1 Copyright © 2017 by ASME Proceedings of the ASME 2016 37th International Conference on Ocean, Offshore and Arctic Engineering, OMAE2018 June 17-22, 2018, Madrid, Spain OMAE2018-77873 LOW FREQUENCY EXCITATION AND DAMPING OF FOUR MODUS IN SEVERE SEASTATES WITH CURRENT Nuno Fonseca Carl Trygve Stansberg Kjell Larsen SINTEF Ocean Trondheim, Norway nuno.fonseca@sintef.no Ctstansberg Marinteknikk Trondheim, Norway ctstansberg.marinteknikk@gmail.com Statoil ASA Trondheim, Norway KLARS@statoil.com Rune Bjørkli Tjerand Vigesdal Oddgeir Dalane Statoil ASA Stavanger, Norway RUNBJO@statoil.com Statoil ASA Stavanger, Norway TJVI@statoil.com Statoil ASA Stavanger, Norway ODDD@statoil.com ABSTRACT 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. The test data is post-processed to identify the surge and sway quadratic transfer functions (QTFs) of the slowly varying excitation, together with the linearized low frequency damping. The post-processing applies a nonlinear data analysis technique known as "cross-bi-spectral analysis" to estimate characteristics of second-order (quadratic) responses from the measured motions and undisturbed incident wave elevation. The empirical QTFs are then compared with numerical predictions to conclude on the role of viscous drift and the applicability of Newman's approximation for calculation of drift forces in irregular waves. Finally, the empirical drift forces, empirical low frequency damping coefficients and low frequency motions statistics are compared for the three MODUs to conclude on the relation between the Semi configuration and the low frequency responses. 1 INTRODUCTION There has been a considerable number of incidents with mooring line failures on Semi-submersibles over the last 15 years. These incidents point to the possibility that mooring lines are overloaded during storms. In fact, several mooring line failures have been reported for North Sea floating structures along the recent years during severe conditions (Kvitrud, 2014). This has uncovered a need to improve methods, procedures and standard industry practice in design prediction of nonlinear wave loadings in high and steep seas. The larger uncertainty in the prediction of floating structures mooring line tensions in severe seastates arises from the estimation of low frequency motions (Stansberg 2015). Two wave-floater interaction sub-problems require improved modelling: the low frequency excitation and the low frequency damping. For semi-submersibles, linear diffraction potential flow codes underestimate wave drift forces, especially in severe seastates (Stansberg 2001, Stansberg et al. 2015). Conditions with combined waves and current add complexity and increase discrepancies. The main reason for the discrepancies seems to be contributions from viscous drag on the low-frequency excitation. For sufficiently long waves, mean viscous drag force on the columns above the still water level gives a contribution to the drift force (Dev and Pinkster, 1994). Faltinsen (1990) discusses an additional contribution from the horizontal component of normal drag forces on the pontoons. Higher than second order potential flow effects may also play a role. The EXWAVE Joint Industry Project (JIP) was initiated with the aim of addressing these problems and come up with