- Hydraulic rigs, such as Maritime Hydraulics' ram unit (shown in profile at left), may replace conventional masts (right) in new generation drilling vessels being considered. Note pipe sections are stacked below the floor and elevator operations take place at the drill floor level in the ram unit.
Sixth generation drilling units will incorporate many design changes
The sixth generation mobile offshore drilling unit probably will contain some drastic changes in both appearance and structure from earlier conventional rigs. Judging by the research underway at present, and the needs of drilling contractors, you can expect some or all of the following:
- Multi-functional: The units will incorporate the ability to readily load on and load off skid-mounted equipment to conduct complete well site operations (rotary and coiled tubing drilling, production, extended well testing, workover, and installation of subsea and pipeline production equipment).
- More compact: Reductions of size and weight will provide a more compact unit, but because of environmental forces, the footprint and freeboard for both jackup and semisubmersible units are unlikely to change much.
- 20-25% less weight: Modular on-and-off movement of equipment to accommodate changing functions will keep deckweight down. New lightweight fire-blast walls, lower accommodations requirements, and wider use of composites will also help.
- Fewer people: Two-person drilling crews and fewer contractor personnel armed with a wide array of computers and video communications gear with experts on line will keep crew numbers down.
- Reduced mast: Hydraulic push-pull systems will replace the conventional derrick and drawworks (see following article in this column), reducing the mast sail area (surface area exposed to overturning forces).
- Lower center of gravity: In addition to a minimized superstructure and drill floor mast, multiple pipe sections will be racked below the drill floor full time and only laid horizontally for testing purposes.
- Reduced keel and draft: Reducing the superstructure, mast, sail area, and lowering the center of gravity will allow designers to focus keel and draft shape for maximum storms instead of drilling.
Key technologies of the future cited
An insight into what technologies drilling company managers see as useful in the coming years was provided recently by Dennis Heagney, president and chief operating officer of Sonat Offshore Drilling, at the Oilfield Breakfast Forum in Houston. The key technologies he outlined include:
- Hydraulic lifting systems to replace the derrick, motion compensator, and drawworks
- Under-balanced drilling with concentric drillstrings
- Subsea under-balanced (riserless) drilling
- Total fluids management
- Hydraulic directional jet drilling without cutters
- Drilling with casing
- Coiled tubing drilling
- Reliable composite materials for tubulars, mooring systems, casing, and risers
- Multilateral completions
- Designer wells.
Manufacturers targeting extended reach limits
There are three limits that drillers are dealing with, or will have to, in extended reach drilling - drillstring rotation, bit steering, and drilling fluid telemetry:
- The maximum projected measured wellbore distance for rotating a drillstring in an extended reach well is calculated to be about 33,000 ft. At that point, torquing of the drillstring creates too many mechanical problems.
- The maximum measured distance for steering the drillbit in an extended reach well is about 22,000 ft. Sliding the drill string becomes nearly impossible past that distance.
- The maximum measured distance for mud telemetry can be as low as 20,000 ft on some wells, but on others, the practical limit has not been encountered. Research is being done on improving the ability to detect very weak pulses and on a substitute telemetry - radio frequency. By the next century, automatic directional control may be sufficiently developed such that the steering is directed by a closed loop system behind the bit.
Regarding the first two barriers, operators and equipment manufacturers would like to push out the drillbit steering envelope to more nearly approximate the drillstring rotation envelope. As drillers hit the maximum limits for steering, Camco and Russell Subsystems are reporting more interest in their steerable rotary drillstring, which incorporates a method of multiple hydraulically-actuated cams to steer the bit in extended reach wells. The method reportedly was troubling to operators at first because of its complexity, but numerous trials have been conducted successfully.
Rotation while steering eliminates sliding, avoids static friction, and improves hole cleaning. Cams, powered by mud flow, contact the casing or formation in rotational sequence to force the drillbit in the desired direction. The cams are operated by a modulated bias unit, independent of drillstring rotation.
The Camco-Russell unit is among a new generation of tools to aid pipe rotation, preferred in practically all circumstances by drillers. If effective, drillers may be able to keep pipe rotating and maintain steerage all the way out to drillstring torque limits.
In order to surpass the barrier where drillstring torque exceeds string weight or push at the surface, a pulling action will be needed behind the bit, possibly some type of drilling fluid powered tractor that would contact the formation or casing wall. This technology is already being examined for coiled tubing drilling in some consolidated formations.
Swath rig may use casing for riser
Statoil is considering the use of casing in place of a riser for its swath-shaped well intervention and slimhole drilling vessel now nearing the fabrication stage. The vessel hull is a single water plane twin hull unit (swath), which has the stability of a twin-hulled semisubmersible drilling unit but can transit from one location to another as a ship would.
The use of casing as a riser would save a great deal of cost, weight, and storage area on the swath unit. Such a system would allow deployment of a blowout preventer on the seabed and at the surface. When experiencing a blowout or kick, both stacks can be closed and the gas circulated out within the top casing while maintaining well control downhole.
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