Analysis of Highly Dynamic Mooring Systems: Peak Mooring Loads in Realistic Sea Conditions

Marine Renewable Energy (MRE) is a promising source of energy for the future. However, it is still under development and many challenges need to be overcome to develop competitive solutions. While the design of the station keeping system of traditional offshore oil and gas structures is driven mainly by their low frequency motions, MRE devices are installed at nearshore locations and move dynamically. Because of these criteria, MRE mooring systems require novel mooring systems and associated standards. MRE mooring standards need to take into account the highly dynamic behaviour of these systems, which can lead to large mooring loads. The nature of these loads needs to be investigated to improve the confidence in mooring design and to improve cost-effectiveness. The aim of this thesis is to develop the understanding of peak mooring loads on highly dynamic mooring systems, in particular, the environmental conditions associated with the loads. In addition, preliminary research into the response of the mooring systems to environmental conditions is presented. Both field tests and tank tests have been conducted. Field tests give insight into the behaviour of a dynamic mooring system in real sea conditions. Measuring the mooring loads and the environmental conditions - wave, and current if available – for several months, a methodology has been developed to detect peak mooring loads and identify the associated environmental conditions in order to compare them with the environmental conditions recorded throughout the field tests. The principal finding is that peak mooring loads occur for sea states with large but not always the highest significant wave height HS. The understanding of the effect of tidal conditions on peak mooring loads requires further work. A tank test of a dynamic mooring system in moderate sea states has been conducted to observe the dynamic behaviour of the mooring system. Tank tests enable detailed observations of the dynamic behaviour of a system in a well controlled environment and allow the calibration of a numerical model. The model can be used to investigate separate physical parameters. The results from this thesis will assist in the development of specific standards for MRE mooring systems. These standards are essential for the evolution of the MRE industry

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

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

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

Assessment of entanglement risk to marine megafauna due to offshore renewable energy mooring systems

This paper defines a methodology to compare different offshore renewable energy (ORE) mooring configurations in terms of the risk of entanglement they present to marine megafauna. Currently, the entanglement of large marine animals is not explicitly considered in environmental impact studies. Recommendations need to be developed, assessing the risk of entanglement of ORE mooring configurations at the beginning of their design process. Physical parameters of the mooring system affecting the relative risk of entanglement have been identified as tension characteristics, swept volume ratio and mooring line curvature. These have been investigated further through six different mooring configurations: catenary with chains only, catenary with chains and nylon ropes, catenary with chains and polyester ropes, taut, catenary with accessory buoys, taut with accessory buoys. Results indicate that the taut configuration has the lowest relative risk of entanglement, while the highest relative risk occurs with catenary moorings with chains and nylon ropes or with catenary moorings with accessory buoys. However, the absolute risk of entanglement is found to be low, regardless of the mooring configuration. This methodology can also be applied to other mooring configurations, arrays or power cables.Scottish National Heritag

Navigating the Valley of Death: Reducing Reliability Uncertainties for Marine Renewable Energy

PublishedTechnology Readiness Levels (TRLs) are a widely used metric of technology maturity and risk for marine renewable energy (MRE) devices. To-date, a large number of device concepts have been proposed which have reached the early validation stages of development (TRLs 1-3). Only a handful of mature designs have attained pre-commercial development status following prototype sea trials (TRLs 7-8). In order to navigate through the aptly named valley of death (TRLs 4-6) towards commercial realisation it is necessary for new technologies to be de-risked in terms of component durability and reliability. Due to a lack of deployment experience a conservative design approach is often adopted utilising existing offshore certification guidance. Developers must therefore balance the competing requirements of designing economically viable and yet robust devices. Reliability assessment (including physical component testing and statistical analysis) enables device developers to determine component suitability and reliability in a cost-effective way prior to full-scale prototype deployment

Flowbec

publication-status: UnpublishedThis document provides an overview of the resources available for the description of the natural environment at the Wave Hub site, and surrounding region. It aims to provide the reader with an understanding of the mechanisms that have led to the collection of the data resources, and details on how to access them. Detailed information for key research areas is then presented. The document does not aim to provide results of the data collection and analysis, and the reader is referred to the data sources reviewed.NERC FLOWBE

Numerical model validation for mooring systems: Method and application for wave energy converters

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 Vol. 75 (2015), DOI: 10.1016/j.renene.2014.10.063The design of wave energy mooring systems is challenging: overdesign incurs a significant cost penalty, underdesign may lead to a premature failure and incorrect design could reduce the power production. Consequently, compliant mooring systems are being developed for wave energy applications. This paper presents tank test results for a scale model of the buoy and mooring used at the South West Mooring Test Facility (SWMTF), an offshore facility developed to conduct long-term sea trials for wave energy device moorings. A compliant three leg catenary mooring system using Nylon ropes in the water column is investigated. Preliminary static, quasi-static, decay, regular and irregular wave tests were conducted on the 1:5 scale model, using the Ifremer basin in Brest. A corresponding numerical model was developed with a time-domain mooring modelling tool, inputting hydrodynamic data from a radiation/diffraction potential modelling program. After the calibration of several hydrodynamic parameters, the numerical model demonstrated good agreement with the experiment. However, numerical results show large differences with the field test results, mainly because of unknowns in the anchor position. The methods and procedures presented will allow the effective validation of numerical models to enable the development of appropriate mooring systems in wave energy applications.MERiFICMARINETPRIMaR

Mooring line fatigue damage evaluation for floating marine energy converters: Field measurements and prediction

publication-status: Publishedtypes: ArticleThe vision of large-scale commercial arrays of floating marine energy converters (MECs) necessitates the robust, yet cost-effective engineering of devices. Given the continuous environmental loading, fatigue has been iden- tified as one of the key engineering challenges. In particular the mooring sys- tem which warrants the station-keeping of such devices is subject to highly cyclic, non-linear load conditions, mainly induced by the incident waves. To ensure the integrity of the mooring system the lifecycle fatigue spec- trum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly as- sessed through numerical modelling of representative load cases. However, for new applications such as floating marine energy converters numerical models are often scantily validated. This paper describes an approach where load measurements from large- scale field trials at the South West Mooring Testing Facility (SWMTF) are used to calculate and predict the fatigue damage. The described procedure employs a Rainflow cycle analysis in conjunction with the Palmgren-Miner rule to estimate the accumulated damage for the deployment periods and individual sea states. This approach allows an accurate fatigue assessment and prediction of mooring lines at a design stage, where field trial load measurements and wave climate information of potential installation sites are available. The mooring design can thus be optimised regarding its fatigue life and costly safety factors can be reduced. The proposed method also assists in monitoring and assessing the fatigue life during deployment periods

OC6 Phase II: Integration and verification of a new soil–structure interaction model for offshore wind design

This paper provides a summary of the work done within the OC6 Phase II project, which was focused on the implementation and verification of an advanced soil–structure interaction model for offshore wind system design and analysis. The soil–structure interaction model comes from the REDWIN project and uses an elastoplastic, macroelement model with kinematic hardening, which captures the stiffness and damping characteristics of offshore wind foundations more accurately than more traditional and simplified soil–structure interaction modeling approaches. Participants in the OC6 project integrated this macroelement capability to coupled aero-hydro-servo-elastic offshore wind turbine modeling tools and verified the implementation by comparing simulation results across the modeling tools for an example monopile design. The simulation results were also compared to more traditional soil–structure interaction modeling approaches like apparent fixity, coupled springs, and distributed springs models. The macroelement approach resulted in smaller overall loading in the system due to both shifts in the system frequencies and increased energy dissipation. No validation work was performed, but the macroelement approach has shown increased accuracy within the REDWIN project, resulting in decreased uncertainty in the design. For the monopile design investigated here, that implies a less conservative and thus more cost-effective offshore wind design.US Department of Energy Office of Energy Efficiency and Renewable Energy Wind Energy Technologies Office, Grant/Award Number: DE-AC36-08GO2830