to grip the exterior of casing joint 45 casing gripper 39 normally has a spring and a piston not shown the spring urges grippers 43 toward the gripping position fluid pressure applied to the piston urges grippers 43 back to the released position mandrel 40 and spear 41 are rotatable in unison with each other while the actuator portion of casing gripper 39 remains stationary typically an anti rotation member not shown extends downward from the stationary part of top drive 29 into engagement with the stationary portion of casing gripper 39 to prevent its rotation the fluid pressure to release casing gripper 39 may be either pneumatic or hydraulic in this example the fluid pressure is supplied by a fluid line 47 from a fluid source 49 fluid source 49 may be the same as source 27 a casing gripper valve 51 is operated by personnel on the rig floor to supply fluid pressure to cause casing gripper 39 to move to the released position it is important that an operator not cause spider 17 to release casing string 15 unless casing string 15 is being supported by casing gripper 39 normally the weight of a single casing joint 45 is sufficient to prevent casing gripper 39 from being.