It has been common practice to incorporate a shearing type of blowout preventer in a blowout preventer stack so that in the event of a problem the shear rams may be closed and in closing shear a string extending through the stack. Also when used on a production string the shear rams shear the production tubing and seal above the sheared tubing so that pressure is not allowed to escape through the production tubing. When a production string was sheared by these prior art devices the upper end of the lower part of the sheared string is flattened so that it is difficult or impossible to pump fluids into the string bore to kill the well and also is difficult to connect an overshot retrieving tool thereto.
U.S. Pat. No. 4,313,496 discloses a shearing type of blowout preventer which is adapted to shear through drill collars of a drill string and is provided with arms secured to the sides of the shear blades which interengage and function to provide a force to resist the forces tending to vertically separate the cutting blades and to fill the space of the guideways outside the vertical bore of the preventer to prevent the forcing of the member being cut into this space.
Examples of other U.S. patents which disclose ram-type blowout preventer which have shear rams are U.S. Pat. Nos. 3,736,982, 3,817,326, 4,132,265, 4,132,266 and 4,132,267, each of which disclose shear rams which engage and shear pipe extending therethrough and crush the upper end of the fish substantially flat after it is sheared and shearing is accomplished with the use of a very substantial amount of force. Also, U.S. Pat. No. 3,863,667 discloses a combination double pipe shearing and plug device mounted on a partially cut pipe line so that a section of the pipe can be removed and then closed with the plug. The cutting edges are pointed but must cut the pipe without deforming it in order to maintain the seal between the housing and the pipe.