A subsea electric actuator includes an electric motor and a telescopic drive connection from the motor to a drive unit that can be moved to and fro and converts rotary motion of the connection to linear motion of an actuating stem. A return spring is operable on the drive unit to urge the actuating stem towards a datum state. An electromagnetic latch is operative when set to maintain the drive unit in a predetermined position so as to decouple the action of the return spring whereby the stem can be advanced and retracted relative to the drive unit free from the action of the return spring. De-energization of the latch allows the return spring to operate on the drive unit to return the actuating stem to the datum state. Various forms of electromagnetic latches are described.

An actuator is driven even if a jammed state, a motor stop, or the like occurs. An electromechanical actuator includes: an electric motor attached to a housing; a conversion mechanism portion having a drive portion that is driven to rotate by the electric motor and a driven portion that is driven by the drive portion and advances and withdraws in a predetermined advancing and withdrawing direction, the conversion mechanism portion being accommodated in the housing; a sliding support mechanism that transmits rotational force of the electric motor to the drive portion and allows the drive portion to slide in the advancing and withdrawing direction with respect to the electric motor; and a piston portion controlled so as to be at a predetermined position in the advancing and withdrawing direction with respect to the housing by means of pressure of a fluid supplied to and discharged from the housing.