Abstract
The majority of fatigue crack growth (FCG) data sets available on steels in air and seawater environments are several decades old and may not be appropriate for structural integrity assessment of offshore wind turbine foundations, which are fabricated using contemporary materials and welding technologies. Therefore, the SLIC joint industry project was formed to investigate the fatigue crack initiation and growth behaviour in offshore wind welded steel foundations. The FCG test data from the SLIC inter-laboratory (round robin) test programme have been analysed using a new proposed shape function solution and the results are presented and discussed. The obtained FCG trends in air and seawater environments have been compared with the recommended trends available in standards. The Paris-law constants and ΔKth values obtained from this programme can be used for defect assessment and remaining life prediction of offshore monopile weldments in air and seawater environments. The results from the SLIC project show that for a given value of ΔK the fatigue crack growth rate, da/dN, is on average around 2 times higher in seawater compared to air for the base metal and weldments. This factor of 2 in the seawater environment is almost half of the crack acceleration factor recommended by standards.
Highlights
Oil, coal and natural gas have powered the world energy demand for many years
The recommendations in BS7910 are for assessment of different metallic materials welded joints and BM operating in air and marine environments with and without cathodic protection
The analysis of the fatigue crack growth (FCG) data presented above and the Paris-law constants summarised in Table 4, were based on the new shape function solution shown in Eq (9)
Summary
Coal and natural gas have powered the world energy demand for many years. Since the U.N. An interlaboratory (round robin) test programme was conducted to obtain comprehensive FCG data sets in air and in seawater, which can be used for defect assessment and remaining life prediction of thickwalled offshore monopile weldments Seawater tests in this programme were carried out in a free-corrosion (i.e. no cathodic protection) environment at a characteristic dominant cyclic frequency typical of that experienced by offshore wind turbine structures. The fatigue crack growth (FCG) behaviour of these offshore welded foundations needs to be accurately characterised in air and seawater environments in order to provide accurate remaining lifetime predictions and efficient inspection plans for offshore wind turbine structures This can be done by performing laboratory scale FCG tests on standard fracture mechanics specimens made of parent material ( known as base metal (BM)) and weldments. Shown in this table is that for each of the materials examined in this round robin test programme, half of the tests were performed in air whereas the other half were conducted in the simulated seawater environment
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