Modelling of Synthetic Fibre Rope Mooring for Floating Offshore Wind Turbines

Abstract

Fibre ropes offer beneficial properties for mooring of floating offshore wind turbines (FOWTs). However, the mooring line’s stiffness is both load-history and load-rate dependent. A quasi-static stiffness is observed for slow loading, with a higher stiffness related to rapid, cyclic loading (dynamic stiffness). Design standards provide different guidelines for how to combine these in the mooring analysis. This paper describes procedures for adapting laboratory test stiffness results to the Syrope and a bi-linear model and investigates the consequence of using the models for load calculations. The Syrope model accounts for the quasi-static and permanent rope elongation, while performing the analyses with the dynamic stiffness. The bi-linear model applies both the quasi-static and dynamic stiffness in the dynamic analyses. Based on fibre rope tests performed by Bridon-Bekaert, a Syrope model and two bi-linear models are adapted to the same fibre rope. Fatigue damage and ultimate loads on the mooring lines of Saitec’s SATH FOWT are calculated. The bi-linear model artificially reduces the tension ranges, particularly if there is a large difference between the quasi-static and dynamic stiffness of the fibre rope. This leads to a longer predicted fatigue lifetime. Differences in the extreme loads are caused by the permanent elongation of the Syrope model. This may be countered if the elongation is known and included in the bi-linear model. Finally, the bi-linear model introduces an amplitude-dependency in the horizontal natural periods.

Characterization and modelling of the dynamic stiffness of nylon mooring rope for floating wind turbines

“…We can also compare the bilinear law with the measurements of Sørum et al, 2023. They proposed a linear law for dynamic stiffness depending on mean tension: Fig. 14 presents their measures in blue superposed with the model they proposed in red, a bilinear law where the coefficients have been fitted to their value in green and a bilinear law using the coefficients proposed here ((Equation ( 10)) in orange.…”

Effects of Mooring Line with Different Materials on the Dynamic Response of Offshore Floating Wind Turbine

“…Zhang and Liu [17] proposed a universal framework for complex structural configurations and applied it to shared mooring for wind farms using OpenFAST (v3.1.0) and AQWA codes (v5.5). Sørum et al [18] described procedures for adapting laboratory test stiffness results to the Syrope model and a bi-linear model and investigated the consequence of using the models for load calculations of polyester mooring lines. Hall et al [19] proposed that the performance requirements of the real-time hybrid test system are suitable for scalemodel floating wind turbine tests and solved the scale incompatibility problem.…”