The ability to perform concurrent operations appeared in 1999 to be the wave of the future for advanced drillship operations. After a hiatus of 11 years, which ended in August 1998, drilling contractors began delivering a series of 16 drillships that promised to push water-depth barriers to 10,000 ft and beyond (Table 1).
Many of these vessels were to be able to:
- Build drillstrings offline while drilling ahead.
- Perform extensive well testing operations on a discovery well while drilling a subsequent delineation well.
- Simultaneously drill out surface casing with one rotary table while running a riser string to bottom on the other.
The Glomar Explorer, Deepwater Pathfinder, and Discoverer Enterprise provide a view of the various technologies incorporated on these vessels.
Glomar Explorer
The Glomar Explorer, operated by Global Marine Inc. on a 30-year lease from the US Navy, was originally built to retrieve a sunken Soviet Golf-class submarine in the Pacific Ocean.1
After completing its mission in 1974, the vessel remained mostly inactive for more than a decade, with sporadic work involving seafloor-based manganese nodule mining. By 1996, however, Global Marine realized that the vessel, with its 2-in. thick, high-grade steel hull, would provide a solid foundation for a next-generation drill rig at a price far below that of a new build.
After 2 years of design and reconstruction work, which involved the addition of an advanced drilling package, a reduction in the size of the moon pool area, and an upgrade to a DPS-2 station-keeping system (see accompanying article), the Glomar Explorer entered the offshore rig fleet at a cost of $225 million.
Pipe-handling
The successful integration of several drilling technologies, involving cooperative synergies among several competitive service companies, allowed the Glomar Explorer to set a drilling water-depth record of 7,718 ft on its first try. These synergies revolved around the following components (Fig. 1):
- Horizontal pipe-racking system.
- Vertical pipe-racking system.
- Top drive.
- Iron roughneck.
- Horizontal pipe stabber.
- Raised backup system.
- Drawworks.
Since 1956, Global Marine`s pipe-handling processes have centered on horizontal pipe-handling techniques in place of traditional stand-it-back in the derrick, vertical pipe-racking technologies.
Byron Hablieb, engineering project manager for Global Marine, said there are two advantages to horizontal rackers. First, the permanent storage of drillpipe and other tubulars in a horizontal position maintains a more stable center of gravity. This is especially important during rough weather when wave, wind, and current motion produce a hazardous condition when the derrick is full of pipe.
Furthermore, if drill pipe is racked back in the derrick, an upcoming storm will force the crews to lay the stands down, a very time-consuming procedure compounded if the riser string must also be pulled.
Second, Hablieb said that 20-30% of the trip time can be saved with a horizontal system.
"When tripping in the hole (vertically), you have to run the elevators up empty, latch onto a stand, make up the connection, then come all the way back down," he pointed out. With a horizontal system, the elevators always remain within a short distance of the next tool joint, whether drilling, making a connection, or tripping.
On the Glomar Explorer, the horizontal pipe racker (Fig. 2), manufactured by Westech HMD of Control Flow Inc. (formerly Western Gear), is controlled through a fully digital electronic control system using programmable logic control (PLC) technology. This control package allows the unit to be operated by a single crew member, usually a competent floorhand.
The horizontal pipe racker has two control stations, one in the driller`s cabin and the other at the far end of the racker. The unit weighs only 350,000 lb yet can store over 890,000 lb of various-sized tubulars or 35,000 ft of standard 51/2-in drill pipe.
Racker operation
Prior to tripping in the hole, the horizontal racker is configured so that the stands roll to the outboard side of the racker, resting up against the right or left-hand side of the vertical conveyor. The outermost stand is then lifted in a horizontal position to the top part of the racker, where it is queued with five to six other stands.
A single stand is then indexed by a ramp arm so that only one stand rolls to the center of the skate track at a time. At this point, the stand lies within the trough, with the pin end cradled by the rolling skate and the box end facing the rig floor.
When the driller is ready for another stand, the operator engages the skate drive and slides the entire stand toward the rotary table, box end first. The box stops just short of the rig floor boundary (Fig. 3), where a lift arm elevates the box end.
At this point, the pipe is bowed, box end high, while the pin end remains horizontal in the skate. The stand is then moved toward the rotary, so that the box end becomes positioned above and slightly past the well-bore center, where it ready to be picked up by the elevators.
The driller lowers the blocks so that the elevators can be latched around the box end. The driller then raises the elevators, and the box end is pulled up into the derrick. Throughout this operation, the skate moves along with the pin end toward the rig floor, in a tailing fashion.
At the rig-floor boundary, the pin end quits moving forward and lifts upward out of the skate. The lower end of the stand is stopped in its forward swing by a bump rail. The pin is now positioned above the rig floor.
Next, the operator initiates the pipe stabber horizontally across the floor, positioning the pin directly over the well-bore center, above the waiting box end. The driller then lowers the blocks and makes up the stand. This sequence is repeated about every 60 sec.
The tripping out sequence is the opposite of this sequence, except that the storage arms are pitched so that the stands roll to the center of the racker.
Evolving philosophy
Global Marine`s pipe-handling philosophy began to change from "purely horizontal pipe-racking systems" to both stand-alone, vertical, and horizontal pipe-handling systems in 1996. In 1998, with the upgrade of the Glomar Explorer, followed by deliveries of the CR Russell Luigs and Jack Ryan deepwater drillships, the company`s concurrent pipe-handling capabilities began a new era of technological innovation that altered the way rig personnel perform their duties.2
"Dual activity, or the ability to handle casing and bottom-hole assemblies (BHAs) offline (while drilling), is fast becoming a way of life," said Randy Wolford, drilling superintendent for the Glomar Explorer. And with day rates often more than $180,000/day for an ultra-deepwater vessel, cost savings can be substantial.
"For instance, when you come out of the hole and have to change out the BHA, you can lose anywhere from 3 to 7 hr," Wolford said. "However, with a dual-activity system, using both horizontal and vertical pipe-handling technologies, the BHA can already be made up and stood back far in advance of getting out of the hole."
Nevertheless, Woford said the key driver behind the automation of drill rigs is safety. In the Gulf of Mexico, for example, "The need for automation is driven by operators such as Texaco and Chevron, not regulations," he explained. "One of the first things an operator looks at is the safety record of that unit. The industry costs per lost time accident is so great-in addition to the incalculable costs of suffering-that you cannot afford to make a mistake."
Thus, the capital investment for advanced drilling equipment can be justified in the safety savings alone. "The thing it gives more than anything else is a safer means of handling drill pipe instead of using a tugger, spinning chain, and manual tongs to make connections. You are simply putting people in harm`s way. But with automated systems, you are removing those people away from the possibility of injury."
Pathfinder
Conoco Inc.`s and R&B Falcon Corp.`s Deepwater Pathfinder spudded its first well Jan. 30, 1999. This is the second next-generation, ultra-deepwater drillship to enter the fleet since the Glomar Explorer. It was initially outfitted to drill in 7,500 ft of water and readily modified for 10,000 ft with a design based on proven double-hulled tanker construction.
The $260 million, DPS-3 vessel began operations in the Magnolia prospect of Garden Banks Block 783, drilling in 4,668 ft of water.
"We felt it prudent to test the rig first before placing it in extremely deep water," said Bill Brister, Conoco`s Gulf Coast regional manager.
While under construction, however, industry analysts felt it might have been a bad investment at the time, especially with oil prices hovering near $10/bbl in late 1998. However, oil prices returned to $20/bbl by July 1999, providing a powerful reminder that long-term investments should be approached with a long-term vision.
On its two first wells in 1999, the drillship made successful discoveries, the first at Conoco`s Magnolia prospect and the second at its K2 prospect on Green Canyon Block 562. "It`s important to keep in mind that even if we discovered a big play (during the 1997-98 downturn), it would take 5 years to see any return on our investment," Brister said.
Advanced drilling package
The 727-ft long, 138-ft wide, and 66-ft deep Pathfinder (Fig. 4), owned and operated by R&B Falcon Inc., contains advanced drilling technologies utilizing fully automated pipe-handling equipment and an active-heave compensating drawworks.
However, in place of the horizontal pipe-handling system, as used by the Glomar Explorer, the Pathfinder uses the vertical pipe-handling concept in which crews build drill stands offline, also without any interference to the drilling process. The sequence:
1. Operator picks up single no. 1 off the conveyor with the pickup-laydown system (PLS-3, Varco Systems).
2. Operator places single no. 1 into the auxiliary mousehole (AMH).
3. Operator picks up single no. 2 at the conveyor with the PLS-3.
4. Operator places and stabs single no. 2 in the AMH then makes up a double using the integrated roughneck.
5. Operator raises and retracts double from the AMH with the pipe-handling machine (PHM-3i, Varco).
6. Operator picks up single no. 3 at the conveyor with the PLS-3 and places it into the AMH.
7. Operator extends the double over the AMH and stabs it into the single.
8. Operator makes up triple with integrated roughneck.
9. Operator raises the triple out of the mousehole and moves it to the setback area using the PHM-3i.
Extended well testing, FPSO
Not only does the drillship incorporate an advanced drilling package, it can also perform simultaneous drilling and testing activities with 100,000 bbl of onboard oil storage capacity.
The extended well tests, in which wellbore fluid is treated and stored onboard, use the inherent health, safety, and environmental aspects of a double-hulled tanker design along with a DPS-3 configuration and zero-discharge provisions.
The vessel could also carry out the next stage of a staged field development by drilling the subsequent delineation well while simultaneously testing the discovery well. The vessel is also designed to allow eventual conversion to a floating production, storage, and offloading (FPSO) vessel.
This involves installation of a full process facility and use of 400,000 bbl of storage capacity. Thus, the unique capability of the vessel to carry out exploration drilling and extended well testing operations, followed by development drilling and conversion to an FPSO, might allow for a drastic reduction in the time and cost to bring a development project onstream as compared to conventional approaches.
Active-heave drawworks
Another innovative technology placed on the drillship includes an active-heave compensating drawworks. The hoisting system supplants two prior technologies:
1. A traditional active-heave compensation system that lifts and lowers the block assembly in response to measured heave.
2. A passive-heave compensation system that employs a dampening effect created by means of compressed air.
In its place, the active-heave drawworks, developed by Hitec Drilling & Marine Systems Ltd., controls the position of the traveling block by varying the motor speed of the drawworks.
The drawworks counteracts heave by winding wire off and on to the drum relative to the observed motion of the vessel. Motion-reference units are mounted on the derrick, dead-line, and hull next to the moon pool.
"At first, R&B Falcon, Hitec, and Conoco were unsure of its ability to handle the tremendous riser and drillstring loads required for deepwater operations. There was a lot riding on it considering this was an untested technology (that was impossible to simulate in the laboratory)," said Michael McKee, staff well operations foreman for Conoco.
Discoverer Enterprise
Operating under a 5-year contract with BP-Amoco PLC, Transocean Offshore Inc.`s Discoverer Enterprise differs from the Glomar Explorer and the Deepwater Pathfinder.
Whereas the latter two drillships can perform offline stand-building activities while drilling ahead, the Discoverer Enterprise can perform two separate operations, such as spudding-in a well while running riser to bottom or even cluster-drilling dual locations.
A fresh idea
Michael Wilburn, engineering manager for the Enterprise class of new builds, said the concept behind the Enterprise class of drilling vessels is to reduce overall deepwater well costs.
"The idea was to build two completely independent rotaries, traveling blocks, and everything else necessary for drilling operations, all under the same derrick (Figs. 5 and 6)."
Thus, it will be possible to perform many operations concurrently, instead of sequentially, providing advantages for both exploratory and development operations.
"We`ve taken a look at how much time it actually takes for an exploratory well, and based on some preliminary studies we save about a quarter of the time using the dual-activity rig as compared to a conventional drillship," said Forrest Estep, manager, drilling engineering, for Transocean.
Furthermore, additional time savings may be attained in batch-type operations. "When you move it to the development scenario, when you`re moving from well to well, we think we can save 40% or even half the time as compared to a conventional operation," Estep said.
"You could be working on the second well while you`re still latched up and drilling, testing, and completing the first well." Thus, operations can be set up so that the blowout preventer (BOP) and riser string are not constantly tripped back and forth to the surface between each well, cutting down on several days of rig time.
The critical path
On the Discoverer Enterprise, the forward and aft rotary tables support each other in an orchestrated manner that maximizes progress along the critical path (Fig. 7). For example, in an exploration scenario, the forward rotary can be used to prepare the BOP stack and riser as the aft rotary prepares the bottomhole assembly and to spud in the well.
Then, once the 36-in. hole section is finished and the 20-in. casing string is cemented, the critical path advances to the forward rotary as the riser and BOP stack, already made up below the water line, are landed at the mud line. This action can be aided by dynamically repositioning the drillship.
"Some ships that have the ability to make up tubulars offline can only do so above the water line. Once they get the tubulars racked back in the derrick, they still have to go through a single rotary," Wilburn said. Thus, on those vessels, it is impossible to drill with one rotary and simultaneously run riser with another.
From this point, the aft rotary table converts to a full service rig for the forward rotary table, reducing time for sequential operations. "We`ll be doing things like making up tubulars, getting ready to run the next casing string, picking up drill pipe for the next hole section, and so on," Estep said.
For example, as the 171/2-in. hole is drilled with the forward rotary (now on the critical path), the aft rotary side will pick up the 133/8 and 95/8-in. casing joints, make them up into 135-ft long stands (quadruples), and rack them back in the derrick. This offline activity serves to reduce the setup time needed to run casing once TD is reached, continuing onward as the hole progresses to smaller casing strings.
Other advantages
The upside for a dual-activity drillship has yet to be exploited. For example, if government regulations outlawed gas flaring, an injection well drilled with the aft rotary could allow for well testing and cleanup. Other applications that can be incorporated into the well-construction process, possibly outside of the critical path, include:
- Running subsea trees to the seafloor while working through the BOP.
- Running a flowline anchor pile.
- Running a pipeline base.
- Running flowline jumpers.
- Washing hydrates off a BOP stack or tree.
- Clearing a cuttings mound away from the well.
- Running flowlines and pipelines.
- Drilling a water injection well for long-term test.
- Drilling a cuttings injection well.
- Having two risers and BOPs deployed and drilling on a template.
- Using a vertical separator to collect liquids from gas well.
Changing work roles
The integration of drilling technologies, such as those implemented on these next-generation drillships, initiated sweeping changes in relation to work-flow processes, affecting every crew member from the roustabout to the drilling manager.
"The toolpusher`s role is now outside, managing the crews, directing the work, seeing that things get done; whereas in the past, this was all done by the driller," Wolford said.
Instead, the driller "is now a captive of the drilling console while the drilling superintendent serves as the liaison between the contractor and operator-between the shore and the rig." Furthermore, there is no longer a need for the derrickman to work the monkeyboard while tripping, since this activity has been fully automated. Thus, the derrickman can concentrate solely on mud-pump and drilling-fluid maintenance.
Consequently, evolving technologies produced a growing need for high-tech personnel.
"Fifteen years ago, your average roughneck could fix anything on the rig floor. However, today, there is very little he can repair," Wolford said, referring to the incorporation in PLC, electronic, and computer technologies on everything from the drilling console to the mud-mixing equipment.
In the 21st Century, Hablieb said there is a "need for people with 1-2 years of college or technical training. We need to pick up personnel with more computer background and experience in sophisticated hydraulic systems."
References
1.Burleson, C.W., The Jennifer Project, Texas A&M University Press, 1997.
2.Dreith, M.W., Garvin, M.D., and Thorson, J.A., "Glomar Hull 456 Class Ultra-Deepwater Drillship," SPE/IADC paper 52858, presented at the SPE/IADC Drilling Conference, Amsterdam, Mar. 9-11, 1999.
3.Munden, A., "Classification of control and power systems for dynamic positioning," presented at the Marine Technology Society`s Dynamic Positioning Conference, Oct. 21-22, 1997.
4.Rokeberg, H., "Presentation of DP Class 2 and Class 3," presented at the Marine Technology Society`s Dynamic Positioning Conference, Oct. 21-22, 1997.
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In contrast to conventional vertical tripping operations in which the derrickman handles drill stands on the monkeyboard, the Glomar Explorer, operated by Global Marine Inc., runs successive stands in the hole from a horizontal pipe-racking system (Fig. 3). Photo by Dean E. Gaddy.
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The Deepwater Pathfinder drillship, owned by a venture of Conoco Inc. and R&B Falcon Corp., is shown drilling its first well in the Garden Banks area of the Gulf of Mexico. This dynamically positioned (DPS-3), double-hulled vessel was designed for extended well-testing operations (Fig. 4). Photo by Dean E. Gaddy.
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Transocean Offshore Inc.`s Discoverer Enterprise was among the most advanced drillship in the 1999 fleet. It contains two independent sets of rotary tables, traveling blocks, and drilling packages underneath one derrick (Fig. 6). Photo by Danny Faulkner, courtesy of Transocean Offshore Inc.
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Dynamic positioning classification
THE AMERICAN BUREAU OF SHIPPING`S (ABS) DYNAMIC POSITIONing (DP) classification system provides a framework for vessel design, most useful in describing DP technology. This system, as follows, depends on the degree of redundancy and sophistication built into the drillship:
- DPS-0: No backup DP system on board.
- DPS-1: Manual centralized control backup, automatic heading.
- DPS-2: Backup for any single fault.
- DPS-3: Backup for combination of any single fault and 1-compartment loss.
Although the ABS system is an industry standard, by no means does it regulate the selection process for vessel design. This is left up to the contractor, with system design typically dependent on whether the ship is an upgrade or new build.
DPS-0 and 1
A DPS-0 rating requires the vessel to have a centralized manual control system and automatic heading control. In this case, the ship uses information from one position reference system, one wind sensor, and one gyro compass. These systems must be located at a main DP control station where the operator is aware of the external environmental conditions and any activities relevant to the DP operation.3
No redundancy is required with DPS-0, and applications typically involve operations where the DP function does not endanger human lives or may cause major damage in case of failure.
If there is a chance that loss of position will result in some pollution and minor economic damage, excluding harm to people, vessel operators typically implement a DPS-1 system.4 This system also does not require comprehensive redundancy.
Instead, the manual position and auto heading control system must provide backup for the fully automatic system, which in turn has sensor redundancy and a more secure power supply. Position reference sensors, wind sensors, and gyros are provided in duplicate.
DPS-2 and 3
If a loss of position can create consequences of severe pollution, large economic damage, or accidents to people, an operator will build a DPS-2 drillship. And where position loss may result in severe pollution or fatal accidents, a DPS-3 system should be used.
Both systems include a power-management system that automatically ensures sufficient power for essential operations. Furthermore, DPS-2 and 3 systems use a consequence analyzer for the continuous monitoring of thrust and to maintain heading and position under prevailing environmental conditions. The analyzer also calculates thrust capability in the event of a single failure.
In relation to DPS-2, a DPS-3 vessel requires:
- An additional backup control station, including a third automatic heading and position control system.
- A third gyrocompass fitted in the backup control station.
- Independent and interruptible power supplies for the backup system, position reference system, and gyro compass.
For a DPS-3 vessel, the backup control station must be situated in a separate compartment, sited and arranged so that no single fault, including fire or flood in one compartment, will render both main and backup control systems inoperable.
Additionally, generators and distribution systems must be sized and arranged in at least two compartments. Thus, if the vessel loses any compartment, sufficient power will remain available to provide essential system loads.
