Assessing the influence of nonlinear mooring restoring forces in the optimization and design of FOWT

Abstract The design of mooring systems plays a pivotal role in the success of Floating Offshore Wind Turbine (FOWT) projects. This process naturally involves iterations, as design and analysis are intricately linked. Despite the inherent nonlinearity of mooring system restoring forces, many designers opt for employing an equivalent linear mooring system stifness matrix to expedite the optimization process in early design stages. This article aims to underscore the limitations associated with relying on the equivalent linear model when compared to accounting for nonlinear restoring forces during optimization processes. To illustrate this point, a case study was conducted using the reference semi-submersible platform VolturnUS-S. The study considered intermediate to large water depths and addressed three different mooring confgurations: catenary, semi-taut, and taut lines. The analysis focused on several critical aspects, including ofset watch circles, line tensions (including those at the anchors), and a cost estimate based on the different models. The findings indicate that, for catenary-based mooring systems, the linear model remains a good approximation, leading to no significant loss of accuracy in the context of early design stages. As the lines become tauter, however, the nonlinearities become more pronounced and the errors involved in the linear model can reach unacceptable levels.