Design and Failure Analysis of Multi-Component Mooring Lines with Non-Linear Polymer Springs for Floating Offshore Wind Turbines

Floating offshore wind turbines (FOWT) stationed in transition depths (50-80 m) offer easier access for installation and maintenance and broaden the spectrum of operational locations when compared to fixed bottom or deep-water structures. The fatigue performance and abrasion resistance of polyester makes it the preferred choice for taut leg systems because this mooring method uses the elastic characteristics of the rope to provide station keeping effects. Adding non-linear polymer springs to this mooring configuration helps reduce peak loads, platform surge, and increase fatigue life. This research studied the effects of changing line pretension, spring safe working load (force exerted at 50% elongation), and spring response curve on peak loads and platform surge first by modeling the system in OrcaFlex at yearly average and 50-year storm environmental conditions. The maximum tension load from the optimized mooring assembly was applied to a modeled section of the 8-strand multiplait rope in ANSYS Workbench to study deformation, stress, and fatigue life. The OrcaFlex simulations showed that a mooring line pretensioned at 1250 kN with a 4500 kN safe working load degressive spring was the optimal assembly. The peak tension of 3717 kN was applied to the section model in ANSYS and yielded a deformation of 0.268 m and maximum stress of 3.20e8 Pa at the inner radius of the weaving section of the rope. This research will continue by analyzing the fatigue life performance of the rope section at the 50-year storm data to predict operational lifetime and at average conditions to validate the design with industry standards.