Drillstring Vibrations: A New Generation Mechanism and Control Strategies

Abstract

Abstract Damaging drillstring vibration has traditionally been associated with resonant behavior of the BHA. As a result modeling has concentrated on harmonic analysis of the drillstring. This has led to strategies for controlling vibrations based on 'tuning' the rotary speed and varying weight-on-bit. Using field and laboratory data we demonstrate that, in fact, once initiated, damaging transverse vibrations persist over a wide range of rotary speeds and that they depend on lithology, BHA design and hole gauge. We also show that downhole vibration can lead to increased torque and impaired ROP. This paper proposes a new mechanism for generating damaging vibration in which the transverse motion of the drillstring couples to the rotary motion during contact with the borewall. This analysis has led to some novel solutions to vibration problems. We present in this paper field examples of downhole shock measurements together with results from a full scale experimental investigation of the transverse vibration of rotating drill collars to support these conclusions. Introduction Severe vibration of the BHA is common while drilling. Apart from the damaging effects of high shock levels, the transverse motion of the drillstring has several other consequences:Backwards whirl will produce very high bending stresses, particularly at the joint between a collar and stabilizer (ref. 1).Impacts with the borehole wall will tend to cause changes in gauge of either the hole or the drillstring components, bit and stabilizers. This can cause problems both with directional control and logging.Impacts of the drillstring with the borehole can lead to an increase in the surface torque required to drill and may adversely affect rates of penetration. In order to effectively reduce damaging transverse vibrations of the Bottom Hole Assembly (BHA) it is necessary to understand the mechanisms by which they are generated. The understanding of transverse drillstring vibration current in the literature is based on harmonic analysis of the BHA and is primarily concerned with the identification of resonant responses of the system, which are considered to be the primary source of vibrations. Using laboratory data from a full scale experiment with an instrumented rotating drill collar in conjunction with downhole vibration measurements it has been shown that resonant phenomena alone do not account for the drillstring behavior. In particular models based oil the harmonic behavior of the drillstring do not explain the response of the drillstring to variations in rotary speed, the effect of lithology, the occurrence of backwards whirl, or the magnitude of the shocks observed. The purpose of this paper is to propose an alternative mechanism which suggests new methods of vibration control. It will be demonstrated that downhole vibrations are highly nonlinear phenomena which makes prediction difficult, and unreliable. P. 353^