Drillship or semi?The choice is not always clear

It is important to address the unique challenges of drilling in deepwater at the beginning of a new project and to have a clear view of the mission of the completed unit.

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By Howard Day,Friede & Goldman Ltd & Charles Springett, GlaobalSantaFe Corproation

It is important to address the unique challenges of drilling in deepwater at the beginning of a new project and to have a clear view of the mission of the completed unit. A mobile unit for ultra deepwater will either be a ship or a semisubmersible; there are no other options.

A ship offers high transit speed, high variable load capacity, large storage volumes, and the ability to store produced fluid. A semisubmersible offers better motion characteristics; equal resistance to wave, wind, and current from any direction; the ability to support a mooring system; and a large deck area.

Simply put, if the primary target is ultra deepwater exploration drilling, the ship is the design of choice. If the primary target is development drilling, the semisubmersible is the design of choice. The key consideration is the mooring system. Some operators demand a mooring system for field development at any water depth. The bigger, more capable ship designs all are dynamically positioned. When Santa Fe Inter-national, now GlobalSantaFe (GSF) after merging with Global Marine in November 2001, approached Friede & Goldman Ltd. (F&G) to design a drilling rig for development drilling, it was this consideration that led to the Development Driller.

However, there are other design considerations, including safety, vessel geometry, dual activity, riser storage and handling, fluid storage, blow out preventer (BOP) handling, tubular handling, well completion, third party equipment, new technology threat, and quarters.

A new semisubmersible capable of running a 21-in. riser and an 18 3/4-in. 15K BOP in 7,500 ft water depth is by definition a big and expensive unit. It will compete with existing units, both new-builds and conversions. Most of the competing vessels were built or upgraded against a contract that covered part of their initial capital cost. The primary challenge GSF and F&G faced was to provide the new rig with operating efficiencies that would allow an operator to justify a higher dayrate on the basis of lower overall field development costs. Such operating efficiencies can be achieved through enhanced mooring system deployment, pre-set mooring attachment capability, riser running, tubular handling, dual load paths with drilling capability in each, BOP handling, and subsea tree manipulation during well completion.

Vessel geometry

History has shown that the simplest and most cost-effective semisubmersible form is four columns arranged on twin pontoons with a water-tight, upper-hull deck box. Such an arrangement allows the redundant alignment of critical structural members in all three elements. The result not only satisfies global strength and fatigue requirements, but does so with a minimum quantity of steel. A case can be made that three columns are even more efficient, but transit speed and available deck space both suffer in a triangular unit.

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Based on model testing, twin hulls provide a reasonable transit resistance, and hence speed, without unacceptably compromising the athwart ship resistance.

Three other issues drive the final configuration:

• Variable deck load at operating draft determines how the unit will manage resupply logistics. As deepwater is by definition further from land, resupply is an important issue, and the ability of the rig to store more consumables increases its attraction to potential clients.

• Total load capacity at transit draft determines how fast the unit will travel when fully laden. If the pontoons are too small, they will be submerged during transit, dropping transit speed. Unfortunately, just making the pontoons as big as possible is not the right solution. This is because as pontoon size increases, so do steel weight, cost, and station-keeping forces. Additionally, variable load capacity at operating draft decreases as pontoon size increases. The importance of determining the optimum size of the pontoons cannot be over emphasized. Making them too large drives up the cost, compromises motion characteristics (bigger stability columns required to restore operating variable load reduce the natural heave period), and increases the power requirements of the dynamic positioning system. Make them too small and the rig cannot deballast with a useful load on board – a fate shared by many older rigs whose weights were underestimated at original construction. This weakness grows as the rig is modified during its working life.

• The environment in which the unit will operate will determine the height of the columns and the air gap between operating water level and the underside of the upper deck box. For a semisubmersible designed to survive in the harsh North Sea environments, the height of the stability columns must be sufficient to ensure that, at survival draft, there is no wave impact on the underside of the deck box. Column height to achieve this is much greater than what would be needed for a rig operating in moderate weather areas such as the Gulf of Mexico, Brazil, or West Africa. On the other hand, a low column height allows a larger variable load, with all other parameters being equal. As a practical matter, during the initial design stage, column size and overall rig size can be optimized to provide the desired variable load and motion characteristics within an affordable envelope.

GSF determined that a 7,716-ton variable deck load would support a full drilling program remote from a supply base. A total load of 5,511 tons with 0.3-meter freeboard on the pontoons would provide adequate flexibility in transit. After researching the subject, the company concluded that a rig purpose-built for moderate environments would find enough demand, and the additional cost required to extend its range to harsh environments could not be justified.

Station keeping

A key factor in the decision to build a semisubmersible is the ability to install a mooring system. On four column semisubmersibles for deepwater, a conventional mooring spread would consist of eight-leg combination chain/wire systems. The rig should be able to anchor with the aid of a conventional anchor handling boat in up to 5,000 ft water depth. To minimize mooring system deployment costs, the rig must be able to store on board all of the chain and wire required for the 5,000 ft water depth. A rig intended to work in 7,500 ft water depth will be larger than most existing drilling units. As a consequence, the mooring system must also be larger to resist larger environmental forces.

Most standard third- or fourth-generation semisubmersibles, with a combination chain/wire system, use 3 1/2-in. diameter wire rope. The breaking strength of this size has increased over time as the manufacturers optimized their products. It is now possible to buy 3 1/2-in. mooring rope with a breaking strength of 1,588 kips. Unfortunately, meeting the latest mooring standards for a true deepwater rig will require either additional legs for the anchor system or stronger wire.

Stronger wire means bigger wire, and the manufacturers have not developed a 3 3/4-in. or a 4-in. wire with the same strength-to-weight ratio as the 3 1/2-in. wire. For example, 3 3/4-in. wire provides only a 7% increase in strength for a 14% increase in cross-sectional area (and therefore weight). It is therefore not a desirable solution to just increase rope size. Not only are the returns (breaking strength) not justified by the increase, but the size of the anchor handling boat required to deploy the system increases. This makes the rig less attractive to an operator in parts of the world where higher capacity anchor handling boats are not readily available.

GSF's solution was to configure the rig with a combination chain/wire system using 3 1/2-in. wire rope and 3 1/4-in. K4 chain. About 95% of the time, in most areas the rig will work, this can keep the rig on station safely and securely. It is the other 5% that must be addressed.

Thruster assist

The company concluded that this 5% could be covered using thrusters in a "thruster-assist" mode. Thrusters allow the mooring system components to be sensibly sized and still keep the rig to remain on station in any combination of weather conditions. A possible exception to this is during tropical revolving storms when the rig is abandoned.

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Artist's conception of the Deepwater Driller.
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The design goal was to provide both redundancy and flexibility in the system without increasing the complexity to the detriment of overall performance. Preliminary calculations for thruster sizing were based on the ability of the system to resist the combined effects of a 10-year Gulf of Mexico winter storm and a 10-year loop current. The sizing was also checked against 10-year Brazil data.

Preliminary thruster sizing was based on computer calculated wind, wave, and current coefficients. Subsequent wind tunnel testing and model testing confirmed that with eight, 3.2 MW thrusters, each capable of 66 tons thrust at zero speed (57 tons thrust at 3 knots), the rig could maintain station in both Brazil and the Gulf of Mexico with two thrusters down. An extensive model test program used correctly scaled thrusters mounted on the model. Applying a full gusting wind spectrum, current and waves collinearly provided confidence that the actual system will perform at least as well as it was designed. Thruster losses due to hull interaction and interaction with each other were fully modeled and accounted for in the results.

The model tests also confirmed that the motion characteristics of the rig at operating draft are what should be expected for a semisubmersible of this size and that wave impact on the under side of the deck box is eliminated at survival draft. Down time, due to motion in the areas for which the unit is designed to work, will be minimal.

Primary power will be provided by eight, 3.8 MW diesel generator sets arranged in two engine rooms. The system is arranged so pairs of engines are connected to individual switchboards that drive thruster pairs located in diagonally opposite corners of the rig. As a result, if any switchboard fails, only two diagonally opposed thrusters are lost. Any single engine failure will reduce power to two thrusters by 50%, and the loss of one engine room reduces total thruster power by 50%. This arrangement is less than DPS-3 but provides considerably more redundancy than is required by DPS-2. In addition, the system will have triple redundant and triple voting control systems, including one system located remotely from the navigation bridge, which is a DPS-3 requirement. The additional complexity and cost required to create full DPS-3 capability could not be justified by either the risk analysis or the FMEA.

In ultra deepwater, it is clear that several of the major operators will insist on a mooring system rather than dynamic positioning. It is almost impossible for a drilling rig to carry a self-deploying system in water depths beyond 5,000 ft. There are, however, some promising systems in use and under development that allow a mooring system to be pre-deployed, such as using suction pile anchors. When the rig is brought onto location, the rig's anchor wires are connected to this pre-laid system. Because only a limited amount of wire is paid out from the rig, the wire line winch should be a traction winch and storage reel for which line pull is independent of the amount of wire paid out. Such a pre-laid system can be supplemented by thruster assist as necessary in the same manner as the conventional system.

Subsea systems

Water depth affects the ultra-deepwater drilling vessel operation in several ways.

The temperature at the seafloor is such that hydrate formation in any gas migrating to the BOP can freeze the system closed. Provision must be made to flush the connectors routinely with BOP operating fluid containing glycol or, as a last resort, to inject methanol remotely from the remotely operated vehicle (ROV).

The pressure at 7,500 ft is 230 bar, or 3,333 psi. All of the BOP equipment must work flawlessly in this cold, dark, high-pressure environment 1 1/2 miles from the drill floor.

The long column of drilling fluid in the riser exerts a pressure on the formation. In shallow water, this pressure is usually far enough from the fracture pressure of the formation that cuttings can be transported out of the hole without exceeding the fracture pressure and invading the formation. This allows longer sections of hole to be drilled before setting casing. As water depth increases, the difficulty of keeping mud weights between fracture pressure and pore pressure causes an effective well design to include multiple casing strings, driving up well complexity and cost.

The riser itself must be strong enough to support choke and kill lines, boost lines, hydraulic lines, dual-gradient lines, and multiplex cables. Yet, it must be light enough to allow a combination of top-tension and buoyancy to support it for a defined range of mud weights and vessel offsets.

The time spent running equipment to the seafloor represents a significant part of the total operational time. It can take days to round trip the BOP in 7,500 ft water depth. During this procedure, the rig is vulnerable to the environment. As a result, the design of the riser, its buoyancy, the BOP, and subsea system are the most critical elements of the rig design. The size and weight of a 21-in. riser and 18 3/4-in., 15K BOP system that dictate the overall size and cost of the rig. The major deepwater operators are aware of this. Extensive research shows how smaller, less capable rigs could drill safely and effectively in the ultra deepwater:

• Dual density drilling – to solve the pressure differential problem
• Slimbore wells to allow the use of a 16-in. riser and a 13 5/8-in. BOP
• Expandable tubing to facilitate slim bore wells (Ref 10)
• Free-standing riser systems
• Artificially buoyant seabeds that bring the wellhead closer to the surface.

All of these technologies hold promise, but they also represent compromises in the primary objective, which is to deliver a quality, high-production well to the operator.

Marine drilling riser

The riser that has been specified for the Development Driller incorporates features that will enable the rig to take advantage of these new technologies if they bear fruit. For example, in addition to the 4-in. inside diameter (ID) choke and kill lines, there will be a 6-in. ID boost line that doubles as a dual-density return line. If dual-density technology reaches its potential, the rig will be better positioned to drill difficult formations with the potential to extend its water depth range.

For the sizes and tensions required for the design water depth, a dogged connector is not strong enough. The two primary choices are flanged and bolted connectors, or the Kværner breech lock or "clip" design. GSF chose the latter because it speeds running time, is particularly suited to the storage and handling configuration described below, and substantially reduces the exposure risk for people and equipment during riser running and retrieval operations.


The first question is: How long should a riser joint be? At one time, most risers came in 50-ft joints. These were easy to transport and handle and were not difficult to store. The problem is, as water depth increases, so does the number of joints that have to be made up and broken out. This additional activity increases the overall BOP deployment time. The solution of longer joints decreases running time but creates the following difficulties:

• Beyond 50 ft, a riser joint is a permit load on most countries' roads and is impossible to transport by land in some countries
• It is hard to bring such a riser onto the rig and to back load it, which means any maintenance must be done on the rig and not in an approved repair shop
• If a riser of this length is picked up by the ends, the sag can be sufficient to damage the buoyancy material
• It is difficult to accommodate such lengths, which tends to compromise other important deck activities on a semi.

Given these points, 75 ft is considered the optimum length for the riser joints. Meaning, 100 joints are required for the design water depth of 7,500 ft.

Most rigs have horizontal riser racks and use handling means ranging from conventional cranes to purpose-built gantry cranes to transport each joint between derrick and riser rack. This requires a motion translation from horizontal to vertical, and subsequently, from vertical to horizontal. Marine drilling riser joints are heavy and awkward to handle, and this method of storage and handling increases exposure to personnel and damage to expensive flotation modules.

The alternative is to store the riser vertically. The advantages are reduced running times, less exposure to people, less damage to buoyancy material, and a smaller storage footprint on the deck. Vertical storage can be in the stability columns, or it can be on the main deck. Clearly the latter increases the vertical center of gravity (VCG) of the rig and the wind area, affecting the intact and damage stability over turning moments. The solution chosen for the Develop-ment Driller is to place the foundation for the riser storage at the bottom of the upper deck box. As a result, the VCG of the stored riser is slightly higher than it would be if stored four high on top of the deck box. This was another reason for limiting the riser joint length to 75 ft.


Because the riser is stored vertically, immediately adjacent to the derrick, it can be transported between rack and derrick by an overhead bridge crane system that is integrated with the tubular handling system in the derrick. The clip connector can be made up and broken out remotely, something that would not be possible with a bolted connection.

Syntactic foam is expensive. Damage to flotation modules, usually as a result of poor handling, is common. Nominal outside diameter (OD) of the flotation is 54-in., the rotary table ID is 60 1/2-in. Improved buoyancy material design is more resistant to impact damage, but care must be taken during running to avoid contact with the rotary table and diverter housing. The unit can handle large OD free-standing riser components, and other subsea construction items through the moonpool.


The combined weight of large-bore choke and kill lines, boost/dual-density lines, and hydraulic supply lines puts a high demand on the tensioning system. A combination of 15-ppg mud, 7,500 ft water depth, the qualifications required under API RP 16Q, and the 10-year Gulf of Mexico environmental condition require a net top tension of 1.9 million lbs. The nominal tension capacity of the rig will be 3 million lbs, which, after allowance for two tensioners down, friction, and lead angle losses will provide a useful net top tension of 2.4 million lbs. This is more than enough for the design water depth with room for future expansion.

Conventional, wire-line tensioners and hydr-aulic in-line tensioners also were considered for this project. The latter hold future promise, but until issues associated with their use have been resolved, it is believed that conventional units would eliminate a potential problem during the early stages of the rig's operation. Also, the moonpool area is kept clearer with a conventional, wire-line tensioner system. In-line tensioners must be moved to clear the moonpool area for some operations.


The buoyancy products on the market today are robust, able to resist the hydrostatic pressures found at design water depths, and resistant to handling damage. Sag in the long riser joints is still a problem, but it is solved by applying six, 11-ft lengths of buoyancy to each joint. For the design water depth, the total buoyancy of the full length of riser is about 90%.

New technologies are aimed at reducing riser weight and the buoyancy requirement. Titanium has been proposed for the riser pipe, as well as the choke and kill lines. Composites also have been proposed, some with a steel liner to improve wear resistance. These alternatives are expensive, and the use of conventional materials ensures that the combination of foam buoyancy and installed riser tension can handle the design range of water depths, mud weights, and environmental forces.

Moonpool access

Good access under the drill floor is important, particularly on a development-drilling rig. The riser and BOP support equipment make this a congested area. The central opening through the upper deck box must be carefully designed. Primary longitudinal structural bulkheads align with the inboard faces of the stability columns. The moonpool cannot extend beyond this bulkhead, limiting the maximum width of the moonpool to 140 ft. The clear opening is 130 ft by 25 ft, which allows access to the moonpool from both sides of the drillfloor. Two-sided access is important. It allows the BOP and riser to be parked on one side while subsea trees are run on the other. The goal is to eliminate congestion on the critical path. The arrangement is also adaptable to the installation of a free standing riser system.

Blowout preventers

Subsea BOP stacks are large, heavy, awkward to handle, and expensive. As currently configured, the six-ram BOP is 60-ft tall and weighs an estimated 772,000 lbs. The decision to store the BOP on the port side of the main deck is unusual. On most semisubmersibles with an upper deck box, the stack is stored in the moonpool area. Main deck storage offers a more user-friendly location for maintenance and an extended rail system for coordinating the movement of BOP and subsea trees into and out of the running elevator.

The BOP elevator in this design is unique. The BOP handling philosophy was driven by a no-wire-rope requirement. The BOP is skidded directly onto the BOP elevator that lowers it to clear the underside of the substructure beams. The elevator is rated at 441 tons and can run the BOP to the starboard side of the moonpool and park it before deployment. Tree staging operations also are handled on the port aft side of the moonpool. The system can of stage up to three trees consecutively, and the elevator can be adapted to run a variety of tree types.

A 716-ton flatbed cart allows the full riser string and BOP to be suspended from the rig clear of the sea floor while either the main or auxiliary hoists install a horizontal tree on a dedicated landing string. Having the auxiliary hoist performing this function maximizes the horizontal separation between riser and tree.

The moonpool area can be used to attach vortex induced vibration fairings to the riser joints using two "cherry picker" man hoists.

Dual activity

A dual activity system saves time during the deployment of tubulars through the water column. The most important component of a dual-activity system is the ability to run two strings of pipe to the seabed simultaneously on hoists horizontally separated enough to prevent interaction below the keel.

This issue has been addressed by others, particularly by Transocean Sedco Forex with their new Discoverer Enterprise class of drillship and Saipem with their Saipem 10,000 drillship, both of which provide dual load paths of equal capacity. Most other deepwater rigs can make up tubulars offline, but very few have two independent load paths, each with rotating capability. The Development Driller design team examined the work flow and concluded that one of the load paths need only be half the capacity of the other to achieve the majority of the savings two load paths offer. This reduces equipment costs, overall size, and cost of the vessel.

The greatest timesavings are realized while drilling top hole. Surface casings can be run on one load path while the hole is being drilled on the other. It is also possible to run the riser and BOP while the top-hole construction is being completed. Once the BOP and riser are in place, the second load path can be used for all tubular make-up in the derrick, including complete stands of casing.

The second major area of timesavings comes through running the horizontal tree on a dedicated landing string. The tree can be run to the seafloor while the BOP and riser are still connected. There is sufficient horizontal separation between the load paths to allow the BOP to be unlatched and suspended from the flatbed cart. The rig is then repositioned over the wellhead and the tree installed before the BOP is retrieved to the rig (or moved to another wellhead).

To obtain maximum benefit from this system, both load paths must have a top drive, and both must be motion compensated. The main load path will always handle the BOP and riser. Thus it must have a dynamic capacity of 1,000 tons. The auxiliary load path can handle all top hole and tree running tasks with a 500-ton dynamic capacity.

Dual activity represents a challenge to the project team. The entire area around the drill floor becomes complicated. This area typically causes shipyards problems and cost overruns. To avoid such problems, GSF opted to assign responsibility for the complete substructure, drill floor, and derrick work area to one equipment supplier. For ease of definition, this work area, including the BOP handling and storage area and the riser storage area and handling system, was designated the Well Activity Centre™ (WAC). GSF contracted Hydralift SA of Norway to undertake overall responsibility for the design of the WAC, the supply of all of the equipment, the supervision of its installation in the shipyard, and the final testing and commissioning. Hydralift accepted full resp-onsibility for the performance of third-party equipment and the integration of those controls with its own proprietary distributed control system.

Drill floor and derrick

Top drives, iron roughnecks, and bridge crane pipe rackers serve both the main and auxiliary hoists. The main hoist also has a fixed diverter below the rotary table.

The derrick for a dual-activity arrangement is big and heavy with a large wind overturning moment. This has a detrimental effect on stability and variable load capacity. It also holds the key to improved performance and operating efficiency. GSF concluded early in the project that the ability to stand back fourbles of drill pipe and triples of casing would provide a substantial productivity boost. F&G factored this into their early hull sizing work, so the derrick meets GSF's operational objectives without compromising operating variable load.

Multiple tubular strings are required to drill an ultra deepwater well. These include drilling assemblies, drill pipe, landing strings, and a number of surface and intermediate casing strings. Most operators prefer a dedicated landing string for wellhead maintenance and casing string landing operations. The capability to make up casing strings off line is essential.

Drilling assemblies, landing strings, and pre-assembled casing strings are made up in the derrick during the course of a drilling program. The set back is therefore sized to accommodate 30,000 ft of drill pipe in sizes from 3 1/2-in. to 6 5/8-in., a string of either 13 3/8-in. or 9 5/8-in. casing, as well as the dedicated landing string. Maximum weight capacity in the set back will be 1,000 tons.

Motion compensation

There are three ways to compensate for heave motion of the drilling rig (four counting bumper subs). The crown mounted compensator and the drill string compensator both are proven products. Their disadvantages are large weight at the top of the derrick in the first case and a requirement for increased derrick height in the second. Both therefore have a detrimental effect on stability. The alternative is an active heave motion compensating drawworks. GSF selected this approach because the combination of moderate environment and large-vessel displacement means heave motions will typically be small.

Pipe handling

Two activity centers in one derrick put a great demand on the pipe-handling systems. They must be simple and reliable, yet have the flexibility to work together so the workflow is not held up unnecessarily. They must be arranged to service either hoist to provide a level of redundancy not found on single hoist rigs. They must have a control system sophisticated enough to prevent interference between any of the equipment components. Above all, they must ensure that personnel working on the drill floor are not exposed to risk of injury. The solution chosen is two bridge-crane rackers, each capable of servicing either hoist.

Mud systems

Several factors combine to demand much greater storage volumes on an ultra deepwater unit. The riser volume in 7,500 ft water depth is nearly 3,000 bbl. Most drilling programs call for two mud systems, which must be kept conditioned simultaneously in the active pits, particularly if the program calls for batch drilling.

Shallow water flows call for large volumes of "disposable" weighted mud to be available. To meet this demand, the Development Driller has 7,000 bbl of active mud pits at main deck level (twice the combined volume of 7,500 ft of riser and the volume of a hole 30,000 ft below the mud line) and a further 12,000 bbl of reserve capacity in the columns.

To take full advantage of this capacity, the system must be configured with hot swap flexibility. Hot swap means that the drilling program can change from one mud system to another immediately, without contaminating either mud. To achieve this, each system can be fully isolated from the other, including dedicated additive mixing facilities.


The authors wish to thank GlobalSantaFe and Friede & Goldman for supporting the publication of this paper.

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