The Corrib field is located approximately 83 kilometres off the coast of County Mayo, in 350 metres of water. The distance from the seabed to the Corrib reservoir is a further 3,000 metres.
The Corrib development is made up of four components:
Six wells have been drilled at the Corrib field. To drill a well more than 3000 metres below the sea bed, the semi-submersible rig drilled section-by-section until it reached the reservoir. When each section was complete, steel casing was installed and held in place with cement. This makes the well strong enough to withstand the pressure from the reservoir.
Each well has a “Christmas tree” on top of it. This is a structure that contains all the necessary equipment to control, monitor and shut off a well (similar to a tap). The “Christmas trees” are covered by a well protection structure to prevent snagging on fishing nets and to protect them from damage from any dropped objects.
The gas from each of the wells is piped through individual sections of flexible flowlines to the production manifold, where it is combined and co-mingled before being fed into the main pipeline for transport to shore.
Subsea facilities are monitored and controlled 24 hours a day from the control room in the onshore gas terminal.
The Corrib gas pipeline is 50 centimetres in diameter. The wall of the pipeline is more than 27 millimetres and is made of high-grade carbon steel. By way of comparison, Gas Network Ireland pipelines running throughout the country are 9 millimetres thick.
The outside of the pipeline has several layers of protective coating, including plastic and concrete, to protect it from external corrosion.
The bundle of cables and small diameter tubes called the “umbilical” carry electrical and hydraulic power to operate the subsea controls. As the name suggests, this is the lifeline to the subsea facilities and connects them to the onshore terminal. To protect the gas pipeline, the umbilical also transports an anti-corrosion chemical and methanol (a type of antifreeze) to the wellheads to mix with the gas and prevent hydrates (ice-like crystals) from forming in the pipeline.
Internal corrosion is controlled by the continuous injection of corrosion inhibitor via the umbilical. External Corrosion is controlled with coatings as the primary protection and secondary is provided by Cathodic Protection. Cathodic Protection and coating condition is monitored by means of Annual Inspection from an ROV Support Vessel. An internal inspection tool, referred to as a ‘smart pig’, is pushed through the pipeline at periodic intervals to gather data on the internal and external condition of the pipeline and monitor the integrity of the pipeline.
As a safety measure, the pipeline was originally designed to withstand 345 bar in the highly unlikely event of pressure increasing above the normal operating pressure.
Advantica were commissioned by the Department of Communications, Marine and Natural Resources, to carry out an independent safety review of the Corrib gas pipeline. The findings of this review were published in May 2006 and concluded that “proper consideration was given to safety issues in the selection process for the preferred design option and the locations of the landfall, pipeline route and terminal.”
Advantica recommended a system be put in place to ensure that the onshore pipeline pressure could not exceed 144 bar, suggesting this to be a practical and effective measure to reduce risk in light of societal concerns that had been expressed.
As part of the design of the onshore pipeline, which was approved by An Bord Pleanála in January 2011, the maximum allowable operating pressure (MAOP) within this section is now 100 bar. Its MAOP under the 2002 design was 150 bar. However, the normal operating pressure in the onshore pipeline will be approximately 85 bar and the pressure reduced as gas in the Corrib reservoir naturally depletes.
In addition, a Landfall Valve Installation has been incorporated into the design, which automatically shuts off the pressure from offshore in the very unlikely event that the pressure in the onshore pipeline should rise towards 100 bar.
The onshore pipeline route is underground along its entire route, and is now 234 metres from the nearest occupied house – more than three times as far away from occupied housing compared to the originally approved route. The route has minimal impact on the local environment and designated conservation sites such as Special Areas of Conservation (SAC) and Special Protection Areas (SPA).
The main purpose of the Bellanaboy Bridge Gas Terminal is to process and dry the gas by removing liquids so that it is suitable to flow into the Gas Networks Ireland (GNI) pipeline network. Natural gas consists principally of methane, and is often found together with water or other hydrocarbon gas and liquids; the gas we use in our homes for cooking and heating has all liquids and most other hydrocarbon gas removed. When the Corrib gas arrives at the terminal it will contain liquid, some of which will be in vapour form, consisting of:
Once the liquids have been removed from the gas, it is ready to be re-compressed to a suitable pressure so that it can enter the GNI grid. Natural gas that has undergone this processing is referred to as dry natural gas. Corrib gas, like all other types of natural gas, has no smell. Before leaving the terminal the gas is “odorised” by adding an odorising agent.
Located 9 kilometres inland near Bellanaboy Bridge, Co. Mayo, the terminal is located in a wooded area so as to cause minimum visual impact to the surrounding area. It has been designed to process up to 10 million standard cubic metres of sales gas per day and is operated 24 hours a day, with the control room controlling the flow of gas from the offshore wells through to the terminal process systems and into the GNI network.
In November 2007, the terminal was granted an Integrated Pollution Prevention Control (IPPC) Licence to operate by the Environmental Protection Agency (EPA). In granting the licence the EPA confirmed that emissions from the terminal “will not adversely affect human health or the environment and will meet all relevant national and EU standards, when operated in accordance with the conditions of the proposed licence”.
The Corrib Gas Partners are committed to ensuring that emissions from the terminal are monitored and analysed in order to demonstrate that emissions are within stringent limits set by the EPA under the terms of the IPPC Licence.
The small amount of water produced from the Corrib reservoir and rainwater that has fallen in areas of the terminal site where contamination could occur, is treated to ensure it meets Environmental Protection Agency standards before being discharged offshore.
The methanol (“anti-freeze” injected to prevent ice formation in the pipeline) is recovered from the gas and regenerated prior to being re-injected back out to the offshore wells, flowlines and manifold.
The Bellanaboy Bridge gas terminal is self-sufficient in terms of energy and power generation. The main source of emissions from the site is from the exhausts of engines that provide compressor power, energy, heat and light for the terminal. These engines are natural gas fuelled using some of the gas produced from offshore. Emissions from the plant are relatively small and only a fraction of what the nearby Bellacorrick power station produced when in operation.
As well as atmospheric and water emissions, light and noise generated from the terminal are strictly controlled to ensure the absolute minimum impact on the local environment and its inhabitants.