"Get deep cheap" goal for new drilling units

The Sedco Express semisubmersible will feature many of the innovations that define the new rig economics. [27,356 bytes] Oil and gas producers and drilling contractors are designing, converting, or refurbishing mobile drilling rigs to address two goals that seem common to all new and redevelopment: increased production rates, lower costs.

Well construction also designed for high production rates

William Furlow
Technology Editor
Oil and gas producers and drilling contractors are designing, converting, or refurbishing mobile drilling rigs to address two goals that seem common to all new and redevelopment: increased production rates, lower costs.

While such talk is cheap, new construction is not. It is one thing to propose a new technology, and quite another to have the confidence to build that technology into a new floating vessel. Thus, a quick survey of some new technologies featured in the latest vessel designs offers a snapshot of where the industry is today in terms of priorities and capabilities.

Get deep cheap

The drilling unit Sedco Express is Sedco's latest contribution to the world's fleet of semisubmersibles. As a fifth generation floater, the rig by definition is focused on achieving the two goals of reaching deepwater targets inexpensively. The new vessel, two are actually in the works, will be capable of mooring in 7,500 ft of water and operating dynamically in water depths greater than that.

The vessel is claimed to reduce well construction cost by as much as 30% and well construction time by 25%. The time reductions come from greater efficiency in pipe handling and tool makeup. The Sedco Express also has built-in systems for wireline, measurement while drilling, logging while drilling, and coiled tubing operations.

Transocean's dual activity drillship design offers an unusual approach to the same challenge of reducing well construction time. The derrick featured on the Discovery Enterprise has two rotary tables so support activities can take place on one table while the other operates over the critical path. Transocean said this new system can increase efficiency on a full exploration well by 15%.

As day rates for all classes of rigs continue to climb, such time saving technologies become more cost efficient. Producers are insisting on the innovations.

These larger drilling units also have increased pipe handling capabilities. They are able to run quads of pipe rather than triples, and both rigs are able to make up and set back 125-ft lengths of casing for time savings. Running quads in deepwater automatically offers a 25% time saving over triples.

Fluid systems

With the high drilling fluid volumes inherent in deepwater drilling and the common problem of lost circulation due to shallow water flows from unconsolidated sands, new mud system designs possess two major characteristics: flexibility, volume.

It is no surprise that ultra-deepwater exploration is going to require more drilling fluid. These higher mud volumes mean bigger tanks, and the longer annulus means more powerful pumps.

The Sedco Express boasts a 4,500 bbl system pumped by a 7,500 psi system, with an additional 8,000 lb mixing reserve. Such powerful systems are not restricted to deepwater. The jackup Rowan Gorilla V will have a 7,500 psi mud system with 5,200 bbl of storage in 12 pits. The drilling unit features increased capacity, even though it is designed to operate in no more than 450 ft of water.

What these rigs gain by beefing up their systems, in addition to added depth, is increased rate of penetration, better hole cleaning, and more fluid options.

Another advantage these new rigs share is the dual mud system. The Rowan Gorilla V has two separate systems. The Sedco Express has the option of splitting the system on the main deck.

This means the option of running two distinct muds. For example, a water-based mud and a synthetic fluid could be made up at the same time. If a driller hits a lost circulation zone with the high dollar synthetic, a switch can be made to the cheaper fluid until the trouble zone is drilled through.

Better deck design

Another area of innovation apparent in the design of new rigs is a more efficient layout on and below the decks. Experience has helped to streamline the drilling process, and these lessons can be seen in several new designs for a variety of water depths.

The Friede & Goldman jackup design JU-2000 has repositioned the crew quarters onto a cantilever arm to balance the extended cantilever used for drilling. This not only increases the deck space for pipe racks and equipment setup and storage, but allows the crew quarters to be fabricated as a independent module.

Likewise, the R-450 jackup designed by Zentech has a wider hull, which means it is more stable and can carry more weight in tow than comparable designs. The cantilever has a 75-ft reach and girders that are mounted 60 ft apart, allowing them to retract around the crew quarters, and provide more engine room, and mud pump space.

The 75 ft reach means the drilling unit can drill wells further off center than the classic Le Tourneau 116-C cantilevered jackup design. With the cantilever partially extended, the drill pipe rack is exposed. Because the girders are designed so far apart, the main deck pipe rack is set transversely, allowing crews to move tubulars directly from the main deck pipe rack to the elevated pipe rack.

The Sedco Express features an even more dramatic redesign, with a lower center of gravity. The semisubmersible can mobilize faster and is more stable. The designers have achieved this by moving most of the heavy components below the waterline. This includes everything from the mud pits to the mooring winches, engine room, and thrusters. This step lowers the rig's center of gravity, frees-up deck space, and allows a lighter deck structure, which in turn allows for a greater variable deck load.

Modular design

The modular design of drilling units also seems to be the wave of the future. This concept means faster and less expensive construction. Modules can be fabricated individually, then assembled together into the final vessel structure.

The individual modular units of these vessels are easier to fabricate, reflecting the man power shortage currently plaguing shipyards.

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