WELL CONTROL

How well control equipment is advancing to meet deepwater needs

Part 1 – A wrap-up of recent developments emanating from brisk deepwater drilling activity gives a quick look at where we are and what's needed

Shawn P. Vigeant, Diamond Offshore Drilling, Inc., Houston

he recent surge in deepwater drilling activity throughout the world, and in particular the U.S. Gulf of Mexico, has prompted oil companies, drilling contractors and equipment manufacturers to develop and implement a wide variety of advanced systems for overcoming the adversities associated with these extreme water depths. Well control equipment and related systems have undergone great transformation and have made some of the most significant technological gains.

This article addresses some recent developments in well control equipment and systems. Space constraints require that some be given a relatively cursory discussion, however, others are discussed in more detail.

Specific examples of advancements summarized within this article include:

• Improved riser tensioning and drillstring compensation systems
• Riser torque tools
• New larger bore diverters
• Improved riser tension rings
• Riser recoil systems and fill-up valves
• Longer riser joints with increased connection ratings.

Beginning with equipment located on the rig and working downward through the water column to the ocean floor, several examples of the recent developments in well control equipment and subsea systems will be described.

Crown Mounted Compensators

With the recent increase in desired compensation weights of up to 1,000 kips, traditional drillstring compensators (DSCs), which are offered with capacities up to 800 kips, are at times under-rated. The crown mounted compensator (CMC) was developed to allow for drillstring compensation to be crown mounted in 600-, 800- and 1,000-kip compensation ratings, Fig. 1 and Fig. 2.

CMCs are offered by several vendors in various configurations. One example is as follows: Two vertically-mounted compression-type cylinders are attached to a rigid frame mounted to the derrick water table. Direct acting cylinders support the crown block above the water table, utilizing all of the derrick height. The crown block is guided by one major guide column, eliminating guide track alignment problems, and a minor auxiliary track balances the system.

The fast line and deadline pass over large diameter sheaves, then are reeved through the traveling block to the deadline anchor. The large diameter sheaves, greater in diameter than the traveling block and crown block sheaves, increase wire rope life by a factor of about two.

The compensator is capable of locking at any point along the compensation stroke. Retracting the cylinders, the crown block comes to rest on the cylinder support beams, eliminating the need for a rotating or extending mechanical lock system. In this mode, with the cylinders not compensating, the fast line and deadline functions remain operational.

Another addition to the system is the speed control valve, which limits the extension speed of the cylinder if the drillstring breaks while the CMC is pressurized. If cylinder extension speed exceeds the maximum operation speed by 15%, the valve closes down to limit extension speed, causing hydraulic back pressure in the rod end.

Most CMCs offer features and benefits such as a single unit of rugged and modular construction; rigidly mounted to derrick water table; compression type cylinders that offer a lower operating pressure; hydraulic lock feature that reduces time during tripping; derrick height kept to a minimum; and adaptability to many derricks.

The stroke of most CMCs is around 25 ft, and with the cylinders retracted / locked, the rating of the 1,000-kip CMC becomes 2,000 kips (1,500 kips for the 600 kip CMC). One additional advantage of the CMC is the fact that DSC weight can be removed from the traveling gear weights and added to the allowable hook load since the CMC is part of the derrick and not part of the traveling gear. This weight is on the order of 65 kips for a 600-kip DSC, and 85 kips for an 800-kip DSC.

A disadvantage of the CMC is clearly the inability to lower the compensator weight in transit or survival conditions. CMC weight, which is about 130 kips for a 600-kip CMC and up to 180 kips for a 1,000-kip CMC, must always remain at the top of the derrick, which increases the overall KG, therefore reducing vessel stability.

Drillstring Compensators

The technology behind the drillstring compensators (DSC) has not changed dramatically over the past few years. However, manufacturers have begun to offer higher ratings for DSCs. The rating range varies from 400 kips to the new 800-kip DSC. To date 1,000-kip DSCs have not been offered, Fig. 3 . Manufacturers are offering upgrades from previous 400-kip compensators to 600-kip compensators with a lower cost and downtime, which is perfect for rig enhancements.

Diverters

Due mainly to the increased water depth, deeper water-depth-rated buoyancy modules are being installed on riser joints. To maintain their optimum buoyant effects, these modules are also growing in OD. As a result, the through bore diameter of the diverter housings on today's deepwater rigs (diverter element removed) need to be larger than the standard 49 in. This requirement is compounded on drillships where motions while running riser are higher than those on traditional semisubmersibles. The through diameters must be increased to prevent inadvertent damage to the riser or buoyancy during operations. Diverter housing diameters on some of the recent upgrades have been as large as 59 in., and are still growing, Fig. 4 .

Some recent developments in these new diverters include a housing that accommodates the large diameter of riser buoyancy modules even in increased sea states; complete open hole shut-off with a 20-in. through bore; ratings of 1,000 psi for closure on 5-in. drill pipe; 500 psi for closure on open hole; high capacity systems available for suspending the riser string from the diverter housing during emergency hang-off and stowage situations; and a 10- to 15-sec closure time on open hole.

Riser Tensioner Rings

Some features of the new tensioner support rings ( Fig. 5 ) include: Riser tensioner lines remain attached to the ring at all times. The ring hydraulically disconnects from the telescopic joint and locks to the mating profile on the bottom of the diverter support housing for convenient out-of-the-way storage. Ring ID is full opening to the diverter housing and may be used with a telescopic "slip" joint fluid bearing, allowing the vessel and tensioners to rotate relative to the riser string.

Riser Tensioners

Loop current conditions, combined with high mud weights and deep water have driven tensioner ratings higher. One manufacturer is now offering a true 250-kip riser tensioner. By true, it is meant that the vertical tension applied to the top of the riser from each tensioner is 250 kips, and losses have been accounted for in the design.

Recent projects have been developed using the design rating of 3.5 million lb for the tensioning capacity and coupling rating for the riser. This has been attributed to extreme water depths and increased choke and kill line sizes that are mentioned in the riser section of this article.

One problem with the higher required top tensions is the space required for installation. To maintain the 3.5-million-lb rating, a rig would need 14, 250-kip tensioners. Space and weight restraints make this nearly impossible for an upgrade or conversion of a smaller existing vessel, and limit the recipients of such a configuration to new build type projects, Fig. 6 .

In addition to the size and weight of the tensioners, the increased diameter and number of turn-down sheaves and the availability of space on the riser tensioning ring at the slip joint for shackles and wire rope jewelry must be considered. Although the weight and footprint space issue is significant, it is the wire rope size that begins to become the limiting factor. The 250-kip tensioner will be outfitted with 2-5/8-in. wire rope, with weights approaching 20 lb per ft. If wire rope sizes get much larger, they will be virtually unmanageable on board the rig.

One solution for this constraint is the automated spooling and motorized wire feed capabilities that make the slip-and-cut procedures much easier. Another solution is pre-cut wire rope lengths. Rather than slipping and cutting after the designated ton-mileage rating is reached, the entire length is swapped out.

As an alternative to buying new, larger tensioners, many contractors have opted to upgrade older 80-kip tensioners to 120 kips. This has proved to be a viable alternative to tension requirement increases on conversions and smaller upgrade projects.

The newest alternative to the traditional wire rope riser tensioner (which is mounted on the side of the drill floor and reeved beneath the floor to turn-down sheaves) is the in-line or direct tensioner. These systems involve mounting tensioning cylinders beneath the drill floor that attach directly to the tensioning ring. This alternative eliminates the numerous sheaves and wire rope problem associated with traditional tensioning systems, and reduces weight on the vessel, which, in turn, improves stability while allowing for much higher top tension limits.

On a semisubmersible, space between the water line and drill floor at drilling draft (air gap) allows these in line tensioners to operate in a virtually dry environment. However, on most drillships, a large portion of these cylinders would be fully emerged in salt water. This certainly needs to be considered before implementing them into a rig package.

Another consideration is the overall stroke of the cylinders. Due to the 4-times reeving factor provided by traditional tensioners, they are able to provide a full 50 ft or more of stroke with a 12-1/2-ft stroke cylinder. The in-line cylinders do not have this advantage and have to be much longer and heavier to accommodate the same functional stroke. With slip joint stroke lengths continually increasing, this too needs to be given careful consideration before equipment selection.

Riser Recoil Systems

With the development of several "new" or converted dynamically positioned vessels for deepwater operations, continued development and improvement of the riser recoil system is imperative. Earlier anti-recoil systems were described by Young, et al., in 1992.¹ Since then, additional work was described by Puccio in 1997.²

The essence of the riser recoil system, which has not changed, is as follows. During an emergency disconnection situation, after unlatching the lower marine riser package (LMRP) / riser connector, the LMRP is lifted off a height adequate to prevent any possible damage to the BOP, while simultaneously stopping the slip joint's outer barrel before it impacts the diverter and substructure. The most recent advances have involved the development of computer models and rig-based software that allow the contractor to optimize top tensions and air pressure vessels (APVs) left on-line prior to a disconnect as weather, currents and riser space out change.

The most recent recoil system development involves the use of an orifice valve that is in-line between the tensioner cylinder and high pressure air bottle. During normal operation, fluids in the tensioner pass through a non-restricted opening, Fig. 7. When the recoil system detects a disconnection signal given (by means of actively monitoring the multiplex system), the orifice valves are engaged. This forces the tensioner fluids to pass through a restricted opening, thus slowing the recoil process to a manageable level.

Equally important, is the isolation of the high-pressure air vessels. If the high-volume, high-pressure bank of air is not isolated, the possibility for the riser to contact the rig increases significantly. In some isolated instances such as high current, low mud weight and high top tension, a relatively small and tolerable impact may still occur. These events may soon be eliminated through the use of variable diameter orifices and / or secondary orifice valves that are being developed.

Marine Drilling Riser

Over the past year, riser coupling ratings (i.e., flange coupling strength ratings) have increased markedly. In the past, the coupling rating utilized was 2 million lb. Several vendors currently offer 3.5-million-lb rated connections, Fig. 8 . This has been driven partially by water depth increases, higher current forces and associated larger top tensions.

The addition of larger diameter choke and kill lines also has driven the connection rating higher, when full pressure rating forces (15k psi) on both lines are considered at the connection during design calculations. As discussed by Actis, et al., line sizes must be increased from the traditional 3 or 3-1/2-in.-ID to a more friction tolerant 4-1/2-in.-ID line.³ This is of course also driven by the gradient between formation and fracture pressures. Where larger differences in pressure occur, traditional line sizes may be used.

In addition to the coupling rating increase, overall riser joint lengths have increased. Previous riser orders usually consisted of 50-ft joints, but now 75 and even 100-ft lengths are the norm. This decreases riser running and retrieving time, greatly reducing operational costs. Simultaneous improvements have been made to the riser running (torque) tools, Fig. 9. Customized tools now offer optimum make-up and break-out speeds and torques.

While most industry focus remains on the optimization of steel risers, one manufacturer is actively pursuing the development of composite riser. While entire strings of composite material risers may be well into the future, lighter, stronger composite auxiliary lines are just around the corner.

Another addition to riser string technology is the use of fairings. Several different fairing designs are being implemented and tested. Most are turret-type fairings that rotate according to current strength and direction. These fairings are intended to reduce the harmful effects of vortex induced vibration (VIV). In addition to protecting against VIV, one operator is testing an improved instrumentation and measurement scheme that will utilize fiberoptics to measure real time stresses and VIV in the string.

Acknowledgments

I thank the following companies for support in writing this article, in addition to providing many of the figures: ABB Vetco Gray, Cameron, Francis Torque Tools, Furon Corp., Hydril, and Varco Shaffer.

This article was adapted from IADC / SPE paper 39298, "Deepwater driven advancements in well control equipment and systems," presented at the IADC / SPE Drilling Conference, Dallas, Texas, March 1998.

Literature Cited

¹ Young, R. D., Hock, C. J., Karlsen, G., and Albert, J. W., "Analysis and Design of Anti-recoil System for Emergency Disconnect of a Deepwater Riser," OTC Paper 6892, Offshore Technology Conference, Houston, Texas, 1992.

² Puccio, W. F., and Nuttall, R. V., "Advances in Control of the Riser Recoil Phenomenon in Deep Water," IADC Deep Water Well Control Conference, Houston, Texas, 1997.

The author

Shawn P. Vigeant is senior mechanical engineer, Subsea Systems, for Diamond Offshore Drilling, Inc. He is in charge of all subsea related engineering for the company fleet and ongoing rig enhancement program. He previously worked as an onsite project engineer and as an engineer in the Technical Services Dept. Before joining Diamond Offshore, Mr. Vigeant was a marine project geophysicist for Digicon Geophysical. He holds a BS degree in ocean engineering from Florida Institute of Technology, and is a member of SNAME, MTS, IADC, SPE and AADE.

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Copyright © 1999 Gulf Publishing Company