Shell U.K. Exploration & Production will install in the U.K. North Sea's Fulmar oil field the world's first commercial submerged turret loading (STL) system.
Shell is replacing the existing Fulmar floating storage unit (FSU) tanker with the new design that employs a new purposebuilt tanker.
The 95,000 dwt Vinga tanker, which arrived Oct. 7 at the Fulmar site, is purpose built to load crude oil using the STL system developed by Norway's state owned Den norske stats oljeselskap AS (Statoil).
STL entails a buoy moored to the seabed and positioned 100 ft below the water's surface, to which a tanker connects and through which a pipeline carries oil.
Statoil said STL enables offshore loading in almost any weather conditions. Conventional loading is limited to a maximum 5.5 m significant wave height.
Shell Expro will use a buoy and tanker to replace Fulmar's existing Single Anchor Leg Mooring (SALM) system and 210,000 dwt Storage tanker.
Vinga is owned by Statoil and chartered to the Shell/Esso U.K. plc combine for a minimum 2 years. It can load as much as 400,000 b/d of oil.
FULMAR DETAILS
At Fulmar, the vessel will load as much as 150,000 b/d of oil from Fulmar, Auk, and Clyde fields. Every 4 days cargo will be discharged into a shuttle tanker.
Fulmar field was developed using two platforms producing via the SALM structure into the FSU. Fulmar A is a drilling-production platform, while Fulmar AD is an unmanned wellhead platform producing from predrilled wells.
The platforms lie on Block 30/16 in 270 ft of water. Fulmar was discovered in December 1975 and started production in February 1982. Peak production was 168,000 b/d in 1986.
Ultimate recovery is estimated by Shell/Esso at 550 million bbl of oil, 60 million bbl of natural gas liquids, and 150 bcf of gas.
Shell expects a lengthy overlap with both FSUs in the field to allow testing. Loading using STL only is expected second quarter 1994 at the earliest.
STL TECHNOLOGY
STL is the brainchild of Kare Breivik, manager of Statoil's department for technical maritime services.
Breivik saw a need for a simpler, cheaper method of offshore loading that was not dependent on sea conditions.
He conceived a development method using a submerged buoy that could be hooked by a grapnel and pulled into the reinforced bow of a tanker. Once in place, the buoy/turret would be secured, and loading could begin.
Breivik's idea was that two buoys could be installed in a field and connected directly to wellheads. Oil would be exported by continuous loading into shuttle tankers. Using one buoy and an FSU with a shuttle tanker, as chosen by Shell for Fulmar, was an option.
Beginning in November 1991, Statoil developed STL in conjunction with Marine Consulting Group (MCG), Trondheim, Norway.
"Because there is no surface equipment to avoid, hook-up to the STL buoy is simple," said Breivik. "Wave tank tests at Trondheim have proven STL's year round capability. Hookups have been performed at 7 m significant wave heights."
Statoil claims hook-up time is 30 min in rough seas. Once moored, the tanker can weather vane, while disconnection in any sea conditions is said to take seconds.
STL PROSPECTS
Breivik said Statoil is in talks with several operators around the world for application of STL. One prospect is Norway's Heidrun field, where operator Norske Conoco AS could reduce development costs by 1 billion kroner ($140 million) by eliminating the need for a storage tanker.
Statoil is talking to BP Exploration Operating Co. Ltd. about using STL for its Forth field development in the U.K., said Breivik.
Statoil and MCG have formed a joint venture to market the STL concept as a leasing arrangement. Statoil/MCG would provide buoys, tankers, and technical support to develop other operators' fields.
Breivik said this approach would be ideal for depleting reservoirs with a 12 year life expectancy. Once a reservoir is depleted, the buoys and tankers could be moved on to another field.
EXPERIENCE
Harald Urheim, Statoil's senior shipping consultant, marine transportation, noted his company has 14 years of experience of shuttle tanker operations. It has a fleet of 13 vessels, with two more due on stream in 1995.
Reliance on offshore loading for Norwegian developments such as Statfjord and Oseberg fields is due to the impossibility of running pipelines across a deep trench on the Norwegian shelf.
Urheim said recent conventional shuttle tanker contracts in the U.K. sector were for Midland & Scottish. Resources plc in Emerald field, Chevron U.K. Ltd. for Alba, Phillips Petroleum Co. U.K. Ltd. for Maureen, Kerr-McGee Oil (U.K.) plc for Gryphon and BP for Forth.
Other contracts have been for backup tankers in Shell's Fulmar and Kittiwake fields, well testing for BP in Mungo, well testing for Texaco Ltd. in Captain, and back-up tanker operations for Mobil North Sea Ltd. in Beryl.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
