Dual activity drillship design prevents deepwater costs spiral

Laying of the keel for the Discoverer Enterprise newbuild drillship at Astano's yard in Ferrol, Spain, March 1997. [27,495 bytes]
Artist's impression of the Discoverer Enterprise. [43,039 bytes]

Exploration successes in the deep waters of the Gulf of Mexico have rekindled interest in dynamically positioned drillships. Numerous vessels have undergone engine refits in readiness for extended duty in DP mode.

For more remote, ultra-deepwater operations however (700 meters and below), a refit alone may not suffice. Assignments at these depths are typically protracted, multi-well campaigns demanding deck space and load capacity not normally attainable on existing drillships.

As an alternative, some contractors are drawn to conversions of larger types of vessels, such as tankers. But the extent of the rebuild and the man-hours involved can push costs to a point where the purchaser might as well go for a newbuilding.

This is the course Conoco/Reading & Bates have followed for their upcoming global deepwater campaign. It was also the solution recommended by Astano to Sonat Offshore Drilling to fulfil long-term deepwater work for Amoco in the Gulf of Mexico.

Sonat had been awarded a $200 million drilling contract by Amoco for a minimum of three years, starting mid-1998. Sonat wanted a unit that could accommodate dual activity drilling - twin derricks and two complete drillstrings - in order to maximize time in, and traveling to, the distant location.

The contractor was looking for a fast-track conversion project when it approached Astano in May 1996. Initially, Sonat wanted to convert an Aframax tanker in its possession, but after a survey study, Astano persuaded them that in terms of cost and construction time, a newbuild would be superior.

Astano's study took account of factors such as infield operation life and the required moonpool installations, which looked somewhat large for the tanker. The drillship would also have to accommodate 200 crew, but here again the tanker looked ill-equipped for such an upgrade. Furthermore, drilling utility installations looked a tall order with twice the normal number of drillpipe risers needed to operate two drillstrings.

Conceptual design for a newbuild drilling monohull unit was completed in-house by the Astano team in just three weeks. The company's previous experience in rig building was the Drillmar I semi in 1991, which was a Friedman & Goldman Pacesetter design, able to work in just 600 meters of water compared to 3,000 meters for the Astano drillship concept.

Following due deliberation, Amoco accepted the newbuild solution, only requesting a 20% increase in the planned crude storage capacity to 100,000b/d. The contract was finally signed on July 24 last year.

Sonat (since acquired by Transocean) also expanded the scope of Astano's work to fabrication and installation of some of the topsides modules to a Transocean design. However, drillfloor items, the Dreco-designed derricks, the BOP machinery module and the mud pump module are being sourced elsewhere and will be installed when the vessel arrives in the Gulf of Mexico.

The vessel, named Discoverer Enterprise, will have an overall length of 255 meters, a breadth of 38m and a design draught of 12m. At that size it will eclipse all other drillships, including the Conoco newbuilds, which are in fact based on existing shuttle tanker designs. Its displacement, at 98,000 tons, will be almost five times that of the current Discoverer-class units.

Dimensions aside, the Discoverer Enterprise's most striking feature will be its two fully-equipped drilling areas separated by 40ft from each other on the 80x80ft drilling floor. From here Transocean will execute a new patented technology called Expedrill, which allows different drilling tasks associated with a single well to be performed simultaneously, rather than in sequence. So while one drilling area is drilling ahead, the other can be used for tasks such as changing BOPs, cementing or logging runs.

Minimizing interruptions this way speeds up drilling operations, particularly in very deep waters where trips are more time-intensive, due to the higher number of connections that have to be made between the drillfloor and seabed. Greater productivity will clearly follow also when two wells are drilled at once. This task is made feasible by the vessel's storage capacity, which will include two active 30,000bbl mud systems, allowing the driller to switch swiftly from oil-based to water-based drilling mud.

Conoco's newbuilds, in contrast, will only be equipped with one drillstring each, so the moonpool will also be shorter. Statoil/Smedvig's new drillship will have two strings - however, one of these will only be used as an auxiliary hook to speed drilling of one well. Unlike the Discoverer Enterprise, it will not be able to drill two wells at once with two risers.

High stability

Discoverer Enterprise's dynamic positioning system will be driven by six Aquamaster thrusters, each with an output of 5.5MW. Power generation will come from six diesel sets, four outputting 7.5MW and two 5MW. All this equipment will be installed by Astano at the vessel's rear.

The vessel will provide a total variable deck load of 30,000 tons -20,000t for the drilling equipment and 10,000t for the topsides modules. Tank tests by the Marin Institute in The Netherlands have proven the vessel's stability in DP mode and during normal speeds of 12 knots, including transit speed. It is crucial to use transit capability to the full in reaching the location, as dayrates are so high (around $180,000).

Astano has designed the two-deck layout as follows. On the upper level, at the rear, the riser rack will sit on top of the engine and power generation rooms. Moving forward, the pipe rack is located above the auxiliary machine space and sack storage areas. In front of these, the central moonpool area is sandwiched between the mud pump and the BOP/forward machinery modules (including crude storage), preceded in turn by the riser rack and the accommodation/navigation area. The lower level comprises mainly water and ballast tanks.

Astano has designed the interface between the topsides and hull to achieve maximum structural integrity and convenient access to utilities. The vessel must be able to supply power, compression, diesel oil, drilling water and cooking water over long stretches of time.

To house two drilling crews simultaneously, Astano is installing living quarters with twin bunks for 200 people. It is also building and installing a helideck.

According to Astano's commercial director Luis M?ndez, the Discoverer Enterprise is designed for 20 years service. "But it can also be drydocked - maintenance and inspection requirements are not as great as with an FPSO. The topsides are designed for 7,500ft of water, but can easily be upgraded to 10,000ft operations."

Cost of the whole project to Astano will be around $240 million, split 50/50 between the topsides and the hull. That compares with $280-300 million for a new 5th generation semi with far lower storage capacity.

Fabrication started last November, with the keel laid this March. Delivery is due in February 1998.

Long-term, Transocean may move the vessel to West Africa, unless Amoco exercises an option to keep it in the Gulf for two more years. "We think there will be a boom for deepwater drilling units", says M?ndez, "although not for this type of unit, because of the high CAPEX/dayrates involved."

Astano is working on a modified version of this design with a turret, which was also requested by Sonat. The solution so far involves dispensing with two of the thrusters, relying instead on a passive mooring system - the aim being to reduce power consumption and, as a result, power costs.

The concept is being targeted at Gulf of Mexico water depths below 800 meters, where dynamic positioning is the norm for drillships. "But the environment there is still mild compared with North Sea conditions," says M?ndez. Numerous drilling contractors have shown interest in Astano's idea.