Comparison of numerical response predictions for a bottom-fixed offshore wind turbine

Numerical simulations are widely used for response calculations on offshore wind turbines. Code-to-code comparisons are frequently used for verification of the codes, as full-scale measurements can be difficult to obtain. However, most code comparisons performed focus on documenting the responses predicted by the different codes, or on the effect of specific differences between the codes. Little insight is provided to how these differences would affect design calculations, such as the fatigue utilization. In this paper, the response predictions of the programs SIMA, vpOne and FAST are compared using the DTU 10 MW reference wind turbine on a monopile foundation. While differences in the models are first highlighted through a number of simplified load cases, a lifetime fatigue evaluation of the model is then performed for the monopile at mudline. In the deterministic load cases the response of all models are quite similar, while some differences become apparent in the stochastic analysis. For the fatigue calculations, a difference of 14 % is found in the damage equivalent bending moment at mudline. This demonstrates how sensitive the fatigue utilization is to small differences is code capabilities and modelling

Modelling of Synthetic Fibre Rope Mooring for Floating Offshore Wind Turbines

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