Riser weight-size-cost hurdle emerging at 7,500-ft water depths

Riser weight and diameter increase substantially in very deep water, posing serious problems for drillers. [70,018 bytes] At a water depth of about 7,500 ft, drilling vessels become involved in a weight-size-cost spiral that imposes huge cost penalties. In order to support and contain a 1-2 mile long column of drilling fluid (at 9-14 ppg, versus 8 ppg for seawater), drilling risers must be sufficiently robust, and so must the drilling vessel at the surface.

Currents, hydrostatic fluid load, deep reservoirs
pose difficulties for current technology

Leonard Le Blanc
At a water depth of about 7,500 ft, drilling vessels become involved in a weight-size-cost spiral that imposes huge cost penalties. In order to support and contain a 1-2 mile long column of drilling fluid (at 9-14 ppg, versus 8 ppg for seawater), drilling risers must be sufficiently robust, and so must the drilling vessel at the surface.

That means a riser with sufficient wall thickness and stiffness, which makes the riser very heavy. To help support a riser weight of 1,500-2,500 tons, large buoyancy modules are needed, which increase the overall diameter of the riser package.

However, as the overall diameter increases, the forces imposed by water currents increase geometrically. Early on, the solutions to this problem were considered costly and burdensome, but not technically critical. Now, the diameter of the riser needed to deal with the huge weight in deepwater, combined with the stiffness needed to withstand water currents much greater than anticipated, is forcing the industry to seriously examine alternatives.

The problems

Drillers are finding current velocities as high as 2-3 knots in the deepwater Gulf of Mexico, and in one case in the Atlantic Margin off Western Europe, as high as four knots. Drillers have had a number of problems latching the drilling riser onto the blowout preventer stack, creating concern for operations in greater depths. The riser could be powered into position with a vehicle, but the forces on the wellhead-riser latch become crucially high, and as the entire riser bows out in response to high current velocities, unacceptable drillstring-riser ID contact can result.

But, the problem doesn't stop with riser weight. The hydrostatic weight of a long column of drilling fluid can easily overbalance sediment pore pressures, resulting in the loss of drilling fluid and plugging of pore spaces around the wellbore. Maintaining a slight over-balance is difficult and so is controlling the well. In fact, maintaining that control margin in deepwater is so complex, the International Association of Drilling Contractors has convened a task force to create guidelines for deepwater drilling. Without some method of relieving the weight of the return mud column, drillers are left with little flexibility in mud weight control.

Compounding the riser weight/casing limitation problem is the reality that geological objectives tend to be found further below the mudline as water depths increase. Since casing programs begin at the surface, rather than the seabed, a large number of casing sets or unusual arrangements are required just to reach 10,000 ft below the mudline. Using conventional riser-casing arrangements, drillers will simply run out of useful casing diameter before encountering deeper formations in the 7,500-ft water depth range. Geological objectives in the 15,000-20,000 ft range below mudline will be unreachable.

Four solutions

The apparent solutions are fourfold: (1) Lighten the riser, reducing the buoyancy requirements and overall diameter of the riser package. (2) Remove the riser altogether, and find another way to transport fluid and cuttings returns. (3) Lighten the drilling fluid weight. (4) A combination of the three. The following is a status summary of the four solutions:

  • Lighten the riser: Researchers are attacking the riser weight problem with composites, which have proven robust enough to replace the body of the riser joint. The composite body is joined to metal flange segments on each end with molded transition elements. Trials of new composite risers are scheduled in the near future.
  • Eliminate the riser: Without a riser surrounding the drillstring, drill fluid and cuttings can be returned to the surface via a separate line about the diameter of the drillstring. In water depths surpassing 9,000 ft, a powerful seabed-level fluid returns pump will be necessary. The method works in principle and design work is underway to develop a working system for trial purposes.
  • Reduce fluid weight: In effect, this is drilling a well in an under-balanced condition. A gas such as nitrogen is injected into the drilling fluid reducing the hydrostatic load of the fluid column. The well will flow in this condition, and drillers can use mud pumping speed and control of fluid returns to control wellbore influx, balance pressure on the formation, and circulate out formation flows. A number of operators are testing under-balanced drilling.
  • Combinations: Composite risers, lightweight but strong riser flanges, and lighter drilling fluids will provide immediate answers to drilling in 6,000-9,000 ft depths. Certainly beyond that depth, and probably in lesser depths, deep currents will begin imposing a penalty in terms of riser size and stiffness, along with the problem of how to latch the riser to the BOP stack. Unless there is some unseen solution, drillers will have to revisit the riser size problem as drilling or water depths increase.
In water depths under 6,000 ft, there is little technical need to alter conventional riser systems. However, once lightweight risers and riser flanges, or under-balanced drilling, and especially riserless drilling, are fully developed for depths beyond 6,000 ft, the systems and processes can be readily adapted to reduce riser weight or vessel size requirement for lesser depths.

The limitation to drilling in ultra-deepwater is so serious that some countries such as the US are taking an extra step. They will accept requests for lease and license extensions for tracts in ultra-deepwater for producers that are actively investigating or researching breakthrough technologies.

However, the most attractive inducement to lightening risers, lightening fluid contents, or doing away with risers entirely comes at the surface. The lower deckload for the drilling vessel at the surface will allow smaller rigs to work in deeper waters. Considering the cost of leasing or building deepwater rigs today, this may be the ultimate payback for developers of these new technologies.

Editor's Note: More details on the prospective solutions to the riser weight-size-cost spiral appear as a four-part series in this and the following issues of Offshore Magazine.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.

More in Deepwater