and beam (at the cost of draft) to improve seakeeping

perfor-mance '(ships 9 and 14).

I"

80 60 10 2.. 35.0 B - 7.52 -1. 35.0 2.26

-00 8-7.52

T6

25LII&

V -20 kn. 0.3g , aO.5I2Lb

0

Ship 210. . --.-- Q

---.-J2

/1/

L

V.30km. ,3C0.3g at 0.522 8 7.52 T 2.26 L 35.0j T -2.26 2. 35.0 , B 7.52 '250 200 250 300 Volume (ml

Figure 6s Operability a. i function of volume. For all ship' C9 0.38 and LCB 46.2,5 LwI..

200 Volume (3) 250 SIUpO 9.20 Sb ph '54 Volume 215 8 5.. 60

I

40

By the -variations in main dimensions and speed8 -a framework has been obtained for the straightforward interpolation of acceleration data presented in tabular form (e.g. Table 1). The interpolated

results for a specific design _{will then.}

be subjected to the calculation proce-dure described in section 5 to determine the operability percentage.

Much attention should be paid _{to a}

proper statistical description of the wave- climate for the intended deployment area. In Figure 7 another statistical

wave description is given; valid for _the

same North Sea area. Operability

percen-tages of. ship 11 are calculated in

a-similar way. The -sum of the frequencies of occurrence in the shaded area of Figure 7 for example gives the

opera--bility at 30 knots in this _{wave climate.}

The more refined wave height intervals and the faired boundary curves (instead of the ladder-like line in Table 2)

4 5 6 7 a 9 10 II 12 13 II IS

Z.rD-upero..inq_wav._p.ria42-(.I

Figure 7 Operability in a wave ecatter diagram /..l for the Mid

- - Ilorth 8ea.-Shown aretimitin Curve. for Ship No _11,

acce'eration criterion a113 C 0.3 g at 0.512 L.

contribute to a higher accuracy. _But

besides this improved sensitivity _the

resulting operability percentages show a

considerable difference with the level

of percentages calculated -for the

-pre-vious wave statistics.

Therefore-, it must be concluded tha-t _the operability figure should be rega-rded as-a relas-ative- meas-asure for the prediciton of

performance unless the wave climate _to

be encountered can be specified with sufficient accuracy.

Nevertheless, by means of the

compara-tive prediction met-hod a useful _{tool is}

supplied to the designer to determine and improve the seakeeping behaviour.

For- a given wave environment his options are changing the main dimensions-,

the-speed, the acceleration criterion _{or its}

location of application (the latter

in-relation with the subdivision of

the-ship).

8. FURTHER RESEARCH AND EXTENSION OF THE

METHOD

In the foregoing -sections _{a number of}

limitations of the current _{investigation}

have been indicated. Future _research

will be aimed at reducing _these-

limita-tions. In a first analysis the

seakee-ping behaviour of small _semi-planing

vessels has -been regarded _{to be}

depen-dent on main dimensions, _{ship speed and}

wave climate.

In general the vertical acceleration

amplitude level is _{representative for}

the operability of sh-ip and crew.

However, human performance degradation

is also depending on the _{frequency of}

ship mat-ion, see Payne [4]. _{The- safety}

of a vessel is ultimately affected by

extreme responses. Harmon-ic motions _and

accelerations can be predicted _nowadays

with a reasonable- deg-ree of _accuracy,

but insufficient prógrss has been made

so far in accurate predictions of _peak

values. H 0 I -I 0 ! I - I I I I 0 2 3 2 H 0 2 3 2 I

33

3 I -- I 4 7

62

-3-S

8

6--I

- 3 II 16 9 2 y&_V - 20 in. I 7 IO24: 8

2)I'

- H \\ 2 6 13 24 29:28 41 29/4'

8,4'

2,/<-V

/

-30 iii. t. I 6 42 50 22 I 6 2/

-/

I

A-

12 67 47 19 .5- I /

-,oAW -

--'a C C 3 0

Under assumption of a voyage scenario (speed and heading distributed over time,) and based on a wave scatter dia-gram for a specific sea area it is possible yet to arrive at a long term prediction for the most probable maximum response amplitude and wave height to be encountered., see Aalbers [5].

Further refinement of the operability prediction could be achieved by adding

the effects of the underwater hull geometry by means of hull form

parameters. The results of a parameter study at constant displacement, like the one performed by Bau [.6], could be

condensed in a' seakeeping index. From composing a regression, equation the influence of hull form parameters on the response level can :be determined, see van Wijngaarden [7].

Apart from the absolute vertical responses also relative motions and related phenomena (deck wetness, added resistance) have to be considered for fast vessels in a seaway. The most

violent vertical reactions occur in head seas, but horizontal responses can af-fect a ship's operation in other

hea-dings. Roll motions and lateral

acce-lerations generally attain their highest amplitudes in beam seas. Together with roil behaviour various stabilization devices could be studied.

As rnentioned in section 7, the opera-bility figure can only become an abso-lute measure when the prevailing wave conditions can be taken into account with sufficient accuracy. Therefore a

reliable and detailed statistical description of the wave climate is in-dispensable.

Further research and extension of the method in the aforementioned directions will upgrade the conception Nadmissible

28]

9. SUMMARY AND CONCLUSIONS

The purpose of this investigation has

been the set-up of a calculation

pro-cedure for the operability of

semi-planing vessels in a realistic _wave

environment. For linear variations of main dimensions and a number of ship speeds the vertical acceleration amplitudes were calculated at several

locations along the, ship's length. The

transition 'to an operability figure has

been established for the North Sea area.

Operability is defined here as the

an-nual time percentage in'wh'ich _{on board' a}

vessel, travelling at a given speed in head waves, a specified acceleration criterion is not exceeded.

The operability percentage can rapidly

be determined' by the calculation

pro-cedure presented on basis of the fol-lowing key design parameters: main di-mensions (volume, length, beam, draft),

ship speed,, wave climate (wave periods and wave heights), acceleration'

cri-terion and it's location of' 'application..

On base of this investigation the fol-lowing can be concluded:

- A beam increase consistently resulted in an operability improvement. A slight beneficial trend with length

increase was found, only valid for the

chosen North Sea wave 'climate,

especially regarding the predominant wave period. The effects of draft were marginal. An increase of ship speed reduced the operability percentage.

- The operability figure should be

regarded as a relative measure for the prediction of seakeeping performance

in preliminary ship design. It can only become an absolute measure when

'the 'prevailing wave conditions can be

sekeepfltbehavjour" to an objective

specified sufficiently accurate. standard in the dëign of semi-planing

- Further research and extension of the method presented here is needed to cover frequency of motion, extreme responses and roll motions in beam

seas.

- Improvement of the operability prediction in ship. design could be

achieved by taking. into account the underwater hull geometry by means of hull form parameters.

REFERENCES

Eames,, .'M'.,C;.. and.

Dr.ummond, P.G.,

"ConceptExplorat'ion - ah.

Approach to Small Warship Design",

Transactions RINA, Vol.. .1L9., London, 1976.

Blok, and. Beukelman, W.,

"The High-Speed Displacement Ship

Systematic Series. Hull Forms -Seakeeping Characteris,,tics!',, Transactions SNAME, New York.,

November 1984.

Bales, S.L., Lee, W..T. and Voelker1

JoM.,

"Standardized Wave and Wind

Environments for NATO-Operational Areas", Report No.

DTNSRDC/SPD-0919-01, DTNSRDC, Bethesda,

July

'1981..

4.. Payne,, P.R.,

"On Quantizing Ride Comfort and AllOwable Accelerations", AIAA/SNAME Advanced Marine Vehicles Conference, Arlington, September 1976.

5. Aalbers, A.B. and

DaIlinga,R.'P., .

"Model Measurements and Design Calculations for the Transport of a Jack-Upon a Barge",. RINA Inter-national Symposium on Offshore

'Transport and Installation, London,

March 1985.

Bau., F..C.,

"Rough Sea Capabilities and Ship Size: A PaEametric Investigation into

the Small 'Warship Area", High-Speed Surface Craft Conference.,. London, May

1983.

'Wijngaarden, A.M. van,

"The Optimum' Form of a Small Hull for

the North Sea Area",

_Vol.

31', No.

'359', mt. 'Shipbuilding Progress,, Rotterdam,. July 1984.

Notation..

Symbol Designation.

a1!3 Significant acceleration

amplitude B 'Beam on waterline 'CB Block coefficient Prismatic coefficient .CWp Waterplane coefficient Fn Froude number g Acceleration due to gravity

H113 Significant wave height

Longitudinal. radius of gyration Lw Length on waterline LCB Longitudinal center' of buoyancy LCF Longitudinal center of flotation T Draft

Modal wave period

V Ship speed