A carriage arranged to walk along an elongate member while carrying a payload includes individually-operable clamps that are spaced axially along a common longitudinal axis. An axially-extensible frame connects the clamps. At least one of the clamps is attached to the frame via a rotationally-displaceable coupling for relative angular movement between that clamp and the frame about the longitudinal axis. The carriage can carry the payload to a subsea worksite by opening and closing the clamps to release and grip the elongate member in a sequence that includes moving the leading clamp forward when the leading clamp is open and moving the trailing clamp forward when the leading clamp is closed. At the worksite, installation force can be applied to the payload in a forward direction by moving the leading clamp forward when the leading clamp is open and the trailing clamp is closed.