Dead-legs remain a potential threat to safe production operations; they merit special attention because of the enhanced corrosion risk that these can often constitute.
Deadlegs are defined as production segments continuously exposed to process fluids but without normal flow or provision for flow, including lines which are closed-off to flow by flanges, welded caps or other fittings. Examples of typical deadlegs that are encountered in practice are summarised as follows:
i. A section of pipework which is not part of the normal flow regime, but which cannot be isolated from the normal flow (e.g., obsolete lines).
ii. Pipework sections that are used intermittently (e.g., bypass lines and non-operating sections of spared equipment).
iii. Pipework sections either upstream or downstream of a normally closed valve.
iv. Drains pipework at the base of vessels.
v. Bridle pipework associated with a vessel.
The fluids contained within dead legs are often stagnant; this leads to a number of potential problems over the longer-term:
Aqueous hydrocarbon production fluids constitute a viable source of nutrients for microbiological entities; the stagnant conditions in deadlegs are also conducive to the formation of sessile colonies of bacteria that may otherwise be less likely to form under flowing conditions. The accumulation and proliferation of sessile colonies of bacteria constitutes a credible threat from microbially induced corrosion (MIC). Local wall thinning leading to eventual perforation of the pipe wall as a result of MIC is a key threat to the integrity of deadlegs.
Aqueous hydrocarbon fluids contain acid gas fractions (CO2 and/or H2S) which are corrosive. When the corrosion process commences, the products which form dissolve in the process fluids, however as deadlegs have no flow, the confined fluids volume is constrained such that saturation with corrosion products will occur relatively promptly. This leads to the accumulation of corrosion products and the eventual formation of debris and/or lose scale, and to the onset of under-deposit corrosion; local wall thinning leading to eventual perforation of the pipe wall as a result of under-deposit corrosion is also a key threat to the integrity of deadlegs.
It should be recognised also that once internal corrosion within deadlegs commences it is often considerably more difficult to prevent its progression via chemical treatments alone using biocides and/or corrosion inhibitors. This is because well-established sessile colonies of bacteria are highly tenacious and will likely persist even where aggressive biocide treatments are employed; and the lack of flow and presence of debris substantially reduces the effectiveness of corrosion inhibitors.
The optimal approach to the management of deadleg integrity is as follows:
1. Deadleg Elimination: using the logic that prevention is better than cure, the elimination of deadlegs is the most effective strategy, by virtue that it will remove all associated risks. Deadlegs should therefore be identified and reviewed regularly to determine whether removal is possible. Removal of deadlegs however, should be conducted in accordance with the management of change process.
2. Deadleg Strategy: in the event that deadlegs cannot be removed, a strategy for managing the integrity of deadlegs should be developed, involving the following:
i. Deadleg Integrity Risk Status (based on the following considerations):
a. Material of construction.
b. Nominal diameter and wall thickness.
c. Flow status (whether flow conditions are ever established).
d. Pressure and temperature
e. Process fluid chemistry
f. Consequence of failure of each dead leg.
g. Location of the deadleg (potential for escalation).
ii. Consideration of whether the pipework can be drained and left empty.
iii. Development of a risk-based scheme backed by data analytics to drive the deadleg inspection program.
iv. Development of an appropriate program for flushing of deadlegs (including treatments using biocides or corrosion inhibitors as appropriate).
WE HELP IDENTIFY YOUR SUSCEPTIBLE SYSTEMS
Although dead legs are considered a potential threat to all process systems, our industrial experience and Veracity data analytics demonstrate that some systems are more susceptible to dead legs than others. If large quantities of dead legs are to be managed, it is important that the appropriate resource and adequate inspection is performed on the most susceptible systems. Through our Veracity platform AIE provides leading risk based and data driving solutions risk-based assessments an data analytics and prioritisation based on a systematic identification of potential damage and an unbiased assessment of the probability of failure.
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Veracity offers an all-in-one solution, designed by engineers for engineers to identify and manage risks associated with dead legs, whether it is identifying additional controls to mitigate risk or utilising the existing controls more effectively. By using advanced analytics, we set the optimal strategy, develop digital dead legs work packs and engage our app for seamless site execution. Our VeracityCCM module is then on hand to define and manage chemicals dosing programs.
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