Rising To The Challenge Of Rigless Slot Recovery

Claxton delivers innovative solution offshore Denmark.

When Maersk contacted Claxton in April 2013 about a slot recovery job, it was apparent that the task in Tyra East field in the Danish sector of the North Sea would be a little more complex than the mechanical removal of casing followed by drilling a sidetrack well. The original well was drilled in the 1980s. During its construction, a large piece of downhole equipment, the bottomhole assembly (BHA), become stuck in it and prevented the well from being completed and contributing to gas production from the reservoir. Maersk’s decision to recover and reuse the slot would involve removing the BHA.

Claxton had an established relationship with Maersk for the supply of whipstocks for slot recovery and, during the initial consultation, Claxton engineers suggested a rigless approach, as Neil Youngs, project engineer, Claxton, explains. “A rigless operation would help to minimise the cost and provide a more flexible approach to project timing. This was critical because the area for the operation had logistical issues around rig moves with the potential to greatly complicate the project.”

Claxton’s team visited the well in May 2013 to assess the situation and develop a recovery plan. The BHA stuck in the conductor pipe was about 15 m long and had a diameter of 8–9 in. About 6–9 m of it was protruding above the seabed.

After working closely with Maersk to fully understand the problem, Claxton brought in the multidisciplinary team needed to deliver the project. This involved close integration and collaboration with specialist cutting companies and a dive support vessel from Royal Boskalis Westminster NV.

The programme started by deploying a specialist casing cutting system to remove a section of the conductor and leave the stuck BHA intact but exposed. This approach called for modification of the cutting equipment and the subsea application of a system that is normally used on the surface.

“Once the conductor was cut and removed, we prepared a bespoke lifting device using an air hoist on deck deployed through an A-frame while divers clamped the lifting gear to the BHA. The BHA was too heavy to recover in one piece and it would have to be cut subsea,” Youngs continues. “We used two sizes of diamond wirecutting machine, the smaller to sever only the exposed BHA and the larger to cut through both the remaining conductor stump and the BHA simultaneously. A key consideration here was making the cut so that it left a smooth and flush surface on the end of the conductor. Once the cutting machine had sliced through the conductor and the BHA, we were able to recover the top section.”

During planning, the orientation of the remaining BHA relative to the conductor stump was uncertain. This required Claxton to create a unique whipstock with a slot beneath the landing plate that would enable it to capture the exposed part of the BHA securely in any orientation. The top half of the whipstock would rotate on this base and enable the new well to be kicked off in any direction. Claxton’s experience and knowledge of whipstock design were vital to providing an effective engineered solution.

“This was a fairly unusual slot recovery project,” Youngs says, “and that made it quite difficult to estimate how long it would take to complete. Our initial plan called for a little over 20 working days, but, as things turned out, we managed to complete it in less than 16 days, a figure that included more than three days waiting-onweather. Naturally, we are delighted to have successfully completed the first rigless recovery of a stuck BHA in the North Sea. The key to the success of this project was teamwork. The simultaneous operations were conducted smoothly and with great efficiency.”

The integrated team led by Claxton delivered the full work scope in only three months, including designing, manufacturing and testing the bespoke equipment. The cooperation between Claxton; Boskalis; CUT UK, which supplied diamond wire; and the Maersk staff involved on Tyra East made it possible to deliver what was needed in a very tight time frame. The project was completed ahead of schedule, despite the weather delay.

Alex Lucas, senior drilling engineer, Maersk drilling department, echoes the significance of the collaborative approach: “This was a very challenging piece of work requiring integrated planning across many departments and companies, not to mention the extensive amount of design, manufacturing and testing of equipment that Claxton designed especially for the job in double-quick time.”

Safety and deadlines were the main concerns for this operation and the rigless approach saved substantial rig time and reduced the associated costs while increasing the flexibility of the project timings. Building on the success at Tyra East, Claxton hopes to win similar work in the near future.

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