Criterion for Allowable Lower Ball Joint Angle in Floating Drilling

Abstract

In drilling from floating vessels, most operators and drilling contractors specify an allowable lower ball joint angle of 3–5 deg for drilling and 1–3 deg for running casing. Presently, no one uses tension in the drill pipe or casing as a factor in setting allowable ball joint angles. The results of the analysis detailed in this paper show that the allowable ball joint angles in use are justified for well depths less than 5000 ft below the ball joint. However, the allowable angle is too high for continual operation for well depths below the ball joint of 10,000 ft or more. The increased drill pipe or casing tension in deeper wells causes high alternating stresses in the drill pipe and high wear rates where the pipe bears against equipment in the ball joint region. The principal conclusion of this paper is that the allowable lower ball joint angle should not be constant, but should vary with the tension in the drill pipe as the well is being drilled.