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A hybrid, porous breakwater—Oscillating Water Column (OWC) Wave Energy Converter (WEC) system is put forward and its hydrodynamic performance is investigated using the fully nonlinear, open-source computational fluid dynamics (CFD) model, OpenFOAM. The permeable structure is positioned at the weather side of the OWC device and adjoined to its front wall. A numerical modelling approach is employed in which the interstices within the porous structure are explicitly defined. This permits the flow field development within the porous structure and at the OWC front wall to be observed. The WEC device is defined as a land-fixed, semi-submerged OWC chamber. A range of regular incident waves are generated at the inlet within the numerical tank. The OWC efficiency and the forces on the structure are examined. Results are compared for the simulation cases in which the porous component is present or absent in front of the OWC chamber. It is found that the incorporation of the porous component has minimal effect on the hydrodynamic efficiency of the OWC, reducing the efficiency by less than 5%. Nevertheless, the forces on the front wall of the OWC can be reduced by up to 20% at the higher wave steepness investigated, through inclusion of the porous structure at the OWC front wall. These findings have considerable implications for the design of hybrid OWC—breakwater systems, most importantly in terms of enhancing the durability and survivability of OWC WECs without significant loss of operational efficiency.

The cost associated to mooring systems of floating offshore wind (FOW) turbines can be reduced by introducing innovative components that lower the design load requirements. An ideal FOW mooring system must possess sufficient stiffness to maintain station-keeping whilst providing the necessary compliance to dampen peak loads. The Intelligent Mooring System (IMS) is a component that provides this combination of desirable stiffness characteristics; it demonstrates a nonlinear stiffness response that not only reduces mooring loads but also loads transferred to other components and subsystems of the turbine. This results in providing an opportunity to reduce the load capacity and associated cost of the mooring system. The key innovation, performance and reliability aspects of the IMS have been physically proven in previous experimental work. This paper presents the results of a numerical modelling study to develop the IMS to Technology Readiness Level (TRL) 6, that is, a demonstration in relevant environment. The IMS is integrated into the mooring system of the IEA 15-MW FOW reference turbine in OrcaFlex using a stiffness profile based on physical lab testing. Coupled hydro-aerodynamic simulations are used to highlight the load reduction potential offered by various configurations of the IMS in different water depths. The results of this study confirm that the load reduction potential of the IMS is larger in shallow water at rated wind speeds relative to deep water sites or extreme weather conditions. The IMS demonstrates a maximum reduction of up to 20%, indicating the possibility of lowering the design requirements for other mooring components leading to cost saving benefits. The paper will be of interest to practitioners and researchers tasked with the design, analysis and installation of mooring systems for floating wind.

In this paper, the hydrodynamic performance of the comb-type breakwater-wave energy converter (CTB-WEC) system with a flange was investigated. Based on the linear potential flow theory, a semi-analytical model for wave interaction with the CTB-WEC system equipped with the flange was developed using matching eigenfunction method. In particular, Chebyshev polynomial was adopted to handle the singularity of velocity at the flange edge. Successful validation of the semi-analytical model was achieved by theoretical examination and comparing with the experimental data. The influence of wave resonance behavior in the confined water region (surrounded by caissons and the flange) was emphasized. It was found that: 1) the wave resonance behavior in the confined water region is modified due to the presence of the flange; 2) the hydrodynamic efficiency and wave attenuation performance of the CTB-WEC system is improved by properly configuring the flange; 3) the presence of piston and sloshing mode wave resonance in the gap between the WEC device and the flange led to the increment of hydrodynamic efficiency.

Though tidal stream energy is a source of electricity with low environmental impact, the manufacture and operation of tidal energy devices has inbuilt environmental impact and cost. Maintenance costs can be significant and reliability challenges have limited the growth of the sector to date. This study considers two designs of tidal stream energy device (seabed-fixed and floating horizontal axis turbines) and estimates the environmental impact of maintenance in the context of sector reliability to date. Lifetime environmental impact of fixed and floating devices were quantified using Life Cycle Assessment methods, based on reliability data for 58 deployments. Maintenance has a significant impact on the total environmental impact of tidal stream energy devices, contributing between 4% and 25% of total embodied emissions. Floating devices offer maintenance advantages over seabed-fixed devices due to easier access and reduced downtime, but these devices are more likely to experience minor failures and curtailments. The results show that standard maintenance contributes more to the overall environmental impact than the mitigation of failure or curtailment, due to frequency and the need for replacement parts. Type and manufacture of parts were found to be a major contributor to environmental impact, making up 97% of the impact of standard maintenance for floating devices and 91% for seabed-fixed devices. Reducing the frequency of part replacement was found to be the best route to reducing the environmental impact of maintenance.

Floating offshore wind Tension-Leg-Platform (TLP) concepts benefit from a reduced hull tonnage and limit response in vertical modes of motion through the use of pre-tensioned mooring lines to resist aero-hydrodynamic forces. However, this design choice requires an accurate prediction of extreme loadings in the mooring cables such as those generated from slack-line events. Literature shows that slacking events are better predicted when including higher-order hydrodynamic loading as these can generate high-frequency resonant responses in TLPs referred to as springing and ringing for second-order and third-order sum-frequency responses respectively. Therefore, understanding the amplitudes and phases of the various harmonics of the forces on TLP platform’s hulls can be an important aspect of their design. This paper compares two common engineering methods for computing second-order loads, the strip theory and second-order potential flow approaches to be applied for a TLP type Floating Offshore Wind Turbine (FOWT). Whilst the strip theory approach is computationally efficient for second-order force calculation on cylinders, its accuracy decreases with high radius to wavelength ratio. On the other hand, commercial BEM solvers compute second-order forces with a much higher degree of accuracy but they suffer from high computational time. Therefore, a third method is proposed in this paper, aiming at combining better accuracy than with strip theory while keeping computational time low, by using semi-analytical equations in the form proposed by Huang & Eatock Taylor [1]. The harmonics of the force signal generated by unidirectional monochromatic waves on a truncated vertical cylinder in finite depth are compared with BEM and strip theory models to understand the difference of force amplitudes and phases between the various models and their exact domain of application. The comparison is then extended by applying the same method on the transition piece of an academic TLP while modelling the remainder of the structure with strip theory. The harmonics of motions and line tensions are then compared for the three models with particular attention given to springing response. The results suggest the viability of applying semi-analytical potential flow solution as an alternative to the traditional strip theory methods for the estimation of springing response in TLP FOWTs in early design optimisation loops. However, the paper highlights the issues of their use on complex structures and proposes further work to determine their full potential.

Introduction of innovative mooring components can reduce the risk and cost associated to mooring systems of floating offshore wind turbines. The Intelligent Mooring System (IMS) is an active, hydraulic, nonlinear mooring component developed by Intelligent Moorings Limited that provides functionality akin to a shock absorber. It offers a combination of desirable stiffness characteristics for floating offshore wind application; an initial compliant response that reduces loads on the structure and a stiffer nonlinear response for larger loads to reduce platform motion and ensure effective station keeping. A salient feature of the IMS is that through variation of the internal pressure, the stiffness of the system can be adjusted in accordance with the prevailing environmental conditions. This paper presents the results of the physical testing of a double braided IMS at the Dynamic Marine Component test facility and compares the stiffness and strength characteristics to a single braid sleeve. The comparative analysis shows that the stiffness profiles of the double braid for the various configurations are consistent with the single braid design. Importantly, the use of a double braid results in a 50% increase of the tensile strength of the IMS. The investigation presented in this paper will aid in the design of a robust IMS for field testing prior to commercial applications in floating wind installations.

This work presents the use of a porous-media approach for computational fluid dynamics (CFD) modelling of wave interaction with thin perforated sheets and cylinders. The perforated structures are not resolved explicitly but represented by a volumetric porous zone where a volume-averaged pressure gradient in the form of a drag term is applied to the Navier–Stokes momentum equation. The horizontal force on the structures and the free-surface elevation at wave gauges around the cylinder model have been analysed for a range of porosities and regular wave conditions. The CFD results are verified against results from a linear potential-flow model and validated against experimental results. The applied pressure gradient formulation produces good agreement for all porosity values, wave frequencies and wave steepnesses investigated. It is demonstrated that an isotropic macroscopic porosity representation used for large volumetric granular material can also be used for thin perforated structures. This approach offers greater flexibility in the range of wave conditions that can be modelled compared to approaches based on linear potential-flow theory and requires a smaller computational effort compared to CFD approaches which resolve the flow through the openings. The approach can therefore be an efficient alternative for engineering problems where large-scale effects such as global forces and the overall flow-behaviour are of the main interest.

A breakwater-WEC system combining heaving body Wave Energy Converters (WEC) and Comb-Type Breakwater (CTB) was investigated. The traditional CTB consists of a distributed array of separated bottom-mounted caissons and wave chambers are located between two caissons. Heaving bodies provide the power take off (PTO) principles that are arranged at the wave chamber of the CTB. The interaction of the CTB and WEC was investigated based on the linear potential flow theory. An analytical model has been developed to examine the hydrodynamic performance of CTB-WEC system. The analytical model is validated with results from an experimental study. Results show that an increase in conversion efficiency is observed when the device is located in the aft end of the wave chamber. A high efficiency (i.e., 77.4%) and qualified wave attenuation performance of the integrated system are achieved for the proposed CTB-WEC system. The wave resonance along the incident wave direction in the wave chamber is beneficial for wave energy capturing. Furthermore, it was found that the critical value kc corresponds to the wave resonance, perpendicular to the incident wave direction, out of the wave chamber. The property of efficiency mitigation at regions of k > kc should be avoided while designing such a system.

Experiments have been undertaken in a wave basin with the hinged attenuator-type wave energy converter M4 in a 6-float configuration with two hinges for power take off (PTO), moored to a single-point buoy. This follows previous experiments with inelastic cables giving very high snap loads. The PTO was disengaged for these tests. The aims are i) to use the elastic mooring cable to reduce the snap loads of the M4 system, with two elastic stiffnesses tested, and ii) to study the system’s dynamic response with a basic mooring configuration under intermediate to large waves. Mooring loads at the fairlead and the bed, and relative pitch angle between floats were measured. Numerical simulations using two different models were carried out, and the results were compared against the experiment results. For very large waves, the floats showed occasional deck submergence (dunking) limiting relative angular motion as wave height increases, in effect providing a passive end stop. The largest peak relative angle was just less than 40o. The extreme snap loads were up to 1/6th of those with inelastic cables. Spectral analyses of relative pitch angle and mooring force were made and shown to be quite different and complex.Keywords: Wave energy conversion; articulated multi-float system; elastic moorings; extreme waves; snap loads.

Marine renewable energy (MRE) is still an emerging technology. As such, there is still a lack of mature standards and guidance for the development and testing of these devices. The sector covers a wide range of disciplines, so there is a need for more comprehensive guidance to cover these. This paper builds on a study undertaken in the MaRINET2 project to summarise recommendations and guidance for testing MRE devices and components, by reviewing the recently published guidance. Perceived gaps in the guidance are then discussed, expanding on the previous study. Results from an industry survey are also used to help quantify and validate these gaps. The main themes identified can be summarised as: the development progression from concept to commercialisation, including more complex environmental conditions in testing, accurately modelling and quantifying the power generated, including grid integration, plus modelling and testing of novel moorings and foundation solutions. A pathway to a standardised approach to MRE testing is presented, building on recommendations learnt from the MaRINET2 round-robin testing, showing how these recommendations are being incorporated into the guidance and ultimately feeding into the development of international standards for the marine renewable energy sector. This research was funded by European Union’s Horizon 2020 research and innovation programme under grant agreement number 731084, project MaRINET2 (Marine Renewable Infras tructure Network for Enhancing Technologies 2).

Floating offshore wind is rapidly gaining traction in deep water locations. As with all new technologies, to gain the confidence of developers and investors, the technical and economic feasibility of this technology must be proven and robust cost estimates are necessary. In this paper, the authors present a methodology to calculate the capital and operational indicators of a floating wind farm over its project lifetime. A set of computational models is used to reduce the uncertainties in the estimation of the technical and economical parameters. In particular, the effect of using detailed operation and maintenance models and strategies allows a better estimation of operational cost. The paper highlights the requirements and specific adjustments considered for floating offshore wind technology. The methodology is demonstrated for two case studies inspired by real floating wind installations in the United Kingdom, namely the Hywind and Kincardine projects. The related input data, gathered from publicly available sources, constitute a reference database for future studies in the floating offshore wind sector. Results are presented for the two case studies. These show that availability and energy production are in line with typical values for offshore wind projects, and highlight the substantial contribution of operational expenses to the cost of energy. Results are also compared against previous estimations for floating offshore wind projects, showing satisfactory agreement for the overall project costs but an underestimation of operation and maintenance costs in previous studies. This highlights the importance of using detailed operation and maintenance models to adequately capture operational expenses. &nbsp

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.

Purpose Additive manufacturing or “3D printing” is a rapidly expanding sector and is moving from a prototyping service to a manufacturing service in its own right. With a significant increase in sales, fused deposition modelling (FDM) printers are now the most prevalent 3D printer on the market. The increase in commercial manufacturing necessitates an improved understanding of how to optimise the FDM printing process for various product mechanical properties. This paper aims to identify optimum print parameters for the FDM process to achieve maximum tensile strength through a review of recent studies in this field. Design/methodology/approach The effect of the governing printing parameters on the tensile strength of printed samples will be considered, including material selection, print orientation, raster angle, air gap and layer height. Findings The key findings include material recommendations, such as the use of emerging print materials like polyether-ether-ketone (PEEK), to produce samples with tensile strength over 200 per cent that of conventional materials such as acrylonitrile butadiene styrene (ABS). Amongst other parameters, it is shown that printing in the “upright” orientation should be avoided (samples can be up to 50 per cent weaker in this orientation) and air gap and raster width should be concurrently optimised to ensure good “inter-raster” bonding. The optimal choice of raster angle depends on print material; in ABS for example, selecting a 0° raster angle over a 90° angle can increase tensile strength by up to 100 per cent. Originality/value The paper conclusions provide researchers and practitioners with an up-to-date, single point reference, highlighting a series of robust recommendations to optimise the tensile strength of FDM-printed samples. Improving the mechanical performance of FDM-printed samples will support the continued growth of this technology as a viable production technique.

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.

Computational fluid dynamics (CFD) modelling of wave interaction with thin perforated structures is of interest in a range of engineering applications. When large-scale effects such as forces and the overall flow behaviour are of interest, a microstructural resolution of the perforated geometry can be excessive or prohibitive in terms of computational cost. More efficiently, a thin porous structure can be represented by its macro-scale effects by means of a quadratic momentum source or pressure-drop respectively. In the context of regular wave interaction with thin porous structures and within an incompressible, two-phase Navier–Stokes and volume-of-fluid framework (based on interFoam of OpenFOAM®), this work investigates porosity representation as a porous surface with a pressure-jump condition and as volumetric isotropic and anisotropic porous media. Potential differences between these three types of macro-scale porosity implementations are assessed in terms of qualitative flow visualizations, velocity profiles along the water column, the wave elevation near the structures and the horizontal force on the structures. The comparison shows that all three types of implementation are capable of reproducing large-scale effects of the wave-structure interaction and that the differences between all obtained results are relatively small. It was found that the isotropic porous media implementation is numerically the most stable and requires the shortest computation times. The pressure-jump implementation requires the smallest time steps for stability and thus the longest computation times. This is likely due to the spurious local velocities at the air-water interface as a result of the volume-of-fluid interface capturing method combined with interFoam’s segregated pressure-velocity coupling algorithm. This paper provides useful insights and recommendations for effective macro-scale modelling of thin porous structures.

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.