Experimental characterisation of small-diameter ropes for representing synthetic moorings in tank testing

• Small-diameter ropes identified to represent commercial ropes at medium tank scales. • Candidates found for 10–15 MW floating wind case-studies, none for wave energy. • Ropes of the same material, scaled mass and diameter may not match scaled stiffness. Offshore renewable energy (ORE) developers are increasingly choosing synthetic ropes in their mooring designs. In hydrodynamic tank testing, the scaled elasticity of these ropes is typically represented by springs, which are attractive for their simplicity but fail to imitate the non-linear, viscoelasticity of synthetic ropes. Employing small-diameter ropes may offer a more accurate portrayal of mooring dynamics for advanced design stages; however, these ropes are rarely produced for engineering purposes and their properties are poorly documented. Consequently, this study characterises a range of small-diameter ropes via tension testing and compares their properties with those of commercial mooring ropes at scales relevant to ORE tank testing (1:25, 1:50 and 1:100). Small-diameter rope candidates were found for commercial polyester ropes used in large (10–15 MW) floating wind moorings at both 1:25 and 1:50 scale, and for nylon ropes at 1:25 scale only. No suitable candidates were found at 1:100 scale or for the smaller commercial ropes used in wave energy. Notably, simply scaling the diameter of a rope of the same material does not reliably reproduce the scaled stiffness. This work offers a means to advance tank-scale mooring designs, thereby increasing the accuracy of experimental hydrodynamic data used for numerical model validation.