Patent GB-1478127-A (GB)
Title: Core barrel head and overshot
IPC: E21B25/02
Filed: 1973-10-04
Abstract: 1478127 Boring earth &e FEDERAL DRILLING SUPPLIES Ltd 31 Dec 1974 [4 Oct 1973] 46319/73 Heading E1F A core barrel assembly 20 provided with an expansible lifting collet 42 is recovered by an overshot assembly 112 provided with a head 138 which enters the lifting collet 42 and a locking collar 160 which moves over the lifting collet 42 when the head 138 has entered the collet 42 to prevent expansion of the collet 42 and thus permit the head 138 to raise the core barrel assembly 20. As described the core barrel assembly 20 (Fig. 1) is located within a hollow drill stem 10 provided with a diamond core bit 11 and includes core receiving inner tube 27 having a core lifter 29. The inner tube 27 remains stationary during drilling and is mounted by bearings 60 on a body 32. Downward movement of body 32 is prevented by a shoulder 31 on the body 32 engaging a ring 33 on the drill stem 10 and upward movement is prevented by latches 50, a lug 100 on the drill stem 10 engaging one of the latches 50 to rotate the body 32 with the drill stem. Passages 66 in the body 32 allow the drilling fluid to by-pass the mating shoulders 31, 33. The lifting collet 42, which consists of spring fingers 148 having inwardly directed ends 152, is connected to a spindle 44 carrying a pin 78 located in cam slots 80 in the latches 50 so that an upward pull on the lifting collet 42 retracts the latches 50 which are urged outwardly by springs 106. When the core reaches the top of the inner tube 27 the upward thrust on the inner tube 27 compresses a pair of rubber packing members 62 which cut off the flow of drilling fluid, thus causing a rapid rise in pressure which can be detected at the surface. The overshot assembly 112 (Figs. 4, 5) includes a head 138 on a shaft 134 fixed to a body having a collet 172. The locking collar 160 is slidably mounted on the shaft 134 and is urged to its locking position by a spring 176. The collar 160 can be held in its non-locking position (shown dotted Fig. 5) by the collet 172. The overshot assembly 112 is connected by a shear pin 132 to a jar assembly 110 which, in turn, is connected by swivel 128 to a wire line (not shown). In recovering the core barrel assembly 20 the overshot assembly 112 is lowered down the drill stem and the head 138 enters the lifting collet 42 by expanding the fingers 148. The ends of the fingers 148 engage the end face 168 (Fig. 5) of the locking collar 160 thus compressing spring 176 until the head 138 has fully entered the lifting collet 42 when the fingers 148 contract and spring 176 moves the locking collar 160 over the collet 42 thus preventing withdrawal of the head 138 from the collet 42. The overshot assembly 112 and core barrel assembly can then be raised by the wire-line. The overshot assembly 112 can be released from the lifting collet 42 by applying a downward blow by the jar assembly 110, thus causing the locking collar 160 to be held in its non-locking position by the collet 172. The locking collar 160 is provided with passages 182 covered by a ring 184 which acts as a one-way valve. When the overshot assembly 112 is being lowered the passages 182 and ring 184 allow the free flow of drilling fluid, but when the overshot assembly 112 is raised rapidly the ring 184 prevents the flow of drilling fluid through the passages thus creating a considerable hydrodynamic pressure on the upper side of thelocking collar 160 which is sufficient  to free it from the collet 172.