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Hypocycloid method for describing regular whirling of bottom hole assembly
MA Ru tao1, JI You zhe2, JIA Tao2, HAN Fei2, ZHU Ying jie2
(1.Post Graduate School of Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China;2.Department of Drilling Machinery of Drilling Research Institute, CNPC, Beijing 100195, China)
Abstract:
On the basis of geometry, a hypocycloid method was proposed to identify and control drillstring whirling. Treating the whirling as a regular pattern, this method was used to describe motion characteristics of a fixed point on drillstring circumference, to solve equations representing bottom hole assembly (BHA) motion trajectories, to define velocities of each point on the trajectory, and to analyze the impacts of rotation and revolution speeds on BHA whirling. The results show that whirling might cause drillstring rolling around borehole wall with a revolution speed much higher than rotating, inducing downhole tools premature failures. The regular motion patterns of a fixed point on BHA circumference can be expressed by geometry measures, and the trajectory equation indicates that the linear velocity plays an important role in tool wear. The whirling equations representing BHA motion can be worked out from inversions of field and laboratory tests and the results properly match documented tests.
Key words:  bottom hole assembly  drillstring whirling  hypocycloid  motion trajectories  process monitoring