Wave Forces and Low Frequency Drift Motions in Extreme Seas: Benchmark Studies

Abstract The paper identifies the wave drift forces and low frequency damping induced by a wide range of extreme sea conditions on two floating units based on model test data, assesses the limitations of today's numerical procedures and proposes solutions to overcome the limitations. Since wave current interaction effects are of utmost importance, a benchmark study is carried out with several state of the art potential flow codes. CFD studies complement the numerical investigations. Empirical wave drift force coefficients are identified by a second order signal analysis technique and the results compared with potential flow predictions and with predictions from a semi-empirical formula. The low frequency damping is a result of the identification procedure as well. On a second stage, time domain simulations are systematically performed for all tested conditions and the results compared in terms of wave frequency and low frequency motion responses. The systematic comparison between calculated responses and model test data identifies the numerical methods limitations with regard to increasing seastates and increasing current velocity. A benchmark study with four wave-current interaction diffraction codes is presented for a Semi-submersible and, given the good agreement of results, it is concluded that the codes may be considered ready for use by the industry. CFD studies are also performed with the aim of demonstrating the present status with regard to this methodology and identify the challenges in terms of numerical modelling.