S150 drill pipe was developed during construction of the scientific drilling vessel Chikyu, and has been used in the past scientific drillings. On the other hand, during past drillings, Chikyu faced harsh environments such as a high-current or a corrosive environment in addition to deep drilling. To reach deep drilling in such a harsh environment, a higher strength and reliability drill pipe, as well as strength evaluation using the high-strength drill pipe, is necessary.

Superior high-strength drill pipes, S155 and S160, possessing high reliability, such as corrosion resistance, have been developed. Pitting corrosion resistance, which causes stress concentration, is required to prevent fatigue failure. Also, consideration on possibility of SSC under low temperature at seabed is required because it would be a cause of fatigue.

Regarding deep drilling and extended reach drilling, required feature is fatigue resistance of not only pipe body but also tool joints. In particular, the tool joint starts to become the cause of wearing out, mud pressure loss, mud bedding, heat cracking, and failure itself.

The concept of the high-strength drill pipe, which we have developed, achieves high toughness and reduction of stress concentration along full string in the salt water to eliminate elements of fatigue phenomenon. The development includes the pipe manufacture process such as welding line performance, heat treatment methodology, the experimental investigation such as fatigue tests under salt water with or without pitting, and analytical investigation.

The superior high-strength drill pipes described in the paper possess not only the high-strength performance but also high fatigue resistance in the corrosive environment. These can exercise the potential capability in the deep drillings, extended reach drillings, and harsh environment drillings.

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