A new seabed pipeline had been struck by an anchor. In addition to being moved out of its position, the pipeline was also deformed.
As this brought the pipeline out of its design intent, the operator had to prove that the pipeline was still fit for the intended purpose.
An oil & gas pipe laying contractor was confronted with the fact that a new pipeline he had put in place on the seabed had been struck by an anchor, which moved it out of its position and deformed it. Another difficulty was that the pipeline had to carry rather sour natural gas and was therefore safety critical. Several remedial measures were considered − the easiest was to drag the pipeline partially back into place. Because this would cause a supplementary deformation of the pipeline in the reverse direction, it was necessary to assess the safety of the pipeline when submitted to a reversible plastic deformation. The characterization had to include the base and weld metal.
A careful selection of mechanical tests was made to pre-deform the materia, age it, and then evaluate the resulting material properties. To determine its resistance to ductile tearing, fracture mechanics tests were conducted on both the weld metal and the heat-affected zone. Coupon tests − including a girth weld with an artificial defect − were performed to simulate the displacement / repositioning of the pipeline.
It was observed that the artificial defect did not extend during the reversible loading, which led to an acceptance criterion for the defect size in the girth weld. Finally, sour testing was carried out in the lab to confirm that the reverse deformation did not degrade the sour resistance of the material.
The engineering critical assessment was validated with a maximum defect size. Based on observed findings, the pipeline has been dragged back into place. The integrity of the pipeline could be confirmed, avoiding the replacement cost.
“By using the real material properties, and looking beyond the standard, we managed to keep our customer’s process running and avoid huge implied costs.”