Criteria for seakeeping performance predictions

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TECHNISCHE UNIVERSITEJT Laboratoslum voor Scheepshydromochanica

Mekelwag 2- 228 co DELET

CRITERIA FOR S:EAK EE PING

PERFORMANCE PREDICTIONS

Tuorno Karppinen

TECNICAL RESEARCH CENTRE

OF FINLAND

SHIP LABORATORY

ESPOO 1987

nStm

ARCHIEF

Helsîngf ors

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SUIU1ARY

Final values of the seákeepiflg criteria established in the nordic co-operative project on seakeeping are given. Grounds for defining thecriteria are discussed. The procedure applying the seakeeping criteria for predicting the percentage of time of operation is reviewed

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CONTENTS

2

Page

INTROÓUCTI0N 3

THE SEAl EEPING PERFORr4ANCE INDEX 4

Percentage operability S

Validity 11

GENERAL SEAKEEPING CRITERIA 12.

Vertical acceleration 15

Läteral accéleration 17

Roll 17

Slamming 21

Deck wetness 22

SEAXEEPING CRITERIA - HUMAN FACTORS 23

CONCLU$ION 28

ACKNOWLEDGEMENTS 28

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IIJTRODUCTION

In a seaway ship performance deteriorates compared with the

performance. in calm water. This degradation of performance due to weather and wave-induced motions may appear as an involuntary speed loss, a voluntary speed reduction or as a lowered working

effectiveness of ship personnel. Violent motions may prevent work onboard which is essential for the mission of the ship thus causing lost working days.

The task of the VTT Ship Laboratory in the nordic co-operative project on seakeeping has been to find a general ranking system for seakeeping performance of ships and to define limits for acceptable wave-induced motions. These operability-limiting criteria with

regard to ship responses, called .seakeeping criteria, are used in the procedure for predicting a numerical value for a seakeeping

performance index. This ifldéx, defined in the nordic co-operative project as the percentage operability, is used as an overall measure of merit, or ranking for seakeeping performance of ships.

Conolly (1974) and Llóyd & Andrew (1977) establish seakeeping

criteria with regard to vertical acceleration, slamming, deck wetness etc by considering the behaviour of a one particular ship during

full-scaléseakeeping trials in severe environmental conditions. In the nordic project the problem has been approached by collecting a large amount of full-scale seakeeping data and proposed criteria

from the litterature. Seakeeping criteria have then been defined on the basis of a critical review of the information available

(Karppinèn & Aitta, 1986) supplemented with oUr òwn observations during full-scale. seakeeping tests. This paper gives the final values of the seakeeping criteria and discusses their basis. First, hwever, prediètion of the seakeeping performance index is

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THE SEAKEEPING PERFORMANCE INDEX

The practical evaluatiön of ship seakeeping performance requires a numerical index by which to measure the seakeeping performance. The index should measure the ability of the ship to fulfil its function in environmental conditions the ship is likely to encounter in its lifetime or over a long-term interval. Since the effect of the seaway and weather degrades the mission performance of the ship in relation to the calm sea performance, it seems natural to formulate the

seakeeping performance index so that it measures the performance degradation. The mission performance in still water is taken as standard of reference.

From the point of view .of the participants in the nrdic co-operative project on seakeeping it is important that a numerical value for the. index may equally well be computed on the basis of basic seakeeping model test data and theoreticallY predicted ship re.sponses. In

seakeeping model tests the number of speeds, headings and recorded resppnseS is always limited compared to theoretical predictions by computer. Thus thé measure of merit should be such that a numerical value can be determined on the basis of nuinimu amount of data on elemental mótion components of the vessel.. When more information on

the seakeeping characteristics of the vessel accumalates., it should be possible to update the v'lue of the seakeeping performance index.

It has often been uggested that seakeeping perförmance of merchant ships could simply be measUred by their ability to maintain .speed in heavy weather. The sustained sea speed reflects the ability of a merchant ship to fulfil its fUnction, i.e. to deliver çargo and

passengers safely and precisely from port to port,, regradless of sea conditions.

In predicting the sustained sea speed both (1) the involuntary speed loss due to added resistance and loss of propulsive efficiency caused

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due to excessive ship motions at the discretion of the ship's master should be considered. It is unusual to measure in seakeeping model tests all the basic data required for à prediction of the sustained sea speed. This limits the use of the sustained sea speed as a seakeeping performance index.

Hösoda et al. (1983, 1984 and 1985) apply methods of reliability engineering for assessing the. seakeeping performance of patrol vessels. The measure of merit used by Hosbda et al., the mission efféctiveness, is well suited for evaluating the seakeeping

perfOrmance when the. effectiveness of the personnel and equipment of the vessel are very important for the task of the vessel.

Percentage operability

In the' nordic co-operative project the percentage of time of

operation has been chosen as the measure of seakeeping performance. This measure of' merit has often been used for assessing the

seakeeping performance of naval ships (e.g. Johnson et al., 1979, Chilò & Sartori, 1979, Bales, 1981, McCreight & Stahl, 1985, and Chilò et al., 1986), but as a comparative measure of seakeeping

performance it is equally well applicable to merchant ships. By naval ships this seakeeping performance index expresses the percent of time the ship is capable to certain operatioñs - according to its mission - on a given sea area over a long-term interval, the operational capability of the vessel is defined in terms of ship motion limits. When ship responses exceed the limiting criteria, the operatioñs cease. Thus, the index value expresses the percent of time the ship motions are smaller than the. operability-limiting criteria.

Figure 1 illustrates prediction of the percentage of operation for naval ships. Many special vesséls such as fishing vessels,' supply ships, tugs and ocean research vessels are similar to naval ships

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PED(CTED RESPONSE PERFORMANCE CRITERIA

j. =

S.P.L=12

SOC SPEED HEADING SEA STATE 6

r

0.1. X - SOI FOR GIVEN ROC

SEAKEEPINO OPERATING ENVELOPES

SEA STATE FREQUENCY IF OCCUR RINCE

SPE ED/HE A DING

TIME PROFILE

Figure 1. _{Development of seakeeping operating envelopes and seakeeping} performance indices, or the percentage of time of operation for _naval ships (Comstock et al., 1980, _{Keane & Sandberg, 1984).}

SHIP RESPONSEJEVENT _PERFO1ANCEFULL ROLL (DEGREES)

PITCH (DEGREES) 3

BOW WETNESSES (PER

HOUR) 30

SLAMS (PER HOUR) 20

X

LUM

_{ROC RE AlIVE}

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The missiOn Of each of these vessels includes different special tasks. carried out onboard and requires operation at different speeds.

With ordinary merchant ships the value of the seakeeping perfòrrnance index at a particular speed and heading may be defined as the percent of time the ship is capable of maintaining the speed and heading on 'a specific sea area. If only voluntary speed' reductions due. to

excessive ship motions are considered, this operation index can be computed for all ships in head seas on the basis of frequency

response functions and phases of heave, pitch and vertical relative motion.. When the frequency response functions are known at several spéeds and headings, all speed and heading

combinations can be weighted according

to importance and the index value is

'--

'--'

-obtained as a weighted 'mean. If the ' INPUT

added resistance and the propulsion Frequency responses

efficiency 'in waves are known too, the

effect of involuntary speed reduction ' '

can be taken into account in the index ' Wave spectra

value.' When the percentage oper-' ISSC and JONSWAP

ability is determined for several ' . .

-speeds 'up to the maximum calm water

speed, it is possible to derive the 'Response rms values avè.rage annual or seasonal speec

shown 'by Chilò & Sartori (1979).

The flow chart in Figure 2 and Figure 3 show the basic steps involved in the procedure for predicting the

percentage of' time of operation at a particular speed and heading. The corner stone of the programmed procedure is the subroutine 'for compùting the root mean square. (rms) values Of ship responses critical

Operability-limiting criteria,

Operability in .%

Figure 2. Prediction of percentage operability.

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RES. AMPI... WAVE AMPL.

X

30

SIGNIFICANT S.A AREA: Y WAVE HEIGHT TRANSFER FUNCTION R1 RESPONSE X SPEEÒ;V kfl u deg WAVE FREQUENCY WAVE FREQUENCY WAVE FREQUENCY RMS1 _CRITERION: KAXIUH ALLOWABLE IISVÀLL

II';

p .tW34

MODAL WAVE PERIOO

OPERABILITY UMITG H DUE TO RESPONSE X CRITERION RMSX PERCENTAGE OF OPERABILITY

Figure 3. Prediction öf the percentage of time of operatiOn at a particular speed and heading.

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from the point of view of ship operations. The response rms values are, determined in ISSC and JONSWAP wave spectra with a significant wave height equal to one metre as a function of a characteristic wave period by applying the principle of linear superpositioñ. Thus the

ship motion response to irregular sea is obtained as a sum of the responses to the individual regular wave components making up the seaway. Ship motion responses to regular waves with different frequencies are definéd in the input by.the frequency response

functiOns. They can be determined either theoretically by :the strip method, for instance, or by model experiments.

Oncé the response rms values for unit significant wave height are known the operability-limiting significant wave heights with regard to critica,] ship responses are easily obtained by the aid of limiting criteria stored in a file. The operability-limiting boundaries are plotted on a wave scatter diagram as Figures 3 and 4 show. The 'wave scatter diagram expresses the join.t probabilities of occurrence for significant wave height and characteristic period in the operation area of the ship. In the computer prograc the wave scatter diagrams are. in the form of Weibull distributions fitted to the. wave data in each period range. The probability of significant wave height not exceeding the operability-limiting value is determined in each wavé period class from the particular Weibul 1 distributiOn. The numerical value of the seakeeping performance index, or the percentage

operability in the specified environment at a, given speed, V, and hading, p, is finally obtained by:

= _{P(T.;V,i)Q(T.),} ( 1)

J J

.J

where P is the probability of significant wave height not exceeding the, operability-limiting value and Q is the percentage of wave observations in the period range with central. period T. The

fore-going procedure can be repeated for several headings at eaçh speed if only the corresponding response amplitude operators are known.

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lo.

7 8 9 lo 11 12

MOÜAL WAVE PERIOD [séc]

Figure 4. _{Operability limiting significant wave heights with regard to} vertical acceleratiòn, slamnïing and deck wetness plotted _{over a wavé} scatter diagram. The Joint, probabilities of occurrence, for significant wave height and modal wave period are in per cent

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Then the percentage operability becomes the weighted mean over ail headings and speeds:

P(Ocean area) = E

kl

Vk)PO(Vk,l). ( 2)

Here is the frequency distribution of speed and f is the conditional frequency distribution of heading given hip speed.

Both frequency distributions are to be defined in accordance with the missiòn profile of the ship. The procedure can easily be extended to include several ocean areas with different wave climates.

Validity

In deriving the formulas for predicting the seakeeping performance index it has been assumed that

(i) the linear superposition principle and (2) the Rayleigh distribution

can be used. Despite of these assumptions also roll motior has been included in the formulas. Roll response of most ships is non-linear, at least at resonance, and this makes the application of t1e linear

superposition and the use of the Rayleigh distribution questionable for predicting the short-term statistics. However, operability-limiting boundaries with regard to roll seem to be so low that at this level of roll motion the non-linearity should bé insignificant.

For heave and pitch and respònses dependent on them, at moderate ship speeds and in moderate seastates, the Rayleigh probability law has been verified in numerous correlation studies in full- and model-scale. Responses of fast small craft may' cOntain non-linearities whiçh make questionable the use of the linear superposition and the Rayleigh probability law.

The fihal resúlt of the prediction, the percentage operability P should not be considered as a deterministic figure defining exactly the number of operational days in a year. From year to year the sea

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conditions that the ship encouñters on a specific sea area vary statistically. Statistical uncertainty of the prediction is further incrèased for instance by human differences iñ characte's Of ship masters. kthiie a da'ing captain may accept up to 5 heavy bottoni

impacts in 00 wave encounters a more careful master slows down after 2to 3/100 slams.

GENERAL SEAKEEPING CRITERIA

Speed is reduced, course is changed or operations are discontinued by the order of the captain if the safety of the ship, cargo or

personnel is threatened, the effectiveness of the personnel has dropped significantly or habitability onboard is reduced. The seakeeping criteria define the level Of ship responses at whih actions are taken by the captain to redUce the magnitude of ship motions. The main concerns of the captain are here (1) excessive vertical and lateral accelerations, (2) slamming, (3) deck wetness, (4) rolling and (5) screw racing. The importance of different ship responses from the point of view of different ship subsystems is shown 1h Table L

In the operability prediction procedure., criteria have been defined with regard to all responses marked with a Ø in Table 1. This chapter considers general operability limiting criteria for ship hull, equipment, cargö and ship operations while the limiting levels òf motion for different jobs onboard and for passeñger cmfort are given in the next chapter. A criterion with regard to pitch has not been considered necessary since on merchant ships large pitch angles in themselves are usually not a problem. The effects of pitch that may be experienced by thepeople onboard are accounted by the

criteria with regard to slamming, deck wetness and vertical

acceleration. Nor has a limiting ieiel been set for the tolerable frequency of propeller emergence. There is only little information available On screw racing in full-scale and it is not clear how

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Table 1. LIMITING CRITERIA VERSUS SHIP SUBSYSTEMS

1 _{For equipment on foredeck.}

2 _{For deck cargo.}

For operations on open lower decks.

Table 2. GENERAL OPERABILITY LIMITING CRITERIA FOR SHIPS

Ship subsystem Slam Deck

wetn. Criteria Vert. acc. with Lat. acc. Roll regard to Pitch Vert. mot. Vert. vel. Rel. mot. Ship hull Propúlsion machinery Ship equipmènt Cargo Personnel effectiveness Passenger comfort Specia.l operations helicopter sonar . lifting . o1 o2 $ ô 0 O Ø $ 0 0 Ø 0 0 o O 0 o o O O O Ô o O O Mérchant ships Nava.i vessels Fast small craft Vert. acc. rms, FP Fïg. .5 0.275g 0.65g

Vert. acc. rms, bridge. 0.15g 0.2g 0.275g

Lati acc. rms, bridge 0,12g 0.lg 0.ig

Roll t'ms 6.0 deg. 4.0 deg 4.0 deg.

Slamming, cnt. prob... Fig. 11 0.03 0.03

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Table 2 givés the basic set of seakeeping criteria used in the operability prediction procedure. _{The slamming and deck wetness} criteria have been defined in terms of critical probability (events per hundred wave encounters). _{All other criteria in Table 2.}

are root mean square values and g is the acceleration _{due to gravty.}

Table 3 below presents the different points of view considered in estimating the limiting magnitudes _{of ship motions in Table 2.}

Table 3. _{POINTS OF VIEW CONSIDERED IN THE CRITERIA}

Thus, in defining the critical level òf vertical _{acceleration at the} forward perpendctilar (VP), the safety öf ship hull and cargo in general have been considered. _{The acceptable level of vertical} acceleration on the bridge is based on crew safety and performance1 The roll and lateral acceleration criteria _{take into account the} working _{conditions of ship personnel and safety of} _{the cargo.}

From the point of view of equipment operationthe _{critical magnitudes of} motion could probably often be somewhat higher _{than in Table 2.}

Table 2 shows a special set of criteria for a fast small craft. This has mainly been defined to stress the different _{nature of motion of a} small vessel at a very high speed. _{The linear methods used in}

Criterion Hull safety Equip. operat. Cargo safety Personnel safety and efficiency Vert. acceleration, FP Vert. acc., brfdge Lateral acc., bridge Roil Slamming Deck wetnéss O $ O O 0 O

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détermining theoperability are not particularly well suited for seakeeping predictions of fast small craft.. For fast craft also the numerical values of the operability-limiting criteria are only

indicative.. Here a fast small craft may be defined as a vessel under about 35 metres in length with speed in excess of 30 knots. A

planing craft is definitely in the category of fast small craft. When a fast craft operates at a low speed, the criteria setfor naval

vessels should be used.

Vertical acceleration

Though ships do not reduce speed or change course as a result of high vertical accelerations at the forward perpendiçular - nobody is at the FP to estimate the magnitude of acceleration criteria have been definèd. alsO with regard to this motion parameter. Criteria at the FP should be. considered rather as criteria fOr cOmparing seakeeping performance of alternative designs than as limiting critéria for reducing speed or changing course in actual service.. The vertical acceleration criterion at the FP reflects the Overall level of

vertical motion of the ship in weather conditions where sla.mïning or deck wetness, the primary reasons for a manoeuvre tO reduce ship motions, may be critical. On the basis of the published data it is believed, that the vertical acceleration criteria are more reliable

than the slamming and deck wetness criteria.

t4ith.merchant ships the operability-limiting rms vertical

acceleration at the FP, shown in Figure 5, decreases with increasing ship length. The critical acceleration boundary is in close agreement with Aertssen's (1968) proposai and correlates well with later

full-scalé observations.

A special criterion of O..1'7g rms vertical 'accelération'at the FP has been dèfiñed for vessels carrying ro/ro-cargo. This limiting

magnitude of acceleratiOn is based on the full-scale data by Ferdinandé & DeLembre (1970) and Lindemann et al. (1977).

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z

D

I-'-<

w

w.

Li

¡

NAVAL'

- iL

ORO/ROE CAR6O 16 NORDIC PROJECT o o

'0

(,,

!)tJU10iUF. L1924J eIv

_-

o !tii&N_L19J6J o o -o

-o n o REFERENCES:

AERTSSEN & vari SLUYS (1972)

FERDINANDE & DELEMBRE (1970)

HANAOKA ET AL (1963) LÍÑOEMAN ET AL (1977) WAHL (1979) WESTIN (1977) - I i - I 100 200 300 LENGTH BTW PP (ml

Figure 5. Criteria established in the nordic projeçt compared with full-scale rms vertical acceleration at the bow of merchant ships in serviceand sorne limiting criteria proposed in the litterature.

L 0.5I 171 REFERENCES: 11) BLEOSOÉ ET AL. (1960) 121 KARPPINEN (1972) 13) HADLER ET AL. (1974) (41 ANDREW & LLOYD (1980) 151 BAKENHAUS (1980)

161 APPLEBEE & BAITIS (1984)

50 .75 100 . 125

LENGTH BTW PP 1m]

Figure. 6. Vertical acceleratiOn criteria for naval ships established in the nordic project compared with full-scale seakeeping trial data..

Measurement station is iven abaft of the FP.

A V, 0,2 I21 1 AlITA (1987) Ill FP 121 0.191 -161 ß.2L ¡3). 121.

I

Ill BRIDGE I1 IS) LLCÜ 121 0.r41 151 o

ii

161 L 0.25. E L. _0,3 I-HcC _0,2 t-J L) LU

>

NAVAL SHIPS BOW 0,4 a BRIDGE

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The criteria with regard to vertical acceleration on the bridge are only to be applied för basic work on the bridge such as steering, observation and navigation. Criteria for other types of work onboard are given in the next, chapter7

The criterion of 0.275g .rms at the FP has often been used in comparisons of naval ship operability, e.g. Bales (1981) and

Walden & Grundman (1985). This criterion, as well as the criterion on the brïdge of naval vessels, 0.2g rms, agree quite well with full-scale observations onboard naval vessels during seakeeping trials, as Figure 6 shows.

The high acceptable vertical accelerations onboard fast small craft aré due to different frequency content of vertical accelerations on

this type of vessels. On fast craft the acceleration spectrum is wider and is located at higher frequencies than ondisplaceífleflt-tYPe

vessels. (Figure 7). Figure 8 shows. that human beings tolerate high frequency vibrations much better than inôtion at low frequencies. Usually also the mission of a fast small craft is such that operation at high speed is only required for a short period of time., making the

high accelerations more acceptable. . . .

Lateral acceleration

The lateral acceleration criterion of O.lg rms on the bridge is equal to the U.S. Navy surface ship criterion estimated on the basis of crew safety and performancé. This motion limit applies for such

operation as transit and combat in general and underway replenishment. Masters on merchant ships seem to permit slightly higher lateral

accelerations than the U.S. Navy criterion, according to some full-scale observations (Áertssen & van Sluys, 1972, and Hoffman, 1976)..

Roll

the roll criterion of merchant ships is mainly based on full-scale data shown in Figures 9 and .10. The data has been recorded on the

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10

Saa

[m2ÌsÌ

I

18

V

21 kn

L

70m

rms=0122g

Ùate: 18.12. 1970

2 3 4 E

CIRC.

FREQ. OF ENC.

[lis]

V=2Okn

L= 21m

rms; 0,37 g

Date: 1. i0 1971

4

CIRC.

FREQ. OF [NC.

ills]

Figure 7. _{Vertkal acceleration spectra measured at} _{the bow of a} corvette and a fast; small craft in head _{seas IKarppinen, 1972).}

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10 8.0 6.3 5.0 4.0 3. 2.5

- 2.0

_i

..

1.6 125 i l.o V, 0.8 4' 1

'j

0.5 0.4 0.315 0.2S 0,2 0.16 0,125 0.1 01 1g 8h

ïL

tiofl sickne.s region

ISO 2631/3 Gota (1983)

- - -

_{icCautey et. at.}

0125 0.16 02 0.25 0.315 0.4 0 5

Frequency (Hil

0 63 0,8 10

Figure 8. The severe discomfOrt boundary according to the International. Standard ISO 2631/3-1985 with regard to vertical acceleration as a fuñct.ion of frequency for exposure times of 30 minuteS, 2 hoUrs and, tentatively, 8 hours. Also shown are Goto's

(1983) proposal for a standard and resuls of laboratory experiments by McCauley et al (1976) corresponding to a lo % motjon sickness

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o

100-E 'S

50

A co

Q

cl:: Q-20

EST. rms ROLL [dég]

2.

3

4 55G

5

I -

.--t

I -5 .

10,.

15

20 MAX ROLL ANGLE [degl.

Figure 9. _{Percentage exceedance ofmaimwii roll}

angle obseíved

onboard container ships at the morflent of _{coUrse change due t'o violent} roiling. Data by Wahl (1979). _{The rms value has been estimated by} assuming that the maximUm is equal _{to the most probable extreme value} in 200 successive oscillations. MAXIMUM (MEASURED) 2*rrns (ESTIMATED BY 2xrms:MAX./163) 2xrms (MEASUREO) WESTIN (1977) AERTSSEN (1968)

AERTSSEÑ & VAN SLUYS (1972)

AERTS SEN (1977) X 'u -J (D

z

V)

;10

20 KITAZAWA U At L,

s.

NOROIC PROJECT loo 200 ₃₀₀ LENGTH 81W PP 1m)

Figure 10. _{The roll criterion for m.nerchant}

ships established in the nordic project compared with full-scale data measured in severe 'sea

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same vessels as the data used for estimating the limiting magnitude of vertical acceleration at the FP. On the basis of safe footing,

for which the critical roll angle appears to be about 14 degrees (St. Denis, 1976), and the fastdegradatiófl of human performance with

increasing roll motion as summarized by Cox & Lloyd (1971), the operabilitv_linflitiflg rms roll could not be nich higher than 6

degrees. Already at 8 degrees rms roll the probabilitY of roll

angle

exceeding 14 degrees is more than one exceedance in five

oscillations. A roll criterion around 4 degrees rrns has often been refered as an upper limit to ensure maximum crew effectiveness onboard naval vessels.

Slamming

The critical slamming probability, or the critical number of slams per 100 wave encounters against length between perpendiculars for merchant ships is shown in Figure 11. The limit has been defined partly on the basis of published. full-scale data and partly so that the operability limiting wave heights obtained by applying the

slamming criterion are sensible compared with the operational limits due to vertical acceleration at the FP.

In the operability predictions, Ochi's (1964) definition of a slam is used. According toOchi a bottom impact is called a slam if the f reship (here at 0.15L abaft of the FP) emerges from water and the vertical velocity relative to water surface at the station exceeds a critical value of

V

= 0.093 (t,

( 3)

where g is acceleration due to gravity and L is length between perpendiculars.

In the average, Ochi's fórmula for the critical re-entry velocity is

sound. In some cases, however, formula ( 3) may predict a much too low treshold velocity and in some other cases a muchtoo high value.

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A better de.firiftjon of the _{critical re-entry velocity would}

take into account at least the suspecttbilfty of

the forebody shape to slamming and. the speed of the vessel. _{For fast small craft the slamming}

criterion should be applied _{with great care.}

With passenger vessels, the _{operability limiting wave, height}

obtained by applying the bottom slamming criterion, may in _{many cases be töo} high. The comfort of the _{passengers may require a speed 'eduction}

or a course alteration already _{on the basis of whipping vibration}

induced by flare slamming

Unfortunately there doesn't seem to be enough data available to define a criterion with regard to flare slamming.

Deçk wetness

Like the slamming criteriOn _{also the deck wetness}

criterion must be used with deliberation for _{small craft.}

On fast vèssels, already thick spray maybe _{an iflcentive for a slow-down.}

In the operability predictions deck wetness is defined _{to take. place when at}

a

particular station the amplitude _{of vertical relative motión}

exceeds the freeboard1, _{This definition does not distinguish diffe,ent}

degrees of deck wetness from spray to green.waer. 22 0.04 I-. -J 0,03 (D

z

0,02 .4 -J -J .4 001 b-U 100 ₂₀₀ -300 LENGTH BTW PP uni

Figure 11. The slamming criterion

for merchant ships _{established in} the nordic project.

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SEAKEEPING CRITERIA - HUMAN FACTORS

Depending on the mission of the ship, the ability of the personnel to carry out a particular job may be critical from the point of viewof operability of the.ship. If ship motions aré sufficiently severe,

they prevent work onboard on the grounds of crew safety.

Wave-induced motions reduce habitability onboard by rendering normal daily activities difficult and cause fatique in people exposed to

severe ship ctions for longer periods of time. In people 1!nadapted to ship motions symptoms Of motion sickness appear after a short exposure when vertical acceleration exceeds a cértain low level.

Seakeeping criteria based on the safety and working effectiveness of the crew and comfort of the passengers have been defined with regard

to: _{vertical and lateral accelerátion and roll.} _{When a ship response}

exceeds the limiting magnitude at the wOrk site or in the passenger spaces, it is assumed in thé seakeeping performance prediction

prócedure that the operational boundary of the ship has been reached. The particular job is discontinued, cannot be carried out or ship speed is reduced pr course is changed to reduce the level of wave-induced motions.

Table 4 presents the seakeeping criteria with regard to rms vertical acceleration and a short description of the corresponding working and living conditions. The same table gives also some references on the basis of which the limiting inagnitude.s have been estimated.

Additional references can be found in Karppinen & Aitta (1986).

Table 5 repeats the vertical acceleation criteria and shows the corresponding criteria with regard to roll and lateral acceleration. The magnitudes are based on the litterature, where more information

is availablé on the effects of vertical acceleration on human performance and comfort than on the effects of roll and lateral acceleration. Thus, in Table 5 the limiting criteria with regard tó vertical acceleration are more reliable than the other criteria.

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Table 4. _{LIMITING CRITERIA WITH REGARD TO VERTICAL ACCELERATION} Vert. acc. rms 0.275g 0. 2g 0.1 5g 0. 1g 0.05g 0.0,2 g 24 DescriptiTon

Simple light work. Most, of the attention must be devoted to keepfng balance. 'Tolerable onÏy for short pèriods on high speed craft. _{Conolly (1974), Bakenhus} (1980).

Light manual work to be carried but by people adapted to ship motions. Not tolerable for longer periods. Causes quickly Iatique. _{Mackay & Schmitke (1978), Applebee.&} Baitis (1984).

Heavy manual work, for instance on fishing vessels and supply ships.

Intellectual work by people not so well adapted to ship motions. For instance scientific personnel on ocean

research vessels (Hutchison & Laible, 1987). _Work of a more demandin,g natüre. _{Loñg-term tolerable for}

the crew according to Payne (1976). _{The'International} Standard ISO 2631/3 (1985) foi- half an hour exposure period for people unused to' ship motions (Figui-e _8).

Passengers on a ferry. _{The International Standard for} two hours exposure period foi- people unused to ship motions. _{Causes symptoms of motion sickness (vomiting)} in approxiiately 10 of unacclimatized adults.' _Goto

(1983), Lawther & Griffin (1985).

Passengers On a cruise liner. _{Oldr people.} _{Close to} the lower treshold below which vomiting is unlikely _to take place. _{Lawther & Griffin (1985).}

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Table 5. CRITERIA WITH REGARD TO ACCELERATIONS AND ROLL

Here the seakeepiflg criteria based on the working effectiveness and safety of the crew have been defined as on/off-type criteria. Either the particular job can be done or it cannot. Some jobs, however, may be such that the job cari be carried out even in very severe sea

conditions but it jUst takes a longer time to complete than in calm sea. With these jobs it would be realistic to consider the

degradation of human effectiveness as a function o ship motions as Hosoda et al. (1983, 1984, 1985) do.

Figures 12, 13 and 14 compäre the limiting magnitudes of motion given in Table 5 with curves for degradation of human effectiveness

estimated by Uosoda et äl. (1984). Figure 12 shows also data by Applebee & I3aitis (1984) and Aitta (1987) frOm full-scale seakeeping tests. The data points by Applebee & Baitis are average estimates of

the crew on their own effectiveness, and results by fitta are based on psychological tests done for the crew in severe sea conditions. The critical motiOn levels of vertical acceleration for heavy manual and simple light work in Table 5 correspond approximately to an

effectiveness of about 55 % on the scales of Hosoda et al. In Figure 14 for roll motion the limtting levels of magnitude correspond to

significantly higher levels of human effectiveness than in Figure 12. Root Mean Square

Vert. acc.

Criterion

Lat. acc. Roll

Description

O.20g O.lOg 6.0° Light manual work

O.15g O.07g 4.0° Heavy manual work

O.lOg O.05g 3.0° intellectual work

O.05g O.04g 2.5° Transit passengers

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However, the seakeeping trial data by Applebee _{& 13aitis (1984) Is} signticant1y below the _{performance degradation}

curve for heavy work

by _Hosoda _{et al.} loo e 2G .-..:HOSODÁ ET AL. (1914) APPLÉBEE&BAJTJS (1984) AITTA ₍₁₉₈₇₎

E1\

_X LU -J

o-z

-I

0,1

_O2

VERTICAL ACCELERATION

_{rms (g]}

Figure 12. _{Seakeeping criteria}

based on human _{factors compared with} seakeeping trial data _{and Ilosoda'sestf,nate}

of degradation of human effectiveness.

¡

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V) V) Lii

X

LU

>

I

L)

-LU u-LAi

z:

=

V) (U (D

z

W V) Vt t-L,,

z

27

J

z

2:

t-(D

J

r

4 s

005 o'io

LATERAL ACCELERATION rms (g]

0:15

Fiyure 13. Seakeeping criteria based on human factors compared with Hosoda's estimate of degradation of human effectiveness as a function

f lateral acceleration. --_HOSODÁ ET AL. (1984) £ APPLEBEE &BA1TIS (1984) .5 5-10 15

ROLL rms (deg]

-Figure 14. Seakeeping criteria based on human factors compared with seakeeping trial data and Hosoda's estimate of degradation of human effectiveness as a fúnction of roll motion.

.-.100

HOSOPA ET Al. (1984)

loo

V) V) LU

z:

Ui

>

I-5O

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CONCLUSI ON

Ship operability limiting criteria with regard to _{accelerations,} slamming, deck wetness and roll fr predicting the percentage operability have been defined. _{In estimating the limiting}

magnitudes both technical and human factors have been _considèred. _On the basis of the available _{data it seems}

- unfortunately - that the seakeeping criteria with regard to vertical acceleration _{are more} accurate than the criteria with _{regard to slamming and deck}

wetness, which are the most important

reasons for a manoeuvre to reduce _the level of ship motions. _{It is very difficult}

to define a single slam or a single event of deck wetness for

the numerical predictions in such a way, that it could easily _{be compared with the}

full-scale observations, so there is very littlé suitable seakeeping _{data, on} which the slamming and deck

wetness criteria could be based.

Best the percentage operability _{is suited to evaluating on a} comparative basis the seakeeping

performance potential of _alternativé designs1 and for revealing _{in the design}

phasepossihie problems in the.seakeeping qualities of the ship. _{The operability}

prediction procedure.couid further be developed _{to take. into account the} probabilistic nature of the _{seake.eping criteria due to human} differences in characters of _{Ship masters.}

A daring captain may accept much more beating on his ship than a relief _{master on his} ship.

ACKNOWLEDGEMENTS

This study forms _{a part of the nordic}

co-operative project on seakeeping. _{rn Finland the project has been financed by}

Nordisk Industriefond, Technology _{Development Centre (TEKES),}

Hollming Ltd, Rauma-Repola Ltd, Wärtsila Marin _{Ltd, FinnIsh Navy,}

Frontier Guard and Technical Research Centre of Finland1 which _{is greatfully} acknowledged. _{I would like to thank my nordic and Finnish}

colleagues in the project for _{a pleasant co-Operative}

atmosphere. 28

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29

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