a sealing device used to close off the annular space around the kelly when 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-pressure, high-volume fluids. The rate of penetration through such formations is usually rapid.
