Evaluation of wire-lay nylon mooring lines in a wave energy device field trial

A 1/7 scale wave energy conversion (WEC) device was installed in Puget Sound for over a year in a realistic scale wave environment. The objectives of the trial were: · Evaluate the behavior of the WEC · Assess the performance of a wire-lay nylon mooring line anchor system. This paper reports the results of the mooring system. The costs of mooring systems for offshore marine renewable energy devices are a critical component in developing an economically feasible installed system. Several factors contribute to this, including the fact that conventional mooring technologies and components have been applied to renewable energy systems with little attention paid to optimizing the overall system. The Puget Sound scale WEC installation offered an opportunity to evaluate the performance of an unconventional mooring line construction for a WEC in field conditions. Offshore WEC systems are typically located in shallow water (depth<;50m), high-energy wave environments. Typically the wave loads drive the mooring configuration, size and cost. In order to minimize dynamic wave loads, options have included using very long or heavy catenary moorings, sinkers, subsurface buoys or long lengths of polyester synthetic lines to lower system modulus. The resulting mooring systems become complicated and costly to install and often occupy a large footprint on the seabed. Use of traditional laid or braided nylon lines provided high elasticity to help dissipate load but at the expense of poor fatigue behavior. Wire-lay nylon offers high elasticity with greatly increased fatigue life. Successful demonstration of wire-lay nylon in WEC moorings had the potential for significant reductions in material and installation cost. Sound & Sea Technology, Inc. designed and installed the mooring with Whitehill Manufacturing Corp. wire-lay nylon mooring lines. Whitehill's wire-lay lines are designed specifically to be used in applications where a combination of low elastic modulus and high fatigue durability are required. The line construction differs substantially from laid ropes and braided constructions. Wire-lay lines have the following advantages compared to conventional line constructions: · No strand crossovers to eliminate internal abrasion in the body of the rope. For braided constructions,internal abrasion at the strand crossovers results in rope strength reduction and poor fatigue performance. · Individual protective strand jacketing to minimize abrasion between strands in the splice region. These jackets also provide protection from UV, foreign particle ingression, and external damage to maximize rope service life. This paper reviews the design criteria for the Puget Sound WEC mooring configuration. The physical characteristics of the wire-lay mooring lines used for the initial modeling and analysis of the mooring design and their influence on the final mooring design are described. The data acquisition system provided wave height, period, and mooring loads. After the WEC was taken out of the water the mooring lines were recovered and tested in Whitehill Manufacturing's laboratory. Residual tensile strength was measured, and any physical degradation was noted. Cyclic tests were conducted to predict remaining rope fatigue life. This field trial demonstrates the utility of wire-lay mooring lines for wave energy, and by extension, other at-sea moored systems that require low elastic modulus, long fatigue life mooring configurations.