Load sequence effects and mixed-mode fatigue crack growth in offshore structures

Abstract

The total fatigue life of a brace in an offshore jacket structure is conventionally considered in four parts. N 1 is the number of cycles to initiate the first discernible surface cracking as noted by any available method. N 2 is the number of cycles to detect surface cracking by visual examination without the use of crack enhancement or optical aids. N 3 is the number of cycles until the first through wall cracking and N 4 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 N 3 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 N 3 data. (In fact it should be noted here that there were very few test results for single sided girth welds available at the time of drafting the HSE guidance; the choice of Class F2 for these joints was therefore based largely on judgement rather than 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. A review by Sharp (Ref.1) concluded that detailed knowledge of the crack shape development after breakthrough together with a value for the ratio N 4 /N 3 are required. 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. The present study was designed to examine these factors for circumferential welds in tubular members, and hence allow the efficacy of the FMD strategy to be assessed. This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy. HSE BOOKS

Airflow Field and Shelter Effect Around Flexible Plants Using Fluid‐Structure Interaction (FSI)‐Large‐Eddy‐Simulation (LES) Simulations

“…When affected by turbulent flow, structures tend to vibrate. Depending on whether a structure is stiff or flexible (elastic), the effect of its vibration on the fluid flow is either negligible (Shepherd & Inaba, 2010) or significant (Benra et al., 2011), and the corresponding simulation is either uncoupled (Tijsseling, 1996) or coupled (Rege, 2019; Woldemariam & Lemu, 2016). In our cases, a flexible plant experiences vibration and significant deformations, resulting in an assured effect on the fluid flow.…”