Swaged pipe in pipe construction has been increasingly used for offshore pipeline system. Double joint pieces of swaged pipe-in-pipe are typically welded together on a J-lay/S-lay vessel. The swaged end joint is fabricated through a cold deformation process to the outer pipe that is then welded to the inner pipe. The pipe-in-pipe system has excellent thermal insulation characteristics. However, the integrity assessment of pipe-in-pipe swage weld is outside the range of validity of existing engineering critical assessment (ECA) codes, and this presents challenges in developing welding defect acceptance criteria, associated welding qualification and automatic ultrasonic testing (AUT).
This paper highlights the assessment and inspection challenges faced and efficient solutions developed for the swage weld system. Comprehensive 3D finite element (FE) analyses were developed to consider mixed mode fracture for defects at the swage weld. Representative fracture testing was carried out as part of welding qualifications.
The methodology was deployed to derive and optimise weld defect acceptance criteria specific to flowline and pipeline end termination (PLET) special joints, considering laydown, initiation and normal lay installation scenarios. The welding defect acceptance criteria were applied to the swage weld through a newly developed AUT system which can fit in small diameter pipelines.
The developed assessment solutions will greatly benefit the future pipe-in-pipe swage weld system assessment and potentially bring significant cost savings to the industry.