Numerical and Experimental Investigation of whipping in Twin Hull Vessels exposed to serve wet Deck slamming

Abstract

An investigation of global structural loads in large twin hull vessels to severe wet deck slamming has been carried out. The study contains two main parts:

• An experimental investigation.

• Development of a numerical method for time domain calculations of global structural loads.

Experiments were carried out in the MARINTEK ocean basin in Trondheim. A flexible segmented test model was used in order to intercept possible hydroelastic effects. Motions, slamming loads and global structural response were measured in the experiments, and significant slam events with corresponding whipping responses were observed for some conditions.

In the numerical model presented a natural mode approach is used to describe the structural response. Vessel motions and structural loads, due to first order steady state wave load, are determined from a linear frequency domain solution. First order motions and surface elevation are used in a time domain calculation of slamming loads and vessel response where phase angels are properly accounted for. The slamming load is determined bu solving a two dimensional boundary value problem (BVP) for each time step. Global structural loads are found by superposition of structural response from continuous wave loading and slam induced structural response (whipping response). The numerical model presented is restricted to head sea and zero forward speed.

The implementation of the method is described and validated by comparison with analytical solutions and results from other numerical tools. Uncertainties in the numerical model are identified and discussed. Parameter studies have been carried out to investigate sensitivity with respect to modelling parameters.

Experimental results are presented and discussed and uncertainties in the experimental results are pointed out. Head-sea results are compared to results predicted by the numerical approach. Reasonable agreement is found between calculated and measured results. Generally the numerical model seems to be capable of describing the slamming force during water entry, but during the water exit phase the agreement between numerical and experimental results is poor. The combined wave- and whipping response is satisfactory described by the numerical model.

With respect to prediction of the slamming force, hydroelasticity due to global structural response seems to be of some importance, but the influence on rigid body motions form the slam is found to be even more important. Both predicted and measured slamming forces were found to be very sensitive to the relative motion between the wet deck and the water surface. .