Large-scale testing of a hydraulic non-linear mooring system for floating offshore wind turbines

The mooring system has been recognised as a key area of expense that needs to be addressed to improve the cost competitiveness of floating offshore wind turbines. The devices installed to date have generally adopted designs from the oil and gas industry using heavy mooring materials, providing the required safety margins but with a significant degree of conservatism. Recent interest in the usage of lighter and more compliant mooring materials has shown that they have the potential to reduce peak line loads, which would in-turn reduce costs. However, the lack of operational experience with such materials has limited their adoption in a risk averse industry. This paper reports on the large-scale physical testing of a hydraulic-based mooring component with non-linear stiffness characteristics. The performance of the device is characterised in a laboratory both statically and dynamically, as well as in conditions representative of operating in a sea state using a combined physical and numerical modelling approach. The results show that the dynamic stiffness of the component is a function of load history and hydraulic pre-charge pressure, while the inclusion of the device as part of the OC4 semi-submersible floating wind platform can reduce the peak mooring line loads by up to 9%. Beyond the physical test results, the calculations suggest that the peak load reduction in the modelled scenarios could be as much as 40% if the device can be scaled further. The paper supports the adoption of innovative mooring systems through dedicated component and performance testing.