Abstract
Abstract Polyamide ropes is a promising component for mooring of floating wind turbines (FWTs) in intermediate water depths, where neither bottom-fixed foundations nor FWTs moored with chain-based catenary mooring systems are economically viable options. However, the mechanical properties of nylon ropes are complex, and no consistent data-sets are available to the general research community on the tension-elongation properties of these ropes. Further, the natural variability in the properties of ropes of the same design is not known. This paper presents the results of two experimental campaigns. First, the complete stiffness characteristics of a polyamide sub-rope intended for permanent mooring applications is presented, based on rope tests performed according to the ABS Guidance Note “The application of fiber rope for offshore mooring”. Quasi-static stiffness values are presented for a new rope, a rope post-installation, and a worked in rope. Further, dynamic stiffness values are given, with coefficients for estimating the dynamic stiffness as function of mean tension and tension amplitude. The hysteresis energy dissipation due to different loading and unloading paths during dynamic cycling is calculated, and a new empirical formulation to estimate the energy dissipation for combinations of mean tension and tension amplitude is given. Finally, ten break load tests are performed on sub-ropes. Based on these, the variability in break load on sub-rope level is estimated to have a coefficient of variation of 1.2%. The same break load tests are used to estimate the variability in sub-rope stiffness as well, with the experimental results estimating the coefficent of variation to be 1.4%.