Stewart & Stevenson develops riser for deepwater dual gradient drilling

Stewart & Stevenson Service Inc, Houston, has introduced the first riser system, shown above, designed for deepwater dual gradient drilling.

Sam Fletcher
OGJ Online

HOUSTON, May 4 -- As others prepare to field test subsea pumps for dual gradient drilling in ultradeep water, Stewart & Stevenson Service Inc.'s Petroleum Equipment Division in Houston has introduced the first riser system designed for that pioneer operation, said company officials.

The company displayed its Drilling Mud Return Marine Riser System (DMRS). at last week's Offshore Technology Conference in Houston.

That system is designed to handle either dual gradient or underbalanced drilling operations without sacrificing its utilization as a conventional drilling riser. "It can take us out to 10,000, 12,000 ft, whatever," said Louis D. Slaughter, subsea systems sales manager for the Houston-based company.

In conventional offshore drilling, the pore pressure of a well is controlled by the column of drilling fluids or mud extending from the bottom of the well up through the riser system to the rig above. "But as we get out in deeper water, the hydrostatic pressure of the mud in the riser is fracing the well," Slaughter said.

The basic concept of dual gradient drilling is to keep the weight of the thousands of feet of dense drilling fluid within the riser, extended from the floating rig to the sea floor, from exerting such pressure within the well bore.

Conventional methods for balancing wellbore pressure and borehole stability sometime require additional strings of casing in deepwater wells. But besides being costly, the additional casing may reduce the borehole size so that, even if it reaches target depth, it's too small to handle high production rates or for horizontal and multilateral completions.

An alternative is the dual gradient system, in which the riser is filled with lighter seawater while wellbore pressure is controlled by a column of drilling fluids extending from the bottom of the hole to the seafloor. But first the industry must develop a subsea pump that can be depended on circulate drilling fluids from the well through a flowline back to the rig.

Three separate industry groups have been working to develop a commercial subsea system capable of pumping that mud from a subsea well to a floating rig in ultradeep waters. In some cases, the system might also separate drill cuttings for separate deposit necessary for dual gradient drilling. At least one, the SubSea MudLift Drilling system developed as a joint industry project by a group of companies headed by Conoco Inc., is now being readied for testing.

However, a direct interface between the heavier mud in the well and the lighter seawater in the riser could allow a dangerous gas kick to bubble up to the rig above.

The DMRS addresses that problem by integrating a 7-in. fluid return line into the riser, along with choke and kill lines, booster line and hydraulic line. It also uses the new Stewart & Stevenson riser Isolation Tool (SSIT) to separate mechanically the seawater in the riser from the heavier mud in the well, providing more control of the dual gradient operation.

As a result, the hydrostatic head created at the mud line by the seawater in the riser is equal to the surrounding pressure on the ocean floor.

"If we can make the well think it's on land instead of in 7,000 ft of water, we can drill it with three less strings of casings. That's a major savings right there," said Slaughter.

By making the well perform as though the drilling rig were sitting on the ocean floor, the dual gradient system improves hole stability and permits deeper drilling without the extra mud pressure triggering fractioning. It also reduces the number of different casing strings required and lowers overall risks.

One immediate advantage is that it takes 63% less drilling fluid to fill a 7-in. flowline than a typical 19-in. ID riser. That reduces the upfront costs of drilling mud and the drilling fluid storage capacity required on the rig. It also cuts the time needed to change fluid weights or to circulate fluid in or out of the well.

Less weight eases the tensioner requirements for the riser and the amount of tensioning capacity required on the rig, while lowering wear and maintenance on the tensioner itself.

With less drilling fluid within the system, the environmental threat is greatly reduced in the event of accidental or emergency disconnections.

Moreover, the smaller fluid return line increases the velocity of the return flow to three times faster than a conventional riser. That makes it easier to carry cuttings out of the well and increases safety overall by reducing the usually sporadic nature of the return flow.

That would be an additional advantage even in conventional drilling and is attracting the interest of some drilling contractors, Slaughter said.

Combining the DMRS components in the riser reduces the number of separate strings of pipe that workers must manage. A multitude of pipe strings also can interfere with a drillship's ability to rotate around a single point in the moon pool as vessel weathervanes in the wind.

Stewart & Stevenson spent "a couple of years" developing its SSIT and DMRS equipment and has patents pending.

"We've completed all tests and are ready to start building as soon as our customers get a pump that they like. We've quoted it to Shell already," said Slaughter.

A unit of Shell Oil Co. is among the groups working to develop a subsea pump for a dual gradient system.

Contact Sam Fletcher at Samf@OGJonline.com

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