The objective of this study is to assess the provision of Post Weld Heat Treatment (PWHT) of girth weld on C-Mn steel submarine pipelines and propose an alternative to avoid PWHT. As per prevailing industry practice, the majority of the existing submarine pipelines handling sour service fluid, limit the steel grade to 450 MPa SMYS (X65). Such limitations on steel grade for sour service application results in high wall thickness especially for high pressure pipelines. This necessitates stress relieving requirements and hence leads to an uneconomical scenario for the project. Performing PWHT for girth weld of submarine pipelines is neither practical nor a cost- effective solution for the project.
The stress relieving requirement of girth weld is linked with pipeline wall thickness and such limitations on thickness varies based on selected governing design code for the submarine pipeline system.
ASME Pipelines Code has exempted PWHT to 32 mm of pipe wall thickness and such a limit is quite often surpassed for large diameter high pressure gas pipelines designed using Grade X65 steel.
To eliminate the need for impractical (in most cases) and expensive offshore PWHT, available alternate options include use of higher strength steel of grade X70 or above. However, in certain cases even higher steel grades do not help in elimination of the stress relieving requirement. Performing PWHT may further result in restricting the larger diameter (i.e. seam welded) pipe material to non-TMCP (Thermo Mechanically Controlled Processing). In such cases, necessary engineering analysis as per the provisions of pipeline design code needs to be performed. This paper discusses a systematic approach based on the provisions made in the Design Standards for waiver of PWHT, with further detailed engineering assessment based on fracture mechanics. A case study has been discussed the feasibility of PWHT waiver.
This paper will address the current limitation of using a higher grade of steel (e.g. X70, X80, etc.) for sour service and submarine applications. Provisions made in Design Standards for waiver of PWHT shall be evaluated by demonstrating an alternate method using fracture mechanics duly supplemented with engineering assessment.
Results from the case study substantiated by detailed engineering critically assessment (ECA) show that the acceptance criteria can be achieved without the requirement of PWHT. The mechanical properties of weldment are also acceptable without having stress relieving of the pipeline girth weld. A suitable methodology is discussed and presented to adopt in case such scenario arises for designing the submarine pipeline system.