Adverse Weather.PDF

GUIDANCE TO THE MASTER FOR AVOIDING DANGEROUS SITUATIONS IN

ADVERSE WEATHER AND SEA CONDITIONS

1 General

1.1 Adverseweatherconditions,for the purposeof the following guidelines,include wind

inducedwaves or heavy swell with a wave length of more than 0.8 x ship length and a wave

heightof more than 0.04x ship length,or, regardlessthe wave length and height, a wave pe-

riod in beam or quartering seiu that may causeharmonic resonancewith the ro11motions of

the ship.

L.2 When sailing in adverseweatherconditions, a ship is likely to encounter various kinds

of dangerous phenomena,which may lead to capsizingor severero11 motions causing damage

to cargo, equipmentand personson board. Although the dpamic behaviour of ships in ad-

verse weather conditions is not yet covered in present stability standards, rnuch progress has

been made in recent yearstowards understandingthe physics of ship response mechanisms

andto identifying potentially critical conditions.

1.3 The sensitivity of a ship to dangerous phenomenawill dependon the actual stability

parameters,hull geometry,ship size and ship speed.This implies that the vulnerability to

dangerousresponse,including capsizing, and its probability of occurence in a particular sea

statemay differ for eachship.

I.4 For those ships,which are equippedwith an on-board computer, the Administrations

are encouragedto employ specially developed software, which would take into account the

main particulars,actualstability and dynamic characteristicsof the individual ship in the real

voyageconditions.Suchsoftrvareshould be approvedby the Ad:rrinistration.

2 Cautions

2.I It shouldbe notedthat this guidanceto the masterhas been designed to accommodate

all types of merchant ships. Therefore, being of a general nature, the guidance may be too

restrictive for certain shipswith more favourable seakeeping properties, or too generous for

certain other ships. Master are requestedto use these guidance with fair obsewation of the

particular featuresof the ship andher behaviour in heavy weather

2.2 It should fillher be noted that this guidanceis restricted to hazards in adverseweather

conditions that may causecapsizing of the vessel or heavy rolling with a risk of d.arnage.

Other hazardsand risks in adverseweather conditions, like damagethrough slamming, lon-

gitudinal or torsional stresses,special effects of waves in shallow water or current, risk of

collision or stranding,&renot addressedin this guidance and must be additionally observed

when decidingon an appropriatecourseand speed.

3 Period of roll at large roll amplitudes :

3.1 The ship's period of roll Tn is the durationof a completeroll motion, i.e. the,tirne span

betweena reversal point of the roll motion and the following reversal point in the same direc-

tion. With amptitudesup to 5o, Tn is usually known by the ship's cofilmand and frequently

referredto asthe natural period of ro11Tq.

3.2 ln dangerousroll motions with large amplitudesof 30o or 40o, the period,of roll Tn

may differ considerablyfrom the natural period of ro11T4 dueto the stability characteristicsof

the vessel. A rough estimation of these differences is facilitated by the still water righting

levercuwe of the vessel.

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3.3 If therighting levercurvekeepsbelow the tangent at zeroheel (seeFigure 1), the pe-

riod of roll at largeamplitudeswill be graterthanthenatural periodof roll T6.

Tangentat

zero heel z/

l l

GM"

,'h

:':i::i::.:::q

:'i;i'.r;rii.::i

-1.,:it.-.i,,'t

W#i:,','i:1

llr;il;::ii. i,?ritlirj

ffi

N-

57'3"

30"

40"

Figure 1: Tn will be greaterthanT4

3.4 If the righting lever curve up to 30 or 40o remains near the tangent at zero heel (see

Figure 2), the period of ro11at large amplitudes will be approximatelyequal to the natural pe-

riod of roll T4.

,/ I

Tangentat

zero heel rz

l l

GM'

.1, ,-,f.*t..j6,

57,3'

400

30.

Figure 2: Tp will be aboutequalto T4

3.5 If therighting levercurveup to 30 or 40orisesabovethe tangentat zeroheel (seeFig-

ure 3), the period of roll at large amplitudeswill be shorterthanthe naflral period of ro11T4.

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Tangentat

zero heel

/,t

il\

i#ffiEff&

;tr,f4tE ilithia&

il!ih?ll\'ti:!i!l

57,3"

40'

30'

Figure 3: Tn will be shorterthan T4

3.6 The magnitude of these differences may be established for the individual ship by

practical on-board observation. Records have shown for a panamax container vessel with

GMc:0.80 m anatural periodof ro11T4 of about 29 secondsanda period of ro11at large ro11

ampiitudesTn around 22 seconds.

4 Dangerous phenomenain following and quartering seas

A ship sailing in following and stern quartering seas encountersthe waves with a longer pe-

riod than in beam, heador bow waves, and principal dangerscaused in such situation are as

follows:

4.I Swf riding andbroaching-to

When a ship is situated on a steepforefront of a high wave in following and quartering sea

condition, the ship canbe acceleratedto ride on the wave. This is known as surf-riding. In this

situationthe so-calledbroaching-to phenomenonmay occur, which endangers the ship to cap-

sizing asa resultof a suddenchangeof the ship'sheading andunexpected large heeling.

The phenomenonof surf-riding is closely related to the ship's speedand becomes'critical if

the speedis sohigh that its component in the wave direction approachesto the phase velocity

of thewave.

The critical speedforthe occurrenceof swf-riding is considered to be 1.8 ' rll- [mots], where

L is the ship's length in rnetres.It should be noted that there is a marginal zonebetween L.4 '

r/t aoOthe critical speed,where a large surgrng motion may occur, which is almost equivalent

to surf-riding in danger.

4.2 Reductionof intact stability when riding a wave crestamidships

When a ship is riding on the wave crest, the intact stability will be decreasedsubstantially

accordingto changesof the submerged hull form. This stability reduction may become critical

for wavelengths within therangeof 0.7 - L up to 2 . L, whereL is the ship'slength:in metres.

Within this rangethe amount of stability reduction is nearly proportional to the wave height.

This situation is particularly dangerousin following and quartering seas,becausethe duration

of riding on thewave crest,i.e. the time spanof inferior stability,becomeslonger.

Figure 4 showsan exampleof the variation of stabilityparameters dr.ring sailing in,longitudi-

nal seas.

Page3 of 10

E 1.0

b 0.8

€ 0.6

0.4

40'

50" 60' 70"

Angleof inclination $

10'

20"

Figure4: Variations of intact stability in longitudinal seas

4.3 Parametric ro11motions

parametric ro11 motions with large and dangerous ro11amplitudes in following or quartering

seasare based on the variations of stability between the position on the wave crest and the

position in the wavetrough.Parametric rolling may occur in two distinguished situations:

The stability varieswith an encounter period Ts that is about equalto the roll period Tn of

the ship (encounterratio 1 : 1). The stability attains a minimum once during each ro11

motion. This situationis characterised by asymmetric rolling, i.e. the amplitude with the

wave crest amidshipsis much greater than the other amplitude. Due to the tendency of

retardedup-righting to* the large amplitude, the ro11 period Tn may adapt to the en-

counter period to a certainextent, so that this kind of parametricrolling may occur with a

wide bandwidth of encounterperiods. In quartering seasa transition to harmonic reso-

nancemaybecome noticeable (seeparagaph 6).

The stability varies with an encounter period Te that is approximately equal to half the

ro11 period Tn of the ship (encounter ratio 1 : 0.5). The stability attains a minimum twice

during eachro11motion. In following or quartering seas,where the encounter period be-

comeslargerthanthe wave period, this may only occur with very large ro11 periods Ts,

indicating a marginal intact stability. The mechanism of this more dangerous 1 : 0.5

parametricresonance

is that the situation "wave crest amidships" with weak stability co-

incideswith the heeling phaseto either side,while the subsequentup-righting takes place

during the phase of increased stability in the wave trough. The result is syrnmetric rolling

with largeamplitudes, again with the tendency of adaptingthe ship response to the period

of encounter due to reduction of stability on the wave crest. Parametric rolling with en-

counterratio 1 : 0.5may also occur in bow andheadseas (seeparagraph5).

4.4 Combination of variousdangerous phenomena

The dynamicbehaviourof a ship in following and quartering seasis very complex. Ship mo-

tion is three-dimensional and various detrimental factors or dangerousphenomenalike addi-

tional heeling momentsdueto deck-edge submerging, water shipping and trapping on deck or

cargo shift due to large ro11motions, may occur in combination with the above mentioned

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phenomena,simultaneouslyor consecutively.This may createextremelydangerouscombina-

tions,whichmaycausea shipcapsize.

4.5 Sailingwith thegroupvelocity of waves

When the ship speedcomponentin the wave direction is aboutequalto the wave group ve-

locity, that is half the phasevelocity of the dominant wave components,the ship will be at-

tackedsucsessivelyby high waves. The maximum wave height in a group or train of waves

may be almosttwice the significant wave height of the existing seastate. In this situation,

surf-riding, reductionof intactstability on a wave crest, parametricrolling or cornbinationsof

thesedangerousphenomenawill be more pronounced and thus the risk of capsizing aggra-

vated.

in head and bow seas

A ship sailingin heador bow waves of a magnitude definedin paragraph1.1 encountersthe

waves in shorterintervals than the wave period. Additionally, the orbital speedof the wave

crestsaddsto the ship'sspeedand thereby increasesthe kinetic energy of wave impact to the

ship'shull or deckstructures. The principal dangersin suchsituation are as follows:

Dangerousphenomena

Damagefrom slammingand shippingof water

5.1

Heavy slamming may causedamageto the ship'sfore bottom plates and to internal equipment

in the fore body. Shipping of water from forward may causeseveredamage to deck equip-

ment, deck cargoor hatch covers due to the high relative speedof the overcoming water.

Slamming or shipping of water may appear particularly when pitch amplitudes are getting

largedueto high speedof the vessel or due to resonance betweenthe encounter period with

the wavesandthe natural pitching period of the vessel. The natural pitching period in seconds

canbe approximated by 0.55 . .lL, where L is the ship'slength in metres. The favourite mea-

sure to reduce slamming or shipping of water is the appropriatereduction of sp'eed.This,

however, ilay haveconsequenceswith regardto parametricrolling.

roll motions

Paramefic ro11motions with large and dangerousro11amplitudes in head or bow seas are

basedon the variations of stabilify between the position of the arnidships body on the wave

crestandin the wave trough. Other than in following or quartering seas,parametric rolling in

head or bow seasmay generally occur with the encounter ratio 1 : 0.5 only, i.e. with an en-

counter period Ts that is approximately equal to half the ro11period Tn of the ship, because

the encounter periods are generally small. '

The stability attainsa minimum twice during each ro11motion. The mechanism of this 1 : 0.5

parametricresonanceis that the situation "wave crest amidships" with weak stability coin-

cides with the heeling phaseto either side, while the subsequentup-righting takes place dw-

ing the phaseof increasedstabihty in the wave trough. The result is symmetric rolling with

largeamplitudes. :

Other than in following or quartering seas, where the variation of stability is solely effected

by thewavespassingalongthe vessel,the frequentlyheavypitching in heador bow seasmay

contributeto the magnitude of the stability variation, in particular due to the periodical im-

mersionand emersionof the flared stern frames of modern ships. This may lead to severe

parametricro11motions even with smaller wave induced stability variations.

6 Dangerous phenomenain beam seas

6.1 Harmonicresonance

Rolling in bearnseasis the result of periodical heeling moments from asyrnmetric buoyancy

in wave slopes.The intensity of this rolling dependson the energytransfer to the ship and can

Parametric

5.2

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