Gas Gatherer - new generation of minimal facilities

25% lower cost touted for repeatable structures

Brown & Root Energy Services is the latest company to introduce a standard design, minimum facilities offshore oil and gas production platform. Called the Gas Gatherer, the company styled the low-cost, lightweight platform primarily for the fast-track development of marginal fields in the southern sector of the North Sea.

Two of the platforms are currently being built by Barmac - the fabricator jointly owned by Brown & Root and J Ray McDermott at its Ardersier yard for Conoco's Viking Phoenix development, which is aimed at accessing additional reserves in the oil company's mature Viking field.

According to Brown & Root, the advantages of the Gas Gatherer's design are its repeatability and ease of fabrication - factors which underlie the claim that the substructure can be built at a cost some 25% less than the nearest competition. Capable of being installed by a jackup rig or conventional lift vessel, the platform's three-legged tower substructure can support either a full-access or a limited-access wellhead.

Brown & Root feels that the latter lower-cost option could be appropriate where, for example, availability is expected to be high or where customer supply agreements are sufficiently flexible to allow platform intervention or planned maintenance using an attendant jackup or support vessel. Conductors can be incorporated either inside the legs of the substructure or externally but still within the platform's footprint. In all, six conductors can be accommodated in the standard design.

Phil Housley of Brown & Root's central projects group, who was heavily involved in the platform's development, described some of the other features of the design. "We gave a lot of thought to the topsides layout, taking account of what our customers were telling us they wanted from an operational standpoint. The triangular configuration of the single-level topsides and cut-back deck enable a very effective interface with the drilling jackup (DJU), allowing the transom of the DJU to be positioned as close as possible to the well positions. This allowed all the slots to be reached from a single location. At the same time, the jacket piles are far enough away from the DJU spud cans so as to minimize foundation interactions.

"We have kept the deck layout as open as possible to minimize blast loading, thereby improving safety and optimizing blast wall requirements. It also gives us good access for fabrication and outfitting, makes commissioning relatively easy, provides scope for mechanical handling operations, and affords the operator adequate deck-space for wire-lining equipment.

Ease of fabrication

"Brown & Root has utilized 3-D CAD over the years and this has paid back in the development of the Gas Gatherer", explained Housley. The system delivers engineering and fabrication materials using Triton, a proven integrated structural system for modeling and drafting offshore platforms. "In addition, we can overlay Triton structural models with those produced by Brown & Root's 3-D process plant design management system (PDMS) to ensure a clash-free design for the entire platform." Linked electronically to Barmac's fabrication yard, engineering data can be transferred directly to the plate cutting machines, improving productivity and accelerating the fabrication process.

Conoco has chosen to employ two of the Brown & Root designs for its Viking Phoenix development. This will bring into production four reserve accumulations in Conoco's Southern North Sea Viking complex: F/Fs (part of Viking 'A' Field). The reserves, in about 25 meters water depth, will be produced by one of the platforms and Wx/Gn (part of the Viking 'E' Field) in 20 meter water depth, by the other. Estimated to contain about 500 bcf of gas, peak production will be of the order of 300 MMcf/d. Export will be via a 12 km flowline to the Viking BD platform - suitably modified with an 800-ton reception module - and thence to the Conoco-operated Theddlethorpe terminal, roughly 100 km away on the Lincolnshire coast.

The two platforms with jacket and topsides combined weights of 600 tons (F/Fs) and 570 tons (Wx/Gn) will be normally unmanned and remotely controlled from the terminal.

Brown & Root is hoping these will be the first two in a line of similar platforms to emerge from the Moray Firth facilities. Several operators in the Southern North Sea have expressed an interest in the platform.

Additionally, the company is targeting developments off the Dutch coast and further afield offshore Egypt. The company accepts there is fierce competition in this sector of the market but recognizes that the gas price is most likely to determine the future prospects for its Gas Gatherer platform.