Abstract

The scientific drilling vessel Chikyu is performing Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a challenging deep drilling activity, for scientific purpose. We faced difficulty to drill deep during past NanTroSEIZE operations due to unstable sediments and insufficient cutting removal. Non-Stop Driller concept is, therefore, applied for the operation of NanTroSEIZE scheduled to start Oct. 2018 to enable continuous circulation of drilling fluid circulation. The Non-Stop Driller concept requires an additional, specially-designed sub called an “NSD sub” with a ball valve for drilling fluid inlet.

Generally, the fatigue strength of a drill pipe is a critical factor governing the performance of challenging deep drilling. This study, therefore, focused on the fatigue failure of the NSD sub due to the bending stress caused by interference with risers including flex joints, ship structure, or drilling equipment resulting from ship motions. The bending stress leads to cyclic stress caused by rotation of the drill pipe. This is especially the case at the Nankai Trough where ocean currents are very strong reaching or sometimes exceeding 4 knots, a high bending stress is assumed to be exerted on the NSD sub.

Full-scale fatigue tests of the NSD sub were first conducted to acquire the actual fatigue curve. Further, the bending stress distribution of a drill pipe, which refers to the locus of the bending stress during the drilling operation, was analyzed by considering interference of the drill string with the structure, drilling equipment, and risers that are deformed by the ocean current.

Time-series ship motions is prepared using the response amplitude operators of the Chikyu for the sea states at Nankai Trough area, and then time-series stress response is obtained by considering the operational conditions such as rate of penetration and rotational velocity of drill pipe. The numbers of occurrence of each stress amplitude can be counted from the time-series stress response. Consequently, the cumulative damage ratio is calculated for evaluating the fatigue of the NSD sub. The results confirmed that the cumulative fatigue is within a safe range.

This study focused on the evaluation of the fatigue strength of the specially designed NSD sub for the challenging scientific drilling operation at Nankai Trough, a harsh environment because of the presence of strong ocean currents. This paper presents the overview of NanTroSEIZE including the Non-Stop Driller concept, and the results of fatigue evaluations.

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