"Snap-Back" - A New Factor In Rig Safety Design
Authors: Eid Nabil M.A. 
Published: March 2010

If a driller Should slick the strung by pulling it into a keyseat while withdrawing it from the hole, or if sloughing formations or other conditions should cause the string to become suck during the drilling operation, immediate efforts should be made to free it.

The main procedure consists of working the string up and down as much as possible, using sufficient tensional and torsional stratus within safe operating practices.

Generally, rotary jais are installed in fishing strings to enable a Driller to strike heavy upward blows against the engaged fish to jar it loose from its stuck position.

All rotary jais a restraining mechanism in them which holds the telescopic elements of the tool in a closed position until sufficient upward pull is exerted to trip the restraining mechanism and allow the telescopic elements to more into their intended position (Fig. 1). In operation, the strain of the upward pull will stretch the drill pipe, and when the jar trips, the upward surge of the drill pipe in retrating to its normal length will cause it to strike a severe blow. Thus in rig design, in order to maintain an adequate factor of study, sufficient compression and tensile stresses should be allowed to account for the sudden application of weight (corresponding to severe blow), and the sudden release of the bit from a stalled position and consequent "snap-back" of the drill pipe.

To compensate for the force of heave of water on the drill string, a drill string compensator (DSC) is used. In offshore drilling, the DSC is used to reduce the tensile and compressive stresses on the drill due to the heave.

The effect of "snap-back" was studied in the DSC design. The results show that "snap-back" is a major factor in drilling technology and specifically ill offshore drilling.

Discussion of the Problem

When a body held in position by elastic constraints is displaced from its equilibrium position by the application of an external force and then released, it commences to vibrate. Work is done by the external force in producing the initial displacement against the internal elastic forces which resist deformation. This work is stored up as elastic or strain energy in the constraints, so when the external force is removed the internal elastic forces tend to restore the body to its equilibrium position.

Neglecting for the moment any resistance to the motion of the vibrating body, the whole of this elastic or strain energy is converted into kinetic energy at the instant the body reaches its original equilibrium position. As a result, the motion of the body continues until the whole of the kinetic energy is absorbed in doing work against the internal elastic forces and the energy in the system is once more strain energy. Again the body begins to return to the equilibrium position and the vibration is released indefinitely, i.e. free vibration.

In practice, however, energy possessed by the system is gradually dissipated in overcoming internal and external resistances to the motion, and the body finally comes to rest in its original equilibrium position, i.e. damped vibration.