West Midlands Gas Alliance, a partnership between National Grid and engineering contractor Morgan Sindall, embarked on a renewal programme of an 800m stretch of pipeline, running from the old gas holder station on Bath Street up to Blowers Road, in line with the HSE directive requiring replacement of metallic gas mains with PE pipes. The road contained a total of three gas mains: a 24” medium pressure main, a 24” low pressure main and an 8” low pressure main. The plan was to complete renewal of the medium pressure and low pressure mains as part of the same project in order to minimise disruption to local homes and businesses and reduce the costs of reinstatement.
West Midlands Gas Alliance’s approach was to complete the medium pressure main replacement first before moving on to renewal of the 24” low pressure main and then finally decommissioning the 8” low pressure main. The programme was due to take place from early November 2010 to the end of February 2011: the peak period for gas consumption. As a result, ensuring that the project was completed efficiently and on time was a high priority.
While the project took place in a built up area, only a few houses are located on Shaw Road itself along with largely light industrial businesses so National Grid was able to secure a closure order for part of the road as this would not affect passing trade and would aid the speed and ease of installation. For the medium pressure main renewal, the West Midlands Gas Alliance team was able to isolate the required section of pipeline as this main feeds the pressure reduction station so a temporary closure of the main was not a problem. Unusually, National Grid was also able to use a dead mains insertion installation method for renewal of the 24” low pressure main, thanks to the presence of the 8” low pressure main in the same location.
Explains Gary Stokes from National Grid: “Ordinarily, we would not have been able to cut off supply to an 800m stretch of low pressure main like this but in this case we were able to divert the services from the 24” low pressure main to the 8” main while we carried out the renewal work. Once the job was completed, we then moved supply back to the 24” low pressure main and decommissioned the 8” main as it is no longer required.”
While a second low pressure main was not required under Shaw Road, West Midlands Gas Alliance was keen to maximise capacity in this main by maximising the internal diameter of the PE pipe used. However, pipe size was not the only consideration: the pipe chosen also needed to offer excellent flexibility to facilitate ease of installation and needed to offer sufficient wall thickness to provide a robust solution that will ensure pipeline integrity in the event of any scoring during installation.
Gary Stokes continues: “We needed to use the largest possible diameter pipe for the PE main because of the low pressure and the specification for the installation called for insertion of the PE pipe directly inside the old metallic main with no liner. The inside diameter of the steel main was 610mm giving us a clearance of just 50mm between the old pipe and the 560mm PE pipe specified for the renewal scheme.”
The pipe chosen for the low pressure main renewal was 560mm PE80 pipe from GPS PE Pipe Systems and the project was the first time that this large diameter pipe had ever been used in the UK for a job of this kind. Not only was the pipe of optimal size and the required PE80 construction, it was also available in a choice of SDR21 or SDR17.
Gary continues: “The SDR of the pipe was critical to the specification. By selecting SDR 21 rather than SDR 17 we were able to maintain the same pressure rating but maximise the internal pipe cross sectional area whilst reducing pipe costs by around 20%.”
Indeed, the increased flexibility of PE80 pipe enabled the project team to dramatically reduce the size of the excavations required for the launch pit. The excavation for the scheme was 1.3m deep and 18m in length: approximately half the length that would have been required had PE100 pipe been specified.
The National Grid team isolated the relevant section of the 24” main and switched supply to the 8” main. As the project took place during the peak period for gas demand, the team also connected a number of temporary feeds to introduce additional gas into the diverted supply system and maintain pressure levels. Once all of this had been completed the National Grid team was able to decommission the 24” low pressure main ready for the renewal programme to begin.
For insertion of the new low pressure main, four insertion pits were dug, one at each end of the 800m span and two further pits at regular intervals along the route. The longest section between insertion pits was 288m long and the selection of locations for the pits was based on which sites would cause least disruption to local residents. At each insertion pit, the installation team cut a section out of the original 24” low pressure main to allow for the installation of the new PE80 pipe.
The next step was to inspect the existing metallic main to fully assess its condition and identify any obstacles. A remote control camera was lowered into the main and guided along it for this purpose.
Gary explains: “With any metallic pipe of this age we would expect some deterioration in its condition, after all, that’s the reason for replacing the old steel pipes with PE. In this case, however, there was more rust than we had anticipated and the low pressure nature of the main results in more debris inside the metallic pipe so we had to remove all traces of rust and any obstructions to avoid the risk of damaging the new pipe as it was fed through the main.”
The team brought in a specialist cleaning company to remove all traces of rust and minor obstructions, safely disposing of all debris off-site and preparing the internal surfaces of the metallic main thoroughly before insertion of the PE pipe began.
Pushing 12.5 tonnes
The PE80 560mm pipe was supplied in 12m lengths and the installation began with the butt -fusion of three 12m lengths of pipe to make a single 36m pipe section. On the front end of this triple length of pipe, the West Midlands Gas Alliance team attached a tapered PE ‘nose’ cone, which served a dual purpose.
Gary explains: “While, theoretically, insertion of the PE80 pipe was a straight line from A to B, in practice the path that the pipe needs to take is far from straight. The nose cone helps to negotiate the path of the PE80 pipe inside the 24” main much more smoothly, minimising the potential for any friction or scratching of the new pipe. The nose cone also provided the meeting point for two pipe sections as we inserted a section from each direction to meet at the insertion pit in the middle. When the two nose cones met we were able to simply remove the cone and join the sections together using Frialen couplers to fuse them securely and connect any tees required for off takes.”
The project was not only the first National Grid renewal scheme to use 560mm PE80 pipe, it was also the first to use a Steve Vick Pipe Handler, fitted with 560mm shells, to push the new pipe into the existing metallic main rather than dragging it through with a winch. The Pipe Handler was attached to a JCB sitting at one end of the trench and the equipment clamped each section of pipe and then pushed it into the main using the hydraulic arm. As each section was successfully put into position a further length of pipe was butt fused onto the end, extending the pipe section until it reached the next insertion pit. This method was used for all four sections of the installation, including the maximum 288m length which saw the pipe handler push a total weight of 12.5 tonnes through the main.
Gary continues: “Our usual approach for a project like this would be to send a cable down through the main using the same bogie as the inspection camera and this cable would then be used to winch the pipe through to the next insertion pit. Using the pipe handler makes the whole process both quicker and safer as it only requires a team at one end of the section of main, reducing the number of man hours required on site and the health and safety risk. It was a fast and efficient method of inserting the pipe and was ideally suited to the flexibility of the PE80 pipe.”
For more information on GPS products, visit www.gpsuk.com