Advancement in Piping Roundness Inspection through Digital Technology Measurement to Proactively Prevent Internal Coating Failure

Introduction

In the oil and gas industry, the internal coating of a small diameter pipe is always a challenge due to the presence of weld protrusions in the weld root that usually cause failures of the internal pipe coating [1]. It was noticed that in several projects the internal girth weld coating failed with the use of a mechanized gas metal arc welding (GMAW) process. The initial investigation revealed that the failure could be a result of pipe misalignments due to the difference in pipe-end ovality. Even with utilizing an internal line-up clamp with shims to correct the ovality, the proper pipe alignment, as per standard requirements, could not be easily achieved to prevent weld protrusion [2]. It was also a challenge to perform visual inspection on weld roots for small pipe diameters, such as 16 inches, to ensure the weld protrusion did not exist prior to commencing coating activity. One case study related to internal girth weld coating failure will be discussed in detail in this article, including the root causes and recommendations. Therefore, it is highly recommended to use an advanced technology, such as a robotic video crawler, for inspection to find these defects prior to coating pipe [3]. Discovering these defects at this stage will be considered after the fact and will require cutting and rewelding all defective joints (i.e., rework which affects cost, project schedule, and quality). A new technology was assessed to resolve this challenge -- advanced digital technology -- which can proactively prevent girth weld coating failures due to weld protrusion and rework by accurately and rapidly measuring pipe-end dimensions, and then match best-fit end-to-end pipes with software-based analysis.

Study on Internal Coating Failure

A study was conducted on a new construction pipeline project which used an internal liquid coated girth weld with a length of 56 km (2,308 joints) and containing 1,260 welded joints. The objective of the study was to identify the root causes of the pipeline’s internal coating girth weld failures during a holiday test; 67% of the joints failed as shown in Figure 1. The collected pipeline data was as follows:

• Mechanized GMAW welding process
• Pipe diameter: 16 inch (406.4 mm) x 14.3 mm thickness
• Service type: Sour gas
• Pipe type: API 5L X60 seamless pipe
• Pipe coating: Internal and external fusion bonded epoxy (FBE) coated pipe, girth weld liquid coating

The investigation revealed that the failure of the internal girth weld coating was due to excessive root penetration (Figure 2). For example, 3.8 mm was observed, which exceeded the allowable acceptable limit as per ASME B31.3 (1.5 mm). Further investigation revealed that the weld protrusion was due to Hi-Lo, i.e., misalignment between pipe ends during the line-up of adjacent pipes, sometimes exceeding 3 mm although an internal line-up clamp (ILUC) was used to correct the misalignment. The misalignment was because of extreme out of roundness (ovality) at the pipe ends. The main root causes can be summarized as follows:

1. Sharp edges and excess root protrusion were caused due to the gap between the copper shoe backing of the internal line-up clamp in the mechanized GMAW (M-GMAW) welding process.
2. No immediate internal visual inspection of the root profile was performed after welding due to lack of coating subcontractor availability at site.
3. Specialized coating contractor services were not finalized prior to start of the welding, which led to delays from welding and nondestructive testing (automated ultrasonic testing [AUT]) acceptance of the coating application.
4. Delay in verification of internal girth welds for coating suitability and dependency on AUT assessment. The contractor proceeded and completed 1,260 joints, which later had to be cut out and rewelded.