Session: 05-02-03 Weld NDE and ECA

Paper Number: 87109

87109 - Post-Heating as an Alternative to Delayed Nondestructive Examination (Nde) 

The current preference of regulators in the United States and Canada is to require a delay time prior to nondestructive examination (NDE) of welds made manually during construction of new pipelines and maintenance activities for existing pipelines.  The reason for this is that hydrogen-assisted cold cracking (HACC), sometimes referred to as delayed cracking, often requires time to occur; specifically, time is required for hydrogen diffusion, the development of residual stresses, and, if cracking occurs, the initiation and growth of HACC to a limit detectable by NDE.  A better approach than imposing a delay time is to prevent cracking, by lowering the bulk hydrogen below a threshold value necessary for cracking to occur.  Once hydrogen is reduced below this level, determining the necessary delay time becomes a moot point.  In other words, if cracking is not going to occur, it does not matter what the delay time is. This project explored the proactive approach of using post-heating immediately after welding to prevent the weld from cracking in lieu of an inspection delay.

An experimental program was carried out to determine the hydrogen diffusion rate in pipe materials, as well as the typical weld hydrogen levels in both a baseline (room temperature) and time and temperature conditions.  Experiments were also conducted to measure hydrogen levels in simulated welds in the as-welded condition, after 24 hours at ambient temperature, and after post-heating.  The experimental program showed that post-heating newly constructed girth welds for approximately 20 minutes at 200 °C (400 °F) and in-service welds for approximately 20 minutes at 120 °C (250 °F) may be used to mitigate the risk of HACC, thereby justifying immediate inspection in lieu of imposing an inspection delay.

Four heating methods were evaluated to determine how effectively the post-heating time and temperature targets could be achieved for new construction welds.  These included gas torches, electric resistance heaters, and induction coils.  For gas torches, both individual torches (“weed burners”) and multiple torches mounted on a bespoke band system were assessed.  Additionally, three heating methods were evaluated to determine how effectively the post-heating time and temperature targets could be achieved for in-service welds, including multiple gas torches mounted on a band system, electric resistance heaters, and induction coils. 

To aid in the successful implementation of post-heating, guidance material suitable for use by field personnel in the application of post-heating, as well as recommendations for industry standards improvement were developed.

Presenting Author: Melissa Gould DNV

Presenting Author Biography: M.S. in Welding Engineering, AWS Certified Welding Inspector, and is currently the Secretary of the API 1104 Committee