Aberdeen is to play a key role in Gateway, the most ambitious gas storage project attempted in the UK, the Rough facility excepted.
However, whereas Rough is a depleted North Sea gas field that was converted to storage, Gateway will centre on a complex of caverns purpose-excavated in massive salt strata beneath the North Irish Sea.
Gateway (GGSP) will boost current gas storage capacity by about 30%, adding a further five days of buffer based on Britain’s average gas demand.
Detailed engineering work is under way, with Amec and Senergy in Aberdeen, plus Parsons Brinckerhoff, contracted to carry out the front-end engineering design (FEED) work for both the offshore and onshore elements of the 1.5billion cu m underground storage scheme that will cost some £600million to create.
The engineering work at Amec is split between Aberdeen and Darlington, with a team of about 30 engineers and other specialists in each location. Amec’s work covers offshore installations, pipelines, the onshore compression station and gas grid connections. Senergy’s task is focused on the offshore infrastructure, installation, logistics and well designs.
Parsons Brinckerhoff is advising on the salt cavern design and construction.
The project team is based in Aberdeen at Amec and, assuming smooth passage, the company driving the Gateway project, Stag Energy, is expected to commit to the construction around the end of this year with a view to the facility becoming operational in 2014.
Aside from creating the underground complex – 20 caverns – by literally dissolving away rock salt, there will be a need for a family of 20 mini-monopod platforms – 19 standardised at 400-600 tonnes, plus a 1,000-tonne unit nicknamed the “Daddy Pod” – infield pipelines and an export line to Barrow in Furness some 15 miles distant.
Amec project director Murray Douglas told Energy: “In total, we’re going to have something like 11,000 tonnes of topsides … 19-20-21 installations … so it’s quite a big fabrication exercise.
During the construction phase, though it’s hard to estimate at the moment, my guess is that around 500 people will be required at peak. We’re looking to be able to place orders at the turn of the year.”
Peter Taylor, a director on the survey and geo-technical board at Senergy, said Gateway represented an opportunity for oil&gas and renewables skills to meld – oil&gas because of the hydrocarbons element and renewables because of the standardised multiple facilities.
He explained that of particular interest to the Senergy team during this early phase is the geology – both soil conditions at seafloor level and down in the salt strata.
He said, too, that data had been harvested from windfarms in the area, plus there was site survey data to hand.
“Also, data was collected from a geo-technical borehole two years ago, which we project managed.”
He pointed out that the Gateway team was very multi-disciplinary and that the offshore oil&gas and wind experiences were valuable to cavern specialist Parsons Brinckerhoff as this sort of work had hitherto been carried out onshore.
Andrew Stacey, director of Gateway Storage, told Energy that the genesis of the project was back in 2003 and that it addressed a number of issues for the first time in the UK, including that it will be the first offshore salt cavern project. Discussions with the Government started in 2004 and became a four-year process.
Describing the hunt for a suitable site, Stacey said: “There is good information in and around the UK as to where salt is located. We identified a few structures offshore in the East Irish Sea because we decided the number of options onshore were very limited and fairly small.
“This salt structure was known about. British Gas drilled a well through it in the late-1970s looking for gas beneath the structure. So we knew there was this unusually good salt structure.
“Given the scale of the project, there is much work to do, but Gateway Storage will bring storage capacity and deliverability to the UK energy market and so help reduce price volatility and risk.”
Gateway will be connected to the National Gas Transmission System (NTS) via a new pipeline to a gas compression station adjacent to the existing Morecambe gas terminals at Barrow.
Twenty wells – one per cavern – will be drilled. These will form the initial phase of the cavern leaching process. The wells will be drilled from a jack-up drilling rig similar to those used to drill oil&gas wells and each should take about 15 days to complete. The wells will be drilled through the monopod substructures prior to installing topsides units.
Once the vertical well has been drilled into the salt layers, the cavern leaching process can start. The caverns will be formed by injecting water under pressure into the selected halite strata, which will form a cavity in the undersea salt bed. This turns the water into brine containing about 30% salt. The brine is then discharged to the sea.
Using this process, the caverns will be gradually formed over a period of about two years. When finished, the caverns will each have a diameter of about 85m (280ft) and a height of 100-260m (330-850ft). The cavern roofs will be at a depth of 750m (about 2,500ft) below the seabed. The salt layer is some 610m (2,000ft) thick.
The leaching equipment will be housed on the monopods and will be controlled remotely from shore.
Cavern leaching was originally programmed to start Q3 2009 and be completed in mid-2013. That has been pushed back to early-2010, completing in 2014.
At the peak of operations, all 20 caverns will be undergoing the solution mining process. This peak period will occur in 2011 into 2012, lasting about seven months.
The main offshore pipeline will be arranged in the form of a large loop running to and from the the Barrow Gas Terminals. It will comprise a “ring main” surrounding the GGSP offshore complex and two 24km offshore import/export lines running from Walney Island to the ring main.
All of these pipelines will have a diameter of 36in. Short lengths of smaller (10in diameter) feeder pipes will connect each cavern to the ring main.