PROCESSING | OIL & GAS
ing pressures – to allow the use of larger diameter PE pipes at higher pressures for a range of pipe sizes and SDRs over the full temperature range – a series of full-scale RCP tests has been implemented at the Spadeadam site. These tests make it possible to calculate the safe
operating pressure for each pipe diameter and wall thickness for operating temperatures from -20 to +20°C. This research has made it possible to design pipeline operating pressures such that the risk of RCP occurring is minimised within the UK PE distribution network: to date, RCP has not been seen as a failure mode in the UK, as the relevant operating codes prevent pipes being operated under conditions that would make them susceptible to this effect.
Above: A blade is used to initiate a crack in the pipe – which is cooled using methyl- ated spirit and dry ice
Current criteria The current pass criteria for RCP, taken from ISO 4437, is:
Pc Pc
> 1.5 x MOP Where:
= critical pressure MOP = maximum operating pressure
MOP is determined from the standard formula knowing the pipe standard dimension ration (SDR), the MRS (minimum required strength) and C (design coefficient).
Once the gas company has determined its MOP, the critical pressure of the pipe must be at least 1.5 times greater. This ‘1.5 factor’ is due to the pre- commissioning leak test pressure, which is gener- ally done at 1.5 times the MOP. Many RCP failures occur during the leak test, because this is when the pipe sees the highest internal pressure – and is most susceptible to RCP. To determine comprehensive, accurate operat-
Summary
RCP is not caused by inferior PE material, or sub-standard pipe quality but by a combination of factors: intrinsic material characteristics, the type of impact or damage, pipe size and SDR, temperature and the presence of pressure in the pipe. We advocate full-scale RCP testing of all non-metallic pipeline systems to determine the RCP critical pressure level and to determine whether the failure mode poses a threat. Robust verification and certification testing – in
accordance with ISO 13478 – provides total confidence that the RCP failure mode can be resisted. This gives the necessary assurance in the integrity and safety of the pipeline asset in the event of third party intervention, damage or poor installation.
n Chris O’Connor is principal consultant, and Richard Beedell is project engineer at DNV GL Oil & Gas. �
www.dnvgl.com
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EXTRAORDINARY TECHNOLOGIES FOR TODAY‘S PLASTICS
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