The hard rock stratum will seriously affect the efficiency of deep-sea drilling and greatly increase the cost of drilling operations due to its complexity. In ocean drilling operations, the design of the bit is critical. Therefore, the core bit should be able to drill in a variety of formations, especially in hard rock stratum, with the function of reducing the need to change the bit due to formation changes and improving the drilling efficiency. In this paper, three different core bits were designed according to the hard rock stratum of the seabed: the roller bit, diamond bit and bionic bit. According to the test results, it was observed that: (1) It is fully proven that the roller core bit has the characteristics of stable drilling in hard strata, small torque and high drilling efficiency. The same method can realize the core in shallow soft strata through the setting of drilling parameters. (2) For the diamond bit, the field test data show that the new formula diamond bit has a good adaptability to the changing hard rock strata in this area, with an average rate of penetration (ROP) of 4.4 m/h and a bit life of 137.75 m. (3) For the bionic bit, field experiment data showed that drilling in the formation lithology was gray green tuff, which had a core recovery up to 100%. The average ROP was 1.35 m/h, and the highest ROP was 1.88 m/h. The design and experiment of three kinds of drill bits have effectively solved the technical problem of efficient drilling in deep-sea hard-rock stratum.

In the process of ocean scientific drilling, borehole accidents (e.g., borehole collapse, burying of drill tools, and sticking of drill tools) are often caused by borehole wall instability in loose and broken strata. It is very difficult to deal with such accidents, and the drill pipe in the borehole has to be discarded in serious cases. Therefore, the stability of the borehole wall seriously affects the drilling quality and efficiency. At present, there are three effective methods of ensuring wellbore stability: casing, drilling fluid, and agent plugging technology. Casing can effectively prevent instability of all types of unstable formations and can isolate different pressure zones. Using the proper drilling fluid can control drilling fluid loss, reduce free water infiltration into the borehole wall, and form a thin and dense mud skin, which is conducive to the stability of the borehole wall. Agent plugging technology, especially chemical gels, is mainly aimed at the broken zone. After entering the formation, it makes the entire broken zone form a single high strength unit, improves the cementation ability of the lost formation, and improves the sealing and bearing capacity in order to achieve the purpose of sealing and plugging of leaks. In this paper, the present situation, technical principles, and technological processes of wall protection technology in ocean scientific drilling are summarized, and the different types of wall protection technology were researched and tested. The results of this study provide a reference for international ocean scientific drilling in the future.