Prediction of the mean heading of a turret moored FPSO in bi-modal and bi-directional sea states

Previous studies by the authors have confirmed that the delicate balance between various mean wave drift forces and moments is the main contributor to the heading stability of a single-point moored ship shaped vessel. It is shown here that in a bi-directional seastate, the interaction between the two wave components can significantly alter the drift loads on the platform. A simple cross-wave correction is added to the time domain simulations to account for the interaction. Implementing this correction brings the numerical results more in line with the experimental findings. The cross-wave correction term is quantified and recommendations are made for better simulations and experiments of such platforms.