Impact pressure acting on bow of large full ships

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Impact Pressure acting on Bow of Large Fi1i Ships Prepared for 13th ITTC

March

₁₉₇₂

ByHitoshi FUJII, Dr. Eng.

Takeshi TAAKASHI

Seakeeping Research

Laboratory,

Nagasaki Technical Institute,

Nitsubish Heavy Industries, Ltd.

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Impact Pressure acting on Bow of Large uflSShIpk

1

.--by Hjtoghi Fujii!.iu:kc

TakeshiTak&iashj

Summary

The impact pressure acting on bow of large and full ship may be

predicted by the following considerations. The ship motions are mainly

due to the waves longer than ship but scarcely affected by the-shorter

waveB. Therefore, the ship cruising in the oóeari is oscillated violently

by the swell (longer waves): and the velocity or the bow motion relative

to wave surface becomes very large. Impacts on bow occur when short waves

are superposed to the longer waves and. collide with such oscillating ship. If the angle between the stem and the wave surface of the short Waves is

small, large impact pressure is produced. The slope of stem and -bluntness

of bow are considered to be main parameters controlling the amount of pressure and. its frequency.

At Nagasaki Eiperimental Tank, impact pressures on the model are

measured in shorter waves superposed onlonger waves or using models

oscillated with frequency and amplitude corresponding to the conditiOn

in longer waves. .

-1. Introduction . . .

-The wave load can be divided into twokinds; the hdro&ynamic pressure

due to ship motions and. the impact pressure due to collision of waves with

the ship hull. As to the former, a method of estimation was proposed by

Prof. Tasai 1) and its availability has been confirmed by model teets.2)

As to the latter, however, the investigations made so far seem to be

restricted only to the problems of slamming or shipping green water because of the difficulties in its treatment.

Impact pressure, that occurs When a bod- collideg with water surface, is observed in many instances such as striking of waves against breakwater or quay, alighting of a flying boat on the water and entry of a inisille

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occur.

into water surface. Many studies have beën,carriedout on thesephenomena

eajdes the problem at ships. it is consired that the bow imact'

phenomenon can be regarded as the same mechanism as those, but the investiga-tion on such problems, In what consiinvestiga-tions the impact occurs and. to what

magnitude the impact pressure reaches and what relation the iTaOt pressure

has to the ship structu.re subjects, ehouldbe dév'eloped onboth ides of the

hydrodynamics and the ship struotural strength.

In this report, the conditions and the intensities of impact pressure :in the bow of the full and large ships were investigated experimentally,

regarding the Ship model' as a rigid .boy.. affects of th mnclination.of

the stem and the bow fullness abovelóa'd water line werô--a1so investigated

by model tests because they seemedtO have'a close relation to the bow

impact.

2. Mechanism of Wow Impact Phenomenon

Many researches have been carried out on the impact pressure acting on a ship, such as slamming and shipping water, but the repeated studies on the impact pressure acting on hull surface above the load water line are only

few. Slamming occurS when the bottom of a ship collides with water surface

and the impact piessure by the shipped waler occurS when sea water collide

with the dOck surfaoe ofa sh, sóthe mechanismof impact has often been studied using the drop test machineV Itis suppósedthat the bow impact is

based on the same mechanism as the above phenomena and occurs under the eriain conditions.

Though the waves in the ooaan are irregular, the condition of it can

be expressed by two scales, the sea scale and the swell scale. Accordingly,

it can be regarded as "sea" of shorter wave length is superposed on "swell" of longer wave length and the height and the length of each wave vary at

random. The amplItudes of ship motions are very small in waves of

less than 0.3 as found in the response functions. Therefore, the large

ship responds to the longer waves and oscillates but does not respond to

the' shorter waves superposed on them. It is considered that there exists

the possibility that the impact phenomenon is prod.uced when the shorter

waves collide with the hull of ship responding to the longer waves. When

the relative velocity of hull surface to the surface of shorter waves is large and the angle between them is small, the high impact pressure must

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3

- in- order to investigate the bow impact phenomenon in model tests, it

is considered- that more frequent chances of impact are given by the

experl-ment in: the superposed _{waves rather in the irregular waves, as shown in}

-

-Pig. 1. _{The same results can be expected by the experiment with the towed}

model In .BhOrter waves, by meanS of. the foroed

oacilliition:-with

_{the same}

-amplitude and frequency of the motions corresponding to the condition of longer waves.

-yr V.

in long waveB(VL).1) ifl Bhor-t wave8('jQ.3) 17% superpoeed- rave.

pig. 1 Ship Motion in Superpoaed Mavea

When a full and large ship having length L?p3lO° is cruising at the

speed V5 16 " in moderate long wave1 the wavelength =.310'" and the

wavheight

45",

the amplitude of relative motion to wave surface is

(P)

6.75'at theP-.P. of thia ship. The velocity (normal to the

tangential plane of stem) of collision of hull surface with wave surf aoe

is calculated as

!1!.8"

at maximum, including the orbital velooity of

water particles of longer wave. When the shorter wave, the wavelength

A4Om

and wave height &wt 2", are superposed on the above swell,

maximum resultant wave slope becomes 12 degrees in aocordance with the

phase conditions of both waves, though the wave slope of shorter wave is

9 degrees. Moreover, the local disturbance wave should be

taken

into

consideration. This local wave, as observed in case of towing of the full

ship model instill water, is generated at the front of bow by the

dis-placement effect of the running of full ship and becomes higher as the

advance speed of ship becomes larger. in the case of test in waves the

slope of incident short waveB Is made Btoeper by the effect of this local

wave. Pig. ? is an example of wave profiles, sketched from photographs,

0 a toWed model It- is obaerved that the elope of the wave in close

front -of the bow beoomee larger as the shorter wave approaches to the ship and the wave slope reaches -two or three times the orieinal incident

waYS-elope. TherOfor8,

even

the shorter wave considered above, the wavealope

beoomes 30

to 35

-degrees just before it collides-with the bow surface.

The inclinatiOn of stem being 40 degrees the angle between thehull surf aoe

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Regarding the bow _{urf ace jn the neighbourhood of the ship center lIne as}

a flat plate,

a8

shown in ISSC

report4

for example, the maximum impact

pressure is estimated as follows.

P= 120

(Head)

ways profile of incident wave

Conductor was developed and it han frequently been used in the hydrodynamic pressure measurement on the hull surface of a model

7t/Lp

.1 20

-Pn _.15

Pig. 2

_{Sketch of Wave Profil}

In Such a manner, the full, and large Bhip oeoiflate _{responding to}

longer waves and collides with large velocity against the surface of

shorter wave. Moreover, as the wiveslope is made steeper by the effect of

looal diaturbanoe wave, the angle between the hull 'surface and the wave

surface bóoomes smaller. These cbvioualy bring about the possibility of

000uranoe.of severe Impaot.

3.

Method of Measurement for Impact Préeèuré

The measurement of h'drodynamio pressure acting on a.hull surface of ship in waves and of impact pressure due to slarnming-.haie often been

conducted in model teats. However, there was someproblema relating to

the reliability of the preseure tranaduoer. Recently9 a miniature sized

pressure' transducer utilising the piezo resistance effect of a semi

ow Porn A

without ship Motion

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shown in Table 1, and it was

confirmed that the typical pattern of impaot pressure profile can be obtained

in the model

teats

designed by the principle of section 2, as shown in Fig.4.

rable 1 COup rtlt of P.upon. ?t..

of Pr.sU. Nosm*rtag Syutuo

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Pig. 6 0rranea'Pnt of Pr005ure Tr,,sducerl:

flow Porl A ,nt.T Lila

As to the wave loads, the theoretical and experimental researches have been performed by SR 131 C'omrnittee of the Shipbuilding Research Aesociation of Jlapan and also the measurements on a full size ship have been carried

out by SR 124 Committee At Nagasaki Experimental Tank, the computer

program for the calculation of the hyd.rodynamic pressure was developed a. few years ago and its availability has been confirmed by model tests. On the other hand, the fundamental experiments on wave impact pressure

were carried out. The following is the results of model test concerning

the wave impact pressure acting on the bow of the full and large ship.

4.1

Teated Model

Two models were selected for the purpose of th following;

To investigate the effects of bow form on the wave impact

Po confirm the modelship correlation on the wave impact

One of them is an entrance part of 210 WT type tanker. (Bow Form "A" ), and

the other. is an entrance part of 150 KDWT type tanker (Bow !orfi. "B" .).

The difference between them are that the bow qrm A

has

wore inolined.ste,

larger fullness and 1arge bulb. than the bow form B, as shown in. Fig.

5.

Bow Pan, A

.0 Bow Pora B

Ti

Front VtS

Body Pill Of Ioet.d !odel,

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As the object of test is mainly concerned with the local portion of the bow above load water line, the wooden partial models were made.

Twelve pressure transducers were fitted on the model as shown in Fig. 6,

for the bow form A. The whole length of model corresponds to L= 45h1

for the bow form A (scale is about

1/70)

but the partial model is the fore

h1' of it. For bow form B, pressure transducers of same number were fitted

on the same position as the bow form A. Each model was bal1ated at the

full load condition and. was towed at Froude number F= 0.10 and 0.15.

4.2 Results of Measurement -

-First, the both models were restrained and towed at F= 0.15 in

regular head wave with short wavelength, /L,, = 0.18 and

_/,Ws

2O.

Records of thepattern of pressure profIle obtained by pressure transducer

PtT-2 mounted on the ship center line are shown in Fig. ₇ for example. For

the bow form A, the impact pressure were observed at e'ery encounter with waves, but f or the bow form B oocurance of impact was few and the values -of h1 (max. 'alue Of the peak -of impact pressure pr-ofile) were small, in

comparison with the ones fOr the bow form A. Next, both mod1 ièz'e towed

at Froudenumber F,=0.15 in regular head wave with short wavelength,

-As/L 0.18 and 15/=20, performing the forced heaving motion which is equivalent tO the relative vertical motion at F.P. in regular head wave

with long wavelength, Au./L 1.0 and. A./.WLo60.. The amplitude and the

frequency of forced heaving were calculated by the strip method under the

above condition. Records of the pattern of pressure profile obtained by

pressure transducer PU-2 are shown in Fig. 8 for examplO. FOr.the bow

form A, large values of h1 were observed at every two or three other interval of the encounter with short wave, but for the bow form B, few impact pre8sure were found and the values of h1 were 60 to 70% ones for

the bow form A at the above condition. The value of h, of the bow form A

corresponds to

50!water

head at the full-scale ship. When the waveheight

is increased to ?/= 13, the values of

h1 corresponding to 80a water head

were observed. - - - -- -- - Bow Farm A BowPOrmA I j

IILLLLLU.LLLLLU

ith.LLwLJ.

-BowForm B BowFocm8

-R.e.rd. of I.p.ct Pp..t. Ptck-up Uo.Z Ftp. S Records oT Iopct Fre,.ur Ptc op P0.2

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Phase angle between the heaving motIon of ship and the shorter wave

changes in every encounter with wave, and the angle between the surface

of ship hull and the surface of short wave.dhange& from one encounter to

another. So the impact pressure is not always generated when the.short

waves collide with the hull as is observéd1n thèé records. It seems

ttthe Impact OCcurs when

reaches a value below a certain critical

value. The ralues of h1 change at random accordingly to the encounter

condition to waves therefore, the alue of h1 at each position should be

expressed statistically. It is àonaidered that the significant values of

Wave impact pressure àan be obtained by the repeated tats for the same

conditIon. . .

4.3

éiults Of,Añä.lysis

To make it clear the difference of the value h for the bow forms A

and B, the results of analysis was. expres5ed by the significant value of hf,

àne-third highest mean value ht() . Fig. 9 is an example of histogram of

h,/h5 value, calculated by using about 60 samples, for the bow form A.

In PU-i fitted on the center line of ship, h*s9)/hwS=l0.0 and in PU-2,

hi()/wsl0.5 then it is observed that the impactIpréssure corresponding

to the value of about ten times wave-height would be measured. Obtaining

hiU4)/hs values for each pressure transducer fitted on the model, the

distribution of impact pressur on the bow surface could be drawn as shown

in Fig. 10 and 11. In both figures are drawn the distribution of significant

impact pressure of the bow formi A and B. Fig. 10 is the case of restrained

models, and Fig. 11 is the caBe with ship motion given by forced oscillation. in the latter case, the impact pressure are distributed in wider range and.

the higher values of hc4)/hv are generated beciuse of the. relative inorease

of wave impact pressure between the bow forms A and B. The values of

h,)/hws near the ship center line on the bow form B are about 70 to 80%

of the àne for the bow form A, and they decrease toward hull side and, become about 50% at 8' buttOck line in comparison with the ones of bow form A.

fthe.towing:speed decreases. the values of hiS)/hw bcorne small, as is a

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Bow Per. A

with Ship Heti

0 5

5

v1thot Ship Hollow

Bow Por. A

Bow Per. B

10 15

with Ship Potion Sc. Per. A

Bow Per. B

10 15

Pig. 9 Exonplel of HistogrSU

9

It is confirmed that the bow form B at which the inclination of stem

is smi]l and the fullness of bow is small as well generates the small wave

impact pressure. As the matter of fact the future investigations should be

conducted in veiw of the response of structural strength to the wave impact

pressure obtained by the present tests, this report gives only the fact that

the amount and the distributiOn of wave impact pressure vary according td

the difference of bow form.

20 Pn 0.15 Xg/Lup 0.10 20 Pr 0.15 Pt%/Lpp 0.18 20 Pu 0.15 Pick-up Ne. 2 No. of 1st. N- 63 Plak-up No. I S N .4 _{- 10.5} No. of dste N- 6

Pig. 10 Pre..up. Di.tributiau ow Hull Surfacs: Pig. 11 P.e,5flr. Di.trtbution on Hull Surfaoei

without Ship NOtion with ShiP Notion

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without Shtp lOtto. 0.10 20 0.123 .13 lark S 0 £ 0 Pick-op No. 2 No. 5 No. h 10 0

with Ship NOtioll

rig. 12 !tt.ct of *I.snOe 5pasd

5.

Concluding Remarks

Model tests were conducted and some keys to the research of the

mecha-niem of bow impact were obtained. The reasons why the bow impact phenomenon

occurs are summerised as fo11aiis;

The full and large ships respond to longer waves but not to shorter

waves superposed on longer waves. Therefore, shorter waves collide with

such oscillating ship that has large relative velocity to the water surface. For the full and large ships, the fullness of entrance is relatively

large and the inclination of stem large also, and therefore the angle

between the hull surface and the wave surface becomes small. The wave

slope of incident shorter wave is affected by the local disturbance wave produced by the ship bow and the slope of the wave in close front of the

bow becomes larger, thus the angle to the hull surface becomes very small.

When the above mentioned conditions are satisfied during the voyage in the ocean, that is, the relative speed of the bow surface to the wave surface becomes maximum and. the angle between the hull surface and the wave surface becomes minimum, severe face impact can occur.

By this experiment, it was confirmed that in the case of ship having

larger fullness (Bow form A), larger wave impact pressures were measured.

However, the statistioal treatment is neoesaary for the analysis of the

wave impact pressure, so the repeated experiment should be conducted in the

future. Although the considerations on the relation between the impact

pressure and the ship structural strength is out of the scope of this report,

the present results might give some keys to the future investigations for

models and full scale ships as well. The authors hope these may serve the

solution of the problem in any way.

a/Lpp 0.i

0.10 0.125 0.15

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- - Acknowledgement

The

authors

wish'tô express their gratitude to Prof. S.Motora, ThIro

University and Dr. K.Paniguchi, the direotor.andthe manager of Nagasaki.

Technical Insiitute,-Mitsubishi Heavy tndusries Ltd. for their continuing

guidance and encouragement.

The authors also withes to express their appreciation to Mr. _H.Kasai

and Mr. K.Hatakenaka who cooperated in carrying out this investigation.

Reference

F.Pi, "Pressure Fluctuation on the1Ship Hull Oscillating in Beam Seas"

J. of;the Society of Naval Arch. of West Japan No. ₃₅ Feb.

(1968)

K.Goda; "Hydrodynamical Pressure on a Midship in Waves"

-J. of the Society of Naval Arch. of West pan

_No.35

Feb.

(196 -.

Grant Lewison and W M. Maclean, "On the Cushioning of Water Impact by Entrapped Air"

Journal of Ship Research, vol 12, No 2 (1968)

J.LC. Verhagen; "On the Rydrodynami.c Impact Problem for !-1atBoomed

Bodies"