She wasn't a conversion but purpose built to extract oil from small wells. Below is an extract from my book on the BP fleets
As BP became increasingly involved in oil exploration, their new company BP Petroleum Development managed by BP Shipping invested with Harland & Wolff on a 50:50 share basis in a purpose-built vessel to undertake with greater efficiency oil exploration as and where required. The vessel was Single Well Oil Production Ship (SWOPS) - called perhaps appropriately after the Gaelic name for a bee - the SEILLEAN. Originally, a converted tanker hull had been used, but as dynamic positioning and production plant became increasingly sophisticated the need for a specialised craft was essential to hold station accurately in dynamic positioning mode (DPM) immediately above an oil well-head. SEILLEAN was based upon a tanker hull format. The result was the construction of an amazing ship containing a range of highly complex and state of art technology. She was completed in April 1990, of 249.7m BP with moulded beam and depth of 37m and 20.5m respectively. The ship was 45,000-sdwt tonnage with a design draught of 11m and a service speed of 9 knots. Extensive trials occurred for a month between October and November 1989 off various parts of the Irish coast, and upon satisfactory completion SEILLEAN returned to Harlands yard for final stages of fitting-out.
The well was marked by seabed transducers and a system of highly accurate radio beacons. Once this unique ship arrived on station, her function was to lower a pipe connected to the seabed wellhead and pump oil from this into shipboard tanks. To achieve this, the ship sat stern-to the weather allowing the flare of the bow to sit downwind. A series of electrically powered thrusters kept her in position, two of which acted also as propulsion for a craft that was not fitted with conventional propeller and rudder. The connection was made by use of a Riser Connection Package (RCP) suspended from a derrick through a moonpool and lowered through the hull of the ship. The fix was obtained by flexible pipe, permitting relative movement of the ship in her sea environment, using hydraulic signals transmitted between the RCP and wellhead. A second smaller moonpool was fitted with a camera and light platform, and equipped with manipulator arms to operate wellhead connections. The entire process could be monitored on board by video camera at the dynamic positioning console in the wheelhouse.
The well fluids were separated into oil, water and gas. Each element was processed through a treatment package after which water was returned clean to the sea, gas burned off, and oil pumped into the tanks. She had a cargo capacity of around 300,000 barrels the equivalent of twenty days production before returning to port for discharging.
SEILLEAN (1990 - 1996)
Diesel-electric and gas-turbine powered processing and storage vessel for offshore installations.
O.N. 149505. 50,928g. 15,278n. 79,608d. 249.70 x 37.63 x 11.540 metres.
Three, 8-cyl. 4 S.C.S.A.(400 x 450mm) M.A.N. 8L40/50 type engines (each 9,900B.H.P) made by the shipbuilder, each driving a generator connected to a 4,079 S.H.P electric motor and three TB5000 type gas turbines by Ruston & Hornsby Ltd., Lincoln, geared to twin shaft with directional propellers. Thwartship thrust propeller and two retractable directional thrust propellers forward.
18.9.1986: Keel laid by Harland & Wolff Ltd., Belfast (Yard No. 1726)
9.3.1990: Registered at Alloa/Dundee.
4.4.1990: Completed by builders renamed Harland & Wolff Shipbuilding and Heavy Industries Ltd. for BP Petroleum Development Ltd., (BP Shipping Ltd., managers).
8.1996: Sold to Reading & Bates Development Company, (Reading & Bates (UK) Ltd., managers), and transferred to Panama registry.
1997: Sold to RB FBSO L. P. (same managers).
1998: R&B Falcon Ltd., appointed as managers.
2001: Transocean Brasil Ltda., appointed as managers.
28.4.2005: Sold to Frontier Seillean KS, Brazil, (Frontier Drilling Brazil KS, managers).
2010: Sold to Noble Drilling Corp, (Noble Drilling Services Inc, managers), Texas and renamed NOBLE SEILLEAN.
9.6.2011: Placed in lay-up.
24.4.2015: Declared as To be Broken up