Comparison of Seabed Friction Formulations in a Lumped-Mass Mooring Model

This paper explores the impact of friction models on mooring line simulations. Seabed friction can play an important role in the determination of mooring loads of slack-moored floating offshore wind turbines. Most mooring models include a relatively simple seabed friction formulation, if any, and little examination of their accuracy is available in literature. Current implementations typically represent seabed contact as coulombic friction with ramping near zero velocity to mitigate instability in the numerical time integration. To assess the impact of this friction model’s use, we compare it against a more sophisticated friction model. This model differentiates between static and kinetic friction, where the former is dependent upon the forces acting on the line and the latter is a function of seabed’s normal response. Both friction models have been implemented into the MoorDyn mooring dynamics simulator and tested under a set of prescribed scenarios including snap loads and oscillatory motion, where the fairlead of a mooring line was driven along both linear and circular paths. Additionally, coupled floating wind turbine simulations using the OC4-DeepCwind semisubmersible show how the friction models affect the platform global response and the extreme and fatigue mooring loads. The results highlight practical differences between the models in terms of both loads prediction and simulation stability/consistency.