Wave loads of the 1976 ISSC

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CONTRIBUTION TO THE REPORT OF COMMITTEE 1.2.

"WAVE LOADS" OF THE 1976 ISSC

BY

F.F. VAN G.UNSTEREN

I

Model experiments on wave loads for small wave length ratios,

hydroelastic phenomer. and slamming impact loads

As reported at earlier Committee meetings model tests for the determination of small wavelength ratio wave forces are under way in the Netherlands at the Deift Shipbuilding Laboratory. A test has been carried out with a Todd

model consisting of ₉ segments in order to investigate the experimental problems related to wave force measurements for small wave length ratios.

The required accuracy and stiffness of the straingages, stiffness and _natural frequencies of various parts of the experimental set-up have been studied. The preliminary results for head waves confirm the trend of an increased decay of the sectional wave force amplitude over the length of the ship with a

decrease in wave length ratio. This trend has been observed earlier (i) and could also be concluded tentatively from shipside wave height measurements (2). In the autumn of

1975

ratio wave forces will be carried 'out. The

_4.65

attached to a very rigid backbone. Vertical forces on the segments and

corresparling phase angels will be measured for the restrained _{model with}

various speeds in head waves of wave leñgth ratios between 0.06 _{and 0.75} A second model consisting of 12 segments attached to a light weight rigid

backbone will be tested in a free pitching and heaving

_condition.

Vertical and horizontal faces as well as torsional moments on the segment

can be measured.

-2-The model set-up and philosophy behind the model tests are described in

reference (3).

In future the experiments in head seas can be extended to include various wave directions. Also experiments with a flexible backbone can be carried out to study the influence of hull flexibility on wave forces.

This last aspect will also be studied at Seakeeping Laboratory of the

Netherlands Ship Model Basin by means of an extension of shear- and bending moment measurements of a 4-segmented model in waves (4),

The three joints will be made flexible to allow the model vertical vibratory deflections. The influence of the vibratory hull deflections on shear forces and bending moments of the model in short waves will be investigated.

At the Delft Shipbuilding Laboratory preliminary seakeeping experiments with a model of a fast cargo ship in ballast condition were carried out to investigate the feasibility of measuring the slamming impact loads by means of five

pressure gauges in the forebody of the model. The top of the pressure peak during a slam could hardly be determined. Future experiments are planned where the pressure measurement will be re'orded on magnetic tape.

The impact energy will be determined by means of a special analysis technique using a spectrum display.

Other experimental investigations on wave loads in the Netherlands _{are carried}

out by the Netherlands Ship Model Basin and the Delft Hydraulics Laboratory. The wave load research of these institutes is mainly directed towards offshore related subjects like driftforces (mooring line forces), wave loads on

vertical circular cylinders etc. These subjects are covered in the Committee report by other members of the Committee and are treated by various papers at

the Symposium referred to in reference (i).

Theoretical investigations on small wave length ratio wave loads and

hydroelastjc aspects.

An extension of the strip theory for small wave length ratios has been given recently by Wahab and Yiik (5).

-3-Extension of the method of Faltinsen (6) from circular sections to ship like sections might be under taken in future parallel to the planned model

test research.

Experiments with segmented model in waves of short waveleri ratios such as reported in references

₍₄₎

(7)

purposes.

Investigation on the effect of bow flare induced second order buoyancy forces on springing has been carried out (s).

The influence of bow flare induced second order added massforces has been mentioned by various authors (i) (2) and might be investigated in future.

Wave loads on the ship's deck

Severe hull structure damage has been reported on various ships due to extreme

loads when sailing in freak waves, A possible _{cause of damage could be the} vertical downward load by the overcoming waves on the deck.

An other example where the wave loads on deck play an important role in the dynamics and thereby the safety of the ship is the transportation of various

material for the offshore activities by supply vessels and pontoons. _These vessels operate with minimum freeboard. Water on deck of this craft is no exception. The water on deck is not only of influence for the structural integrity of the ship but also for the dynamic behaviour (motions) _{of the} ship in the seaway. In the Netherlands this subject has been _{given some}

attention.

Calculations of shipping of water of a supply vessel in following waves

have been carried out. From these results the probability of shipping a certain volume of water on deck is calculated according to an approximate method given

in reference (9). The results of these calculations _{are not yet verified by} experiments, It is planned to carry out a research programme at the Deift Shipbuilding Laboratory to develop a method for the calculation _{of the}

probability of a certain volume on deck. For private account experiments have been carried out at the Netherlands Ship Model Basin with models of dock type ships in waves. The amount of water in the holds without hatches was investi-gated. These results in connection with specially designed _{experiments at the} Deift Laboratory will be used for the development of the above described

Although the main reason for this research _{is the study of shipping of water,} the rolling behaviour of the ship and its effect on seakeeping qualities,

the problem is mentioned here in view of the fact that the water on deck as a wave load plays an important role in the safety of.a ship and should not be omitted in the Committee report (or _{the report of Committee 1.3.).}

Wind loads

The mandate of our Committee indicates _{a survey of wind load research.} This subject is not covered in the work _{distribution over the members.} A short review on some literature on wind loads _{on ships is given now.} Application of practical information on aerodynamic drag (io) for marine purposes is only valid for simple forms and does not _{taken the mutual} interference between wind and wave surface and _{the complex ship forms into} account. Extensive model tests have been carried out in the windtunnel of.

the "Hamburg University Institut fUr Schiffbau".

This model test research and other research on wind loads on ships have been reported (ii) (12) and have been analysed for the use of prediction

of wind resistance (13).

Interesting results have been obtained from measurements of forces on ship's super structures using the model _{basin facilities in stead of a}

REFERENCES

1. Guntereri, F.F. van

Kumai, T.

Wereldsma, R.

Tan Seng Gie

Wahab, R. and Vink, J.H. Faltinsen, O. 7, Daizell, J.F. ánd Chiocco, M.J. Gunsteren, F.F. van Hansen, H.J.

ome fvrther cicnlatione of

duc ship wi1 vitcj'tioPaer no

Symposium on the Dynamics of Narine Vehicles and Structures in Waves, April

1974, London.

"On the Exciting Force and Response of

Springing of Ships". Paper no 28, Symposium

see ref. (i).

"Normal Mode Approach for Ship Strength Experiments, a Proposal". Paper no. 29, Symposium see ref. (i).

"Wave load measurements on a model of a large containership". Netherlands Ship Research Centre TNO Report No 1735, December 1972.

"Wave induced motions and loads on ships in oblique waves". Netherlands Ship Research Centre TNO Report No 193S, May 1974.

"Wave Forces on a Restrained Ship in Head

sea Waves"

9th ONR Symposium, ParIs, August 1972.

"Wave Loads in a model of the SL-7 Container ship running at oblique headings in regular

waves"

Ship Structure Committee Report no.239,

1973.

"Bow flare induced springing" Netherlands Ship Research Centre TNO, Report no.176S,

May 1973.

"Uber die Vorhersage von Decksbelastungen durch GrUnea Wasser". Schiff und Hafen,

10. Hoerner, S.F. "Fluid-dynamic drag"

Published by the author, 1965

Wagner, B. Wieghardt, K. Isherwood, R.M. Brummer, G.M.A. Voorde, W.R. van de Wijk, W.R. van Glansdorp, C.C. "WindkrUf te am Uberwassersehiffe"

Jahrbuch der Schiffbautechnischen Gesell-schaft,

_1967,

"On wind resistance"

International Jubilee Meeting,

40th Anniversary of the NSMB, Wageningen,

1973.

"Wind resistance of merchant ships" Transactions of the Royal Institution of Naval Architects, vol. 115,

1973.

"Simulation of the steering- and manoeuvring characteristics of a second generation

containership". Netherlands Ship Research