Aug 2018
Gas Transmission
Investigate integrity of plain dents in grade L555 pipe
Aug 2018
Aug 2020
National Grid Gas Transmission
Paul Connolly
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Network Innovation Allowance
GT - Reliability and maintenance
Gas Transmission Networks
The first use of high strength grade L555 pipe (also commonly referred to as X80) in the UK onshore gas industry occurred in 2000. National Grid (NG) now operates 863Km of 1220mm L555 pipe. It is anticipated that this high strength material will continue to be a preferred choice for future large capacity feeders required for bringing in supplies from new areas, and on future reinforcement projects.

Because of the large diameter to wall thick thickness ratio, 1220mm diameter L555 pipe tends to be more susceptible to denting, particularly during the pipeline construction phase. The acceptance limits currently available - as set out in NG’s procedure for the inspection assessment and repair of pipeline defects procedure, T/PM/P/11 - are based on a limited amount of evidence and are thought to be overly conservative for L555.

The project looks to address gaps in evidentiary acceptance limits, provide validation for an increase to the superficial limit for grade L555 pipe, and to confirm the impact of smooth dent damage on the fatigue life of the pipeline. This will provide NG and other gas network licensees with complete, accurate information for the assessment of dent damage on grade L555 pipe. Thereby ensuring that instances of unnecessary remedial action are avoided. Remedial action involves the assessment, repair and replacement of buried pipeline and requires excavation at the site of a dent. Enabling an avoidance of unnecessary excavation will remove the potential for disruption of service for our customers, a disruption to the local environment, and the potential for an average carbon savings of 21,000 tonnes of CO2e per excavation.
Carry out comprehensive testing on plain dents to determine tolerance levels of grade L555 pipe. Incorporate findings into inspection procedures and associated training. Remove any unnecessary remedial action that would impact on local environment and cause service disruption.
This project looks to promote a reduced plan for excavations based on an increase to the superficial limit for grade L555 pipe, which could have the following environmental benefits:
  • Less local environmental nuisance from noise, dust, light pollution, road traffic.
  • Less use of natural resource, such as backfill, fuel for plant, fuel for staff transport.
  • Less risk of damage to wildlife habitats
  • Less risk of impacts on areas of outstanding natural beauty and Sights of Special Scientific Interest (SSSI)

The benefits derived from a removal of unnecessary remedial actions will be realised through cost avoidance in the region of £200k to £10m. The figures are dependent upon level of disruption of service, where excavation requires outages or an agreement for reduction of pressure as outlined below. An environmental benefit would also be realized from the prevention of pipeline excavation, delivering a carbon savings of 21,000 tonnes of CO2e per repair.