The Need for Pressure De-rating Prior To In-Service Welding

Completed

Fossil Fuels: Oil Gas and Coal(Oil and Gas, Refining, transport and storage of oil and gas)

Applied Research and Development

ENGINEERING AND TECHNOLOGY (Mechanical, Aeronautical and Manufacturing Engineering)

Not Cross-cutting

Project Contact
National Grid Gas Transmission

Network Innovation Allowance

Ofgem

01 October 2012

01 May 2014

19 months

£85,000

Energy

London

Network Innovation Allowance

Project Contact, National Grid Gas Transmission

This project to aims to confirm whether hot-tap welding to pipelines operating at pressures above 70 bar can be carried out: With a sufficient margin of safety to prevent loss of material strength. Minimising consequent local bulging due to the effect of high pipe wall temperatures and the internal gas pressure acting upon the pipe wall. Ensuring that considerations of the volume of product flow or gas flow rate are quantified as this has a significant affect on the rate of heating or cooling of the pipe wall during such an operation. The programme will provide a better understanding of the mechanical behaviour of pipe sections under the circumstances of hot tap welding operations which will either: Confirm the existing safety margins already specified in the relevant National Grid standard T/SP/P/9 are appropriate. Will provide supporting evidence to be incorporated in an updated T/SP/P/9 standard.

In service welding is a common maintenance and installation connection practice, generating economic and environmental benefits by not removing pipelines from service. The relevant National Grid standard (T/SP/P/9) specifies that during the welding operation(s), pipeline pressures are "to be reduced to the minimum practicable". This programme is a benchmarking study, which looks to address whether elements of this procedure should be amended in light of material and technological developments over the past 20 years. The de-rating assessment will be performed covering the following topic areas: The scope of the P/9 standard will be reviewed to determine the underlying assumptions. Other published standards such as BS 6990 and EEMUA 185 will be reviewed. Previous studies on hot tap welding and pipeline repair by local deposition of weld metal such as those carried out for the Pipeline Research Council International (PRCI) and the Australian Pipeline Industry Association will be reviewed to determine likely temperature distributions during welding under these operations. BMT Fleet Technology’s industry standard hydrogen diffusion modelling tool will be utilised to study a range of bead on plate and weld geometries to generate typical thermal histories. Data on the variation of material properties with temperature will be collected for typical pipeline steels. Using these properties and the calculated temperatures, the extent of the softened area and the possible strength reduction will be estimated. High temperature tensile testing to confirm material property assumptions. Metallography examination of welded specimen cross-section to determine molten metal penetration depth. Results will be used to determine the load bearing pipe wall thickness during welding. Results from high temperature tensile testing data and metallography examination will be used in FE analysis to determine whether pressure de-rating during P9 is required for this selected pipe diameters and wall thicknesses. The review and the numerical results will be analysed to determine whether P9 should be amended to include pressure reductions when hot tap welding on National Grid transmission or distribution pipelines. The results will form the basis determining derating magnitudes and implementation of these within the current National Grid T/SP/P/9 standard.Note : Project Documents may be available via the ENA Smarter Networks Portal using the Website link above

17/12/18