A wellbore system, comprising: wellbore equipment comprising a central bore for connection to a wellbore tubing string at a subterranean location; an annular seat located in the central bore of the wellbore equipment, wherein the annular seat has a bore formed therethrough; a seat release mechanism holding the annular seat in axial position in the wellbore equipment until an axial force on the annular seat exceeds a first amount; and an obturator for engaging the annular seat, comprising: an elongated body of a shape to pass through the annular seat; an annular member of a size and shape to engage the annular seat, arranged at the up hole end of the elongated body, wherein the annular member comprises a central passageway formed therethrough; and a release mechanism connecting the annular member to the elongated body until an axial force on the body exceeds a second amount; wherein the system is configured such that with the obturator in the wellbore equipment with the annular member in engagement with the annular seat; increasing fluid pressure acting on the obturator until the resulting force acting on the seat exceeds the first amount causes the seat release mechanism to release the seat to shift axially together with the obturator; and thereafter increasing the fluid pressure acting on the obturator until the resulting force acting on the body exceeds the second amount causes the release mechanism to release the elongated body to shift axially and pass through the wellbore equipment. wellbore equipment comprising a central bore for connection to a wellbore tubing string at a subterranean location; an annular seat located in the central bore of the wellbore equipment, wherein the annular seat has a bore formed therethrough; a seat release mechanism holding the annular seat in axial position in the wellbore equipment until an axial force on the annular seat exceeds a first amount; and an obturator for engaging the annular seat, comprising: an elongated body of a shape to pass through the annular seat; an annular member of a size and shape to engage the annular seat, arranged at the up hole end of the elongated body, wherein the annular member comprises a central passageway formed therethrough; and a release mechanism connecting the annular member to the elongated body until an axial force on the body exceeds a second amount; an elongated body of a shape to pass through the annular seat; an annular member of a size and shape to engage the annular seat, arranged at the up hole end of the elongated body, wherein the annular member comprises a central passageway formed therethrough; and a release mechanism connecting the annular member to the elongated body until an axial force on the body exceeds a second amount; wherein the system is configured such that with the obturator in the wellbore equipment with the annular member in engagement with the annular seat; increasing fluid pressure acting on the obturator until the resulting force acting on the seat exceeds the first amount causes the seat release mechanism to release the seat to shift axially together with the obturator; and thereafter increasing the fluid pressure acting on the obturator until the resulting force acting on the body exceeds the second amount causes the release mechanism to release the elongated body to shift axially and pass through the wellbore equipment.