Dynamic Stresses in Long Drill Pipe Strings

Abstract

A 15,000-ft string of Grade E drill pipe is capable of stretching 37 ft without yielding. If the pipe were infinitely rigid so it would not stretch, it would break at the slips as soon as they were set. Fortunately, the pipe is not rigid, but is capable of stretching without permanent elongation provided the stress developed does not exceed the yield point of the pipe material. This ability to stretch allows the top of the pipe to be stopped suddenly while the bottom of the pipe continues to move. Whenever slips are set on moving drill pipe, dynamic stresses are added to the stresses already in the string. Parting or permanent elongation could result from these stresses depending upon the string length and upon the velocity of the pipe when the slips are set. An investigation into these stresses shows what are the critical slip setting velocities and how much effect slowing down the pipe velocity can have on the magnitude of these stresses. This study also reveals that the expense and inconvenience of shock-absorbing devices can hardly be justified for protection of drill pipe since only relatively small shock reduction is practical.