The April branch meeting was joint with The Welding Institute, and Dr Patrick Lydon, IACS Corrosion Engineering, gave a presentation on “The Corrosion and Safety Hazards Associated with AC and DC Interference on Onshore Pipelines”. This was a hybrid meeting, with 23 attendees in person, and 20 online, approximately half of these from oversees.
Pat explained that one of the main concerns associated with AC, and also DC, stray current interference on pipelines is that it can in certain circumstances result in high rates of corrosion, even if a pipeline system has fully effective levels of cathodic protection (CP).
In the case of AC corrosion, the highest reported rate of corrosion in the UK was 2.45mm/yr and that resulted in through-wall corrosion and loss of containment within 2.5 years of the pipeline construction.
A situation was described where a powerline operator had changed the electrical configuration on a power cable system routed close to a pipeline, whilst maintenance work on the cable system was being carried out. This resulted in a sudden increase in AC voltage on the pipeline from 2V to about 20V which resulted in an increase in the AC corrosion rate on the pipeline from 0.02mm/yr to 4.0mm/yr.
Pat pointed out that it is not only higher rates of corrosion that pipeline operators should be concerned about but also the electrical safety risks to personnel working on pipelines from short term and long-term AC interference. AC voltages can be induced on pipelines from low frequency inductive coupling between buried and above ground pipelines, where they are routed in close proximity to high voltage powerlines.
Historically pipeline designers did not consider the proximity to electrical substations and powerline pylons when selecting a pipeline route. Thus, in the event of a ‘phase to earth fault’ on the electrical systems there could be an enhanced touch potential risk to personnel working on a pipeline from the resultant ground potential rise (GPR) effects during short term interference events. The fault currents can result in high localised voltage gradients, which can transfer potentially fatal touch voltages to pipelines, presenting a safety risk to personnel working on a pipeline system.
Pat then pointed out that the standard that addresses the touch potential limits is BS EN 50443 “Effects of electromagnetic interference on pipelines caused by high voltage AC electric traction systems and/or high voltage AC. power supply systems”. However, the voltages specified in this standard are in excess of those adopted in other countries and in other industries. The limits stated in BS EN 50443 only apply to electrically instructed personnel, and for a specified footwear insulation resistance, which may not be achievable when working on a pipeline. It was pointed out that current guidance is to use an alternative to this standard to specify maximum touch potential limits.
Examples of pipeline damage due to lightning or an arc strike, where loss of containment has occurred where shown. Lightning damage can result in through wall perforation in a relatively short time period. Figure 2 shows an example of a high voltage surge damage to 3-layer polyethylene coated pipeline.
An example of the damage that an arc strike from a 400kV power cable onto a buried gas main was also given. The arc strike occurred because a fire had occurred in a wooded area underneath the powerlines.
The fire changed the dielectric strength of the air and resulted in an arc which hit a pipeline and caused the damage shown on Figure 3.
The basic safety guidance for work on pipelines located within the vicinity of overhead and buried powerlines was discussed, and information provided on recommended safe practices when working near HV power cables, together with details on plant and personnel separation and clearance distances.
It was pointed out that when pipelines are strung out above ground for welding, close to HV overhead power cables, there is the risk of inductive and capacitive coupling creating relatively high AC voltages on the pipeline sections, which can affect welding operations but also create a touch potential risk. To mitigate this risk, each end of the above ground pipeline section should be earthed.
The presentation included the factors that pipeline operators and designers need to consider when assessing and mitigating the risks from electrical interference on pipelines which relate, not just to touch potential risks, but also the ignition risks from pipework or cables carrying electrical current.
Pat also explained that pipeline operators often isolate the pipeline CP system before performing welding operations to ensure that when pipelines are cut, there is no spark potential risk from the CP current. However, the magnitude of the AC current flowing in a pipeline where it is routed close to powerlines can be at least an order of magnitude greater than the DC current and can result in an incendive ignition risk when pipework is cut.
It is important that not just the DC current, but AC current flowing in a pipeline, is considered. This is because switching off a CP system may not remove all ignition sources. It is important to ensure a solid bond is in place before pipework is cut or removed. There should also be sufficient slack cable in the bond to allow for pipe movement when a pipe is cut, which could otherwise result in cable detachment.
Surge protection on pipelines was also discussed and information provided on how surge protection on cross country pipelines may compromise plant earthing systems. DC decoupling devices e.g polarisation cell replacements (PCR)s can facilitate the flow of AC current through earth cables within above ground installations (AGI)s and compromise the specific earthing arrangements in AGIs.
The presentation concluded with examples of situations where DC stray current corrosion has occurred due to incorrect welding operations. High rates of corrosion were observed within a relatively short time.
There was a lively question and answer session, and Pat was then thanked for his excellent presentation and presented with a pen on behalf of London Branch, and a paperweight from TWI.
The next branch technical evening will be on 13th October, and details of the speaker/topic will be given in the summer magazine, and also on the ICorr website.
Figure 1, Typical AC corrosion on FBE coated pipeline.
Figure 2, Lightning Strike on 3-Layer Polyethylene Coated Pipeline.
Figure 3, Damage to pipeline caused by arc strike.
