The LRC concept is based on constructing a cavern and then lining it with a sandwich wall construction that consists of a layer of concrete to dissipate the pressure and a gas-tight steel lining. The intention is that a typical storage complex will consist of two to four caverns providing over 60 million m³ of total working gas capacity. The store will be used for equalisation purposes and to provide a reserve supply of gas.
The Halmstad project is being undertaken by a JV comprising the gas business sector of the Swedish power company, Sydkraft, and French national gas company Gaz de France, which have formed a company called LRC Demo AB.
The facility now being constructed near Halmstad is a demonstration plant that will have a storage capacity of approximately 10 million m² at a maximum storage pressure of 25MPa. It is being partly financed by the EU’s Thermie research and development programme. In addition to the underground store there will be a surface plant with equipment for handling the gas. The two parts of the facility will be linked by a vertical shaft containing pipes for the intake and outflow of gas and an access tunnel for transport to and from the rock cavern during the construction process.
The cavern was designed by Sycon Energikonsult, a company in the Sydkraft Group, and the contract for construction of the rock cavern was awarded to Skanska Sverige AB. Quality documents have been drawn up for both the design and construction stages, including a comprehensive inspection programme and a programme for quality assurance. Work began in November 1998 and the plant will be commissioned in the autumn of 2001. The construction costs have been estimated at $20.9m and total costs at around $23.3m.
The Halmstad cavern is in the form of a vertical cylinder 35m in diameter and 50m high, having a spherically shaped dome at the top and a rounded off cup shaped base. Rock cover is 115m.
The entire cavern will be surrounded by a dual-purpose drainage system, partly to drain water from the rock and partly to drain any gas that should leak from the cavern. The drainage system consists of a network of perforated pipes mounted against the rock face and then cast into the concrete wall. The system will have a lower outlet leading to a pump pit and a top outlet via the vertical shaft to ground level.
The rock is mainly composed of good quality gneiss, equivalent to 189MPa, with elements of amphibolite. Although the depth of the cavern is not great, low spalling or minor rock bursts have been experienced, which means that more scaling and shotcrete reinforcement has been undertaken than was originally anticipated.
Dr Robert Sturk, production manager for rock contracts in Skanska Underground Construction & Bridges, points out that this phenomenon has been experienced in other tunnelling work undertaken in this area and says that it appears to be related to the type of gneiss present.
The access tunnel to the rock cavern has a cross-sectional area of 28m² and slopes downwards at a gradient of 1:7. It consists of four main sections: an approach tunnel; a tunnel above the cavern for access to the shaft; a lower cavern tunnel at the base of the cylinder; and an upper cavern tunnel at the base of the spherical dome, which will form the top of the storage cylinder. These tunnels have a total length of 1041m and were driven between November ’98 and November ’99.
A 1m diameter, 90m long shaft to link the gas storage cavern with the surface installation has been raisebored by Skanska Raiseboring from the shaft tunnel to the surface. The pilot hole for the raiseboring operation was drilled from the surface down to a level equal to the bottom of the cavern, a total of 157m. The complete operation was carried out between October 5-28 1999.
The tunnels were drilled and blasted using an Atlas Copco Robot Boomer 185 computer controlled 3-boom rig and Prillit A, Dynamex, Emulite and Gurit explosives. An Akkerman H7 excavator was employed for scaling and a Cat 963B loader for mucking out. Shotcrete was applied as necessary and grouting took place when the water inflow was above 10 litres/min/100m of tunnel, although very little water inflow was experienced during tunnel construction. Stabilator rigs are being used for both grouting and shotcreting on site.
The rock quality was good during tunnel excavation except at one point where a zone of crushed and poorly weathered rock was traversed where the upper cavern tunnel branched off the access tunnel. Careful excavation was carried out in this section using a pilot tunnel and stoping. The rock was reinforced with spiling, shotcrete arches and bolts. 30 000m³ of rock was removed from the tunnels in total and 45 000m3 will be excavated from the rock cavern. When work is complete, the tunnel opening will be sealed with concrete and the tunnels filled with groundwater.
When T&T International visited the site in March, the dome was being excavated. The rock in the dome had been sealed by grouting carried out from the shaft tunnel but other grouting will be undertaken from the face of the cavern as it is excavated. Two spiral tunnels have been driven from the upper cavern tunnel along the periphery of the dome in opposite directions. Where they meet, a ramp has been constructed to reach the very top of the dome.
Great precision is required in the excavation of the cavern. A maximum of 0.3m may be excavated outside the theoretical profile of the tunnel and the damage zone to the rock must not exceed a further 0.3m beyond. The spherical shape of the dome is a complicating factor.
The process adapted is as follows: first, the positions of the drill holes are marked out on the rock surface manually. Measurements of each position are taken using a Geotronics servo driven, direct reflex measurement total station and the results fed into a Geopad field computer which uses software designed by Svensk ByggnadsGeodesi (SBG) to calculate the length of the hole required for each position. These lengths are then fed into the drill rig computer and the holes drilled automatically to the exact length and inclination, thus eliminating human error and facilitating precise results.
At the time of T&T International’s visit, 25% of the dome had been excavated and Sturk was satisfied that the requirements for the profile were being met and a very even profile was being achieved. “To a certain extent, the rock controls how we work and this is really part of the charm of excavating in rock,” he says. “You have to be imaginative in order to obtain a positive result.”
Fibre reinforced shotcrete and 6m long grouted bolts are being applied to the dome for reinforcement purposes.
Twenty-three people are engaged on site, working three shifts. The site workers are very experienced in cavern construction and their views are taken into account at the planning stage.
Water is removed from the construction area by four pumps, three underground and one at the entrance to the tunnel to drain the surface water inflow. When wastewater was first drained from the tunnels it was found that the sedimentation pond was not working adequately and the receiving water, a small stream, became polluted with suspended solids. To mitigate the adverse environmental impact, a treatment plant was installed consisting of: an equalisation pond; a lamella separator; a final sedimentation pond; and an oil separator.
After primary sedimentation in the equalisation pond, the water passes through the lamella separator with chemical coagulation. This induces the suspended solids to coagulate and agglomerate, creating uniform particles with favourable settling characteristics. The particles are separated both in the lamella separator and in the final sedimentation pond.
The separated sludge is removed from the lamella separator at regular intervals by a sludge pump and stored in a sludge container prior to disposal. After passing through the final sedimentation pond, the water enters two gravity separators placed in series. Automatic oil skimmers remove the free phase oil by means of a rotating drum.
When the excavation of the cavern is complete, the rock will be lined with a 1m thick layer of concrete to provide an even base for the 10-12mm thick fully welded steel plate lining. The concrete will also equalise and distribute loads and deformations. Deformation in the rock will be monitored by extensometers, which are currently being drilled into the rock by TGB under a sub-contract. Rock stress measurements are also being undertaken to verify the design and to confirm the measurements that were made before work started.
The natural gas store will be linked to the existing main network by a 3km long, 400mm diameter high pressure pipe. Exhaustive tests will be made before the plant is commissioned. An advanced and operationally reliable monitoring system will continually check to ensure that no leakage arises. If, despite the precautions, a leakage does occur the storage cavern can quickly be emptied into the main supply network.
“The aim of the LRC Demo is that the project should be a commercially viable new storage option, not just for Sweden but other countries as well,” explains Sturk. “Interest has been expressed in the US and a study has already been set up to see if this technique can be adapted to American conditions.
“The LRC concept is suitable for many different rock types, although weak sedimentary rock, such as limestone and clayey pyroclastic rocks, is not suitable. What is special about lined gas storage caverns is the pressure and how the rock can be used to absorb the forces,” he added.
Related Files
Figure 2: 3-D representation
Figure 1: Grouting sequence
