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. The plug was ejected at high velocity from the access fitting (pipeline pressure 103 bar), during the operation to ease the plug using a ring spanner to a maximum of ¼ turn (as per procedure) and before the service valve and retrieval tool were installed. A high volume of crude oil spilled from the pipeline via the access fitting. Fortunately, the two technicians escaped the access pit without injury from the plug projectile or crude oil release.
The Venture is still in the process of conducting the incident investigation. Based on their findings to date, the most probable cause is that 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 coupon 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 repair (chasing) of worn threads on access fittings is performed using an original equipment manufacturer supplied thread tap assembly service tool.
- 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 plugs.
- In this incident, the original equipment manufacturer supplied thread tap 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.
Lessons learnedAs yet, there is no standard method to determine internal thread condition of on-line corrosion probe/coupon original equipment manufacturer access fittings. Thread condition is not easily inspected.The risk posed by long term use of thread tap assembly service tools on access fittings, has not been previously identified.
Action taken in originating company
Temporarily suspend all corrosion coupon retrieval operations on pressurised lines furnished with threaded access fittings in the 6 o’clock position (bottom of pipeline). This provides time to complete the investigation and complete work with the original equipment manufacturer to develop clear guidance on the maximum number of retrieval cycles.
Source: IMCA Safety Flash