Due to potential leakage, it was necessary to replace two 3” steel gas service risers in a block of some 35 flats at the point where they entered the building. To avoid disrupting residents’ gas supply, with the subsequent need to test, purge and relight each dwelling, it was decided to carry out the operation live using Live Riser Transfer. This technique allows a gas riser to be partly renewed or transferred to a new main without first decommissioning it.
The services entered the building in two adjacent ground floor flats through a service duct between the bathroom and kitchen.
The site had been prepared by Transco contractors, McAlpines. After 2 site surveys, the first task for the Steve Vick International Special Contract Service two-man team was to fit special under pressure tees and valves to the portion of the old riser remaining in commission (at point A on diagram 1.) and test them in accordance with Transco procedures.
Transco contractors then pieced up the pipework from the new supply to the under pressure tee. The tee was re-tested by SVI and the old riser drilled using a special under pressure drill. The new supply was then pieced in and commissioned by Transco personnel.
Angled under pressure tees were fitted in two places on each riser (points B and C) and tested. The old risers were drilled to allow the introduction of Foambags™. Transco personnel fitted a sacrificial under pressure tee to the riser and supported this with an acrow (point D).
SVI injected expanding resin foam at points B and C. After allowing the foam to cure for 15 minutes, a pressure gauge was attached to a 1” BSP service connection to check the Foambags™ had made a gas tight seal.
With a 100% gas tight seal, the riser was the cut at point C and disconnected at the tee (point E). The angled under pressure tee was removed. A 3” BSP plug was fitted to the tee at point E.
The under pressure tee was removed at point B and a 3” repair clamp fitted.
Date published – 11th July 2003