Drill String Strength Evaluation for Deep Earthquake Zone Drilling

The scientific drilling vessel Chikyu was designed to have the deep drilling capability to reach the deep earthquake zone. To realize such deep drilling, a drill pipe with higher than ever strength and reliability is of necessity. A strength evaluation of such high strength drill pipe is also necessary. Around Japan, the earthquake zones are widely located under the seabed in deep water. For example, the Nankai Trough located beneath the ocean off the southwest coast of Japan is one of the most active earthquake zones where large-scale earthquakes have occurred repeatedly throughout history. Thus, the Chikyu has started the first scientific drilling at the Nankai Trough as the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). NanTroSEIZE targets the megasplay fault zone at 3500 m below the seafloor and finally 6000m deep drilling into the seismogenic zone and across the plate interface into the subducting crust at water depths of around 2500m. In addition, a huge earthquake zone is expected to be located about 1000m below the seafloor at water depths of around 7000m. For this drilling task, the riserless drilling technique should be applied. To realize such deep drilling with both riser and riserless techniques, a S150 drill pipe was developed during the construction phase and has been used in the past scientific drillings of the Chikyu. For deeper drillings in the future and drilling operations in harsh environments, we are developing superior high strength drill pipes, S155 and S160, possessing high reliability including corrosion resistance to achieve high toughness and reduction of stress concentration. An evaluation of the maximum possible stress was conducted. In the maximum strength evaluation, we considered dynamic stress and bending stress due to the current and the vessel inclination. This paper describes the development of superior high strength drill pipes and the strength evaluation of such drill pipes for deep earthquake zone drilling.