Next-generation drilling equipment pushes back water depth barrier

Dean E. Gaddy
Drilling Editor

Revolutionary and evolutionary improvements in drilling equipment are pushing back water-depth barriers.

With this equipment, drillers will be able to meet the operational requirements needed to hoist and brake heavy loads, ensure efficient mud circulation hydraulics, and provide redundancies for essential operations in ultra-deep water.

The industry's water-depth drilling capability will soon surpass 7,500 ft with the delivery of 21 newly built floating rigs now under construction (Table 1 [31,370 bytes]). These rigs are being equipped with heavy-duty drawworks, top drives, rotary tables, and mud pumps needed to fulfill current lease obligations.

According to Shell Deepwater Development Inc., of the 5,298 leases available from 6,000 to 10,000 ft of water, 889 have been sold as of January 1998. Back in September 1996, there were only 320 leases sold in waters deeper than 2,950 ft (U.S. Minerals Management Service).

With drilling-equipment technologies now at the point where operators can explore and develop plays in water depths approaching 10,000 ft, operators will be able to take the next step in deepwater exploration.

5,000 hp drawworks

To meet the challenge of hoisting riser loads in excess of 7,000 ft, a next-generation drawworks has been developed by Continental Emsco. The Electrohoist V is designed to lift a 2 million lb hook load with a 12 line configuration (Table 2 [4,615 bytes]), surpassing the capabilities of conventional drawworks.

Six of these drawworks will be installed on five ultra-deep water floating rigs: three to Falcon Drilling, two to Transocean Offshore Inc., and one to Diamond Offshore Inc.

The standard configuration for this skid-mounted drawworks consists of four 1,250 hp, dc-traction (or ac) motors, providing a total of 5,000 hp to the unit (Fig. 1 [115,221 bytes]). Two motors are mounted on each side of the drawworks, forming mirror copies of each other.

For comparison, the next-largest conventional drawworks has a total of 3,000 hp, 40% less power than the Electrohoist V. This chain-driven, four-speed drawworks is capable of pulling 1.5 million lb with 12 lines and is the standard workhorse for deep drilling rigs. The motors for this drawworks are mounted behind the unit.

According to Ken Kondo, manager of engineering for Continental Emsco, other than power, the most important difference between the 5,000 hp and 3,000 hp models is that the former has redundancy integrally built into the drawworks (patent pending).

On each side of the Electrohoist V, there are three shafts. One shaft connects the main drum to the gearbox, and two shafts connect the gearbox to each motor (Fig. 1 ). Each side can work independently or together. In addition, if a motor goes off-line while drilling or pulling a heavy riser load, the remaining three motors will be able to run both sides, with two motors providing power to one side, and one motor to the other.

Thus, there are four or more possible operational modes:

All four motors can operate together with both sides of the drawworks working in tandem.
Three motors can work together with both sides of the drawworks working in tandem-one motor is down.
Two motors can work together on one side of the drawworks while the other side is nonoperational-two motors are down.
One motor on each side of the drawworks can operate both sides of the drawworks-two motors are down on opposite ends.

The drawworks can be modified to add additional motors. Fig. 2 [94,453 bytes] shows an arrangement where another motor is mounted on the left side of the drawworks (Motor E), above Motor B. The motor is connected to the gearbox with an individual shaft and may remain as an empty slot until needed.

One of the unique features of the Electrohoist V is that if necessary, most drilling operations can be performed with only one side of the drawworks in operation. In addition, a motor can be pulled from the right side and placed in an empty slot on the left side. With three motors on one side, a total of 3,750 hp is available to pull riser.

According to Jerry Van Pelt, vice-president of sales for Continental Emsco, not only does this allow routine maintenance to be performed on one side of the drawworks without significantly decreasing total hoisting capacity, it provides a very high degree of redundancy above and beyond what is already designed into the unit.

Innovations in braking

Conventional braking systems use drum-disc or band-disc brakes in combination with eddie-current brakes to slow down and park a load. According to Kondo, however, the conventional braking system has been supplanted on the Electrohoist V.

This drawworks utilizes three different types of brakes:

Regenerative brakes
Pneumatic, computer-controlled, water-cooled disc brakes
Drum disc brakes.

The primary, regenerative braking system works by rotating the motors in reverse, resulting in drag. However, because the regenerative brakes cannot continually hold a load, they are used in combination with the pneumatic water-cooled disc brakes to park and hold.

In comparison with conventional braking systems, the regenerative brakes replace the more conventional eddie-current brakes. In addition, because each motor is coupled into a transmission by itself, each serves as a singular, independent brake, or conversely, they all can work together.

The secondary, pneumatic water-cooled disc brake system uses multiplate friction discs to progressively engage the friction element sections under the supervisory control of a computer. The computer reads the driller's control inputs and engages additional friction plate sections as required to slow or stop the load. These brakes operate independently of the regenerative brakes.

The pneumatic, water-cooled disc brakes are sized to provide redundancy to the regenerative brakes. In addition, a drum disc-brake system is also available. This system can be added to the other two or can replace the pneumatic water-cooled disc brakes.

Drum disc brakes are similar to conventional braking systems, except discs are exclusively used instead of bands. This technology was developed in the 1980s and has evolved to its present accepted form.

It consists of two major components, an operating system and an emergency system. Multiple sets of calipers provide the braking force for all drilling operations including parking. Multiple emergency calipers provide fail-safe braking in the event of complete rig-power failure.

Two, hard-surfaced, water-cooled discs replace the conventional drawworks' brake rims. The number of calipers on each disc is determined by the drawworks' maximum rated line-pull capacity.

Each side of the system is actuated by a separate control valve and is capable of holding the full-rated load. This provides twice the required braking capacity, but also retains full capacity in the event one side should become inoperative.

The fail-safe emergency system consists of, spring-actuated, hydraulic pressure-released caliper for each disc. The emergency system can be actuated by the operator, or is automatically actuated should hydraulic pressure or air failure occur.

According to Kondo, the braking systems are designed to provide redundancy in case any of the brakes fail and can be matched to fit the operator's needs.