Neil PotterPhillips has pioneered the use of sophisticated fiber optics technology in bringing onstream its new subsea well in the Little Dotty satellite gasfield in the UK Southern Gas Basin. The project also demonstrated the value of technology transfer from the military sector to the offshore subsea scene.
The concept appears to be simple - a single well tied into an existing 10 km pipeline from the Della subsea well 48/30-11 (part of the Little Dotty Field), which has been producing since 1979 to the Hewett 48/29-A platform. But the Phillips project team in fact faced numerous challenges, commercial as well as technical.
In the first place, when the project started, Little Dotty producer well 48/30-15 had not been drilled. Having evaluated newly acquired data, however, the team felt that a well in a different part of the reservoir would be viable. Eventually the location was fixed at 250 meters from the Della pipeline.
The well was spudded last November and completed this January. By then, the team was already looking at several development options. Since Della was in decline (production had fallen from 60 MMcf/d to 20 MMcf/d) and there was also spare capacity in the 10-in. pipeline, the best solution appeared to be a mid-line tie-in.
Varying tariffsBefore this solution could be adopted, however, contractual difficulties had to be overcome. Gas from the Little Dotty well had been sold to British Gas under the original Hewett sales agreement. But the Della well production was sold outside this agreement under a different tariff. The answer was deemed to be accurate subsea monitoring and metering of the two streams to assure the customer that good, reliable data was being received.
Gas from Della feeds into a test separator on the platform. For the Little Dotty development, a subsea venturi flowmeter was installed at a point in the Della pipeline before it is joined by the Little Dotty pipeline, and the flows co-mingled.
In this way, output from the Della well is measured accurately. When compared with the output recorded by the test separator, production from the Little Dotty well can also be measured.
ISA Controls of Shildon, UK supplied its Seastream flowmeter, with dual pressure cells and isolating manifolds for dual redundancy. The unit was supplied complete with upstream and downstream metering tubes and temperature and pressure transducers for compensation. The whole meter tube assembly was calibrated at a British Gas site using natural gas to determine the flow characteristics of the meter.
Clearly for accurate metering to follow, it was essential to have good, clear data transmission. The Della hydraulic control system was subject to interference, so Phillips canvassed manufacturers for innovative solutions. Some of the major subsea control system suppliers were not, it has to be said, over-interested. One company that did respond, Wormald Lintott, had experience of hydraulics and electronics, but had never before designed a complete control system. Nevertheless, Phillips accepted its suggestion of a fiber optics communications transmission system.
This in turn created a need for a wet, mateable fiber optic connector which divers could simply plug in. Ocean Design (ODI) of Florida, which has close links with the US military in the use of fiber optics, had spent two years developing a range of multiple channel wet-mateable electro-optical connectors. It believed that these would provide enabling technology for the use of fiber optics in the subsea environment.
Stuart Barlow of ODI described the connector at OTC in May. "In both connector halves, the contacts are contained within oil-filled, pressure-balanced chambers. As the connectors are mated, rolling seals on the mating faces of the respective connector halves squeeze together to displace any water between them.
"Further mating causes the seals to rotate, and displace any debris trapped between them. This rotating action also opens ports between the oil-filled chambers of the connector halves. Continued mating then causes the pin contacts to pass through the seals and enter into the receptacle contacts.
"As the electrical pins enter the sockets, seals are formed to their dielectric shafts, thus creating sealed secondary oil chambers around each contact pair. During the mating sequence the contacts are never exposed to the outside environment." Wormald Lintott's system used 39 wet-mateable connectors, seven oil-filled hose-type jumpers and three umbilicals.
Phillips project team leader Brian King says: "This has been a truly fast-track development - ten months from conception to production. The success of the project was that when the Little Dotty well came onstream on time on January 28, it was producing 60 MMcf/d without a hitch. The innovative connectors now provide us with a useful weapon in our armor for future developments in the area.
"The configuration of the system, and its flexibility, provide for future needs. This is particularly important as this system is not cost-effective for just one well. 3D seismic is pinpointing undeveloped pockets of gas in the area and our philosophy is to bring these onstream as quickly as possible. This equipment allows the refiguring of the complicated subsea system, and when required, efficiently and with accurate monitoring. That in turn will allow us to add in an additional well, beyond Della, probably in 1998.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.