Mooring systems for marine energy converters

Abstract

This paper discusses several new technologies for mooring floating marine energy converter (MEC) devices, such as wave energy generators, tidal current turbines and floating wind turbines.The principal mooring component is a special nylon fiber rope which provides cyclic tension fatigue endurance much superior to that of conventional nylon ropes. The nylon fiber is treated with a new proprietary coating which has excellent wet yarn abrasion properties. The parallel-subrope type rope construction further reduces internal abrasion.Extensive laboratory testing was carried out on this new nylon rope design. Cyclic tension fatigue tests were conducted at mean loads and load amplitudes typical of actual service conditions and at higher mean loads and amplitudes. These tests demonstrate that the special nylon rope has essentially the same, desirable stretch characteristics as conventional nylon rope and has much better endurance performance.The mooring connection to the floating MEC device consists of a high-modulus fiber rope pendant which passes through a low-friction bell-mouth nylon fairlead on the MEC device. This eliminates the use of heavy, unreliable chain in this critical connection.A unique bag anchor system would be used on sand, clay, rock and other sea beds in which conventional drag embedment anchors and driven piles are impractical. The bag anchor consists of a large abrasion resistant carcass with lifting straps and top closure. The bag is transported to site in a collapsed form and is filled with local sand or aggregate to provide ballast weight. Several or many such bags are enclosed within a fiber rope net for deployment and are grouped together for connection to the mooring line.The paper will be of particular interest to designers of moorings for MEC systems in shallow water and severe wave environments. It will also be of interest for other mooring applications.

Fatigue of improved polyamide mooring ropes for floating wind turbines

“…The results from yarn-on-yarn loading abrasion tests show a significant increase in abrasion resistance by the addition of a specially developed coating. Furthermore, the long lay length rope construction enables these polyamide ropes to exhibit very good fatigue performance, similar to that demonstrated recently for another rope product by Ridge et al (2010;Banfield, 2017;Flory, 2016) .…”

“…There is a widely perceived idea that polyester ropes have superior fatigue resistance than nylon ropes. Quite recently, however, Ridge et al (2010;Banfield, 2017) have shown significantly improved fatigue results for twisted nylon ropes; those authors qualified these ropes as ideal for wave energy convertor moorings and Flory et al (2016) also discussed these applications. Two major differences compared to the earlier fatigue work were the use of twisted ropes with long lay lengths and the application of improved fibre coatings.…”

Development of a Multi-Objective Optimization Tool for Screening Designs of Taut Synthetic Mooring Systems to Minimize Mooring Component Cost and Footprint

“…Taut-synthetic systems provide restoring force to the platform by the elastic properties of the rope [3]. By comparison a chain catenary system provides restoring force through the geometry of the mooring system and the weight of the chain [4]. For a chain catenary system to be functional in the intermediate depths a large amount of chain needs to be used to provide the necessary restoring force for the platform.…”

Round robin testing of synthetic fibre ropes for application in marine renewable energy

“…Mooring systems of dynamic floating MRE devices are required to provide effective station-keeping whilst reducing the combined effects of metocean loads on the system. Conventional station-keeping solutions, composed of steel wire and chain systems, fail to provide the necessary compliance to minimise peak loads for a highly dynamic system (Flory, JF;Banfield, SJ;Ridge, 2016). This reduces system reliability, where a mooring component failure may lead to interruption of operations and incur high offshore repair costs.…”