17 as shown in fig5 d the rov 18 is then used to disconnect the tether connection 9 from the support structure 7 again a diver or other suitable means may be used to perform this task the motor unit 2 then powers the winching device 4 to retract the tether 10 although some inspection or maintenance of the pga 1 may be performed while the pga 1 floats above the support structure 7 it will be more usual for the pga 1 to be towed by the vessel 16 to shore or to a suitably equipped ship for such maintenance the invention has the particular advantage that no large surface vessel with a heavy lift crane winch is required to deploy the pga 1 onto the support structure 7 or subsequently to retrieve it this obviates the need for heavy vessel lifting equipment and eliminates problems associated with the motion heave of the vessel pulling in an unpredictable manner on the pga consequently it is possible to deploy or retrieve the pga 1 in moderately severe weather conditions a further advantage is that no mechanical guide arrangement is required whilst lowering the pga down through the water column since the natural.