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Future offshore wind technology solutions will be floating to facilitate deep water locations. The EUH2020 funded project FLOTANT (Innovative, low cost, low weight and safe floating wind technology optimized for deep water wind sites) aims to address the arising technical and economic challenges linked to this progress. In particular, innovative solutions in terms of mooring lines, power cable and floating platform, specifically designed for floating offshore wind devices, will be developed and tested, and the benefits provided by these components assessed. In this paper a purpose-built Failure Modes and Effect Analysis (FMEA) technique is presented, and applied to the novel floating offshore wind components. The aim is to determine the technology qualification, identify the key failure modes and assess the criticality of these components and their relative contributions to the reliability, availability and maintainability of the device. This will allow for the identification of suitable mitigation measures in the development lifecycle, as well as an assessment of potential cost savings and impacts of the specific innovations. The methodology takes into account inputs from the components developers and other project partners, as well as information extracted from existing literature and databases. Findings in terms of components innovations, their main criticalities and related mitigation measures, and impacts on preventive and corrective maintenance, will be presented in order to inform current and future developments for floating offshore wind devices.

This paper explores the role of wave spectral characteristics and wave time history on the estimation of extreme mooring loads on floating offshore wind turbines. This research is applied to the DeepCwind semi-submersible platform located at the BiMEP test site in the North of Spain. Extreme sea states are selected using the inverse first-order reliability method (I-FORM). Mooring loads are modelled by quasi-static and dynamic numerical approaches. Different wave time series are generated numerically for each sea state to investigate the variability in predicted peak loads. All cases simulated incorporate the combined effect of wind and waves. Differences of approximately 30% in peak loads are found for the mooring system, reaching 40–79% for the most extreme sea states. Safety factors are proposed to account for sensitivity to wave groupiness in modelling loads under extreme work conditions of the DeepCwind platform (e.g., pitch and velocity control). A comparison between theoretical and real-sea wave spectra is also modelled to investigate possible differences due to the presence of multiple spectral peaks associated with swell and wind seas. In general, results show differences below 12% in the prediction of loads between both assumptions.

Synthetic fibre ropes are in widespread use in maritime applications ranging from lifting to temporary and permanent mooring systems for vessels, fish farm, offshore equipment and platforms. The selection of synthetic ropes over conventional steel components is motivated by several key advantages including selectable axial stiffness, energy absorption and hence load mitigation, fatigue resistance and low unit cost. The long-term use of ropes as safety critical components in potentially high dynamic loading environments necessitates that new designs are verified using stringent qualification procedures. The International Organization for Standardization (ISO) is one certification body that has produced several guidelines for the testing of synthetic ropes encompassing quasi-static and dynamic loading as well as fatigue cycling. The paper presents the results of tension-tension tests carried out to ISO 2307:2010, ISO 18692:2007(E) and ISO/TS 19336:2015(E) on 12-strand rope with embedded electric cable constructions manufactured by Ashimori Industry Co. Ltd from Vectran fibres. The purpose of the tests was to characterise the performance of a novel strand construction (SSR) and compare this to a conventional 12-strand construction. Utilising the Dynamic Marine Component test facility (DMaC) at the University of Exeter several key performance metrics were determined including; elongation, minimum break load (MBL), quasi-static, dynamic stiffness and embedded cable resistance. During the ISO 2307:2010(E) test programme the samples were tested dry and during the ISO 18692:2007(E) and ISO/TS 19336:2015(E) test programmes the samples were fully submerged in tap water after being soaked in water for at least 24 hours. Two methods were used to quantify sample extension: i) an optical tracking system and ii) a potentiometer. Axial compression fatigue and cyclic loading endurance tests were also carried out on Vectran sample. Failure of the Vectran sample or embedded cable did not occur during tests carried out using DMaC. Further tests and sample analysis were also carried out by Ashimori Industry Co. Ltd. Quasi-static bedding-in at 50% MBS and cyclic load endurance test with 6000 cycles between 3.57% MBS and 53.6% MBS was completed. The Effective Working Length (EWL) was 3.821 m before testing and 3.974m after testing. The resistance of the cable increased from 9.6962 Ω to 9.7693Ω during the test and importantly the embedded cable did not fail. Each tensile loading cycle of the rope caused a measurable variation in wire resistance; approximately 0.01Ω. The data obtained during these tests will provide insight into the behaviour of these materials, which will be of use to rope manufacturers and mooring system designers, in addition to offshore equipment and vessel operators.

Synthetic mooring lines are becoming a popular alternative to conventional chain mooring systems. For marine renewable energy devices, they have been considered as an enabling technology for this nascent sector, given their reduced costs and ease of deployment. However, the extreme operating environment has led to an increased interest in the &lsquo, insitu&rsquo, condition monitoring of these mooring lines. This paper considers the use of polymer fibre optic technology and the optical time domain reflectometry (OTDR) technique for the condition monitoring of synthetic mooring lines. To establish the operating envelope of the fibres, Polymethylmethacrylate (PMMA) polymer optical fibres are mechanically tested. Additionally, an OTDR is used to monitor fibres whilst under elongation using a tensile test machine, and the sensitivity of the system in monitoring strain is established. At the lowest strain rate, the average proportional limit and yield points of the fibres are found at 1.16% strain and 5.41% strain, respectively. Fatigue exposure of fibres up to 1.25% strain identifies no measurable effect on fibres&rsquo, proportional limit or yield point. The occurrence of significant creep is identified for fibres strained beyond 1.5%. The OTDR system is able to identify strains at and above 4%. The study identifies important criteria that should be considered in the integration of polymer optical fibres for mooring applications. Limitations are discussed and suggestions for progressing this technology are provided.

The coupled motion characteristics of a tunnel element, which is suspended from a twin-barge and moored to the seabed during the installation process, has been investigated using a 1:50 scaled model. Response characteristics are obtained for multiple regular wave conditions and three different immersion depths. Experimental investigation includes studies to identify system properties of individual arrangements (tunnel, twin-barge) and for the coupled tunnel & twin-barge configuration. Investigation of motion characteristics includes i) experimental studies of the tunnel element from a fixed suspension point and barge, ii) experimental studies with and without a mooring arrangement from the tunnel element to the seabed, iii) experimental study of the fully coupled tunnel & twin-barge configuration, and iv) numerical investigation of the fully coupled tunnel & twin-barge configuration using a commercial fully dynamic mooring simulation software (OrcaFlex™). The experimental investigations were carried out in the State Key Laboratory of Coastal and Offshore Engineering at Dalian University of Technology (DUT), using the ‘6-D Measurement System’ (6D-UMS) to obtain six degree of motions for both the tunnel and twin-barge. For the numerical study hydrodynamic properties were obtained from the diffraction/radiation potential code WAMIT for simplified tunnel and twin-barge elements and used to derive fully coupled motion behavior using the time-domain mooring simulation software OrcaFlex™. The results are presented in order to provide insights into the motion characteristics for the different configurations studied. The main findings indicate that the sway and roll motions for the coupled tunnel & twin-barge configuration decrease with increasing wave incidence angle and immersion depths. The use of additional mooring lines to restrain the tunnel element to the seabed played a further role in reducing the motions of the tunnel element, particularly when subjected to large amplitude and long period waves.

The design of reliable station-keeping systems for permanent floating structures such as offshore renewable energy devices is vital to their lifelong integrity. In highly dynamic and/or deep-water applications, including hydrodynamics and structural dynamics in the mooring analysis is paramount for the accurate prediction of the loading on the lines and hence their dimensioning. This article presents a new workflow based on EDF R&D's open-source, finite-element analysis tool Code_Aster, enabling the dynamic analysis of catenary mooring systems, with application to a floating wind turbine concept. The University of Maine DeepCwind-OC4 basin test campaign is used for validation, showing that Code_Aster can satisfactorily predict the fairlead tensions in both regular and irregular waves. In the latter case, all of the three main spectral components of tension observed in the experiments are found numerically. Also, the dynamic line tension is systematically compared with that provided by the classic quasi-static approach, thereby confirming its limitations. Robust dynamic simulation of catenary moorings is shown to be possible using this generalist finite-element software, provided that the inputs be organised consistently with the physics of offshore hydromechanics.

Marine renewable energy devices require mooring and foundation systems that suitable in terms of device operation and are also robust and cost effective. In the initial stages of mooring and founda...

Offshore renewable energy systems are generally required to operate in exposed offshore locations for long deployment periods at low cost. This requires innovative new mooring system solutions to g...

This paper presents the experimental results for a 1:50 scaled model of an immersed tunnel element suspended from a twin-barge in tank tests. A compliant four leg catenary mooring system using steel link chains in the physical tests is investigated. The corresponding numerical model is developed to simulate the mooring characteristics and the fully coupled dynamic responses of the tunnel-barge system, using the hydrodynamic properties from the diffraction/radiation potential code. To validate the numerical model, preliminary static, quasi static, decaying test and regular wave test were carried out for the comparison. The main findings demonstrate that the mooring tensions of the tunnel-barge system are comparatively lower than that of the tunnel suspended from a fixed platform, particularly when subjected to long period irregular waves. The maximum mooring load of the tunnel element is significantly affected by the natural frequencies of both the tunnel and twin-barge. For the method of the tunnel element immersion presented in this paper, large local peaks in mooring tensions occur at excitation periods of waves close to the tunnel-barge resonance of roll, which should be avoided during design for an appropriate mooring system.

Existing mooring systems for floating offshore wind turbines are largely based on designs from the oil and gas industry. Even though these can ensure the safe station keeping of the floating wind platform, the design of the mooring system is currently largely conservative, leading to additional expense in an industry striving to achieve cost reduction. Recent interest in the usage of mooring materials with non-linear stiffness has shown that they have the potential to reduce peak line loads, ultimately reducing cost. This paper reports on the combined physical testing and numerical modeling of a hydraulic-based mooring component with these characteristics. The results suggest that the inclusion of the component as part of the OC4 semi-submersible platform can reduce the peak line loads by 10%. The paper also discusses a number of challenges associated with modeling and testing dynamic mooring materials.

Floating wind technology is being developed rapidly with the aim of harvesting high-energy wind resources in medium and deep water areas, unreachable using fixed bottom solutions. Given the complexity of these systems, the interactions between the structure and incident hydro-aerodynamic forces need to be well understood. While numerous solutions are being explored, an optimal design is yet to be established within the industry. This study explores the effects of tendon inclination on the dynamic behaviour of a 10MW tension-leg platform (TLP) floating offshore wind turbine (FOWT), and the interaction of different design solutions with higher-order hydrodynamic loading. The model was subject to an extreme sea state in order to capture second and third-order wave effects, and the nonlinear waves were generated via the high-order spectral (HOS) method. The analysis was performed using the hydrodynamic engineering tool CALHYPSO, in-house developed by EDF Lab. Second and third order inertial hydrodynamic loads were included in the time-domain simulations in order to capture low frequency loads and ringing effects respectively. Results show that difference-frequency second order effects have a negligible impact on motions and tendon tensions of the analysed floating wind turbine model, while third order terms can significantly enhance the dynamic response of the system to extreme incident waves. While inclined-leg floater configurations presented improved motion and tendon tension responses under linear loading, the inclusion of quadratic and triple-frequency contributions showed that tendon inclination can in fact increase tension variations in the mooring lines when subject to extreme wave climates. This can lead to slacking in the mooring lines being observed more frequently in inclined-leg configurations. The results therefore suggest that neglecting third order effects, as commonly done in industry, can lead to significant underestimations of motion and tendon tension responses of tension-leg platform wind turbines.

The aim of this paper is to study the mooring tension characteristics on a tidal energy converter (TIC) platform considering i) a horizontal and ii) a vertical tidal turbine. The study examines numerically the feasibility of a catenary mooring line for a modular tidal energy platform. A modular platform is designed and modelled with two floating hulls and anchored by studlink catenary mooring chains on the seabed. Vertical and horizontal axis turbines which have similar Cp are selected and modelled separately. The effect of those turbines on the mooring system are compared and the results informs lifetime of the mooring component for each turbine connection. The hydrodynamic model with no turbine is firstly developed and validated against an experiment with 1:12 scale ratio. The starboard fore mooring line tension, platform surge and pitch displacements are validated against the experiment. The model results show identical signal frequency with slightly different magnitude from the experiment. The mooring tension under vertical and horizontal tidal turbine operations in the particular environment is further examined. The result shows that the mooring line using selected vertical axis turbine experiences higher tension. For platform motions, the horizontal turbine generates slightly larger displacement in surge. However the pitch motion record shows equal displacement under both turbine operations. The selected vertical axis tidal turbine also produces longer lifetime mooring components.

This paper uses a coupled FAST-OrcaFlex model in order to explore the impact of simulation duration on model convergence. The work analyses both operational and extreme cases, assessing the estimated fatigue and extreme loads experienced by a floating offshore wind turbine and its mooring system. Considering an OC4 semi-submersible deployed with the NREL 5 MW turbine, the case study performs a parametric sweep over a range of wind speeds, sea states, and simulation durations. Through this sweep, the paper establishes the impact of the simulation duration for this particular floating offshore wind turbine and characterizes the convergence properties of the loads and excursions as a function of the simulation duration. The results inform the selection of simulation durations to be used in coupled aero-hydro models and optimization frameworks for floating offshore wind applications and can be used to aid the development of guidance and standards for coupled floating offshore wind turbine models.

The mooring system for a floating offshore wind turbine is a critical sub-system that ensures the safe station keeping of the platform and has a key influence on hydrodynamic stability. R&D efforts have increasingly explored the benefits of nonlinear mooring systems for this application, as they have the potential to reduce the peak mooring loads and fatigue cycling, ultimately reducing the system cost. This paper reports on a hydraulic based mooring component that possesses these characteristics, attributable mostly to the non-linear deformation of a flexible bladder. This is not a typical hydraulic component and, as a consequence, modeling its dynamic performance is non-trivial. This paper addresses this by introducing an analogy to numerically model the system, in which the functionality of the mooring component is compared to that of a hydraulic cylinder. The development of a working model in Simscape Fluids is outlined, and is subsequently used to simulate the IMS in a realistic environment. It is found that the numerical model captures a number of the dynamic performance characteristics observed in a previously tested prototype of the IMS.

Mooring systems constitute a fundamental component of all floating systems. For port authorities and device developers, improvements in moorings and foundations allow for savings in capital costs and increased revenues. In this work, a cost effective and secure mooring system for the station keeping of commercial vessels is presented and developed. A modular arrangement constituted by a series of connecting lines and clump weights is proposed, in order to allow for its deployment by small or craft vessels, and reduce the capital, installation and maintenance costs with respect to those of traditional mooring systems. The mooring system is calibrated according to the environmental conditions of Falmouth Harbour, UK. A numerical model of the mooring system is implemented using the offshore dynamics analysis software OrcaFlex. Experimental demonstration is provided through testing of a scaled prototype. Both numerical and experimental investigations aim at a preliminary evaluation of the design proposed. The outcomes indicate the feasibility of the arrangement as a suitable alternative to conventional and more expensive mooring systems. Further areas of improvement are identified and outlined.

Reliability and storm survival of Marine Energy Converters are critical to their commercial development and deployment. The Intelligent Active Mooring System (IAMS) is a novel device intended to minimise extreme and fatigue loading in mooring lines through a non-linear load–extension curve that is variable in operation to adjust to the prevailing metocean conditions. An analytical model of IAMS, validated by physical model tests at the Dynamic Marine Component test facility at the University of Exeter, is used in a numerical simulation of the performance of IAMS as part of the mooring system of the South West Mooring Test Facility buoy. A 10 m length of IAMS can reduce the rms line tension in normal operating conditions by up to 21% and the peak line tension in storm conditions by up to 21% when compared to braided nylon mooring lines. Peak line tension reductions of over 50% can be achieved if a longer IAMS unit is used. The resulting mooring system can be optimised to give load reductions in a wide range of metocean conditions; while variable pre-tension could be used for tidal range compensation or to ease access for installation and maintenance.

Innovation in materials and test protocols, as well as physical and numerical investigations, is required to address the technical challenges arising due to the novel application of components from conventional industries to the marine renewable energy (MRE) industry. Synthetic fibre ropes, widely used for offshore station-keeping, have potential application in the MRE industry to reduce peak mooring line loads. This paper presents the results of a physical characterisation study of a novel hybrid polyester-polyolefin rope for MRE mooring applications through a round robin testing (RRT) approach at two test facilities. The RRT was performed using standard guidelines for offshore mooring lines and the results are verified through the numerical modelling of the rope tensile behaviour. The physical testing provides quantifiable margins for the strength and stiffness properties of the hybrid rope, increases confidence in the test protocols and assesses facility-specific influences on test outcomes. The results indicate that the adopted guidance is suitable for rope testing in mooring applications and there is good agreement between stiffness characterisation at both facilities. Additionally, the numerical model provides a satisfactory prediction of the rope tensile behaviour and it can be used for further parametric studies.

The mooring system has been recognised as a key area of expense that needs to be addressed to improve the cost competitiveness of floating offshore wind turbines. The devices installed to date have generally adopted designs from the oil and gas industry using heavy mooring materials, providing the required safety margins but with a significant degree of conservatism. Recent interest in the usage of lighter and more compliant mooring materials has shown that they have the potential to reduce peak line loads, which would in-turn reduce costs. However, the lack of operational experience with such materials has limited their adoption in a risk averse industry. This paper reports on the large-scale physical testing of a hydraulic-based mooring component with non-linear stiffness characteristics. The performance of the device is characterised in a laboratory both statically and dynamically, as well as in conditions representative of operating in a sea state using a combined physical and numerical modelling approach. The results show that the dynamic stiffness of the component is a function of load history and hydraulic pre-charge pressure, while the inclusion of the device as part of the OC4 semi-submersible floating wind platform can reduce the peak mooring line loads by up to 9%. Beyond the physical test results, the calculations suggest that the peak load reduction in the modelled scenarios could be as much as 40% if the device can be scaled further. The paper supports the adoption of innovative mooring systems through dedicated component and performance testing.

The mooring system for a floating offshore wind turbine ensures that the platform stays within pre-defined station keeping limits during operation, while it provides sufficient restraining forces in storm events to guarantee survival. This presents a challenge during the design process, since the cost of the mooring system is proportional to the peak loads, i.e. those that occur infrequently in extreme conditions. Mooring designs are governed by extreme and fatigue loads which determine the required Minimum Breaking Load (MBL) of the system. If uncertainties in the environmental loading or hydrodynamic coupled response exist, additional safety factors are required. This paper explores the application of a hydraulic based mooring system that enables a variable, non-linear line stiffness characteristic that cannot be achieved with conventional designs. This non-linear load-response behavior could function like a ‘shock absorber’ in the mooring system, and thus reduce the line tensions, enabling a more efficient mooring system that necessitates a lower MBL and thus lower cost. These claims are evaluated through numerical modelling of the NREL OC3 spar buoy and OC4 semi-submersible offshore wind platforms using the FAST-OrcaFlex interface. The simulations compare the dynamics with and without the inclusion of the hydraulic mooring component. The results suggest that mean mooring line loads can be reduced in the region of 9–17% through a combination of lower static and dynamic loads, while the peak loads observed in extreme conditions were reduced by 17–18%. These load reductions, however, come at the expense of some additional platform motion. The paper also provides an outlook to an upcoming physical test campaign that will aim to better understand the performance and reliability of the mooring component, which will provide the necessary evidence to support these load reduction claims.

This paper explores geometry optimization of an offshore wind turbine’s mooring system considering the minimization of the material cost and the cumulative fatigue damage. A comparison of time domain simulations against frequency domain simulations is made to explore the suitability of these methods to the design process. The efficient design options, the Pareto front, from the frequency domain study are also re-evaluated using time domain simulations and compared against the time domain Pareto front. Both the time and frequency domain results show optimal results utilizing similar design philosophies, however, the frequency domain methods severely under predict the fatigue loads in the mooring system and incorrectly class infeasible solutions as feasible. The frequency domain is therefore not suitable for optimization use without some external means of applying engineering constraints. Furthermore, re-evaluation of the frequency domain solutions provides guidance to the uncertainty and the necessary design fatigue factors required if implementing frequency domain methods in design.

Offshore foundations are typically designed to resist mooring loads coming from a single direction. This paper provides some insights into the geotechnical challenges associated with anchor sharing (i.e. where mooring loads are coming from multiple directions to one anchor) for Offshore Renewable Energy (ORE) applications. Firstly, the multidirectional mooring load conditions on shared anchors are characterised for cases with one, two and three mooring lines connections. Secondly, multi-directional loadings applied on a caisson foundation in a geotechnical centrifuge facility showed large capacity reduction and large displacement of the foundation at large loading angle. Thirdly, multi-directional simple shear tests have showed that volumetric contraction increases as the degree of shear stress reversal increases. Lastly, the results from the soil element tests are brought into discussion with the physical modelling results to identify the foundation’s response to multi-directional loading.

Synthetic 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.

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.

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.

This paper describes the development of a framework using a genetic algorithm in order to aid in the design of a mooring system for offshore renewable energy devices. This framework couples numerical models of the mooring system and structural response to cost models in order for the genetic algorithm to effectively operate considering multiple objectives. The use of this multi-objective optimization approach allows multiple design objectives such as minimum breaking load and the material cost to be minimized simultaneously using an automated mathematical approach. Through the application of this automated approach, a wider set of designs will be considered allowing the system designers to select a design which appropriately balances the trade-off between the competing objectives. In this work, a set of mooring designs that represent efficient solutions for the stipulated constraints are found and presented. The developed framework will be applicable to other offshore technology subsystems allowing multi-objective optimization and reliability to be considered from the design stage in order to improve the design efficiency and aid the industry in using more systematic design approaches.

Synthetic fibre ropes are in widespread use in maritime applications ranging from lifting to temporary and permanent mooring systems for vessels, offshore equipment and platforms. The selection of synthetic ropes over conventional steel components is motivated by several key advantages including selectable axial stiffness, energy absorption (and hence load mitigation), fatigue resistance and low unit cost. The long-term use of ropes as safety critical components in potentially high dynamic loading environments necessitates that new designs are verified using stringent qualification procedures. The International Organization for Standardization (ISO) is one certification body that has produced several guidelines for the testing of synthetic ropes encompassing quasi-static and dynamic loading as well as fatigue cycling. This paper presents the results of tension-tension tests carried out to ISO 2307:2010, ISO 18692:2007(E) and ISO/TS 19336:2015(E) on three different 12-strand rope constructions manufactured by Ashimori Industry Co. Ltd from polyester and Vectran® fibres. The purpose of the tests was to characterise the performance of a novel 12-strand construction and compare this to a conventional 12-strand construction. Utilising the Dynamic Marine Component test facility (DMaC) at the University of Exeter several key performance metrics were determined including; elongation, minimum break load (MBL) and quasi-static and dynamic stiffness. During the ISO 2307:2010(E) test programme the samples were tested dry and during the ISO 18692:2007(E) and ISO/TS 19336:2015(E) test programmes the samples were fully submerged in tap water after being soaked for at least 24 hours. Two methods were used to quantify sample extension: i) an optical tracking system and ii) a draw-wire potentiometer. Axial compression fatigue and cyclic loading endurance tests were also carried out on two Vectran® samples. Further load-to-failure tests and sample analysis were also carried out by Ashimori Industry Co. Ltd. It was found that the MBL of the samples exceeded the values specified by the manufacturer (by 7.7–29.5% for the polyester samples) with failure occurring at the splices in all cases and minor abrasion noted in several locations. The measured MBL of the novel polyester Straight Strand Rope (SSR) construction was up to 16% higher than the conventional construction with increases of quasi-static and dynamic stiffness of up to 6.8%. Differences between the viscoelastic and viscoplastic behaviour of the samples were also noted. The data obtained during these tests will provide insight into the behaviour of these materials and different rope constructions which will be of use to rope manufacturers, mooring system designers in addition to offshore equipment and vessel operators.

One 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 induced 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 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 identified as one of the key engineering challenges. In particular the mooring system 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 spectrum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly assessed 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.

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.

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 dynamic response of the mooring line will be a dominant factor to consider in their use for the station keeping of a wave energy converter (WEC). Due to the relatively small size of WECs and their being moored in relatively shallow waters the effect of waves, tide and current can be of greater significance than for other floating offshore systems. Axial line stretching and high-frequency ‘top-end’ dynamics can importantly modify damping and top-end loading. If a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere and this will have great significance for the loading experienced by the line. One must also consider how the mooring system might change the response of the WEC and so alter its ability to extract power from the waves. Unlike a typical offshore system, the design of moorings for a WEC device must consider reliability and survivability, and the need to ensure efficient energy conversion. The design and operation of a chain mooring for a WEC is considered here. Generic experimental measurements of mooring line damping were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The measurements were conducted on a single mooring line for surge motions and include the study of axial stretching and high top-end dynamics. The laboratory procedures were designed to resemble tests undertaken earlier at ‘full’ scale in 24 m water depth. The measurements were also compared with numerical studies. The experimental findings for WEC devices, supports the conclusion that dynamic mooring line motion will be an important variable, needing to be considered carefully within the design.

In order to design marine renewable energy mooring systems which are both economical and durable it is necessary to establish the lifecycle performance of individual components. In parallel with numerical tool development, physical component testing utilising realistic load cases is pivotal in achieving a greater understanding of performance variations including the contribution of degradation mechanisms. Building upon previous experimental tests conducted by the authors, tension–tension tests were conducted on a sample used in first part of the study and samples extracted from a mooring line which was deployed for 18 months with the South West Mooring Test Facility. In agreement with the first part of the study it was found that sample axial stiffness and damping are influenced by load history and instantaneous strain. The increased compliance, lower load bearing capacity and reduced tension–tension fatigue performance of aged specimens are symptomatic of fibre-on-fibre abrasion damage sustained in service. Visual inspections of the rope and yarns including scanning electron microscope analysis of fibres revealed that abrasion wear was accelerated by debris found within the rope structure, highlighting the importance of preventing particle ingress. Datasets are provided to facilitate the development of rope and mooring system simulation tools.

The 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.

To be effective, wave energy converter devices (WECs) must be installed in unsheltered locations with high wave energy density. Such regimes are more likely to be at offshore locations and much current interest is focused on offshore floating devices. Since the wave energy industry has little practical long-term experience with deployed WEC systems there is no historical experience upon which to base the formulation of appropriate station-keeping arrangements. The development of generic station-keeping arrangements that will allow reliable, yet cost-effective, mooring systems presents a challenging task. This paper raises issues pertinent to the station-keeping of floating WECs and discusses a preliminary design procedure to identify suitable mooring arrangements.

The utilisation of synthetic mooring ropes for marine renewable energy (MRE) devices is a recent occurrence. Despite current use in the offshore industry, MRE mooring components are typically subjected to highly dynamic loads, necessitating the detailed characterisation of operational and long-term component performance for lifecycle analysis and operations management. To address the uncertainties associated with synthetic mooring components in this application, tension experiments have been conducted on nylon 6 parallel-stranded rope samples at IFREMER, France and the University of Exeter, UK under the consortium MERiFIC (Marine Energy in Far Peripheral and Island Communities). Measurements are reported from harmonic loading tests with different initial bedding-in levels used to investigate the influence of load history on the immediate dynamic properties of the rope. Two irregular load regimes were also applied based on mooring tensions recorded by the South West Mooring Test Facility (SWMTF). Datasets are provided to facilitate the development of rope modelling tools. For the load regimes studied it was found that the operational performance of the rope is strongly influenced by the instantaneous load-strain characteristic. This study provides unique insight into the stiffness and damping properties of synthetic rope in the context of loading regimes relevant to MRE devices.

The reliability and integrity of components used in the marine offshore environment is paramount for the safety and viability of offshore installations. The engineering challenge is to design components that are robust enough to meet reliability targets whilst lean enough to minimise cost. This is particularly the case for offshore marine renewable installations which operate in the same, possibly harsher, environment as offshore oil and gas installations, and are subjected to highly cyclic and dynamic wave, wind and operational load conditions. The cost of electricity produced has to compete with other means of electricity generation and does thus not offer the same profit margins available as oil and gas commodities. As a result, components for marine renewable installations have to meet the target reliability, without the application of costly safety factors to account for load and environmental uncertainties. Industries with similar design tasks such as the aviation or automotive industry have successfully used a service simulation test approach to develop robust yet lean designs. This paper builds on an approach to establish and validate the reliability of floating renewable energy devices in which dedicated component testing using the purpose built Dynamic Marine Component test rig (DMaC) plays a pivotal role to assess, validate and predict the reliability of components in the marine environment. This paper presents a test rig for both static and fatigue tests of marine components such as mooring lines and mooring shackles under simulated or measured load conditions and provides two case studies from recently conducted mooring component tests. This includes an investigation into the load behaviour of synthetic mooring ropes and the ageing of mooring shackles.

One of the key engineering challenges for the installation of floating marine energy converters is the fatigue of the load-bearing components. In particular the moorings which warrant the station-keeping of such devices are 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 spectrum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly assessed 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 experimental approach, where load measurements from tank tests are used to estimate the lifecycle fatigue load spectrum for a potential deployment site. The described procedure employs the commonly used Rainflow cycle analysis in conjunction with the Palmgren-Miner rule to estimate the accumulated damage for individual sea states, typical operational years and different design lives. This allows the fatigue assessment of mooring lines at a relatively early design stage, where both information from initial tank tests and the wave climate of potential field sites are available and can be used to optimise the mooring design regarding its lifecycle fatigue conditions.

This paper describes the test facilities developed within the Peninsular Research institution for Marine Renewable Energy (PRIMaRE) group and discusses the approach of the group to mitigate risk for marine renewable energy installations. The main consideration is given to the reliability assessment of components within mooring configurations and towards power umbilical for typical renewable energy sites. Load and response data from sea trial will be used to highlight the importance of these research activities, and a Dynamic Marine Component Test rig (DMaC) is introduced that allows four degree of freedom fatigue or destructive tests. Furthermore it is discussed how this facilities could also aid in the reliability assessment of wider offshore applications.Copyright © 2011 by ASME

This paper presents results from the third phase of large scale mooring investigation at the Scapa Flow in Orkney, Scotland, for wave energy applications. It considers the suitability of three slack mooring line configurations, namely, i) compliant chain, ii) complaint hybrid and iii) taut fibre rope arrangement. This investigation is extended with a numerical study analysing the station keeping criteria for the three slack line configurations and that of a taut system for wave energy converter installations. The results are used to discuss the suitability of the individual mooring line configurations and that of the taut arrangement. This paper is partially to provoke a discussion on the advantages of a taut mooring system and raise the issues that need to be solved if they are to be practicable.Copyright © 2008 by ASME

The design and operation of a chain mooring for a wave energy converter (WEC) is considered. Experimental measurements of a mooring line were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The laboratory procedures were designed to resemble tests undertaken earlier in the year at ‘full’ scale in 24 m water depth. This paper describes and compares these measurements and relates the results to earlier work on mooring lines by Webster [1]. Measurements of both the damping and response frequencies of the mooring are described. Although the present results support partly the conclusions of the earlier work, care must be taken in how these are applied when one is considering mooring line design for WECs. It is concluded that there are significant differences for a WEC for both operational and limit state design in comparison with a more conventional offshore system such as an FPSO or CALM. Although the primary requirement is still one of station-keeping two further considerations may be of great importance. Firstly if a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere with one another and this will have great significance for the loading experienced by the line. This can be exacerbated by variations in tidal range which will have a larger effect in comparison with a conventional deepwater mooring. A second factor may arise if the mooring system response is critical to the WEC energy extraction process. If the mooring becomes part of the ‘tuned’ system then changes in the mooring properties of damping and natural frequency could seriously affect energy conversion efficiencies.Copyright © 2006 by ASME

The research leading to these results has received funding from the European Community's Seventh Framework Programme for DTOcean Project.