A novel mooring tether for highly dynamic offshore applications; mitigating peak and fatigue loads via selectable axial stiffness (conference paper)

Abstract

Highly dynamic floating bodies such as wave energy convertors (WECs) require mooring lines with particular mechanical properties; the mooring system must achieve adequate station keeping whilst controlling mooring tensions within acceptable limits. Currently, fibre ropes are commonly used but many mooring designers are seeking alternative solutions that can offer more favourable mechanical properties. The compliance offered by a mooring system will depend largely on the axial stiffness of the mooring lines. Whilst fibre ropes can offer lower axial stiffness than alternatives such as chain and wire rope, there remains a fundamental conflict which prohibits the free selection of axial stiffness properties. This conflict exists because the axial stiffness is strongly governed by the minimum breaking load (MBL) of the rope. The specified MBL must be sufficient to accommodate the predicted peak tension loads with an appropriate factor of safety (FOS) to cater for uncertainties and degradations. In achieving a sufficient MBL, the designer is often forced to accept a higher axial stiffness than is preferred. A potential benefit of reducing the axial stiffness of a mooring line is the reduction of peak loads and fatigue loads. This allows a reduction in mass of both the floating body and the mooring system, thus reducing costs and improving system reliability. This work describes the 'Exeter Tether', an innovation in mooring tether design which decouples the axial stiffness properties from the MBL of the tether. Removing this constraint allows a tether to be specified according to both MBL and axial stiffness. The principals behind the novel tether design are introduced along with an outline of 10 prototype tether variants manufactured in collaboration with Lankhorst Ropes. Results from the proof of concept tests at the University of Exeter's Dynamic Marine Component Test Facility (DMaC) are presented together with preliminary findings from sea trials at the South West Moorings Test Facility (SWMTF). The anticipated load mitigation introduced via the mooring tether is investigated and the implications for system design are discussed.