Abstract

This paper concerns the application of design standards to the design of offshore wind turbines, focusing on the United States. Offshore wind turbines (OWTs) are those wind turbines whose support structures are subject to hydrodynamic loading. A considerable number of OWTs have been installed in Europe, but so far none in the United States. Interest in offshore wind energy is growing in the U.S., and it is expected that projects will reach the design stage in the near future. This paper will facilitate development in the U.S.

The paper consists in three highly related parts. The first part gives the background for the development of offshore wind in the U.S. in terms of external conditions and expected design standard requirements.

The paper's second part provides a summary of the method and recommendations of the committee draft of the OWT design standard IEC 61400-3 [1] that is believed to represent state-of-the-art. It is placed within the context of other relevant standards. Of particular note are both wind turbine specific standards and standards that are presently used to design offshore structures in the U.S. and internationally. This summary is crucial for the appreciation of the principal differences between designing OWTs, where dynamic response to wind loads is most often dominant, and typical bottom fixed offshore structures.

The last part of the paper discusses the level of structural reliability implied by the design rules of IEC 61400-3. There is a long tradition in design of wind turbines to use 50-yr return period values for extreme environmental conditions with associated load safety factors. The tradition in design of offshore structures is to use 100-yr return period values, with or without load factors depending on code format. When developing design standards for OWTs this difference has to be addressed. The paper explains how this has been done in the IEC 61400-3 CD and what the philosophy behind it is been in terms of choice of reliability level. This includes an estimation of the structural reliability under extreme North European environmental conditions. Especially the uncertainty model assumptions are presented in some detail. For hurricanes the uncertainty model will be different possibly implying different load factors if return period is maintained. The paper discusses what should be considered in this regard, e.g. if it is feasible to aim at the same reliability as for U.S. offshore structures. Some simple tools are provided that can be of use for initial assessments structural reliability acceptance criterion.

Introduction

The initial European work on offshore wind systems were concerned with the scaling and structural and economic optimization of bottom-mounted offshore wind farm systems. Two early examples of this type of design work include the Opti-OWECS (Optimization of Bottom-Mounted Offshore Wind Energy Converters) study [2] and the Dutch Offshore Wind Energy Converter (DOWEC) project [3].

This content is only available via PDF.
You can access this article if you purchase or spend a download.