Scheme design and performance analysis of thin-walled pressure-retaining coring tools for seafloor drills in the deep-sea operating environment

Abstract To meet the demand for high-quality pressure-retaining samples for natural gas hydrates research, a pressure-retaining design scheme for a thin-walled coring tool is proposed to suit the characteristics of seafloor drills and meet the requirements of 3000 m water depth. Under the working pressure of 8–30 Mpa, theoretical analysis and a test verification of the pressure-retaining performance of the drilling tool were carried out. Then, the adaptability test, which was between the drilling tool and seafloor drills, and the drilling test were carried out. The results show that the pressure-bearing capacity of the pressure-retaining drilling tool exceeds 30 Mpa; note that this drill is designed based on the direct sealing principle of the core tube. After 12 h, the pressure loss inside the drill tube was not more significant than 10%. In the theoretical calculation of the pressure loss of thin-walled tubes under the pressure of 8–20 Mpa, the calculation accuracy can be improved by considering the influence of the tube outer diameter deformation. The core sample with a diameter of 45 mm can be obtained through a process of 75 mm drilling. The matching performance between the drilling rig and the drilling tool meets the actual requirements of the seafloor drills for small diameter and a large hole depth drilling operation. The related research can enrich the existing technology and theory of natural gas hydrates pressure coring and provide theoretical support for the popularization and application of thin-walled coring tools in the field of seabed pressure exploration.