Three dimensional finite element based ECA is needed for non overmatching welds during reel-lay. Two procedures are presented. One procedure is based on a tearing-low cycle fatigue concept which is included in BS7910. The other procedure disregards low cycle fatigue but treats the blunting as part of tearing. This procedure is described in DNV-RP- F108. This paper compares reeling crack growth between the two approaches using an 18" diameter alloy overlay pipe. The comparison reveals that both methods give comparable crack growth. Detailed comparisons on safety margin are discussed. This work helps industry to appreciate the confidence level in reeling FE based ECA.
Offshore pipeline typically requires an over-matching girth weld. However, the increasing selection of high strength steel and corrosion resistant alloy (CRA) clad/ lined pipes has created the need to address the issue of partially over-matching or fully under-matching girth welds. During reel-lay, strain concentration occurs in the weld region with the weld partial over-matching or full under-matching. The fracture integrity of the weld cannot be addressed through the typical failure-based diagram (FAD) based engineering critical assessment (ECA). 3D finite element (FE) analyses are required in this scenario.
Two distinct ECA procedures based on 3D FE analyses for reel-lay are presented in the public literature. One procedure is based on a tearing-low cycle fatigue concept in BS7910 (BSI, 2019). Tearing is calculated when the crack driving force exceeds the blunting limit (crack initiation) and the low cycle fatigue mechanism is used to account for the crack growth between multiple reeling cycles. The total crack growth is the summation of the tearing and low cycle fatigue. In contrast, the other procedure disregards the low cycle fatigue due to multiple reeling cycle but treats the blunting as part of tearing. This procedure is followed in DNV-RP-F108 (DNV, 2021). Both methods have been widely used in the industry.
