Pressure and flow control in drilling operations

A method of maintaining a substantially constant desired pressure during a well drilling operation, the method comprising the steps of:

• dividing flow of drilling fluid between a line in communication with an interior of a drill string and a line in communication with an annulus formed between the drill string and a wellbore;
• the flow dividing step including permitting flow through a first flow control device interconnected between a pump and the interior of the drill string;
• and
• the flow dividing step including changing a second flow control device interconnected between the pump and the annulus, from preventing flow to permitting flow through the second flow control device, while flow is permitted through the first flow control device and while maintaining the substantially constant desired pressure in the wellbore.

A method of making a connection in a drill string while maintaining a desired bottom hole pressure, the method comprising the steps of:

• pumping a drilling fluid from a rig mud pump and through a mud return choke during the entire connection making method;
• determining a desired annulus pressure which corresponds to the desired bottom hole pressure during the entire connection making method;
• regulating flow of the drilling fluid through the mud return choke, thereby maintaining the desired annulus pressure, during the entire connection making method;
• increasing flow through a bypass flow control device and decreasing flow through a standpipe flow control device, thereby diverting at least a first portion of the drilling fluid flow from a line in communication with an interior of the drill string to a line in communication with an annulus;
• preventing flow through the standpipe flow control device;
• then making the connection in the drill string;
• and
• then decreasing flow through the bypass flow control device and increasing flow through the standpipe flow control device, thereby diverting at least a second portion of the drilling fluid flow to the line in communication with the interior of the drill string from the line in communication with the annulus.