GEAR-DRIVEN DRAW WORKS HAVE HIGH RELIABILITY

Peter Heinrichs
Wirth GmbH
Erkelenz, Germany

Gear-driven draw works have several advantages over conventional chain-driven draw works.

Gear-driven draw works have a longer life and working availability (reliability) than chain-driven units, and these factors lead to lower operating costs because of fewer rig shut downs for repairs.

The gear shift allows higher speeds and a better adaptation to the performance curve for greater operational efficiency. Additionally, more variable speed selection is possible with direct shifting from 1st to 4th speed.

On failure of a gear shift, operation can be continued, with reduced performance, with the autodriller which is integrated directly into the gear shift. A special gearing design prevents overload at the hook. (In case of incorrect shifting, the system automatically shifts back to 1st speed.)

Additionally, the gear-driven unit makes less noise and vibration and has a compact construction for reduced weight.

The oldest gear-driven draw works have been in use without any gear failure for 27 years, and other gear-driven draw works have worked for many years with no failure.

KTB

The 4,000 hp gear-driven draw works used by Ultratief-Bohrge-sellschaft for the Kontinentales Tief-bohrprogramm (KTB) project in Windischeschenbach, Germany, have been in operation since 1990. These draw works have made nearly 500 round trips to depths of about 9,000 m. Many of the trips were in the deeper third of the well; the loads were equivalent to the loads on a conventional draw works on more than thirty 5,000-m wells. Despite this work load, these prototype draw works on the KTB well had a rate of availability of more than 99%. Comparable chain draw works without modifications are not as reliable.

CHAIN DRAW WORKS

Chain draw works of 500-3,000 hp have been built by various manufacturers worldwide for more than 40 years. The simple, solid, inexpensive chain design has several pluses. Chain handling and maintenance are simple. The chain can bridge large distances between the motors and draw works, and chains can tolerate slight misalignments. Because of standardization, chains are inexpensive.

More attention must be paid to these two last arguments, however. For higher loads, larger chains ire needed, and these larger chains are more sensitive to misalignment. The advantage of the chain as an inexpensive component becomes less important taking into account the long life of the draw works. During its operational life, various chain exchanges may be necessary at certain intervals because of the lengths required and not because of fatigue. (Most chains have high fatigue strength.) Gears can be designed with high fatigue strength without any problems.

GEAR-DRIVEN DRAW WORKS

The drum base frame holds the drum support, drum shaft, brake cylinder fixture, eddy-current brake, and compound gear (Fig. 1) (163109 bytes). The drivers are 740-kw dc motors. In the gear shift, there are three hydraulic disk clutches.

The compound gear output shaft transmits power positively onto the draw works drum. The draw works drum has a Le-Bus system and for normal operation uses only three rope layers. The draw works are equipped with a disk brake. A positive-locking toothed clutch for the eddy-current brake is mounted at the second drum shaft end.

The draw works can be equipped with an autodriller, which permits operation of the drilling equipment (sensitively controlled) with a speed range of 0-42 m/hr for lowering operations. The autodriller is mounted on the drive carriage at both gear shifts. The autodriller can lift a string of up to 250 tons to pull out of the hole (for example, during emergency operations such as a failure of the dc motors). The draw works and autodriller are controlled remotely from the driller's stand.

SAFETY

The appropriate speed corresponding to the suspension load is selected by remote control. The 1st speed is shifted via spring force for safety (to still allow shifting in case of shifting pressure failure or current failure) and then is hydraulically eased.

In the other speeds, the suspension load is controlled via a pressure switch at the driller's stand. In case of overload in one of these speeds, this pressure switch actuates the respective solenoid valve for the speed selection, so that 1st speed is automatically shifted, and simultaneously the brake is actuated. This system ensures that the load is kept under control.

Speed selection is only possible during stoppage. The individual gears are kept synchronized by hydraulic pressure and pressure oil shifted disk clutches. The control console indicates the response of the pressure switch to an overload.

The autodriller also takes control of the load by a safety connection, and the speed can be adjusted at the operation cabin. The drillstring can be pulled when the autodriller is connected.

The draw works are equipped with a stop brake actuated by spring force. This brake is hydraulically eased during each starting procedure and on connection of the autodriller. The stop brake is automatically actuated by spring force (fail-safe unit) after each stopping procedure or a current supply failure.

In case of a main failure, the eddy-current brake has a battery back-up system for a short period of emergency operation.