Well construction costs in deeper waters, it is said, could be drastically reduced, if rigs operated on the seabed instead of at the sea surface, and this has been confirmed in a recent DeepStar Benchmark Study; but operators hesitate to take the plunge.

Necessity, rather than economics, is the mother of invention. So, it now appears that the first application of the new seabed rig may be in shallower waters, under moving ice, where there is no real alternative.

Background

Seabed rigs, designed during the last 20 years, have mainly been "marinized" land rigs mounted on submersibles with jackup legs. The unresolved problems being:

• The sheer size, requiring enormous moon-pools in the support vessels;

• A difficult interface with soft seabeds in deeper waters;

• A difficult interface between seawater and all drilling fluids; and

• The need for a reliable method of handling all tubulars and tubular assemblies.

Overall, the operators concluded that these four problems were technical "show stoppers" and, also, there would be little cost saving compared with conventional floating drilling. Hence, the focus in the last ten years has been on chipping away at floating drilling costs by using surface blowout preventers (BOPs), dual-density drilling, coiled tubing developments and casing drilling; all of which have their merits. However, a far greater saving would still be secured by simply locating the rig on the seabed.

The current Maris seabed rig design is: compact enough to deploy through a normal moonpool, light enough to eliminate interfacing with the seabed and capable of continuously segregating seawater from drilling fluids (Figure 1). And, the innovative mechanical handling of tubulars facilitates reliable automation. So, if it is now technically feasible, what about the economics?

DeepStar benchmark analysis

Two benchmark cases were taken in 5,000 ft (1,524 m) of water and in 10,000 ft (3,280 m) of water, code-named Cottontail and Coyote, respectively, by DeepStar. The total well construction cost for the seabed rig was compared with that of conventional floating drilling rigs (Table 1). Benefits not included in these results are:

• The reduced sensitivity to weather, which is considerable in hostile locations outside the Gulf of Mexico;

• Reduction in typical drilling problems due to continuous circulation, (estimated to be greater than 50% in a quality and reliability assessment (QRA) study); and

• Increased safety to personnel, equipment and the environment.

Even without the weather, drilling, and safety factors the well construction cost savings over conventional methods are 22% in 5,000-ft and 34% in 10,000-ft water depths, respectively. Sooner or later, operators are going to want to economize. Meanwhile, the heavy investment by the industry in very expensive deepwater rigs will still have to be paid for or written off.

One of the key advantages of seabed drilling is the improved casing program. Comparing a typical Gulf of Mexico well, provided by DeepStar, drilled with a floating rig and with a seabed rig, the saving in time, materials and cost of completing with three casings, instead of six, is very significant. Alternatively, the reach of the lateral wells could be very much extended, with a larger diameter production tubing, or a smaller initial borehole (Figure 2). There is no other known way to achieve this - dual density drilling came close - but was only half way there. The seabed rig controls the well and the mud at the seabed in both directions and can "recycle" mud in deeper waters.

Under-ice drilling

Back in 1999, earlier Maris seabed rigs were designed for Werner Offshore to drill for gas in the Russanovkoye and Leningradskaye fields, under the moving ice of the Kara Sea. It was then considered that the ice was too thick for even the larger Russian icebreakers; and so, the seabed rig was to be controlled from a submarine. A third seabed installation was to be the innovative mud processing unit (MPU).

Since then and currently, attempts are still being made to develop an alternative, a "one atmosphere subsea drilling rig;" but co-locating drilling personnel and hydrocarbons in the same confined space is intrinsically unsafe in air; and a nitrogen environment is also high risk for humans. Separating drillers from the hydrocarbons is possible; but then one might as well make the machinery wet at ambient pressure; it is also best to simplify it and modularize it, for ease of construction and replacement; and that, essentially is the basis of the Maris seabed rig design.

With less ice now in the winter months, it is feasible to use an ice breaker, to provide a passage for a "drilling" vessel. This is particularly so for a seabed rig, since the ice breaker, followed by the support vessel, has considerable latitude to seek a path through the moving ice, to avoid the thicker ice hummocks and ridges. If the flexible risers become overly taut, the vessel can unreel more riser and, if the vessel has to move out of the "riser envelope," the risers can be disconnected at the seabed rig, as a last, but safe, resort. The vessel only needs to be "above" the rig about twice a day to change out tubular containers. Floating drilling, with rigid marine risers, in this situation would be a far more limited and risky operation.

Almost 'off the shelf'

Now, in 2005, most of the necessary components are available, almost off-the-shelf. The segregation of seawater from drilling fluids is achieved with a Continuous Circulating System (CCS), produced by National Oilwell Varco. It is now in operation on drilling rigs in Italy onshore, in Egypt offshore and soon in the Gulf of Mexico (this CCS design will require additional development to handle a wider range of tubular diameters). The interface with the seabed is eliminated by reducing the total rig weight, including tubulars, to less than 400 tons, and by ensuring that the foundation conductor can take the lateral load imposed by tidal currents and flexible risers. The interface with the vessel is limited to deploying all components at a cross section of 8.5 ft by 8 ft (2.6 m by 2.4 m), (being the standard dimensions of a 40-ft (12.2-m) isometric container). The handling of tubulars in one operation from storage to the borehole, and back, has simplified the automation and increased the reliability of repeatedly making and breaking tool joints.

Complementary technologies

Additionally, there are complementary technologies. Apart from the National Oilwell Varco CCS, there are Tesco's designs for handling a range of casings by internal gripping; casing drilling itself; subsea wireline units; Huisman-Itrec's designs for automatic rig operations, Mærsk's containerization technology for the worldwide transportation of all tubulars by sea, rail and road.

The technology is there, ready to be integrated into one of the most significant "game changes" in the drilling industry. "The Rig of the Future" will simplify and organize the handling of tubulars, from the supplier to its downhole destination, and from well to well across the globe, sooner or later and, preferably, as soon as possible.