Scheme design and flow performance analysis of double-bits combination wire-line coring technology for seafloor drill

In order to meet the requirement of high-quality samples for evaluating marine mineral resources reserves, a double-bits combination wire line coring scheme for seafloor drill is proposed. The Bernoulli math model reflecting the drilling fluid pressure variation inside the drilling tool is established. The influence of the drilling tool's water circuit parameters on the drilling fluid pressure variation is clarified. The drilling fluid's flow performance during the drilling process is analyzed. The drilling performance of this scheme is tested through engineering application. The application results indicate that: When the inner tube bit's water channel cross-section A1 reaches 30mm2, A2 reaches 20mm2 and the A3 reaches 20mm2, the drilling fluid pressure ΔP in the drilling tool tends to stabilize. The influence of PVC tube outside diameter d8 on drilling fluid pressure ΔP is small within the allowable design range of 62–65mm. The simulation results show a high-speed area in the first section of the water channel in the inner tube bit. Selecting pump displacement in 50–90 L/min has more engineering application value. The engineering application in complex seabed strata shows that the average core sampling rate is more than 85% within the pump displacement of 50-90 L/min and rotation speed of 350–450 r/min.