document

Summary
While pre-heating the surface casing landing mandrel in preparation of welding to repair a leak on Prudhoe Bay well P2-15, the wellhead suddenly and unexpectedly moved downward approximately 18 inches. This failure resulted in an injury and required a rig workover to repair the well.

The wellhead was cut below the surface casing landing mandrel and sent to a company specializing in metallurgical analysis to determine the cause of movement. A section of the surface casing was identified with reduced wall thickness due to drilling wear. As heat was applied to raise the wellhead to welding temperature, the reduced wall thickness section of the surface casing experienced a temperature increase greater than the design pre-heat temperature. This thin-walled section of the casing began losing tensile strength, eventually resulting in the surface casing collapsing inward and allowing the wellhead to rapidly subside.

This paper discusses the causes of the failure and presents recommendations to prevent future failures of this type.

Discussion of Incident
Prudhoe Bay well P2-15 is in WAG (wateralternating- gas) injection service. It was drilled in 1994 and completed using 10-3/4 inch surface casing and 20 inch conductor casing. In 2004, leakage from the lower connection of the surface casing landing mandrel was identified. The well remained in injection service until September 2006, then was shut-in pending repair of the leak.