Wind turbine reliability characteristics and offshore availability assessment

Abstract

Since the early 90s, wind energy industry has stepped offshore due to a number of interesting characteristics compared to onshore wind harvesting. At least when initially realized, offshore wind farms were not really different than onshore ones, since the same wind turbine technology with slight modifications was directly applied to the new environment. In these early cases, low water depths and small distance to shore may have justified this option. For future large scale offshore wind farms however, it is doubtful whether simply applying the existing onshore wind turbine technology will lead to the desired outcomes, e.g. high energy yield and revenues. Technical reliability of wind turbines affects the economics of an offshore wind farm since failures increase maintenance costs and decrease operational time, thus energy production. In the beginning of this report, the most important aspects that drive modern wind turbine design are investigated, followed by a review of different design choices for the various parts of a wind turbine as well as overall wind turbine topologies. A thorough reliability analysis, focused on comparison of the most prominent architectures, is then carried out using failure data from two German databases, LWK-SH and WMEP. Total annual failure rates of 2 to 5 failures per wind turbine per year were observed but no specific topology seemed to clearly stand out in terms of reliability. Most critical components proved to be blades, electrical and electronic systems, as well as hydraulics. Gearboxes, generators, and shafts and bearings are also important not because they fail very frequently, but due to the high associated downtimes. Using a widely used reliability growth model, there seem to be more failures during the first years of operation for most subassemblies. In order to identify the most important parameters affecting offshore wind farm availability and O&M costs per kWh, several Monte-Carlo simulations were performed using the software tool CONTOFAX. Results showed that availability is not only affected by technical reliability, but also availability of heavy lifting equipment and spare parts, site accessibility, crew and vessel strategy, and distance of the wind farm to shore.