Under the marine environmental load, the stress concentration is easily occurred at the joint for the structure of Offshore Wind Turbine (OWT) welded by the steel pipe. Through several years of monitoring flexible ice-resistant structures in Bohai Liaodong Bay, it was found that the flexible structures occurred obvious ice induced vibration, and the occurring structural vibrations can contribute to the overall fatigue of the structure. Three types of ice force characteristics were observed when the ice velocity varies from low to high: quasi-static, steady-state and random vibrations. The steady-state and random vibrations can contribute to almost all fatigue of structure caused by ice. In this paper, the fatigue caused by ice induced vibrations was divided into two processes. Fatigue induced by steady-state vibrations are based on time history analysis, and fatigue induced by random vibrations are based on spectral analysis method. Based on a proposed wind farm in Bohai Sea, the finite element model of tripods OWT is established by ANSYS finite element software. The Plamgren Miner linear fatigue cumulative damage rule and S-N curve are used to calculate the fatigue life of OWT foundation. The fatigue life of OWT foundation under dynamic ice load is calculated by the established fatigue analysis process in this paper. The proportion of fatigue damage caused by dynamic ice load on the OWT foundation is compared with that caused by wind load and wave load. It was cleared that the fatigue damage caused by dynamic ice load must be considered in the design of OWT foundation in ice regions. The fatigue analysis process established of this paper can more quickly analyze the fatigue damage of OWT caused by ice.
As a kind of renewable and green resources, wind energy is widely developed by most countries. Offshore wind energy resources are more stable, greater reserves, and smaller impact on the surrounding environment than the wind energy on land (Zhou, 2011). The Offshore Wind Turbine (OWT) is a tall and slender structure, and the dynamic response of the OWT structure is remarkable under the loading of the marine environment (Wang, 2016). Most of the OWT structures are typically flexible structures, and the strong vibration will cause fatigue damage of structural tubular joints. Ice induced vibrations has been observed at oil and gas platforms in Bohai Sea for decades. The maximum vibrations appear in the frequency lock-in, which have been measured on some vertical platforms (Yue and Guo, 2007). The significance of ice-induced dynamic responses has been remarkably revealed for years (Blenkarn, 1970; Palmer, Yue and Guo, 2010). Three regimes of ice-induced vibrations are distinguished based on measurement results in the Bohai Sea: intermittent ice crushing, frequency lock-in, continuous brittle crushing (ISO 19906,2010). IEC (IEC 61400–3,2009) gives the method of simulating the dynamic ice loads on vertical foundations of OWTs. Barker et al. (2005) and Gravesen et al. (2003, 2005) carried out a large number of model testing to investigate the ice-induced vibrations in OWTs in Danish waters. Gravesen and Kärnä (2009) focused on the static ice crushing occurring on a vertical structure of an OWT in the South Baltic Sea. Hendrikse et al. (2014) studied the fatigue damage accumulation of the combined ice and aerodynamic load case. Hendrikse et al (2017) focused on the predictions of the frequency lock-in state by ice induced. On the other hand, from the monitoring and research of offshore platform in Bohai for many years, it is necessary to carry out the analysis of ice induced fatigue life in the design of ice-resistant structure (Yue et al., 2008). Therefore, the assessment of fatigue life of OWT structure in ice regions should consider the wave loads, wind loads and ice loads.
