Brigantine, United Kingdom
Brigantine is a cluster of three fields (A, B and C), located offshore North Norfolk, 110km NNE of Bacton. The fields lie in the southern North Sea, in block 49/19, with Brigantine B slightly extending into 49/18. The fields were discovered by wells 49/19-4, 49/19-6 and 49/19-7, which were drilled in 1986, 1997 and 1998 respectively.
Brigantine was granted government planning permission in July 2000 and the stream came on stream in 2001. Shell Expro owns 50% of the fields; the other half is owned by Esso Exploration. Shell is the operator.
The project will develop 280 billion ft³ of gas at a total cost of £100 million. Production rates of 130MCFD were achieved soon after the start-up.
The latest technological advances were employed in the development, with state-of-the-art reservoir analysis techniques and the use of extended-reach horizontal wells. The reservoir is of Permian age. Three horizontal wells are used to drain the long, narrow reservoir structures of the A and B fields and to overcome potential compartmentalisation. One slanted well is also planned for the C structure.
These four wells are produced into the two platforms Brigantine BG and Brigantine BR. The Maersk Enhancer was contracted to drill the wells, commencing with Brigantine BG.
The £5 million contract for the procurement, fabrication and onshore commissioning of the two platforms was awarded to KYE Limited of Lowestoft. The contract included the procurement, fabrication, onshore commissioning and load-out of the new platforms.
Shell specified low-cost normally unattended installations (NUIs) for both the Brigantine BG and BR platforms, and it selected a Trident platform of the same design (that achieved initial success with the Skiff development, in early 2000). This design was considered to be between around a third to a half cheaper than a more traditional one.
The Trident jacket is a four-leg cross-braced steel structure, weighing approximately 400t. Designed for installation in 26m of water, each unit stands approximately 470m high.
Within the platform jacket perimeter, the conductors from the four gas wells will be employed as pile elements. The Trident design relies upon these, to provide a considerable proportion (possibly as much as 100% subject to soil conditions) of the foundation capacity. Skirt piles have been used to supplement the conductors as necessary. The conductors are first driven to a terminal depth (for Skiff this was 74m) using drill/drive techniques.
Once the jackets were set, the topsides were installed. The minimal 1,500t duplex topsides require only the minimal instrumentation. Power is provided by the Galleon PN platform, via subsea cable.
For the first time in the UK, the platform design does not include a helideck. Any unscheduled intervention is planned to be carried out by an operations crews - typically a team of four - gaining marine access to the platform, instead of by air. The boat would remain attached docked as a means of escape.
Planned maintenance is carried out annually, using a jack-up barge or a conventional marine-installation vessel. This cuts operating costs by up to 66%.
The lack of a helideck and support facilities means that the platform’s total weight and dimensions are sufficiently reduced, to enable installation by means of a drilling rig. This obviates the need for a heavy-lift crane vessel. While the heavy-lift vessel Thialf was used to install the Brigantine BG in early August and the BR platform in late September 2000, the Skiff platform was installed by the the jack-up Maersk Endurer.