STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 813
As drill bits, drill collars and drill stem subs are discussed elsewhere this
section regards shock absorbers, jars, underreamers, and stabilizers.
Shock Absorbers
Extreme vertical vibration throughout the drill string are caused by hard,
broken or changing formations, and the drilling bit chafing against the bottom
formation as it rotates.
In shallow wells, the drill string transmits the vibration oscillations all the
way to the crown block of the drilling rig. The affect can be devastating as welds
fail, seams split and drill string connections break down under the accelerated
fatigue caused by the vibrations. In deep holes, these vibrations are rarely
noticed due to elasticity and self-dampening effect of the long drill string.
Unfortunately, the danger of fatigw still goes on and has resulted in many
fishing operations.
The drill string vibration dampeners are used to absorb and transfer the shock
of drilling to the drill collars where it can be borne without damaging or
destroying other drill string equipment. Their construction and design vary with
each manufacturer. To effectively absorb the vibrations induced by the drill bit,
an element with a soft spring action and good dampening characteristics is
required. There are six basic spring elements used: (1) vulcanized elastometer,
(2) elastomeric element, (3) steel wool, (4) spring steel, (5) Belleville steel springs,
(6) gas compression.
Types of Shock Absorbers. There are eight commonly used commercial
shock absorbers.
Drilco Rubber vpe. See Figure 4-169 and Table 4-102 [56]. Shock is absorbed
by an elastometer situated between the inner and outer barrels. This shock
absorbing element is vulcanized to the barrels. The torque has to be transmitted
from the outer into the inner barrel. This tool is able to absorb shocks in axial
or in radial directions. There is no need to absorb shocks in the torque because
the drill string itself acts like a very good shock absorber so the critical shocks
are in axial directions. These tools cannot be used at temperatures above 200°F.
Though they produce a small stroke the dampening effect is good [56].
Christensen Shock-Eze. See Figure 4-170 [57]. A double-action vibration and
shock absorber employing Belleville spring elements are immersed in oil.
The tool features a spline assembly that transmits high torque loads to the bit
through its outer tube, while the inner assembly absorbs vibration through a series
of steeldisc springs. The spring system works in both suspension and compression.
The high shock-absorbing capabilities of this tool are attained by compression
of the stack of springs within a stroke of five inches. The alternating action of
the patented spring arrangement provides a wide working range, under all
possible conditions of thrust and mud pressure drop.
Placement of Shock Absorber in Drill String. Many operators have their own
way of placing shock absorbers in the drill string (see Figure 4-171) [57]. In
general, the optimum shock absorbing effect is obtained by running the tool
as near to the bit as possible. With no deviation expected, the tool should be
installed immediately above the bit stabilizer as shown in Figure 4-171C.
In holes with slight deviation problems, the shock absorber could be run on
top of the first or second string stabilizer. For situations where there are severe