New ultra-deepwater rig with dual rotaries will reduce costs

Jon C. Cole, Robert P. Herrmann, Robert J. Scott
Transocean Offshore Inc. Houston

John M. Shaughnessy
Amoco Corp. Houston

The Discoverer Enterprise, a next generation, ultra-deepwater drill ship with a dual rotary system, will decrease drilling and completion costs by reducing bottom hole assembly (BHA) and tubular preparation time.

Transocean Offshore received a contract from Amoco Corp. to build the ultra-deep floating rig and is scheduled to spud its first well in July 1998. It will generally work in water deeper than 6,000 ft.

The rig design involves a new approach that addresses the overall well-construction process and equipment required to decrease significantly deepwater drilling time.

The Discoverer is the first ultra-deepwater rig designed specifically for handling subsea completions and extended well tests.

Increased deepwater rig demand

Advances in deepwater production technology, tax and royalty relief, and the development of deepwater infrastructure are stimulating development activity in the Gulf of Mexico, Brazil, and West of Shetlands.

This has significantly increased demand for rigs capable of drilling in deep water, particularly in the Gulf of Mexico where more than 700 blocks have been leased in greater than 5,000 ft of water.

Currently, only three rigs in the Gulf have drilled in water deeper than 5,000 ft, including two dynamically positioned drillships and a moored semisubmersible.

The increasing demand for deepwater rigs has increased day rates and improved contract terms for the drilling contractor. In response, contractors have begun upgrading existing equipment and are converting nondrilling semisubmersible units in order to meet operator demand.

Because capital costs for conversion have increased rapidly, new construction is becoming a viable alternative.

The Richardson, an anchored semisubmersible, was the last floating rig built capable of operating in 5,000 ft of water.

Delivered in 1988, the vessel cost $90 million. Building the same rig today may cost more than $250 million. In order to compensate the capital costs, day rates of $180,000/day are needed to provide an adequate return to the owner for this level of investment.

If the Richardson was duplicated, the technology and drilling process would not be improved significantly; therefore, a more cost-effective drilling approach was needed to justify the $180,000 day rates.

An in-house operations and engineering team was formed to reduce well-construction costs by 40%. At this level of efficiency, an operator can justify the day rate required for new construction because faster well construction will offset higher daily costs.

The Enterprise development team analyzed the entire well-construction process and determined the equipment requirements needed to improve drilling efficiency.

"Critical path"

Rig activity normally revolves around a single rotary table. Therefore, any additional activity that requires ancillary use of the rotary will place itself within the "critical path" of completing the well.

The operator pays for that activity through the rig day rate and spread costs including remote operated vehicles (ROVs), measurement while drilling (MWD), personnel, boats, helicopters, shore base, rental tools, and communications.

By utilizing two rotaries, two top drives, and two draw works, the Enterprise becomes capable of removing many typical tubular activities out and away from the "critical path." The result is greater flexibility, enhanced efficiency, and lower cost to the operator.

Rig construction costs

A prime determinant of the day rate for new rig construction is the capital cost.

Semisubmersible as well as ship-shaped designs were considered before the team settled on a North Sea shuttle tanker hull form. Use of this hull allowed a much larger drillship with more capabilities to be delivered for the same price as a semisubmersible.

A purpose-built hull from Astano (Astilleros Españoles) was selected because there were fewer cost uncertainties in the estimate.

While the hull is under construction, the drilling modules will be built simultaneously. Building in parallel will reduce delivery time, allowing it to begin generating revenue sooner.

Conventional construction time for a deepwater drillship would require 6 months more than required for the Enterprise.

Design of the drilling modules will minimize the number of lifts required for installation.

As shown in the views of the Discoverer Enterprise (Fig. 1 [41589 bytes]), individual modules include the substructure and derrick, mud pump room, shaker house, quarters, pipe racks, well and mud logger units, warehouse, and machinery rooms.

Because the drilling modules will be completely outfitted and tested in the construction yard, it will require only 2 months after hull delivery for the completed Discoverer Enterprise to be fitted and ready for work.

Estimated cost of the completed unit, including capitalized interest and mobilization, will be $280 million.

Drillship design

The large size of a shuttle tanker hull will improve the motion characteristics compared to existing drillships and will even approach those of fourth generation semisubmersibles in all but the most severe environment.

The rig is expected to heave less than 7 ft in a 50-year Gulf of Mexico storm while the rig's station-keeping system is designed to stay on location through such an event.

The reduced weather sensitivity and ability to store all of the riser onboard will reduce the watch circle for approaching storms. Along with a direct reduction in weather downtime, the Enterprise will be able to minimize lost time for hurricane abandonment by using its onboard storage capacity and rapid transit speed.

The rig will be positioned with six 7,000-hp azimuthing thrusters. The latest dynamic positioning and power management systems are expected to consume fuel in amounts similar to existing drillships for day-to-day fuel consumption.

The large hull will provide unprecedented space and payload. Table 1 [39185 bytes] compares the Enterprise to one of the world's largest drillships, Discoverer 534, and the fourth-generation semisubmersible, Transocean Richardson. Fig. 2 [25894 bytes] shows a profile of the Discoverer Enterprise compared to the Discoverer 534 and the Richardson.

The large variable deck load will allow the rig to carry tubulars and consumables for three or more wells.

Space and load capacity for 10,000 ft of 21-in. drilling riser and 10,000 ft of completion riser will be available.

The use of a shuttle tanker hull will allow onboard storage of up to 100,000 bbl of crude and will provide dedicated deck space for processing equipment.

The Enterprise is capable of working West of Shetlands or the Voering Plateau during the summer season. In the fall it could be loaded with consumables needed to drill through the winter off the west coast of Africa, thereby minimizing the operator's shore-base requirements.

The vessel's 200-man quarters will allow the operator to station most personnel offshore.

Dual rotary

A unique feature of the Enterprise will be the two drilling areas set 40 ft apart on an 80 ft by 80 ft rig floor (Fig. 3 [31928 bytes]). Both drilling areas will be equipped with a top drive, 4,000-hp draw works, and crown-mounted motion compensators.

Automated pipe-handling equipment will allow pipe to move from either rotary to the set-back area.

As planned, the drilling riser will run off the primary (forward) rotary. The auxiliary (aft) rotary could be equipped with a drilling riser, but is not planned at this time. The auxiliary rotary can be used to drill riserless on a template.

The large rig floor will provide adequate storage area for tools needed in each step of the drilling and completion operations. Because the moon pool area will be 80-ft long by 30-ft wide, it will provide ample room for working with the blowout preventor (BOP) and subsea trees.

The auxiliary rotary will reduce costs by moving operations out of the "critical path." For example, it will be able to stand back all of the tubulars, and will allow the pretesting of downhole tools prior to running through the main rotary.

When the rig arrives on location, the auxiliary rotary can begin drilling riserless, while the primary rotary runs the BOP and riser at the same time. Because the latter operation will be moved out of the "critical path," it will become, in effect, cost free.

An improvement in drilling efficiency comparable to that accomplished by batch drilling is forecast for the Discoverer Enterprise. Efficient planning will be required to optimize use of the second rotary and drilling crews with emphasis on support activities for the primary rotary.

The "flat-line time," or the time on the drilling curve from which there is no drilling progress should be substantially reduced.

Finally, the auxiliary rotary will be used to run subsea trees and other construction activities, including pipeline pull-in or running of flow line jumpers, all performed outside of the "critical path."

Tubular steel derrick

The Enterprise derrick will be designed like an offshore platform jacket. It will be lighter, stronger, and have less wind resistance than a similarly sized conventional derrick.

The rectangular crown will span both rotary tables. Dual, crown-mounted motion compensators will be located more than 240 ft above the rig floor.

The height of the derrick will allow drill pipe stands to be stood back as quadruples (four joints), rather than as triples (three joints). Casing will be stood back as triples prior to running, as opposed to the normal practice of running singles.

Thus, the number of drill pipe and casing connections will be reduced by one-quarter and two-thirds, respectively.

The large set back area can accommodate 80,000 ft of pipe.

Use of the secondary rotary and the large setback area will allow subsequent bottom hole assemblies to be prepared for each hole section. In addition, all tubing can be picked up and stood back prior to completion operations.

Riser

The Enterprise's riser will be constructed of 80,000 psi material with a 21-in. OD and a 13/16-in. W.T. It is designed to have adequate strength for use with 17 ppg mud in 7,000 ft of water.

Riser buoyancy will be syntactic foam. The last 2,000 ft of riser will be run without buoyancy to improve riser stability in a disconnected mode. Special considerations for station keeping and riser disconnect have been factored into the overall subsea design.

The weight of the riser and mud column with 17-ppg mud will require approximately 2 million lb of tensioning capacity.

Mud Systems

The rig will be equipped with four 2,200-hp mud pumps. Space is available for two more pumps in the event the auxiliary rotary is equipped to drill an independent well.

The Enterprise will have two separate mud systems, either of which can be active. Thus, no time will be lost in preparing pits to change from a water-based mud system to a synthetic mud.

The total mud storage volume will be 15,000 bbl, allowing for two or three complete mud systems to be stored on board. In addition, the dual system will save mud losses by 10% to 20%, which are normally lost while transferring to shore.

There is sufficient capacity to store the entire mud volume in the riser in the event of a storm. A 7,000-ft, 21-in. riser will hold 2,500 bbl of mud.

Well construction

The Discoverer Enterprise will arrive on a new location with all drill pipe, structural casing, and conductor casing picked up and ready to run. The 30-in. casing, guide base, and 26-in. hole drilling assembly will be run off the auxiliary rotary table while the primary rotary table runs the BOP and riser (Fig. 4 [49785 bytes]).

After the 20-in. casing string is run and cemented through the auxiliary rotary table (Fig. 4), the BOP will be in position to land on the well head. Thus, one more operation will be moved out of the "critical path."

While testing the BOP, drilling assemblies for the 171/2-in. hole will be picked up and stood back in the derrick. Next, the 171/2-in. hole section will be drilled using the primary rotary, while the auxiliary rotary is used to make up and stand back the 133/8-in. casing.

Because the casing will be made up in triples, only one-third as many connections will be required as in a traditional operation. Thus, the time normally used to rig up for casing will be eliminated. In addition, inspection of casing threads and the hanger running tools will also be done outside the "critical path."

The 121/4-in. hole drilling assemblies will be picked up while cementing the 133/8-in. casing. The MWD tools and mud motors can be checked on the auxiliary rotary, thereby minimizing premature failure of downhole tools.

While the bit is being pulled to run 95/8-in. casing, the wire line sheaves and open hole logging tools will be rigged up and hung in the derrick. Because casing can be run earlier, a hole-conditioning trip following logging may be eliminated and the chances for "sticking" the casing are reduced.

A total of 2 days can be saved through pre-preparation at routine casing points. Because reducing the time to evaluate and case a hole will also reduce hole problems, there will be intangible improvements.

Fig. 5 [27804 bytes] shows the timelines for a generic well comparing dual and single-rotary operations. Excluding intangible improvements in efficiency, it is estimated that the Discoverer Enterprise equipped for dual activities can drill a deepwater exploratory-type well in 30% less time than a traditional single-activity rig for the reasons detailed in this section.

At the end of the well, drill pipe can remain racked in the derrick saving time when spudding the subsequent well. After operations on the well are suspended, the BOP can be pulled and serviced in one piece underneath the rig's substructure.

The use of two rotaries greatly reduces the nonproductive time generally associated with running tubulars and riser in deepwater depths. The result is a shorter "critical path," faster well times, and a lower effective day rate to the operator.

Development drilling

Amoco expects to utilize the Discoverer Enterprise primarily to develop fields discovered by other rigs. While the rig would be cost effective in exploratory drilling, it will have a significant cost advantage over conventional rigs for development work because completion and testing activities are incorporated into its design.

On a template drilling operation, while the first well proceeds in a similar method to the exploratory well described in the previous section, the auxiliary rotary can be used to drill additional template slots by setting 20-in. casing strings. At the end of each well, the BOP can be moved to another slot without pulling it to the surface.

Because of the Enterprise's mud-storage capacity and the two 3,000-bbl active systems, the rig can store more than one mud system on board. If a water-based mud is used in the 171/2-in. hole, no time would be lost preparing the rig for a synthetic mud to be used in drilling out the 121/4-in. hole section.

After the hole is ready, water-based mud would be circulated to the pits by pumping synthetic mud from the second active pit system. Storing the mud onboard eliminates mud losses due to handling as well as transportation costs.

Because the Enterprise will have the ability to switch mud systems through a manifold, the mud of choice for drilling or reservoir evaluation can be used without loss of rig time.

Tree deployment

The rig's substructure will have 60 ft of clearance above the moon pool deck. This will allow handling the BOP in one piece and will accommodate the tallest known subsea tree.

A major improvement will be the Enterprise's systems for handling and deploying subsea trees. Initially, Amoco plans to use horizontal spool trees for its completions.

First, the trees will be lifted off the work boat in one piece and placed on a moonpool trolley system. Because of the space and height under the substructure, the trees will always be handled in one piece. Therefore, the trees will be tested and prepared for running outside of the "critical path."

While the primary rotary finishes drilling and casing the well bore, the auxiliary rotary will prepare and run the tree to the sea floor. Then the BOP will be released and picked up by the main draw works and rotary.

The rig will then position the tree over the well and land it on the wellhead. The BOP will then be landed on top of the tree. The completion operations (perforating, gravel packing, etc.) will all be run through the tree.

Table 2 [22059 bytes] compares the "critical path" time for the Discoverer Enterprise and a conventional deepwater rig to run subsea trees in 6,000 ft of water. The Enterprise will save 7 days if there are no mechanical problems. In the event that there are problems with the subsea systems, savings will compound quickly.

Completion and testing

The rig's payload capacity will allow storage of up to 100,000 bbl of produced crude in the hull. This will significantly reduce the operator's cost to test and complete in deepwater.

When completing through a subsea tree, a longer or more complete production test will increase the chance of making an effective completion and decrease the chance of having to do remedial work.

Without the ability to produce and store crude in the hull, a dynamically positioned tanker would be required. Offloading crude to a shuttle tanker during the test period while flaring gas at the same time would pose significant safety and logistical problems. The Enterprise will not have these complications because a tanker will be brought along side only after testing is completed.

The entire load of crude can be unloaded within 24 hr which will reduce the required weather window.

The rig will have a five-ram subsea BOP to facilitate testing equipment. The upper ram cavities will contain blind shear rams, while the lower two cavities will have variable bore rams to accommodate drilling equipment. The fifth ram will be sized to close on the test tree, thus reducing the need to pull the stack in order to adjust the ram sizes for test tools.

Another feature of the Enterprise's large payload and deck space will be a dedicated well-testing area. Process equipment designed to handle up to 20,000 bo/d and 70 MMcfd will be permanently installed on deck because the vessel will store all its tubulars on dedicated racks. On conventional rigs, compromises are invariably made to accommodate testing equipment because of a shortage of deck space and variable load.

The rig's dual-active mud systems will eliminate the time lost generally needed to clean the mud pits prior to taking on clean completion fluids. Because the completion fluid will be held in one of the 3,000-bbl active systems, mud displaced from the hole can be stored in the other 3,000-bbl system. Thus, completion activities can continue without delay once all the mud is recovered.

Additional activities

The immense size and capabilities of the Enterprise provides opportunities for additional well construction activities. The vessel could be used to intervene quickly and cost effectively in a subsea well that needs repairs.

The rig can travel at 1-2 knots with the riser suspended below the hull. Consequently, the time and cost to work over a subsea well may be less compared to mobilizing a smaller rig with a lower day rate, which will likely require far more time to restore production.

The rig's 80-ft by 30-ft moon pool, the two draw works, and 70-ton cranes will facilitate running templates, flow lines, or subsea control bundles. The large deck space and second riser rack will provide an ample work platform from which to J-lay pipelines or flow lines.

Cost justification

At this stage in the project, it appears that the goal of reducing well construction time by 40% for Amoco's proposed drilling and completion program is achievable. A summary of the major innovations and area of cost impact is shown in Table 3 [17133 bytes].

The potential cost advantages for efficient operations are shown in Fig. 6 [32044 bytes]. It shows a typical deepwater well with the total cost expressed as a percentage of the entire well cost on the Y axis. In other words, with no efficiency gains, the total well cost is 100%. The total cost is split into three categories consisting of rig costs, spread costs (boats, services), and fixed costs (well heads, casing).

The effective day rate, defined as the actual total day rate less spread cost savings divided by the expected days for the well at a zero percent efficiency rating, decreases rapidly as drilling efficiency improves.

A 20% efficiency improvement will lower the effective day rate by 25%, while a 40% efficiency improvement will reduce the effective day rate by about 54%. The disproportionate savings are due to corresponding savings in the spread cost for the more-efficient operation.

The goal of Transocean Offshore and Amoco is to achieve the targeted 40% efficiency improvement, thereby satisfying the operator's need for a cost-effective drilling rig while generating sufficient income for the contractor to justify its large investment in building the Discoverer Enterprise.

The Authors