A novel mooring tether for highly dynamic offshore applications; mitigating peak and fatigue loads via selectable axial stiffness (journal article)
Thies, Philipp R.
Journal of Marine Science and Engineering
Multidisciplinary Digital Publishing Institute
All articles published by MDPI are made immediately available worldwide under an open access license (Creative Commons Attribution License). This means: everyone has free and unlimited access to the full-text of all articles published in MDPI journals, and everyone is free to re-use the published material if proper accreditation/citation of the original publication is given.
© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).
Highly dynamic floating bodies such as wave energy convertors require mooring lines with particular mechanical properties; the mooring system must achieve adequate station keeping whilst controlling mooring tensions within acceptable limits. Optimised compliant mooring systems can meet these requirements but where compliance is achieved through system architecture, the complexity of the system increases together with the mooring footprint. This work introduces the ‘Exeter Tether’, a novel fibre rope mooring tether providing advantages over conventional fibre ropes. The tether concept aims to provide a significantly lower axial stiffness by de-coupling this attribute from the minimum breaking load of the line. A benefit of reduced axial stiffness is the reduction of mooring system stiffness providing a reduction of peak and fatigue loads, without increasing mooring system complexity. Reducing these loads improves system reliability and allows a reduction in mass of both the mooring system and the floating body, thus reducing costs. The principles behind the novel tether design are presented here, along with an outline of eight prototype tether variants. Results from the proof of concept study are given together with preliminary findings from sea trials conducted in Falmouth Bay. Results demonstrate that the Exeter Tether can be configured to achieve a significantly lower axial stiffness than conventional fibre rope and that the stiffness is selectable within limits for a given breaking strength. Strain values greater than 0.35 are achieved at 30% of line breaking strength; this represents more than a threefold increase of the strain achievable with a conventional rope of the same material. The tether was subjected to six months of sea trials to establish any threats to its own reliability and to inform future design enhancements in this respect.
UK Centre for Marine Energy Research (UKCMER) through the SuperGen programme funded by the EPSRC
IFREMER’s Materials in a Marine Environment Laboratory, funded through the MARINET Programme
Open access journal
Special Issue: Selected Papers from the ASRANet International Conference on Offshore Renewable Energy
Vol. 3(4), pp. 1287-1310