Hydraulic overshot repairs conductor damage

When a support vessel accidentally struck a production platform in the North Sea the resulting damage was fortunately limited. One effect of the collision, however, and an indication of its force, was a severe kink in one the platform’s 20-in. conductors.

The impact of this could have been worse had the well served by the affected conductor not been suspended. Nevertheless, the operator still had the problem of how to gain entry to the well to abandon it completely.

Asked to suggest a solution, Claxton recommended first tensioning the conductor in an effort to reduce the severity of the kink. The company proposed then cutting the damaged pipe and its internal casing strings below the affected area, before using an overshot connector attached to a new section of conductor to re-establish access to the well.

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Bespoke hydraulic overshot being installed

Though relatively simple in theory, the project was not without challenges, as Blair McKnight, the design engineer on the project, explains: “To get back to straight pipe, the damaged conductor had to be cut in the splash zone; further, as part of this operation, a conductor guide in the area had to be removed. Consequently, our connector had to be unusually robust. In fact, the specification called for a bending capacity of 990 kN.m at a pressure of up to 10 kPa. In addition, we soon discovered that the cut pipe was not perfectly round: the outside diameter ranged between 502 and 513 mm. Finally, because we were in the splash zone, the new connection had to be completed remotely.”

The overshot connector designed by Claxton had several features to cope with these various requirements. The features included a self-centralising mechanism using weight-activated colletts; hydraulically operated slips to enable the connector to grip the cut pipe and lock onto it and a remotely controlled sealant injection system to make the two gallery seals between the connector and the cut pipe.

McKnight reports that the attention to detail in the design of the connector paid dividends offshore when the operation to reconnect the conductor was accomplished without any undue difficulty. The connector, which had an internal diameter of 520 mm and overshot the cut pipe by about 1.5 m, was engaged easily and an over-pull test was carried out to check the mechanical integrity of the joint. The sealant was then injected, and shortly afterwards the seals were pressure tested to ensure the joint was fit for its intended purpose.

The operator subsequently ran a 13-in. casing through the repaired conductor and successfully abandoned the well. The upper conductor and its connector are now back in Claxton’s yard.

“Projects like this are never going to make headline news,” says McKnight. “But there can be serious repercussions from getting things wrong. Customers return to Claxton for this kind of work because they trust us to come up with sound, practical solutions that, most importantly, can be reliably implemented offshore.”

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