Catastrophic rupture of dead-leg pipe-work
Health and Safety Executive - Safety alert
Chemicals, Explosives and Microbiological Hazards Division (CEMHD)
CEMHD2 - 2019
Operators of Process Plant (and associated inspection bodies) which may have pipe-work dead-legs on toxic, flammable, dangerous to the environment or other critical services.
- Oil and gas (onshore / offshore)
- Chemical processing and production
- Power production
This safety alert highlights the increased risks of primary containment loss associated with dead-legs that may be present on process pipe-work. Dead-legs are lines containing process fluids under stagnant, no or low flow conditions.
In a recent incident there was a significant leak of hot hydrocarbon under pressure when a dead-leg failed catastrophically, leading to a full-bore rupture of the pipe. This resulted in the loss of an estimated 75 tonnes of boiling hydrocarbon to atmosphere, but a major catastrophe was avoided as the resulting vapour cloud did not ignite. The investigation found that significant pipework wall loss had taken place in the failed dead-leg, but that this was highly localised and had been missed by the pipework examination regime; various thickness checks were routinely being made, but these were either side of the locally corroded area. This incident has highlighted the risks of dead-leg corrosion, and that existing integrity management regimes may be inadequate to manage the threat of highly localised internal degradation, particularly when a thermal gradient, liquid interface or other feature may be present along the dead-leg.
The purpose of this safety alert is to highlight the risks of corrosion in dead-legs and where they can't be eliminated, remind operators of the guidance on how to inspect these areas to avoid loss of containment.
Pipe-work dead-legs have long been considered as an elevated risk; corrosion can be aggressive, leading to accelerated thinning of the pipe wall, and they often receive less attention than line pipe. A recent incident of localised corrosion in a dead-leg resulted in catastrophic full-bore failure of the line and a large loss of containment of extremely flammable material.
This safety alert is to remind operators of relevant good practice for dead-leg inspection to help avoid similar incidents.
A recent failure on a UK refinery involved an 8" diameter vertical relief line, approximately 5m in length, which catastrophically failed during normal operation part way along its length, releasing approximately 75 tonnes of extremely flammable material at elevated temperature and pressure. Connected to an insulated process header and vessels, the failure involved complete separation of the relief line (which was not insulated).
Subsequent examination of the line revealed excessive internal thinning which was very local to the failure point, but relatively normal wall thickness elsewhere along its length. The inspection regime included regular thickness testing at four locations, but not at the area local to the failure. Notably, an adjacent relief line was also found to have very similar localised and severe internal corrosion, which had also gone undetected.
The incident is similar in nature, and on related plant, to an incident in 2003 at another UK oil refinery. In that instance, internal corrosion resulted in thinning along a horizontal section of a relief line, which again went undetected and resulted in catastrophic failure. Liquid partially filling the line resulted in a particularly aggressive interface, whilst condensation local to pipe supports was also a factor. In this and the recent incident, harm was avoided because personnel weren't in the vicinity, and escalation was avoided because the resulting vapour cloud dispersed before finding a source of ignition.
Industry Guidance and Relevant Good Practice
API 570  defines dead-legs as:
"Components of a piping system that normally have little or no significant flow. Some examples include blanked (blinded) branches, lines with normally closed block valves, lines with one end blanked, pressurized dummy support legs, stagnant control valve bypass piping, spare pump piping, level bridles, pressure relieving device inlet and outlet header piping, pump trim bypass lines, high-point vents, sample points, drains, bleeders, and instrument connections. Dead-legs also include piping that is no longer in use but still connected to the process."
This incident has again highlighted the risks of dead-leg corrosion, and that existing integrity management regimes may be inadequate to manage the risk of highly localised and aggressive internal corrosion, particularly when thermal gradients, deposits, liquid interfaces and condensation locations (such as supports and clamps) may be present along the dead-leg.
Operators of plant have a duty to maintain the pipework in accordance with Regulation 5 of the Provision and Use or Work Equipment Regulations (PUWER). In addition, regulation 6 of PUWER mandates regular inspection where deterioration can take place. There are additional requirements for operators of plant subject to the Control of Major Accident Hazards Regulations (COMAH) to 'take all measure necessary' to avoid incidents (including risks to the environment), and in the offshore regime the Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations 1995 have similar safety requirements to COMAH. Both sets of regulations for the major hazard sector require operators to adopt a hierarchal approach, seeking elimination of risk ahead of prevention, control and mitigation strategies.
Guidance on the inspection of pipework, including dead-legs is contained within API 570, API RP 574  and advice published by the Energy Institute . These publications are regarded as relevant good practice for the purposes of reducing risks to as low as reasonably practicable (ALARP), particularly for process plant on a COMAH establishment or offshore installation. Operators of such plant should use the advice they contain when they consider the risk of failure of pipework and derive a suitable examination regime.
Advice relating to dead-leg inspection can be found within Section 6.6.2 of API 570 and Annex I.6 of the Energy Institute guidance, and includes comment on the level of rigour that needs to be applied; over and above regular pipework examination. The API guidance includes the following points pertinent to the cases outlined:
- Operators should consider the removal of non-essential dead-legs;
- Dead-legs can require special attention due to increased corrosion rates;
- Risk assessment should be employed for lines at risk of accelerated corrosion, or which may be difficult to isolate. Any thermal gradient, liquid interface or other feature that could exacerbate corrosion should be included in the risk assessment as this may lead to highly localised deterioration.
- Various non-destructive testing methods are available depending on the size and duty of the pipework, but for high risk dead-legs, a reliance on spot UT is not considered adequate, and operators should ensure that a significant proportion of the entire dead-leg is tested for remaining wall thickness to help identify local corrosion.
Pipe-work minimum thickness / retirement limits are provided in API 574.
Note that the Energy Institute Guidance suggests '…a dead leg is any segment of piping extending below the horizontal plane of the pipe…' – this does not take in to account the vertical nature of relief piping, which is accommodated in the API guidance and should not be overlooked.
Operators of process plant should ensure that their pipework examination regime takes account of dead-legs, and in particular that:
- Dead-legs are eliminated wherever possible, and minimised thereafter;
- The risk assessment and associated examination regime takes account of conditions which may be peculiar to dead-legs, including thermal gradients, interfaces, solid/corrosion deposits, condensation points etc., and that the frequency of examination and level of scrutiny reflects potentially aggressive and localised deterioration. Where there is additional environmental threat, an absence of secondary/tertiary containment should influence the risk assessment.
- The integrity regime should monitor pipework wall thickness so that localised deterioration is captured, checks are undertaken with sufficient frequency, and repair or retirement and replacement is undertaken in good time;
- Process lines which are redundant but retained for future use are left in a safe state, and thoroughly checked for integrity before being reinstated.
Relevant Legal Documents
Health and Safety Executive, Redgrave Court, Merton Road, Bootle, Merseyside, L20 7HS.
- API 570 Piping Inspection Code 4th edition, February 2016, plus addendum May 2017
- API RP 574 Inspection Practices for Piping System Components, 4th Edition January 2016
- Energy Institute: 'Guidance for corrosion management in oil and gas production and processing', Second edition, March 2019.
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