During drilling operation in strong current, drill string will be constantly bent. And the drilling operation by rotating the drill string will lead a bending stress to a cyclic stress. Consequently it may cause the fatigue damage. Fatigue strength curve expressed with moment M and the number of cycles to failure N is obtained from the fatigue test. And it is modified by applying the modified Goodman diagram concept considering an axial stress which is always exerted. The moment distribution of drill pipe is analyzed under the assumption that the drill string is inclined in a certain angle due to the current. Then, fatigue damage level can be obtained by expressing with cumulative damage ratio.


Japan Agency for Marine-Earth Science and Technology (JAMSTEC) constructed the scientific deep-sea drilling vessel Chikyu - Japanese word for Earth. Chikyu was designed to be capable of operating in water depths up to 2,500 m for riser drilling and 7,000 m for riserless drilling and also drilling down to 10,000 m total vertical depth for both riser and riseless drillings (Inoue, Nishigaki, Setta and Wada, 2006). Chikyu was also designed to enable continuous coring and recovering scientifically worthy core samples (Inoue, Wada, Kyo and Miyazaki, 2006). The first scientific drilling of Chikyu under the international organization IODP (Integrated Ocean Drilling Program) has commenced at the Nankai trough located beneath the ocean off the southwest coast of Japan since September 2007. The main purpose of the expedition at the Nankai trough is to drill deep into the Earth to observe the earthquake mechanisms because there is one of the most active earthquake zones. However, it is recognized that the Nankai trough is a harsh environment area for drilling operation because the strong current due to Kuroshio (Black Stream) always runs there.

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