AOGCC issues emergency NS well safety order after BP DS-2 spill
On Oct. 30 the Alaska Oil and Gas Conservation Commission issued an emergency order to all North Slope oil and gas operators to shut in by the end of the year any wells that have a similar design to that of a Prudhoe Bay well which leaked oil and gas in April. BP determined, and AOGCC has agreed, that the April leak from the wellhead of well 02-03 on Drill Site 2 resulted from a combination of the well design and the thawing and subsidence of the permafrost around the wellhead. The problem related to the wellhead geometry, in which the well’s outer casing shoe was set in the permafrost, AOGCC says.
AOGCC requires all wells with a similar construction geometry to be shut in and that each operator submits a list of the impacted wells to the commission.
In July, following its determination of the cause of the spill, BP shut in all 14 of its wells that have the problematic well design. At that time the company said that the impacted wells are old wells that have a particular rigid three-casing design with the base of the upper casing located within the permafrost. The two-casing design, used for most Prudhoe Bay wells does not present the same problem because the well tubing in this design is inherently more flexible than in the three-casing design, the BP investigation found.
Spray of gas and oilThe Drill Site 2 wellhead incident began on April 14, when gas started venting from the wellhead of well 02-03. The gas leak generated a spray of oil that impacted the well pad. It turned out that there were two leaks in the wellhead: an oil leak at the pressure gauge assembly at the top of the wellhead structure, and a gas leak farther down the wellhead. In response to the leaks, the well was eventually killed on April 17.
Apparently the melting and subsidence of permafrost around the wellhead had generated a load that had broken the surface well casing. Subsequently, the detached upper portion of the well had moved upwards, causing the wellhead to strike the wellhead housing. The pressure gauge at the top of the wellhead consequently broke off, resulting in an oil leak, while the striking of a valve handle lower in the wellhead against the side of the well house caused the gas leak.
- ALAN BAILEY