Low Frequency Wave Loads and Damping of Four MODUs in Severe Seastates With Current

Model tests have been performed with four mobile offshore drilling units (MODUs) with the aim of identifying wave drift forces and low-frequency damping. The MODUs configuration is different, namely on the number and diameter of columns; therefore, the sample is representative of many of the existing concepts. The model scale is the same as well as the wave and current conditions. The experimental program includes irregular waves with systematic variations of the significant wave height, wave peak period, current velocity, and vessel heading. A nonlinear data analysis technique (cross bi-spectral analysis) is applied to identify the surge and sway quadratic transfer functions (QTFs) of the slowly varying excitation, together with the linearized low-frequency damping. The paper also presents a semi-empirical formula developed in the scope of the EXWAVE JIP to correct potential flow mean wave drift force coefficients of Semis in high seastates with current. The empirical QTFs are then compared with numerical predictions. Comparisons with potential flow coefficients lead to conclusions on the role of viscous drift. The semi-empirical formula is assessed based on comparisons with test results and concluded that it provides a significant improvement compared to potential flow predictions.