Robust Contact and Friction Model for the Fatigue Estimate of a Wire Rope in the Mooring Line of a Floating Offshore Wind Turbine

Station keeping of Floating Wind Turbine (FOWT) is ensured by mooring lines. They may be composed of steel wire ropes, which are particularly difficult to design against the Fatigue Limit State, because the standard Tension-Tension rules cannot capture accurately the influence of the frictional contact interactions between the wires when the rope is bent. We propose here a new model linking the tension and curvature time series computed by a global scale model to a micro-scale model simulating the fretting fatigue at an inter-wire contact location. This new model of a detailed part of rope relies on the use of a new contact element, which allows to gain robustness and CPU time. This is of crucial importance for the large number of simulations required by a fatigue life estimate. A case study is presented considering a FOWT equipped with three pairs of catenary mooring lines. The computed tension and curvature obtained for a severe sea state are transferred to the detailed model of the wire rope, with periodic boundary conditions representing the rope continuity. The time series of sliding and contact forces are finally reported at different locations within the rope, providing possible input data for a fretting fatigue analysis.