After a casing head or spool and Blow Out Preventers, (BOP), have been installed on a well a pressure integrity test for these items must be conducted. In order for the test to take place, a test plug should be set in the casing head or spool.
A BOP test plug can be installed by running it down through the BOP stack and landing it on the load shoulder in the head
The test plug is usually run on a joint of drill pipe
Once in place the hydraulic rams of the BOP can be closed on the pipe and a hydrostatic test performed
This will insure the integrity of the well control equipment prior to the restart of drilling operations