Comparison of Design Guidelines for Offshore Wind Energy Systems

Abstract

Abstract This paper provides a brief history and general comparison of different guidelines that may be used for the design of offshore wind turbine (OWT) support structures. These guidelines have been developed by organizations such as the American etroleum Institute (API), Det Norske Veritas (DNV), Germanischer Lloyd (GL) and, International Electrotechnical Commission (IEC). The guidelines developed by API and IEC are compared in general terms based on the extent of their applicability and fundamental differences for OWT support structure design. The API and IEC are also compared on the basis of the levels of reliability inherent in their design methodologies for one example application (type of support structure and limit state condition). The inherent reliability is calculated for a conventional monopile designed according to the criteria provided in API and IEC. Probability distributions are specified for the random variables and uncertainties to represent model errors. Limit state equations are developed for bending failure in the monopile based on extreme wind and wave loading conditions. Based on normal practice, it is assumed that the system will be shut down for this condition, therefore, the dynamic effect between the turbine and support structure is not considered. The intent of this paper is to examine the range of applicability of the different design guidelines and to illustrate how these may be compared to assess their applicability to the design of OWT support structure for U.S. wind farm applications. Introduction Renewable energy is becoming an increasingly important element of the total energy resource in the U.S. Energy generation using renewable resources such as wind has yet to realize its full commercial potential. A number of utilities have begun to recognize this and are diversifying their generation into renewable sources such as wind and solar power. One utility, Florida Power and Light, has achieved approximately 30% [1] generating capacity from wind farms. There is a long history of on-shore wind power development in the U.S., however, offshore wind power resources remain untapped. Offshore wind farms are a relatively new resource. Offshore wind farms can capitalize on very large wind turbines (5 MW units are now in operation offshore) that utilize advances in the areas of control systems and advanced aterials to produce significant power output and efficiency. At the time of publication, there are a few offshore wind farms proposed for U.S. waters, however, none have been constructed. Offshore wind farms have been built and are in operation in several locations in Europe. The Energy Policy Act of 2005 established the U.S. Minerals Management Service as the lead regulatory authority for offshore wind power developments on the U.S. Outer Continental Shelf (OCS). There are currently no guidelines that have been accepted by the MMS or other U.S. agencies for the design of offshore wind power generators in U.S. waters. There are codes and guidelines that have been developed for offshore wind power development overseas; however, they have a limited history of use and have not been reviewed in terms of their applicability for the conditions that exist on the U.S. OCS or for the levels of safety that would be required by the MMS and other U.S. agencies.