Selection on seakeeping

SELECTION ON SEAKEEPING

R.P. Daflinga

Maritime Research Institute Netherlands (MARIN), P.O. Box 28, 6700 AA WAGENINGEN, The Netherlands

ABSTRACT

The performance in wind and waves is an important aspect in the design of ships and structures. Originating in the offshore industry the corresponding analysis is called _{an "operability" analysis. It establishes the degree to whichthe vessel or} platform meets the requirements of a given mission in given climatological circum-stances.

The present work shows how the results of an operability analysis _{can be used in} ship _{selection and design. BasEd on results for a typical ferry the impact of an} operability _{analysis on the layout of passenger areas, the design of fin} stabiliz-ers and powering aspects are discussed;

1. INTRODUCTION The objective quantify the senger vessel * safety

- structural integrity of the hull - stability in steep waves

- passenger and crew safety - seafastenings loads * economy

- sustained speed - crew performance

- passenger and crew comfort - seasickness

- mobility

- ability to relax/sleep Performance

In the offshore industry it is quite common to express the performance of a given design in terms of the average fraction of time that criteria for various motion components are ex-ceeded In a design "wave-climate". It is re-ferred to as "operability" (non-exceedance of criteria) or "downtime" (exceedance of criter-ia). Although the downtime is not the only and final measure for the seakeeping quality of a design it is an important figure because it relates the three most important ingredients of a performance analysis. These are:

* ship characteristics * prevailing wave climate * criteria.

Obviously the foregoing performance figure is affected by a number Of ship parameters like speed and heading. These "mission" related uantities, as well as the ability of the ships crew to avoid or adapt to bad weather of an operability analysis is to performance in waves. For a pas-this performance is governed by:

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Laboratorlum voor

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Arch let

Mekelweg 2, 2628 CD

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have to be taken into account when judging and comparing performance figures.

Theoretical Frame Tiork

The evaluation of operability requires a pre-diction of the dynamic behaviour for every possible wave condition constituting a wave climate. For this reason the basis Of most methods is linear (or linearized) seakeeping theory. In the following we will adhere to this approach, also because it yields valuable insight in the nature of the operability prob-lem.

2. SHIP CHARACTERISTICS

In principle there are two ways to quantify the motion characteristics of a particular design: calculations or physical experiments. Theoretical calculations by means of computer programs _{offer great flexibility and speed at} relatively low costs. Experiments with a scale model offer relatively large accuracy and an independent check on the calculated results. In _{many cases the designer relies on the}

syn-ergy of both methods.

2.1. Linear Theory

Within the framework of linear seakeeping theory _{linear transfer functions are used to} dEscribe the first-order motion character-istics and quadratic transfer functions to describe _{the added resistance, both in} (regu-lar) _{waves. The prediction of thE response in} an arbitrary (irregular) wave condition con-sists of a convolution of the characteristics of the motions and added resistance and the wave characteristics, in terms of the energy spectrum.

Viscous Effects

Linear theory makes it possible to tackle the behaviour of floating structures and ships by means of potential theory. This approach only accounts for wave making effects, viscous and other non-wave making effects are neglected. Contrary to predictions based.on potential theory, experiments with scale models may suffer from exaggerated viscous effects. Neglecting or exaggerating viscous effects

affects the prediction of the motion com-ponents with little wave, making damping. This is in particular the case for the roll charac-teristics of normal ship sections and the heave _{characteristics of ship sections with a} small beam-to-draft ratio. The latter can be found on special structures like catamarans and SWATH (Small Wãterplane Area Twin Hull) ships.

Regarding the roll damping of rnonohulls vari-ous _{empirical methods are available to obtain} an impression of the contribution of vortex shedding _{from the bilges and combined vortex} shedding and lift from bilge keels, rudders and skegs. Most of this information relies on experiments with scale models.

Notion Control and Roll Stabilization

The laëkóf roll damping of monohulls is one of the main reasons for large roll amplitudes. This implies that with relatively small forces the roll motion can be rEduced drastically. Prerequisite is of course that these forces counteract the roll velocity.

Solutions outboard the hull to obtain more roll dmping are designed to create eddy damping (bilge keels, skegs) or lift (fin stabilizers or rudder-roll stabilization systems). In-board systems consist of spe-cially tuned active or passive free-surface tanks ("flume" or "U"-shaped) or mechanical systems (moving weights).

Fin based stabilizer systems are sometimes al-so used to obtain more heave and pitch

damp-ing; in particular on catamarans and SWATH ships.

2.2. Experiments with Scale Models Test Strategy

The results of experiments with scale models in waves can be used in two ways.

First of all the direct test results are used as design values. In the case of non-linear system characteristics this requires careful modelling of the "design" wave condition. Typ-ical results for ferries concern the stability in high waves and broaching phenomena and im-pact phenomena in the bow flare and below the. keel.

Secondly the results can be used for a more general operability analysis: assuming linear-ity of the test results (in particular re-sponse _{functions) are used to predict the be} haviour for a wide range of sea states. Test-ing in long crested irregular waves (to ac-count partly for non-linear effects but still obtaining response functions) is attractive for this purpose. Typical results concern the motion response in normal operational condi-tions and related acceleration levels. Also the stabilizer angles in non-stalling condi-tions can be described in terms of a response function.

2.3. Motion Characteristics of Nonohulls Calculations and experimental work yield in-formation on the motion characteristics of a ship. Figs. 1 to 3 indicate some important characteristics of a typical ferry design, they concern the roll motions and the trans-verse and vertical accelerations in the for-wardhalf of the ship.

The results were generated with the strip theory program SHIPHO, the presence of fin stabilizers was accounted fOr in a schematic way. All results are presented in terms of response functions, the response in regular waves of unit height. Since this response is a function of wave frequency (wave length) and wave _{direction (ship heading) use is made of} so-called contour plots.

Roll

In order to obtain gentle roll motions normal ferries are built in such a way that the transverse stability is not much larger than safety requires. In practice this leads to a relatively _{long natural period of roll which} generally falls beyond the wave frequency range _{occurring normally at sea. At non-zero} speed stern quartering seas provide conditions with frequencies of wave encounter come close to the natural frequency of roll. For these wavE _{directions the relatively low damping in} the roll mode yields the largest roll angles. Transverse Accelerations

The _{transverse accelerations are the product} of the transverse motion amplitudes and the square of the frequency of wave encounter. However _{they also contain a gravity component} along the deck due to finite angles of roll. As a consequence two peaks in the response are

observed. See Fig. 2. Vertical Accelerations

Forward speed affects the frequency of wave encounter, which is relatively high for waves coming in over the forward half of the ship. This affects the magnitude of the vertical accelerations, which is the product of the local vertical motion amplitude and the square of the frequency of wave encounter. Fig. 3 shows that consequently the highest vertical

0.0

OPERABILrTY ANALISYS ON A FERRY

ROLL RESPONSE AS A FUNCTN O HEAPING AND WAVE FREQUENCY

SPEED - 18.00 kn

05

10

WAVE FREQUENCY in radls

30 20 15

-WAVE PERIOD inS 5

OPERABILITY ANALISYS ON A FERRY

LATERAL ACCELERATION CAS A FUNCTION OF HEADING AND WAVE FREQUENCY

SPEED ..18.00kn

05

10

WAVE FREQUENCY In rad/s

i-0 50 30 20 15

WAVE PERIOD Ins

135

OPERABIUTY ANAUY ON A FERRY

VERTICAL ACCELERATION AS A FUNCTION OF HEADING AND WAVE FREQUENCY

SPEED 18.00 kn I I - I I 50 30 20 15 1.00 10 WAVE PERIOD In s 5

Fig. 3 Vertical acceleration as a fuiiction of heading and wave frequency

BEAM

STERNO

STERN 13

0.0

05

lo

WAVE FREQUENCY In iad/s

HEAD

BOWO

45

20. IS. E C -J -C -C C) > 10.0 0

)

1 C 0 C a, U, 5.0 8eOufàrt. nurber

I-

-I 5 6

Sea BLOI.o number

Fig. 4 _{Beaufort Number, sea state and wave height}

Although often used in ship operations the above approach fails tp recognize that one wind coñdifion can show different wave height and period characteristics These characteris-tics depend strongly on the stage of growth of the waves, which is governed by the fetch and duration of the wind, Since the wind speed and direction are not constant it means that the waves are hardly ever in an equilibium condi-tion. An example of the related scatter in wind and wave data is illustrated in Fig. 5.

e0.p 60.0 a) 0 C -x C 10.0 . a) a) 0. a) c 20.0 0.0 Wove I he.ghL

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RoLL, North ALLOU.0

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acceleration levels are observed in head and bow quartering waves.

3. CLIMATE BeaüfortNumbers

The oldest and simplest way to characterize an offshore _{environment is to characterize the} wind climate, for instance by the frequency of occurrence of the various Beaufort numbers These _{wind classes are then related to} "aver-age" wave conditions. Fig. 4 _{summarises some} commonly used relations.

> 20 -U 10-S

0_

C 0 u-Bf 4 (11-15 kn) 663 case Bf 6 (2227 kri) 433 cases 10-C) r

0.

Dg0

WAVE HEIGHT AND BEAUFORT NUMBER NORTH SEA C P .r

C.

-Wave height Hs )

I-Wave height H

Fig. 5

Vave height and Beaufôrt number

_oOO

0-Bf3

_-50 (7-10 kn) 373 cases -40 -30 -20

OD_o

LI P Lf' C Bf 5 (16-21 kn) -30 731 cases -20

__Lo0

_Z

LI Ô L

0 i

C L C C

0

LI Sf 7 (28-33 kn) 234 cases 20 60- Bf 0-1 (0-3 kn) 50- 90 cases Bf 2 (2-4 kn) 4o 127 cases U 30- 20-Bf 8 (34-40 kn) 93 cases 20 S 30-)- 20-U F0

-In the situation of a high wind speed and growing wave height most of the energy input will take place at the high-frequency tail of the wave spectrum, resulting in a low, growing steep wind wave. This implies that the average period viii be relatively short. If the wind speed drops the continuing non-linear transfer of wave energy from higher to lower frequen-cies viii turn the wind wave into a relatively high, decaying long period swell.

Neglect of the wave period information is ag-gravated by the fact that the behaviour of ships and offshore structures is quite sensi-tive to wave height, wave period (motion response, drift forces) and wind speed (speed loss, dynamic positioning).

Scatter diagrams

In the ôfUhôè industry the _{availability of} wave measurements has led to the introduction and _{use of so-called wave "scatter" diagrams,} which reflect the joint statistics of wave height and average wave period. The wave con-dition at a particular instant is character-Ised by only two parameters, the significant wave height and the average. period (most fre-quently the average zero-uperossing period). A "typical" single-peaked spectral shape (for instance the JONSWAP fotniulation, see Fig. 6) is used to model the wave spectrum.

Period Information

When ôdnsideTing the left hand side of the wave scatter diagram the wave height/period is limited by the steepness of the individual waves. It is the domain in which simple single peaked parametric wave models (with a spectral shape like the JONSWAP formulation) are quite successful.

The wave period information reflected in the lower right hand side of the wave scatter diagram should be regarded with some reserve, especially when considering swell conditions (relatively low wave height and longer wave periods). In real life double peaked wave spectra are quite common in these conditions (a mix of wind driven sea and a long period swell). The selected period definition (the "average" zero-uperossing period) masks the period of the swell. From these facts it may be concluded that the schematization into a single _{spectral peak can yield unreliable} re-sults.

Wind Information

Empirical information used by Janssen et al. [11 in the formulation of a parametric wave model may be useful to describe "average" conditions in non-statiOnary wind conditions. The relation is:

= 9.25

U0.376

(1)

in which U represents the wind speed at 10 m height, T the peak period of the spectrum and the significant wave height. Numerical evaluation of this result for a family of Kruseman wave spectra yielded the folloving results with regard to the average zero-up-crossing period T2 (see also Fig. 7):

B C T

VwAT2Hs

(2)

with V = wind speed at 10 m height in rn/s T = zero-uperossing period in s and:

Hs = significant wave height in m A = 80.443

B =-1.8421 C = 1.6012 D =-0. 0474 16

High wind speeds create short steep waves. Because short waves introduce relatively large wave drift forces, the combination with high wind loads implies that the positioning problem will be relatively severe in the left hand side of the scatter diagram.

4. CRITERIA 4.1. Introduction Safety

With regard to the safety of ships and struc-türes at sea one is inclined to relate cri-teria to the threats to human life and the natural environment and to structural damage. Some of these can be expressed in terms of the global and local structural integrity and dynamic stability in extreme circumstances. Considering the published information it be-comes clear that very little general informa-tion is available on these subjects. A compli-cating factor is the limited applicability of analytical design tool-s to describe the behav-iour in extreme. circumstances. Consider for instance the problem of the stability and course keeping of ships in steep following seas or the hydro-elastic nature of impact pressures as a result of slamming.

However, some aspects of the safety of ships and structures can be tackled with the avail-able tools. For instance, it is possible to formulate criteria for the design of sea fastenings (strength of lashings), vertical accelerations (human tolerance), green water on deck or transverse accelerations.

Economy

With regard to ships and structures the most obvious economical criterion is related to the performance of the task they are designed for. For a ferry this implies a transport and an entertainment "mission".

cJ N E 3

1.00.

T

0,75

0.50

0.25 0

ThEOREtjcA WAVE SPECTRUN (JONSWAP)

Significant wave height. = 1.0 rn

18 $ 14 s 12 s 11 8 s

AiAA&L&

11iW\WA

IJi4rM

0 - 0.5

_10

21t/ _{.in r&d/s}

Fig. 6 _{Theoretical wave spectra (JONSWAP)}

RELATION BETWEEN WAVE PARAMETERS AND WIND SPEED AREA NORTh SEA

-I

5 10

zERO-UP cROSSING PERIOD ins

Fig. 7 _{Relation between wave parameters and wind speed}

(North Sea) 15

Transport

The transport function, depends strongly on the capability to maintain a particular schedule. Delays are introduced by added resistance and a voluntary speed reduction. The "involuntary" speed loss is a result of the added resistance due to wind and waves. The installed power, together with the engine characteristics, is a direct measure for the sustained speed. Some-times a "voluntary" power reduction is applied to avoid high impact loads or excessive accel-eration levels.

The _{present evaluation method is based on the} thrust balance, neglecting the effects of ship motion and propeller ventilation on the pro-peller efficiency.

Comfort

Although hard to quantify accurately the oc-currence of seasickness among the unadapted passengers _{or an experienced crew may be} re-garded as a relevant criterion for comfort. The occurrence of seasickness is mostly related to vertical acceleration levels [2], however, there are various indications that this aspect of the motion response is not the only important indicator. Fig. 8 relates ver-tical acceleration levels to the occurrence of seasickness as reported on basis of laboratory experiments by McCauley [3] and observations on _{board of ships by Goto [4]. It also} indi-cates the magnitude of the Subjective Motion 'magnitude as proposed by Lloyd [5]. The figure shows that human tolerance is relatively low in _{a frequency range which is quite common in} ship motions.

In addition to seasickness the ability of crew and passengers to move about the ship seems an important _{criterion for passenger comfort and} crew performance. Recent work by Graham 16]) relates mobility to the "tipping" phenomenon; if _{the effective gravity angle (EGA) becomes} too _{large a person will have to seek support.} See Fig. 9. The effective gravity angle EGA is governed by the transverse and vertical accel-eration levels and their mutual phasing. It's instantaneous value is given by:

EGA =

tan_1{J

(3)

The above quantity is rather non-linear in character. In order to obtain a prediction in an arbitrary wave condition the local (linear) vertical and transverse acceleration levels are expressed temporarily in irregular time histories with the correct mutual phasing. The EGA is calculated and expressed in terms of the ms value.

Fin angles

The effectivity of fin stabilizers is based on the generation of lift. The performance breaks

down if mechanical actuators reach their maxi-mum exôursions or the effective angle of at-tack _{exceeds the stalling angle. High} perfor-mance retractable stabilizers are often of the "flap" type with a rectangular geometry. Work by Kervin [7]), supported by in-house experi-mental work, suggests that an effective angle of attack in the order of 20-25 degrees is an appropriate measure for the stall angle. Typi-cal mechanical restrictions are of a similar magnitude.

Recent experimental work with a scale model with instrumented fins indicates that the effective angles of attack are considerably larger than the angles introduced by the fin control. Main sources of the increases are the orbital motions in the incoming and.diffracted wave, hull-to-fin interference and the heave and roll motions of the vessel,. Fig. 10 shows typical _{results. Note the differences between} the windward and leeward fin. The observed ratio of the mechanical and effective angle of attack was used to obtain the effective angle of attack from the current calculated results. 4.2. Adopted Criteria

In order to demonstrate the typical results of an operability analysis criteria regarding seasickness, mobility, fin effectivity and sustained speed were adopted. The criteria proposed _{for the vertical and transverse} ac-celerat-ion levels in the Nordforsk study [8] for transit passengers were adopted as well as a criterion for the _{effective gravity angle} and the roll angle. Table 1 suminarises them in terms of the rins value, the significant double amplitude and the average single amplitude. The criterion for the EGA and the roll are

based on the notion that, if the vertical ac-celerations would be negligible, the results should correspond with the results obtained

for the transverse accelerations. 5. OPERABILITY ANALYSIS

5.1. Introduction

The operability analysis presented in this work were performed with the program JASC0. This program was developed in 1992 on behalf of the Netherlands Foundation for the Coor-dination of Maritime Research [9].

A wave climate typical for western Mediter-ranean Sea is accounted for, it was derived from BMT's "Global Wave Statistics" [10]. The result of the analysis is the average fraction of time that the criterion is ex-ceeded (the sum of all wave statistics above the lines indicating maximum conditions) for the _{wave direction and speed under} considera-tion. It is expressed in fractions out of thousand.

= 10. 5. 1.0 0.2 0.1 ISO 2631/3

GOTO (1983) 10% severe seasickness, on board observations Lloyd (1978) Subjective motion magnitude

- McCauley (1976) Severe seasickness in 2 hours, lab. exp.

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L

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.5 1.0 .2 0 5.0 10.0

Wave frequency _wDeak in rad/s

20

Fig. 9 Effective gravity angle

0

20

Fig. 10 Fin angles and effective angles of attack

cc Large fins deg/degis deg/deg/s 1.2 deg/deg large .52 srnall . 0.40 BC BC l0 SO (.ind..n,-d) Control angle ¼ PS (leerd) cc

-BC

S.ill fins degldeg/s degldeg/n 58 (wIndward) 2.2 deg/deg liege 4.9 sall 2.80 BC 10 Control t}(1eeiiard) 0.5 in radiI 1.0 15 OS uinrsd/s lB 1.5

5.2. Results Mobility

Fig. 11 indicates the frequency of exceedancë of the criteria adopted for the roll response, the transverse acceleration accelerations and the effective gravity angle EGA in the most unfavourable vave direction, stern quartering seas. It shows that for this wave direction the roll motions and transverse accelerations yield very similar operability figures.

Intro-duction of the EGA, accounting for the verti-cal accelerations, hardly affects the results

for this wave direction. Considering the mag-nitude of the vertical accelerations (see Fig. 3) this is no surprise.

In beam seas and bow-quartering seas (see Fig. 12) the relation between the transverse accel-èrations and the roll motions is less obvious; also for these wave directions the introdlic-tion of- the EGA hardly affects the operability figures.

Seasickness

Fig. 13 indicates the results of the operabil-ity analysis regarding the vertical accelera-tions over the length of the ship in bow-quartering _{seas. Five locations, ranging from} station 0 (aft) to stat-ion 20 (forward) are accounted for. The results illustrate. the well known fact that the vertical accelerations are lowest just aft of midships. Fig. 14, indicat-ing the downtime figures over the length of the vessel, demonstrates this point again. It stresses the benefits of a carefully designed arrangement of the working/passengers areas on the ship.

Stabilizers Limits

Based on. criteria for the mechanical "control" angle

c and the effective angle of attack a the wave height was established above whic lift degradation and loss of effectivity may be expected. The results are shown in Fig. 15, they show that in stern quartering seas the maximwn wave height is relatively low.

Table 1 _{Notion criteria transit passsengers 18]}

Sustained Speed

The character of the speed loss die to added resistance from wind and waves is indicated in Fig. 16 for one given poi.ier. _{It shows that in} general the involuntary speed loss is substan-tial. In practice the sustained speed depends strongly on the design point in calm water. Fig. 17 shows that low powered ships require a relatively large service margin, especially if the reliability of the service is important. Taking a 10% level of exceedance as an "ex-treme" value the vessel with 22 knots calm water speed suffers a speed loss of about 5 knots, the vessel with 18 knots in calm water suffers a speed loss of around 6.7 knots. 5.3. Interpretation of the results

The result of the analysis is the average fraction F(x>c) of time that variable x exceeds criterion c. The probability of exceeding it on a given trip depends on the length of the trip.

On short trips the weather will be more or less constant and the above fraction is a di-rect measure for the probability of excee-dance.

On longer trips the weather will not be con-stant. Statistically speaking one is forced to more than one "drawing". Usually one adopts an "independence" [11J period as a measure for the number of drawings. The _{probability of} not-exceeding

NE the criterion in N drawings becomes:

= (1_F(x>c))N

(4)

Taking (arbitrarily) an independence period of three hours this implies that on a 24 hour trip with an average operability of 90% the probability of non-exceedance is only 43%. Ships requiring a good performance, on a year-round basis require very high average operability to make sure that the work can be performed without interruptions.

Designation Unit -RNS Significant double amplitude Average 1/nth single amplitude -N=1 N=3 N=1O Transverse acceleration m/s2 0.4 1.6 0.50 0.80 1.02

Effective gravity angle deg 2.5

Vertical aôceleration m/s2 0.5 2.0 0.63 1.00 1.28

Roll Angle - deg 2.3 9.3 2.94 4.65

5.95 Stabilizers

Mechanical angle deg 7.5 30.0 9.4 15.0 19.1

Effective angle deg 7.5 30.0 9.4 15.0 19.1

- PERFORMANCE OF A FERRY IN WAVES SHIPMO/WASCO OPERABILITY ANALYSIS Ferry in sternquartering seas

DOWNTIME ANALYSIS SCATTER DIAGRAM: Medit.Sca

LINETYPE IDENTIFICATION CRITERION AVERAGE DOWNTIME

THOUSANDTh

5 10 15

ZERO-UP CROSSING PERIOD in s

Fig. 11 DomtiEne in stern quartering seas

TAcceI.15 1.60 iWs°2 57

EGA 15 2.33 deg 57

- PERFORMANCE OF A FERRY IN WAVES

SHIPMOPWASCO OPERABILITY ANALYSIS

- Ferry in Bowq. Seas

DOWNTIME ANALYSIS SCATrER DIAGRAM: McditSea

LINFYPE

5 _{10 15}

ZERO-UP CROSSING PERIOD in s

Fig,.

12

Downtime In bow quarter-in

seas

IDENTIFICATION CRITERION AVERAGEDOWNTIME

THOUSANDTH T.AcceL 13 1.60

nVs2

0 TACCCI. 15 010 ITVs'2 25 EGA 15 233 deg 0 EGA 15 1.16 4ç 28 ROLL 9.34 deg 0 ROLL 4.67 deg 0

0

PERFORMANCE OF FERRY IN WAVES SHIPMO/WASCO OPERABILITY ANALYSIS Ferry in Bowq.Seas

DOWNTIJ ANALYSIS SCATrER DIAGRA1i4: MediLSca

LINETYPE IDENTIFICATION -CRITERION AVERAGE DOWNTIME

ThOUSANDTh

I _- 1

5 10

ZERO-UP CROSSING PERIOD in $

Fig. 13 Vertical accelerations in bow quartering seas

15 Vfi.ccel. 0 200 th/si*2 ₁₀₇ V.Accej.5 2.00 nVs!2 16 VAcceJ 10 2.00

ns2

30 V.Accel.15 2.00 nVs"2 141 V.AccI.20 2.00 th/s"2 258

100

50

0

N

N

DISTRIBUTION OF VERT ACCELERATION LEVELS OVER THE LENGTH

F ( av > av* )

_{Frequency of exceedance or adopted criterion}

Pa ( 24 Hr )

Probability of exceedance on a 24 hour trip

Bow Quartering seas

Pe

Head seas

Pe

Bow Quartering seas

F

Head seas

/

/

/

/

,-

-10 STATION 15

Fig. 14

_{Distribution of vertical acceleration levels over the ship length}

20

PERFORMANCE OF A FERRY IN WAVES - SHIPMOPWASCO OPERABILITY ANALYSIS

Ferry in steñiquartering seas

DOWNTIMEANALYSIS SCATFER DIAGRAM: MdiLSea

LINETYPE IDENT1HCATION CRITERION AVERAGE DOWNTIME

ThOUSANDTh 3O m 33 30.cO rn 219 IO. deg 20 ALFAC ALFA_E ROLL 0 5 10 ₁₅

ZERO-UP CROSSING PERIOD in s

1O-Q

>

z

- PERFORMANCE OF A FERRY IN WAVES - SHIPMO / WASCO OPERABILrrY ANALYSIS

SustainedSpeedinHeadSeas (2Oknotsmca1mwat)

DOWNTIME ANALYSIS scArrER DIAGRAM: MediLSea

I I I 4 2 I 15-0 - I 5 10

ZERO-UP CROSSING PERIOD ui s

Fig. 16 _{Sustained speed in head seas}

15 I

/_'

'Z

5,32

60 140 I

--

-66 102 I

-50 35 I

--.u.

-20 7

...._t

.-6 1 -I -1

LINETYPE IDENTIHCATION CRITERION AVERAGE DOWNTIME

ThOUSANDTH 20 knots 10.31 iyi/s 1000 19 knots 9.80 nils 604 18 knots 9.28

ns

321 I6knots 8.25 nih 145 l4knots 722 oils 90 l2knots 6.19 oils 61 I0knôts 5.16 oils 46 I I

-I

-I - I I _-!

-100

I

__ 50

Trip duration

The distance covered at a given time after departure is the sum of the distances covered in each "drawing". The effective speed after N 3 hr time steps becomes:

N

E (3.V )

VE = I

(5)

The duration of the trip is the value of N for which the travelled distance covers the re-quired distance.

In practice the effective speed and the trip duration are affected by the coherence between two subsequent time steps (V1) and the limited duration of storms. Time domain simulations provide a way to tackle this problem. This problem and other operability aspects involv-ing "memory" effects are currently under in-vestigation.

6.

CONCLUSIONS

In the foregoing a basis for the quantifica-tion of the performance of a ship in a wave climate was discussed. Sample results were shown.

The interpretation of the operability or downtime figures suffers somewhat from un-certainties _{regarding the prevailing criteria} and details of the wave climate. Despite these limitations it may be concluded that it. is possible to predict the operational perfor-mance _{of a ferry in waves in a meaningful and} quantitative way. The application yields in-teresting observations regarding the optimum layout of passenger areas and the design of fin stabilizers. Besides aspects related to

10 15

SUSTAINED SPEED V

Fig. 1.7 Distribution of stistathed speed _{pr three different calm water speeds}

the motion response also the involuntary speed loss as a result of combined wind and waves can be tackled in terms of an operability analysis. It provides important support for the design of a relevant service margin. Last but. not least, an operability analysis yields valuable insight in the relative impact of the various criteria on the performance of a _{design, offering opportunities to arrive at} a well-balanced design for service.

REFERENCES

11 Janssen P.A.E.M, Komen G.J. and Voogt V.J.P.; "An Operational Coupled Hybrid Wave. Prediction Model", Journal of Geophysical Research, Vol. 89, pages 3635-3684, 1984

[21 "Assessment of Ship Performance in _a

Seaway", The Nord-ic Cooperative Project "Seakeeping Performance of Ships", ISBN87-982 637.

McCauley, N.E., Royal, J.W., Wylie, C.D., O'Hanlon, J.F. and.Nackie, R.R.; "Motion Sickness Incidence: Exploratory Studies of Habituation, Pitch and Roll and the Refinement of a Mathematical Model", Human Factors Research Inc., Technical Rept.1733-2, 1976

Goto, D.; "Characteristics and Ealua-tion of notion Sickness Incidence on-board Ships", PRADS 83, 2nd mt. Symp., Tokyo & Seoul, 1983.

Lloyd, A.R.J.M. and Andrew R.N.; "Criteria for Ship Speed in Rough Weather", 18th Aierican Towing Tank Conference, 1978.

Graham, R.; "Motion-Induced Interrup-tions as Operability Criteria", Naval Engineers Journal, March 1990.

22 knots

20 knots

18 knots

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-

67 kn -/

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20 25

Kervin, J.E., Handel P. and Lewis, S.D.; "An Experimental Study of a Series of Flapped RUdder", Journal of Ship Researëh, Dec. 1972.

"Seakeeping Performance of Ships", The Nordic Cooperative Project, ISBN 87-982 637-1-4.

CM0 Project Code 88 B.;2;20,. HARIN Report 49790-i0E.

Hogben, N., Dacunha,, N.M.0 and Oliver, Global Wave. Statistics", BMT, London, 1986.

Hutchisson, B.L.; "Risk and operability Analysis in the Marine Environment

SNA}fE transactions, Vol. '89, pp.