Buoy-based production systems developed for small remote fields

Aug. 1, 1999
Deep water versions in design
Click here to enlarge image

A spread of buoy-based production systems has been designed by Mentor Subsea and Ocean Resources. The basic concept is 20 years old, but has recently progressed to a variety of configurations, including long-distance tieback of marginal subsea fields in deepwater.

Ocean Resource, located in Chepstow (UK) patented its first remotely controlled concept in 1979. In the 1990s, it has engineerd three buoys for offshore production:

  • Three-tether flare buoy for Zafiro off Equatorial Guinea
  • Sea Commander buoy for East Spar off Northwest Australia
  • Larger buoy for Mossgas' current E-M development off South Africa.

Under the new partnership, five different buoy concepts have been grouped together into The Universal Buoy System range (TUBS). Of these, only the Sea Commander is proven offshore. This is designed to provide control of remote satellite wells, with facilities for chemical storage and injection. The buoy itself is operated via telemetry links.

Sea Commander

The standard Sea Commander, for 250 meter water depths, is tethered to a 1,600-ton reinforced concrete gravity base on the seabed by twin spiral strand wire mooring lines. The buoy is a cylindrical, spar-type structure 53 meters tall, with five internal decks, the bottom half being submerged in water. The upper section structure is a column that includes satellite or VHF communications antennae, plus access decks for maintenance or intervention teams arriving by boat or helicopter.

Onboard power comes from four specially designed diesel generators, only one of which operates (normally) at any one time, providing AC and DC power to control systems, pumps, and other equipment.

In standalone production mode, the Sea Commander can accommodate 8-10 km subsea step-out wells, using a local distribution manifold. "For smaller oil accumulations, the buoy could also act as a hub, controlling several drill centers up to 15 km away," says Mentor Subsea's Tony Cousins.

Studies by Mentor and Ocean Resource suggest that the longer the step-out, the bigger the saving in capital expenditures (capex), compared with a conventional subsea tieback of similar length. The "crossover" point is viewed as 20 km, with 9% capex savings predicted for a 40-km tieback. Cost of a standalone, five-well Sea Commander development is put at £60 million.

Mossgas' Sea Commander will be towed out to the E-M field horizontally, then upended, but there are plans for larger buoys with heavier equipment to be towed vertically. Once on site, installation is effected by a combination of deballasting, pull-down wires and hydraulic release rams connected to the gravity base. A torquing system insures correct load distribution after buoyancy/tethering has been accomplished.

A remotely operating installation which is not fixed presents numerous safety issues, which the designers are dealing with. According to Mentor Subsea's Chris Shaw, the buoy is "very resistant to hazards - it's compartmentalized, so spillage from broken pipes or flanges can be contained within that area. Any escaping vapors will dissipate. There should not be enough oxygen to start a fire."

The buoys are designed to be manned only during maintenance visits. Normal escape time, via the central access tower, is estimated at 6 minutes. To avoid collisions, the supply boat is moored downweather of the buoy.

Sea Booster buoy

The Sea Booster buoy has provision for water injection or reservoir pressure maintenance, using either downhole ESPs or mudline-supported multiphase pump systems. Power would have to be supplied from a host facility, with a modal control center on the buoy. Access to a pigging line has been included, to enhance flow assurance.

A heat tracer onboard the Sea Buoy can be applied to the subsea trees, thereby avoiding hydrate formation. Upfront engineering costs for the Sea Booster appear high, Cousins admits, but CAPEX savings should increase as tieback distances lengthens. He cites 35% savings for a 100-km five-well subsea tieback, compared to a conventional umbilical tieback to a host platform. On shorter tiebacks, savings of $1-2/bbl could be achieved, he claims.

The C-FAST (combined filtration and seawater treatment) buoy is an initiative started by Amerada Hess, involving use of a raw seawater injection facility developed by J Ray McDermott and CAPCIS - the idea being to eliminate topsides water treatment, pumping, and subsea flowlines (thereby saving 40% on comparable topsides capex). C-FAST comprises a subsea pump and filtration module installed adjacent to the subsea water injection christmas tree, powered and controlled via an umbilical.

Initially, Cousins says, the oil company sponsors showed reluctance to deploy a large pump set housing the equipment on the seabed, because of the associated high operating expenditures. But attempts are now being made to locate the treatment filters and pumps within the buoy itself. No cost estimates are yet available, however.

Sea Producer

The Sea Producer is a small production facility within the buoy itself, operating in the 20,000-50,000 b/d range. Up to 300,000 bbl of produced oil could be stored in an associated facility stationed 500 meters away, but production could also be exported to a host platform. This buoy has three equipment decks in its main hull, the lowest of which houses first and second stage separation. The upper deck would support heat generation and process control systems.

Oil would be directed to the buoy from one or more subsea trees via a semi-dynamic riser. Any associated gas could be used to provide heat for separation and to generate power for on-board equipment. A large access tube from the buoy's hull would provide a route from the surface for equipment maintenance, plus air intake. The tube also supports a helideck and power generation module, well above the highest wave crest.

Intervener Buoy

Typical Sea Producer Buoy development configuration.
Click here to enlarge image

The Intervener Buoy allows integration of the manifold and gravity base, with a mini-moonpool in the buoy's central access tower allowing limited intervention on the manifold's control pods, accumulators and valves. Downhole operation would also be feasible using coiled tubing or wireline systems, deployed through the central access tower from an intervention vessel.

Deep Sea versions of these buoys are now under conception, using an artificial seabed at a pre-determined water depth (probably 125 meters), tethered by poly propylene lines to an anchoring system on the seabed. Studies have been undertaken for a Sea Commander in 500 meters water depth in the Gulf of Mexico for Exxon (involving a 30-km stepout), for Reading Bates in 4,000 meters water depth with a 40-0km stepout, and for Arco in 1,250 meters water depth West of the Shetland Islands.

A Sea Producer version for 2,000 meters water depth was investigated for Amerada Hess. All studies were deemed feasible to the satisfaction of the clients, according to Ocean Resource's Eduardo Freire. The systems appear to be stable, with reliable motions, and do not pitch or roll.