The advantage of the vertically restrained buoyancy can is that the riser tensions are independent of the hull. Dry or wet tree wells can be accommodated from the buoyancy can, providing field development flexibility.

Water depth is not the only challenge to deepwater drilling and production in the sub-salt GoM, nor is it the greatest. Reservoirs are at record depths, with high temperatures and pressures, making for daunting drilling and completion problems. Seismic imaging is extremely difficult in the Gulf, and reservoir drive mechanisms, continuity/compartmentalization, and recovery efficiencies are largely unknown in the deeper, older strata.

Compounding these drilling challenges is the fact that managers are contending with very high well costs, ongoing appraisal drilling required to hold their leases, extended subsea and production equipment delivery schedules, and high costs and limited availability for derrick barges.

These factors leave operators facing very large, very risky development bets or continued costly drilling and well testing to reduce uncertainties. Another stumbling block is a lack of consensus on production facility concept, design basis, and cost.

A new answer to an old problem

Horton Deepwater Development Systems has developed a new floating production and drilling unit called the Multi-Column Floater (MCF), which offers a way out of this logjam. The unit incorporates the proven advantages of past successful designs while overcoming their associated design constraints.

The deep-draft [150 ft (46 m)] MCF semisubmersible offers full production and deep drilling systems (in the full capability configuration). The vessel and all associated riser and subsea systems have been engineered to be reconfigurable, relocatable, and reusable and will be made available on a leased basis.

The MCF provides flexibility for
deepwater asset management teams, enabling “phased adaptive” field appraisal and development with improvements that include:
• Reduced drilling costs through increased use of lower-cost platform rigs, dry trees, and minimized weather/loop current downtime;
• Early, sustained production and phased development to evaluate and manage reservoir risks — as well as to accelerate production revenues and first oil;
• Decreased development capital requirements;
• Expedited, lower cost well interventions and sidetracks; and,
• Release or replacement of the vessel late in field life when a smaller, cheaper version would better fit with field requirements.

System design

The MCF includes a production system design that delivers high capacity and product flexibility (range of specific properties and gas-oil-water ratios). The dual-train system can accommodate third-party production through a segregated second production train for accurate production monitoring. System capacity is based on an analysis of US Minerals and Management Service production history and actual decline rates in the GoM.

New riser technology breakthroughs incorporated in the MCF will advance the seafloor layout of the riser-to-wellhead interface and riser operating pressure ranges and will increase the number of production riser options from a single production floater.

Production and drilling risers, the riser tensioning system, surface blowout preventers (BOPs), the surface trees, and the high-pressure production chokes and manifolds are all resident and supported on the MCF’s vertically restrained well deck buoyancy can.

The buoyancy can resembles a tension-leg multi-cell single structure, an integrated buoyant column that is tensioned to the bottom by redundant tendons or by the drilling and production risers. Decks on the buoyant can create safe barriers between the drilling and production systems.

The advantage of the vertically restrained buoyancy can is that the risers are tensioned by a single buoyant structure with multiple buoyant cells that allow riser tension to be adjusted when additional risers are installed or when risers are removed for well workover, sidetracking, or maintenance. Riser tension loads are supported by the buoyancy can, which is vertically independent of the MCF hull and restrained laterally in the hull center. With this arrangement, the vertical riser tension loads no longer have to be supported by the topsides structure or the buoyancy in the system’s hull.

Traditional tensioning systems constrain the space used for other systems. With this new arrangement, multiple single-riser hydraulic tensioners or single-riser air tension cans, which compete for space in the hull and topside design, are no longer needed.

A single buoyant can riser tensioning system produces no relative movement among the individual risers, surface BOPs, and trees, which creates a safer environment for the crew during riser and tree installation and during maintenance activities. Integrating the surface BOP reduces risk by facilitating access and by increasing visibility and safety. This arrangement is also easily reconfigurable for direct vertical access to the wells. The MCF buoyancy can is large enough to accommodate the production chokes and manifolds; so produced oil and gas can be flexibly piped to the production system on the MCF with 3,000 psi or lower pressure production jumpers, which has proven to be a significant safety improvement for high-pressure/high-temperature developments.

System options

The MCF is sized in various configurations and topsides capacities (5,000 through 50,000 tons) for wet tree, dry tree, direct well access, satellite subsea tree production, enhanced oil recovery and drilling, or a combination of all methods.

The MCF configurations deliver much larger production and drilling decks, which were not practical on the tension-leg platforms and spar designs. The MCF’s two-level larger deck areas allow segregation of the production system compartments from the drilling system and lower the center of gravity of the topsides. Production high-pressure piping no longer has to be commingled with high-pressure drilling systems in the same space on the topsides. The MCF has been tested in wave model basins to the new API Central Gulf of Mexico criteria.

The 270 ft by 270 ft (76 m by 76 m) deck areas combined with the heavy lifting capacity of the onboard cranes allow the reservoir team flexibility to add enhanced oil recovery systems at a later date, based on actual production and drawdown history of the reservoir. The deck area can be configured for future expansion to allow power, pump, filtration and additional future vapor recovery units required for potential waterflood, gas lift, and seabed and downhole electric service pump scenarios.

Construction and installation

The topsides production and drilling systems will be integrated in the topsides and fully commissioned when the topsides is onshore. The completed topsides will be installed using the floatover method near shore and close to the topsides and hull fabrication yard. The hull and topsides have very few interconnections, which shortens the time for offshore system integration. The floatover also eliminates exposure to downtime associated with high current contingencies and requires no high-cost, high-capacity floating crane vessels to lift the topsides onto the hull.

The MCF can be locally constructed for production only, drilling only, or production with drilling. When the MCF is built purely as a production system, the workover and completion rig slot allows the rig to be mobilized and demobilized on the asset manager’s schedule.

The large drilling rig on the production and drilling MCF is built in modules and integrated with the topsides on land.

The modularized drilling rig can be demobilized later, on the asset manager’s schedule, using the two large MCF deck-mounted, 165-ton capacity cranes.