Physical testing of a non-linear active damper developed for offshore renewable energy

Abstract

The ambitions of large-scale offshore renewable energy deployment can only be realised if technological and logistical challenges are resolved to reduce the levelised cost of energy. The effective station keeping during device lifetime is a significant challenge that can be addressed through innovation in mooring systems. To increase confidence in the performance of the innovative components and systems prior to field deployment, lab based physical testing must be conducted. The Intelligent Mooring System (IMS) is an innovative non-linear component that is designed to provide active control over the load response of the mooring system to reduce peak loads. To improve the seaworthiness of the system, design changes were made and the resulting IMS is composed of a braided Dyneema sleeve housing an internal accumulator. This paper characterises the static and dynamic load response of the improved design through physical tests conducted at the Dynamic Marine Component test facility. Results indicate that the initial internal pressure is the primary driver of the IMS stiffness profile relative to the water/air ratio. A comparison between the quasi-static and dynamic stiffness characterisation shows that quasi-static stiffness provides a good first-estimate for individual configurations. While the Dyneema fibre displays a hysteretic behaviour for loading and unloading, it improves the strength of the IMS by 47% compared to the previous Vectran build. The presented stiffness curves of the IMS can be used in conjunction with available offshore renewable energy system models to demonstrate the effectiveness of including the IMS in the mooring system to reduce peak loads. Future work includes the field demonstration of a scaled prototype at the U.S. Navy Wave Energy Test Site in Hawaii.