The present study is aimed at investigating the turbulent wind effect on FOWT through the usage of a high-fidelity computational fluid dynamics (CFD) method. This method is believed to resolve the wind field, giving us a more in-depth examination into the aerodynamics of FOWT. The work is built upon our previous studies on the modelling of a coupled aero-hydro-mooring FOWT system under regular wave and uniform wind. In the present study, we replaced the previously uniform wind with a temporal and spatial variable turbulent wind field using a time-varying spectrum. The turbulent wind is generated with Mann’s wind turbulence model while the Von Karman wind spectrum is used to represent wind turbulence. The present study shows that when turbulent wind is present, there may be fluctuations of the rotor thrust and power outputs, causing the non-uniform wake region. Despite this, both the dynamic motions and the mooring tensions of the floater are not significantly influenced by the wind turbulence under the present inflow wind conditions.

As of now, only a small number of offshore foundations, related to offshore wind energy, were decommissioned in Europe. With a diameter up to nine meter, an embedment of about 40 meter and a set up effect over 25 years, the necessary force to pull the pile out of the seabed can be assumed, if at all determinable, to be enormous. The piles that were decommissioned were cut beneath the mud line, which leaves approximately one third of the foundation permanently in the seabed. Different methods and techniques for a complete removal of offshore pile foundation are currently investigated within the project DeCoMP. Vibratory extraction aims for a reduction of the pile skin friction by creating a layer of less density between the pile shaft and pending soil. During the design and planning process for vibratory installation or extraction a drivability prediction is a key element. In order to identify and characterize soil parameter for the numerical simulation of a drivability prediction, large-scale tests are performed by the Institute of Geomechanics and Geotechnics of the Technische Universität Braunschweig (IGG-TUBS) [1]. In this paper first results of pilot tests with two vibrators are presented and key elements such as crane uplift, frequency and acceleration displayed.

A new fatigue lifetime assessment approach for offshore jacket structures is presented. It combines a previously developed numerical framework for automated determination of stress concentration factors in tubular joints and a multidimensional finite element modelling approach. The approach is explained based on a case study of an OC4 type offshore jacket. To determine the fatigue life, a directional wave spectrum is combined with the JONSWAP spectrum. The fatigue life of the jacket is assessed for two different sea states. Based on the fatigue analysis the most fatigue critical wave direction is identified. The hot spot stresses in one of the most critical joints are determined and compared to stresses obtained with the Efthymiou equations. The shortcomings of these equations are highlighted and it is shown how the numerical framework can be used to improve the current fatigue design philosophy for offshore jackets which relies on the Efthymiou equations for stress concentration factors in the welded tubular joints.

Offshore wind structures are being designed for seismically active areas. Load calculations can be performed in an integrated way or using a superelement approach. This study aims to demonstrate that these two approaches can give equivalent results when earthquake excitations are included as an applied load in the superelement approach. Models of a generic 7MW wind turbine on a jacket support structure are defined in aeroelastic code Bladed and strength assessment software Sesam. Analysis is performed on the stand-alone jacket and support structure, and also including the rotor nacelle assembly. Load conditions of earthquakes, waves and wind are considered. Time domain results show that the superelement and integrated methods for modelling earthquake loading are equivalent in terms of the motions and loads at key points of the support structure. When the tower and RNA are included, an excellent match is seen for all studied variables: interface and tower top motions, loads at the interface and in the jacket, and displacements of jacket nodes. Analysis of the stand-alone jacket highlights differences in the approaches with regard to coupling of the hydrodynamic loading to the earthquake motion, which is absent in the superelement method. However, this does not appear to be significant for realistic cases including the tower and RNA.

Offshore wind turbines are poised to become a vital part of the global energy landscape — particularly the floating types which give access to a much greater wind power resource. The design possibilities for floating turbines are so different from onshore and offshore fixed-bottom turbines, that a cost-reducing re-imagining may be justified. Apart from the expense of an offshore transmission cable and substation, the costs of hardware for offshore bottom-fixed wind turbines (CAPEX) are roughly twice as much as for onshore. Much of the extra cost can be attributed to the mass of the above- and underwater support and expensive installation. In this paper, we reconsider two aspects of present-day offshore turbines: (a) maintaining a land-turbine architecture (a slender tower with the rotor cantilevered from a yawing, equipment-filled nacelle); (b) seeking to minimize wave-induced motion and loads of the above-water plant. Our aim is an offshore floating design that is potentially less expensive than offshore fixed-bottom units. We outline the preliminary structural analyses that underlie a design focused on weight, cost reduction, and ease of manufacturing. This includes lattice towers, tubular hub and axle, and needle-roller bearings. The biggest concerns about the proposed lightweight system involve motions and forces induced by waves. Shallow-draft floats will follow the waves, leading to greater rotor translation and precession; and lattice towers may be subject to impact loads, ice buildup, and fouling. We present analysis of some of these motions and forces, along with resulting estimates for required structural weight as a preliminary investigation into the feasibility of this lightweight concept.

It has been observed that, although submarine power cables have a critical role to wind power arrays and power export to shore, they are often overlooked at early stages of projects and oversimplified during late stages. This leads to lack of attention given during cable design and planning, as well as pressured schedules during manufacturing, testing and installation. The significant number of incidents attributed to offshore submarine cables during construction has increased overall project risk, lowered system average power availability and increased insurance costs. Lack of proper routing can also result in an inability to maintain asset integrity for the project design life. Despite the attention that submarine power cables have received over the past few years, the number and cost of incidents does not appear to be decreasing. A comparison can be made between offshore HVAC and HVDC cables used for wind power and offshore umbilicals and MV cables used in the oil and gas sector. These umbilicals are often similar in weight, size and bending stiffness, and have similar design, manufacturing, routing and installation challenges, but with a fraction of the incidents observed with offshore wind array and export cables. An additional caveat is that the offshore oil and gas sector has achieved a reliable track record while installing and maintaining these umbilicals and cables in fully dynamic conditions (ultra-deep water) as well static conditions. One primary difference between how the oil and gas sector executes these systems are design, planning and specification from an early stage of the project. Significant attention is given at an early stage to quality control, including offshore routing and umbilical testing specifically to avoid incidents resulting in umbilical damage due to the tension and crushing forces during installation as well as ambient seawater and seabed interaction. Management of these risks are documented, and optimal mitigation strategies are implemented early in the design phase. This paper will discuss the types of incidents which have been observed during construction and installation of submarine HVAC/HVDC cables in the wind power sector and how they could have been prevented by normal practices of the offshore oil/gas sector from early design and planning all the way to installation and commissioning.

Floating wind is now entering a commercial-stage, and there are a significant number of commercial projects in countries like France, Japan, UK and Portugal. A floating wind project is complex and has many interdependencies and interfaces. During all stages of the project several participants are expected to use a numerical model of the whole system and not only the part the participant has to design. Examples of this are the mooring and floater designer requiring a coupled model of the whole system including also the wind turbine, the operations team requiring a model of the system to plan towing and operations. All these stakeholders require a coupled model where the hydrodynamics, aerodynamics and structural physics of the system are captured with different levels of accuracy. In this paper, we will concentrate on a simplified model for the aerodynamic loading of the turbine in idling and standstill conditions that can be easily implemented in a simulation tool used for floater, mooring and marine operations studies. The method consists of using a subset of simulations at constant wind speed (ideally close to the wind speed required for the simulations) run on a detailed turbine model on a rigid tower and fixed foundation — normally run by the turbine designer. A proxy to the aerodynamic loads on the rotor and nacelle (RNA) is to take the horizontal yaw bearing loads. The process is then repeated for a range of nacelle yaw misalignments (for example every 15° for 360°). A look-up table with the horizontal yaw bearing load for the range of wind-rotor misalignments investigated is created. The simplified model of the aerodynamic loads on the RNA consists of a fixed blade (or wing) segment located at the hub, where aerodynamic drag and lift coefficients can be specified. Using the look-up tables created using the detailed turbine model, drag and lift coefficients are estimated as a function of the angle between the rotor and the wind direction. This representation of the aerodynamic loading on the RNA was then verified against full-field turbulent wind simulations in fixed and floating conditions using a multi-megawatt commercial turbine. The results for the parameters concerning the floater, mooring and marine operations design were monitored (e.g. tower bottom loads, offsets, pitch, mooring tensions) for extreme conditions and the errors introduced by this simplified rotor are generally lower than 4%. This illustrates that this simplified representation of the turbine can be used by the various parties of the project during the early stages of the design, particularly when knowing the loading within the RNA and on higher sections of the tower is not important.

The wind field at sea is of considerable interest to identify suitable sites and for designing offshore wind energy production facility. However, the reliability of wind information suffers from the relative scarcity of offshore wind measurements to validate the wind models used in assessments. This paper presents a comparison of the publicly available NOAA-CFSR global re-analysis data set against offshore wind measurement collected in West Africa, Mediterranean Sea, Barents Sea and NW Australia, with the goal to investigate — in widely different meteorological conditions — the overall model reliability, in term of statistical indices of performance: Moreover, an attempt has been made to ascertain the representative averaging duration of model wind and the reliability of engineering formulas used to correlate wind of different averaging durations.

The natural surge and pitch frequencies of semisubmersible offshore wind platforms are typically designed to be below the wave frequencies to avoid direct excitation. However, surge or pitch resonance can be excited by the nonlinear low-frequency loads generated by irregular incident waves. Second-order potential-flow models with added Morison drag have been found to underpredict this low-frequency excitation and response. As part of the OC6 project 1 , the authors performed computational fluid dynamics (CFD) simulations to enable a better understanding of the low-frequency loads and the limitations of lower-fidelity models. The focus of this paper is to set up a computationally cost-effective CFD simulation of a fixed semisubmersible platform to investigate nonlinear difference-frequency loads and establish the corresponding uncertainty in the results. Because of the high computing cost, CFD simulations of irregular waves can be challenging. Instead, simulations were performed with bichromatic waves having a shorter repeat period. A preliminary comparison with quadratic transfer functions from second-order potential-flow theory shows that CFD models consistently predict higher nonlinear wave loads at the difference frequency, likely because of flow separation and viscous drag not accounted for in potential-flow theory.

We study deep water ocean wind-driven waves in strait, with wind directed orthogonally to the shore, through exact Hassel-mann equation. The strait has “dissipative” shores, there is no any reflection from the coast lines. We show that the wave turbulence evolution can be split in time into two different regimes. During the first regime, the waves propagate along the wind, and the wind-driven sea can be described by the self-similar solutions of Hasselmann equation. The second regime starts later in time, after significant enough wave energy accumulation at the down-wind boundary. Since this moment the ensemble of waves propagating against the wind starts its formation. Also, orthogonal to the wind waves, propagating along the strait, start to appear. The wave system eventually reaches asymptotic stationary state in time, consisting of two co-existing states: the first, self-similar wave ensemble, propagating with the wind, and the second – quasi-monochromatic waves, propagating almost orthogonally to the wind direction, and tending to slant against the wind at the angle of 15° closer to the wave turbulence origination shore line. Those “secondary waves” appear only due to intensive nonlinear wave-wave interaction. The total wave energy exceeds its “expected value” approximately by the factor of two, with respect to estimated in the absence of the shores. It is expected that in the reflective shores presence this amplification will grow essentially. We propose to call this “secondary” laser-like N onlinear O cean W aves A mplification mechanism by the acronym NOWA .

Offshore wind is increasingly utilised as a renewable energy source. A growing number of bottom fixed wind turbines installed offshore are supported by suction caisson foundations. The suction-assisted installation remains a source of uncertainty towards the in-service performance due to the unknown post-installation soil plug state. Cone penetration tests within the suction caisson can help to improve the reliability. Therefore, cone penetration tests were employed in centrifuge tests to investigate the plug state in a previously installed suction caisson. However, the performance of a cone penetration test in a small-scale experiment is connected to uncertainties: A relatively large diameter device is required to conduct the cone penetration test — especially in a centrifuge test. Different finite element models are developed in order to visualise and investigate a cone penetration test inside a suction caisson. The numerical analysis results are validated through the back-calculation of centrifuge cone penetration tests. The results of the simulated cone penetration tests inside a suction caisson are evaluated and compared to the centrifuge experiments. This investigation reinforces the scope of the centrifuge experiments and emphasises a considerable effect of the pressure transferral through the caisson lid in the soil plug state. Hence, the results of this study reduce existing uncertainties regarding possible suction installation effects on the in-service performance of caisson foundations.

Subsea export power transmission cables for offshore wind farms are being installed more extensively year-on-year due to the increasing demands for power output from renewable sources. With the increasing number of installations, the number of cable failures during installation has increased. One failure mode involves the temporary or permanent radial deformation of armour wires otherwise known as armour wire bird caging which occurs from a combination of twist, bending and compression loads which build up in a cable. This failure mode can lead to significant remediation costs and schedule delays for projects affected. In this paper, the authors present a method for predicting armour wire bird caging for generic installation configurations based on a review of the root causes from several historical bird caging failure instances. Various numerical models and analyses which simulate the installation conditions are described. The models simulate key response mechanisms including bending-induced twist and inter-layer separation within a cable. Cable loading conditions are compared with cable bird caging limits and the parameters which influence the onset of bird caging are identified. Based on a range of sensitivity analyses, handling curves to assist with installation are developed and a full-scale test validation programme is proposed. This work was performed for a project which received financial and technical support from The Carbon Trust’s Offshore Wind Accelerator (OWA), a collaborative R&D programme funded by nine leading offshore wind developers (EnBW, Equinor, Innogy, Ørsted, RWE, SPR, Shell, SSE, Vattenfall) and the Scottish Government.

Leading edge erosion (LEE) of a wind turbine blade (WTB) is a complex phenomenon that contributes to high operation and maintenance costs. The impact between rain droplets and rotating blades exerts cyclic fatigue stresses on the leading edge — causing progressive material loss and reduced aerodynamic performance. One of the most important parameters for erosion modelling and damage prediction is the relative impact velocity between rain droplets and rotating blade and depends upon the environmental conditions. The environmental condition, in general, could vary for onshore and offshore wind turbines (OWTs) — for instance, the presence of wave-induced loads along with less turbulent wind and varying rainfall conditions in the offshore environment. The present paper tries to provide guidelines whether all these parameters need to be included for LEE modelling. Aero-hydro-servo-elastic simulations are carried out for a rotating blade based on the NREL 5 MW turbine by considering realistic environmental conditions for a land-based wind turbine and monopile-supported OWT. Further, the impact velocities and erosion damage rate, evaluated using a surface fatigue model, are analysed and compared for different environmental conditions. It is found that rainfall intensity and turbulence intensity influences the impact velocity minorly, however, has a substantial effect on the overall erosion damage rate. For instance, for the investigated load cases, an 8% increase in the impact velocity is observed when the turbulence intensity increases from 6% to 26%, which indicates an increase of erosion damage rate by more than 40%. Furthermore, no substantial influence is found due to the effects of wave-induced loads on the wind turbine.

The numerical fatigue analysis of floating offshore wind turbines (FOWTs) must account for the environmental loading over a typical design life of 25 years, and the stochastic nature of wind and waves is represented by design load cases (DLCs). In this statistical approach, combinations of wind speeds and directions are associated with different sea states, commonly defined via simplified wave spectra (Pierson-Moskowitz, JONSWAP), and their probability of occurrence is identified based on past observations. However, little is known about the difference between discretizing the wind/wave direction bins into (e.g.) 10deg bins rather than 30deg bins, and the impact it has on FOWT analyses. In addition, there is an interest in identifying the parameters that best represent real sea states (significant wave height, peak period) and wind fields (profile, turbulence) in lumped load cases. In this context, the aim of this work is to better understand the uncertainties associated to wind/wave direction bin size and to the use of metocean parameters as opposed to real wind and sea state conditions. A computational model was developed in order to couple offshore wind turbine models with realistic numerical metocean models, referred to as numerical prototype due to the highly realistic wind/wave conditions in which it operates. This method allows the virtual installation of FOWTs anywhere within a considered spatial domain (e.g. the Mediterranean Sea or the North Sea) and their behaviour to be evaluated in measured wind and modelled wave conditions. The work presented in this paper compares the long-term dynamic behaviour of a tension-leg platform (TLP) FOWT design subject to the numerical prototype and to lumped load cases with different direction bin sizes. Different approaches to representing the wind filed are also investigated, and the modelling choices that have the greatest impact on the fidelity of lumped load cases are identified. The fatigue analysis suggests that 30deg direction bins are sufficient to reliably represent long-term wind/wave conditions, while the use of a constant surface roughness length (as suggested by the IEC standards) seems to significantly overestimate the cumulated damage on the tower of the FOWT.

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.

Floating offshore wind turbines (FOWTs) are considered as a promising technology for installing turbines in deeper sea sites. One of the difficulties is that the maintenance cost tends to be high because of the limited accessibility to the site. Therefore remote monitoring and estimation of various physical variables of a wind turbine in real-time is important for health assessment and fault diagnosis before a critical failure occurs. One promising way to do this is to have a wind turbine model in computer and estimate the various physical variables of the turbine in real-time by using the available noisy sensor measurements. In this paper, we consider a 5MW FOWT and design a Kalman filter to deal with the estimation problem of the turbine state variables and effective wind speed. A blade pitch controller that uses the estimated state feedback and wind feedforward is also designed. The proposed estimation and control scheme is evaluated using the turbine simulation code, FAST, and results show that the state variables are well estimated except for the blade flapwise moment.

The simplified numerical models commonly employed for the pre-design of floaters for offshore wind only include linear wave loads, due to the higher computational effort required by second-order methods. Second-order hydrodynamics, on the other hand, need to be considered from an early stage, since they cause resonance of the moored structure. In the present study, we introduce a new method to include second-order inviscid hydrodynamic loads at a computational cost similar to linear loads. We compare the accelerated method to standard second-order diffraction theory and to second-order Rainey forcing with Sharma & Dean wave kinematics. The comparison, based on the loads and response of a spar floating wind turbine in surge and pitch, is carried out for three different sea states. We find that a good prediction of the second-order resonant response can be obtained with the accelerated method for medium and severe sea states, while the match is not as good for the mild sea state. The accelerated method is between 400 and 850 times faster than commonly used second-order approaches, for an 1-hour realization of a given sea state. This speed up allows the application of the load model in the floater pre-design, where efficient numerical models are the key to achieve optimal designs and the consequent reduction in the cost of the floater.

The fully coupled analysis model of a jacket offshore wind turbine (OWT) is established based on the governing equation of motion of the structure which is derived in accordance with the blade element momentum theory (BEM), Morison formula and theories of structural dynamics. The dynamic characteristics and structural responses of the jacket OWT under the different combined seismic cases are analyzed. It can be seen that the interactions of the wind and wave loads are non-negligible in the seismic analysis of an OWT, and the abundant dominant frequencies of the responses of the support system under the combined seismic cases are discovered. Meanwhile, the passive control method of tuned mass dampers (TMD) is applied to the support system of the OWT under the earthquakes, and the influence of the TMD parameters on the reduction of the responses of the support system are investigated. Furthermore, according to the reduction of the structural responses, the suggestions for the design of a TMD under the combined seismic cases for the bottom fixed OWT are summarized.

During single blade installation in offshore wind farms, relative motion between nacelle and blade root due to wind and wave excitation pose a significant challenge. Wave excitation can be modelled considerably well by employing state-of-the-art simulation tools and can, therefore, be included in installation planning. Other phenomena, such as flow-induced vibrations are hard to capture and hence challenging to account for when defining installation procedures and limitations. Here, we present measurements conducted during the installation of an offshore wind farm consisting of multi-megawatt turbines installed on monopile foundations in the North Sea. A custom-built sensor capturing linear & angular acceleration and GPS-data was deployed atop the nacelle. Both partially and fully assembled turbines displayed complex oscillation orbits, swiftly changing amplitude and direction. Mean nacelle deflection correlated strongly with significant wave height as well as mean wind speed. As wind speed and significant wave height showed a strong correlation as well, it is difficult to discern which load drives the observed relative motions. While wind loads are significantly smaller than wave loads on partially assembled turbines under installation conditions, additional momentum induced by vortex shedding may prove sufficient to cause the observed effects.

With wind turbine blades becoming longer and slender, the influence of structural deformation on the aerodynamic performance of wind turbine cannot be ignored. In the present work, the actuator line technique that simplifies the wind turbine blades into virtual actual lines is utilized to simulate the aerodynamic responses of wind turbine and capture downstream wake characteristics. Moreover, the structural model based on a two-node, four degree-of-freedom (DOF) beam element is adopted for the deformation calculation of the wind turbine blades. By combing the actuator line technique and linear finite element theory, the aeroelastic simulations for the wind turbine blades can be achieved. The aeroelastic responses of NREL-5MW wind turbine under uniform wind inflow condition with different wind speeds are investigated. The aerodynamic loads, turbine wake field, blade tip deformations and blade root bending moments are analyzed to explore the influence of blade structural responses on the performance of the wind turbine. It is found that the power output of the wind turbine decreases when the blade deformation is taken into account. Significant asymmetrical phenomenon of the wake velocity is captured due to the deformation of the wind turbine blades.