A method has been developed with which the rigid-body motions, stress and fatigue damage response of a TLP-type FOWT are determined in time domain, with the aim of assessing the influence of non-linear hydrodynamic response on the fatigue damage. The dynamics are described by means of a system of coupled 6DOF-EoMs and solved in Matlab, after which the validity of the method is confirmed by means of validation and verification using OrcaFlex models.
Subsequently, a method has been proposed to linearize the dynamics of the developed model. For linearization of quadratic damping, three different methods have been proposed, after which the best performing method is determined by means of a comparative study. Subsequently, using the fully linearized model, the difference in long-term damage w.r.t. the nonlinear model is calculated and the influence of multiaxiality is investigated.
The results have shown that the linear approach is suitable for approximation of long-term fatigue damage in hotspots that are located in the connections between the central column and the pontoons of the TLP. However, in hotspots with relatively low stress response, the long-term damage is underestimated by 48% due to the high sensitivity of the damage to differences in the stress response. Finally, the presence of multiaxiality and non-proportionality has been demonstrated for a number of hotspots and during both operational and extreme conditions.