Session: 04-03-02 Rigid Pipelines II

Paper Number: 79401

79401 - Integrity Assessment of Electrically Heat Traced Pipe-in-Pipe Plug Weld 

To ensure the integrity of two 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. In this system developed by Subsea 7 and ITP InTerPipe, the annulus is designed to be vacuumed to reduced pressure so that a premium insulation performance can be achieved. Typically, one or two vacuum plug welds are planned between bulkheads. In one recent project, two plug welds were planned to cover every 3km length of EHTF Pip section. Although the number of vacuum plug welds appear to be low, these welds have been a high focus area for the EHTF system study. This paper provides an overview of the ECA development and execution.

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 a significant challenge to plug weld integrity. Due to the unique design and function of the 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 life cycle. The allowable defect size from the ECA is also crucial to validate the accuracy of the inspection method. If the acceptance criteria are below the detection threshold, the plug weld may not be used until a sufficient welding and inspection tool is deployed – this potentially could prevent EHTF system from being used on a project.  

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. For fatigue crack growth during installation and operational phases, there is no out-of-shelf empirical stress intensity factor (SIF) solutions for a plug weld; and the relatively close feature of a fillet weld empirical SIF solution is found over-conservative. A bespoke SIF solution for a plug weld was developed and verified via 3D FEA. A case study of a plug weld ECA using the developed methodology is presented. Segment testing of a plug weld to ascertain the reeling defect growth is also described in this case study.

Presenting Author: Lingjun Cao Subsea 7

Authors:

Daowu Zhou Subsea 7
Lingjun Cao Subsea 7