Anasuria is the first purpose-built floating production storage and offloading (FPSO) vessel to be constructed for Shell UK Exploration and Production (Shell Expro) in the UK North Sea. The vessel was designed and constructed by Mitsubishi Heavy Industries (MHI) in Nagasaki, for the combined development of the Teal, Teal South and Guillemot A fields.
The wellstream enters the vessel through a swivel and is offloaded to shuttle tankers using a stern mooring system.
The hull has a length of 234.2m, a breadth of 44.8m and a draught of 23.8m. The structure consists of seven cargo tanks, which can store 850,000 barrels, and two slop tanks surrounded by ballast tanks. The tanks are formed by longitudinal and transverse bulkheads, including a longitudinal centreline wash bulkhead.
The hull has a single bottom for protection of the cargo tanks – particularly against collision damage from moored shuttle tankers. Strong points were welded on to the structural interface between topsides and vessel deck, allowing topsides pillar supports to be included.
The topsides facilities were designed as separate modules. Amec used the largest single pedestal crane in Europe, a PC9600 from Grayston, White and Sparrow, to lift two modules a day, including the 555t lower section of the turret and the 340t switch-gear module.
The twelve mooring lines are anchored to the seabed by piles in six pairs. The mooring and anchoring work was carried out by HeereMac and Aker Marine. HeereMac installed the anchor piles in 89m of water using its derrick barge DB 102.
The piles weighed 87t, were 37m long and 2.1m in diameter. Each pile was penetrated to a depth of 38m. Attached to the top of each one was 150m of 6in diameter chain.
Pumping is carried out by hydraulically-driven, 1,400m³/h cargo pumps, powered by high-voltage motors. When offloading, four pumps are used simultaneously (5,600m³/h). Inert gas supply and tank purging closed systems obviate the release of tank gas on the maindeck area.
The topside process design consists of a conventional three-stage crude oil stabilisation train. Wellhead fluids are received separately by two identical first-stage separators, each designed to cater for 55,000bpd. The associated gas is dehydrated and compressed prior to export. Produced water is treated to an oil-in-water specification of 20ppm by means of hydrocyclones.
The separators are aligned parallel with the vessel’s longitudinal axis and located amidships, where pitch and heave accelerations are lowest. The stabilised crude is routed to the wet crude reception tank for final dehydration prior to storage. Water injection facilities are provided to inject treated seawater for reservoir pressure maintenance.
The turret and swivel anchor the facility to the seabed, acting as the entry point for the well fluids and the exit points for gas exports and water injection.
They were designed by SBM and built by Amec and IHC. The FPSO is designed to accommodate 14 risers for production, water injection and gas export, as well as control umbilicals.
Individual production lines from each field, along with the water injection lines, are manifolded in order to reduce the number of flowpaths required through the swivel stack. With a height of 11.8m and a weight of 57t, the swivel stack assembly is one of the largest to date.
The main power generation system consists of three generator sets, of 10MW each, driven by Solar Mars gas turbines. An emergency 750kW diesel engine starts automatically on loss of main power, providing power to the accommodation systems and starting the main generators.