American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
A discussion of the buckling of drill collars and drillpipe in inclined (directional) boreholes is presented for the purpose of selecting maximum drillbit weights from drill collars such that the section of the drillpipe within the drillstring above the drill collars will not be buckled. The theories of A. Lubinski, R. Hoch, and the work of M. Rollins pertinent to drill-collar applications are pertinent to drill-collar applications are reviewed. The mechanics of drill-collar and drillpipe bending and buckling in inclined boreholes and long-standing misconceptions are discussed. A mathematical equation for the analytical calculation of a maximum permissible drillbit weight from drill collars in inclined (directional) boreholes is derived and presented with solved example problems. A convenient nomograph is given.
Drill collars are a major component of the drillstring within the rotary drilling system. They are thick-walled, hollow steel tubes about 30 ft in length with thread connections at either end. A series of drill collars within the drillstring are called a drill-collar string and the common lengths of a string are between 90 and 500 ft. Common wall thicknesses are between 1 and 4 in., which yields linear weights in the range of 20 to 368 lb/ft. The drill-collar string may weigh as much as 100,000 lb. It occupies a position within the drillstring between the drillbit and the drillpipe.
Drill collars were introduced in the Seminole oil fields of Oklahoma in the early 1920's for the purpose of maintaining more vertical boreholes. More vertical boreholes give higher assurance that boreholes will remain within lease boundaries, penetrate the correct location within the geologic structure, and reduce interference between wells during drilling and producing operations.
For three decades the selection of drill-collar strings with respect to their diameters and lengths was an art given to errors arising from misunderstood concepts and misinterpreted experiences. In 1950, Lubinski published his paper, "A Study of the Buckling of Rotary paper, "A Study of the Buckling of Rotary Drilling Strings," which placed drill-collar string selection into the realm of science. His theory is often called the "pendulum theory", or the "one-stabilizer theory" and contends with the problem of analytically aggregating the following variables pertaining to drill collars and the earth's structure:
