This invention relates to winches, in particular winches used in a marine environment. It provides a means for maintaining a substantially constant tension on the winch rope despite the fact that the load and the winch is moving relative toeach other. It further provides a means for safely raising a load from, or lowering a load onto, a vessel moving on the sea surface relative to the crane which is raising or lowering the load.
There are a number of applications in which it is necessary or desirable to maintain a constant tension on a winch rope, despite the fact that the winch and the load are moving relative to each other. Examples are preventing shock loads on amarine crane when lifting is initiated, maintaining a constant pressure on a drill stem being operated from a floating rig and maintaining a steady tension on a marine anchor.
A number of servo-controlled systems intended to automatically operate the winch so as to follow the movement of the load and thereby maintain a constant tension are described in the prior art. Examples can be found in U.S. Pat. Nos. 2,249,947 and 3,753,552. However, all such mechanical or electro-mechanical control systems have an inherent lag between input of a command to alter the operation of the winch motor and the alteration actually occurring. This lag means that the tensioncannot be maintained constant, particularly at a point where the relative direction of motion changes. The conventional response to this problem is to design the control system so as to have as small lag as possible under the circumstances. However, inthe types of application described above, the winch loads are typically large, and to control such loads the winch motor and controller must be of a design that typically has a relatively long lag time.
The above noted and other disadvantages of the prior art are overcome by providing a wave motion compensator which synchronizes the movement of the winch rope with the movement of the load without significant lag, thereby maintaining asubstantially constant tension on the winch rope. There is further provided a hoist control which automatically initiates lifting or completes lowering at the optimum point for the particular sea state experienced at that time.
The present invention provides a wave motion compensator for a marine winch, in which the winch drive motor has a servo controller. The controller is actuated so as to apply a predetermined tension to the winch rope. A means for making a firstand second determination of the direction of vertical motion, vertical displacement, vertical velocity and vertical acceleration of a load on the winch rope relative to the winch and for comparing these determinations is provided, and from thiscomparison and standard sea state data a suitable means predicts the vertical displacement, vertical velocity and vertical acceleration of the load relative to the winch at a time in advance of the time at which the prediction is made at least as long asthe lag time of the controller and winch drive motor. The controller is actuated according to this prediction so as to effect changes in the winch speed and direction such that the motion of the winch rope substantially corresponds without significantlag to the motion of the load, thereby maintaining substantially constant tension on the winch rope.
The present invention further provides a marine crane hoist control, which applies a tension less than the weight of the load to be hoisted or lowered to the hoist rope while the hoist rope is connected to the load and while the load is on avessel from which it is to be hoisted or onto which it has been lowered and which automatically maintains this tension constant as the load rises and falls with the vessel relative to the position of the crane. There is also provided means fordetermining data representing the direction of vertical motion, vertical displacement, vertical velocity and vertical acceleration of the vessel relative to the crane, and for comparing this data to standard sea state data to determine the optimum timefor initiating lifting or completing lowering of the load. When it is determined that this optimum time has been reached, the lifting is initiated or the lowering completed automatically by electronic means.
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