Room:
Egbert-von-Hoyer Hall
Topic:
H. Offshore wind energy
Form of presentation:
Oral
Duration:
120 Minutes
Chaired by: M. Muskulus, P. Schaumann
13:30
Analytical fatigue reassessment for lifetime extension of offshore wind monopile foundations
Lisa Ziegler | Ramboll | Germany
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Authors:
Lisa Ziegler | Ramboll | Germany
Michael Muskulus | Germany
Offshore wind is still a young industry where most wind farms have less than 10 years of operational experience. However, in the soon future, the first larger offshore wind farms will reach a mature age (e.g. Anholt, London Array). It then becomes increasingly important to reassess the structural integrity of wind turbine support structures in order to optimize the operation for exploitation of structural reserves and to decide on lifetime extension. Already today, an important question for the offshore wind industry is: how to determine the turbine-specific fatigue damage accumulated throughout the operational life? This paper analyses the suitability of numerical fatigue reassessment for monopile substructures. In a case study, residual fatigue lifetimes are calculated for expected variations (upper and lower bound) of environmental, structural and operational conditions. Conclusions on important parameters to monitor are drawn from a global sensitivity analysis. Sensitivity measures are calculated with the elementary effects method in order to reduce the sample space for the computational expensive simulations. The generic OWT assembly defined in Phase I of the OC3 project was used for the case study. This assembly consists of the NREL 5 MW reference wind turbine supported by a monopile in 20 m water depth. Results show that corrosion, turbine availability, and turbulence intensity are the most influential parameters. This can vary strongly for other settings (water depth, turbine size, etc.) making case-specific assessments necessary. The elementary effects method offered more insight on the importance of variables than standard single-parameter variations while the computational effort is kept small. The method is strongly recommended for further use in academia and industry.
13:50
A Comparison Study of Offshore Wind Support Structures with Monopiles and Jackets for U.S. Waters
Dr. Katherine Dykes | NREL | United States
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Authors:
Dr. Katherine Dykes | NREL | United States
Rick Damiani | United States
George Scott | United States
U.S. experience in offshore wind is limited, and high costs are expected unless innovations are introduced in one or multiple aspects of the project, from the installed technology to the balance of system (BOS). The substructure is the main single component responsible for the BOS capital expenditure (CapEx) and thus one that, if improved, could yield significant levelized cost of energy (LCOE) savings. For projects in U.S. waters, multimember, lattice structures (also known as jackets) can render required stiffness for transitional water depths at potentially lower costs than monopiles (MPs). In this study, we used a systems engineering approach to evaluate the LCOE of prototypical wind power plants at six locations along the eastern seaboard and the Gulf of Mexico for both types of support structures. Using a reference wind turbine and actual metocean conditions for the selected sites, we calculated loads for a parked and an operational situation, and we optimized the MP- and jacket-based support structures to minimize their overall mass. Using a suite of cost models, we then computed their associated LCOE. For all water depths, the MP-based configurations were heavier than their jacket counterparts, but the overall costs for the MPs were less than they were for jackets up to depths of slightly less than 30 m. When the associated manufacturing and installation costs were included, jackets resulted in lower LCOE for depths greater than 40 m. These results can be used by U.S. stakeholders to understand the potential for different technologies at different sites, but the methodology illustrated in this study can be further employed to analyze the effects of innovations and design choices throughout wind power plant systems.
14:10
Breaking phase focused wave group loads on offshore wind turbine monopiles
Amin Ghadirian | DTU | Denmark
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Authors:
Amin Ghadirian | DTU | Denmark
Henrik Bredmose | Denmark
Martin Dixen | Denmark
The current method for calculating extreme wave loads on offshore wind turbine structures is based on engineering models for non-breaking regular waves. The present article has the aim of validating previously developed models at DTU, namely OceanWave3D and Waves2FOAM, against the measurements of phase focused wave group impacts on a mono-pile at DHI. The focused 2D and 3D wave groups are reproduced and the free surface elevation and the in-line forces are compared to the experimental results. In addition, the pressure distribution on the monopile is pictured at the time of maximum force and discussed in terms of shape and magnitude.
14:30
A model for Quick Load Analysis for monopile-type offshore wind turbine substructures
Dr. Signe Schløer | Technical University of Denmark | Denmark
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Authors:
Dr. Signe Schløer | Technical University of Denmark | Denmark
Laura Garcia Castillo | Denmark
Morten Fejerskov | Denmark
Emanuel Stroescu | Denmark
Henrik Bredmose | Denmark
A model for Quick Load Analysis (QuLA), of an offshore wind turbine substructure is presented. The dynamic structural response is represented by the first mode only and is computed in the frequency domain using the equation of motion. The model is compared against the advanced aeroelastic code, Flex5. The first comparison shows good similarity between the two models.
14:50
Value of information of repair times for offshore wind farm maintenance planning
Helene Seyr | Norwegian University of Science and Technology | Norway
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Authors:
Helene Seyr | Norwegian University of Science and Technology | Norway
Michael Muskulus | Norway
A large part of the total cost of energy in offshore wind farms is due to maintenance costs and different approaches to lowering these costs have been investigated in recent years. Decision support models for maintenance scheduling exist already, dealing with different influencing factors. Our contribution deals with the uncertainty in the repair times. Given the mean repair times for different turbine components, we make some assumptions regarding the underlying repair time distribution. We compare the results of a decision support model for the mean times to repair and those repair time distributions. The value of lowering the uncertainty in the repair time is calculated and we find that using distributions significantly decreases the availability, when scheduling maintenance for multiple turbines in a wind park. Detailed information about the repair time distribution may influence the results of maintenance modeling and might help identify cost factors.
15:10
Surrogate based wind farm layout optimization using manifold mapping
Kaja Kamaludeen Shaafi Mohamed | Delft university of technology | Netherlands
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Authors:
Kaja Kamaludeen Shaafi Mohamed | Delft university of technology | Netherlands
Sander van Zuijlen | Netherlands
Hester Bijl | Netherlands
High computational cost associated with high fidelity wake models such as RANS or LES serves as primary bottleneck to perform direct high fidelity wind farm layout optimization (WFLO) using accurate CFD based wake models. Therefore, a surrogate based multi-fidelity WFLO methodology (SWFLO) is proposed, which uses manifold mapping (MM) to build the surrogate model. As a verification, optimization of spacing between two staggered wind turbines was performed using the proposed surrogate based methodology and the performance was compared with that of direct optimization using high fidelity model. Significant reduction in computational cost was achieved using MM: a maximum computational cost reduction of 65%, while arriving at the same optima as that of direct high fidelity optimization. The similarity between the response of models, the number of mapping points and its position, highly influences the number of high fidelity model runs. As a proof of concept, realistic WFLO of a small 7-turbine wind farm is performed using the proposed method. Two variants of Jensen wake model with different decay coefficients were used as fine and coarse model. The proposed SWFLO method arrived at the same optima as that of fine model with very less number of fine model simulations.