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Renewable energy allows electricity generation with lower environmental and resource impact than generation from fossil fuels. However, the manufacture, use and ultimate disposal of the equipment used to capture this energy has an environmental impact, which should be minimised. Tidal turbine blades are currently primarily manufactured from glass-fibre reinforced polymers. Such blades cannot be recycled at the end of their life, and are disposed of in landfill or by incineration. As the tidal energy industry grows, the volume of non-recyclable waste is a potential problem. Here we consider the environmental impact of ten combinations of material and disposal method for tidal stream turbine blades, including recyclable options. Our findings suggest that glass fibre blades have greenhouse gas emissions of around 15,500 kgCO2e for the scope considered, and a significant environmental impact in all impact categories, which would be increased by changing to carbon fibre (99% mean increase from glass fibre across impact categories) or steel (134% mean increase from glass fibre across impact categories) blades, but that composite materials using flax fibre and recyclable resin may have lower impact (26% mean decrease from glass fibre across impact categories), provided they are treated correctly after use. These materials may also offer the potential for lower cost blades in future.

The offshore installation, logistics, and commissioning activities are currently estimated to make up 20% to 30% of the capital expenditures (CAPEX) of offshore wind projects. Technical and geographical factors affect both the CAPEX during construction and the installation schedule, such as a lack of supporting port infrastructure, the availability of specialized vessels, the distance from the wind farm to shore, accessibility, water depths, and seabed conditions. In addition, there are significant risks during the construction phase, such as uncertain durations due to the sensitivity of marine operations to weather conditions. Identifying supply chain requirements is critical in the early stages of project planning in order to avoid time delays and cost overruns during the transport and installation process. This study explores and analyzes the logistic requirements and installation methods of a floating offshore wind (FOW) technology. Using an advanced forecasting and decision support tool, realistic case scenarios are simulated at a variety of potential sites for FOW deployment across the UK. Technical risks associated with installation strategies are identified and classified. The results provide a comparison of key installation performance indicators of each case scenario (e.g., installation rate per wind turbine, weather downtime). This study is of interest to researchers, offshore wind project developers, service providers, and other key stakeholders seeking to optimize planning and logistics to drive down CAPEX costs, reduce the construction downtime, and minimize risks during marine operations.

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.

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.

Floating wind turbines are becoming an important part of renewable offshore power generation, offering an opportunity to deliver green energy. The floating nature of the substructures permits wind turbine placement in deep water locations, probably out of sight of land. This paper presents the tow out design method requirements for the installation of floating offshore wind turbines. Most existing floating offshore wind turbines substructures are barge, semi submersible, TLP and Spar types and their installation methods have been developed from those used on offshore oil and gas structures. Whilst the turbines are derived from those used on fixed bottom offshore wind turbines. The paper summarises the weather window limitations for the various substructure types and installations phases, including the transportation to and from the offshore site and during the connection of mooring lines and electrical cables. The choice of construction materials i.e. steel or concrete, influence the draft of the substructure at the fit-out quay and hence during the tow offshore. Semi submersible and barge types are of shallow draft and can be fitted out alongside a quay. Spar types typically require deep water for construction in sheltered inshore waters. The Tension Leg Platform (TLP) floating wind turbine has minimum water plane area and hence has low intact stability during ocean tow and thus TLPs may require modified crane vessels for offshore installation. The paper will present recent advances in the tow out requirements of floating offshore wind turbines. Data will be provided on intact stability, damage stability, tow forces and motions during tow from fit out port to the offshore location.

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.

Accurate reliability prediction for tidal turbines is challenging due to scarce reliability data. To achieve commercialization, it is widely acknowledged that reductions in maintenance costs are vital and robust component reliability assessments can help drive this. For established technologies, reliability prediction either involves a statistical assessment of historical failure data, or a physics of failure approach based on dedicated accelerated testing. However, for low/mid Technology Readiness Level tidal developers these common approaches are difficult. Thus, developers require a method of making reliability predictions for components in the absence of tidal turbine specific failure data and physical testing results. This paper presents a failure rate model for a tidal turbine pitch system using empirical Physics of Failure equations, with associated uncertainties. Critical component design parameters are determined and their effects on the failure rate investigated via a sensitivity analysis. The modelled failure rate is then compared with wind turbine failure data from a series of turbines. The tidal turbine failure rate is approximately 50% lower, however high reliability requirements mean this is unlikely to be acceptable. The developed model can assist turbine developers in estimating failure rates and determining reliability critical design parameters for the failure critical pitch system.

This article presents a metocean modelling methodology using a Markov-switching autoregressive model to produce stochastic wind speed and wave height time series, for inclusion in marine risk planning software tools. By generating a large number of stochastic weather series that resemble the variability in key metocean parameters, probabilistic outcomes can be obtained to predict the occurrence of weather windows, delays and subsequent operational durations for specific tasks or offshore construction phases. To cope with the variation in the offshore weather conditions at each project, it is vital that a stochastic weather model is adaptable to seasonal and inter-monthly fluctuations at each site, generating realistic time series to support weather risk assessments. A model selection process is presented for both weather parameters across three locations, and a personnel transfer task is used to contextualise a realistic weather window analysis. Summarising plots demonstrate the validity of the presented methodology and that a small extension improves the adaptability of the approach for sites with strong correlations between wind speed and wave height. It is concluded that the overall methodology can produce suitable wind speed and wave time series for the assessment of marine operations, yet it is recommended that the methodology is applied to other sites and operations, to determine the method’s adaptability to a wide range of offshore locations.

D.C., J.M., P.T. and S.W. acknowledge the financial support of the Tidal Stream Industry Energiser project (TIGER), which is co-financed by the European Regional Development Fund through the Interreg France (Channel) England Programme. A.A. acknowledges the support of NERC through the Industrial Innovation fellowship grant no. NE/R013209/2. D.G., B.S., T.S., D.W., R.W. and B.W. acknowledge EPSRC for supporting part of this work through EP/S000747/1. G.H. and C.S. acknowledge support by the Scottish government-funded Marine Mammal Scientific Support Programme MMSS/002 with additional resources from the Natural Environment Research Council grant nos. NE/R014639/1 and NE/R015007/1. M.L. acknowledges the support of the Smart Efficient Energy Centre (SEEC) at Bangor University, part-funded by the European Regional Development Fund (ERDF), administered by the Welsh Government, and the EPSRC METRIC fellowship grant no. (EP/R034664/1). L.M. acknowledges the financial support of an EPSRC PhD studentship award, grant no. EP/R513052/1(18-22). J.McN. acknowledges the financial support of R.Willden’s EPSRC Advanced Fellowship grant no. EP/R007322/1. S.N. acknowledges the support of SEEC (Smart Efficient Energy Centre) at Bangor University, part-funded by the European Regional Development Fund (ERDF), administered by the Welsh Government. This review provides a critical, multi-faceted assessment of the practical contribution tidal stream energy can make to the UK and British Channel Islands future energy mix. Evidence is presented that broadly supports the latest national-scale practical resource estimate, of 34 TWh/year, equivalent to 11% of the UK’s current annual electricity demand. The size of the practical resource depends in part on the economic competitiveness of projects. In the UK, 124 MW of prospective tidal stream capacity is currently eligible to bid for subsidy support (MeyGen 1C, 80 MW; PTEC, 30 MW; and Morlais, 14 MW). It is estimated that the installation of this 124 MW would serve to drive down the levelized cost of energy (LCoE), through learning, from its current level of around 240 £/MWh to below 150 £/MWh, based on a mid-range technology learning rate of 17%. Doing so would make tidal stream cost competitive with technologies such as combined cycle gas turbines, biomass and anaerobic digestion. Installing this 124 MW by 2031 would put tidal stream on a trajectory to install the estimated 11.5 GW needed to generate 34 TWh/year by 2050. The cyclic, predictable nature of tidal stream power shows potential to provide additional, whole-system cost benefits. These include reductions in balancing expenditure that are not considered in conventional LCoE estimates. The practical resource is also dependent on environmental constraints. To date, no collisions between animals and turbines have been detected, and only small changes in habitat have been measured. The impacts of large arrays on stratification and predator–prey interaction are projected to be an order of magnitude less than those from climate change, highlighting opportunities for risk retirement. Ongoing field measurements will be important as arrays scale up, given the uncertainty in some environmental and ecological impact models. Based on the findings presented in this review, we recommend that an updated national-scale practical resource study is undertaken that implements high-fidelity, site-specific modelling, with improved model validation from the wide range of field measurements that are now available from the major sites. Quantifying the sensitivity of the practical resource to constraints will be important to establish opportunities for constraint retirement. Quantification of whole-system benefits is necessary to fully understand the value of tidal stream in the energy system. Publisher PDF

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.

Nacelle-mounted, forward-facing Light Detection and Ranging (LIDAR) technology is able to provide knowledge of the incoming wind so that wind turbines can prepare in advance, through feedforward control. LIDAR can aid in improving wind turbine performance across the full operating range, assisting with torque control in below rated wind speeds, pitch control in above rated wind speeds and yaw control for correctly aligning the turbine rotor with the incoming wind direction. The motivations are for decreasing structural loads, resulting in reduced maintenance and extended lifetimes of turbines and their components, and increasing power capture, both of which can lead to reductions in the levelised cost of energy. This paper provides a review of control strategies that have been employed for LIDAR-assisted turbine control. This paper reviews the computational and practical studies that have been performed for both bottom-fixed and floating turbines and the journey that the field has undertaken since its conceptualisation. Detail is provided of the key differences between fixed and floating offshore turbine dynamics. The paper concludes with guidance for future work within the field, with a focus on floating turbines, as the extent of the literature is scarce when compared to bottom-fixed. Suggestions are offered for how the future studies can better account for the current and future industry landscape. Opportunities for testing of LIDAR-assisted floating turbine control in the field, its benefits for floating substructure design, and the steps needed to be taken to ensure its increased utilisation on industrial projects are also discussed.

Offshore renewable energy installations are moving into more challenging environments where fixed foundations are not economically viable, forcing the development of floating platforms. Subsea cables are critical for transfer of the power generated back to shore. The electrical capabilities of subsea cables are well understood; however, the structural capabilities are not, subsea power cable failures accounting for a significant proportion of insurance claims. Cables are challenging to repair, with specific vessels and good weather windows required, therefore making operations very costly. A good understanding of the internal structure of a subsea cable, and interaction between the layers, is integral to the development of robust and reliable, high voltage, dynamic, subsea cables. A requirement therefore exists for non-destructive examination (NDE) of live subsea cables to determine locations, and identify the causes, of faults and classify their type. An NDE framework such as this would assist in planning operations and reduce the risk and cost inherent to delivering offshore power. Improved understanding of subsea cable failure modes and mechanisms could also be achieved through us of NDE during onshore, dry, experimental testing. Three currently available NDE methods are considered, developed for use in other disciplines, for the purpose of structural monitoring of subsea power cables during onshore evaluation testing. The NDE methods were: (a) thermography, (b) eddy current testing (ECT), (c) spread spectrum time domain reflectometry (SSTDR). The methods are assessed with regards to the information that could be obtained from both a static and oscillating cable in pilot physical tests. The results of the testing were promising, with cable motions and interlayer movements being detected by all techniques to various degrees.

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

Support structure fatigue is a key component in determining the structural lifetime of an offshore wind turbine (OWT). As the currently installed assets age, turbine operators are exploring options for lifetime extension to potentially increase the long-term financial return. Strain monitoring at critical points on a turbine is commonly performed to improve understanding of structural integrity and ultimately reassess its remaining useful life. Reliable application of the findings of a time-limited structural monitoring programme in predicting structural response over a turbine’s lifetime requires a good understanding of the representativeness of the dataset. Uncertainties arise in fatigue damage estimations due to the stochastic nature of the environmental loading. Statistical treatment of the environmental loads and the corresponding structural response is made by defining measured load cases (MLCs), within which the turbine operational state and associated range of environmental parameters are specified. For OWTs, this leads to a multi-dimensional problem, as both wind and wave parameters need to be accounted for. The complexity of the analysis is thus increased, requiring identification of the critical external and operational parameters that influence overall fatigue. The associated statistical uncertainty can then be estimated by considering repeated measurements throughout the monitoring period. The work presented in this paper investigates the application of statistical resampling techniques in evaluating the uncertainty in total measured fatigue damage experienced by an offshore wind turbine. Direct fatigue computation over the given measured dataset has been contrasted with statistical approaches applying probability distributions of MLCs to give indication of the influence of the key environmental parameters. Strain monitoring data from a 2.3MW OWT was utilised in conjunction with the corresponding operational and environmental measurements. The methods and outcomes of this study can be used to improve the remaining fatigue life prediction of installed turbine foundations, by assessing the representativeness of strain measurements. As structural design uses industry defined safety margins, comparison of design predictions against operational measurement data will allow verifying that these safety margins are not exceeded, within the bounds of the given uncertainties. Finally, an understanding of data uncertainties will allow estimates to be made regarding the reliability of the consequent fatigue lifetime reassessment or of the numerical model validation procedures. Such information is useful to wind turbine operators as it provides the first step towards data-driven lifetime extension and informs on measurement campaign utilisation.

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.

Numerical modeling of currents and waves is used throughout the marine energy industry for resource assessment. This study compared the output of numerical flow simulations run both as a standalone model and as a two-way coupled wave–current simulation. A regional coupled flow-wave model was established covering the English Channel using the Delft D-Flow 2D model coupled with a SWAN spectral wave model. Outputs were analyzed at three tidal energy sites: Alderney Race, Big Roussel (Guernsey), and PTEC (Isle of Wight). The difference in the power in the tidal flow between coupled and standalone model runs was strongly correlated to the relative direction of the waves and currents. The net difference between the coupled and standalone runs was less than 2.5%. However, when wave and current directions were aligned, the mean flow power was increased by up to 7%, whereas, when the directions were opposed, the mean flow power was reduced by as much as 9.6%. The D-Flow Flexible Mesh model incorporates the effects of waves into the flow calculations in three areas: Stokes drift, forcing by radiation stress gradients, and enhancement of the bed shear stress. Each of these mechanisms is discussed. Forcing from radiation stress gradients is shown to be the dominant mechanism affecting the flow conditions at the sites considered, primarily caused by dissipation of wave energy due to white-capping. Wave action is an important consideration at tidal energy sites. Although the net impact on the flow power was found to be small for the present sites, the effect is site specific and may be significant at sites with large wave exposure or strong asymmetry in the flow conditions and should thus be considered for detailed resource and engineering assessments.

Floating offshore wind turbines are an emerging technology with notable prototype and demonstration projects being installed in recent years. Future floating wind farms enable the access to significant new wind resources in expanses of water too deep for conventional, bottom-fixed farms. Spar, barge, TLP and semisubmersible types have been deployed as demonstration units. Pre commercial units have been installed off the coasts of Portugal and the East Coast of Scotland. This paper discusses naval architecture requirements for the temporary conditions of construction, ocean transport and installation. The requirements of Marine Warranty Surveyor guidelines for temporary conditions and the Classification Society requirements during the temporary phase are also taken into account. It is expected that major offshore maintenance will be carried out on floating offshore wind turbines by towing them back to more sheltered port locations. This paper is a literature and guideline review with additional information on intact stability during the tow to the offshore location. Thus, marine operations play a pivotal role throughout all phases of a floating wind farm’s life cycle. In particular, uncertainties associated with offshore installation work can extend construction schedules and increase the capital expenditure required for the project. This paper will be of interest to researchers and practitioners in the floating wind sector, considering innovative installation methods as a means to reduce overall project lifecycle cost.

This paper presents a methodology for the reliability and power flow analysis of islands or off-grid power networks for situations of scarce data and information. It offers a new and pragmatic approach to derive the required power network information, including the power cable parameters and the load at each power node. The paper aims to present the developed methodology, and to illustrate its application, using Ushant Island as a case study. The assessment of the current power network status and the reliability analysis are presented. Grid performance parameters are further compared for conventional diesel generator operation and renewable energy generation scenarios, demonstrating the advantages of replacing the existing diesel units with renewable energy sources in terms of system reliability. The analysis shows that by introducing renewable energy systems to the island’s grid, the reliability of the grid increases by up to 50% and cable capacity usage reduces by up to 30%. Furthermore, this work suggests that it would not be necessary to modify the grid cables when substituting the diesel generator. The paper will be of interest to network planners, community stakeholders, project developers and decision-makers concerned with renewable energy investment on islands and in remote rural areas.

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.

This paper details a hydrodynamic model based on Blade Element Momentum Theory (BEMT) developed to assess ’conventional’ 3-bladed tidal stream turbines (TSTs), adapted here to analyse an ’unconventional’ case of a ducted and open centre device. Validations against a more detailed coupled Reynolds averaged computational fluid dynamics (RANS-BEM) model shows excellent agreement, of within 2% up to the peak power condition, with associated computational times in the order of a few minutes on a single core. The paper demonstrates the application of hydrodynamic forces into a structural analysis tool, in order to assess blade stress distributions of a generic hubless turbine. Incorporation of parameters such as non-uniform inflows and blade weight forces are investigated, with their effects on stress profiles presented. Key findings include: i) the adapted BEMT model replicates the majority of turbine performance characteristics estimated through previous CFD assessments; ii) the proposed model reduces the computational effort by several orders of magnitude compared to the reference coupled CFD, making it suitable for engineering assessments iii) blade stress distribution profiles are quantified, detailing concentration zones and cyclic values for use in fatigue analyses. This work forms part of a greater project aimed to develop a suite of analytical tools to perform engineering assessments of bi-directional ducted TSTs.

Quantitative reliability, availability, and maintainability (RAM) assessments are of fundamental importance at the early design stages, as well as planning and operation of marine renewable energy systems. This paper presents an RAM framework adaptable to different offshore renewable technologies, conceived to provide support in the choice of the device components and subsequent planning of the O&M strategies. A case study, characterizing a pilot farm of oscillating water column (OWC) wave energy converters (WECs), is illustrated together with the method used to obtain reliable estimate of its key performance indicators (KPIs). Based on a fixed feed-in-tariff for the project, economic figures are estimated, showing a direct relationship with the availability of the farm and the cost of maintenance interventions. Consequently, the probability distributions of the most relevant output variables are presented, and the mutual correlations between them investigated using principal components analysis (PCA) with the aim of discovering the relationships influencing the performance of the offshore farm. In this way, the contributions of the individual factors on the profitability of the project are quantified, and generic guidelines to support the decision-making process are derived. It is shown how this type of analysis provides important insights not only to ocean energy farm operators after the deployment of the devices, but also to device developers at the early design stage of wave energy concepts.

One of the key objectives for renewable energy technologies is to reduce the environmental impact of energy generation. Floating offshore wind technologies have been developed in recent years to exploit the wind energy resource available at deep waters where bottom-fixed technologies are not economical. However, few studies exist that analyse the environmental impact of such technologies. Particularly, offshore activities such as those required for Operation and Maintenance (O&M) are not represented in detail in previous studies. The present study addresses these gaps by performing a Life Cycle Assessment using an advanced O&M model to quantify the environmental impact of a floating offshore wind farm. Different O&M philosophies – assuming towing to shore for major operations vs. performing all operations on site – and their impact are evaluated and discussed for two case studies inspired by real pilot park deployments. The results show mean Global Warming Potential (GWP) values between 25.3 and 45.2 gCO2 eq/kWh depending on the assumed O&M strategy and vessels, with the contribution of O&M to GWP ranging from 26 to 49%. Assuming O&M strategies to be the same for fixed and floating offshore wind could result in a 20.0% underestimate of GWP, whereas the vessel choice resulted in up to 29.6% variation in the estimated GWP. An environmental impact perspective provides key insights on the choice of different designs, operation strategies and asset management, and thus should be used in the decision-making process.

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...

Tidal stream energy has the potential to contribute to global renewable energy generation, but this remains largely untapped. Technical developments have helped create a nascent industry with several pre-commercial installations. However, planned projects experience a cautious investment climate due to perceived risk of failure, and maintenance and repair cost. This paper reviews 58 tidal stream energy deployments between 2003 and August 2020. The analysis reviews commonalities, success and engineering issues, to inform current and future projects. The work classifies each deployment by type, rated power, number of devices, grid connection and foundation. In each case, project status and (if appropriate) failure mode is identified. Failure modes are compared to deployment classification to identify potential relationships. Most deployments were of horizontal axis turbines. The majority (54%) of deployments performed well. 18% failed, 14% were withdrawn from service, and 14% generated less power than planned. The most common failure cause was blade failure, followed by generator and monitoring failures. Ducted devices and devices in high velocity locations were more likely to fail, suggesting that flow velocity is a key factor. Most blade failures were attributed to underestimation of loads during design. Floating deployments were less likely to fail than fixed deployments, but more likely to be curtailed. Off-grid and grid connected deployments showed similar failure rates, suggesting sector immaturity. Tidal stream energy has accumulated around 1.4 million operating hours. Analysis shows a falling empirical failure rate, and likelihood of failure similar to that experienced by the wind industry at a similar stage. This work will be useful for project planners, developers and technology companies and investors in de-risking future project efforts.

Combined Autonomous Surface Vehicles (ASV) and remotely operated underwater vehicles (ROV) inspection and intervention systems can contribute to future asset management of offshore renewable energy. This paper presents the design and performance of the winch system which couples the ASV and ROV and deploys/recovers the ROV. The hydrodynamic models and control algorithms are developed and solved with analytical and numerical approaches. The winch performance needs to meet a range of operational profiles, including i) ASV following/not following the ROV ii) winch operating in speed control iii) winch operating in tension control iv) varying ROV distance and depths targets. For a representative ASV/ROV configuration, the work determines the required umbilical length for different ROV targets and suitable winch speeds. The results show that the strategy where the ASV follows the ROV can reduce the umbilical tension, but conditions of compression should be carefully managed. The umbilical tension can also be decreased by tension control and shows to be very effective in larger sea states. This study also models the accidental limit case, where a malfunctioning ROV is recovered. The estimated increase of umbilical tension during the recovery stage of a malfunctioning ROV can thus be incorporated into the design calculations.

This study investigates the processes of occasional sea surface temperature (SST) fronts and their impacts on chlorophyll-a concentration (chl-a) in the South China Sea (SCS), based on satellite remote sensing and in situ observations in 2009–2013. The SST fronts were detected by an entropy-based edge detection algorithm method from satellite-derived SST images with a 0.011° grid size. Three offshore SST front case studies (S1, S2 and S3) at the peripheries of eddies in the northern SCS were studied. In case S1 in September 2013, two SST fronts were detected with gradient magnitudes (GMs) greater than 0.06°C km–1 in the cyclonic eddy and 0.08°C km–1 in the periphery waters, and the fronts only existed for one and two days, respectively. After three and seven days, the high chl-a was found in the strong SST front waters which were about 51 and 54% higher than the concentration in the surrounding waters. The depth of the maximum chl-a elevated from the subsurface (50 m) to the surface. In case S2 in...

In order to accelerate the access into the energy market for ocean renewables, the operation and maintenance (O&M) costs for these technologies must be reduced. In this paper a reliability-based simulation tool for the optimization of the management of an offshore renewable energy (ORE) farm is presented. The proposed tool takes into account the reliability data of the simulated devices and estimations on the energy produced to create a series of results in terms of availability and maintainability of the farm. The information produced supports operational and strategic decision making regarding the O&M for offshore farms. A case study simulating a conceptual tidal energy project, consisting of an array of two tidal turbines located off the north coast of Scotland, is presented to show some of the results achievable with this model. The proposed methodology, although adopted for a tidal farm here, is generally applicable to other kinds of ORE farms.

The target of Indonesia's national primary energy supply in 2025 and 2050 are estimated to be at least 400 MTOE and 1000 MTOE accumulated from a total of 34 Provinces in Indonesia to fill the target. At the same years, target for per capita primary energy utilization of 1.4 TOE and 3.2 TOE and per capita electricity utilization of at least 2,500kWh and 7,000kWh. This study aims to determine the magnitude of the achievement targets of national energy policies from several provinces in Indonesia by taking samples, namely the Nusa Tenggara Timur, West Java, and Central Java Provinces. The quantitative approach for the year 2010 to 2015 data periods of the NTT Province have taken by considering parameters, among others the economic structure, environmental conditions, and specific energy conditions. All data are processed through LEAP assistance. The study results have shown a dilemma in achieving national energy policy targets represented by three sample provinces.

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 offshore wind turbine (FOWT) installations are progressing from the R&D stage to commercial installation projects. The prospective sites are situated in increasingly deeper water and further away from the shore. This paper presents the Deep Turbine Installation-Floating (DTI-F) concept, an innovative hybrid spar buoy-based FOWT capable of being able to raise and lower the tower and nacelle, which simplifies construction, installation, maintenance and decommissioning. The study is focused on the hydrodynamics of the moored floating system, and it is based on experimental and numerical modelling work. A 1:45 Froude scaled model of the DTI-F wind concept was tested using three different mooring configurations: i) three mooring lines, ii) four mooring lines, and iii) three mooring lines with a delta connection. Free decay and stiffness decay tests were carried out together with regular and irregular wave tests. The numerical study comprises diffraction (ANSYS AQWA) and time-domain modelling (OrcaFlex). The experimental hydrostatic and hydrodynamic results are compared with the numerical simulations based on the as-built scale model. Considering the natural frequencies results obtained for the three mooring configurations, the three lines configuration without delta connection was selected as the most suitable design. The obtained results for the three mooring lines configuration show good agreement between the experiment and numerical simulations. The presented analysis of the design concept indicates a high degree of technical feasibility.

Improving the reliability of marine renewable energy devices such as wave and tidal energy convertors is an important task, primarily to minimize the perceived risks and reduce the associated cost for operation and maintenance. Marine systems involve a wide range of uncertainties, due to the complexity of failure mechanism of the marine components, scarcity of data, human interactions and randomness of the sea environment. The fundamental element of a probabilistic risk analysis necessarily needs to rely on operational information and observation data to quantify the performance of the system. However, in reality it is difficult to ascertain observation of the precursor data according to the number of component failures that have occurred, mainly as a result of imprecision in the failure criterion, record keeping, or experimental and physical modelling of the process. Traditional reliability estimation approaches such as Fault Tree, Event Tree and Reliability Block Diagram analysis offer simplified, rarely realistic models of this complex reliability problem. The main reason is that they all rely on accurate prior information as a perquisite for performing reliability assessment. In this paper, a hierarchical Bayesian framework is developed for modelling marine renewable component failures encountered the uncertainty. The proposed approach is capable to incorporate the conditions, which lack reliable observation data (e.g. unknown/uncertain failure rate of a component). The hierarchical Bayesian framework provides a platform for the propagation of uncertainties through the reliability assessment of the system, via Markov Chain Monte Carlo (MCMC) sampling. The advantages of using MCMC sampling has proliferated Bayesian inference for conducting risk and reliability assessment of engineering system. It is able to use hyper-priors to represent prior parameters as a subjective observations for probability estimation of the failure events and enable an updating process for quantitative reasoning of interdependence between parameters. The developed framework will be an assistive tool for a better monitoring of the operation in terms of evaluating performance of marine renewable system under the risk of failure. The paper illustrates the approach using a tidal energy convertor as a case study for estimating components failure rates and representing the uncertainties of system reliability. The paper will be of interest to reliability practitioners and researchers, as well as tidal energy technology and project developers, seeking a more accurate reliability estimation framework.

The rotor of a horizontal-axis floating offshore wind turbine is more frequently misaligned with the oncoming wind than that of a fixed offshore or onshore wind turbine due to the pitch and yaw motions of the floating support structure. This can lead to increased unsteady loading and fatigue on the components beyond those considered in the standard load cases. In this work, the Simulator fOr Wind Farm Applications (SOWFA) tool within the CFD toolbox OpenFOAM is used to perform simulations of a wind turbine at different stationary angles to the oncoming wind flow that a floating wind turbine may experience, so that the impact of misaligned flow on power production and blade loading can be studied. The turbine is modelled using an actuator line method which is coupled with NREL’s aeroelastic code FAST to compute the structural response. The results of this study will be used in future work to optimise the rotor geometry of a floating offshore wind turbine.