Patent Publication Number: US-2009232625-A1

Title: Motion compensation system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Application No. 60/993,759, filed Sep. 14, 2007. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to motion compensation systems, and more particularly to a heave compensation system for a crane mounted on a vessel to isolate the load being raised or lowered by the crane from the vertical movement or heave of the vessel. 
     BACKGROUND 
     Heave compensation in marine vessels has historically been carried out by either an active heave compensation system or a passive heave compensation system, or a combination of the two systems. An active heave compensation system relies on motion reference sensors that are mounted on the deck of the vessel to measure the amount of heave. The correction of motion due to the heave of the vessel is accomplished by movement of a hydraulic cylinder that drives a multi-wire rope sheave assembly through the output signals derived from the motion reference unit sensors. 
     A limitation of active heave compensation systems is that the heave displacement correction is limited by the hydraulic cylinder stroke. In order to compensation for significant levels of heave, very large hydraulic cylinders are required resulting in significant weight as well as significant associated components of the system. Also, an active heave compensation system cannot sense the level of frequency of the load being experienced by the load-carrying line. Thus, it is possible to have the same natural frequency of the vessel as well as the load during lowering, which can lead to catastrophic results. 
     A passive heave compensation system typically relies on the compression of a compensating cylinder. During the heave of a vessel, the compensating cylinder has one end connected to the vessel and the other (rod) end connected to the load-carrying line. As the vessel heaves, the piston in the cylinder moves up and down to compensate for the heave. The pressure within the cylinder is adjusted to a correct level using gas-filled accumulators. As would be appreciated, the cylinder pressure must change with different load levels being handled. The drawback of passive heave compensation systems is that their response time may be relatively slow and thus not capable of “keeping up” with the heaving action in rough seas. 
     Thus, a heave compensation system that can overcome the drawbacks of existing active and passive compensation systems, while still being of relatively simple and straightforward design, could be greatly beneficial. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     A heave compensation system for a lift mechanism mounted on a floating vessel, with the vessel subject to periodic and transient short-term vertical movements. The lift mechanism includes a boom having a proximal end at the vessel and a distal end extendable beyond the vessel. A load line extends along the boom and is supported by the boom. The load line has a distal end connectable to a load and a proximal end engageable with the winch carried by the vessel. The winch is powered to pay out the load line and to reel in the load line. A heave compensation system comprises an accelerometer mounted on a distal end of the boom to sense the vertical movement of the distal end of the boom and transmit signals related to such movement. A winch control system controls the operation of the winch. A heave compensation control system receives signals from the accelerometer and converts such signals into control signals that are transmitted to the winch control system to cause the winch control system to control the operation of the winch to compensate for the heave being experienced by the vessel. 
     In accordance with a further aspect of the present invention, the winch control system controls the speed that the winch pays out the load line and reels in the load line. 
     In a further aspect of the present invention, the heave compensation control system converts signals from the accelerometer into control signals transmitted to the winch control system to cause the winch control system to control the direction and speed of operation of the winch to compensate for the heave being experienced by the vessel. 
     In another aspect of the present invention, a harmonic load control subsystem detects the occurrence of a harmonic load condition on the load line and adjusts the speed of operation of the winch to eliminate such harmonic load condition on the load line. 
     In another aspect of the present invention, the harmonic load control subsystem detects the load on the load line as a function of time and adjusts the speed of operation of the winch if a harmonic load condition is detected. The means for detecting the load on the load line may include transducers incorporated into the winch. 
     In another aspect of the present invention, the lift mechanism includes a crane mountable on a floating vessel. The crane has a boom with a proximal end mountable to the vessel and a distal end deployable beyond the vessel. A powered winch is carried by the crane and a load line extends from the winch along the boom to a distal end that is connectable to a load. A motion sensor is mounted on a distal end of the boom to sense the vertical movement of the distal end of the boom and transmit signals relative to such movement. A winch control system controls the operation of the winch. A heave compensation control system receives signals from the motion sensor and converts such signals into control signals to cause the winch control system to control the operation of the winch to compensate for the heave being experienced by the vessel. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a pictorial view of a crane utilizing a motion/heave compensation system of the present disclosure; 
         FIG. 2  is a view similar to  FIG. 1  with the crane shown in a different position and with the location of the components of the heave compensation system illustrated; 
         FIG. 3  is a schematic view showing the movement of the crane of  FIGS. 2 and 3 ; 
         FIG. 4  is a schematic view of the motion compensation system of the present disclosure; 
         FIG. 5  is a view similar to  FIG. 4 , but showing the hydraulic supply in more detail; and 
         FIG. 6  is a schematic of the hydraulic system for the crane. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, and in particular  FIGS. 1-3 , a lift mechanism in the form of a crane  20  is mounted on a vessel or ship  22 . The crane  20  has a base structure  24  composed of a base platform  26  mounted on the upper end of a rotatable pedestal  28  that is receivable within the corresponding structure on the ship  22 . The pedestal allows the crane to be rotated about the pedestal by a rotational drive system that can be of a standard construction. 
     The crane  20  further includes a lower arm  30  having a lower end pivotally mounted on spaced-apart mounting ears  32  extending upwardly from base  24 . A cross pin  33  is carried by the mounting ears  32 , and engages through a cross hole formed in the lower end portion of arm  30 . The arm  30  can be raised and lowered by actuation of fluid linear actuators that may be in the form of hydraulic cylinders  34  having their lower ends pinned to base  24  and their upper ends pinned to an intermediate location along the length of the arm. 
     The crane  20  also includes an upper arm  36  having a lower end portion pinned to the upper end portion of the lower arm  30 . The upper arm  36  is pivotally connected relative to the lower arm  30  by linear actuators that may be in the form of fluid  38  having lower end portions pivotally pinned to an intermediate location along the lower arm  30  and upper end portions pivotally pinned to an intermediate location along the length of the upper arm  36 . 
     Operation of the linear actuators  34  and  38  enables the crane  20  to be raised and lowered as well as to be extended and retracted. In this regard, see  FIG. 3  for the range of motion of the crane. 
     Referring specifically to  FIGS. 1 and 2 , a main load winch  50  is mounted on the lower end portion of arm  30  to pay out or reel in a main load wire rope cable or other type of line  52  which is wound about a spool  54 . A hook  56  is attached to the distal end of line  52 . Between spool  54  and hook  56 , the line  52  extends over a guide sheave  58  mounted on the distal end of arm  30 , a further guide sheave  60  mounted on the lower or proximal end of arm  36 , and a distal sheave  62  mounted on the distal end portion of arm  36 . 
     Crane  20  further includes an auxiliary winch  70  also mounted on the lower or proximal end of arm  36  used to pay out or reel in auxiliary line or cable  72  which extends along the length of arm  30  and arm  36  to carry at its distal end a hook  74 . As in cable  52 , cable  72  is also guided by a sheave  76  adjacent sheave  58 , a sheave  78  adjacent sheave  60 , and a sheave  80  located at the distal end of an arm extension portion  82  extending beyond sheave  62 . 
     Referring additional to  FIGS. 4 and 5 , a heave or motion compensation system  100  is employed on crane  20  to compensate for the heave experienced by ship  22  during operation of the crane. The heave compensation system includes a motion reference unit  102  in the form of an accelerometer mounted on the distal end portion of crane arm  36 . The accelerometer  102  measures the movement (acceleration) of the distal end of arm  36 . This information is transmitted to a microprocessor in the form of a programmable logic control processor  104 . The control processor  104  controls a winch control system that, in turn, controls the operation of the winch. The winch control system includes electric control valves  106  and  108  that control the flow of hydraulic fluid or other fluid medium to winch  50 , thereby operating the direction of rotation of the winch spool  54  as well as the speed of rotation of the spool. 
     Accelerometers such as accelerometer  102  are articles of commerce. Suitable accelerometers for use with motion compensation system  100  are used in the aerospace industry. 
     Hydraulic fluid flows from control valve  106  to winch  50  through line  110 , whereas hydraulic fluid flows from control valve  108  to winch  50  through line  112 . It will be appreciated that if hydraulic fluid is flowing to winch  50  through line  112 , the fluid is also simultaneously flowing from winch  50  through line  110 , and vice versa. Hydraulic fluid flows from valve  106  to hydraulic supply  114  through line  116 , whereas hydraulic fluid flows between valve  108  and supply  114  through line  118 . 
     The hydraulic fluid is illustrated in  FIG. 5  as stored in separate reservoirs  120  and  122  for lines  116  and  118 , respectively. However, the hydraulic fluid could be instead stored in a single reservoir. The hydraulic fluid in reservoirs  120  and  122  is cooled using ambient water that is pumped through heat transfer devices  124  and  126  in reservoirs  120  and  122  via cooling water pump  128 . 
     The motion compensating system  100  also includes an encoder  140  mounted on the distal end of boom arm  36  to measure the rotation of sheave  62 , and thus the position and movement of line  52 . A second encoder  142  is mounted on winch  50  to monitor and measure the rotation of spool  54 , and thus the speed of movement and extent of pay out or reeling in of line  52 . This information is transmitted to the logic control processor  104 . 
     The control processor  104 , as noted above, receives signals from the accelerometer  102 , which signals are related to the movement at the distal end of the crane arm  36 . Such movement is due to the heave being experienced by the vessel as well as the movement of the crane arm. The information from the encoders  140  and  142  is also transmitted to the control processor  104 . With this information, the control processor controls and detects the payout of the load line  52  as well as the payin of the line and the speed thereof. Such payout and payin is coordinated with the heave being experienced by the vessel as well as the operation of the crane itself. 
     Moreover, the control processor can be programmed to recognize a trend in the amplitude and frequency of the heave being experienced by the vessel, thereby enabling the motion compensating system of the present disclosure to better compensate for such heave. In essence, the control processor  104  is able to model the “shape” of the heave, which may be in the form of a sine wave or other wave form. 
     The motion compensation system  100  of the present disclosure also includes pressure transducers  144  incorporated into the winch  50  to detect the load on the line  52 . Such pressure transducers measure the hydraulic pressure at an applicable location in the winch that operates hydraulically. If the winch was electrically operated, suitable transducers could be incorporated into the drive system or drive mechanism of the winch. 
     As is not uncommon, when a load, such as load  150 , shown in  FIG. 4 , is attached to the end of line  52  and is being lowered or raised, the heave being experienced by the vessel  22  can result in harmonic loads being imposed on the line  52 . If a resonance condition in the line were to develop, the resulting stress imposed on the line could lead to failure of the line. Thus, the transducers  144  are utilized to measure the load on the line  52  over time. 
     In operation, the heave compensation system  100  measures the vertical movement of the distal end of the crane boom not only due to heave, but also due to the operation of the crane itself, by the use of the accelerometer  102 . Signals corresponding to such movement is transmitted to the controller processor  104 . The controller processor  104  processes this information and determines what adjustments must be made in the pay out or reeling in of line  52  to compensate for the heave and other movement being experienced at the distal end of the crane. In this regard, the controller processor operates electric control valves  106  and  108  to control the direction of rotation of the winch spool  54  as well as the speed of such rotation. In this manner, the correction of the operation of the crane due to heave of the vessel  22  is accomplished by use of the crane winch  50 , rather than requiring other additional heave-compensating apparatus or equipment, as is typical. As such, heave compensation through the present disclosure is accomplished in a more straightforward manner not requiring additional complicated or expensive apparatus or equipment. 
     Also, the motion compensating system of the present disclosure, as noted above, is able to detect oscillating load levels in the line  52 , including whether a resonance situation is developing. If so, the motion compensation system can adjust the speed of the movement of the spool  54  and thereby eliminate the development of a resonance condition. For example, applicants have found that changing the speed of the movement of line  52  by as little as 15% can eliminate a resonance condition in the line. 
       FIG. 6  illustrates a hydraulic schematic for crane  20 . 
     While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.