Challenges in Integrity Assessment of TMCP Pipeline Girth Welds in Severe Sour Service

Abstract

Abstract In recent years, the number of developed gas and oil fields with an increased content of hydrogen sulphide is increasing. Metallic materials such as carbon steel and weldments can become brittle and crack in sour environments due to a combination of chemical, physical and electrochemical factors. Following the industry experience with premature failure of Thermo-mechanically Controlled Process (TMCP) steel pipes handling sour gas, stringent requirements have been established for integrity assessment of linepipe longitudinal seam and girth welds. Commonly used reference is DNV-RP-F108 with certain project specifications from operators. This paper identifies the gaps and potential conservatisms from multi-disciplinary inputs used for integrity assessment using fracture mechanics, commonly referred as Engineering Criticality Assessment (ECA). This work also briefly elucidates the resulting fabrication challenges associated with a stringent girth weld acceptance criterion. Integrity assessments of flaws in pipeline system girth welds is typically performed based on a fitness for purpose approach using fracture mechanics principles. Commonly used standards are DNV-RP-F108 and BS7910 for pipelines and risers. Parametric fatigue and fracture assessments have been performed using Crackwise software. These results have further been compared with crack driving force from the Finite Element Analysis software, ABAQUS. Solid 3D FE modelling and assessment of crack driving force for fracture has been performed by evaluating the J-integral. The influence of key parameters such as fatigue and fracture characterization of parent pipe and girth weld, associated loading, weld geometry, and Non-Destructive Evaluation (NDE) inspection accuracy has been discussed. Potential impact to welding acceptance criteria, associated inspection challenges, and risks to overall project schedule has been discussed. Gaps to DNV-RP-F108 have also been identified and potential solutions recommended. This paper provides a holistic view based on recent experiences in sour service application that can be used as basis for revising DNV-RP-F108 for integrity assessment of girth welds. It can be used as a basis for standardization of sour service tests. This work is of interest to stakeholders, including field owners, EPCI contractors, standardization committees and consultants.