The world's first subsea wireline intervention on a horizontal subsea tree was completed late last year on Amoco (UK) Exploration's Arkwright Field in the North Sea when one of the three subsea wells was converted from an oil producer to a water injector. Cost savings were made in this intervention by using a workover vessel, the CSO Seawell, in dynamic positioning mode, rather than a jackup or semisubmersible drilling or production rig.
The Amoco Workover Support Team, Coflexip Stena Offshore Ltd. and ABB Vetco Gray engineering teams worked together to re-enter the subsea well with a combination of existing and new technology. The key to being able to use the CSO Seawell was that the original subsea completion design allowed the use of a DSV. The operation involved two wireline plugs that were set in the tubing hangover above the production flow path and in the 18 3/4-in. internal tree cap.
The plug design replaces the vertical bore swab valves used on conventional dual bore trees. The two-plug configuration allowed access to the well using a subsea wireline lubricator to accommodate the 4 1/2-in. and 5-in. plugs, with no requirement to remove the internal tree cap, which would require a rig with an 18 3/4-in. BOP.
Functionality testPrior to the offshore intervention, a complete offshore stack-up test for full equipment functionality was completed. On completion of the vessel DP trials, divers from the CSO Seawell were used to configure the subsea manifold and tree valves to the high-pressure pipework by the host platform.
A winch was used to remove the tree debris cap with overtrawlable frame, before the divers attached the vessel guide wires to the guideposts on the tree stucture. The subsea BOP and 45-ft intervention lubricator assemblies were deployed from the CSO Seawell through the moonpool to the tree in two stages using the vessel's 65-ton winch and motion-compensated derrick arrangement. The pre-installed guide wires and utility frames achieved final subsea alignment.
Once latched to the tree, the subsea intervention system was fully functional and pressure tested. Divers were used to disconnect the platform umbilical controls from the tree to allow the CSO Seawell's own hydraulic workover control systems to be used to operate the tree, sub-surface safety valve, and subsea lubricator system.
Leaving the subsea lubricator latched to the tree, slickline tools were run from the CSO Seawell into the lubricator using a standard wireline winch located onboard. A retrievable stuffing box was run with the tools and hydraulically latched into the lubricator. Once the two well barrier plugs were removed and bore protector installed, access to the 4 1/2-in. tubing and reservoir was obtained allowing recovery of a venturi insert from the permanent downhole gauge mandrel.
The intervention was successful and completed in 10 days, including nine slickline runs, 16 perforating runs (421 ft.) with micro-pressure and temperature memory gauges. A multi-rate wireline log was also obtained while injecting into the well from the host platform.
Operational downtime due to adverse weather was limited to less than 6%. Downtime due to operational variances was less than 8%.;
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