Mechanical Properties of Nylon Yarn for Use in Wind Turbine Moorings at Various Water Temperatures

Abstract The elasticity and flexibility of nylon mooring ropes offer many advantages, including a potential to reduce dynamic loads and by that fatigue damage and extreme loads in mooring elements. However, there is very limited experience with use of nylon ropes in permanent mooring of floating structures. To ensure their integrity during a lifetime of operation in wind and waves, it is important to study short- and long-term stiffness properties, strength, and elongation of nylon ropes in varying sea temperatures. Nylon ropes for moorings are made of high tenacity PA6 multifilament yarn. These fibres hold a high tensile strength to weight ratio and are known to have good abrasion resistance. However, their non-linear stress-strain behaviour at different temperatures in the saturated wet state, is not fully documented. Mechanical testing of mooring ropes is expensive and time consuming due to their large dimensions and high strength, and limited access to relevant test equipment. Thus, when feasible, it will be beneficial to perform extensive testing and parameter studies on the nylon yarn used in rope production. This includes studying the effect of different water temperatures on stiffness and strength of nylon yarn. This paper presents a test set-up and procedure for tensile testing of yarn in a water tank with varying temperatures. Results from tensile testing of two different batches of PA6 multifilament yarn submerged in water temperatures of 4, 15 and 25 °C water are given. Both quasi-static stiffness and strength properties are presented based on multiple tests replicates. Results showed that mechanical properties of PA6 yarn are affected by water temperature, as both strength and stiffness increased as temperature was reduced. Yarn breaking load increased from an average of 126 N at 25 °C to 143 N at 4 °C, while yarn stiffness (given as a function of strain) at low strain increased from an average of 212 N at 25 °C to 259 N at 4 °C. The tests revealed minor differences in mechanical properties between different spools of yarn.