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tion has to extend over some 2,000 time steps.

The operational conditions that can be an- alysed include the starting position and speed of the lifeboat, wind force and direction, water currents, water depth, wave profiles and di- rection of propagation, and any combination thereof. A fifth-order Stokes’ theory is used for modelling the waves. The initial velocity field is also determined from this theory.

To test this simulation procedure, a vali- dation study was performed in cooperation with Norsafe AS. The study examined the in- fluence of the hull shape on the movements of the lifeboat and on the accelerations act- ing upon the occupants. Three different hull shapes were considered: the original version, one with a modified aft section and one with modified aft and bow areas. By making use of the symmetry, the calculations took account of three degrees of freedom: horizontal and vertical motion, and rotation about the trans- verse axis. The computation mesh consisted of approximately 350,000 cells.

Comparisons of the results obtained from the experiments and simulations (see dia- grams right) demonstrate: the development of the accelerations, both in the bow and stern areas, was predicted very well for the critical phase (the first two seconds). The bow hits the water first, experiencing a deceleration of ap- proximately 5 g; the water resistance at the bow causes a tilting moment, which is why the stern is initially accelerated (up to about 3 g over the acceleration due to gravity). Only when the stern comes into contact with the water is a deceleration of up to 6 g introduced for a very short period of time.

The usual quantitative measure for accelera- tion can be expressed as the so-called CAR val- ue, which is determined as a combination of the accelerations in three directions. For all three hull shapes, the accelerations in the afterbody are higher than in the forebody, but the differ- ence decreases with the changes to the original design. What is more, the absolute level of the CAR values is reduced with each modification.

SIMULATION I. Lifeboat position and free surface shape during water entry (50 ms between frames, from top to bottom).

Wave of difficulty

The hull with a modified stern and bow yields a decrease in the stern CAR values of almost 20 per cent in relation to the original design. The reduction in the bow CAR values is much lower

  • approximately 5 per cent.

These comparisons show that the simula- tion results are plausible from both the qual- itative and the quantitative standpoint: they reflect the influence of design changes in the same way as the experiment.

Wave of Difficulty

For the entry into waves, the direction of wave propagation and the entry point relative to the wave crest both play an important role. To in- vestigate these effects, a lifeboat (which is ap- proximately 15 m long and has a mass of about 20 t) was launched from a height of 32 m with

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