A novel mooring tether for peak load mitigation: Initial performance and service simulation testing

Abstract

One of the main engineering challenges for floating marine renewable energy devices is the design of reliable, yet cost-effective mooring solutions for the harsh and dynamic marine environment. The mooring system must be able to withstand the ultimate limit state during storm conditions as well as the fatigue limit state due to the highly cyclic wave motions. This paper presents the performance and service simulation testing of a novel mooring tether that combines the material properties of elastomeric and thermoplastic elements. This allows to 'tailor' the load-extension curve to exhibit a low stiffness response for the expected normal, operating, load conditions and a high stiffness response for the envisaged extreme, storm, conditions. The experimental results demonstrate the working principle of the mooring element and show good agreement between the theoretical load extension curve and the conducted performance tests with a distinct hysteresis effect caused by the thermoplastic element. The hysteresis is dependant on the applied pre-tension and load cycle amplitude of the element and to a lesser extent on the cycle frequency. The relaxation of the elastomeric element is quantified, giving insight into the expected longterm performance of the tether. The demonstrated working principle and the possibility to tailor the mooring response allows engineers to load- and cost-optimise the mooring system of floating marine energy converters.