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A subsea payload such as an AUV (autonomous underwater vehicle) or an AUV garage is lifted from an underwater location by flying a latch unit through the water to the payload. The latch unit carries a lift line toward the payload. The latch unit is then attached to the payload and the payload is lif

A subsea payload such as an AUV (autonomous underwater vehicle) or an AUV garage is lifted from an underwater location by flying a latch unit through the water to the payload. The latch unit carries a lift line toward the payload. The latch unit is then attached to the payload and the payload is lifted using tension applied through the lift line via the latch unit. The latch unit can also be used on a lift line to lower a payload and then to release the payload at an underwater location. The lift line is supported by a heave-compensating winch on a surface vessel that effects z-axis movement of the latch unit. The winch maintains tension on the lift line to prevent the lift line falling on the payload. Movement of the latch unit on x- and y-axes is effected by on-board thrusters.

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1. A method of lifting a payload from a seabed location, comprising: flying a latch unit through the water to the payload, the latch unit being attached to a taut lift line supported by a heave-compensating winch on a surface vessel to carry the lift line toward the payload;wherein flying the latch

1. A method of lifting a payload from a seabed location, comprising: flying a latch unit through the water to the payload, the latch unit being attached to a taut lift line supported by a heave-compensating winch on a surface vessel to carry the lift line toward the payload;wherein flying the latch unit comprises moving the latch unit on x- and y-axes horizontally to align the latch unit above the payload before attachment of the latch unit to the payload, while controlling a z-axis position of the latch unit using the heave-compensating winch;attaching the latch unit to the payload;progressively increasing tension applied through the lift line via the latch unit to detach the payload from a seabed location; andlifting the payload using the tension applied through the lift line via the latch unit. 2. The method of claim 1, comprising lowering the latch unit on a z-axis toward the payload by paying out the lift line from the winch during movement of the latch unit on the x- and/or y-axes. 3. The method of claim 1, comprising determining underwater positions of the latch unit and the payload and flying the latch unit while monitoring its changing position to converge with the position of the payload. 4. The method of claim 1, wherein the latch unit is suspended from the lift line with negative buoyancy. 5. The method of claim 1, wherein heave compensation by the winch is activated when the latch unit is at a mid-water position nearing, but vertically spaced from, the payload. 6. The method of claim 5, comprising detecting attachment of the latch unit to the payload and, in response to attachment, applying tension to the lift line using the winch. 7. The method of claim 6, comprising sensing local tension in a portion of the lift line closely adjacent the payload to control the winch. 8. The method of claim 1, preceded by lowering the latch unit from the surface vessel on the lift line. 9. The method of claim 1, preceded by lowering the payload to the underwater location attached to the latch unit attached, in turn, to the lift line; detaching the latch unit from the payload; and lifting the latch unit clear of the payload using the lift line. 10. The method of claim 9, comprising lowering the payload from the surface vessel through a splash zone while damping movement of the lift line. 11. The method of claim 1, wherein the payload is an AUV or an AUV garage. 12. A system for recovering a payload from an seabed location, the system comprising: a lift line;a thruster-equipped latch unit attached to the lift line and steerable in x- and y-axes horizontally through water to carry the lift line toward a payload at an underwater location, the latch unit having a latch formation to attach the latch unit to the payload, and being arranged to transmit lifting tension from the lift line to the payload via the latch formation to recover the payload;a heave-compensating winch which in use pays out the lift line to lower the latch unit toward the payload, controls a z-axis position of the latch unit during lowering, andincreases the tension applied through the lift line via the latch unit is progressively increased to detach the payload from a seabed location. 13. The system of claim 12, wherein the lift line is attached to one side of the latch unit and the latch formation is disposed on an opposite side of the unit to transmit tension from the lift line through the unit to the payload. 14. The system of claim 12, comprising; a position-detection apparatus for detecting underwater positions of the latch unit and the payload; anda control apparatus acting on the winch and the latch unit to direct the latch unit to the payload. 15. The system of claim 14, wherein the control apparatus is arranged to control one or more thrusters of the latch unit to move the latch unit on x- and/or y-axes and to control the winch to lower the latch unit on a z-axis by paying out the lift line from the winch. 16. The system of claim 14, further comprising a lift line tension sensor adjacent to or integrated with the latch unit whose signal is used by the control apparatus to control the winch. 17. A surface support vessel having the system of claim 12.