Fatigue of drill pipe

Field failures of drill pipe can have serious economic consequences, and the drilling industry reports that such failures are increasing in number. The basis of existing design guidelines for the fatigue strength of drill pipe is work that was presented in the early 1950's. Although the experimental data upon which the rules were based appear to be valid, the physical models used do not correspond to modern drilling conditions in a number of significant ways. Accordingly, an investigation of the fatigue strength of drill pipe manufactured in accordance with today's standards was conducted. The effects of stress range, mean stress, corrosion, and pipe geometry were investigated. Because of the number of parameters involved, it was not practical to conduct a full factorial experiment, however. Full-size drill pipe was tested in rotating bending under conditions of axial preload and with or without corrosion present. These test conditions correspond closely to the situation in the field. The program included 29 tests conducted in air and 21 tests conducted in a 3.5 percent NaCl solution.;The effects of both stress range and mean stress were found to be significant, and this was true in both non-corrosive and corrosive environments. The change in pipe geometry that exists in the upset region between the pipe body and the tool joint did not influence the fatigue strength for the cases examined. With one exception, the fatigue failures that occurred in the tests took place in the drill pipe body. The exception was a crack that occurred in the upset region and it started from a slag inclusion on the outer surface of the pipe.;It is required that all drill pipe be inspected before it is released for service. One result of the inspection procedure is light grinding on the surface of the pipe, done in order to remove small imperfections. The effect of this grinding was found to be detrimental to the fatigue life of the pipe. About 50 percent of the test specimens failed as the result of fatigue cracks that originated at grinding marks. The reason is twofold. The notch effect created by the grinding affects the fatigue strength; this phenomenon is well known. Furthermore, X-ray diffraction studies showed that beneficial compressive residual stresses on the surface of the pipe are removed by this grinding and replaced by high tensile residual stresses. The latter promote fatigue crack growth.;Fractographic examinations and a fracture mechanics analysis of the failed drill pipe both indicated that once a fatigue crack has grown through the wall thickness, the fatigue crack will propagate rapidly. Thus, rupture of the drill pipe is likely to occur shortly after a fatigue crack has been detected in the field operation.;A critical evaluation of the current design guidelines of the American Petroleum Institute for drilling in a non-corrosive environment shows that those rules overestimate the fatigue life of drill pipe. They predict less fatigue damage than predicted by the results of the investigation reported herein. A similar examination shows that, when drilling in a corrosive environment, the existing guidelines are conservative under conditions of high axial load in the pipe and unconservative under conditions of low axial load.