Sam Weller, Peter Davies, Lars Johanning, and Stephen Banfield – MERiFIC Work Package 3, October 2013
This report is a deliverable of MERiFIC Work Package 3: ‘Dynamic Behaviour of Marine Energy Devices’ involving the collaboration of IFREMER (Institut français de recherche pour l’exploitation de la mer) in France and the University of Exeter in the United Kingdom. Although synthetic ropes have been used for the station-keeping of offshore structures for the past two decades predominantly by the oil, gas and shipping industries, there is considerable interest in their utilisation for the station-keeping of marine renewable energy (MRE) devices. Differences in application between typically small, highly responsive devices (e.g. Wave Energy Converters or WECs) and large slow-moving platforms necessitate a unique approach to mooring system design and dedicated mooring component test programs, both guided by relevant certification standards. It is the intention of this report to provide an introduction to synthetic mooring ropes in the context of previous usage in the offshore industry and also to highlight factors which should be considered for their use in MRE mooring systems.
The document begins by setting the scene to give background on the fundamental differences between previous applications of synthetic mooring ropes and MRE devices. In Section 2 a brief overview of commercially available ropes is then given. The distinct properties of synthetic materials and rope constructions are summarised with emphasis placed on issues which are likely to be relevant for MRE devices. In the absence of specific advice for this emerging industry, conventional approaches to applying safety factors to synthetic ropes are then introduced. Section 3 highlights in-service considerations relevant for the different lifecycle stages of ropes, from installation and operational procedures (such as maintenance and inspection) to decommissioning. Specific modelling approaches for synthetic ropes are then summarised in Section 4, followed by a summary in Section 5. This document is not intended to be an exhaustive account of all aspects of synthetic mooring ropes and in light of this further references are provided for the interested reader.