Continuous drilling system

A continuous drilling system comprising:

• a subsea wellhead and casing which are installed at a borehole drilled in a seabed;
• a tubular tool stem which is connected to a drill string hung from a ship/floating rig;
• an external tool body which is fitted to the tool stem and which is provided inside the subsea wellhead so as to be capable of being attached to and detached from the subsea wellhead;
• a locking protrusion which protrudes radially outward from the tool body and locked to an inner surface of the subsea wellhead;
• a projection which protrudes radially inward from the tool body and locked to the tool stem;
• and
• a lock plug and an unlock plug which are inserted into a plug installation position where the tool body and the subsea wellhead are mounted in an insertion hole of the tool stem, wherein
• the lock plug is inserted into the plug installation position in the insertion hole, and when a fluid has flowed into the insertion hole, the locking protrusion protrudes and is locked to the inner surface of the subsea wellhead and the projection protrudes and is locked to the tool stem in a lock state, and
• the unlock plug is inserted into the plug installation position in the insertion hole in place of the lock plug, and when the fluid has flowed into the insertion hole, the locking protrusion is separated from the subsea wellhead and the projection is separated from the tool stem to release the lock state.