Failure of threads – Follow up to “Corrosion coupon plug ejected from pressurised pipeline”

The IOGP published safety alert number 273 in May 2016, regarding an incident in which a corrosion coupon plug was ejected from a pressurised pipeline. This was passed on to IMCA members as Stored pressure: Corrosion coupon plug ejected from pressurised pipeline.

This forms a follow up to the previous alert and is part of a summary of the main lessons learned, relating to the wearing of threads on bolts.

What Went Wrong?

• 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 – 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.

The main lessons learnt were summarised thus:

• 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.

The full report can be found on the IOGP’s website at safetyzone.iogp.org/SafetyAlerts/alerts/Detail.asp?alert_id=276