are illustrated in Figure 33-1. The toggle type consists of alternate long and 

 short fingers arranged to dig into it and break off a hard core. Above this unit 

 is the spring-ring core catcher, consisting of flexible fingers that bear against 

 the core. These serve two purposes: (1) they hold a broken core, or (2) if the 

 core is soft, the fingers bite in, cut off, and retain the core. The inner barrel 

 extends through the outer barrel to the bearing at the upper end which presses 

 against the upper connection. The upper end of the inner barrel is a vent sub 

 containing a check valve, which allows fluid in the barrel above the core to be 

 pushed out as the core advances into the barrel. The upper part of the valve 

 sub contains the upper bearing and ports which allow the circulating fluid 

 coming down through the center of the upper connection to pass into the annulus 

 between the inner and outer barrels and flow to the drill bit. As the core is 

 pushed into the upper barrel, the force on the inner barrel is generally upward. 

 The rubber bearing at the top permits this load to be transferred to the upper 

 connection with minimum drag on the inner barrel. This permits the inner 

 barrel and the core catcher assemblies to remain stationary with the core while 

 the outer barrel and the cutter head rotates. The elimination of motion by 

 parts in direct contact with the core prevents breaking up of the core, thus 

 materially increasing the percentage of core recovery. After about 18 feet of 

 section has been cored, rotation is stopped and the drill string raised. This 

 causes one or the other core catchers at the bottom to engage the core, break it 

 off, and block its passage out of the barrel. The drill string then is pulled and 

 the core barrel brought to the surface, where the core is removed. 



Some variations of the conventional core barrel are the outside-vent core 

 barrel, the drop-ball type, and a combination of the outside-vent and drop-ball. 

 Since the check valve in the upper end of the core barrel opens into the interior 

 of the core barrel, the fluid passing through the check valve must overcome the 

 pressure of the circulating fluid at the top of the barrel. In soft cores the 

 additional load on the core might cause it to swell and stick in the barrel, 

 thereby preventing additional core entry. One way of combating this situation 

 is to use an outside-vent sub, which is a special section inserted between the outer 

 barrel and the upper connection. This sub then allows the fluid from the inner 

 barrel to be vented to the exterior of the barrel where pressure is lower. This 

 outside-vent core barrel allows the fluid above the core to be much more easily 

 displaced, thus facilitating the cutting of very soft material. When the barrel may 

 become clogged with contamination on the way down, or when it is necessary 

 to flush the bottom of the bore hole very strongly in order to clean it of junk, 

 the standard barrel may be used as a drop-ball type. The valve ball and the 

 vent tube (fig. 33-1) are removed when the barrel is run and before coring 

 operations are started, mud is circulated through the barrel to clean it as well 

 as to flush the bottom of the hole. Then just prior to coring, the ball is dropped 

 through the drill pipe and seats at the top of the barrel. 



698 



