A Dynamic Approach to Evaluating the Effect of Slamming on a Jacket Foundation Template Lowered Through the Wave Zone

Abstract

The joint venture between Boskalis and VolkerWessels is responsible for installing 70 jacket foundations for the Wikinger offshore wind farm. The critical phase in this installation procedure is lowering a jacket foundation template through the wave zone. This installation phase will induce large dynamic loads on the hoisting wire. Lowering through the wave zone is a difficult process due to the occurrence of a wave slamming forces on the template. This slam force is highly uncertain and rather difficult to estimate. The aim of this thesis to investigate the effect of the wave slamming force on the dynamic behavior of the template and the dynamic tension in the hoisting wire.

The wave slamming force on the template and the dynamic tension in the hoisting wire was analyzed using the OrcaFlex software. In order to determine the wave slamming force a range of characteristic slam coefficients were used. The range of the slam coefficients were investigated for different wave periods. A total of 4 test were performed. The first test focused on the behavior of the wave slamming force. The second test was dedicated to the influence of the vessel motion. In the third test the effect of the buoyancy, inertia and drag forces was analyzed. Finally, the effect of irregular waves was evaluated.

The performed tests showed a large influence of the wave slamming force on the dynamic tension of the hoisting wire. The influence decreased when the vessel motions and the other wave forces were taken into consideration. The influence of the slam coefficient decreased as well. In regular waves a scatter of 25 % was found in the prediction of the dynamic amplification factor (DAF) for all considered slam coefficients. When comparing the scatter with results obtained by the methods used in guidelines a possible maximum error of 18 % was found. Due to the extreme environmental conditions used in the performed tests this was considered to be within the project safety margins. It was concluded that the wave slamming force will not limit the operational condition in the Wikinger project. Furthermore it was concluded that specialized analyses like CFD will not significantly improve the prediction of the range of the wave slamming force.