The total fatigue life of a brace in an offshore jacket structure is conventionally considered in four parts. N1 is the number of cycles to initiate the first discernible surface cracking as noted by any available method. N2 is the number of cycles to detect surface cracking by visual examination without the use of crack enhancement or optical aids. N3 is the number of cycles until the first through wall cracking and N4 is the total number of cycles to the end of test or final separation of the member. The majority of fatigue tests conducted on tubular connections or on girth welds in brace members obtained only N3 results, it being common practice to stop testing when a through wall crack was present. In the HSE Guidance the S-N curves for tubular connections and girth welds in braces are therefore based on N3 data.
In UK waters, flooded member detection (FMD) by ultrasonic inspection with a remotely operated vehicle is used to check whether through cracks are present; however, in practice, fatigue cracks are likely to continue to grow around the brace circumference after breaking through-wall. From the structural safety viewpoint, there is clearly a need to quantify the rate of fatigue crack growth after development of a through wall crack, but prior to the point at which final separation becomes a possibility. In this way the remaining safe life of the structure after through-wall cracking can be assessed, and hence the efficacy of the FMD strategy examined.
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