Trepannig is a hollow boreing tool that does not reduce the center portion of the work to chips. The center comes out like a core sample.
It is use mostly to make heavy wall tubeing or drill stem from solid bar stock.

It is a destructive type of testing taking a sample out of the fabrication instead of a test sample. When trepanning is used the repair could be much worse than the original undisturbed weldment.

drilling a hole in your skull, but that might not be the definition you're looking for.

In 1104, it means cutting a hole in the pipeline and inserting the radiographic guide tube into the pipe and making a panoramic exposure. It is usually then plugged with a threaded Gamma Plug and seal welded. This is not allowed by API-1104.

Trepanning is the removal of a window in a weldment, usally a pipe., that is opposite of the defect used to facilatate a repair. The problem is that if QC was not made aware of it the window can be closed, welded back., and the area never inspected. hence the reason 1104, or any other code,. doesn't allow it to be done