Design Load Cases Required for Offshore Wind Turbines in Hurricane-Prone Regions
Authors: Yu Qing , Kim Kunho , Lo Tzu-Wei 
Published: April 2012

Abstract
This paper discusses the design load cases that are critical for offshore windturbines installed in hurricane-prone regions on the US Outer Continental Shelf(OCS). Results of a literature review on the research on the design load casefor hurricane conditions are first presented in the paper, followed by themodels and results of the case studies using the conceptual designs of amonopile, a tripod and a jacket type support. The characteristic structuralresponses of offshore wind turbines under hurricane conditions are evaluatedwith the consideration of the dynamic interaction among the turbinerotor-nacelle assembly, support structure and foundation. The correlationsbetween those responses and various design parameters including the turbineoperating mode, design environmental condition, site location and supportstructure configuration are of particular interest. Based on the literaturereview and the case study results, recommendations are made for the hurricanedesign load cases.

Introduction
A number of studies have been undertaken from various perspectives to examinethe gaps between existing offshore wind turbine design standards andestablished practices for designing offshore oil and gas platforms on the USOCS. Some of those studies explored the means of extending the coverage of IEC61400-3 (2009), which is primarily developed based on experience from Europe, to hurricane-prone regions. Efforts have also been made to adapt the designcriteria in API RP 2A-WSD (2007) for the design of offshore wind turbinessubjected to hurricane risks.
As part of the 2011 Technology Assessment and Research (TA&R) " DesignStandards for Offshore Wind Farms" project funded by the Bureau of Safety andEnvironmental Enforcement (BSEE, formerly BOEMRE) of the US Department of theInterior, ABS performed a comprehensive study on the strength design criteriafor offshore wind turbines to be deployed on the US OCS. The study expanded thescope of previous studies, particularly those carried out during thedevelopment of the ABS Guide for Building and Classing Offshore Wind TurbineInstallations (2010), where a set of design criteria were established to takeinto account the risk of tropical storms on the US OCS. The paper presents themajor research findings of this BSEE-funded project. The following summarizesthe main contents of the paper. Further details can be found in the projectfinal report available on BSEE's website (Yu et al., 2011a).A literature review of the study on the design load cases for hurricaneconditions and the modeling of hurricane wind and wave fieldsCase studies for the three typical support structure configurations includinga monopile, a tripod and a jacket. The representative regional site conditionsin the Gulf of Mexico as well as the Mid-Atlantic and South Atlantic sectionsof the US East Coast are considered.Parametric study of the correlations between the responses of turbine supportstructures and the main design parameters including return period of designenvironmental conditions, normal and abnormal turbine operating modes, misalignment between wind and wave directions, hurricane wind models, type ofsupport structures and site variations.Conclusions and recommendations drawn based on the findings obtained in thisstudy.