Upgraded lightning protection for remote corrosion monitoring

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Abriox has upgraded the lightning protection on its remote corrosion monitoring solutions for pipelines, with the help of the lightning test consultancy services of Cobham Technical Services. The degree of protection of the system against high energy surges has been substantially enhanced by a development exercise incorporating advice on the nature of coupling between lightning power surges and ground-based equipment, and characterisation studies including destructive testing.

The protection has been implemented on Abriox's Merlin cathodic protection (CP) monitor. This telemetry-based instrument is one of the most widely used field devices for remotely monitoring the anti-corrosion CP systems that are installed on pipelines, storage tanks and other buried metal infrastructure used in onshore oil and gas networks.

"Pipelines are a significant attractor for lightning, and in some regions of the world strikes can occur frequently. We've always had lightning protection on our corrosion monitor, but the only feedback we ever got on how it performed in the real world tended to be when a burnt-out unit arrived back from the field," says Jason Hanlon, technical director of Abriox. "How much energy was present, what the shape of the energy surge was, whether it arrived directly or indirectly remained a mystery - and we decided it would be a good idea to better understand the risk by talking with lightning experts."

Abriox has its design centre in the UK, and after investigating the high voltage testing market, selected UK-based Cobham Technical Services, because its lightning unit is one of a tiny number of organisations in the world that specialise in lightning testing and consultancy and is able to give practical advice, rather than simply testing against standards.

An initial review considered the particular installation conditions and environments of the Merlin CP monitor, and a typical catastrophic field failure. A destructive test at Cobham's test facility in Abingdon was performed. It became clear the corrosion monitor was most likely dealing with power surges that arrived following direct strikes on the pipeline itself, or the supply to the electrical rectifiers that provide the impressed-current cathodic protection system. Unlike some of the areas that Cobham works in - particularly aircraft protection - there are no standard lightning test waveforms for this type of nearby strike to ground-based equipment, but that did not prevent Cobham from creating a representative waveform specifically for this testing purpose.

The destructive test exercise also demonstrated to Abriox that some of the external lightning surge protection devices originally selected for use with Merlin did not actually perform in the way the manufacturer's datasheet indicated. Although other aspects of the Merlin design provided a good degree of protection, the Abriox designers sought further improvement.

After the exercise, Abriox gained a better understanding of the nature and energy levels of lightning-related power surges, and decided to re-engineer the system to increase the protection level. This exercise involved both uprating the surge protection circuitry, using different components and changing the physical layout of parts of the embedded electronics system.

To speed the design phase, Abriox constructed its own simple low-power generator that could provide a high voltage pulse, to test switching times and clamping characteristics. However, when the final protection design was settled on, Abriox took a monitor to Cobham to fully characterise its performance against lightning pulses.

Cobham subjected the equipment to increasing levels of lightning strikes using a range of pulse shapes and durations that represented the kind of surges that would be experienced in typical installation scenarios. The revised protection worked perfectly, and continued to operate successfully beyond its target energy level protection rating corresponding to a 12 kA transient waveform. Cobham used a 30 kA-rated generator to test the equipment, and in the very final test step, the strike energy was increased to the maximum. Although this destroyed the front-end protection circuitry, the Merlin monitor itself survived and continued to function.

"With Cobham's help, we now know exactly what our lightning protection system is capable of," adds Jason Hanlon of Abriox. "It's impossible to protect against every conceivable lightning strike, but we know that our equipment will be resilient when faced with the majority of the real-world energy surges that could be encountered."

"This particular project was very interesting. Our understanding of the nature of the lightning threat means that we were able to simulate the type of waveform expected by Abriox's monitor in the field," says Dan Brown of Cobham Technical Services. "This type of pipeline installation makes it highly likely that power surges arrive indirectly, from the pipe or power supply, making it important to consider protection for the design as a whole - rather than just the system inputs. It's easy to blow up a device in our lab; what's more of a challenge is to do it in a representative way."

 

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