A study on engineering critical assessment (ECA) of subsea pipeline girth welds for reeling installation
MetadataShow full item record
Reeling is offshore pipeline installation method which delivers fast and cost effective pipeline laying. Each of the pipeline segment are welded onshore, the long section of pipeline then spooled onto a large diameter of reel. However, reeling installation method causes large plastic strain to the pipeline girth welds. Due to the existing cracks commonly found in the girth welds, the plastic strain will cause possible crack growth. To derive the acceptance criteria for pipeline girth weld defects and sustain the integrity of pipeline during reeling installation, an Engineering Critical Assessment (ECA) is required. The objective of this thesis is to perform Engineering Critical Assessment (ECA) of pipeline girth welds during reeling installation particularly in spooling on and reeling off stages using LINKpipe and CRACKWISE software and also to perform the ECA for clad/lined pipes using LINKpipe. CRACKWISE is one of the software that can be used for the flaw assessment of pipeline girth welds during reeling installation. In order to reduce the conservatism of existing failure assessment methods, SINTEF recently have developed a new failure assessment approach which depends on finite element calculations of pipeline model. LINKpipe is based on four-node ANDES shell elements and a non-linear line-spring element. The software established an efficient and adequately accurate model even for large level of strain, thus it has potential as an alternative ECA tool for pipelines subjected to plastic strains. Moreover, the new bi-metallic shell elements that were developed in LINKpipe making it capable analyzing defect assessments on clad and lined pipes. Based on the analyses performed for the thesis work the influence of misalignment for the critical crack size curve is less significant compared to the effect of residual stress. On the contrary, pipe misalignment in LINKpipe ECA simulations can show the effect of increasing the Crack Driving Force very significantly, which makes the critical crack size, became smaller. Whereas the residual stress showed little influence in the prediction of the critical crack size using LINKpipe. When the maximum possible misalignment (which is 1.95mm) along with the residual stress is applied, the critical crack size curves resulted from CRACKWISE and LINKpipe, are relatively close to each other. However, CRACKWISE tends to be conservative for long crack lengths (>90mm) compared to LINKpipe, whereas for short crack lengths (<90mm) CRACKWISE yields less conservative critical crack sizes.
Master's thesis in Offshore technology