HOUSTON—With production in water depths greater than 2,744 m (9,000 ft), operators already are planning for water depths beyond that, higher temperatures and pressures, and seabed processing.

The DeepStar study was the first major evaluation of 13 alternative deepwater floating platforms and riser systems with dry tree and direct-vertical access (DVA) capability, said Rajiv Aggarwal, Granherne Inc., during the technical session “Advances in Deepwater Technology” on May 4 at the OTC 2015.

In SPE Paper 26033, which he coauthored, he noted that the participants in the study provided designs for three regions—the Gulf of Mexico (GoM), West Africa and offshore Western Australia—and three water depths—914.6 m (3,000 ft), 1,829 m (6,000 ft) and 2,439 m (8,000 ft). The designs included semisubmersible-shaped and hybrid hulls. Tension-leg platforms (TLPs) and spar hulls were used as baselines for comparison.

Evaluation Parameters

The life of the marginal field was 10 years. Reservoirs for the GoM and West Africa were 40 MMboe to 100 MMboe (oil), requiring six producing wells and three water-injection wells. Western Australia’s reservoirs were about 28 Bcm to 85 Bcm (1 Tcf to 3 Tcf) of gas, requiring six producing wells. Water depths were 4,573 m (15,000 ft) in the GoM, 1,829 m off West Africa and 2,134 m (7,000 ft) off Western Australia. Topside varied with design and requirements.

Group I consisted of four-column, semisubmersible-shaped hulls vs. TLPs. These included a dry-tree design by Aker Solutions; the OPTI-DRI hull design from Exmar Offshore; a damper chamber column hull design by INTECSEA; and a heave-motion and vortex-induced-motion suppressed hull design by Technip.

Group II were hybrid designs, which included the OctaBuoy design by Moss Maritime; the extendable semisubmersible design by FloaTEC; and the free hanging solid ballast by INTECSEA.

Group III focused on low-payload, DVA designs. The design with compliant vertical-access riser was from Granherne. The OPTI designs from Exmar were evaluated for DVA. A three-column MiniFloat-V from Marine Innovation and Technology also was studied.

Platform Conclusions

Comparative assessments and technology readiness reviews were included in the study. The project confirmed the feasibility of both the semisubmersible and hybrid hull designs as low-cost, dry-tree solutions for deepwater marginal fields.

In 914.6-m water depths, TLP designs are a low-cost option. Offshore Western Australia semisubmersible hulls with suction anchors would be a competitive solution. Novel semisubmersible and hybrid designs showed increased value for water greater than 1,524 m
(5,000 ft).

As Aggarawal pointed out, “Some solutions require more work.”

Deepwater Installation With Synthetic Fiber

The trend toward ultradeepwater development is forcing operators to further evaluate the use of steel wireline for installation systems.

During the same session, Gregor McPherson of Caley Ocean Systems, in SPE Paper 26059, said that deploying subsea processing equipment in deepwater in excess of 1,500 m (4,920 ft) challenges the industry’s preference for using conventional steel wire rather than synthetic fibers.

As the water depth increases, the weight of the steel cable combined with the weight of the payload is critical.

At 3,000 m (9,840 ft) the weight of a 5-in. wire rope is about the same as its 170-ton payload. At 6,000 m (19,685 ft), the capacity of the steel wire is entirely used by its own weight, thus no payload.

Even though advances in winch design are useful, the industry needs to develop a synthetic fiber rope with “steel-like” qualities. Until the advent of steel-like fiber rope, the industry will continue to design ways to increase the working depth for conventional steel wire.

This article is based on information from SPE papers 26033 and 26059.