Cost Of Offshore Wind Energy Research Articles

Study of a monopile with pre-tensioned tethers for offshore wind turbines at deeper waters

Currently, monopiles are the industry's first choice to support offshore wind turbines (OWTs) due to the complexities associated with fabrication, transportation and installation of various foundation solutions. These are, however, considered suitable for water depths up to ∼30–35 m, beyond which they become massive and complex to handle. Increasing the application range of monopiles without making them too large by adopting some suitable means may significantly impact the rapid growth of OWTs at intermediate water depths (∼50 m), where existing solutions such as jackets or concrete gravity foundations involve large capital expenditure. In this work, a novel approach of utilising pre-tensioned tethers along with a 6.0 m wide monopile was explored for 50 m water depth. Under an extreme load case, the stress ratio was found to be reduced from 1.17 to 0.44 when pre-tensioned tethers are used. Similarly, it was found that by adjusting parameters associated with the proposed concept, fatigue damage can be brought down from a very high value to 0.63 or less. The results thus show that the proposed approach can open new avenues to bring competitiveness to the cost of offshore wind energy at intermediate water depths.

Comparative dynamic analysis of two-rotor wind turbine on spar-type, semi-submersible, and tension-leg floating platforms

Multi-rotor floating offshore wind turbines have been recently proposed as an innovative technology to further reduce the cost of offshore wind energy. Even though examples of commercial prototypes are present, the literature lacks studies on the dynamic performance of such systems. This work presents a comparative analysis of a two-rotor wind turbine concept mounted on spar-type, semi-submersible, and tension-leg platforms. Their short-term performance is assessed by considering six different load cases considering directionally congruent turbulent wind profiles and irregular sea states. The analysis is carried out through an in-house fully-coupled code developed in Modelica. AeroDyn v15 within FAST v8 by NREL is coupled to the Modelica code to achieve blade-element momentum capabilities. Results indicate that platform yaw motion is an important dynamic mode of the systems, particularly for the spar configuration. Stiffer station-keeping lines and longer fairlead distance to the platform centerline reduce significantly yaw motion, as in the case of the semi-submersible and tension-leg configurations. Large tower base bending moment standard deviations and the associated concentration of energy at the platform heave and pitch motion frequencies indicate an increased risk for fatigue damage for the TLP configuration, especially at above-rated wind speeds. Moreover, large tendon loads can pose concerns in terms of fatigue and limit state performance. Large mean platform pitch angle and yaw standard deviation contribute to the reduction of electric power output quality. Extreme storm conditions greatly increase the response standard deviation, especially for the semi-submersible configuration.

Floating offshore wind turbine fault diagnosis via regularized dynamic canonical correlation and fisher discriminant analysis

Over the past decades, Floating Offshore Wind Turbine (FOWT) has gained increasing attention in wind engineering due to the rapidly growing energy demands. However, difficulties in turbine maintenance will increase due to the harsh operational conditions. Fault diagnosis techniques play a crucial role to enhance the reliability of FOWTs and reduce the cost of offshore wind energy. In this paper, a novel data-driven fault diagnosis method using regularized dynamic canonical correlation analysis (RDCCA) and Fisher discriminant analysis (FDA) is proposed for FOWTs. Specifically, to overcome the collinearity problem that exists in measured process data, dynamic canonical correlation analysis with a regularization scheme, is developed to exploit the relationship between input and output signals. Then, the residual signals are generated from the established RDCCA model for fault detection. To further classify the fault type, an FDA model is trained from the residual signals of different training faulty data sets. Simulations on a FOWT baseline model based on the widely used National Renewable Energy Laboratory FAST simulator are carried out to demonstrate the feasibility and efficacy of the proposed fault detection and classification method. Results have shown many salient features of the proposed method with potential applications in FOWTs.

Post‐installation adaptation of offshore wind turbine controls

AbstractThe cost of offshore wind energy can be reduced by incorporating control strategies to reduce the support structures' load effects into the structural design process. While effective in reducing the cost of support structures, load‐reducing controls produce potentially costly side effects in other wind turbine components and subsystems. This paper proposes a methodology to mitigate these side effects at the wind farm level. The interaction between the foundation and the surrounding soil is a major source of uncertainty in estimating the safety margins of support structures. The safety margins are generally closely correlated with the modal properties (natural frequencies, damping ratios). This admits the possibility of using modal identification techniques to reassess the structural safety after installing and commissioning the wind farm. Since design standards require conservative design margins, the post‐installation safety assessment is likely to reveal better than expected structural safety performance. Thus, if load‐reducing controls have been adopted in the structural design process, it is likely permissible to reduce the use of these during actual operation. Here, the probabilistic outcome of such a two‐stage controls adaptation is analyzed. The analysis considers the structural design of a 10 MW monopile offshore wind turbine under uncertainty in the site‐specific soil conditions. Two control strategies are considered in separate analyses: (a) tower feedback control to increase the support structure's fatigue life and (b) peak shaving to increase the support structure's serviceability capacity. The results show that a post‐installation adaptation can reduce the farm‐level side‐effects of load‐reducing controls by up to an order of magnitude.

Design of a Superconducting DC Demonstrator for Wind Generators

The trend to reduce the cost of offshore wind energy is to develop large wind turbines. Turbine sizes of 6–8 MW have already been seen in the wind market. Even larger turbine sizes are managing to pave their way from studies to market. Due to the high current-carrying capability and no dc losses of the superconductors, superior power to weight/volume ratio of a superconducting generator can be achieved. Hence, in this paper, a superconducting dc generator is proposed. In this generator, the superconducting coils are used mainly to generate the main air gap flux density and silicon steel iron cores are employed by the stator and rotor. In order to study the feasibility of the proposed generator and to design the generator, the critical current of the superconducting material and electromagnetic properties of the silicon steel iron core are first measured. Then, a demonstrator of the superconducting dc generator is designed through the developed process, which integrates the analytical method into the particle swarm optimization. As a preliminary step toward the demonstrator, a test for a key component of the demonstrator, the superconducting coil, has been made.

Life cycle cost-benefit analysis of offshore wind energy under the climatic conditions in Southeast Asia – Setting the bottom-line for deployment

Offshore renewable energy resources are options to address a country’s energy and climate objectives without having to occupy vast amount of land space. Offshore wind power production, among the various offshore renewable options, has gained popularity albeit its dependence on the deployment location’s wind conditions, technical challenge in construction and grid connection, and potentially high capital cost. The wind conditions in Southeast Asia are generally much less favourable as compared to other parts of the world, but several economies in this region consider offshore wind energy as a long term solution to decarbonize the electricity sector and to diversify the source of electricity. There is thus a need to evaluate the true benefits of offshore wind energy under the region’s suboptimal climatic conditions. In response, this study employs the life cycle analysis approach to conduct a cost-benefit analysis of offshore wind energy in the context of Southeast Asia. Findings from study suggest that the cost of offshore wind energy remains high at the moment for Southeast Asia. The findings further contribute to the debate on offshore wind energy by setting the bottom-line for deployment under unfavourable climatic conditions in the tropical region and world-wide. However, offshore wind farm could still represent a plausible option through technology innovation in wind turbine designs and reductions in the cost of wind turbine and submarine power cable. As suggested in this study, there is potential for the cost of offshore wind energy to reach parity against onshore renewables such as solar energy in the context of Southeast Asia.

Integrated structural optimisation of offshore wind turbine support structures based on finite element analysis and genetic algorithm

By accounting for almost 25% of the capital cost of an OWT (offshore wind turbine), optimisation of support structures provides an efficient way to reduce the currently high cost of offshore wind energy. In this paper, a structural optimisation model for OWT support structures has been developed based on a coupled parametric FEA (Finite Element Analysis) and GA (Genetic Algorithm), minimising the mass of the support structure under multi-criteria constraints. Contrary to existing optimisation models for OWT support structures, the proposed model is an integrated structural optimisation model, which optimises the components of the support structure (i.e. tower, transition piece, grout and monopile) simultaneously. The outer diameters and section thicknesses along the support structure are chosen as design variables. A set of constraints based on multi-criteria design assessment is applied according to standard requirements, which includes vibration, stress, deformation, buckling, fatigue and design variable constraints. The model has been applied to the NREL (National Renewable Energy Laboratory) 5MW OWT on an OC3 (Offshore Code Comparison Collaboration) monopile. The results of the application of the integrated optimisation methodology show a 19.8% reduction in the global mass of the support structure while satisfying all the design constraints. It is demonstrated that the proposed structural optimisation model is capable of effectively and accurately determining the optimal design of OWT support structures, which significantly improves their design efficiency.

Improved Methodology of Weather Window Prediction for Offshore Operations Based on Probabilities of Operation Failure

The offshore wind industry is building and planning new wind farms further offshore due to increasing demand on sustainable energy production and already occupied prime resource locations closer to shore. Costs of operation and maintenance, transport and installation of offshore wind turbines already contribute significantly to the cost of produced electricity and will continue to increase, due to moving further offshore, if the current techniques of predicting offshore wind farm accessibility are to stay the same. The majority of offshore operations are carried out by specialized ships that must be hired for the duration of the operation. Therefore, offshore wind farm accessibility and costs of offshore activities are primarily driven by the expected number of operational hours offshore and waiting times for weather windows, suitable for offshore operations. Having more reliable weather window estimates would result in better wind farm accessibility predictions and, as a consequence, potentially reduce the cost of offshore wind energy. This paper presents an updated methodology of weather window prediction that uses physical offshore vessel and equipment responses to establish the expected probabilities of operation failure, which, in turn, can be compared to maximum allowable probability of failure to obtain weather windows suitable for operation. Two case studies were performed to evaluate the feasibility of the improved methodology, and the results indicated that it produced consistent and improved results. In fact, the updated methodology predicts 57% and 47% more operational hours during the test period when compared to standard alpha-factor and the original methodologies.

Iterative tuning of feedforward IPC for two-bladed wind turbines

At present, the cost of offshore wind energy does not meet the level of onshore wind and fossil-based energy sources. One way to extend the turbine lifetime, and thus reduce cost, is by reduction of the fatigue loads of blades and other turbine parts using Individual Pitch Control (IPC). This type of control, which is generally implemented by feedback control using the MultiBlade Coordinate transformation on blade load measurement signals, is capable of mitigating the most dominant periodic loads. The main goal of this article is to develop a self-optimizing feedforward IPC strategy for a two-bladed wind turbine to reduce actuator duty cycle and reduce the dependency on blade load measurement signals. The approach uses blade load measurement data only initially for tuning of the feedforward controller, which is scheduled on the rotor azimuth angle and wind speed. The feedforward strategy will be compared to the feedback implementation in terms of load alleviation capabilities and actuator duty cycle. Results show that the implementation is capable of learning the optimal feedforward IPC controller in constant and turbulent wind conditions, to alleviate the pitch actuator duty cycle, and to considerably reduce harmonic fatigue loads without the need for blade load measurement signals after tuning.

Towards a Risk-based Decision Support for Offshore Wind Turbine Installation and Operation & Maintenance

Costs of operation & maintenance, assembly, transport and installation of offshore wind turbines contribute significantly to the total cost of offshore wind farm. These operations are mostly carried out by specific ships that have to be hired for the operational phase and for duration of installation process, respectively. Duration, and therefore ship hiring costs is, among others, driven by waiting time for weather windows for weather-sensitive operations. Today, state of the art decision making criteria for weather-sensitive operations are restrictions to the significant wave height and the average wind velocity at reference height. However, actual limitations are physical, related to response of equipment used e.g. crane wire tension, rotor assembly motions while lifting, etc. Transition from weather condition limits to limits on physical equipment response in decision making would improve weather window predictions, potentially reducing cost of offshore wind energy. This paper presents a novel approach to weather window estimation using ensemble weather forecasts and statistical analysis of simulated installation equipment response.

Dynamic Models for Load Calculation Procedures of Offshore Wind Turbine Support Structures: Overview, Assessment, and Outlook

One of the main mechanisms for driving down the cost of offshore wind energy is to install ever larger wind turbines in larger wind farms. At the same time, these turbines are placed further offshore in deeper waters. As a result, traditional monopile foundations are not always feasible and multimembered foundations, such as jackets and tripods are required. Typically, thousands of load cases need to be simulated for the design and certification of offshore wind turbines (OWTs). As models of such foundations are significantly larger than their monopile counterparts, model reduction is often applied to limit the computational costs. Additionally, the foundation design is generally done by a specialized company, which bases its design on the results of the load simulations. Hence, an accurate estimation of the stresses in load simulation is essential to predict the integrity and the lifetime of different designs. The effect on the load accuracy of both the model reduction as well as the postprocessing method used by foundation designers (FDs) are investigated in this paper. A case study is performed on a jacket-based wind turbine model to verify and quantify the findings. First, it is observed that the effect of the reduced foundation model on the wind turbine loads is negligible. However, both the reduction method and the postprocessing method applied by the FD have a large influence on the fatigue loading in the jacket. It is shown that the popular Guyan reduction results in significant errors on the fatigue damage and that a static postprocessing analysis leads to serious underestimations of the fatigue loads. Finally, an outlook is given into future developments in the field of load calculations for OWT foundation design.

Advanced Concept Offshore Wind Turbine Development

Development of novel offshore wind turbine designs and technologies are necessary to reduce the cost of offshore wind energy since offshore wind turbines need to withstand ice and waves in addition to wind, a markedly different environment from their onshore counterparts. This paper focuses on major design challenges of offshore wind turbines and offers an advanced concept wind turbine that can significantly reduce the cost of offshore wind energy as an alternative to the current popular designs. The design consists of a two-blade, downwind rotor configuration fitted to a fixed bottom or floating foundation. Preliminary results indicate that cost savings of nearly 25% are possible compared with the conventional upwind wind turbine designs.

Improving Gearbox Design and Analysis for Offshore Wind Turbines

The paper outlines challenges in improving wind turbine availability to reduce the cost of offshore wind energy. It reports the development of a design software tool for wind turbine gearboxes. It facilitates efficient conceptual designs of wind turbine gearboxes with combinations of epicyclic and parallel gear trains. Field loading conditions are obtained by analysing SCADA (Supervisory Control and Data Acquisition) data to support fatigue damage estimation based on Miner’s rule. Results show that considerable overloading conditions are present which would result in significant fatigue damages to the gearbox.

Towards a mature offshore wind energy technology—guidelines from the opti-OWECS project

The article reviews the main results of the recent European research project Opti-OWECS (‘Structural and Economic Optimisation of Bottom-Mounted Offshore Wind Energy Converters'), which has significantly improved the understanding of the requirements for a large-scale utilization of offshore wind energy. An integrated design approach was demonstrated for a 300 MW offshore wind farm at a demanding North Sea site. Several viable solutions were obtained and one was elaborated to include the design of all major components. Simultaneous structural and economic optimization took place during the different design stages. An offshore wind energy converter founded on a soft–soft monopile was tailored with respect to the distinct characteristics of dynamic wind and wave loading. The operation and maintenance behaviour of the wind farm was analysed by Monte Carlo simulations. With an optimized maintenance strategy and suitable hardware a high availability was achieved. Based upon the experience from the structural design, cost models for offshore wind farms were developed and linked to a European database of the offshore wind energy potential. This enabled the first consistent estimate of cost of offshore wind energy for entire European regions. Copyright © 1999 John Wiley & Sons, Ltd.