By C. Fraser, Hitec Products Drilling; G. White, Rowan Drilling; and E. DePeuter, LeTourneau Technologies
The top drive has become one of the biggest breakthroughs in drilling technology since it was first commercialised in the 1970s. Its advent brought vast time savings through drillers being able to drill with 90-ft stands; elimination of connections and their impact on circulation and the wellbore; being able to rotate and circulate when tripping out, providing the ability to get through tight spots, and much more.
Since then, apart from the switch from DC motors to AC motors, its basic functional design has remained the same, that is, one or two large motors connected to a gearbox, driving a main shaft. A new type of top drive has been developed with possibilities for significant drilling cost savings due to improvements in reliability and easier maintenance.
A major drilling contractor who had significant experience using top drives on their rig fleet conducted an analysis of where most of their rig downtime occurred. They found that 40% of all their top drive failures were related to gearbox issues (gears, bearings and seals). Another significant failure was with motors, particularly on smaller top drives in their land rig fleet.
These types of failures had been expensive for the drilling contractor and the operator with significant frustration, lost drilling time and lost revenue. Problems were exacerbated by spare parts not being in stock and service technicians not being readily available. This led to the drilling contractor deciding to help develop a new direct-drive top drive with less parts, no gearbox and improved motor.
Figure 1: Early top drives used a single-speed transmission gearbox and a single DC motor. This provided enough continuous torque for the wells of the 1970s and early ’80s but became inadequate as wells got deeper and with the advent of horizontal drilling.
These systems have been field-trialed extensively for a number of years onshore and offshore, with significant success by this drilling contractor and others in the United States, Middle East, Russia, North Africa and Australia. The drilling contractor is in the process of replacing all top drives on its complete fleet of existing and newbuild rigs, and the first DNV-certified 750-ton version has been retrofitted on the Rowan Gorilla V jackup in the North Sea. After the rig went on location, the top drive performed satisfactorily with no downtime on the first well for TOTAL E&P.
Since the early 1980s, the top drive has replaced older methods of drilling that used a kelly. On a conventional top drive, one or two large motors are coupled to a gearbox and turn a hollow main shaft (or drive stem). The main shaft connects via a series of in-line blowout preventer valves and a pipe sub to the drill pipe. In drilling mode, mud is pumped through the shaft down the drillstring while the motor or motors turn the drive shaft via a gearbox.
The top drive is also used while tripping to make and break stands (typically 3 x 30-ft lengths of joined drill pipe). One of the biggest benefits of using a top drive is this ability to drill with stands, typically eliminating two-thirds of all connections. This saves time and reduces the chance of downhole problems. Other benefits of drilling with a top drive include:
• The possibility to backream, allowing full rotation and circulation while tripping and reducing stuck-pipe incidents.
• Improved well control since stabbing is instant and the well can be shut in at any position in the derrick.
• Increased safety due to one back-up tong being required and fewer pipe connections.
Most offshore and many onshore drilling rigs now drill with top drives. When the first top drives were introduced, the maximum continuous torque was typically in the range of 31,000 ft-lb with a single speed transmission gearbox and one single DC motor (Figure 1). That provided enough torque for the wells of the late ’70s and early 80s.
In the late ’80s, as drilling got deeper and more horizontal wells were drilled, a demand developed for 45,000 ft-lb top drives that used a higher-torque DC motor and a two-speed transmission (Figure 2). Since then, various manufacturers have provided more powerful units using two DC motors or one AC motor and up to 67,000 ft-lbs continuous torque, according to their published data.
In 2004, LeTourneau Ellis Williams Co (LEWCO) and Oilfield Electric Marine (OEM), encouraged by their sister company Rowan Drilling, assembled a team with the intention of designing the next generation of top drives. Those two companies are known today as LeTourneau Technologies Drilling Systems Inc (LTDSI).
When their engineers looked at the top drives available at that time, they found an opportunity for improvement. So they developed a new top drive with the potential to increase drilling performance and significantly reduce maintenance. One of the most dramatic of innovations was eliminating the maintenance-demanding gearbox. As described, this was driven by Rowan Drilling, which had experienced significant difficulty with a variety of top drives throughout their rig fleet.
DIRECT-DRIVE TOP DRIVE
Figure 2: In the late ’80s, new models used a higher-torque DC motor and a two-speed transmission to deliver more power.
The new direct-drive top drive comes in five models: 250 ton, 350 ton, 500 ton, 750 ton and 1000 ton. Unlike other top drives, the direct-drive top drive has no gearbox, which cuts down on maintenance. Almost all previous top drives on the market have either a two-speed gearbox or a single-speed gearbox. On this new top drive, the main component is a hollow-shaft, air-cooled AC motor. A pneumatic counterbalance system protects threads when disengaging from the pipe, and there is a pipe-handling system that features a dual-drive, 360° rotational capacity with elevator and link tilt.
The pipe-handling system has a “grab” assembly with a capacity of 90,000 ft-lbs, with air-over-hydraulic operation. The pipe-spinning and torquing is achieved through the use of the AC motor. The mud path is via the gooseneck and wash-pipe system through the main stem, which runs through the centre of the motor.
The main shaft/drive stem assembly can be removed without taking off the motor or rigging down, and there is a redundant speed-sensing capability. Both the hoisting capacity and drilling capacity are rated at 750 tons on the Rowan Gorilla V unit. The unit has continuous 3,000 hp from 150-350 RPMs. The system uses 57,000 ft-lbs static brake capacity. It has a 4-in. diameter mud passage at 7,500-psi working pressure. It has 15,000-psi air-actuated upper and manual-operated lower IBOPs. Also, there is an 8 5/8 regular RH pin rotary stem connection. An electrical service and air/hydraulic service loop is required for carrying power and control signals to and from the top drive. The service loops are terminated at junction boxes at the derrick monkeyboard level, and cables are run down the outside of the derrick to a power house that contains the electrical drive systems.
Figure 3: The 750-ton direct-drive top drive has no gearbox.
Its main component is a hollow-shaft, air-cooled AC motor.
The approximate overall stack-up height is 29 ft, 5 in. with 132-in. links. The maximum width is 100 ft and the depth 58 ft. The weight is approximately 65,000 lbs, excluding the retract-dolly (Figure 3).
The direct-drive top drive is manufactured and tested in the United States in accordance with API 8C PSL 1. During initial design testing, the load test was witnessed and certified by ABS.
TOP DRIVE MOTOR
To achieve the higher torque and RPM without a gearbox, a 1,500-hp motor was designed with high-density windings and more copper and iron in the winding slots. This and the high-volume air cooling provided by two mounted blower motors help to withstand temperatures associated with continuous operation and increase efficiency and durability.
Special consideration is given to insulation, with additional insulation provided through the use of carefully controlled dipping processes (Figure 4). During initial testing, the motors were even immersed in water to ensure insulation was adequate. This all helps to deliver up to twice the peak torque as conventional AC motors, from 0 rpm through rated rpm.
The 750-ton direct-drive delivers 72,000 ft-lbs of continuous torque at 0-116 RPM and up to 100,000 ft-lbs of intermittent torque at 0-118 RPM. Mounted behind the motor is an integral disc brake system.
Much of the data was collected in the form of interviews with manufacturing and engineering staff during the manufacture of the Rowan Gorilla V top drive and interviews with the drilling contractor’s drill crew and onshore operational staff.
Figure 4: Additional insulation was provided on the
new top drive via carefully controlled dipping processes.
Visits were made by the authors during the installation and commissioning in the yard in Dundee, Scotland, and interviews were conducted with the installation engineers and drill crew. Aspects the authors wanted to explore were: how difficult it would be to change from one top drive to a new one; the drilling crew’s experiences with the previous top drive; the drill crew’s reaction to the new top drive; and what issues were faced during installation and commissioning.
Rowan Drilling had three jackups (Gorilla V, Gorilla VI and Gorilla VII) built between 1998 and 2002 that have been working in the North Sea almost solidly since then. All three have had issues with top drive failures, and major failures where there was extensive downtime were all related to gearbox gear failure.
On the Gorilla VII, built in 2002, the top drive has had to be laid down three times and gearbox components replaced each time. This also occurred at least twice on the other two rigs. On one rig alone, the drilling contractor reported this had cost the company in excess of $10 million in lost revenue and in excess of $1 million on spare parts and manpower hours. The drilling contractor also found it difficult to obtain spare parts locally.
The drilling contractor also previously experienced top drive gearbox failures on its US Gulf of Mexico jackups, as well as on its land rigs in the United States and Middle East. The failures were not concentrated on one manufacturer or one model of top drive.
Since the development of the new direct-drive top drive, 90 top drives have been ordered with 95% of these in the field around the world. Rowan Drilling is the biggest user, with approximately 21 500-ton units in operation and 14 750-ton units in the field.
In June 2007, Rowan decided to replace the top drives on their North Sea fleet and planned for installation during the next five-yearly yard inspection. Several surveys of the rig were conducted and data fed back to the drilling contractor and manufacturer’s engineering departments.
Special consideration was given to the interface of the new top drive with the existing retractable dolly and the mounting of a new power house just above the windwall on the outside of the derrick. Software engineering was also required to enable all the existing PLCs to communicate with one another, and the existing driller’s console and controls had to be adapted to fit the requirements of the new top drive.
A decision was also taken on where to mount the power control container, which housed the new required variable frequency drive (VFD) for the top drive. When installing an AC top drive, the existing DC-type SCR system cannot be used and has to be replaced by a VFD system. The existing SCR system was located in the hull of the jackup, approximately 500-m cable-running distance away. New power cables were required, so to save time and cost, mainly due to cable running, it was decided to opt for a VFD system mounted in a purged container. It was decided this container would be mounted above the wind wall on the outside of the derrick.
Extensive testing of the motor was conducted at the factory. Testing was done vertically with the top drive in a purpose-built test stand. A VFD was connected and power applied for 3.75 hours, and 72,000 ft-lbs of torque was applied. Temperature was monitored using sensors built into the motor.
Various other parameters were also monitored, including motor frame temperature, air temperature, bearing temperature, volts, amps, frequency, motor stator winding resistance, humidity and more. The top drive motor was tested and certified in accordance with ATEX Zone 1 for use in a hazardous area. DNV also performed a design verification and monitored the manufacturing and testing of the system.
INSTALLATION ON RIG
In August 2008, the rig went into the shipyard in Dundee, Scotland. The yard stay was a total of 59 days. A variety of other work was ongoing at the time of the top drive installation, such as spud can inspection, overhaul of BOP stack and installation of a rail-mounted access basket in the derrick. Twenty-five days were allowed and used for installing the top drive. Most of this time was used for installing the VFD house and running cable.
The existing rails and retract system were utilised. During the installation, an issue was found with the top drive alignment, which was attributed to wrongly sized dimensions on the adaptor plate. This resulted in the adaptor plate having to be removed from the top drive twice, then a third time to be modified onshore (Figure 5). The drilling contractor stated that dimensions of the existing structures had not been clearly defined and was an engineering issue related to them and not the manufacturer. Alignment was found to be accurate once this was modified. In the course of modifying the adaptor plate, it was felt that the strength may have been impacted, so a new one was ordered.
Training was conducted by a full-time instructor teacher for two days for all the drilling crew while in the yard.
Since this was the first such top drive installed in the North Sea, the decision was taken to have two units manufactured and have one standing by onshore for the first two wells. After the Gorilla V is proven, this second top drive is to be installed on the Gorilla VI.
Figure 5: The direct-drive top drive is installed on the existing retractable dolly on the Rowan Gorilla V jackup.
There was some expectation by the drilling contractor and operator that there would be limited downtime due to the top drive installation. After drilling the first 30-in. top hole section to 815 ft, a two-hour maintenance break was taken to thoroughly inspect the top drive. Nothing was reported, so drilling continued. The maximum torque they had seen at the time was is 25,000 ft-lbs, which is the value of the pre-set limit in order not to overtorque the pipe. The top drive had no reported downtime as of writing.
A small issue arose with the spin routine on the driller control console. This was software control system-related, and a workaround was available.
During the first well, a top drive electrical and mechanical technician was kept onboard for extra insurance.
This drilling contractor has had extensive experience with top drives over many years. They had identified that 40% of their top drive failures were related to the gearbox. In addition, they had similar experiences and motor failures on their land rigs as well. Due to downtime on the Rowan Gorilla V in the North Sea, they took the decision to replace it with a new direct-drive system.
There were challenges during installation, such as running cable, fitting a power house and having to modify the dolly. With training, the crew reported that they found the change-over easy, and no problems or downtime had been encountered on the first well drilled so far.
The drilling contractor believes it cost them in the range of $4 million to change to the direct-drive top drive, including the price of the hardware and installation.
Although this was a large cost, it was felt by the drilling contractor that it was worthwhile. Should performance continue to be good, the drilling contractor will replace the top drive on the sister rig working in the Norwegian sector of the North Sea.
This article is based on SPE/IADC 119402, a presentation at the 2009 SPE/IADC Drilling Conference, 17-19 March, Amsterdam.
Additional graphics and photos for this article can be found online at www.drillingcontractor.org.
Acknowledgements: Special thanks to R. Smith of Rowan Drilling in the UK for valuable input and allowing us to survey on the rig. Also special thanks to all the LeTourneau Technologies Drilling Systems engineers and technicians who perfected the design, installed the top drive and reduced the cost of drilling for both drilling contractor and operator.
“An Overview of Top-Drive Drilling System Applications and Experiences.” by G.I. Boyadjieff, Varco Intl. Inc, IADC/SPE 14716.
“Successful High Angle Drilling in the Statfjord Field,” by R.C. Willis and D.N. Willis, SPE 15465.