For a better understanding and evaluation of shock absorber performance, detailed data upon their relevant physical parameters, such as spring characteristics and energy dissipation properties, have been measured in a test jig. Down hole tests where well defined sinusoidal vibrations are generated in a 1000 m vertical drill string means of a down hole exciter were also performed. The drill string response above the shock absorber is measured by a hard wired MWD tool giving high frequency data on accelerations, WOB, torque and pressures. Examples of test results for different pressures. Examples of test results for different types of absorbers are presented and discussed for both laboratory and downhole tests.
Drill string shock absorbers have found increased use in drilling as they are claimed to reduce drill string vibrations and to increase penetration rate. Especially in offshore drilling penetration rate. Especially in offshore drilling operations this might give considerable economical benefits. Several papers have discussed drill string vibrations and the effects upon these when a shock absorber is used in the drill string. It is generally accepted that excessive drill string vibrations are detrimental to economical drilling, mainly due to reduced rate of penetration and to increased equipment wear. The penetration and to increased equipment wear. The main function of the shock absorber is accordingly to reduce vibrations in the drill string, and its performance may be measured as the degree to which performance may be measured as the degree to which it is able to do this under actual drilling conditions.
Vibrations are mainly generated at the drill bit by the interaction between the formation drilled and the bit. Since the drill bit is coupled to the drill string, its behaviour will to a large extent depend upon the vibration properties of the drill string, such as resonant frequencies, damping properties etc. Changing these properties, for properties etc. Changing these properties, for instance by mounting a shock absorber in the drill string, may considerably change the vibrations generated. There are thus two possible mechanisms whereby a shock absorber may reduce drill string vibrations.