Abstract: The components required for changing out a bottom hole assembly are stored within a submerged storage chamber adjacent the lower end of a riser extending up from the bottom of a body of water. A vertical, revolving magazine holds the bottom hole assembly components within the storage chamber. A pressure lock between the storage chamber and the interior of the riser is opened to permit the bottom hole assembly components in the chamber to be exchanged with those forming the drilling string assembly within the riser. The storage chamber may be selectively sealed from the body of water and the internal riser area as the bottom hole assembly components are moved into and out of the chamber. A remotely operated mechanism assembles and disassembles the bottom hole assembly components of the drilling string assembly.