Abstract
To ensure the integrity of three recent electrically heat traced flowline (EHTF) projects, an innovative engineering critical assessment (ECA) of the vacuum plug weld has been performed. Vacuum plug welds are used to seal the annulus of the EHTF pipe in pipe (PiP) after vacuum draw down operations. Plug welds are a high focus area for the EHTF system study.
EHTF PiP in offshore application is installed by a reel-lay method. The plug weld in the carrier pipe is subjected to large plastic deformation, followed by cyclic fatigue in lay-down and operation. These loading conditions pose significant challenges to plug weld integrity management. Due to the unique design and function of the EHTF plug weld, a fit-for-purpose ECA is crucial for demonstrating the weld quality is sufficient to guarantee the integrity of the carrier pipe throughout the entire EHTF life cycle. The allowable defect size from the ECA is also crucial to validate the accuracy of the inspection method.
This paper presents the development of the plug weld ECA methodology. The complexity of the plug weld requires a bespoke ECA to address defect tearing during plastic deformation from reeling, as well as fracture check against operational conditions. In terms of fatigue crack growth for installation and operational phases, there is no off-the-shelf empirical stress intensity factor (SIF) solutions for a plug weld. A bespoke SIF solution for plug weld was developed and verified via 3D FEA. A case study of plug weld ECA in an EHTF using the developed methodology is presented. Segment testing of plug weld to ascertain the reeling defect growth is also described in case study.
