Fatigue Rig Destructive Testing

High pressure steel pipelines are essential in enabling a safe natural gas transportation network, an overly engineered solution tried and tested over several decades, proving the NTS to be a robust nationwide asset. The National Transmission System is used to flow gas every day to keep the lights on and our homes heated by connecting large scale industry, cities and towns where the network is dynamic allowing for flexibility and adaptability to various flow demand scenarios. This is done so by utilising over 5000 miles of varying grades and differing sizes of pipelines, where the gas can flow, build line pack for high energy demand areas and provide a mass energy storage solution.

The NTS is used to limit gas loss, manage flow direction, facilitate maintenance, repair, modification, testing and commissioning to enable safe and effective start-up and shutdown of our pipelines. We now must further evidence pipeline steel material integrity when subjected to high pressure hydrogen gas, this can be done by expanding upon the existing fatigue rig standalone testing at DNV Spadeadam.

Although some pipelines materials that we use today have seen blends and 100% hydrogen within the HYNTS Phase 1 test facility, what we have not done, is post hydrogen fatigue cycling non destructive testing of materials that have been subject to prolonged high pressure hydrogen. One of the welds that make up the fatigue rig has a known weld defect within it, NGT aims to have the welds and the weld defect analysed through various methods of testing such as magnetic particle inspection followed by if necessary standard ultrasonic testing.

In 2022 small scale mechanical characteristic tests were conducted to characterise the mechanical properties of the materials used within the construction of the fatigue rig, this testing commenced outputting a standard mechanical property data set, the new end of test data post hydrogen exposure will be compared to the original data set from 2022 at the end of fatigue cycling. Testing will establish the effect of trapped hydrogen on ‘standard’ mechanical properties measured, To facilitate this, DNV will remove all girth welds, selected seam welds and fitting welds, and store them at low temperature to mitigate loss of hydrogen from within the trap sites..

A technical note will be prepared comparing the results of the weld inspections (internal and external inspections). The note will be used to confirm defect removal for metallographic examination.

A technical report will be prepared, summarising the macro and microscopic examinations undertaken, confirming defect size (to that reported by UT) and whether the defect was an original feature else created due to the pressure cycle duty of the test vessel and the hydrogen environment.