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.
Benefits
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.
Learnings
Outcomes
Progress 2018/2019:
To date the following tests have been carried out:
Trialled new former base on denting frame with positive results.
Performed dent trials and produced calibration curve for 14.3 mm WT pipe
Welding of two test ends onto two 3-m test sections
Denting of test sections on plain pipe. Confirmation of denting calibration curve or 15.8 mm WT pipe achieved.
Progress 2019/2020:
Progress was as follows:
Static dent tests: all 10 have been completed.
Fatigue tests: All dents have failed by fatigue.
Numerical analyses and fatigue life predictions have been completed
Metallurgical examination of dented and undented samples has been completed.
Test results have been presented to National Grid
Reports are being progressed, which will provide detail on the results of the tests and additional metallurgical and numerical analyses undertaken, and their impact on managing dent damage in grade L555 pipelines. The results are positive and a significant step forward in understanding the behavior of dent damage in high strength steel pipelines.
Closure 2020/2021:
Reports have been completed
Guidance has been developed for inclusion in Management Procedure, T/PM/P/11, which exceed the current limits, without compromising the integrity of the pipeline.
Consistent with the factors of safety included in the damage limits and fatigue guidance used within T/PM/P/11, the following guidance has been included for smooth dent damage affecting grade X80 (L555) pipelines,
Smooth dent damage in plain pipe
Acceptance limit, H≤6.0%D
Applicable for BOL$ and TOL+ dents
Fatigue life of smooth dents of depth H≤6.0%D,
BOL dents : 1,100 cycles of 125N/mm2 hoop stress range
TOL dents : 550 cycles of 125N/mm2 hoop stress range
Smooth dent damage affecting a seam weld
Not permitted
Smooth dent damage affecting a girth weld
BOL dents,
Acceptance limit, H≤2.0%D
Fatigue life of 1,100 cycles of 125N/mm2 hoop stress range
TOL dents,
Not permitted
The work also demonstrated that the recommendation to assess fatigue life using the EPRG method to be inappropriate.
These observations will be shared with EPRG
If a more detailed assessment is required, a method developed by DNV GL is capable of accounting for the observed variation in fatigue life as a function of increasing dent depth.
The test results have provided a basis for recommendations to be made to ASME, UKOPA and EPRG to update extend the range of application of their dent acceptance limits to grade X80 (L555).
There are discussions regarding further investigative work. A key area concerns the integrity of smooth dent damage affecting a seam weld. These concerns are highlighted in the limits that have been included in T/PM/P/11.
$ BOL is bottom of line (a dent located on the bottom 1/3 of the pipe, from 04:00 to 06:00 to 08:00)
+ TOL is top of line (a dent located on the top 2/3 of the pipe, from 08:00 to 12:00 to 04:00)
Lessons Learnt
Progress 2018/2019:
Further pressure/burst testing pertaining to welding (2-off vessels) was to take place in April, however it was noted that welder’s availability was likely to become problematic from April onwards, therefore the tests were brought forward.
Progress 2019/2020:
Progress with the fatigue test was slower than anticipated. Each pressure cycle was taking approximately 100s to complete. As the dents failed a strategy of patch welding was used to enable testing to continue as soon as possible. Later, it was possible to cut out a section of the vessel to produce a shorter test vessel, which reduced the cycle time to approximately 60s. This has impacted on the estimated completion date for the test. Testing has now been completed though and the test reports are being finalized.
Closure 2020/2021:
All testing had been completed during last reporting period. Test reports were drafted and issued during this period.
