Oct 2012
Gas Transmission
The Need for Pressure De-rating Prior To In-Service Welding
Oct 2012
May 2014
National Grid Gas Transmission
Joanne Harris
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Network Innovation Allowance
Gas Transmission Networks
The expected design pressure was limited to 70 barG with a specified maximum yield stress (SMYS) of 72%. Now National Grid have pipelines that may operate up to 80% of SMYS and pressures for some pipelines have increased to a maximum of 94 barG. To quantify the current specified safety factors, additional analysis, using the latest modelling techniques, is required to ensure that the mechanistic behaviour of the welding of hot-tap connections is better quantified and incorporated in best practice.

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.