PROCESSING | OIL & GAS
This research gave rise to the full-scale PE RCP test facility and methodology. The latter was to become the ISO 13478 standard for RCP pipe testing. This test is the ultimate reference for assessing RCP performance, proving that the RCP will not occur and providing assurance that the pipe will arrest the failure mode. The legacy test facility, which is now owned and
operated by DNV at its Spadeadam site, provides for further analysis of RCP, and verification and certification services for global manufacturers of PE and other non-metallic materials, pipe manufactur- ers and PE pipeline operators. As well as verifying the performance of new PE grades, it can handle other systems including polyamide and reinforced thermoplastic plastic (RTP) pipe structures.
Above: An RCP crack typically forms a sinusoidal pattern along the pipe
Impact of failure Several instances of RCP have occurred in the US and continental Europe, but gas companies are reluctant to share information on these. The potential for uncontrolled, large-scale release of gas could be disastrous, which demonstrates the importance of assessing and understanding RCP. Simulated RCP failure of buried PE pipe show
that the failure event can be explosive, generating significant pressure waves. During pipe fracture, significant fragmentation may occur, which can travel at high velocity over distances of 50m. Typical features of an RCP crack are a sinusoidal
crack path along the pipe, though it can also split into two directions. RCP can propagate through butt fusion joints, though research suggests that cracks are arrested by electrofusion couplers. Propagation through the coupler has been seen in non-PE systems.
RCP research The development of high pressure steel transmis- sion pipes has resulted in intensive research into the mechanism of RCP – providing a comprehensive understanding that can be applied to PE networks. In the early 1970s, the threat of RCP restricted the use of pipe larger than 315mm in diameter. To overcome this, British Gas used its knowledge of crack propagation in steel pipelines to develop a test methodology for PE pipe. It calculated critical pressures below which any initiated fracture would not propagate within a known pipe diameter. Using an appropriate safety factor, it was then possible to identify safe operating pressures.
20 PIPE & PROFILE EXTRUSION | July/August 2018
Test methodology The RCP test facility consists of a trench equipped with a cooling unit to condition the test sample at any temperature from ambient down to -15°C and a large steel pressure vessel to simulate a pressur- ised gas network. The test involves the initiation of a crack in a 24m length of PE pipe. The test pipe is first fused, stringed and clamped in a trench with one end connected to the gas reservoir. The pipe is then buried in gravel and cooled to a reference tem- perature of 0°C. Following pressurisation, a blade driven by a fast-acting pneumatic piston is used to initiate the crack in an area of the pipe. The make-up pipe, in which the initiation is performed, can be extruded with a less resistant resin to ensure this initiation. To assure initiation of a brittle crack, a groove is machined in the initiation zone which is cooled under the glass transition temperature of PE using a mixture of methylated spirits and solid carbon dioxide prior to testing. Repeating the experiment at different pressures determines – at a given temperature – the charac- teristic critical pressure beyond which the crack propagates over more than 90% of the tested pipe length.
PE materials used for the construction of natural gas networks are of the third generation type with elevated mechanical characteristics. The European draft standard for PE gas networks provides a maximum operation pressure up to 10 bar for such materials. Since the risk of RCP becomes more critical at growing pressures, the importance increases to control this phenomenon thoroughly. For this reason, gas industry standards for PE gas pipelines such as ISO 4437, EN1555 and GISPL2 include an RCP requirement.
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