National Gas are the owner and operator of the >7,500km UK national gas transmission system which, in accordance with good practice, is required to be examined using in-line inspection (ILI) in a timely manner with technology appropriate to the identified pipeline threats.

The UK’s interest in ILI began in the late 60’s when it became apparent that the Code requirement of hydrotesting pipelines operating > 24 barg every 5 years was logistically challenging and the need for a mobile NDT system was identified. No such system existed in the world at the time.

In the UK, the optimum frequency of ILI is determined through understanding the initial pipeline resistance to deterioration and the distribution of deterioration rates in its operational life. Risk based inspection was codified in the UK in 1993, recognising the good practice which had been applied.

This paper will briefly explain the initial development of ILI in the UK and the prioritisation tool which was used to specify the ILI frequency. The methodology has been substantially updated over time and the current approach will be detailed.

Managing the ILI program for a >7,500km network has significant challenges, e.g dynamic flow rates, network constraints etc. Any indications identified by ILI require careful management with respect to immediate pressure reduction requirements / excavation / damage measurement / damage assessment / repair. The examination of buried pipework at installations presents additional obstacles, National Gas are commencing a project that will aim to deploy robotic technologies to examine the condition of key installations. In the paper, National Gas’s current approach to ILI will be summarised including how they address specific challenges and their plans for inspection of installations.

Finally, looking to the future the paper will discuss the relevant performance considerations for operators for an ILI tool in hydrogen service.