Auxiliary brake ( drilling rig ) – Readyzone

Auxiliary brake

During tripping-out operations, the drill pipe is removed in stands ( ̴93 ft, depending on size of derrick etc. ) ion such a way that the pipe is decelerated during the last 5-10 ft before being arrested in the rotary table with the slips. Thus, during tripping out operations, the main brakes are not required to absorb excessive energy.

During tripping-in ( or RIH ) operations without the aid of an auxiliary brake, the main brake must be applied continuously in order to lower the load at a controlled speed. As each stand of pipe is added, the load becomes greater. To use the band brake for this almost continuous operation would quickly cause fading and overheating. In practice, the main brakes are assisted by an auxiliary brake which is only engaged during tripping-in operations. The auxiliary brake provides a continuous retarding torque to control speed of lowering. The main brake is used for final stopping of each stand before being held in rotary table by the slips.

The auxiliary brake is cradle-mounted on the draw works frame and is connected to the hoisting drum shaft by means of a sliding coupling. Two types of auxiliary brakes are in use: hydrodynamic and eddy current.

Draw - works

Fig. 3 – Typical water cooling system for draw works and eddy current brake.

Hydrodynamic ( or Hydromatic ) auxiliary brake

The hydrodynamic brake is built somewhat like a centrifugal pump using water to provide a cushioning effect, rather than pumping and producing pressure. The brake shaft connect with the draw works drum shaft through a sliding coupling. A braking effect is produced by the resistance to agitation of water circulated between the veined pockets of the rotor and stator elements. The mechanical energy of the rotor is transferred to water in the form of heat energy. The magnitude of mechanical energy absorbed and, in turn, braking effect produced is proportional to the velocity of water in the brake chamber. Also, the braking effect is dependent on the water level inside the brake chamber, with the greatest braking effect being possible when the chamber is completely full of water. Braking effect can be varied by changing the water level inside the chamber.

The hoisting drum friction braking cooling water is discharged through a passageway in the auxiliary brake shaft. A separate supply branch provides continuous flow of cool water to the auxiliary brake thereby preventing water in the chamber from overheating.

Draw - Works

Fig. 4 – Schematic of auxillary brake; (a) hydromatic brake; (b) eddy brake. (Courtesy of Oil and Gas Journal)

Eddy current auxiliary brake

The eddy current brake uses magnetic forces to slow the rotation of the hoisting drum. It consist of an iron rotor connected to the brake shaft, the rotor being surrounded by a stationary member stator which provides a controlled variable magnetic field ( Figure 4 b ). The magnetic field of the stator is provided by coils excited by an outside source of d.c. electricity. The stator induces a magnetic force in the rotor which opposes the rotor motion and, in turn, provides a braking effect. The magnitude of the braking effect can be varied by varying the intensity of the electromagnetic field in the stator.

The induced magnetic field also produced, within the rotor, eddy currents which, in turn, generate heat. This heat is removed by a continuous flow of cool water provided by the brake cooling system (shown in Figure 3).

Comparison of hydromatic and eddy current brake

With the hydromatic brake the braking effect increases exponentially with speed of rotation. Thus, in lowering stands of pipe, the braking effect increases as the pipe tries to drop faster. As the band brake is applied to slow the stand near the rotary table, the hydromatic brake has less effect, until all braking is being done by the band brake. Hydromatic brakes are generally used on rigs where electrical power supply is limited. Where electrical power is readily available, as on a diesel-electric rig, it is normal to use an eddy current brake, which has the advantage that the braking effet is dependent upon the intensity of the electromagnetic field and is controlled by the driller through a small operating lever,. Thus, the driller has more precise control of the braking effect.

Related Links:-

*  Derrick             *  Drill collar            *  Kelly Drive

*  Drill pipe          *  Rotary table         *  Swivel

*  Tool joint          *  Draw works         *  Drum shaft group

*  Cat shaft and coring reel group       *  Hoisting drum


Reference : H. Rabia, Oil Well Drilling engineering-Principles and Practice


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