Design Considerations for Offshore Wind Turbines in US Waters - The American Way

Abstract

Abstract This paper addresses unique design considerations for the support structure of an offshore wind turbine in US waters and how the Working Stress Design (WSD) approach can be applied. The Load and Resistance Factor Design (LRFD) criteria specified in IEC 61400–3 are primarily developed based on the experience from offshore wind turbines installed in European coastal areas. Due to the higher variability of the wind and wave climate in hurricane-prone areas, offshore wind turbines in US waters would not achieve the same level of safety as those in European waters if the existing IEC design criteria were applied. To address this, a set of acceptance criteria, accounting for the unique design consideration related to tropical storms, has been developed and are published in the ABS Guide for Building and Classing Offshore Wind Turbine Installations. Extensive calibrations have been carried out to verify the adequacy of design load conditions and structural design requirements of the ABS WSD-based design criteria. Metocean conditions in the US Gulf of Mexico (GoM) and East Coast were used in the calibrations to calculate the aerodynamic and hydrodynamic loads as well as the structural responses of the typical designs of offshore wind turbines. Introduction The US offshore oil and gas industry has extensive experience with hydrocarbon-related offshore structures installed on the US Outer Continental Shelf (OCS). A series of Recommended Practices (RP) has been published through American Petroleum Institute (API) under its Subcommittee on Offshore Structures. The main document is API RP 2A-WSD [2], which provides a comprehensive basis for the design of offshore structures subjected to wave, wind, current and earthquake loading conditions on the US OCS. What is not included in API RP 2A-WSD, however, is the definition of environmental and load conditions that can account for unique response characteristics of offshore wind turbines. Several design and certification guidelines have been developed primarily based on the experience from offshore wind turbines installed in European coastal areas. Of these guidelines, the International Electrotechnical Commission (IEC) 61400–3 Wind turbines - Part 3: Design Requirements for Offshore Wind Turbines [12] embodies the industry's state-of-the-art knowledge for offshore wind turbines. Although a framework to define the site-specific Class S wind turbines is included in IEC 61400–1 Wind turbines - Part 1: Design Requirements [11] and further referred to by IEC 61400-3, the applicability of IEC standards to the offshore areas subject to the risk of tropical storms remains unanswered. Further validations and possible modifications are important to determine to what extent the IEC61400-3 design criteria can be adopted for the US OCS.