Nylon fibre rope moorings for wave energy converters

The wave energy market has traditionally employed mooring concepts adopted from the mature technology of the oil and gas industry. However, this approach may not always be appropriate to satisfy the particular requirements of Wave Energy Converter (WEC) mooring systems, especially the need for very compliant moorings. Specific challenges are: the huge 1
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order motion induced forces generated by storm waves in shallow water; the potential for slack-snap loading on a mooring; and, the fatigue performance of the mooring components. This paper describes the benefits of employing high compliance fibre rope in mooring systems of less than 100 m water depth. Nylon fibre ropes are a very attractive proposition because of their low modulus and cost but have previously been thought to have a poor wet fatigue performance. A specially designed long-lay-length Superline subrope has been tested both for compliance and for wet fatigue performance. The rope compliance is about twice that of parallel-subrope construction polyester rope. The fatigue performance is less than that of parallel-subrope polyester rope but is comparable to that of conventional construction polyester rope and to that of the chain and steel wire ropes which must also be used as mooring components. The high compliance of the nylon subrope construction together with the more than adequate fatigue performance demonstrated in this study qualify such nylon ropes as ideal for WEC mooring systems.