UK GAS network company Transco has completed the replacement of a major gas artery in Lincoln, eastern England.
The work, which began in June 1999, involved installing 2.7km of 315mm diameter polyethylene pipe into an old 406mm cast iron main running on either side of the main street. Some 1.9km of 63mm, 90mm and 125mm service pipe was also installed to maintain supplies during the project.
Transco employed directional drilling contractor Steve Vick International to carry out the work. The company used its patented Foambag 'foaming-off' technique to isolate sections of the low pressure main before inserting the polyethylene pipe.
The method involves inserting two specially made semi-porous fabric bags into the main via a standpipe in a 50mm borehole and filling them with resin foam to seal off a section of the pipe.
A small bypass fitted either side of the sealed off section ensures that pressure remains balanced either side of the foam bags as they fill. A vent between the bags allows any gas that may have passed the first bag to escape.
'We find that using foam bags gives us a 50% saving over the alternative inflatable bag system, ' said Transco network officer Dave Turner. 'We don't need large excavations and we can use standard 50mm bores.' The first of 11 foaming-offs took place in a pedestrian shopping precinct at the top of the main street. Work was done at night to avoid disruption to stores and customers. It proceeded down the east side and then the west side of the street, working on 240m to 300m sections, depending on the time of the year - longer sections could be tackled in the summer when gas demand falls.
Construction paused at the end of November 2000 and restarted in early February 2001 to keep the area work-free during the busy Christmas shopping and January sales periods.
As each section was installed, a mixture of 63mm, 90mm and 125mm PE pipe was installed in the pavement to maintain domestic and business supplies.
As it expanded, the foam seeped through the bag to adhere to the pipe wall, forming around any irregularities such as rust deposits. Once the foam had fully cured after about 30 minutes, a Venturi pipe was used to clear gas out of the dead section of main allowing it to be cut out and sealed off with a mechanical cap.
A Steve Vick hydraulic pushing machine was used to insert the new 315mm polyethylene pipe in 8m lengths into the dead section from an excavation 300m along the main street. Pushing, rather than winching in the pipe, avoided the risk of stretching the polyethylene and eliminated the need to wait while the pipe relaxed back to its original length.
Transco claims the cost of replacing 2.7km of the cast iron main and installing 4.6km of new pipework has been kept to a minimum by using methods such as foaming off and mains insertion.
'We have kept all consumers on gas except for the shortest possible time when we connected their individual services, and we've managed to win commendation from the council for the way the job was carried out, ' said Turner.