Abstract: An improved offshore drilling method and apparatus are disclosed which are useful in preventing formation fracture caused by excessive hydrostatic pressure in a drilling riser. Gas is injected into the riser to provide the lift necessary to return the drilling fluid to the surface and to reduce the density of the drilling fluid. The rate of gas injection overlifts the drilling fluid to the extent that the pressure of the fluid is reduced to less than that of the seawater surrounding the riser. Seawater is permitted to flow into the lower end of the riser in response to the differential pressure between the drilling fluid and seawater so that the pressures of the drilling fluid and the seawater approximately equalize.

Abstract: An improved offshore drilling method and apparatus are disclosed which are particularly useful in preventing formation fracture caused by excessive hydrostatic pressure of the drilling fluid in a drilling riser. One or more flow lines are used to withdraw drilling fluid from the upper portion of the riser pipe. Gas injected into the flow lines substantially reduces the density of the drilling fluid and provides the lift necessary to return the drilling fluid to the surface. The rate of gas injection and drilling fluid withdrawal can be controlled to maintain the hydrostatic pressure of the drilling fluid remaining in the riser and wellbore below the fracture pressure of the formation.

Abstract: Method and apparatus for determining on board a heaving drilling vessel, the flow rate of drilling fluid flowing from a well hole and into a telescoping marine riser connecting the well hole with the vessel. A measuring apparatus is associated with a drilling fluid return conduit of the riser in such a position that the measuring apparatus and the portion of the conduit between the measuring apparatus and the riser are at all times filled with drilling fluid. The measuring apparatus generates a first signal proportioned to the flow rate of drilling fluid flowing therethrough, measures the drilling fluid level in the riser and generates a second signal proportional to the change in volume of the drilling fluid contained therein above the point at which the conduit intersects the riser. The telescoping movement of the riser is measured and a third signal is generated proportional to the change in volume of the flow path provided by the riser between the well hole and said point.