WE Student Seminar: Alejandro Alvarez, Joining of Internally Clad API Grade X65 Pipes using Low Alloy Steel Filler Metals

ABSTRACT

Pipe-reeling is a cost effective solution for the O&G industry to install pipelines and risers. DNV-OS-F101 specifies that for pipe-reeling applications, the deposited girth weld metal shall overmatch the base metal yield strength by 100 MPa with a maximum hardness of 250 HV10. These requirements ensure that deformation is limited to the base material while also rejecting the formation of brittle microstructures that may become susceptible to hydrogen assisted cracking. Industry currently joins internally clad API Grade X65 pipes using Alloy 625. Alloy 625 girth welds however, are unable to meet the overmatching strength requirement and are a topic of continued discussion as premature failures have been attributed to the fusion boundary of the dissimilar metal weld (DMW). As a result, industry is now considering the use of other filler materials to develop such girth welds. Low alloy steel (LAS) girth welds appear to be a cost effective solution but entail overcoming several weldability challenges. Such a procedure implies depositing a material that has a higher melting temperature (LAS) over a substrate (Ni-overlay) that has a lower melting temperature. Due to the large differences in melting temperature, it is anticipated that the DMW will experience high dilutions, high hardness, and poor weldability. This project utilizes thermodynamic computational modeling, characterization techniques, and mechanical as well as weldability testing to determine if LAS girth welds can be made to meet DNV requirements. It is also used to explain the solidification behavior of DMW’s that are made using materials that have large differences in melting temperature. If defect-free welds that meet DNV requirements are achieved, such a welding procedure would provide an enormous reduction in expenses to the O&G industry.