Seabed Drilling Rig Feasible, Says Recent Study

Abstract: A novel, though comparatively practical, drilling concept is being developed in a joint industry project; the difference is, a prototype could emerge by 2006.

Analysis: Drilling offshore wells from the sea bottom actually is an older concept than drilling from the water's surface. Patents for such rigs were filed as far back as the 1840s. But then, concepts are just that--ideas that are only half-baked until somebody turns up the heat.

And somebody's turning it up.

In practice, offshore drilling began at the surface and has remained there ever since. Locating a drilling rig on the ocean floor--though the more practical of the two notions for lots of reasons, the main one being steadily increasing water depths for offshore operations--has been considered pretty much only in the academic sense. So far, bottom-supported oilfield equipment has been employed mainly for post-drilling, production-related operations.

But times--and drilling water depths--have changed. Today, advances in marine drilling technology have outdistanced even the deepwater prospects actually available to offshore producers. Though one or two ultradeepwater wells have been drilled so far in up to 9,000 ft (2,745 m) of water or so, floating rigs for exploratory drilling--semisubmersibles and drillships--already are being designed and built to work in water depths to 10,000 ft (3,000 m), with even newer designs aimed at the 15,000-ft (4,675-m) and even 20,000-ft (6,100-m) marks. Problem is, as water depth capability increases, so does the size and cost of the rig itself as well as that of all the equipment needed to connect it to the ocean floor. The overall cost for rigs and terminal equipment necessary for drilling in those water depths has slipped past the $300 million mark and there seems to be no end in sight. But storms and heavy seas can still shut it all down for days, even weeks, at a time.

Re-enter the concept of the seafloor rig.

A couple of years ago, with the aid of some 200,000 British pounds (US $325,000) in "kick-start" grants and assistance from the U.K. government, a group of six multinational oil companies and several offshore equipment manufacturers formed a joint industry project (JIP) aimed specifically at coming up with an economical way to drill offshore wells from the seabed. Such a rig would be relatively unaffected by topside weather, and there would be no need for rigid riser connections between the rig and the surface, either.

To date, the group has established that not only is such a rig feasible, but it's possible to have a prototype in operation by late 2005 or early 2006. Vitally important among the benefits afforded by such a rig would be significant drilling cost reductions and the ability to plumb the earth for oil and gas in water depths that are considered impossible, even with today's deepwater drilling equipment.

Maris International Ltd., a veteran marine engineering and project management company based near London, is running the subsea rig project under the JIP. The company was on hand at the recent Offshore Technology Conference in Houston, and featured a model of the JIP's Seabed Drilling Rig (SDR) concept at their exhibit booth. Also, Maris officials delivered a technical paper dealing with the recently positive feasibility study. Understandably, both drew lots of interest from conference attendees, particularly since the study claims that use of such a rig could gain well construction cost reductions of up to 30 percent at water depths of 3,500 ft (1,100 m) to 7,500 ft (2,300 m).

The preliminary system design includes the SDR, which is a modular, remotely controlled rig that uses standard automated drilling procedures and jointed casing and drilling tubulars, with control of both applied and downhole pressures and drilling fluid circulation at the seabed, rather than at the surface, as is the case with floating rigs connected to the well by rigid riser systems. Pipe handling is automatic, as is the top drive system for rotating the drillstring. The most obvious advantage of a rig stationed on the seabed would be its relative insensitivity to weather and, of course, its water depth.

The SDR is connected to a Rig Support Vessel (RSV), a ship-shape vessel stationed at the surface by flexible risers for power and remote control of the seabed rig. A shipboard hoist handles lowering and raising of the rig, which is only a third of the size used aboard conventional floaters. This hoist also conveys pipe, drilling fluid, and power modules to and from the surface to keep the drilling operation active. The hoist operates through a conventional moon pool in the ship's hull.

Since there is no rigid marine riser and drillstring between the vessel and the seabed, there would be no need for large tensioners and heavy heave compensation gear aboard the RSV. Therefore, the ship could be significantly smaller in size than, say, a deepwater drillship.

According to Maris' OTC paper, with no rigid risers in place, the RSV would not require heavy chain/wire anchoring systems to keep it above the seabed rig. Instead, it would use dynamic positioning--a thruster-driven station-keeping technology already well developed--to stay in place above the rig. What's more, the flexible risers would allow the RSV much wider latitude for excursion due to wind and wave action. This would allow it to operate in much more critical sea conditions than conventional floating rigs.

Jim Neffgen, JIP coordinator for Maris, said funding of the seabed rig JIP by U.K. government entities was obtained through a program designed to identify and evaluate step-change technologies. Members of the JIP also contributed both money and in-kind services and facilities use.

But while the initial JIP study determined that it is feasible to put a riserless rig on the sea floor and drill and complete wells in ultradeep water, a number of basic questions need final solutions before a rig would be built, said Neffgen.

"Some technology questions remain to be answered," he said, indicating that the rig probably could be used both for exploratory and development drilling, provided that ongoing "monobore," drilling, with expandable casing technology and smaller drillstring components, continues to be advanced in the industry. Additionally, the SDR's automated pipe-handling system would be based on existing systems already in widespread use aboard floating rigs. Also, JIP participating companies are working to further develop the rig's drilling fluid control system, combining it with a continuous circulation system that already is in joint development by Maris with U.S.-based Varco International Inc. This system would allow well fluid circulation even during "trips" in and out of the hole.

Additionally, he said, JIP participants are investigating enabling technologies to be used in a seabed mud conditioning system that would separate drill cuttings from the well return flow, filter out and store unwanted well fluids, and turn the reconditioned mud back into circulation in the well.

The JIP participants have kicked off a detailed design phase, which includes a more precise cost analysis and more specific description of new or improved technology needs. Maris International itself has filed several patents for components that could be used in conventional drilling applications, as well. Licensing fees would help enable the company in its future development activities. Additionally, the company is seeking participation in the project from other entities, including manufacturers, and perhaps would entertain new technology development grants from other governments, as well.

Neffgen noted in the end, the system probably would be offered by a third-party entity, given Maris International's dedication to design engineering and new technology development.

"We aren't ruling out the obvious, of course, which is the contract offshore drilling industry," he said. "But it's likely that the market will reveal the type of company needed to perform the service, and they will fund its final development."

He also noted that the JIP's more practical approach to development of the SDR/RSV system is helping to attract more interest in the industry.

"The whole point of moving ahead with this project is to include emerging technologies in the mix," said Neffgen. "Breakthroughs are expected in a number of equipment areas already incorporated into the preliminary design. We will include them in the detailed design as they mature. In that sense, the SDR and RSV are still a work in progress."

And then, there's the obvious demand for a seabed rig. "The industry has an increasing need to explore for oil and gas in deeper water, yet drilling costs continue to escalate with water depth," said Neffgen, "so step changes in drilling methods and equipment are needed now. The SDR/RSV system provides a practical and economic solution to reducing well construction costs, particularly for water depths beyond the reach of floating rigs attached to the bottom by expensive rigid connections. Extending this technology would allow producers to drill wells economically in 10,000 ft of water and beyond--in water depths that make up half of the world's surface but which currently are out of technical or economic reach."