A drillstring for drilling oil or gas wells is one of the most slender structures known, and hence is liable to experience vibration and stability problems. The larger upper part is held in tension, so one may genuinely refer to the structure as a string rather than as a beam, but the lower part is usually kept in compression in order to enhance the penetration of the bit. This lower part is composed of a series of thick-walled tubes (collars) and coarsely grooved elements with a larger diameter (stabilizers). The string may perform axial, torsional and bending vibrations, which are coupled. Especially bending vibrations of the lower part are hard to observe at surface, but down-hole measurements have shown their importance .
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