The scientific drilling vessel Chikyu is performing challenging deep and harsh drilling activities. Generally, the fatigue strength of a drill pipe is a critical factor governing the performance of challenging deep drillings. Authors therefore established the evaluation method of the cumulative fatigue of the drill pipe and the drilling programs including the drill string configurations are planned based on the evaluation. The bending stress distribution of a drill pipe, which refers to the maximum locus of the bending stress during the drilling operation, was analyzed considering interference of the drill string with the structure, riser or drilling equipment. The occurrence probability of ship inclination due to ship motions and ocean current, the occurrence probability of the bending 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.

This study focused on the strength evaluation of the drill pipe for challenging scientific drilling operation at Nankai Trough that is a harsh environment because of the strong ocean current. From the results, we confirmed how severe the challenging scientific drillings are.

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