When hurricanes Katrina and Rita thundered through the Gulf of Mexico in 2005, they had an impact on 3050 platforms. While most structures performed adequately, many suffered major damage above or below water.
Damage to offshore platforms had been relatively uncommon before the two powerful hurricanes struck, so S2S talked to Tom Kennedy, InterAct PMTI’s president, about the engineering challenges posed by repairing or decommissioning the damaged wells and production facilities.
“Increasingly, our role is about devising solutions to problems that have perhaps never been faced before,” says Kennedy, who has more than 30 years of experience in the oil and gas industry. “This is an environment in which cost is paramount, so service providers must leverage their experience to avoid or at least optimise the use of expensive repair and maintenance equipment.”
He adds, “It is often impossible to obtain vital data on the field conditions in advance; therefore, teams have to be nimble so that they can take an alternative approach if the need arises as the project develops.”
Kennedy cites the example of a recent initiative to bring a damaged 36-in. caisson back on production in the Gulf of Mexico. The simple free-standing caisson (which supports a single well completion with a minimal deck and is tied back to surface facilities) was leaning at 52°, but it was not on the seabed and the wellhead was still showing. The water depth was 12 m.
The field operator was keen to get the well back into production because abundant oil reserves remained. InterAct PMTI’s role was to remove the leaning structure, jet below the mudline and locate straight pipe so that the conductor could be tied back to surface.
Despite comprehensive upfront planning, Kennedy expected the crew to have to think on its feet when it was offshore. The organisation’s engineering studies and finite element and soil analyses suggested that the conductor was bent at a point 8 m below the mudline. However, it was impossible to confirm this. According to Kennedy the plan was: “Let us go out and see what we have. If we are correct in all our assumptions, then here is your procedure. But if we are not, then we are prepared to adjust it as we go.”
InterAct PMTI mobilised on a 200-class lift boat. The planned methodology involved removing the top section of caisson down to the mudline and then jetting down to the targeted 8 m using a submersible pump. To mitigate the risk of caveins, the hole had to be sloped to a diameter of around 24 m.
However, the pipe at 8 m still did not have sufficient integrity. “So we had to keep jetting,” says Kennedy. “At about 9 m, cave-ins became a major issue. When we had overcome those, we eventually found straight pipe at 12.5 m – you can imagine the size of the hole!”
Having located round, straight pipe, Kennedy’s team was able to jet down the cofferdam enclosure that would provide the dry work environment, make a cut on the caisson and move the 9-m tall cofferdam into position. The team made two cuts on the conductor to prepare it for the overshot, and installed a centraliser around the conductor before installing a 72-in. caisson around the 36-in. conductor. The well could then be tied back to surface.
This project illustrates the challenges posed by many of the Gulf of Mexico’s hurricane-damaged structures, which are frequently bent below the mudline. Regardless of whether the operator plans to bring the well back on production or to plug and abandon it, the service provider has to get below the bend and find straight pipe that has integrity. Projects based on incorrect assumptions can incur major cost overruns.
Says Kennedy, “During the planning stage, it is impossible to pinpoint where a conductor is damaged. And there is no science that can predict this to a high degree of accuracy; we have seen everything from platforms falling down on themselves to platforms being twisted. InterAct PMTI’s experience shows that, although some valuable engineering studies and analyses can be performed in advance, essentially the team must be prepared to respond cost-efficiently to the situation that presents itself during project execution.”