Gas flaring reportedly costs Nigeria US$2.5 billion a year, as well as contributing to air pollution, heat and rainforest damage.
In a project for Shell Petroleum Development Company, Mott MacDonald carried out detailed engineering design, including preparing specifications and requisitions for all materials to be purchased by engineering, procurement, construction and commissioning (EPCC) contractor, Daewoo. Mott MacDonald also provided all specialist technical support required by the contractor during construction and commissioning.
Recovering 200 million standard cubic metres from the Cawthorne Channel, Awoba and Krakama oil fields located in the eastern swamp area of the Niger delta translates into US$600,000 on a daily basis. The project involved construction of three gas plants, one booster compressor station and approximately 71km of gas gathering and export pipelines.
Ingenuity
Solving the engineering challenges involved taking mostly standard designs and putting them together in unique ways. The compressors themselves were an unusual configuration, with gas coming on to the first and second stage compressors. Between the third and fourth stage there was dehydration, and a liquid petroleum gas (LPG) extraction plant.
Furthermore, the project used two compressors running in parallel. Normally there would be one master running at full power, with the second machine part loaded to compress the balance of gas. With only two 50% machines to get the throughput, our team balanced the delivery pressures to operate on the same duties in parallel. This meant we could keep the costs of the project down by eradicating the normal requirement for a third 50% standby machine.
The Krakama station also had some remarkable features, representing the largest trial to date of a new style of membrane dehydration unit operating between the second and third stages of compression. The compressed gas is passed along the surface of cylindrical membranes, where the gas is allowed to pass and the water is disengaged from the gas. A counter flow of purge gas sweeps the membrane elements to remove the water.
As well as being more economical for smaller gas plants than the usual thermoelectric generator (TEG) systems, the membranes should also run indefinitely, and require less maintenance due to the lack of moving parts. Minimising maintenance was an important part of the project due to the remote and difficult location of the plant.
Innovation
The project represents one of the biggest implementations of concepts developed by the EPCC contractor and Mott MacDonald, for Shell, for the elimination of gas flaring.
Building gas plants in the middle of mangrove swamps is a challenge in itself, and it was down to the resourcefulness of the team to come up with solutions to enable the project to succeed.
The gas plant at Awoba was constructed on concrete barges. Modules of the plant were assembled in the USA on these barges, which were then transported to Nigeria and slotted into place, instead of spending two years working in the swamp on site. In this way, the surrounding mangrove area was spared a great deal of disruption.
At Cawthorne the plant was built on a sand pad, as three years before the project started Shell dredged sand in the swamp and built a pad for the plant. The plant at Krakama took a different approach again, being built on a steel platform.
In essence, the Cawthorne channel - associated gas - gathering project will alleviate a great deal of environmental damage, and it will generate income for Nigeria by eliminating a massive amount of daily waste.
Through this project, Mott MacDonald demonstrated highly innovative and practical approaches not only to solve a major environmental problem, but also to make a significant economic contribution by recovering the gas. The project also demonstrated effective approaches and solutions to reducing flaring in other sensitive locations worldwide.
Our work was honoured at the British Expertise Awards 2006, where we were given the joint large consultancy firm of the year award in the ‘tangible visible project’ category.
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