Applicability of offshore mooring and foundation technologies for marine renewable energy (MRE) device arrays

Published onlineThe marine renewable energy (MRE) sector is progressing from single device units to device arrays. Currently, the mooring/foundation technologies used in MRE are based on offshore oil/gas industry practices. For MRE arrays to reach commercialization, several issues need to be addressed including the hy-drodynamic array layout, electrical infrastructure, operations, maintenance, control, moorings, foundations, installation and logistics. The DTOcean (The Optimal Design Tools for Ocean Energy Arrays) project is aimed at accelerating the industrial development of ocean energy power generation knowledge, and providing design tools for deploying the first generation of wave and tidal energy converter arrays. In this paper, the ap-plicability of offshore mooring/foundation technologies for marine renewable energy (MRE) device arrays are assessed. The paper introduces the criteria which can be used to appraise technologies and approaches rele-vant to MRE devices. Existing mooring/foundation technologies used in the offshore industry are summarized with examples given of MRE device deployments. The guidance from certification agencies which is used for the design and analysis of mooring/foundation systems is summarized. If not addressed, the failure to opti-mize the design of ocean energy arrays and failure to properly understand economic, environmental, or relia-bility impacts of individual components could have significant consequences for the overall project and sec-tor. The function and type of mooring and/or foundation system are determined by a number of factors including the cost, site characteristics, expected environmental loading and environmental or legislative con-straints and these factors are discussed

Long-term distributions of individual wave and crest heights

This is the final version of the article. Available from Elsevier via the DOI in this record.This paper considers three types of method for calculating return periods of individual wave and crest heights. The methods considered differ in the assumptions made about serial correlation in wave conditions. The long-term distribution of individual waves is formed under the assumption that either (1) individual waves, (2) the maximum wave height in each sea state or (3) the maximum wave height in each storm are independent events. The three types of method are compared using long time series of synthesised storms, where the return periods of individual wave heights are known. The methods which neglect serial correlation in sea states are shown to produce a positive bias in predicted return values of wave heights. The size of the bias is dependent on the shape of the tail of the distribution of storm peak significant wave height, with longer-tailed distributions resulting in larger biases. It is shown that storm-based methods give accurate predictions of return periods of individual wave heights. In particular, a Monte Carlo storm-based method is recommend for calculating return periods of individual wave and crest heights. Of all the models considered, the Monte Carlo method requires the fewest assumptions about the data, the fewest subjective judgements from the user and is simplest to implement.This work was partly funded through EPSRC grant EP/R007519/1

Sub-Natural-Linewidth Quantum Interference Features Observed in Photoassociation of a Thermal Gas

By driving photoassociation transitions we form electronically excited molecules (Na$_2^*$) from ultra-cold (50-300 $\mu$K) Na atoms. Using a second laser to drive transitions from the excited state to a level in the molecular ground state, we are able to split the photoassociation line and observe features with a width smaller than the natural linewidth of the excited molecular state. The quantum interference which gives rise to this effect is analogous to that which leads to electromagnetically induced transparency in three level atomic $\Lambda$ systems, but here one of the ground states is a pair of free atoms while the other is a bound molecule. The linewidth is limited primarily by the finite temperature of the atoms.Comment: 4 pages, 5 figure

The influence of load history on synthetic rope response

Although used for the station-keeping of offshore equipment for several decades, synthetic ropes have only recently been used for marine renewable energy (MRE) devices. The fundamental mooring load differences between these two applications necessitate the detailed quantification of mooring component performance. Of particular importance for lifecycle analysis, installation and maintenance operations is the evolution of synthetic component performance over time due to load history and fatigue mechanisms. Changes to the stiffness and damping properties of these materials will affect the global response of the device if the mooring system and device responses are closely coupled. To address these uncertainties, tension experiments have been conducted on Nylon parallel-stranded rope samples at IFREMER as part of a MERiFIC (Marine Energy in Far Peripheral and Island Communities) consortium. Measurements are reported from tests involving three new samples subjected to a mixed creep/relaxation and harmonic loading regime. Different initial bedding-in levels are used to investigate the influence of load history on the immediate quasi-static and dynamic properties of the rope. For the load regimes studied, it is found that the rope condition with respect to the load-strain characteristic has a strong influence on the performance of the line.The authors would like to acknowledge the support of the MERiFIC project partners. The project is funded by the European Regional Development Fund through the Interreg IV-A programme

Best practice report – mooring of floating marine renewable energy devices

MERiFIC is an EU project linking Cornwall and Finistère through the ERDF INTERREG IVa France (Manche) England programme. The project seeks to advance the adoption of marine energy in Cornwall and Finistère, with particular focus on the island communities of the Parc naturel marin d’Iroise and the Isles of Scilly. Project partners include Cornwall Council, University of Exeter, University of Plymouth and Cornwall Marine Network from the UK, and Conseil général du Finistère, Pôle Mer Bretagne, Technôpole Brest Iroise, IFREMER and Bretagne Développement Innovation from France.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. It is anticipated that the International Electrotechnical Commission’s guidelines Marine energy - Wave, tidal and other water current converters - Part 10: The assessment of mooring system for marine energy converters (MECs) will be published by the end of 2013. Although there are several guidance documents in the literature regarding the mooring of marine renewable energy (MRE) devices, the IEC document is one of the first to be produced on this subject, with guidance also available in documents produced by Det Norske Veritas. This document is intended to provide a concise introduction to mooring systems for MRE devices with reference given to guidelines and standards which may be applicable to the design of moorings for marine renewable energy (MRE) devices. The document begins by setting the scene to give background on the fundamental differences between conventional offshore equipment and MRE devices. In Section 2 design considerations are introduced, including cost, geometry and the importance of conducting risk analysis. Section 3 then gives an overview of moored system numerical modelling. Key findings of the report are then summarised in Section 4.MERiFIC was selected under the European Cross-Border Cooperation Programme INTERREG IV A France (Channel) – England, co-funded by the ERDF

A Markov chain model to enhanced the weather simulation capabilities of an operations and maintenance tool for a wave energy array

Operations and maintenance is a vital area of research in the push to make wave energy a commercial reality. A tool has previously been developed by Pelamis Wave Power to obtain reliable estimates for operational expenditure and ensure smooth running of wave energy arrays. Wave Energy Scotland is now tasked with the future development of this operations and maintenance tool. One of its key inputs is the wave and wind data used to simulate weather windows suitable for marine access. This paper details the creation and validation of a Markov Chain Model to enhance the weather simulation capabilities of the tool. This will ensure that the operations and maintenance strategy of wave energy arrays is modelled more realistically, resulting in an increased confidence in cost estimates and logistical arrangements.The author would like to thank the academic supervisors of this IDCORE project for their contributions, advice and support. Similar thanks must go to the engineers, past and present, who have dedicated their time to the Pelamis project. The industrial supervisor deserves a special mention, without her experience and guidance this study would not have been a success. The author would also like the IDCORE programme and its funding bodies, in particular the ETP (Energy Technology Partnership), for their support