SS: Offshore Wind Energy Special Session: On Hurricane Risk to Offshore Wind Turbines in US Waters
Authors: Jha Alok , Dolan Daniel K. , Musial Walter , Smith Charles 
Published: July 2010

With the increased focus in the US on green technology, there are several offshore wind farms being planned in the US waters to meet the goal of utilizing renewable energy sources for consumers. The offshore wind industry can benefit substantially by leveraging the design and analysis experience from oil and gas platforms in the United States and from offshore wind turbines in European waters. However, no offshore wind turbines have ever been installed in U.S. waters where several unique challenges must be addressed, particularly with respect to how tropical storms may alter the design basis. This paper presents the engineering challenges that need to be addressed in analyzing offshore wind turbines to include the metocean loads imposed where both the aerodynamic and hydrodynamic loads may be equally significant with particular attention to U.S. characteristics. Analysis techniques available for offshore oil and platforms can be utilized for assessment of offshore wind turbine structures. Specifically, the engineering insights on ultimate strength analysis and the implicit safety levels as warranted in design codes can be used to inform the design process for offshore wind turbines, giving due regard to the hurricane risk present in U.S. waters that changes with geography.

Introduction and Background

This paper presents key results on safety level assessments for offshore wind turbines from recently completed studies, References (1) and (2), that compared two design guidelines for applicability to U.S. waters. The methodology and examples presented can be used to better analyze and design, for adequate structural safety, offshore wind turbine structures to be considered for deployment in US waters.

The objective of this paper is to present the range of metocean loads present at a U.S. offshore site, and the nonlinear dynamic response of a monopile- foundation offshore wind turbine, when subject to operational and extreme loads. Insights into shallow water depth effects, soil-pile interaction modeling, modeling of aerodynamic wind loads along with concurrent hydrodynamic wave and current loads, and the analyzed physical behavior of the structure will be presented.

This paper presents the following sequence of studies to develop the reliability assessment for offshore wind turbines in the United States:Direct comparison of safety factors in the two design guidelines (American Petroleum Institute (API) vs. International Electrotechnical Committee (IEC); more on this follows) studied,Comparison of reliability levels achieved for a generic offshore wind turbine structure for the two guidelines, for four sites/regions, andComparison of reliability levels for two types of offshore wind turbine substructures at a specific site designed based on both API and IEC guidelines API vs. IEC - A comparison of safety levels for US Waters

The two design guidelines being studied here are:The American Petroleum Institute guidelines for fixed offshore platforms based on working stress design (WSD) methodology; see Reference (3).International Electrotechnical Committee guidelines for offshore wind turbines (OWTs) based on load and resistance factor (LRFD) methods primarily developed for use in European conditions, see Reference (4).