Apparatus and method for wave motion compensation and hoist control for marine winches

A method of wave motion compensation for a marine winch comprising:

• actuating a control means for the winch drive motor so as to apply a predetermined tension to the winch rope,
• making a first 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,
• making a second determination of the direction of vertical motion, vertical displacement, vertical velocity and vertical acceleration of the load on the winch rope relative to the winch, said second determination being at a later time than said first determination,
• comparing the results of said first and second determinations,
• using this comparison and standard sea state data to predict 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 as the lag time of the control means and winch drive motor and,
• actuating the control means 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 to the motion of the load, thereby maintaining substantially constant tension on the winch rope.

A hoist control for a marine crane, comprising:

• means for applying a tension less than the weight of a 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 a vessel from which it is to be hoisted or onto which it has been lowered;
• means for maintaining such tension constant as the load while on the vessel rises and falls with the vessel relative to the position of the crane;
• means for determining data representing the direction of vertical motion, vertical displacement, vertical velocity and vertical acceleration of the vessel relative to the crane;
• means for comparing this data to standard sea state data to determine the optimum time for initiating lifting or completing lowering of the load;
• and
• electronic means for automatically initiating such lifting or completing such lowering at the determined optimum time.

A method of lifting a load from a vessel which is in motion relative to the lifting crane, comprising:

• connecting the load to the crane hoist rope;
• applying a tension to the hoist rope less than the weight of the load to be lifted;
• maintaining this tension constant as the load on the vessel rises and falls relative to the crane;
• determining data representing the direction of vertical motion, vertical displacement, vertical velocity and vertical acceleration of the load relative to the crane;
• comparing this data to standard sea state data to determine the optimum time to initiate lifting of the load;
• and
• automatically initiating such lift at the determined optimum time by electronic means.

A method of lowering a load onto a vessel which is in motion relative to the lowering crane, comprising:

• lowering the load into the vicinity of the vessel;
• determining data representing the direction of vertical motion, vertical displacement, vertical velocity and vertical acceleration of the vessel relative to the load;
• comparing this data to standard sea state data to determine the optimum time to complete lowering of the load onto the vessel;
• automatically completing such lowering at the determined optimum time;
• maintaining a constant tension on the hoist rope after the load has been lowered onto the vessel until the load is disengaged from the hoist rope.