A new guide for building and classing offshore wind turbine installations is out. Classification society ABS says its the first guide to address design considerations for the bottom founded support structure of a turbine situated in tropical storm prone areas on the Outer Continental Shelf.
Previous guides have been primarily based on experience from European coastal waters. However, ABS says its guide takes into account the International Electrotechnical Commission (IEC) 61400 series of standards for wind turbines, the American Petroleum Institute’s Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms (API RP 2A), and ABS’ offshore rules and guides.
“The direct application of design criteria in existing standards, such as IEC 61400-3, is not sufficient for the offshore wind turbines in US waters,” explains ABS Managing Principal Engineer, Qing Yu who principally authored the guide. “We incorporate additional requirements based on calibration studies that use regional and site-specific conditions of US waters,” he adds.
The guide incorporates refinements to the design environmental conditions and load cases required by IEC 61400-3 to account for the effects of tropical hurricane conditions. Yu points out the principle of site-specific design are more directly addressed in the definition of the design load cases. “Omni-directional wind conditions are required for the design load case in the event an offshore wind turbine loses its connection to the electrical power grid during a tropical hurricane.”
The guide also specifies a unique set of strength design criteria for the steel support structure of an offshore wind turbine based on the commonly accepted working stress design (WSD) approach. “Allowable stresses are defined for a variety of design conditions including normal, abnormal, transport, and installation, as well as earthquake and other rare conditions,” says Yu. “In addition, equivalent load and resistance factor design (LRFD) criteria are defined as an alternative to the WSD-based criteria.”
ABS offers a class notation for those turbine support structures complying with the requirements and conditions of the guide. The notation A1 Offshore Wind Turbine Installation will be listed in the ABS Record. Other optional class notations are S (years) for the return period of maximum design environmental conditions and FL (years) for the design fatigue life.
The guide includes the requirements for the survey during construction and installation and the survey after construction taking into consideration the uniqueness, such as serial fabrication and installation, of offshore wind turbines.
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