A Comparison Between CFD-Based Aerodynamic Models and BEM Theory-Based Models Applied in Coupled Simulations of Floating Offshore Wind Turbines

Interest in floating offshore wind as a renewable energy source is growing, as it offers the potential to access deeper waters than those suited to bottom-fixed offshore turbines. A key design challenge for floating offshore wind turbines (FOWTs) is capturing the aerodynamic behaviour using numerical models, which is significantly more complex than for bottom fixed turbines due to the motions of the floating platform that result in unsteady relative wind flow at the rotor. Many of the engineering models available for analysing wind turbine aerodynamics such as the blade element momentum (BEM) method were designed for fixed turbines and require empirical corrections to account for unsteady aerodynamic effects, and may not be suitable for analysing the more complex aerodynamics associated with FOWTs. Higher order modelling approaches including computational fluid dynamics (CFD) may offer improved accuracy as they capture more of the flow physics, however, they can have extremely high associated computational costs. In this thesis, the performance of different aerodynamic models for FOWTs is investigated by studying the motion and load response of a FOWT in a range of load cases covering operational and extreme conditions using a BEM method and two different CFD-based models. Firstly, the BEM method used in the wind turbine engineering tool FAST is compared with an actuator line model (ALM) from the CFD wind turbine code package SOWFA for a range of load cases. Comparisons are made in load cases that have specific challenges for FOWTs and where the BEM method has known limitations, including rotor misalignment with the wind due to yaw, and varying wave conditions. The two modelling approaches are then used to study FOWT behaviour in realistic operational and extreme environmental conditions, and the model results are compared against available field data from full scale FOWT demonstration projects. The impact of using high order large eddy simulation (LES) to generate a turbulent wind field is also compared against a lower order statistical approach. Finally, a high order modelling approach is proposed that couples a geometry-resolved CFD model of a wind turbine blade with a structural model based on 3D finite element analysis (FEA) to enable two way coupled fluid structure interaction simulation. This model provides detailed information on the loading and deformation of blades, and is compared against FAST for studying a large flexible wind turbine blade in the parked and feathered position. This research provides improved understanding of the impact that the choice of aerodynamic and wind models have on the predicted response of a floating offshore wind turbine. ALM predictions are found to diverge from BEM predictions in increasingly large rotor yaw misalignment angles. Turbine loads and platform motions are found to be sensitive to the atmospheric stability condition, with stable conditions having a significant effect, however the use of high fidelity LES modelling of neutral conditions has little effect on turbine response (in either operational or extreme conditions) compared to using more efficient statistical modelling of turbulence using the Kaimal model. The results of the presented comparisons in this work are used to make recommendations on the use of different models in the design process for FOWTs. It is found that FAST is suitable for the majority of load cases, and may provide improved predictions of a FOWT in extreme conditions over an ALM that may underestimate aerodynamic loading on the tower. However, an ALM may provide improved predictions for a yawed turbine. The use of a high fidelity coupled CFD-FEA approach has potential to be a useful tool for analysing the detailed response of highly flexible blades where low fidelity methods are less reliable, though further work is needed to validate the modelling

Simulating the Effects of Floating Platforms, Tilted Rotors, and Breaking Waves for Offshore Wind Turbines

Offshore wind energy is a rapidly expanding source of renewable energy worldwide, but many aspects of offshore wind turbine behavior are still poorly understood and are not accurately captured by low-cost engineering models used in the design process. To help improve these models, computational fluid dynamics (CFD) can provide valuable insight into the complex fluid flows that affect offshore wind turbine power generation and structural loads. This research uses CFD simulations to examine three main topics important to future offshore wind development: how breaking waves affect structural loads for fixed-bottom wind turbines; how platform motions affect power generation, wake characteristics, and downwind turbine behavior in floating wind turbines; and how rotor tilt angles affect wake characteristics when interacting with earth\u27s surface. These high-fidelity simulations can help inform future improvements to engineering models like wake models, power prediction models, and breaking wave models, which are integral to designing and financing both offshore turbines and offshore wind farm arrays. First, breaking wave limits and slam force models are evaluated using CFD simulations of shoaling and breaking waves impacting monopile foundations, for environmental conditions representative of U.S. East Coast offshore wind sites. Second, floating turbine wakes are characterized by the velocity deficit, turbulent kinetic energy, and wake centerline location using large eddy simulations (LES) coupled via an actuator line model to the multidynamics turbine modeling tool OpenFAST. These wake metrics are compared for different floating platform types, atmospheric stability types, and environmental conditions. Third, the power generation of spar and semisubmersible floating turbines is simulated using OpenFAST with LES inflow, with different platform motions isolated. These power results inform a new analytical model for power generation in floating turbines. Fourth, downwind turbines with different platforms are simulated in OpenFAST using an upwind floating turbine\u27s LES wake as inflow, to study how floating-turbine wakes affect a downwind turbine\u27s power, blade loads, and towertop displacements. Finally, LES with an actuator disk model of a tilted wind turbine are performed for different tilt angles and blade-to-surface gaps, to characterize tilted rotor wakes and how they interact with the sea or ground surface

Waves and Ocean Structures

Ocean Structures subjected to actions of ocean waves require safety inspection as they protect human environment and everyday lives. Increasing uses of ocean environment have brought active research activities continuously. The newly developed technology of ocean energy even pushed the related needs forward one more step. This Special Issue focuses on Analysis of Interactions between wave structures and ocean waves. Although ocean structures may cover various practical and/or conceptual types, we hope in the years to come, the state-of-the-art applications in wave and structure interactions and/or progress review and future developments could be included. There are fifteen papers published in the Special issue. A brief description includes: Lee et al. [1] presented a concept of a water column type wave power converter. Li et al. [2] considered submerged breakwaters. Lin et al. [3] studied an ocean current turbine system. Thiagarajan and Moreno [4] investigated oscillating heave plates in wind turbines. Chiang et al. [5] proposed an actuator disk model. Tseng et al. [6] investigated Bragg reflections of periodic surface-piercing submerged breakwaters. Lee et al. [7] analyzed caisson structures with a wave power conversion system installed. Yeh et al. [8] reported motion reduction in offshore wind turbines. Wu and Hsiao [9] considered submerged slotted barriers. Tang et al. [10] studied floating platforms with fishnets. Chen et al. [11] calculated mooring drags of underwater floating structures with moorings. Jeong et al. [12] estimated the motion performance of light buoys using ecofriendly and lightweight materials. Zhang et al. [13] considered vibrations of deep-sea risers. On the other hand, Shugan et al. [14] studied the effects of plastic coating on sea surfaces

Simulating the Hydrodynamics of Offshore Floating Wind Turbine Platforms in a Finite Volume Framework

There is great potential for the growth of wind energy in offshore locations where the structures are exposed to a variety of loading from waves, current and wind. A variety of computer-aided engineering (CAE) tools, based largely on engineering models employing potential-flow theory and/or Morison\u27s equation, are currently being used to evaluate hydrodynamic loading on floating offshore wind turbine platforms. While these models are computationally inexpensive, they include many assumptions and approximations. Alternatively, high-fidelity computational fluid dynamics models contain almost no assumptions, but at the cost of high computational expense. In this work, CFD simulations provide detailed insight into the complex fluid flow that has not been captured experimentally, nor can be attained with reduced-order models. This work includes a thorough validation of the various CFD toolboxes necessary for simulating offshore floating wind turbine platforms in the ocean environment, from numerical wave propagation to fluid-structure interactions. The fundamental physics of flow around complex structures is examined through various studies to better understand the effects of a fluid interface, truncated ends, structure size, multi-member arrangements and environmental conditions. These factors are explored in terms of drag, lift and frequency of the loads. Additionally, motion of structures in free decay tests and waves are investigated. The work provides insight into the complex fluid flow around floating offshore structures of small draft in a variety of environmental conditions. CFD simulations are used to assess assumptions and approximations of reduced-order engineering models, and explain why, and in which conditions, these models perform inaccurately. Finally, the work provides suggestions for improvements to engineering tools often used for hydrodynamics modeling of floating offshore wind turbines

Long-term research challenges in wind energy – a research agenda by the European Academy of Wind Energy

The European Academy of Wind Energy (eawe), representing universities and institutes with a significant wind energy programme in 14 countries, has discussed the long-term research challenges in wind energy. In contrast to research agendas addressing short- to medium-term research activities, this eawe document takes a longer-term perspective, addressing the scientific knowledge base that is required to develop wind energy beyond the applications of today and tomorrow. In other words, this long-term research agenda is driven by problems and curiosity, addressing basic research and fundamental knowledge in 11 research areas, ranging from physics and design to environmental and societal aspects. Because of the very nature of this initiative, this document does not intend to be permanent or complete. It shows the vision of the experts of the eawe, but other views may be possible. We sincerely hope that it will spur an even more intensive discussion worldwide within the wind energy community

Currents, Waves and Turbulence Measurement: A view from multiple Industrial-Academic Projects in Tidal Stream Energy

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordTidal Stream Energy is considered a regular, predictable and dense energy source with potential to make a significant contribution to our future energy needs. Development of the industry, from resource assessment to device design and operation, requires characterisation of the flow environment at a variety of spatial and temporal scales at tidal energy sites. Demand for flow characterisation arises from companies developing, installing and operating tidal turbine prototypes or small arrays in locations from Scotland to France to Canada. Flow characterisation for tidal stream applications relies on the measurement of water velocity at the relevant scales, yet given the non-uniformity of the flow field, no single instrument measures all the necessary data inputs required by the sector. This paper provides an overview of a variety of current, surface wave and turbulence metrics of industrial relevance to tidal stream and discusses methods employed to secure these datasets. The use of variants of acoustic current profilers is presented, which have been utilised and developed on previous and ongoing industrialacademic projects, including ReDAPT (ETI, UK), FloWTurb (EPSRC, UK) and RealTide (EC H2020, EU). These variants feature differing numbers of acoustic transducers and varying geometrical configurations with installations at both seabed locations and atop operating tidal stream energy converters. Ongoing development of advanced sensor configuration is discussed, aiming to achieve resilient, high resolution threedimensional measurement of mean and turbulent flow tailored for tidal energy applications. The paper gives practitioners and researchers an overview of tidal stream flow characterisation and practical lessons learnt.Engineering and Physical Sciences Research Council (EPSRC)European Union Horizon 202

CFD Simulation of a Floating Wind Turbine in OpenFOAM: an FSI approach based on the actuator line and relaxation zone methods

Floating offshore wind turbines (FOWTs) have the potential to harness wind resources in deepwater, which is so far prohibitive for conventional approaches. This, however, comes at a cost: the platform’s extra degrees of freedom (DoFs) introduce complex aerodynamic and hydrodynamic behaviours. Therefore, FOWTs must be accurately modeled to reduce load uncertainties that ultimately prejudice their economic viability. This project implements a framework for the coupled, high-fidelity simulation of FOWTs in OpenFOAM. The tool is built upon two existing libraries: turbinesFoam [1] —for rotor modeling based on the actuator line method— and waves2Foam [2] —for wave-field generation and absorption based on the relaxation zone method. The multi-phase simulation uses the interFoam solver in combination with a morphing mesh technique and rigid-body model to represent the platform. The mooring restraints are computed with a quasi-steady, catenary model from waves2Foam. The turbinesFoam library, targeted at bottomfixed turbines, is modified so that it can accommodate arbitrary motions along the rigid-body DoFs. The platform-turbine FSI coupling follows a serial sub-iterating strategy based on the PIMPLE scheme. The simulation framework is built in a sequential style. First, the propagation of second-order waves in an empty tank is studied, followed by the decay oscillation of floating buoys from the experiments by Ito and Palm et al. [3, 4]. Then, the modified version of turbinesFoam is tested for the conditions from the OC6 Phase III campaign —a series of wind-tunnel tests carried out at Politecnico di Milano that analyzed the performance of a scaled 10-MW turbine under prescribed motions in pitch and surge [5]. Lastly, the coupled simulation of a 2-DoF (surge and pitch) semi-submersible FOWT under combined wind-wave conditions is achieved. The presented framework proved capable of modeling the aerodynamic performance of turbines under prescribed motion and produced plausible results for a semi-submersible FOWT under combined wind and wave conditions. Once carefully validated, this tool will have the potential to serve as a reliable technique for the advanced modeling of FOWTs

Development of numerical and data models for the support of digital twins in offshore wind engineering

Error on title page. Date of award is 2022.As offshore wind farms grow there is a continued demand for reduced costs. Maintenance costs and downtime can be reduced through greater information on the asset in relation to its operational loads and structural resistance to damage and so there is an increasing interest in digital twin technologies. Through digital twins, an operational asset can be replicated computationally, thus providing more information. Modelling these aspects requires a wide variety of models in different fields. To advance the feasibility of digital twin technology this thesis aims to develop the multi-disciplinary set of modelling domains which help form the basis of future digital twins. Throughout this work, results have been validated against operational data recorded from sensors on offshore structures. This has provided value and confidence to the results as it shows how well the mix of state-of-the art models compare to real world engineering systems. This research presents a portfolio of five research areas which have been published in a mix of peer-reviewed journal articles and conference papers. These areas are: 1) A computational fluid dynamics (CFD) model of an offshore wind farm conducted using a modified solver in the opensource software. This work implements actuator disk turbine models and uses Reynolds averaged Naiver Stokes approaches to represent the turbulence. This investigates the impact of modelling choices and demonstrates the impact of varied model parameters. The results are compared to operational site data and the modelling errors are quantified. There is good agreement between the models and site data. 2) An expansion on traditional CFD approaches through incorporating machine learning (ML). These ML models are used to approximate the results of the CFD and thereby allow for further analysis which retains the fidelity of CFD at comparatively negligible computational cost. The results are compared to operational site data and the errors at each step are quantified for validation. 3) A time-series forecasting of weather variables based on past measured data. A novel approach for forecasting time-series is developed and compared to two existing methods: Markov-Chains and Gradient Boosting. While this new method is more complex and requires more time to train, it has the desirable feature of incorporating seasonality at multiple timescales and thus providing a more representative time-series. 4) An investigation of the change in modal parameters in an offshore wind jacket structure from damages or from changing operational conditions. In this work the detailed design model of the structure from Ramboll is used. This section relates the measurable modal parameters to the operational condition through a modelling approach. 5) A study conducted using accelerometer data from an Offshore Substation located in a wind farm site. Operational data from 12 accelerometers is used to investigate the efficacy of several potential sensor layouts and therefore to quantify the consequence of placement decisions. The results of these developments are an overall improvement in the modelling approaches necessary towards the realisation of digital twins as well as useful development in each of the component areas. Both areas related to wind loading as well as structural dynamics have been related to operational data. The validation of this link between the measured and the modelled domains facilitates operators and those in maintenance in gaining more information and greater insights into the conditions of their assets.As offshore wind farms grow there is a continued demand for reduced costs. Maintenance costs and downtime can be reduced through greater information on the asset in relation to its operational loads and structural resistance to damage and so there is an increasing interest in digital twin technologies. Through digital twins, an operational asset can be replicated computationally, thus providing more information. Modelling these aspects requires a wide variety of models in different fields. To advance the feasibility of digital twin technology this thesis aims to develop the multi-disciplinary set of modelling domains which help form the basis of future digital twins. Throughout this work, results have been validated against operational data recorded from sensors on offshore structures. This has provided value and confidence to the results as it shows how well the mix of state-of-the art models compare to real world engineering systems. This research presents a portfolio of five research areas which have been published in a mix of peer-reviewed journal articles and conference papers. These areas are: 1) A computational fluid dynamics (CFD) model of an offshore wind farm conducted using a modified solver in the opensource software. This work implements actuator disk turbine models and uses Reynolds averaged Naiver Stokes approaches to represent the turbulence. This investigates the impact of modelling choices and demonstrates the impact of varied model parameters. The results are compared to operational site data and the modelling errors are quantified. There is good agreement between the models and site data. 2) An expansion on traditional CFD approaches through incorporating machine learning (ML). These ML models are used to approximate the results of the CFD and thereby allow for further analysis which retains the fidelity of CFD at comparatively negligible computational cost. The results are compared to operational site data and the errors at each step are quantified for validation. 3) A time-series forecasting of weather variables based on past measured data. A novel approach for forecasting time-series is developed and compared to two existing methods: Markov-Chains and Gradient Boosting. While this new method is more complex and requires more time to train, it has the desirable feature of incorporating seasonality at multiple timescales and thus providing a more representative time-series. 4) An investigation of the change in modal parameters in an offshore wind jacket structure from damages or from changing operational conditions. In this work the detailed design model of the structure from Ramboll is used. This section relates the measurable modal parameters to the operational condition through a modelling approach. 5) A study conducted using accelerometer data from an Offshore Substation located in a wind farm site. Operational data from 12 accelerometers is used to investigate the efficacy of several potential sensor layouts and therefore to quantify the consequence of placement decisions. The results of these developments are an overall improvement in the modelling approaches necessary towards the realisation of digital twins as well as useful development in each of the component areas. Both areas related to wind loading as well as structural dynamics have been related to operational data. The validation of this link between the measured and the modelled domains facilitates operators and those in maintenance in gaining more information and greater insights into the conditions of their assets