The deployment of wind power plants in cold climate becomes ever more attractive due to the increased air density resulting from low temperatures, the high wind speeds, and the low population density. However, the cold climate conditions bring some additional challenges as itt can easily cause wind turbine blades to freeze. The frizzing ice on blades not only increases the energy required for the rotation of blades, resulting in a reduction in the power generation, but also increases the amplitude of the blades’ vibrations, which may cause the blade to break, affecting the power generation performance of the wind turbine and poses a threat to its safe operation. Current published blade icing detection methods focus on studying the blade icing mechanism, building the model and then judging if it is iced or not. These models vary with different wind turbines and working conditions, so expertise knowledge is required. However, deep learning techniques may solve the abovementioned problem based on their excellent feature learning abilities but until now, there are only few studies on wind turbine blade icing detection based on the deep learning technology. Therefore, this paper proposes a novel blade icing detection model, named two-dimensional convolutional neural network with focal loss function (FL-2DCNN). The network takes the raw data collected by the Supervisory Control and Data Acquisition (SCADA) system as input, automatically learns the correlation between the different physical parameters in the dataset, and captures the abnormal information, in order to accurately output the detection results. However, the amount of normal data collected by SCADA systems is usually much larger than the one of blade icing fault data, leading to a serious data imbalance problem. This problem makes it difficult for the network to obtain enough features related to the blade icing fault. Therefore the focal loss function is introduced to the FL-2DCNN to solve the aforementioned data imbalanced problem. The focal loss function can effectively balance the importance of normal samples and icing fault samples, so that the network can obtain more icing-related feature information from the icing fault samples, and thus the detection ability of the network can be improved. The experimental results of the proposed FL-2DCNN based on real SCADA data of wind turbines show that the proposed FL-2DCNN can effectively solve the sample imbalance problem and has significant potential in the blade icing detection task compared with other deep learning methods.

LCOE reduction in FOWTs is heading to larger wind turbines in order to increase power production and capacity. NREL and DTU have recently developed a 15MW reference wind turbine, which can be used to validate the platform concepts for the next generation of wind turbines. Increasing the power of wind turbines leads to larger platforms due to the need to withstand the increase of the weight of the Nacelle Rotor Assembly, as well as the increase of the wind forces and pitching moment. Moreover, the larger the turbines and platforms the larger surge/sway and yaw unbalanced forces, which will need to be hold up by the mooring system. The mooring system has to be designed to balance the wind and wave forces and provide the stiffness needed to the FOWT for a proper behavior. Moreover, the mooring system has to achieve enough reliability to prevent line failure that could lead to a chain reaction within a floating wind farm, and thus huge loses. Then, a complete and detailed fatigue analysis should be performed in order to guarantee the performance of the FOWT during its service life. Within the CoReWind EU-2020 project, the Windcrete platform is upscaled to withstand the new EIA 15MW reference wind turbine. As concrete is used as a main material, the mass and inertia are larger than steel counterpart which leads to stiffer and more loaded mooring system. In this paper, the fatigue analysis of the Windcrete mooring system is assessed and compared using different methods.

The scope of this work is to investigate if and how it is possible to estimate the incident wave elevation on a floating wind turbine, with the purpose of improved control strategies. A Kalman based algorithm is proposed, which receives as input the rigid motions of the floater and estimates the wave elevation hitting the floating platform. The structure of the observer is described and the estimator is tested numerically on the OC3-Hywind platform coupled with the 5-MW reference wind turbine from NREL. Limitations to the estimation procedure are discussed. Finally the algorithm is tested on experimental data coming from a wave basin experimental campaign on a floating wind turbine model. The algorithm still needs improvements, but results are encouraging in the development of this technology.

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.

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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.

This paper investigates the efficacy and reliability of three different state-of-the-art rotation speed estimation techniques on a very large set of experimental vibration data originating from thirty offshore wind turbine gearboxes. The three methods include the multi-order probabilistic approach, the phase demodulation method based on the frequency-domain energy operator, and the multi-harmonic demodulation technique. The goal is twofold: to assess statistically the performance of present-day vibration-based rotation speed estimation techniques on challenging experimental data, and to establish indirect rotation speed estimation through vibration data as a viable alternate solution to the conventional solution involving a direct measurement with a physical device such as a tachometer or an angle encoder. The results show that while all three techniques attain satisfying results, the multi-harmonic demodulation technique produces the most accurate speed estimate for the majority of all measurements whilst also being flexible in its usage.

The relatively large motions experienced by floating wind turbines and wave energy converters pose a challenge for power cables, whose internal components provide significant bending resistance and are sensitive to deformation. The behavior and associated design considerations of power cables in these highly dynamic applications make coupled analysis relevant for design. Bending stiffness capabilities have recently been added to the lumped-mass mooring dynamics model MoorDyn to enable simulation of dynamic power cables. MoorDyn is a common modeling choice for floating wind energy simulation (often coupled with OpenFAST) and floating wave energy converter simulation (often coupled with WEC-Sim) but the model’s previous line elasticity formulation only considered axial stiffness. To properly capture the dynamics of power cables, a bending stiffness model has been added that approximates cable curvature based on the difference in tangent vectors of adjacent elements. The resulting bending moment is realized by applying forces on adjacent nodes, enabling cable modeling while leaving the underlying lumped-mass formulation unchanged. In this paper, the new bending stiffness implementation is verified in static conditions against analytical solutions and then in a dynamic power cable scenario in comparison with the commercial simulator OrcaFlex. The dynamic scenario uses prescribed motions and includes wave loadings on the cable. Results indicate correct implementation of bending stiffness and show close agreement with OrcaFlex.

This paper deals with the numerical design of a floating offshore wind turbine outdoor large-scale prototype based on the DTU 10MW. The objective of this work is to develop a numerical simulation environment for the design of an outdoor scaled prototype. The numerical model is realized coupling the preliminary designed Blue Growth Farm large-scale turbine model with a traditional floater, the OC3 spar buoy. The numerical model is used to evaluate the loads associated with the wind turbine when combined to a floating foundation, with the focus on the coupling between the dynamics of the control system and the one of the floating platform. In addition to this, also the consistency of loads on crucial turbine components is an interesting test bench for the evaluation of the dynamical effects and drives the final design of the physical model.

Floating offshore platforms motions induced by currents are quite complex phenomena, in general. In particular, VIM, Vortex-Induced Motion, is a type often encountered in platforms with circular columns. Recently, VIM has been observed in towing tank tests with a small-scale model of a Floating Offshore Wind Turbine (FOWT), the OC4 Phase II floater, a 3+1 columns platform. The present paper proposes a reduced-order mathematical model (ROM) to assess VIM of a FOWT. The ROM is derived on the horizontal plane, including yaw motions and nonlinear mooring forces. Current forces are represented through ‘wake variables’, adapting phenomenological models firstly used for VIM of mono-column platforms. The ROM is built upon a set of eleven generalized coordinates, three for the rigid body motion on the horizontal plane and a pair of wake variables for each column, resulting in a system of eleven nonlinear second-order ODEs. The pairs of wake variables obey van der Pol equations, and use hydrodynamic coefficients and parameters obtained from previous experiments with small draught cylinders. Hydro-dynamic interferences among columns or heave plates effects on the flow are not considered, for simplicity. The validity of the proposed model is assessed having the mentioned small-scale experimental campaign as a case study. The simulations are carried out at three different current incidence angles, 0, 90 and 180 degrees, spanning a large range of reduced velocities. The simulations reproduce well the oscillations observed in the experimental tests. A good agreement in transverse oscillations is found, including lock-in regions. The simulations also depict a possibly important phenomenon: a resonant yaw motion emerging at high reduced velocities.

Large diameter monopiles are widely used for the installation of offshore wind turbines in relatively shallow water depth applications. Monopiles are considered as a cost and time-efficient foundation solution, taking advantage of the experience gained from the Oil&Gas industry. Due to their relatively large diameter, monopiles are susceptible to scour effects that threaten their structural integrity and necessitate the application of high-cost mitigation measures. The mitigation measures are also associated with heavy environmental impact at the local subsea environment, as well as increased CO 2 emissions for their application. The present work examines the effect of scour on the structural integrity of monopiles using integrated simulation models that combine advanced finite element analysis (FEA) and computational fluid dynamics (CFD) techniques. A typical monopile geometry and North Sea representative metocean data are used in a case study. In this example analysis emphasis is given on the structural integrity deterioration resulting from the scour development over time. Advanced techniques are employed to simulate the soil-pile interaction. Furthermore, fluid-structure interaction effects due to the water flow around the monopile are examined using CFD techniques. Finally, fatigue life predictions of sensitive areas are conducted based on the “S-N methodology” and are enhanced by employing crack-growth analyses based on linear elastic fracture mechanics principles.

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.

To help the selection of suitable sites for development of offshore wind projects in the US on the coasts of California, Oregon and Hawaii, the Bureau of Ocean Energy Management (BOEM) funded this study to assess the potential geo-hazards in this region. First, a comprehensive review of potential threats to the sites based on historic events is provided. The geospatial indexing for the call areas are then calculated based on weights associated with inputs, consisting of: sea floor slope, soil type, and seismicity (peak ground acceleration data of 500 year event). Finally, suitability indices are provided for each region. To perform a suitability analysis using geospatial indexing, all input factors are first standardized into a common scale, then a weighted overlay function is applied. Each of the criteria in the analysis is multiplied by the weights defined based on their importance in the region and then added together and suitability maps for each lease block are developed. Comprehensive maps of geohazards and geological data, suitability index maps and suitability rankings for the area of interest are being generated and presented online. This paper focuses on the floating windfarm call areas offshore California, including Humboldt, Morro Bay and Diablo Canyon, and presents the new approach for evaluating, integrating and indexing geospatial geohazard data for offshore windfarms, and the state-of-the-art suitability analysis approach. This new method can be also beneficial in the other parts of the world (e.g. East Asia), and similar concept can be implemented to evaluate the suitability of sites, based on the hazards in the area of interest.

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.

Utilization of Computational Fluid Dynamics (CFD) codes to perform hydrodynamic analysis of Floating Offshore Wind Turbines (FOWTs) is increasing recently. However, verification studies of the simulations that quantifying numerical uncertainties and permitting a detailed validation in a next phase is often disregarded. In this work, a verification study of CFD simulations of a semi-submersible FOWT design under regular waves is performed. To accomplish this goal, Response Amplitude Operators (RAOs) are derived from the computational results of the heave, surge and pitch motions. Four grids with different grid sizes with a constant refinement ratio are generated for verification of spatial convergence. Three different time increments are paired with each grid for verification of temporal convergence. The verification study is performed by estimation of the numerical errors and uncertainties using procedures proposed by Eca and Hoekstra [1].

The goal of the new Wind Energy with Integrated Servo-control (WEIS) toolset under development is to provide the offshore wind industry and research communities with an open-source, user-friendly, flexible tool to enable true controls co-design (CCD) of the physical design of a floating offshore wind turbine together with the controller. WEIS will use a multifidelity library of models built on the foundations of WISDEM ® and OpenFAST formerly known as FAST). This paper presents the WEIS development plan, including the functional requirements of WEIS (including improvements to WISDEM and OpenFAST) together with associated rationale for their establishment and a qualitative description of the modeling approaches that will be implemented to address these functional requirements. The development of WEIS is a project under the Aerodynamic Turbines Lighter and Afloat with Nautical Technologies and Integrated Servo-control program funded by the U.S. Department of Energy Advanced Research Projects Agency – Energy.

The energy ship concept has been proposed as an alternative wind power conversion system to harvest offshore wind energy. Energy ships are ships propelled by the wind and which generate electricity by means of water turbines attached underneath their hull, The generated electricity is stored on-board (batteries, hydrogen, etc.) It has been shown that energy ships deployed far-offshore in the North Atlantic Ocean may achieve capacity factors over 80% using weather-routing. The present paper complements this research by investigating the capacity factors of energy ships harvesting wind power in the near-shore. Two case studies are considered: the French islands of Saint-Pierre et-Miquelon, near Canada, and Ile de Sein, near metropolitan France. The methodology is as follows. First, the design of the energy ship considered in this study is presented. It was developed using an in-house Velocity, and Power Performance Program (VPPP) developed at LHEEA. The velocity and power production polar plots of the ship were used as input to a modified version of the weather-routing software QtVlm. This software was then used for capacity factor optimization using 10m altitude wind data analysis which was extracted from the ERA-Interim dataset provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). Three years (2015, 2016, and 2017) data are considered. The results show that average capacity factors of approximately 40% and 40% can be achieved at Ile de Sein and Saint-Pierre-et-Miquelon with considered energy ship design.

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.