Forced motion experiments (WHIP-1 project)

CRS’s research efficiently combines numerical, experimental and full-scale monitoring activities, which are all used together in order to ensure the validity and reliability of the final numerical tools. Several working groups are taking care of the different aspects of hydro-structure interactions, using both the simpler but faster potential flow-based models, and the more complex but computationally expensive CFD models.

Hydro response Calculating loads on ships and, thereby, the motions in waves has been done since the sixties. At first this was done using a 2D approach (strip theory) and this was corrected for the effect of forward speed. In the late seventies and eighties 3D methods were developed, very often these methods also needed some tricks to include forward speed effects. An example of a development of the latter option is the CRS program PRECAL. The common approach in these methods was the assumption of small disturbances relative to the equilibrium position. This assumption resulted in a linear program.

A number of accidents (e.g. sinking of the Estonia in 1994) showed that extreme loads are clearly very important and also that these are outside the scope of programs

based on a linear approach. These conclusions led to the development of the time domain program PRETTI, which uses the linear hydrodynamic results of PRECAL and adds nonlinear effects due to the actual immersion of the hull.

It was assumed that in deploying such a simplified approach - ignoring the nonlinear effects in the dynamics - the main components of the nonlinear loads were captured. Although this approach is still state-of-the-art for long duration simulations, it excludes slamming events. Therefore, a long-term research programme was started resulting in a string of CRS projects: SLAM, ELAST, WHIP-1,2 and WHAM. The work evolved from drop tests on 2D ship sections to several model test campaigns using 3D segmented models and flexible beams to model the structure of the ship. At the same time, a software development programme was started to include hull bending modes in both PRECAL and PRETTI. This evolved into a full restructuring of both codes – which was quite a project on its own.

15 years of slamming studies The slamming problem was, and still is, a very hard nut to crack. Two approaches were developed, one 2D method based

on a strip theory type approach and a 3D momentum method. Both methods give good results for head seas cases, but impacts in quartering waves appeared to be much harder to predict. In fact, it appeared that really extreme impacts were caused by relatively short and steep waves. It is not necessary that the complete bow emerges just before such an impact. This implies that approaching the slamming problem by a drop test simulation has its limitations. After studying the slamming problem for some 15 years, we had to conclude that the model of the incoming wave (linear Airy model) also needed to be improved to properly describe the velocity in the crest of steep waves.

SLAMFLOW The new approach to tackle the slamming problem is to make use of CFD calculations; this is done in the SLAMFLOW project. Today’s large computers have no problems handling grids with a number of cells in the order of 107

. Such

grids can solve the local flow problem sufficiently accurately to have a good impression of impact pressure and duration.

However, extreme values cannot be determined by only CFD. Long-duration simulations with CFD are totally unrealistic. Therefore, approximate methods are required that are able to select the critical events that

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