Reliability-based operation of offshore wind turbines

Abstract

The offshore wind industry is growing significantly these and coming years at offshore places far away from the coasts, where wind and wave loads as well as fatigue, corrosion and wear of the substructures are main sources of uncertainty. These uncertainties are important for the design, installation, operation and maintenance of the offshore wind infrastructure. Due to the location, it is important to reach the optimal plans for operation of offshore wind turbines in order to reduce the costs, coming from inspection and maintenance activities. Offshore wind turbines (OWTs) are operated with specific power production specifications which are inherently linked with the reliability levels for the fatigue loads due to wind impact and depending of the component to be analyzed. When OWTs are operated over its design capacity influenced by changes of generator, gearbox, blades or simply by the control configuration, they will be imposed to higher wind velocities and obviously larger fatigue loads. Besides operating over the design power capacity, the location at wind farms can play a detrimental role for the fatigue performance of OWTs. Wind turbines with a wind farm will face up (larger) turbulence coming from wakes generated by surrounding OWTs. Therefore, the structural components of offshore wind turbines have to be able to withstand large fatigue loads during the design life. The operational conditions are characterized by the wind speeds at the site, the wind turbine capacity, the operational modes, and the wind farm layout. Changes in the control configuration of the operational states imply an influence on the fatigue loads of the structural components. Varying the production periods, operational wind speed and desired energy production may therefore have a significant effect on the reliability of the wind turbine components. This paper addresses the influence of the operational configuration on the structural reliability of offshore wind turbines by assessing its life cycle during operational periods with different operational configurations. This implies different influences on the load stress ranges at different wind speeds. The influence of the operational control on the load may be important when offshore wind turbines are intended to be fully exploited in their fatigue life. In the same manner, the wind farm location can vary the operational conditions and add fatigue load. The in-wind farm location and associated wake effects is taking into account by a code-based reliability approach. The stochastic model will be explained in detail and it will be shown how the influence of the operational conditions influence the fatigue reliability by setting up different load-stress ranges, wind intensities and wind turbulence for the case of single/alone and in-wind farm locations. Therefore, a reliability-based approach is used and a probabilistic model has been developed where strength and load uncertainties are described by stochastic variables. SN-curve / Miner’s rule and fracture mechanics approaches are considered to model the fatigue life. Design and limit state equations are established for the accumulated fatigue damage for single (no wake effects) and in-wind farm condition (with wake effects). The acceptable reliability level for optimal fatigue design of OWTs is discussed and results are presented. Further, the influence of inspections is considered in order to extend and maintain a given target safety level. The probabilistic basis for the analysis of fatigue reliability is using the fatigue model, proposed by Sorensen et al. (2008). Welded steel joints in the support structure are considered in this paper. An application example is described for the impact of the operation configuration (control configuration due to over-rate power production) in the life cycle reliability for single (no wake effects) and in-wind farm condition (with wake effects).