Fatigue Strength Investigation of Drill Pipe for Challenging Scientific Deep Drilling and Utilization of Drilling Data to Estimate Cumulative Fatigue

摘要

The scientific drilling vessel Chikyu is performing challenging deep drilling activities, such as the JapanTrench Fast Drilling Program (JFAST), for conducting research on the Tohoku earthquake, whichaccompanied the tsunami that devastated northern Japan in March 2011. Generally, the fatigue strengthof a drill pipe is a critical factor governing the performance of challenging deep drillings. This studytherefore focused on the fatigue failure of a drill pipe, which is caused by an axial dynamic tensionresulting from ship motion and drilling operation under the condition that a bending stress is exerted, inchallenging deep drilling or drilling operations in harsh environments. Fatigue tests of actual drill pipeswere conducted to acquire the actual fatigue curve. Further, the bending stress distribution of a drill pipe,which refers to the maximum locus of the bending stress during the drilling operation, was analyzedconsidering interference of the drill string with the structure or drilling equipment. The occurrenceprobability of ship inclination due to ship motions and ocean current, the occurrence probability of thebending stresses, and the axial dynamic stress and its occurrence probability were analyzed. Consequently,the cumulative damage ratio and the possible duration of operation were obtained. From the results, weconfirmed how severe the deep drillings of JFAST are. Another important issue is to estimate the actualfatigue accumulated in drilling operations for further usage of the drill pipe. We therefore attempted toestimate the fatigue that accumulated during operations by using the abovementioned fatigue analysisalong with drilling data acquired during the operation.