Failure of oilwell drillstrings is very costly in terms of money and time. There are many reasons for drillstring failure, such as vibration, fatigue, and buckling. Stick-slip vibration has received considerable attention in recent years with increasing use of polycrystalline diamond compact (PDC) bits in harder formations, and has motivated extensive research on this type of drillstring vibration. This paper addresses the advantages of a linear quadratic regulator (LQR) controller, compared to a spring-damper isolator, for stick-slip and bit-bounce mitigation in an oilwell drillstring. A bond graph model of a drillstring has been used for simulation that predicts axial vibration, torsional vibration, and coupling between axial and torsional vibration due to bit-rock interaction.
Lateral whirl vibrations in long sections of horizontal oilwell drillstrings, which are essentially enclosed shafts lying on the low side of the wellbore, are potentially destructive to the bit, pipes and downhole tools. Forward or backward whirl can lead to impact with the borehole, and stick slip and bit bounce can cause tool joint failure, twist-off, and bit damage. A complete deviated drillstring has been modelled by having decoupled axial and torsional segments for the vertical and curved portions, and nonlinear three-dimensional multibody segments with lateral vibration in the final horizontal section ending at the bit. The model can predict how axial and torsional bit-rock reactions are propagated to the surface, and the role that lateral vibration near the bit plays in exciting those vibrations and stressing components in the bottom-hole-assembly. The proposed model includes the mutual dependence of these vibrations, which arises due to bit-rock interaction and friction dynamics between the drillstring and wellbore wall.
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