Towards Understanding Reliability Levels for Offshore Wind Turbines
Authors: Agarwal Puneet , Manuel Lance 
Published: May 2011

Abstract
Achieving uniform reliability is a desired intent of design standards based on Load and Resistance Factor Design (LRFD) methodology. The design standard for offshore wind turbines, the IEC 61400-3 (IEC, 2009), also follows the LRFD. This standard is based on European experience and it may not necessarily represent offshore environment in the US waters, where several offshore wind farms are being planned. For these reasons, it is of interest to investigate factors that may influence the reliability of offshore wind turbines under various levels of wind and wave loads.
We evaluate the reliability index, which is a measure of the probability of failure, for the ultimate limit state associated with the fore-aft tower bending moment at the mudline for offshore wind turbines with fixed support structures. We compare reliability index for various values of characteristic levels and coefficients of variation of wind and wave loads, as well as for various ratios of hydrodynamic to aerodynamic loads. These parameters represent different sites and turbine designs. Using the combined wind and wave load effect model for offshore wind turbines proposed by Tarp-Johansen (2005), we show that the range of reliability levels are reasonably uniform under various combinations of wind and wave loads. This meets the intent of codes based on the LRFD. Since large diameter monopile support structures tend to be dominated by inertia forces, while jacket support structures with smaller diameter members are often dominated by drag forces, we also employ a more general wave load formulation that models both drag and inertia forces. We include this wave load model in the combined wind and wave load effect model for offshore wind turbines, and estimate resulting reliability levels. Results show that dragonly case results in smaller estimates of reliability index than the inertia-only case. Moreover, for inertia-dominated cases, reliability levels are found to be even more uniform than those for drag-dominated case.

Introduction
Several design codes, including the IEC 61400-3 (IEC, 2009) standard for offshore wind turbines, are based on the Load and Resistance Factor Design (LRFD). Designs of an entire class of structures based on the LRFD methodology tend to have a uniform reliability. On the other hand, different designs in a class of structures may have quite different reliability levels if working stress design methodology is followed. LRFD is particularly useful for loads that exhibit considerable variability. Offshore wind turbines are influenced by wind as well as wave loads, both of which can exhibit significant variability. Therefore, it is of interest to understand influence of various factors on the reliability of offshore wind turbines under various levels of wind and wave loads.
Understanding reliability levels is even more important for relatively new industries like offshore wind energy for which the design practice is still evolving. The first edition of IEC 61400-3, the Design Requirements for Offshore Wind Turbines (IEC, 2009), was issued only recently in 2009. This standard is based more on the European experience, where the offshore wind industry is about a decade old. Lately, there has been considerable interest in developing offshore wind farms in the US waters. Care has to be used in applying IEC standards in the US where offshore environment is quite different than that in Europe. The offshore oil and gas industry is quite mature in the US, and American standards for design of fixed-bottom offshore structures exist, such as API RP 2A-LRFD (API, 2003) and API RP 2A-WSD (API, 2005). However, these standards were not intended for wind turbines, which are active machines instead of passive structures, and for which loads on support structures are mainly dominated by aerodynamic loads.