COMPARISONS OF DRILL STRING CYCLIC LOADING DUE TO VIBRATION AND DOG-LEGS

This paper presents an investigation of the cyclic loading on oilfield drill strings due to longitudinal, torsional and lateral vibrations a nd compares this with the 'statical' cyclic loading caused by a deviat ed string at a dog-leg. Such cyclic loading is a principal contributor to drill string fatigue failures. The methodology of the paper uses n umerical dynamic analyses of longitudinal and torsional vibrations tog ether with quasi-static models for lateral vibration and dog-legs to c ompare the cyclic loading induced separately by each of these effects. A linear damage law (that is Miner's rule) is used to calculate the r esultant fatigue damage from the cyclic loading. The results of this i nvestigation show that longitudinal and torsional vibrations may induc e 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 d amping coefficients and can induce significant cyclic loading and rapi d accumulation of fatigue damage on drill pipes and collars.