Pipeline In-Service Fillet Weld Inspection Delay Time

Abstract

Traditionally, in-service welding procedures have been developed to minimize the risk of hydrogen cracking by considering the weldment cooling rate and chemistry to control the susceptibility of the resulting microstructure. To further ensure that weld hydrogen cracks do not enter service, weldment inspection is completed. The BMT Hydrogen Diffusion and Cracking Model has been used to develop a means of conservatively estimating the delay time for hydrogen cracking in multi-pass welds. The hydrogen cracking delay time estimate is developed based upon the Time To Peak Hydrogen (TTPH) concept that is evaluated numerically considering the hydrogen diffusivity in the weldment. CSA Z662 indicates that the pipeline operator should delay weld inspection until the risk of cracking is over. This requirement includes a suggested delay time of 48 hours after weld deposition. The BMT Hydrogen Diffusion Model and TTPH concept were used to define conservative inspection delay times for pipeline repair sleeve end circumferential fillet welds deposited in-service. This paper describes the investigation results and the effect of variations in welding, environmental, material and pipeline characteristics on the recommended inspection delay time. These delay times are compared to those recommended by CSA Z662 to illustrate this novel approach to establishing weldment inspection delay times.