This is an excerpt from the Third Quarter 2013 edition of the Wind Program R&D Newsletter.
The design basis for an offshore wind farm establishes the conditions, needs, and requirements to be taken into account in designing the facility. To address design knowledge gaps and facilitate safe deployment of U.S. offshore wind projects in areas along the U.S. Atlantic Coast, the U.S. Department of Energy (DOE) is funding research by a team consisting of DOE's Savannah River National Laboratory, Coastal Carolina University, MMI Engineering, and DOE's National Renewable Energy Laboratory. This research effort will advance the state of the art for the methods of determining the design basis related to "slam loads" from breaking waves. Improved design methods will reduce uncertainty of the design basis for offshore wind power generation systems and result in more robust designs, while reducing the cost of construction and ultimately, the cost of energy.
|Water Depth||Depth Range||Foundation Class||% of U.S. Offshore Atlantic Wind Resource|
|Shallow||0–30 meters (m)||Fixed foundation, traditionally a monopile structure||42%|
|Intermediate||30-60 m||Fixed foundation, typically jacket structures||29%|
|Deep water||> 60 m||Floating foundation (e.g., semisubmersible type)||29%|
In the early stages of the offshore wind industry, the shallow water resource will most likely be developed first as fixed monopile foundation, which is the platform technology typically used for this resource (Table 1) because it is mature and has been used extensively in Europe. Further, the shallow water resource is located off the coast of the fastest-growing regions in the United States, in terms of population and electricity demand. (Figure 1 illustrates the distribution of offshore wind resources along the U.S. Atlantic Coast as well as the depth of the seabed where that resource exists.)
Offshore wind farms deployed in shallow water in areas prone to hurricanes, such as the South Atlantic Coast, may experience extreme breaking wave conditions. Little is known about the characteristics of these types of waves, which can be the dominant structural loads that a wind turbine will be subjected to over its design lifetime. In addition, existing equations for forces from waves (e.g., the Morrison equation) may not be applicable in these conditions under which large diameter monopile foundations are subjected to breaking waves. Furthermore, no design codes or standards for offshore wind power generators have yet been adopted by the Bureau of Ocean Energy Management for application in the hurricane-prone shallow waters of the Outer Continental Shelf off the United States.
The DOE-funded research aims to improve design methods in four phases:
- Using the Earth System Modeling Framework, the team assembled a dynamically coupled computational system of public domain software, the DcRWS-metocean model, which consists of the Weather Research and Forecasting model, Regional Ocean Modeling System, and the Simulated Waves Nearshore Shallow Water Waves model to analyze the spatial and temporal variability of breaking waves. The model output will be used to determine the key characteristics of breaking waves.
- In addition, researchers are using geographic information system tools to assemble data from existing metocean data sources, enabling integration with modeling results for model validation and mapping of established indices related to breaking waves, such as wave steepness and the International Electrotechnical Commission 61400-03 breaking wave parameter. Use of temporal data from the coupled model enables statistical analysis of breaking wave conditions and generation of inputs to the reliability-based design process for offshore wind turbine foundations.
- Modeling and mapping results were used to identify a site for deployment of a research buoy to collect data to verify breaking wave modeling and test a new technology for identifying breaking waves. The research buoy, equipped with an extensive suite of monitoring equipment for studying the test site, was deployed off the coast of South Carolina in August 2013. The buoy is equipped to measure many ocean parameters related to waves, currents, and tides as well as meteorological data, including directional wave spectra and the water current, which can affect the shape of waves, as well as the sea surface height and echo intensity within the water column, both of which can be used to identify breaking waves.
- Finally, a computational fluid dynamics (CFD) model will be used to simulate breaking wave loads on a monopile to analyze time variation of horizontal and vertical forces and associated moments in water depths where breakers are anticipated. The CFD model will use a numerical representation of a wave tank with a flap-type wave generator to create virtual breaking waves to analyze slam forces on a hypothetical monopile foundation installed at the test site (see Figure 2 for example). Field data from the test site will be used to calibrate the CFD model of waves. Results from this numerical study will be used to assess the applicability of slam equations developed for smaller diameter structural members to large diameter members such as monopiles.