Apply Non Destructive Testing for Assessment Hydrogen Cracking in Joining Procedure of Split Tee to Pipeline in Hot Tapping

Proceedings Publication Date:

24 Apr 2019
Meysam Rasooly
Meysam Rasooly, Ebrahim Mohseni Homagerani
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Hot tapping is a process by which ramification of in service pipelines are done. One of the phases of this process is to join split tee fitting to the pipe. Joining of this component is mainly done by welding. This component has axial and circumferential welds to pipe. Circumferential welds are under higher stress in compare with axial welds. They are also directly joined to pipe; therefore these welds are subject of this investigation. Since pipeline is in service, and fluid is passing through it, in high flow rates, heat sink rate in welding zone is high and this leads to increase of weld cooling rate that may cause hydrogen cracking which will be propagated in steels with low ductility and low tensile strength. After welding phase, fitting joints will be under stress in next phase, and this makes them more sensitive because in the case of cracking, pipeline gets leakage. Codes and standards like API 1104 suppose solutions to prevent this defect. Although by choosing proper type of electrode, welding procedure, fluid flow rate, welding positions, pre and post heat treatments, it is possible to reduce risks of hydrogen cracking, it is needed to investigate presence of cracks by proper Non Destructive Tests. In this investigation, formation of hydrogen cracking is assessed by appropriate NDT in split tee ramification on an in service pipe containing gas with specific pressure (maximum pressure is considered) and effect of Magnetic Particle Testing and Ultrasonic Testing specifications are approved to get optimum results in diagnosing the defect. Material type of pipe is API 5L X62 and welding process is Shielded Metal Arc Welding with electrode type E7018. Practical results of this investigation show the importance of applying Non Destructive Tests to control the mentioned variable parameters.

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