Parameter Estimation for Synthetic Rope Models

With widespread market penetration in the offshore sector, synthetic rope materials offer a range of axial compliance that can be exploited for mooring design, ranging from stiff (e.g. high modulus polyethylene) to soft (e.g. nylon). For new applications, such as the marine renewable energy (MRE) sector synthetic ropes are potentially an enabling technology due to their load reduction properties and relative low cost compared to conventional mooring component materials. Existing design, test and certification procedures for ropes were developed for the station keeping of large offshore equipment. Arrays of small, reactive MRE devices will have an entirely different set of mooring load cases with the mooring system potentially an integral part of the power take off system. Larger devices are also likely to be subjected to complex loading as MRE deployment sites are typically highly energetic in terms of wind, wave and/or tidal energy. Consequently devices and associated subsystems are subjected to dynamic load cases in a wide range of complex environmental and operating conditions. Underpinning research is therefore required to understand the behaviour of synthetic materials subjected to highly dynamic loading regimes in terms of operational performance and long-term durability as these factors could have a significant impact on coupled device performance and availability. At present conventional mooring analyses is based on the use of simplified rope characteristics, although several studies including a Joint Industry Project (Syrope) have investigated these aspects. However, at present an industry-standard method to account for the time-varying behaviour of synthetic ropes for offshore equipment moorings does not exist. The SynMaRE (Synthetic ropes for Marine Renewable Energy mooring systems) project aims to develop a time domain analytical model that can adequately represent the time dependent and non-trivial behaviour of synthetic ropes. Instead of being a stand-alone tool, it is intended that the model will be adopted for incorporation in (or with) commercial mooring system software allowing the prediction of mooring loads and device responses to an increased level of accuracy. This paper will present findings from an initial assessment of parameter estimation techniques utilising a simplified viscoelastic and viscoplastic model and validation scenarios based on physical tension-tension test data featuring load cases relevant to MRE mooring systems.