IMCA has presented a follow up to a previous safety alert published by IOGP in May 2016, regarding an incident in which a corrosion coupon plug was ejected from a pressurised pipeline.
A routine corrosion coupon retrieval operation was being conducted on a 28” crude oil pipeline. Two retrieval technicians were located in a below ground access pit, to perform the operation. The operation involved removal of the corrosion coupon carrier ‘plug’ from its threaded 2” access fitting on the pipeline. Whilst easing the plug using a ring spanner to a maximum of a ¼ turn (as per local procedure in use at the time) and before the service valve and retrieval tool were installed, the plug was ejected at high velocity from the access fitting (pipeline pressure 103 bar). A high volume of crude oil spilled from the pipeline via the access fitting. Fortunately, the two technicians escaped the access pit without injury either from the plug projectile or the crude oil release.
Causes:
- The threads of the access fitting were worn down to such an extent, that they were unable to restrain the plug upon minor disturbance (the ¼ turn of the plug);
- The access fitting was installed during pipeline construction in 1987. It is estimated to have been subject to over 140 retrieval and installation cycles;
- Bottom-of-pipeline debris can cause galling of threads on stainless steel plugs, which in turn can damage the threads of carbon steel access fittings;
- The Original Equipment Manufacturer (OEM) supplied thread tapping assembly service tool had been used routinely for every plug coupon retrieval and installation cycle without the use of flushing oil to remove debris from the threads;
- In the presence of bottom-of-pipeline debris and thread damage, the repetitive removal of internal thread material can lead to ever smaller contact surfaces, increasing contact stress, increasing wear rates and/or galling;
- Smaller thread contact surfaces reduce the ability of the access fittings to restrain the plug;
- Absence of a tool to determine the internal thread condition under pressurised conditions meant that the internal thread condition was unknown.
Lessons Learnt:
- Threaded access fittings, which are subject to frequent use of a thread tapping assembly service tool (used to clean internal threads from debris and galling damage) can suffer from reducing thread contact surfaces. This mechanism was previously not identified;
- The internal thread condition was neither confirmed after each use of a thread tapping tool nor periodically
verified because this was an unknown degradation mechanism.
Further details may be found by reading the report here
Source : IOGP
