Abstract
The continued growth of the offshore wind industry will depend essentially on reductions in wind energy production cost. Large cost reductions can be achieved through efficient, economic and optimised wind turbine support structures. To achieve maximum offshore wind adoption beyond 2020, significant industrial and research efforts are being made in optimised material selection and application. Fatigue and corrosion damage are the greatest challenges today in design and life estimation of wind turbine support structures. S355 steel is currently used in fabrication of most wind turbine monopile support structures. Clear understanding of their corrosion-fatigue properties and accurate steel selection will support the optimisation and economic design of extra-large wind turbines. This paper presents the fatigue crack growth test results of advanced S355 TMCP steel in air and seawater, and compares the results with studies on commonly available S355 steel. The results show that S355 TMCP steels generally offer higher fatigue damage tolerance than normalised S355 steels in air and the factor decreases and tends towards a common value with increase in stress intensity factor range. However, in seawater there is no significant difference in fatigue crack growth rates for all the S355 ferritic steels considered in this study.
Highlights
The growth of the offshore wind industry will depend essentially on reductions in wind energy cost
Due to scarcity of corrosion-fatigue data for advanced S355 steel subgrades that are currently being used to design extra-large (XL) wind turbines (WTs) support structures, this paper presents a comparative study of corrosion-fatigue characteristics of some of these steels by performing new sets of experiments and comparing the results with the existing data on other sub-grades of S355 steel
Note that fatigue crack growth rate (FCGR) in air is found to be approximately constant in the range 0.1 Hz and 5Hz [51], whereas FCGR is sensitive to cyclic frequency in seawater environment
Summary
The growth of the offshore wind industry will depend essentially on reductions in wind energy cost. Previous studies have shown that further cost reduction could be achieved through efficient, economic and optimised turbine support structure, the dominant being the monopiles [3]. Many newly licensed wind farms in Europe, especially in UK Round 3 and Germany, to be developed before 2020 are at water depths below 40m, offshore activity is progressing towards deeper waters, requiring new design concepts to be considered Despite these changes, monopile structures are maintaining interest from developers especially for large 10 MW concept designs [10,11,12,13,14]. Fracture mechanics approach is frequently employed in the prediction of the loads at which cracks of given length will grow and cause failure This knowledge enables safe design and aids in economic and effective preventive maintenance. Due to scarcity of corrosion-fatigue data for advanced S355 steel subgrades that are currently being used to design extra-large (XL) WT support structures, this paper presents a comparative study of corrosion-fatigue characteristics of some of these steels by performing new sets of experiments and comparing the results with the existing data on other sub-grades of S355 steel
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