The concept of structural redundancy is implemented in the fatigue analysis of an offshore wind turbine jacket structure. The analyzed jacket is a real life example. Time domain analyses are performed for the most representative design load case. The uni-directional and multidirectional simulations of the offshore wind turbine system are carried out using a coupling of the aero-elastic code and the finite element code. Fatigue analyses are performed using hot spot stress approach and Miner's rule. Comparative studies show that considering structural redundancy leads to expanded fatigue life of the offshore wind turbine jacket structures.
Offshore wind turbines (OWTs) are typically designed according to respective standards and guidelines for a lifetime of 20–25 years. The design of support structures is usually fatigue-driven since OWTs are exposed to long-term and variable-amplitude loading. Lifetime extension is one of the priorities of wind industry in order to reduce levelized cost of energy (LCOE). However, there is not much experience regarding this issue. DNV-GL (2016) recommends renewed lifetime calculation combined with the assessment through inspection. Ziegler and Muskulus (2016) identified environmental, structural and operational parameters which are important for fatigue lifetime of monopiles and which should be considered in the lifetime extension decision. Fatigue reliability analysis of OWT jacket support structures has been performed by Dong et al. (2012). They observed that allowable cumulative fatigue damage can be increased with the implementation of a relevant inspection strategy.
The aim of this paper is to investigate the influence of structural redundancy on fatigue life of OWT jacket structure. High redundant capacity is an advantage of jackets compared to monopiles and this can be of great interest in order to extend designed lifetime. Nevertheless, this advantage is not made use of in design practice. In this study, the investigation is based on a real-life Senvion 6.2M126 OWT with a jacket structure. Load simulations of the OWT system are performed using the coupled simulation tool ASAS/Flex5. Load case sets are reduced in comparison to what is generally used in design practice. The load-time histories obtained from simulations are post-processed applying rainflow counting (RFC) and damage equivalent loads (DELs) are derived. Based on this, fatigue life of all welded tubular joints within the jacket is estimated using Miner's rule and the structural stress approach. Crack initiation and crack propagation phases are not accounted for and it is assumed that components which reach a cumulative fatigue damage of 1.0 (according to Miner's rule), fail. These components are considered as non-load carrying and they are released (no load transfer) in the numerical model in further OWT simulations. In order to understand the influence of the loss of the structural component within the jacket structure, several parameters are analyzed: eigenfrequencies, DELs, fatigue damages. Moreover, the most critical joints for fatigue design and potential spots for crack initiation are identified. Wind/wave misalignment and multidirectionality are taken into account as well.