Sensitivity Analysis of Non-Linear Air Gap and Wave Slamming Load with Respect to Current for Semi-Submersible Platform Design

A design of offshore platform is mainly based on the extreme response analysis due to the critical environment condition. It is important to predict accurately air gap and wave slamming load in order to check the strength of local structures which withstand the wave slamming due to negative air gap. Due to the factors of working depth, the size of mooring system model, sizes of experimental tank and the equipment of tank, it is difficult to simulate the wave, wind and current loads at the same time, accurately. When the platform suffers the rough wave condition, the current is also harsh. The main aim of this paper is to present sensitivity analysis results of non-linear air gap and wave slamming load with respect to the current load for the design of semisubmersible platform. In this paper, the wind tunnel test and the seakeeping model test have been performed. The linear air gap analysis can be carried out in frequency domain without the effects of wind and current. But the results of linear analysis and model tests were diverging. It was decided to investigate the non-linear hydrodynamic analysis considering the effects of wind and current. In the non-linear simulation, the wetted surface of the platform is updated for each time step. Both excitation and restoring forces are based on the instantaneous wetted surface. According to model test results, a numerical model is tuned and validated. Then, sensitivity analysis studies of the non-linear air gap and the wave slamming load with respect to the current load are performed in time domain. The sensitivity analysis method and results are valuable for the floating platform design.