Rotating head:

A sealing device used to close off the annular space around the kelly in drilling with pressure at the surface, usually installed above the main blowout preventers.

A rotating head makes it possible to drill ahead even when there is pressure in the annulus that the weight of the drilling fluid is not overcoming; the head prevents the well from blowing out.

It is used mainly in the drilling of formations that have low permeability.

The rate of penetration through such formations is usually rapid.