DEVELOPMENT OF MORE POWERFUL TOP DRIVES PROGRESSES

Feb. 17, 1992
Keith Rappold Drilling Editor The increase in drilling of deep extended reach and horizontal wells has stimulated a number of design improvements in overhead rotary system technology. Overhead rotary systems, or top drives, are being developed with larger, more powerful motors capable of producing output torque up to 60,000 ft-lb. The load ratings of top drive systems have increased to improve rig floor safety and to allow running of larger casing strings.
Keith Rappold
Drilling Editor

The increase in drilling of deep extended reach and horizontal wells has stimulated a number of design improvements in overhead rotary system technology.

Overhead rotary systems, or top drives, are being developed with larger, more powerful motors capable of producing output torque up to 60,000 ft-lb. The load ratings of top drive systems have increased to improve rig floor safety and to allow running of larger casing strings.

Overhead rotary drilling systems are referred to as derrick drillers, top drives, or overhead power swivels, depending on the manufacturer. All of the systems perform similar functions and have numerous advantages over rig floor rotary drilling with a kelly:

  • Because these systems can drill with 90-ft stands, the number of connections is reduced. Crew safety improves because fewer connections decrease the chance of accidents on the drill floor.

  • Another improvement in safety is the ability to control kicks quickly while the drill pipe is tripped out of the hole. The driller can set the slips and come back down with the assembly to screw into the pipe. A remotely operated valve then closes the well, and the driller has control of the well in 20-30 sec.

  • Because the driller can rotate in and out the hole, sticking the pipe is minimized. Reaming out doglegs is easier with 90-ft stands than with single joints. Forward reaming while tripping in the hole helps eliminate bridges and tight spots. With a rotary table, picking up the swivel and kelly is time consuming and therefore sometimes neglected.

  • Overhead drive systems have found a niche in deep wells with long extended reach or horizontal sections. Directional drilling has improved from the use of these systems because circulation in 90-ft stands reduces downhole motor orientation activities.

CURRENT MARKET

The market for overhead rotary drilling systems has remained healthy because of the increase in international offshore drilling markets and the financial restructuring of several offshore drilling contractors. According to a Journal update of a study by Howard, Weil & Associates Inc. the top four suppliers have installed top drives on 419 rigs worldwide. There are still about 350 offshore rigs without top drive systems that are candidates for retrofitting. This does not include the much larger market of small platform and land rigs.

To date, Varco BJ Drilling Systems is the largest manufacturer of top drive systems and has sold 347 units, all of which are electric drive. Fifteen of these are the two original models made by Bowen Oil Tools, and eight are BJ side drive models. Varco shipped 60 top drive units in 1991 and plans to ship about 60 top drive systems in 1992. Under current market conditions, Varco expects the sales rate to continue at the current pace.

Varco builds its top drive systems in Orange, Calif., and services them in a number of locations worldwide. The majority of Varco's top drive units work on fixed derrick rigs in the Gulf of Mexico and the North Sea. It currently has 6 units operating in Venezuela, 23 in West Africa, 4 in China, and several in Brazil. By June, Varco expects to have installed top drive units on at least 9 land rigs working in Colombia. At present there is one unit drilling in Colombia.

Varco sees the Far East, West Africa, South America, and the former Soviet Union as good opportunities for top drive systems in the near future. The North Sea has only a handful of rigs without top drive systems, and the Gulf of Mexico drilling industry is not growing.

Maritime Hydraulics AS, a Norwegian company, has sold 23 hydraulic-driven top drives and 19 electric-driven top drives. The majority of its derrick drilling systems have been installed on rigs operating in the North Sea, with a few in the Far East and Russia.

Maritime Hydraulics expects to de-Ever 15 top drive units by the end of 1992, and it plans to expand its business in the Far East.

National-Oilwell has sold more than 15 overhead power swivels since entering the market 1-1/2 years ago. It expects to improve this pace slightly in 1992-93. National-Oilwell's power swivels operate on rigs worldwide, with about one third of them sold to land rigs.

At present, National-Oilwell builds its power swivels in Garland, Tex., and has the capability of servicing or upgrading them at many of its worldwide supply facilities.

A number of other companies have manufactured various types of top drive units. GEC Alsthom ACB has sold 15 top drive units (as of Mar. 31, 1991). Canadian Rig Ltd. has installed five small, electric top drive systems: one in France, one in China, and three in Canada. Triten Corp. has produced a lightweight hydraulic top drive drilling system, and West Coast International By, a Dutch company, has manufactured a short, ball-piston hydraulic top drive system.

SYSTEM DEVELOPMENTS

A drilling contractor must consider a number of factors before installing an overhead rotary drilling system:

  • Loading on the derrick during top drive operations may require additional bracing and structural supports to the derrick.

  • The position of the track within the derrick must allow proper alignment of the drilling system over the full travel length. This includes checking the full path for any interference with service loops and equipment lines.

  • There must be suitable clearance between the bottom of the crown block and the top of the traveling block with the top drive raised to its highest position when making connections.

Typical top drive units cost about $1-1.3 million with a total installation time of about 3 weeks. Most manufacturers can install the top drive unit and associated support loops in 3-5 days, with the bulk of the time spent on adding derrick extensions and support bracing.

One of the biggest problems with retrofitting rigs with top drives is the need for extensive derrick modifications, especially on typical 147-ft derricks. The derrick modifications often last 2-3 weeks and can add up to $250,000 to the cost of installation. Three weeks of rig down time and the added expense of derrick work have forced some drilling contractors to delay the installation of top drives on rigs.

Typical top drive systems add about 24 ft to the traveling assembly. This has encouraged the development of shorter top drives. The use of integrated swivels cuts a few feet off the standard design.

Fig. 1 is a schematic of the general arrangement of Varco's fully integrated top drive system (IDS). The entire setup can be installed without the hook by short-coupling the swivel bail directly to the block. This can give up to a 9-ft clearance on a 136-ft derrick or mast without modifications.

Two of the IDS units have been sold to date: one for a platform rig in Trinidad and the other for a jack up in Egypt.

Varco's IDS system is the beginning of a trend in the development of smaller, less expensive top drives that are well suited to installation in the masts of small land rigs. Varco estimates that the small rig market may reach three times the level of the current market for large top drives.

National-Oilwell and Maritime Hydraulics have both developed integrated swivel designs for their drilling systems. Their respective designs eliminate swivel-stem fatigue problems associated with separate-swivel top drives.

The first top drives were 500-ton units with a 1,000 hp electric motor capable of generating 27,000 ft-lb of torque. As operators became more efficient with the use of top drives to drill in triples, higher torque models were needed for drilling longer, high-angle holes. Varco upgraded the torque rating on these units first by using a 1,250-hp General Electric high-torque motor (TDS-3 model top drives) and later by use of a manual two-speed transmission. The two-speed TDS-4 can reach 40,000-50,000 ft-lb of torque. Another model, the TDS-5, uses a larger gear ratio on a one-speed transmission to achieve about 40,000 ft-lb.

Varco then developed the two-motor TDS-6. With two 1,230-hp motors, this top drive is capable of 60,000 ft-lb of torque, which is higher than the torque rating of most drill pipe. In addition to greater drilling capabilities, a benefit of a two-motor system is the ability to continue suitable drilling rates with one 1,250-hp motor if the other motor breaks down.

The standard TDS-3 was sold widely in the North Sea. Rather than replace the entire top drive to obtain the higher torque, many contractors have upgraded from a single to a double transmission and replaced the original motor with the high-torque motor. The upgrade eliminates duplication of many of the service loops and electrical systems. Because the systems have the same footprint, they can be interchanged quite easily. This takes only a few days of rig time, and most work occurs while the rigs are in transit between locations or wells.

National-Oilwell's power-swivel design originated from a complete rig automation program begun in the 1980s. It currently manufactures two models of power swivel. The first power swivel has a one-speed transmission, has a 500-ton rating for both the hoisting and drilling load paths, and can reach 40,000 ft-lb of torque. The second model is similar but uses a two-speed transmission to achieve 40,000-50,000 ft-lb of torque.

An optional integral power-swivel block can be added to increase casing load capacity.

The load path design on the National-Oilwell power swivels is unique (Fig. 2). The casing and tripping loads are carried separately from the drilling loads. This path avoids the swivel stem, swivel bearings, and APT rotary shouldered connections, significantly reducing the ton mileage on them. A recent top drive safety report by the U.K. Department of Energy cited the dual load path as a safe, high-quality design.

Maritime Hydraulics currently manufactures four top drive systems: an electric unit with one motor and a one or two-speed transmission, an electric unit with two motors and a one-speed transmission, a high-torque hydraulic unit, and an ultra high-torque hydraulic unit.

Although Maritime Hydraulics builds electric and hydraulic systems, it favors the hydraulic drives because they are shorter, lighter, and better balanced. The hydraulic units have better redundancy; four hydraulic pumps and four hydraulic motors can be independently isolated for maintenance and repair. Other benefits include the lack of high voltage cables looped in the derrick and the elimination of electrical sparks.

Maritime Hydraulics' electric derrick drilling motors have 500 or 650-ton ratings, can achieve speeds up to 197 rpm, and can produce up to 36,000 ft-lb of torque. Its ultra high-torque system has a 650-ton rating, can reach speeds of 230 rpm, and can produce torque up to 47,000 ft-lb.

One of its design features includes a load bearing shaft forged in one piece, which avoids numerous threaded connections subject to metal fatigue. The company has geared much of its development to reducing the length of its systems to minimize weight and cost and to eliminate the need for derrick extensions.

All of the top drive manufacturers have started incorporating APT Specification 8C into their designs where practicable. This specification on drilling and hoisting equipment was issued in January 1990 to replace the eleventh edition of Specification 8A issued in 1985.

This specification calls for frequent, high levels of inspection. Adherence to 8C specifications has increased the rotary drive costs, but most companies are making efforts to use these standards.

Copyright 1992 Oil & Gas Journal. All Rights Reserved.