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Corrib gas: Black starts, intelligent pigs and the mechanics of extraction

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Corrib gas: Black starts, intelligent pigs and the mechanics of extraction

Screen Shot 2015-06-11 at 19.31.15Peter Murtagh: Monday, 29 June 2015

The mechanics of extracting gas from the Corrib field appear simple enough. It is only when one gets into the detail of the engineering that the complexities emerge.

This is an environment in which a relatively small number of people can spend years – in remote places such as Sakhalin Island in the far east of Russia, in the Middle East, or on rigs in the North Sea – living intense lives in dangerous conditions and sometimes speaking a language alien to others.

They talk about “slug catchers” and “intelligent pigs”, about Christmas trees at the bottom of the sea, and about “black starts” – no one wants a black start, but if you are going to operate a terminal like the one now being tested at Bellanaboy in Mayo, someday for sure, you are going to have to do a black start.

So. Better practise now.

There is a simulator room on the first floor of the Shell offices in Belmullet. It is a little like a radio studio: on one side of a glass wall, technicians tap instructions into their desktop keyboards.

On the other side of the glass, the consequences of their actions appear on the screens of the mock gas terminal control room.

The two sides communicate by walkie-talkie and the trainees cannot see the instructors, but Gary Killen from Coleraine and Jim Munro from Scotland (and now married in Mayo), have to work out what to do.

In front of them are four computer screens, each showing a diagrammatic representation of the gas terminal operation. Visually, it has all the simplicity – and complexity – of the London Underground map.

“We run the guys through an emergency drill – what we think can’t possibly happen, but let’s make it happen anyway; something like in the law of averages it’s probably got a 1 per cent chance of doing it, but we make it happen,” Pat Connett, a trainer with Lowestoft College in Suffolk who previously worked for Shell for 29 years, explained during a visit to the simulator this spring.

Shut down safely

“So then these guys have to react, they have to call the emergency services, get their emergency teams together and deal with the whole thing. Shut down safely, keep people safe, and we practice that over and over again. In fact we do a major exercise four times a year. We practice again, and again and again.”

Co-trainer Steve Fennell, who was in Corrib for about three years working on operation readiness, but has since transferred to Oman, explains the terminal’s power supply system, the most backed-up part of the whole operation.

The plant has three gas-powered generators that are independent of the ESB system, which is also available. Two of the generators are always running in parallel, the third available if one of the other two fails. If two generators fail, the plant can be run on the remaining one, plus the ESB.

If all fail, “we would then have to start worrying about our safeguard and back-up systems because we’ve lost power, we wouldn’t have fans running in sensitive areas where we need to have airflow, so then the emergency generator would kick in, which is diesel driven”, says Fennell.

At this point, the terminal operators would depressurise the system, using the flare stack – that is, set alight, through the top of a stack which is taller than any other part of the plant, any gas that was still in the system.

It’s called a “flare off” as opposed to just releasing the gas into the atmosphere, which is more environmentally damaging.

Gas is not stored at the terminal; the inventory is just what is in the pipes and vessels being processed at any given time.

“The emergency generator would give us enough power for escape lighting for people and it would power battery chargers for some of our safety systems, such as fire and gas detectors, what we would call the critical stuff. And we have battery back-up.”

All vehicles at the terminal are parked at right angles, front pointing forward, for rapid exit in emergency.

And the worst case scenario?

Fix the problem

“Total loss of the plant,” says Connett. “Black start. When you lose all your power and everything shuts down, the whole lot, and your batteries start to run down, then you’ve got to find the problem, fix the problem and before the batteries completely discharge, start the whole thing up again. Its like you’re the middle of the night, everything is pitch black. That’s a black start.”

The gas, which is 97 per cent pure, comes from inside a vast pile of 250 million year old sandstone about 3km below the seabed and some 83km out in the Atlantic. The gas is trapped in the rock by a cap of salt.

Inside the rock, the pressure of the gas is at 350bar – that is, 350 times greater than normal air pressure at sea level, or about 5,000 pounds per square inch (PSI) – and is held in the rock similar to the way gas is held inside an aerosol tin. Press the button and the odourless gas comes out. The “button” in this instance being a wellhead on the seafloor with a tap – really a series of valves – that is called a Christmas tree – because it looks like one.

Gas leaving the sub-sea reservoir is pressure reduced to 120bar and methanol is mixed with it to prevent freezing. By the time it reaches landfall at Glengad, the pressure is 85bar to 95bar. Entering the terminal, it is reduced further as the gas is processed and conditioned, before being increased again to 85bar for pumping into the national gas pipe network, after the methanol and any other impurities such as water have been removed.

There are six wells drilled into the Corrib reservoir to extract the gas. They are called P1, P2, P3 – through to P6.

Flow lines from each of the well heads merge and combine into a single pipeline which is inside a seabed junction that is known as the central manifold. This is the start of the Corrib gas pipeline.

It runs along the seabed for approximately 83km and, when it makes landfall at Glengad, it then runs for 8.3km along along a trench and tunnel to the terminal.

The rate of flow from the well heads is regulated from the control room inside the terminal. However, production technologist and engineer Dena Cullen, who designed two of the wells, can simulate the reservoir’s performance using her laptop from just about any location in the world.

‘A little baby’

“All our systems have these tags on them which, if I log onto a system, you can see the wells wherever you are,” she says. “I still look at my old wells that I used to work on in Holland to see how they are doing, how they are behaving. You kind of become attached to a well. They’re like a little baby. . . I’m very sad [the Corrib wells are] in the sea. A lot of the wells that I’ve worked in the past are on land and you can actually go and see them.”

Cullen, who has worked in Russia, the North Sea and the Netherlands, says the Corrib gas field is among the best she has experienced. “If God could design a nice field, I think its Corrib,” she says. “It is high pressure, it is dry gas, it’s a beautiful, beautiful field. It’s got six wells so it’s quite easy to manage.”

Cullen’s job is to monitor the reservoir, the pressure at the well head and the central manifold, the temperature and flow rates of the gas, and to ensure there are no hydrates (that is, hydrocarbon ice) entering the offshore pipeline.

The pipeline that has come over 90km from the wellheads rises out of concrete and gravel and, via a cluster of valves and taps, plugs into a spaghetti-like maze of smaller pipes, leading the gas through the brief process in which impurities are removed, the smell added and pressure altered before passing through a meter and into the Bord Gáis network.

Any accumulation of liquids in the gas are known as “slugs” and inside the terminal, there is a “slug catcher”, a system of parallel pipework where the liquids are separated from the gas.

The gas is also odourised as the final part of the terminal process. (An intelligent pig is a robot or drone that can made to travel through the pipe, cleaning the inside of it and inspecting it for any corrosion. Like the slug, it too is removed by a catcher – a pig catcher in this case.)

Since spring, control room training has moved from the simulator into the terminal proper as the plant is commissioned, section by section, ahead of full production, or first gas as it is known.

Recently, The Irish Times and two journalists from the Norwegian newspaper Stavanger Aftenblad were shown around.

Antiseptic charm

The terminal looks rather like the Pompidou Centre in Paris, but without the building – just the pipework. The only solid structure on the site is the concrete and stone reception building holding the control room, a stark, windowless place, bright but with the antiseptic, cold charm of an operating theatre – all airconditioning hum, tiled floor and swivel chairs.

There are 14 computer screens, each displaying a part of the inner workings of the system. They all look unintelligible to the untrained eye.

Set in the middle of the screens is another, a portrait-shaped panel showing lights and with the words subsea, gas terminal and firewater pumps written on it.

On the wall to the left of the operators is another diagrammatic screen, a map of the plant itself. Over on the left side are four more desktop type screens showing various views from outside the plant, and the perimeter surrounding it – views from the security cameras.

In the middle of all this sits trainee control room operator Gary Killen, still being put through his paces, but now several steps closer to the real thing.

Stuart Basford, the plant’s commissioning, start-up and operations manager, and Fenna Snater, the senior operations and start up engineer, explain how everything works. Bosford has been on the Corrib project for 10 years and will retire when the terminal is up and running; Snater, who is Dutch and a chemical engineer, came to Erris in August 2014 via Sakhalin and a stint with MSF in Zimbabwe and the UN’s Habitat programme in Nepal.

Few problems

She has been surprised during the commissioning process at how few problems have been encountered, although flaring – which has just concluded – went on longer than expected initially.

“I would have expected more [problems] because of the duration we’ve been on hold here and I honestly thought we would have a whole set of unique problems here, different from other projects but, up till now, the equipment has been starting OK.”

After all the delays and subject to one outstanding permit, the Corrib gas pipeline and terminal appear ready to start production.

SOURCE

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