In-line inspection (ILI) of the main 48” × 21 mm wall thickness API 5L grade X70 export gas pipeline of an offshore oil and gas production field was performed. The ILI using a high resolution Magnetic Flux Leak (MFL) pig revealed an anomaly in a girth weld in the onshore portion of the pipeline. A dig campaign was organized to carry out detailed inspection and sizing of the detected anomaly by external Phased Array Ultrasonic Testing (PAUT) and Time of Flight Diffraction (TOFD). The inspection confirmed the presence of a deep external, surface breaking, girth weld crack. The presence of the defect raised concern for the integrity of the pipeline and it was a question if it was necessary to shut the line down for immediate repair or if repair could be performed at a later date.
CTOD fracture toughness testing was performed on a reproduced girth weld. The joint tested was welded using pipe remaining from installation with virtually identical chemistry and mechanical properties as the pipes joined at the cracked girth weld location according to the mill certificates. For the welding, the same welding consumables, original WPS, and the welding system (CRC-Evans system using internal line up clamp and root pass) were used.
Based on the fracture mechanics test results and the analyses performed, it was recommended to repair the pipeline as soon as practical. It was decided to repair the cracked girth weld using a full encirclement snug fitting (Type -B), welded split sleeve according to the PRCI Repair Manual. To assess the risk of welding the split sleeve onto the “live” pipe, which could increase the crack driving force due to the thermal stresses from welding and preheating, a combined 3D non-linear thermal and fracture mechanics FEA modelling of the girth weld crack and thermal distributions were performed in Abaqus. As for all welding onto “live” pipelines, the overall risk associated with burn-through and hydrogen induced heat-affected zone cracking (HAZ) was also assessed.
This paper describes the methodology applied for the assessment and qualification of the repair and the analysis of the thermal, operating and residual stresses on the crack driving force to establish if the repair operation was safe to undertake.
