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Rock release


One of the world's biggest underground gas storage caverns has just started operations in south east China.

First deliveries have recently begun of liquefied natural gas (LPG) from five giant caverns hewn out of coastal hills near the major city of Ningbo in eastern China. The scheme is one of a series of energy import facilities being created around the country as its economy expands and local infrastructure is modernised.

For British oil giant BP and the recently merged Amoco, which is China's western investment and management partner for this scheme, the rock cavern choice was obvious. Rock voids deep underground can store much greater volumes of gas than steel pressure cylinders above ground. They do not require extensive safety quarantine zones around them, nor are they subject to summer heat problems.

BP project manger Herbie Redpath says that at full capacity the five caverns will hold over 500,000m 3of the liquefied gas.

For construction BP/Amoco turned to Swedish contractor NCC which has built up wide experience in cavern design and excavation. Underground storage has been developed extensively in the last 25 years, particularly in Norway and Sweden where the predominant hard rock geology lends itself to the method. The firm bid for the $60M work in joint venture with Japanese process engineering specialist Chiyoda under an engineer procure and construct form of contract working to a front end design by French specialist design consultant Geostock. BP used the consultant for site investigation and feasibility and as construction manager for the project when excavation began in 1999.

Three of the caverns at Ningbo store propane 125m below ground, while two more at 65m to the cavern crown are used for for butane, which has a longer hydrocarbon chain and is therefore less volatile.

The LPG must be kept under pressure to remain in a liquid state. Groundwater all around the cavern provides a seal. 'A small amount must run into the cavern all the time, ' says Lars-Olof Dahlstr÷m, head of NCC's rock mechanics department. A shallow 'pond' in the reservoir base forms a floor seal and levels are maintained automatically by pumping the surplus water from a monitored sump point, out through the gas inlet point in the cavern roof.

Part of the design challenge for creating such caves lies in ensuring a rock mass of the right quality, and with a sufficient groundwater level to surround the cavern and maintain pressure. Modern designs do not leave this to chance and at Ningbo one of the first tasks was to create a network of small parallel tunnels above cavern roof levels.

These are used to bore a interlocking 'umbrella' of drill holes through which water is injected at carefully monitored pressures during the facility's lifetime to ensure that rock cracks are always saturated. The rock at Ningbo itself is a conglomerate with pyroclastic materials.

'You need to create the umbrella before the main excavation, ' says Linus Levinson NCC's technical manager for the project, because it is hard to ensure total saturation otherwise, he explains. Surprisingly this causes little in the way of water inflow problems during the work.

The 14m 2tunnels were drilled by early 2001 using Tamrock rigs, by which time excavation was under way on the larger caverns. Atlas Copco jumbos were used on this work with the 560,000m 3of spoil hauled out in 35t Kiruna dumptrucks. Br°yt face shovels were the main tool used for loading and Toro loaders worked in the smaller tunnels.

'Spoil was deposited primarily in a reclaim area just along the sea coast, ' says Levinson. Part of the contract involves formation of a jetty area in this deepwater coastal area and the reclaim and its roads are part of the permanent facility.

Though NCC brought in equipment, it has also worked with local subcontractors. The Chinese state owned 6th Engineering Bureau of Hydropower & Water Conservancy took on shaft construction and much of the drill and blast work with some 400 workers on site.

Partly at BP's insistence NCC pushed a strong safety culture on the site. 'The teamwork with the Chinese firms was good, ' says NCC's China manager Anders S÷reklint 'and we achieved 3.8 million man hours without one lost time incident.'

Cavern excavation was relatively straightforward, he says, with better than expected rock quality. The two parallel butane caverns, each 290m long 22m high and 20m wide, and three smaller but deeper propane caverns were formed at the same time.

All five were completed by October 2001 with handover of the complex, ready for operation in June 2002.

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