Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Ralph Nichols (1) F P Paul Gayes (2) Len Pietrafesa (2) Tingzhuang Yan (2) Shawu Bao (2)
(1) Savannah River National Laboratory, Aiken, United States (2) Coastal Carolina University, Conway, United States
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Ralph Nichols is a Senior Fellow Engineer at the Savannah River National Laboratory and is the Wind Energy Program Manager. He received an M.S. degree in Civil/Environmental Engineering from the University of Oklahoma. Ralph has been working on offshore wind energy for 10 years. He has primarily worked on improving methods for assessing met-ocean conditions and resulting hydrodynamic loads on fixed foundations for wind turbines. His research interests include improved methods for determining design loads incorporating better field techniques and advanced computer simulation of met-ocean processes and interactions between offshore wind turbine structures and waves.
Dynamically coupled met-ocean model for integrated design of offshore wind turbines
Approximately 70 percent of the United States offshore wind energy resource exists along the east coast on the outer continental shelf in water < 60m deep. This same area is highly susceptible to tropical cyclones with wind speeds ranging from 18m/s for tropical storms to > 70ms for category 5 hurricanes. The combination of high wind speeds and shallow water can lead to steep and breaking waves. Steep and breaking waves can produce large slam loads on offshore structures and can be the controlling factor in the design of fixed foundations for offshore wind turbines.
New tools are need for integrated met-ocean design of wind turbines are needed to ensure turbines, towers, and foundations can withstand the large dynamic loads that can result from tropical cyclones on the outer continental shelf. A dynamically coupled met-ocean model (DCMoM) was developed from open source models of the atmosphere, waves, and ocean using a software coupling tool Earth System Modeling Framework (EMSF). The atmospheric model WRF provides wind forcing to SWAN (waves) and ROMS (ocean); ROMS feeds sea-surface temperature (SST) to WRF, SWAN feeds wave parameters to WRF and ROMS, and ROMS and SWAN provide wave breaking simulation.
Main body of abstract
The DCMoM was used to simulate 2 hurricanes in the South Atlantic Bight which extends from Cape Hatteras, NC USA to West Palm Beach, FLA USA. Data from observational buoys operated by the NOAA National Data Buoy Center was used to validate the model.
A nested grid was used to increase the resolution of the model around to particular areas of interest for offshore wind energy development. The coarse mesh had an aerial resolution of 18km and the finest mesh had a resolution of 0.72 km and vertical discretization with 10 layers for the atmosphere and 3 layers for the ocean. Results from the coarse mesh model show the spatial variability of design parameters such as significant wave height, wave period, and water depth. These results are presented in the form of maps of significant wave height, slam force, and wave energy dissipation due to wave breaking.
Fine mesh models were used to study the temporal variability of design parameters significant wave height, wave period, and water depth for 2 sites of interest for offshore wind energy development. Histograms of various wave parameters and slam force were prepared to determine the likelihood of different wave load conditions. Output from the fine mesh was used to study the spatial variability of wave height and slam force throughout the proposed project area. Temporal profiles showing the change in significant wave height over time at various distances from shore were developed to further study the temporal variability of wave height at different locations.
Results from the DCMoM show the variability of wave parameters and loads and other met-ocean parameters used in the layout/design of offshore wind turbines and support structures. As storms move westward from deep water up onto the Atlantic continental shelf the rapid change in water depth produces steep waves which break offshore beyond potential offshore wind energy sites. Maps prepared using DCMoM output can be used to identify high risk areas and improve our understanding of the met-ocean environment.
Conference delegates will learn about a state-of-the-art integrated met-ocean model being developed in the U.S to improve the design basis for offshore wind turbines. Delegates will also become familiar with results from the model which show the spatial and temporal variability of wave parameters and slam forces from breaking waves on the continental shelf. Results show the large variability of met-ocean conditions resulting from 2 different hurricanes that crossed the study area.