DEEPWATER TECHNOLOGY

Upcoming technology developments

n his presentation, which he called "Delivering tomorrow," Paul Goodfellow covered: deepwater GOM technology drivers; use of subsea pumps to create dual-density drilling; pre-installed moorings; and applications of surface BOPs on floating rigs. He then summarized the status of the technology he discussed, along with potential cost savings. He also discussed the evolution of slender wells using expandable tubulars – this subject is covered in more detail in a separate presentation, as are surface BOPs.

Deepwater GOM technology drivers. Several basic problems common to deep water include:

• Wells in geopressured environments require many casing strings.
• The number of casings, hole size and completion needs drive BOP and wellhead size.
• Riser, mooring and deck loads drive hull size, thus drill vessel size.
• Well costs are driven primarily by: 1) rig dayrates (for 5th generation rigs); and 2) time on the well (for 10 – 12 strings of pipe).

The opportunity is – reduce drill vessel costs and improve drilling efficiency.

And well difficulty is increasing. Deeper water and deeper well depths are requiring large hole diameters – up to 10-5/8-in. – at TD for optimum production. These wells present higher temperatures / pressures. There is little or no difference between drill fluid density needed and formation fracture gradients. And field developments in "frontier" areas are more challenging / expensive, after successful exploration efforts.

A summary of deepwater technology development is illustrated in Fig. 1. In the overall well / rig illustration, large-diameter production casing is needed for high-rate completions in 20,000 – 30,000-ft wells. Casing expansion is a method to maintain larger hole diameters. Dual gradient drilling using subsea pumps enables drilling with fewer casings.

Fig. 1. Overview of SEPCo’s deepwater well technology development program and objectives.

Smaller BOPs are required, due to fewer casing strings and hole diameter maintenance. The riser is lighter / smaller and serves only as a "guide" for tubulars, as drill fluid is returned through the attached conduit. Mooring systems, suction anchors and synthetic lines provide mooring in deeper water.

The technology pieces introduced above result in reduced riser loads / size, less tubular handling, lower drill-fluid volumes, improved mooring and a surface BOP. All of this enables a "compact" rig, which reduces drill vessel costs and improves drilling efficiency.

Subsea pump, dual-gradient drilling. Fig. 2 shows schematically how a subsea pump takes drilling mud returning up the drill pipe-casing annulus and pumps it up a separate conduit to the rig floor for processing. The riser above the pump is filled with seawater, so only weight of the depth of seawater is added to mud weight in the hole, not the 7,000 ft or 10,000 ft of drill mud in the riser’s annulus.

Fig. 2. Schematic of subsea pump returning drilling fluid up separate conduit to the rig from the mudline to create dual-gradient drilling scenario.

This effectively produces a mud gradient in the hole relative to the mudline, vs. a gradient relative to surface. The bottom line is that it widens the distance between mud weight and fracture pressure and allows greater distance between casing strings.

SEPCo’s subsea pumping system is currently being constructed and will be deployed on the Transocean Sedco Forex Deepwater Nautilus semi in 3rd or 4th quarter 2002. The pump features: minimal moving parts, limited subsea control, a patented passive annular control and patented subsea mud separation.

Besides producing fewer casing strings, in the process the subsea pump helps manage shallow flow problems that plague deepwater drillers. With subsea pumping, a compact semi can be used. The initial part of the well can be drilled riserless with mud / cuttings pumped to surface. Deeper portions can be drilled conventionally. Potential cost savings of up to 40% are possible.

Pre-installed mooring. As introduced above, this is a method of making smaller vessels practical. Moorings for deepwater floaters using steel wire and suction anchors have been successfully installed in 7,790-ft water. These systems require multiple boats and submersible buoys. Steel-line weight limits water depth.

The first system with synthetic rope has been deployed in 9,100-ft water, using a single boat; and the Deepwater Nautilus semi was hooking up in late 2001. Advantages of the system are that: smaller rigs can be utilized; it extends the rig’s capabilities; and moored rigs have deepwater capability formerly achievable only with dynamic positioning, with less environmental emissions and fuel usage.

Other applications include suction anchor placement for mooring system interference and seafloor congestion around installations such as subsea trees, umbilicals and flowlines.

	Shell’s deepwater program: A broad-based technology commitment
	Don Jacobsen, Manager, Drilling & Completions, Shell Exploration & Production Co. (SEPCo), New Orleans;
	Paul Goodfellow, Drilling & Completions Operations Manager, Shell International Exploration E&P Inc., New Orleans;
	Ken Dupal, Sr. Staff Drilling Engineer, SEPCo, New Orleans; and
	Graham Brander, Sr. Drilling Engineer, Shell International E&P Inc., Houston. Overviews of their presentations follow.