Reducing Peak & Fatigue Mooring Loads: A Validation Study for Elastomeric Moorings

Abstract

Fibre ropes are often specified for floating wave and tidal energy device mooring systems. The relatively low axial stiffness goes some way towards mitigation of the peak and fatigue mooring loads. However, the minimum breaking load (MBL) of a fibre rope dictates its axial stiffness and hence the free selection of low axial stiffness is not possible with conventional rope. The resulting mooring stiffness is often sub-optimal, giving rise to elevated peak and fatigue loads. Elastomeric, nonlinear mooring elements solve this by partially de-coupling the axial stiffness from the MBL and offering an initial soft response with increasing stiffness for higher strains. These nonlinear elastomeric moorings have the potential to reduce the peak and fatigue mooring loads as indicated by numerical studies. This work uses a validated numerical model to quantify the load reduction achievable by substituting a novel elastomeric tether in place of a conventional fibre rope. Field data is used to validate the base case model of the highly dynamic South West Moorings Test Facility (SWMTF). The base case mooring design utilises Nylon ropes which are subsequently replaced with elastomeric tethers in the validated model. The results show that the peak mooring loads are reduced substantially upon substituting the elastomeric tethers for the conventional ropes. Subsequently this allows a downward iteration of MBL and axial stiffness towards an optimal condition, providing the lowest achievable load case. In most instances, the optimum iteration outcome also allows a reduction in catenary chain weight. The reduction in peak tension is accompanied by an increase to the buoy excursion in surge. However, the mean peak excursion increase is 21% whilst the mean peak tension reduction is 66%.