Remotely operated external tieback connector

An external tieback connector secures to a lower end of a driller riser. The tieback connector has a locking element that engages an external profile on the wellhead housing and an actuating piston within a piston chamber. A hydraulic fluid accumulator is in communication with the piston chamber through a hydraulic circuit having valves. An umbilical extends from a floating platform to the accumulator. Sending a signal through the umbilical opens the valves to supply hydraulic fluid pressure from the accumulator to the piston chamber. An acoustic signal receiver also connects to the hydraulic circuit. An acoustic transducer deployed subsea on a transducer cable will emit an acoustic signal that is received by the receiver. The receiver opens the valves to apply hydraulic fluid pressure to the piston chamber.