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New wind turbine technologies and designs are being explored in order to reduce the cost of energy from offshore wind farms. Two potential routes to a lower cost of energy are the X- Rotor Concept (XRC) and Multi-Rotor System (MRS) turbines. A key cost saving for both Novel concepts included in this paper is operation and maintenance (O&M) costs savings. The major component replacement cost for conventional horizontal axis, XRC and MRS turbines are examined and the benefits of the concepts are provided in this paper. A review on existing decision support systems for offshore wind farm O&M planning is presented with a focus on how applicable these previous models are to novel turbine concepts, along with analysis of how the influential factors can be modified to effectively model XRC and MRS.

Combining multiple-types of Wave Energy Converters (WECs) and integrating them into in-development or pre-existing marine platforms can reduce the total Levelised Cost of Energy (LCoE) by sharing infrastructures and maintenance costs. The current study proposes an innovative multi-purpose solution by deploying an Oscillating Buoy (OB) device inside the chamber of an Oscillating Water Column (OWC) WEC integrated into a π-type floating breakwater. A fully non-linear time-domain model based on the Higher-Oder Boundary Element Method (HOBEM) is established to investigate the hydrodynamic performance of the proposed concept. The non-linear time-domain model is generalised to incorporate the OWC (aero and hydrodynamics coupling) and multi-body interaction. A series of simulations are performed to examine the hydrodynamic performance of the proposed hybrid concept. Results were compared with an isolated breakwater and an OWC-integrated breakwater, demonstrating that the proposed hybrid concept has a beneficial impact on both wave energy conversion and transmitted wave attenuation. In addition, long-period waves enter into the chamber more easily, which leads to a larger inner water motion and pressure fluctuation in the chamber. Importantly, there exists a coupled resonant motion between the OB device and the water surface in the chamber, which enhances the maximum capture efficiency and the efficiency frequency bandwidth. The asymmetric OB with a triangle-shaped bottom is found to absorb the wave energy along with the water depth more effectively. Despite the better performance, the current design does not increase the characteristic system volume and has no external moving part, making it ideal for array deployment.

Meeting climate and air quality targets, while preserving the focus on the reliability and cost effectiveness of energy, became a central issue for offshore wind turbine engineers. Floating offshore wind turbines, which allow harnessing the large untapped wind resources in deep waters, are highly complex and coupled systems. Subsystem-level optimisations result in suboptimal designs, implying that an integrated design approach is important. Literature saw a few attempts on multidisciplinary design analysis and optimisation of floating wind turbines, with varying results, proving the need for an efficient, and sufficiently accurate, integrated approach. This paper reviews the state-of-the-art approaches to multidisciplinary design analysis and optimisation of floating support structures. The choice of the optimisation framework architecture, support platform design variables, constraints and objective functions are investigated. The techno-economic analysis models are closely examined, focusing on the approaches to achieving the optimum accuracy-efficiency balance. It is shown that the representation of the fully coupled system within the optimisation framework requires the introduction of a more complex multidisciplinary analysis workflow. Methods to increase the efficiency of such frameworks are indicated. Nonconventional support structure configurations can be conceived through the application of more advanced parametrisation schemes, which is feasible together with design space size reduction techniques. The set of design criteria should be extended by operation and maintenance cost, and power production metrics. The main technical limitations of the frameworks adopted so far include the inability to accurately analyse a diverse range of support structure topologies in multiple design load cases within a common framework. The cost approximation models should be extended by the chosen aspects of pre-operational phases, to better explore the benefits of the floating platforms.

A large deployment of energy storage solutions will be required by the stochastic and non-controllable nature of most renewable energy sources when planning for higher penetration of renewable electricity into the energy mix. Various solutions have been suggested for dealing with medium- and long-term energy storage. Hydrogen and ammonia are two of the most frequently discussed as they are both carbon-free fuels. In this paper, the authors analyse the energy and cost efficiency of hydrogen and ammonia-based pathways for the storage, transportation, and final use of excess electricity from an offshore wind farm. The problem is solved as a linear programming problem, simultaneously optimising the size of each problem unit and the respective time-dependent operational conditions. As a case study, we consider an offshore wind farm of 1.5 GW size located in a reference location North of Scotland. The energy efficiency and cost of the whole chain are evaluated and compared with competitive alternatives, namely, batteries and liquid hydrogen storage. The results show that hydrogen and ammonia storage can be part of the optimal solution. Moreover, their use for long-term energy storage can provide a significant, cost-effective contribution to an extensive penetration of renewable energy sources in national energy systems.

Combing floating breakwaters with wave energy converters (WECs) and integrating them into very large floating structure (VLFS) can provide a viable option to explore economically offshore wave energy resources and simultaneously to protect marine structures. In this paper, the time-domain numerical model is developed based on the modal expansion theory with nonlinear consideration to optimize the design and layout of an integrated system of modular WEC-type floating breakwaters and a pontoon-type VLFS, with emphasis on the effects of the WEC geometric size and shape, the WEC-VLFS gap distance and the wave nonlinearity. A hybrid finite element (FE)-boundary element (BE) method is presented to simulate the structures as Mindlin plate elements and the water waves as fully nonlinear potential flow boundaries, respectively. Breakwaters as WECs with deeper draft and larger length are found to more fully interact in phase with long-period waves, and receive more wave energy extraction and larger hydroelastic response reduction. The addition of breakwaters has a favorable effect on the wave energy extraction, but a destructive effect on the hydroelastic reduction. Importantly, wave resonance induced by the multi-modal scattering waves in the WEC-VLFS gap leads to multiple peaks of the power capture efficiency. Compared to the symmetric-shape WECs, the asymmetric-shape WECs strengthen the gap resonant effect, which improves both the wave energy extraction and hydroelastic reduction for a broader frequency bandwidth. The findings of this study indicate the synergistic benefits of wave energy exploitation and transmitted wave attenuation at the fore-end of VLFSs.

There is increasing competition for space in coastal seas as new industrial sectors, such as Marine Renewable Energy (MRE) and Aquaculture, seek to expand. Multi-Use - involving sharing of space and, in some cases, facilities - can lessen competition and reduce industry costs if societal and economic challenges can be overcome. An example societal challenge is that of gaining Social Licence to Operate (SLO) for 'Multifunctional Offshore Installations' (MOI) combining fish farming with MRE (from wind and waves) in a large floating structure. This article reports a mixed-methods study at two potential MOI deployment sites in 2019, aiming to understand the local context for SLO. A survey was carried out in Reggio Calabria, Italy, with 108 respondents, and in Islay, Scotland, with 127 respondents. Questions concerned opinions about MRE and fish-farming, separately and combined. A facilitated workshop in Reggio Calabria provided additional qualitative data. Most findings were the same in both places. Respondents thought better of MRE than fish-farming but remained moderately likely to eat fish produced in MOI. The majority distrusted regulators to control environmental impacts of the technology. The main differences were that respondents in Reggio Calabria anticipated local benefits from MOI industrial activity, and were more likely to accept development by non-local owners than were people on Islay. We interpreted the findings in a conceptual framework that combines theory for SLO with theory for Action Situations, hypothesising that a community’s diffuse and perhaps heterogenous opinions might ‘crystalise’ around an issue during an Action Situation. The hypothesis will be tested when a prototype MOI is deployed near Reggio Calabria in 2021.

The development of novel energy technologies to meet net zero carbon emission is essential in the provision of solutions to realize an increasing worldwide demand for renewable energy. Floating Offshore Wind Turbine (FOWT) is one of the emerging technologies to exploit the vast wind resources available in deep waters within the offshore wind sector. However, as a result of the complexity of a FOWT system, bringing FOWT technology up to speed requires a detailed understanding of the different disciplines within the system and the relationship between the FOWT system and the dynamics of the marine environment; hence, the need for Multidisciplinary Design Analysis and Optimization (MDAO) of the system. This paper reviews the MDAO of FOWT substructures / platforms proposed in the literature. This review covers an overview of floating offshore wind turbine substructures’ concepts, the design using geometric shape parameterization techniques and the analysis approaches (time and frequency domain) for response assessment of the FOWT system. It also provides a comprehensive review of MDAO frameworks for FOWT substructures. Regarding the optimization aspect, a review of some optimization algorithms used for floating offshore wind turbine substructure is provided, i.e., from the global search heuristic and meta-heuristics algorithms to the local search gradient-based optimization algorithms. This work further identifies the research gaps in MDAO for FOWT substructures. The main proposed future research areas to address these gaps are: increasing design space richness by adopting more advanced parametrization techniques to represent the platform geometry (and other characteristics), utilize surrogate/meta models to replace the most computationally expensive high-fidelity models needed for quick sensitivity studies before detailed analyses on selected models are conducted, and exploring the upscaling of the geometric design parameters of an optimal shape parameterized FOWT platforms derived from existing designs which can be coupled with new generation highly rated and heavier turbines.

The number of installed floating offshore wind turbines (FOWTs) has doubled since 2017, quadrupling the total installed capacity, and is expected to increase significantly over the next decade. Consequently, there is a growing consideration towards the main challenges for FOWT projects: monitoring the system’s integrity, extending the lifespan of the components, and maintaining FOWTs safely at scale. Effectively and efficiently addressing these challenges would unlock the wide-scale deployment of FOWTs. In this work, we focus on one of the most critical components of the FOWTs, the Mooring Lines (MoLs), which are responsible for fixing the structure to the seabed. The primary mechanical failure mechanisms in MoLs are extreme load and fatigue, both of which are functions of the axial tension. An effective solution to detect long-term drifts in the mechanical response of the MoLs is to develop a Digital Twin (DT) able to accurately predict the behaviour of the healthy system to compare with the actual one. Moreover, we will develop another DT able to accurately predict the near future axial tension as an effective tool to improve the lifespan of the MoLs and the safety of FOWT maintenance operations. In fact, by changing the FOWT operational settings, according to the DT prediction, operators can increase the lifespan of the MoLs by reducing the stress and, additionally, in the case where FOWT operational maintenance is in progress, the prediction from the DT can serve as early safety warning to operators. Authors will leverage operational data collected from the world’s first commercial floating-wind farm [the Hywind Pilot Park (https://www.equinor.com/en/what-we-do/floating-wind/hywind-scotland.html.)] in 2018, to investigate the effectiveness of DTs for the prediction of the MoL axial tension for the two scenarios depicted above. The DTs will be developed using state-of-the-art data-driven methods, and results based on real operational data will support our proposal.

This paper investigates the limiting wave conditions at which a wind turbine technician can complete maintenance activities safely and effectively on a 15MW floating offshore wind turbine. Through linear, frequency-domain statistical analysis of floating turbine motion and applying acceptable motion limits for technician working, significant wave height and peak wave period limits are investigated. It was found that over the range of wave conditions considered, the turbine nacelle motion did not exceed the motion limits for technician working considered in this analysis. Further analysis found that the turbine nacelle motion increased with increasing significant wave height and was also significantly influenced by peak wave period. The impact of differing wave characteristics is also investigated through the use of different wave energy spectra and also found to have an impact on turbine nacelle motion.

This paper presents the key operations & maintenance (O&M) modelling inputs for fixed-bottom wind (FBW) and highlights the adaptations required for floating offshore wind (FOW) uses. The work also highlights major repair strategies such as tow to shore (T2S) and discusses the limitations and constraints which arise in an operational context. The technical and economic feasibility of such O&M strategies requires rethinking of weather risks and constraints, new vessel technologies and operational costs. The work also collates and reviews existing FBW models which have been adapted for FOW uses and analyses O&M inputs for a tow to shore operation. Findings show that there is ambiguity in literature for tug speeds and disconnection/reconnection times of the turbine system. A performed case study investigates the sensitives of both parameters through a weather window analysis of ScotWind sites. Recommendations for future practises, including additional O&M modelling considerations and inputs for FOW uses are given.

Integrating multi-type Wave Energy Converters (WECs) enables wave energy to be extracted from multiple harvesting manners simultaneously, which presents a more competitive energy conversion technology. In this study, an Oscillating Water Column (OWC) and an Oscillating Buoy (OB) are combined to devise a hybrid WEC system that serves as a floating breakwater. The two devices are aligned in line with the incident wave direction, where the OB is deployed upstream from the OWC. A series of physical experiments are conducted to understand the working principle of the hybrid WEC system. After a fully non-linear time-domain model is validated by the measured data, the comparisons of the hybrid system and its respective isolated devicesare emphasised. Then, the hydrodynamic dependence of the hybrid system is comprehensively tested. The results indicated that the hybrid system design provides higher energy conversion as well as better wave attenuation performance compared to the isolated OWC and OB devices. Additionally, the OB outperforms the OWC in terms of wave energy conversion. The OWC-OB gap resonance has a negative impact on wave energy extraction but has a negligible effect on wave attenuation. The OWC with a deeper draft and symmetric wall performs better both in terms of wave energy extraction and wave attenuation in long-period waves. The overall conversion efficiency decreases with the increase of the OB draft, but the transmission coefficient follows an opposite trend. The PTO dampings of the OB and the OWC dominate the overall conversion efficiency and the transmission coefficient, respectively.

Wave energy converters(WECs) integrated into floating breakwaters or, more generally, their designs also used as wave absorbers can provide a space- and cost-sharing solution to enhance the efficiency of coastal protection. In this study, the hydrodynamic performances of an oscillating buoy(OB)-type single-pontoon floating breakwater(SPFB) and an oscillating water column(OWC)-type dual-pontoon floating breakwaters(DPFB) are evaluated, respectively, and are comprehensively compared with each other. A series of physical experiments were conducted to investigate the effects of wave parameters, geometrical dimensions, gap distances and PTO damping coefficients. Numerical simulations based on the fully nonlinear potential flow theory cross-check the measured data, and help to further understand the contribution of higher-order waves. It is found that both the wave attenuation and energy conversion of the OWC-type DPFB with a non-uniform draft (i.e. shallow front-pontoon draft and deep back-pontoon draft) are better than those of the OB-type SPFB with the same displacement. Meanwhile, for an OWC-type DPFB, a larger ratio of the chamber width and the total pontoon width has a beneficial effect on the wave attenuation, but has an adverse effect on the maximum energy conversion efficiency. Under the premise of the same pontoon draft, the maximum conversion efficiency of the OWC for the optimal opening ratio is higher than that of the OB for the optimal PTO damping, but the effective frequency bandwidth is almost identical between two devices. Strong wave nonlinearity can generate more energy dissipation induced by the viscous effect and the vortices, resulting into the reduction of energy extraction.

Aquaculture and marine renewable energy are two expanding sectors of the Blue Economy in Europe. Assessing the long-term environmental impacts in terms of eutrophication and noise is a priority for both the EU Water Framework Directive and the Marine Strategy Framework Directive, and cumulative impacts will be important for the Maritime Spatial Planning under the Integrated Maritime Policy. With the constant expansion of aquaculture production, it is expected that farms might be established further offshore in more remote areas, as high-energy conditions offer an opportunity to generate more power locally using Marine Renewable Energy (MRE) devices. A proposed solution is the co-location of MRE devices and aquaculture systems using Multi-Purpose Platforms (MPPs) comprising offshore wind turbines (OWTs) that will provide energy for farm operations as well as potentially shelter the farm. Disentangling the impacts, conflicts and synergies of MPP elements on the surrounding marine ecosystem is challenging. Here we created a high-resolution spatiotemporal Ecospace model of the West of Scotland, in order to assess impacts of a simple MPP configuration on the surrounding ecosystem and how these impacts can cascade through the food web. The model evaluated the following specific ecosystem responses: (i) top-down control pathways due to distribution changes among top-predators (harbor porpoise, gadoids and seabirds) driven by attraction to the farming sites and/or repulsion/killing due to OWT operations; (ii) bottom-up control pathways due to salmon farm activity providing increasing benthic enrichment predicated by a fish farm particle dispersal model, and sediment nutrient fluxes to the water column by early diagenesis of organic matter (recycled production). Weak responses of the food-web were found for top-down changes, whilst the results showed high sensitivity to increasing changes of bottom-up drivers that cascaded through the food-web from primary producers and detritus to pelagic and benthic consumers, respectively. We assessed the sensitivity of the model to each of these impacts and the cumulative effects on the ecosystem, discuss the capabilities and limitations of the Ecospace modeling approach as a potential tool for marine spatial planning and the impact that these results could have for the Blue Economy and the EU’s New Green Deal.

Floating offshore wind turbines are subjected to higher tower fatigue loads than their fixed-to-seabed counterparts, which could lead to reductions in turbine life. The worst increases are generally seen in the tower axial fatigue, associated with the tower fore-aft bending moment. For a spar type platform, this has been shown to increase by up to 2.5 times and for a semi-submersible platform, by up to 1.8 times. Reducing these loads would be beneficial, as the alternative of strengthening the towers leads to increase in cost. Here, two offshore floating wind turbine systems, of the spar type, are analyzed and selected responses and tower fatigue are compared: one incorporates a variable-speed, variable-pitch-to-stall blade control system and a back twisted blade, and the other a conventional pitch-to-feather control. The results are then compared with those obtained in an earlier study, where the same turbine configurations were coupled to a semi-submersible platform. A weighted wind frequency analysis at three mean turbulent wind speeds of 8, 13, and 18 m/s highlights that the impact of the back twist angle magnitude and initiation point on tower axial fatigue life extension was the same for both platform types. Compared with their respective feather base models, an increase in the tower axial fatigue life of 18.8% was seen with a spar platform and 10.2% with a semi-submersible platform, when a back twist angle to the tip of −6 deg was imposed along with the variable-speed, variable-pitch-to-stall control.

The hydromechanics analysis of floating offshore wind turbines is a fundamental and time consuming part of the design process, traditionally analysed with methods of computational fluid dynamics. In this work, an alternative computational framework is suggested, able to significantly accelerate the design process with minimal accuracy loss. Through the use of a state-of-the-art potential-flow code, a surrogate model is developed with the aim to approximate the Response Amplitude Operators of any arbitrary floating offshore wind turbine of the spar buoy type. The results, measured in terms of accuracy and computational effort, demonstrate that this approach is able to approximate the potential-flow solver with very high accuracy at a fraction of the computational cost.

Multi-purpose floating platforms are emerging as a promising concept in ocean engineering applications, thanks to their capability of ensuring system integration, cost reduction and modularization. However, their increasing complexity requires the development of numerical tools, which need to be validated experimentally through adequate physical models. New challenges hence arise, since the subsystems integrated in the structure generally follow different scaling laws and may need relatively large physical models to achieve a reliable similitude between the full-scale structure and its physical model counterpart. The latter issue can be critical, because indoor tests in wave tanks and basins constrain the scale factors to the size of the available facilities. Open-sea experiments, albeit challenging because of the uncontrolled environmental conditions, could be a valid complement to the traditional indoor tests. This article proposes a review of the multi-physics scaling strategies for the subsystems usually embedded in multi-purpose floating platforms, i.e. floating support, mooring system, wind turbine, wave energy converter and aquaculture facilities, by providing a critical analysis on the relevance of the scaling factor and of the scaling strategy. The paper may also serve as a guide for practical applications involving one or several of the considered subsystems.

Dimensioning and assessment of a specific wind turbine imply iterative steps for design optimization, as well as load calculations and performance analyses of the system in various environmental conditions. However, due to the complexity of wind turbine systems, fully coupled aero-hydro-servo-elastic codes are indispensable to represent and simulate the non-linear system behavior. To cope with the large number of simulations to be performed during the design process of a wind turbine system, automation of simulation executions and optimization procedures are required. In this paper, such a holistic simulation and optimization framework is presented, by which means iterative simulations within the wind turbine design assessment and development processes can be managed and executed in an automated and high-performance manner. The focus lies on the application to design load case simulations, as well as the realization of automated optimizations. The proper functioning and the high flexibility of the framework tool is shown based on three exemplary optimization tasks.

The development of novel energy technologies to meet net zero carbon emission is imperative in the provision of solutions to realize an increasing worldwide demand for renewable energy. Floating Offshore Wind Turbine (FOWT) is one of the emerging technologies to exploit the vast wind resources available in deep waters within the offshore wind sector. However, as a result of the complexity of a FOWT system, bringing FOWT technology up to speed requires a detailed understanding of the different disciplines within the system and the relationship between the FOWT system and the dynamics of the marine environment; hence, the need for Multidisciplinary Design Analysis and Optimization (MDAO) of a FOWT system.

Offshore wind turbines are now a mature technology to produce renewable energy on a vast scale, nonetheless several design and maintenance planning challenges remain. There have been attempts to investigate the impact of marine growth on fixed offshore wind turbine structures, but only few adopted a whole dynamics approach. This work presents a methodology to capture the influence of marine growth on the dynamic response of a tripod substructure, supporting the NREL 5 MW reference offshore wind turbine, under combined dynamic loads from wind and waves, and including soil-structure interaction by means of the spring-to-ground model. Marine growth is modelled as prescribed by DNV and API, evaluating the effects of variation of its thickness, roughness, and distribution. It is here demonstrated that marine growth thickness and roughness impact significantly on the loads acting on wind turbines' structures and its dynamic response, and that heterogeneity in marine growth thickness profiles vs depth available in literature lead to substantially different results. Tower top displacement becomes 24% higher when marine growth thickness grows from 0 to 200 mm. On the other hand, the changes in the natural frequencies of the support structure with an increase of marine growth's thickness are almost negligible (0.3%).

Offshore wind energy is currently the leading offshore renewable energy technology and plays an essential role in achieving a low carbon economy in Europe. Offshore wind turbines, as a result of the harsher offshore environment, suffer from high O&M costs, and as such a high overall LCOE. These higher O&M costs can be reduced by focusing on the most critical subassemblies of the OWT, by either improving the reliability of the subassembly or by better understanding the failure mechanism, and thus optimizing the O&M strategy. The power converter is identified as one of the most critical subassemblies as risk of the operation of the OWT in terms of maintainability and availability. The reliability of the power converter is heavily influenced by both its steady-state and its dynamic thermal behavior, and as such great attention will be paid to the literature related to the thermal behaviour of the power converter. This paper seeks to provide a comprehensive overview on how the typical mission profile of an OWT influences the thermal loading. The paper seeks to propose a comprehensive and up-to-date literature review related to the lifetime modelling of the power converter, and to highlight some of the main challenges yet to be tackled in the reliability analysis of the power converter, suggesting future areas of research.

Offshore renewable energy (ORE) sources, such as offshore wind turbines, wave energy converters, and tidal and current turbines, have experienced rapid growth in the past decade. The combination of wave, wind, and current energy devices in hybrid marine platforms that use synergies through proper combinations has been a recent scientific focus. The new concepts and structures being investigated require developing new design and analysis approaches that implement novel numerical modeling tools and simulation methods, thus advancing science, technology, and engineering. ORE structures may be subject to complex loads and load effects, which demand comprehensive and accurate numerical modeling representations of the physics underpinning the problem. Important factors that affect design, functionality, structural integrity, and performance of offshore structures include (but are not limited to): fluid–structure interactions, controller actions, intense dynamic effects, nonlinear loadings, extreme and harsh weather conditions, and impact pressure loads. Furthermore, these factors cannot be considered in isolation, since each factor is potentially coupled with another, requiring fully coupled models. To enable further growth in reliable ORE technologies, more advanced numerical tools and nonlinear modeling are needed.

A novel multi-purpose platform (MPP) has been proposed for providing renewable energy to an offshore fish aquaculture system. After having previously analyzed its dynamic response in operational conditions, it is essential to check its survivability under extreme environmental conditions, focusing on wind speeds and wave heights and periods, for checking serviceability criteria and human factors’ requirements. An environmental contour method is applied to derive the 25-year return period extreme metocean conditions, choosing as location the west coast of Scotland (longitude −7°, latitude 56.5°), based on a database of hindcast wind and wave conditions for this location from the start of 2008 till the end of 2017, with a 6 hours granularity. Then, an aero-hydro-servo-elastic coupled model of dynamics of the MPP, in the time domain, is developed, and the dynamic response of the MPP to the 25-year return extreme environment conditions are assessed. The survivability of the MPP under the most extreme environment conditions, have also been checked. The response of the MPP with respect to barge motion and the hub acceleration of wind turbines were observed to fall within the criteria specified in literature.

Combined floating offshore wind platform and Wave Energy Converters (WECs) systems have the potential to provide a cost-effective solution to offshore power supply and platform protection. The objective of this paper is to optimize the size and layout of WECs within the hybrid system under a given sea state with a numerical study. The numerical model was developed based on potential flow theory with viscous correction in frequency domain to investigate the hydrodynamic performance of a hybrid system consisting of a floating platform and multiple heaving WECs. A non-dimensional method was presented to determine a series of variables, including radius, draft, and layout of the cylindrical WEC at a typical wave frequency as the initial design. WECs with larger diameter to draft ratio were found to experience relatively smaller viscous effects, and achieve more wave power, larger effective frequency range and similar wave power per unit weight compared with those with the smaller diameter to draft ratio in the same sea state. The addition of WECs reduced the maximum horizontal force and pitch moment on the platform, whereas the maximum vertical force increased due to the increasing power take-off force, especially at low frequencies. The results presented in this paper provide guidance for the optimized design of WECs and indicate the potential for synergies between wave and wind energy utilization on floating platforms.

Expansion of marine aquaculture into more remote areas will likely accelerate over the next decade. Integrating Marine Renewable Energy (MRE) generation technologies (e.g., wind turbines) into remote, off-grid aquaculture sites will reduce reliance on fossil fuels by allowing localised low-carbon power generation, but may also result in novel environmental pressures. In this study, we undertook a thought experiment to assess the potential for increased collision risks to local marine and coastal bird species of integrating small wind turbines (4 units, combined capacity of 200 MWh) into a generalised marine fish farm in western Scotland (UK). Potential risks to bird species were assessed using a bespoke Sensitivity Index (SI) based on 12 factors, including population size, adult survival rate, UK conservation status, flight manoeuvrability, nocturnal flight activity, habitat preference, sensitivity to wind farms, attraction to fish farms for feeding and/or resting, and attraction to other marine anthropogenic structures/activities. SI scores varied substantially between species, but large gulls (Larus sp.) and European shag (Phalacrocorax aristotelis) were expected to be at the greatest potential risk. The general lack of information on interactions between birds and fish farms represented a significant knowledge gap, and greater focus on these interactions is needed to improve future risk assessments.

Wind power plays a key role in reducing global carbon emission. The power curve provided by wind turbine manufacturers offers an effective way of presenting the global performance of wind turbines. However, due to the complicated dynamics nature of offshore wind turbines, and the harsh environment in which they are operating, wind power forecasting is challenging, but at the same time vital to enable condition monitoring (CM). Wind turbine power prediction, using supervisory control and data acquisition (SCADA) data, may not lead to the optimum control strategy as sensors may generate non-calibrated data due to degradation. To mitigate the adverse effects of outliers from SCADA data on wind power forecasting, this paper proposed a novel approach to perform power prediction using high-frequency SCADA data, based on isolate forest (IF) and deep learning neural networks. In the predictive model, wind speed, nacelle orientation, yaw error, blade pitch angle, and ambient temperature were considered as input features, while wind power is evaluated as the output feature. The deep learning model has been trained, tested, and validated against SCADA measurements. Compared against the conventional predictive model used for outlier detection, i.e. based on Gaussian Process (GP), the proposed integrated approach, which coupled IF and deep learning, is expected to be a more efficient tool for anomaly detection in wind power prediction.

“Blue Growth” and “Blue Economy” is defined by the World Bank as: “the sustainable use of ocean resources for economic growth, improved livelihoods and jobs, while preserving the health of ocean ecosystem”. Multi-purpose platforms (MPPs) can be defined as offshore platforms serving the needs of multiple offshore industries (energy and aquaculture), aim at exploiting the synergies and managing the tensions arising when closely co-locating systems from these industries. Despite a number of previous projects aimed at assessing, from a multidisciplinary point of view, the feasibility of multipurpose platforms, it is here shown that the state-of-the-art has focused mainly on single-purpose devices, and adopting a single discipline (either economic, or social, or technological, or environmental) approach. Therefore, the aim of the present study is to provide a multidisciplinary state of the art review on, whenever possible, multi-purpose platforms, complementing it with single-purpose and/or single discipline literature reviews when not possible. Synoptic tables are provided, giving an overview of the multi-purpose platform concepts investigated, the numerical approaches adopted, and a comprehensive snapshot classifying the references discussed by industry (offshore renewables, aquaculture, both) and by aspect (technological, environmental, socio-economic). The majority of the multi-purpose platform concepts proposed are integrating only multiple offshore renewable energy devices (e.g. hybrid wind-wave), with only few integrating also aquaculture systems. MPPs have significant potential in economizing CAPEX and operational costs for the offshore energy and aquaculture industry by means of concerted spatial planning and sharing of infrastructure.

For a turbine mounted on a floating platform, extreme induced loads can be increased by up to 1.6 times those experienced by a turbine situated on a fixed base. If these loads cannot be reduced, towers must be strengthened which will result in increased costs and weight. These tower loads would be additionally exasperated for a pitch-to-feather controlled turbine by a phenomenon generally referred to as “negative damping,” if it were not avoided. Preventing negative damping from occurring on a pitch-to-feather controlled floating platform negatively affects rotor speed control and regulated power performance. However, minimizing the blade bending moment response can result in a reduction in the tower fore-aft moment response, which can increase the tower life. A variable-speed, variable pitch-to-stall (VSVP-S) floating semi-submersible wind turbine, which does not suffer from the negative damping and hence provides a more regulated power output, is presented. This incorporates a back twist blade profile such that the blade twist, starting at the root, initially twists toward stall and, at some pre-determined “initiation” point, changes direction to twist back toward feather until the tip. Wind frequency weighting was applied to the tower axial fatigue life trends of different blade profiles and a preferred blade back twist profile was identified. This had a back twist angle of −3 deg and started at 87.5% along the blade length and achieved a 5.1% increase in the tower fatigue life.

Floating offshore wind turbine (FOWT) systems are a fast-evolving technology, however, still have to gain economic competitiveness to allow commercial market uptake. Design optimization, focusing on cost reduction while ensuring optimum system performance, plays a key role in achieving these goals. Hence, in this work, an approach for optimizing a floating concept, utilizing global limit states, is developed. The optimization is carried out in Python, linked with Modelica and Dymola for modeling and simulation. For the FOWT design, the over-dimensioned OC3 spar-buoy is utilized. This is modified during the optimization regarding its geometrical dimensions and ballasting. The optimization criteria stability, mean and dynamic displacements, and tower top acceleration are used for formulating the objective functions. The optimization is carried out for one design load case, which is most critical for the considered criteria. Based on an initial study, NSGAII is chosen as optimizer. The convergence of the optimization is examined and the optimum design solution selected. In post-processing analyses, the overall performance of the optimized FOWT system is approved. The presented approach shows one example for the design optimization of a FOWT system and should deal as basis for more advanced design optimization tasks, including local characteristics and reliability aspects.

The complexity of floating offshore wind turbine (FOWT) systems, with their coupled motions, aero-hydro-servo-elastic dynamics, as well as non-linear system behavior and components, makes modeling and simulation indispensable. To ensure the correct implementation of the multi-physics, the engineering models and codes have to be verified and, subsequently, validated for proving the realistic representation of the real system behavior. Within the IEA (International Energy Agency) Wind Task 23 Subtask 2 offshore code-to-code comparisons have been performed. Based on these studies, using the OC3 phase IV spar-buoy FOWT system, the Modelica for Wind Turbines (MoWiT) library, developed at Fraunhofer IWES, is verified. MoWiT is capable of fully-coupled aero-hydro-servo-elastic simulations of wind turbine systems, onshore, offshore bottom-fixed, or even offshore floating. The hierarchical programing and multibody approach in the object-oriented and equation-based modeling language Modelica have the advantage (over some other simulation tools) of component-based modeling and, hence, easily modifying the implemented system model. The code-to-code comparisons with the results from the OC3 studies show, apart from expected differences due to required assumptions in consequence of missing data and incomplete information, good agreement and, consequently, substantiate the capability of MoWiT for fully-coupled aero-hydro-servo-elastic simulations of FOWT systems.

From an operation & maintenance (O&M) point of view, it is necessary to model the aero-hydro-servo-elastic (AHSE) dynamics of each wind turbine but, on the other side, wind farms generally include hundreds of wind turbines. Simply using and linking several advanced, single wind turbine models of dynamics to represent a wind farm can be computationally prohibitive. To this end, this paper developed a reduced-order model (ROM), able to capture the relevant dynamics of the system for a specific failure, having a lower computational cost and therefore more easily scalable up to a wind farm level. First, a nonlinear AHSE model is used to derive the time-domain response of the wind turbine degrees of freedom (DOFs). The failure mode, its relevant DOF, and the relevant operational conditions during which the failure is likely to occur are identified. A linearisation of the nonlinear aero-hydro-servo-elastic-drivetrain (AHSE-DT) model is then carried out. Subsequently, a number of linear ROMs are developed based on the linear full-order system but excluding high-frequency states using the modal truncation (MT) method. For the targeted DOF (rotor torque signal) and the load cases simulated, the results from the linear ROMs showed that the blade modes are important to capture not only the DOF of extreme values, but also the DOF of high-frequency responses (above 1.5 Hz). The results from nonlinear ROMs showed that the ROM eliminating all the tower modes (rigid tower) is acceptable to capture the DOF of low-frequency response (below 0.5 Hz), as it has almost the same spectral responses as the full-order nonlinear model.

Despite several potential advantages, relatively few studies and design support tools have been developed for floating vertical axis wind turbines. Due to the substantial aerodynamics differences, the analyses of vertical axis wind turbine on floating structures cannot be easily extended from what have been already done for horizontal axis wind turbines. Therefore, the main aim of the present work is to compare the dynamic response of the floating offshore wind turbine system adopting two different mooring dynamics approaches. Two versions of the in-house aero-hydro-mooring coupled model of dynamics for floating vertical axis wind turbine (FloVAWT) have been used, employing a mooring quasi-static model, which solves the equations using an energetic approach, and a modified version of floating vertical axis wind turbine, which instead couples with the lumped mass mooring line model MoorDyn. The results, in terms of mooring line tension, fatigue and response in frequency have been obtained and analysed, based on a 5 MW Darrieus type rotor supported by the OC4-DeepCwind semisubmersible.

In order to identify rigid body motions of floating offshore structures, output-only techniques are very useful for developing low-cost intermediate-scale experimental activities directly into the sea, instead of wave tanks. A crucial parameter, however, is the length of the response records used as input for the identification process, since short records may result in significant loss of accuracy, while long ones may be incompatible with the assumption of stationarity of the sea state. This work presents a sensitivity study conducted on a numerical model of a spar structure, identified by means of Enhanced Frequency Domain Decomposition method. An overview on the efficiency of the method is given for various lengths of response record, along with practical indications on the minimum values acceptable.

A multi-purpose platform (MPP) is an offshore systemdesigned to serve the purposes of more than one offshoreindustry. Indeed, over the past decades, a number of industrieshave expanded, or are expanding, from onshore to offshorelocations (renewables, aquaculture, tourism, mineralextractions, etc), and the research on these type of platform isincreasing. In the present work, a MPP able to accommodatewind turbines, wave energy converters, and aquaculturesystems is considered. For an overview of the MPP platformconsidered and its research context, please refer to the EUH2020 project overview (OMAE 2019-96104). This workpresents the second part (Part B) of the analyses of thedynamics of the floating support structure for this MPP,focusing on the hydro-elastic analysis, while its complementaryrigid-body hydrodynamic analysis is presented in Part A(OMAE2019-96212). The aim here is to assess if the support platform structuralelasticity has a substantial impact on the dynamic response ofthe platform. A beam model and a 3D solid model of thesupport structure have been developed, and the inertial forces,hydrodynamic added mass forces, hydrostatic and mooringrestoring forces have been considered in the hydro-elasticanalyses performed. The results show that the dynamicresponse to the wave loads is not substantially influenced bythe elasticity of the support structure, and that, at firstapproximation, a rigid-body approach is acceptable.