High Density Polyethylene (HDPE) pipes have been a substitute for carbon steel pipes for a number of years in the oil, water and gas industry due to their attractive properties of aging resistance, corrosion immunity and lower cost.

Electro-Fusion (EF) and Butt-Fusion (BF) welding processes are most commonly used to join sections during manufacturing and installation of HDPE pipelines. The nature of defects in HDPE welds can be caused by a variety of factors and manifest themselves in different ways. The recognised defect mechanisms in BF welds include embedded planar flaws due to poor welding processes, discrete planar flaws caused by imperfections on the fusion face, cold fusion welds caused by incorrect welding procedures, inclusion of particulates and weld mismatch. In comparison, common problems with EF welds during manufacturing include voids, lack of fusion and incorrect heating cycle resulting in a weak joint due to the incorrect crystallographic structure of the joint.

The quality of the joint significantly influences the overall operational safety of the system. Systems that are currently in place to provide quality assurance of HDPE welds includes the approval of welding procedures and personnel, monitoring of welding conditions and visual inspection. The requirement to evaluate the integrity of the weld using Non-Destructive Evaluation (NDE) by visual inspection does not cater for detection of embedded defects. Other methods used to test the integrity of an HDPE weld is through destructive testing; either sectioning a sample and examining the quality or subjecting the weld to a tensile test.

The importance of having an NDE method to qualify the integrity of HDPE welds cannot be understated for the oil and gas industry. A large scale pipeline NDE validation program was undertaken to evaluate the use of Ultrasonic Time-of-Flight Diffraction (TOFD) complimented by Phased Array Ultrasonic Testing (PAUT) for volumetric inspection of BF welds. In addition, PAUT was used for the volumetric inspection of EF welds during the same program.

This paper aims to supplement the understanding of pipeline construction through identification of improvements to pipeline fusion processes and offers the NDE capability to monitor in-situ and on-line pipelines.

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