Comparisons of Drill String Cyclic Loading Due to Vibration and Dog-Legs

Abstract

This paper presents an investigation of the cyclic loading on oilfield drill strings due to longitudinal, torsional and lateral vibrations and compares this with the ‘statical’ cyclic loading caused by a deviated string at a dog-leg. Such cyclic loading is a principal contributor to drill string fatigue failures. The methodology of the paper uses numerical dynamic analyses of longitudinal and torsional vibrations together with quasi-static models for lateral vibration and dog-legs to compare the cyclic loading induced separately by each of these effects. A linear damage law (that is Miner's rule) is used to calculate the resultant fatigue damage from the cyclic loading. The results of this investigation show that longitudinal and torsional vibrations may induce significant cyclic loading comparable to that from typical dog-legs whereas the influence of lateral vibration is small. The longitudinal and torsional vibrations are strongly influenced by rotary speed and damping coefficients and can induce significant cyclic loading and rapid accumulation of fatigue damage on drill pipes and collars.