Physical measurement of a slow drag of a drag embedment anchor during sea trials

Anchor drag during operation of offshore structures could significantly alter the initial load design characteristics of a mooring system. Hence an estimation of anchor positions during operation is essential to identify whether slow or abrupt anchor motion occurs and might require the redeployment of an anchor. During storm conditions, monitoring of mooring tensions and structure motions at the South West Mooring Test Facility (SWMTF) revealed the slow drift motion of one anchor. This facility is a surface buoy with a three-legged, compliant mooring system designed to investigate mooring system behaviour for Marine Renewable Energy (MRE) devices. This paper presents i) some methods to identify the deployment anchor positions: numerical model, acoustics diver survey, and towed sonar ii) the analyses procedure, and estimations of slow drift anchor motion. The findings indicate that one drag embedment anchor moved slowly during a moderate but prolonged and isolated storm, before embedding again. The work demonstrates that anchor position can be accurately monitored and that anchor motion is not necessarily due to excessive peak loads

Component test facilities for marine renewable energy converters

This paper describes how the PRIMaRE group at University Exeter is engaging in the establishment of appropriate reliability methods suitable for application to marine renewable devices with a key area being the production of suitable failure rate data for the marine renewable energy industry. This activity seeks to mitigate uncertainties and cost implications associated with the reliability assessment of marine energy converters (MECs) due to an omnipresent lack of applicable failure rate data. The capability of two facilities, namely i) the South Western Mooring Test Facility (SWMTF) and ii) the Dynamic Marine Component Test facility (DMaC), to perform specimen and accelerated component testing is discussed. A case study, using data from wave tank tests and numerical simulations performed for the SWMTF, serves to illustrate how evidence of component reliability under operational conditions could be provided.The authors would like to acknowledge the support of the South West Regional Development Agency through the PRIMaRE institution. They would also like to acknowledge the European Community's Sixth Framework Programme HYDRALAB III, Contract no. 022441 (RII3). The second author would like to acknowledge the funding support from the Engineering and Physical Sciences Research Council (EPSRC) under the SUPERGEN Marine Doctoral Programme. Thanks also to Orcina for provision of their Orcaflex software

On peak mooring loads and the influence of environmental conditions for marine energy converters

Mooring systems are among the most critical sub-systems for floating marine energy converters (MEC). In particular, the occurrence of peak mooring loads on MEC mooring systems must be carefully evaluated in order to ensure a robust and efficient mooring design. This understanding can be gained through long-term field test measurement campaigns, providing mooring and environmental data for a wide range of conditions. This paper draws on mooring tensions and environmental conditions that have been recorded (1) for several months during the demonstration of an MEC device and (2) over a period of 18 months at a mooring test facility. Both systems were installed in a shallow water depth (45 m and 30 m, respectively) using compliant multi-leg catenary mooring systems. A methodology has been developed to detect peak mooring loads and to relate them to the associated sea states for further investigation. Results indicate that peak mooring loads did not occur for the sea states on the external contour line of the measured sea states, but for the sea states inside the scatter diagram. This result is attributed to the short-term variability associated with the maximum mooring load for the given sea state parameters. During the identified sea states, MEC devices may not be in survival mode, and thus, the power take-off (PTO) and ancillary systems may be prone to damage. In addition, repeated high peak loads will significantly contribute to mooring line fatigue. Consequently, considering sea states inside the scatter diagram during the MEC mooring design potentially yields a more cost-effective mooring system. As such, the presented methodology contributes to the continuous development of specific MEC mooring systems.The work described in this publication has received funding from the Technology Strategy Board (TSB), Project Number 100855. The authors would like to acknowledge the support of the South West Regional Development Agency for its support through the Partnership for Research in Marine Renewable Energy (PRIMaRE) institution. They also gratefully acknowledge Fred Olsen for supplying the measurements of mooring loads

Reliability verification of mooring components for floating marine energy converters

PublishedThis paper was presented at SHF – conference on MRE – Brest (F), October 2013.Safety factors are critical to device reliability and are applied during device development to protect against early failures. At each stage of a development a designer may apply their own safety factor in relation to the criticality of the component or subassembly for which they are responsible. This paper seeks to understand how different assessment techniques can assist the design process by refining safety factors, with the aim of reducing device costs and improving economic viability. To achieve this, a methodology is presented to assess and verify the fatigue performance of mooring components. The paper draws on field data and introduces a combined approach of modelling, service simulation and field tests to validate the reliability of components. A shackle is used as a case study to demonstrate the methodology. Results from finite element analysis (FEA) and accelerated service simulation testing on the Dynamic Marine Component test facility (DMaC) are presented and discussed, including fatigue damage and failures. FEA is found to accurately predict areas of weakness within a component, however it underestimates component strength due to unrealistic stress concentrations at applied boundary conditions. Static and fatigue tests demonstrate the complex nature of reliability estimation, with static component safety factors of 8.6 being reduced to less than 3.7 under a fatigue loading regime. Service simulation testing is found to be important in refining initial reliability estimations from S-N curves and FEA models. The effect of mean stress on fatigue failure is also found to be significant.The authors would like to acknowledge the support of the UK Centre for Marine Energy Research (UKCMER) through the SuperGen programme funded by the Engineering and Physical Sciences Research Council

Wave Conditions Inducing Extreme Mooring Loads on a Dynamically Responding Moored Structure

The aim of this paper is to determine which wave conditions are inducing extreme mooring loads on a highly dynamically responding moored structure. Currently, the design of a mooring system for a typical oil and gas offshore structure is based on the prediction of the extreme mooring loads for a limited number of wave conditions along the envelope of a wave scatter diagram. During the design process, an inappropriate choice of wave conditions could lead to an incorrect estimation of extreme mooring loads, which may result either in the loss of the mooring system or in a costly overdesign. This paper draws on mooring tensions and wave conditions that have been recorded at a mooring test facility using a multi-leg catenary mooring system. The mooring loads have been assessed to identify extreme mooring loads, which have been analysed in respect to the corresponding wave conditions. Further, joint probability distributions of wave conditions that results in extreme mooring loads have been determined. The most important finding is that extreme mooring loads were not necessarily identified to occur on the envelope of the wave climate parameter scatter diagram

A novel mooring tether for peak load mitigation: Initial performance and service simulation testing

Copyright © 2014 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in International Journal of Marine Energy. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. The ‘In press‘ version is available at http://dx.doi.org/10.1016/j.ijome.2014.06.001One of the main engineering challenges for floating marine renewable energy devices is the design of reliable, yet cost-effective mooring solutions for the harsh and dynamic marine environment. The mooring system must be able to withstand the ultimate limit state during storm conditions as well as the fatigue limit state due to the highly cyclic wave motions. This paper presents the performance and service simulation testing of a novel mooring tether that combines the material properties of elastomeric and thermoplastic elements. This allows to 'tailor' the load-extension curve to exhibit a low stiffness response for the expected normal, operating, load conditions and a high stiffness response for the envisaged extreme, storm, conditions. The experimental results demonstrate the working principle of the mooring element and show good agreement between the theoretical load extension curve and the conducted performance tests with a distinct hysteresis effect caused by the thermoplastic element. The hysteresis is dependant on the applied pre-tension and load cycle amplitude of the element and to a lesser extent on the cycle frequency. The relaxation of the elastomeric element is quantified, giving insight into the expected longterm performance of the tether. The demonstrated working principle and the possibility to tailor the mooring response allows engineers to load- and cost-optimise the mooring system of floating marine energy converters.Engineering and Physical Sciences Research Council (EPSRC)Peninsula Research Institute for Marine Renewable Energy (PRIMaRE)European Regional Development Fund (ERDF)South West Regional Development Agency (SWRDA

Component reliability testing for wave energy converters: Rationale and implementation

Copyright © 2013 European Wave and Tidal Energy Conference10th European Wave and Tidal Energy Conference, Aalborg, Denmark, 2 – 5 September 2013The reliability of marine renewable energy (MRE) converters is a key issue that has to be addressed and included in a whole system approach, in order to make the energy extraction from these sources a viable option. At the current development stage of MRE converters, an increasing number of devices are being field tested at pre-commercial demonstration scale, yielding field experience and load data useful for refining, demonstrating and improving the reliability of devices. This paper gives a brief review of the most advanced technologies and common reliability aspects that provide the rationale for dedicated component testing. It describes a service simulation test approach and the development of a unique large-scale component test facility. The test rig is capable of replicating the forces and motions experienced by components for a range of floating marine applications. The replication of motion angles is demonstrated in this paper. The service simulation test of a marine power cable is presented as a case study on how component performance can be assessed and demonstrated prior to long-term field deployments in order to ensure the reliability of crucial sub-systems and components in the harsh marine environment.Engineering and Physical Sciences Research Council (EPSRC)PRIMaR

Wave conditions inducing extreme mooring loads on a dynamically responding moored structure

PublishedThe aim of this paper is to determine which wave conditions are inducing extreme mooring loads on a highly dynamically responding moored structure. Currently, the design of a mooring system for a typical oil and gas offshore structure is based on the prediction of the extreme mooring loads for a limited number of wave conditions along the envelope of a wave scatter diagram. During the design process, an inappropriate choice of wave conditions could lead to an incorrect estimation of extreme mooring loads, which may result either in the loss of the mooring system or in a costly overdesign. This paper draws on mooring tensions and wave conditions that have been recorded at a mooring test facility using a multi-leg catenary mooring system. The mooring loads have been assessed to identify extreme mooring loads, which have been analysed in respect to the corresponding wave conditions. Further, joint probability distributions of wave conditions that results in extreme mooring loads have been determined. The most important finding is that extreme mooring loads were not necessarily identified to occur on the envelope of the wave climate parameter scatter diagram.The work described in this publication has received funding from the European Commission under the 7th Framework Programme (FP7) through the MARINET initiative, grant agreement no 262552. It also received funding from the Technology Strategy Board. The authors would like to acknowledge the support of the South West Regional Development Agency for its support through the PRIMaRE institution.http://hdl.handle.net/10871/1430

Assessing loading regimes and failure modes of marine power cables in marine energy applications

publication-status: PublishedHighly reliable marine power cables are imperative for the cost-effective operation of marine energy conversion systems. Cable manufacturers and installers have considerable experience with marine power cables when deployed to operate under static or dynamic load conditions, but highly dynamical power cables for marine renewable energy converters have large uncertainties. The mechanical loadings of a power cable attached to a floating marine energy converter will be considerably different to the present applications like remotely operated vehicles (ROVs) or oil and gas umbilicals. The floating structure responds to the wave action and transfers this dynamic motion to the attached power cable. Moreover the frequency of response will be at the wave period (linear case) leading to considerable cyclic effects. At present the loading regime in such applications is not well understood, due to a lack of field experience. The paper describes the parameters and results of a dynamic computational model that investigates the umbilical load conditions for a generic wave energy converter. Two geometric configurations of a double armoured power cable are considered, a catenary and a Lazy Wave shape. The model is set up using the dynamic analysis package OrcaFlex and uses top-end motions measured in 1:20 wave tank tests. While the simple catenary shape experiences high tensional forces at the attachment point and considerable compression, the maximum tensional forces can be significantly reduced and compression is avoided with a Lazy Wave shape. For this configuration the highest tension occurs near the attachment point and at the transition points of the buoyancy section. For the modelled conditions, the power cable accumulated a significant number of tension and bending load cycles, indicating that power cables in floating marine energy applications will operate in a high cycle regime (in the order of 10^6 cycles) likely to accumulate several million load cycles during a single year of operation.EPSRC; SuperGen Marin

Synthetic mooring ropes for marine renewable energy applications

Copyright © 2015 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Renewable Energy. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Renewable Energy (2015), DOI: 10.1016/j.renene.2015.03.058Synthetic mooring ropes have a proven track record of use in harsh operating conditions over the past two decades. As one of the main users of ropes for permanent mooring systems, the oil and gas industry has opted for these components because they possess performance characteristics and economies of scale which are in many respects superior to steel components. Given this accrued experience, it is unsurprising that several marine renewable energy (MRE) device developers have utilised synthetic ropes, motivated by the need to specify economical, reliable and durable mooring systems. Whilst these components are potentially an enabling technology for the MRE sector, this is a new field of application which can feature highly dynamic mooring tensions and consequently existing certification practices may not be directly applicable. Based on the expertise of the authors, this paper provides a state-of-the-art overview of synthetic ropes in the context of MRE mooring systems, including key information about aspects of specification (performance attributes, classification and testing) as well as application (installation, degradation, maintenance, inspection and decommissioning). It is the intention of this review to provide valuable insight for device developers who are considering using ropes in the specification of fit for purpose mooring systems.European Regional Development Fund through the Interreg IV-A programm