Abstract
Synthetic fibre rope materials such as polyester and nylon are used frequently in the offshore industry due to their low weight and flexible properties. A drawback when utilizing these synthetic fibre rope materials is how to account for their complex, non-linear tension-stretch behaviour, which is a combination of both instant-elastic, visco-elastic, instant-plastic, and visco-plastic (creep) material behaviour. This complex behaviour has previously been simplified as a linear stiffness based on an upper bound or lower bound criterion when performing time domain analyses. These stiffness bounds are based on design tension and platform offset criteria. Today it is recommended to use a bi-linear stiffness curve to include both criteria in one, although the model is still a crude approximation of the real behaviour. Recently, the Syrope JIP has developed a method to account for the non-linear behaviour more accurately, which depends on the mean tension, the load rate, and the load history. This Syrope model is utilized in the present study for mooring system design analyses of a floating offshore wind turbine (FOWT) concept and compared to the previously mentioned methods applied in the industry today. The comparison encompasses both fundamental static and dynamic properties of the concept, as well as design tension based on ultimate limit state (ULS) conditions.
