Load handling device and method for using the same

Abstract

A load handling device is for lifting and lowering a load. The load handling device includes an elongated member adapted to be connected to a load and a capstan, including one or more sheaves, through which the elongated member is running, the capstan defining a low tension side and high tension side of the elongated member. The load handling device further comprises a tension regulating member adapted to operate at the low tension side of the elongated member. A method is for lifting and lowering a load via a load handling device.

Claims

1. A load handling device for lifting and lowering a load, the load handling device comprising: an elongated member configured to be connected to the load; a capstan, through which the elongated member is running, the capstan constituting one of a force amplifier and a force reducer used in lifting and lowering the load, and having a plurality of sheaves that are each separately, directly and actively rotatably driven by an external force and configured to modify a force distribution in the elongated member, the capstan defining a low tension side and high tension side of the elongated member such that the force distribution in the elongated member along a sheave in the plurality of sheaves is different when the load is lifted compared to when the load is lowered; a load sensing device provided at the high tension side of the capstan; a tension regulating member; and a control unit that regulates the tension regulating member based on sensed loads from the load sensing device such that the tension regulating member actively regulates the tension on the low tension side of the capstan in response to the tension measured by the load sensing device on the high tension side of the capstan so as to reduce a difference between the force distribution in the elongated member along the sheave in the plurality of sheaves in the capstan when lifting the load and the force distribution in the elongated member along the sheave in the plurality of sheaves in the capstan when lowering the load and thereby reduce tension-tension fatigue in the elongated member; wherein the control unit separately drives the plurality sheaves.

2. The load handling device according to claim 1, further comprising a load sensing device on the low tension side of the capstan.

3. The load handling device according to claim 1, wherein the tension regulating member comprises a storage drum on which at least a part of the elongated member may be stored.

4. The load handling device according to claim 1, wherein the tension regulating member comprises a separate tension control system.

5. The load handling device according to claim 1, wherein the plurality of sheaves includes individually controllable sheaves, and wherein the tension regulating member comprises at least one of sheave engaging and a disengaging unit.

6. The load handling device according to claim 1, wherein the sheave in the plurality of sheaves is at least partially made from a material with a higher friction coefficient than steel.

7. The load handling device according to claim 1, wherein the control unit is connected to a storage unit.

8. A vessel comprising a load handling device for lifting and lowering a load, the load handling device comprising: an elongated member configured to be connected to the load; a capstan through which the elongated member is running, the capstan constituting one of a force amplifier and a force reducer, and having a plurality of sheaves that are each separately, directly and actively rotatably driven by an external force and configured to modify a force distribution in the elongated member, the capstan defining a low tension side and high tension side of the elongated member such that the force distribution in the elongated member along a sheave in the plurality of sheaves is different when the load is lifted compared to when the load is lowered; a load sensing device provided at the high tension side of the capstan; a tension regulating member; and a control unit that regulates the tension regulating member based on sensed loads from the load sensing device such that the tension regulating member actively regulates the tension on the low tension side of the capstan in response to the tension measured by the load sensing device on the high tension side of the capstan so as to reduce a difference between the force distribution in the elongated member along the sheave in the plurality of sheaves in the capstan when lifting the load and the force distribution in the elongated member along the sheave in the plurality of sheaves in the capstan when lowering the load and thereby reduce tension-tension fatigue in the elongated member; wherein the control unit separately drives the plurality sheaves.

9. A method for lowering and lifting a load via a load handling device for lifting and lowering a load, the load handling device comprising: an elongated member configured to be connected to the load; a capstan through which the elongated member is running, the capstan constituting one of a force amplifier and a force reducer, and having a plurality of sheaves that are each separately, directly and actively rotatably driven by an external force and configured to modify a force distribution in the elongated member, the capstan defining a low tension side and high tension side of the elongated member such that the force distribution in the elongated member along a sheave in the plurality of sheaves is different when the load is lifted compared to when the load is lowered; a load sensing device provided at the high tension side of the capstan; a tension regulating member; and a control unit that regulates the tension regulating member based on sensed loads from the load sensing device such that the tension regulating member regulates the tension on the low tension side of the capstan in response to the tension measured by the load sensing device on the high tension side of the capstan so as to maintain a predetermined force distribution in the elongated member within the capstan; the method comprising: measuring the tension on the high tension side of the capstan via the load sensing device; and actively regulating, via the tension regulating member, the tension on the low tension side of the capstan in response to the tension measured by the load sensing device on the high tension side of the capstan so as to reduce a difference between the force distribution in the elongated member along the sheave in the plurality of sheaves in the capstan when lifting the load and the force distribution in the elongated member along the sheave in the plurality of sheaves in the capstan when lowering the load and thereby reduce tension-tension fatigue in the elongated member; wherein the control unit separately drives the plurality sheaves.

10. A method according to claim 9, further comprising: adjusting the tension on the low tension side of the elongated member via the tension regulating member so that the force distribution in the elongated member in the capstan when lifting the load is essentially equal to the force distribution in the elongated member in the capstan when lowering the load by: increasing the tension on the low tension side of the elongated member when lowering the load compared to when lifting the load; and lowering the tension on the low tension side of the elongated member when lifting the load compared to when lowering the load.

11. The load handling device according to claim 1, wherein the tension regulating member comprises a sheave engaging and disengaging unit for engaging and disengaging one or more sheaves in the plurality of sheaves, wherein the controller is configured to alternately control the sheave engaging and disengaging unit to engage the one or more sheaves to thereby cause the one or more sheaves to rotate and to disengage the one or more sheaves to thereby allow the one or more sheaves to freely rotate.

12. The load handling device according to claim 1, wherein the tension regulating member comprises a storage drum configured to store a portion of the elongated member and three sheaves in the plurality of sheaves, wherein in a direction from the capstan towards the storage drum, the elongated member travels 90 degrees over a first sheave of the three sheaves, the first sheave being in a fixed position, then 180 degrees over a second sheave of the three sheaves, which second sheave being movable up and down by a drive unit, and then 90 degrees over a third sheave of the three sheaves, the third sheave being in a fixed position, wherein the tension regulating member regulates tension in the elongated member by moving the second sheave.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following examples of preferred embodiments are shown with reference to accompanying drawings where:

(2) FIG. 1 shows schematically a rope running through a capstan as used with a load regulating device according to the present invention;

(3) FIG. 2 shows in a perspective view a capstan as used with a load regulating device according to the present invention;

(4) FIGS. 3-10 are graphs showing the force distribution on the rope in the capstan as a function of the number of half turns;

(5) FIG. 11 shows schematically a first embodiment of a load handling device according to the present invention; and

(6) FIG. 12 shows schematically a second embodiment of a load handling device according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(7) In the following the reference numeral 1 indicates a load handling device according to the present invention. Identical numerals refer to identical or similar parts, and the figures are shown schematically and simplified.

(8) FIGS. 1 and 2 show a capstan 2, schematically and in perspective, respectively. The capstan 2 acts as a force amplifier or a force reducer for an elongated member 3 in the form of a rope running through the capstan 2. The capstan defines a low tension side 31 with a rope force S2 and a high tension side 33 with a rope force S1. When lifting a load 5, see FIGS. 11 and 12, the rope 3 enters the capstan 2 on the high tension side 33 and exits on the low tension side 31. Contrary, when lowering the load 5, the rope 3 enters the capstan 2 on the low tension side 31 and exits on the high tension side 33.

(9) The rope 3 in the capstan 2 of FIG. 2 travels five full turns, i.e. 10 half turns, around sheaves 21 of the capstan 2, which is of a double drum type. The angle in the exponent of the Eytelwein formula will thus be 10 times (31.4). A friction of 0.125 will be used in the following examples, hence the amplification factor S1/S2 will be approximately 51 in our examples.

(10) The graph in FIG. 3 shows the rope force F given in tons (t) in a capstan 2 in which the rope 3 travels seven full turns, i.e. 14 half turns, as a function of the number of half turns N, i.e. the number of sheaves 21 over which the rope runs. If setting the rope force on the low tension side 31 to 1 ton (1 t), the rope force on the high tension side 33 could be as high as 244 t before the rope 3 starts sliding, i.e. the amplification factor is 244. The exemplary system is designed for lifting loads 5 with a rope force S1 of 50 t, which would require five full rope turns around the capstan 2 only.

(11) The graph of FIG. 4 shows a theoretical force reduction along the rope arcs around the capstan 2 from the low tension side 31 to the high tension side 33. The rope 3 enters on the high tension side 33 and travels over sheaves 14-11 with a force equal to that on the high tension side 33, i.e. 50t. According to the Eytelwein formula, the rope force will reduce according to the exponential function as described above until it reaches the lower force S2 of 1 t.

(12) According to established theory which is the basis for the design of all capstans according to prior art, the maximum rope force in the system will always be found on the entrance of the capstan 2 when lifting a load 5 and on the exit of the capstan 2 when lowering a load 5. In practice, however, force distributions like the one shown in FIG. 5 can be found. An exponential function builds up from both sides of the capstan 2, creating a force peak inside the capstan 2. In this specific example the force peak on the 12.sup.th sheave 21 will be more than twice as high the rope force on the high tension side 33. The rope 3 travelling through the capstan 2 will thus not only do four unnecessary half turns around the sheaves 21 of the capstan 2, the bending cycles will be done under loads higher than what is considered to be a maximum rope force of the system.

(13) The graph in FIG. 6 shows the difference in force distribution on the rope 3 in the capstan 2 when lifting (solid line) compared to when lowering (dashed line) a load 5. The rope force on the high tension side 33 is 50 t, while the rope force on the low tension side 31 is 1 t. The capstan 2 may be working in heave compensation mode continuously switching between lifting and lowering the load 5. When lifting the load 5, the load 5 will enter the capstan 2 on the 14.sup.th sheave 21 and travel with the same rope force over sheaves 14-11 until the force on the rope 3 falls off exponentially to it on the 1.sup.st sheave 21. When lowering the load 5, the rope 3 will enter on the low tension side 31 of the capstan 2 and travel over sheaves 1-4 with a constant, low rope force of it until the force on the rope 3 increases exponentially to 50 t at the exit of the last sheave 21.

(14) When lifting, the rope force on the 10.sup.th sheave will have a rope force of 50 t, as indicated with the with letter A in FIG. 7. However, when lowering, the rope force on the 10.sup.th sheave will be only 10.4 t, indicated by the letter B, meaning that the rope force on the same sheave 21 will be almost fivefold when lifting compared to when lowering. With the rope 3 repetitively lifting and lowering, the rope section around the 10.sup.th sheave will be either in point A or in point B on the graph. As described above, this will lead to a great amount of tension-tension fatigue and additional abrasion on the rope 3 and on the sheaves 21.

(15) According to the present invention, one solution to overcome the above mentioned drawbacks is to regulate the tension on the low tension side 31 of the rope 3, so as to reduce for instance the big gap between points A and B in FIG. 7. Several possible solutions for regulating the tension on the low tension side are discussed above. A result of adjusting the force on the low tension side 31 can be seen from the graph in FIG. 8. The force on the high tension side 33 of the rope 3 is still 50 t, while the force on the low tension side 31 is reduced to 0.4 t. The points A and B still show the force on the 10.sup.th sheave when lifting and lowering, respectively. The force in point A is now 20.3 t while the force in point B is still 10.4 t.

(16) If the force on the low tension side 31 is lowered as much as to 0,205 t, the curves for lifting and lowering, and thus the points A and B will coincide like shown in FIG. 9. The coinciding curves imply that no substantial changes in rope force will occur when changing from lifting to lowering. The two curves become identical when the force on the low tension side is equal to the force on the high tension side divided by the amplification or reduction factor of the capstan:

(17) $\frac{S_1}{S_2}{e}^{}$

(18) This means that the system is operating at or near the force on the low tension side for which the rope 3 starts sliding.

(19) It may be beneficial to vary the rope force on the low tension side 31 of the rope 3 when the motion of the rope is reversed. In FIG. 10 an example of a force distribution in a capstan 2 is shown where the rope force on the low tension side is increased when lowering the load 5 compared to when lifting the load 5. The force distribution in the rope 3 will only change slightly over the capstan 2 and the rope 3 will be operated away from the sliding limit. Compared the dashed lines, indicating the force distribution when lowering the load 5, in FIGS. 7 and 8, the dashed line in FIG. 9 represents an increased overall load on the rope 3 when lowering the load 5. However, the changes in load levels when reversing the rope is significantly reduced. If the load level on the low tension side is reduced to a minimum when lifting and then raised when lowering the load 5, the overall load level in a capstan system can be smaller than in a system without these control mechanisms, and the changes in load level when reversing the motion will be reduced as well. Operating near a load level which creates the same load distribution for lifting and for lowering (coinciding curves like shown in FIG. 7), and slightly increasing the load level on the low tension side when lowering the load will bring both the load level and the changes to a minimum and still guarantee safe operation.

(20) FIG. 11 shows a first embodiment of a load handling device 1 according to the present invention. The rope 3 is stored on a storage drum 7 and runs through a guide sheave 6 before it enters the capstan 2 on the low tension side 31. The rope 3 exits the capstan 2 on the high tension side 33 and runs through also a second guide sheave 6. A load 5 is suspended from the end of the rope 3 on the high tension side 33. In the shown embodiment the storage drum 7 itself acts as a tension regulating member by adjusting the tension of the rope 3 on the low tension side 31. This embodiment may be beneficial for use in lifting and lowering operations not requiring heave compensation due to the potential large inertia of the storage drum 7. The sheaves 6, 6 are provided with load cells 8, 8 for measuring the load on the rope 3 at both the low tension side 31 and the high tension side 33. The load cells 8, 8 may further be connected to a not shown control unit 34 adapted to control the motion of the storage drum 7 based by means of a drive unit, at least partially based on the loads sensed by the load cells 8, 8.

(21) FIG. 12 shows an alternative embodiment of the load handling device 1. A separate tension control system 9 is provided between the storage drum 7 and the capstan 2 for regulating the tension on the rope 3 on the low tension side 31 as explained above. The tension control system 9 is adapted to respond quickly to the motion of the load, so as to adjust the tension of the rope 3 on the low tension side also in heave compensation operations. In the shown embodiment the tension control 9 system comprises three sheaves 6, 6, 6. In the direction from the capstan towards 2 the storage drum 7, the rope 3 travels 90 over a first sheave 6 with a fixed position. The rope 3 then travels 180 over a second sheave 6 which can be moved up or down by a not shown drive unit, such as a hydraulic cylinder. Finally the rope 3 travels 90 over a third sheave 6 with a fixed position. The rope tension can be increased by lifting the middle sheave 6 by means of the drive unit, thereby stretching the rope 3 and increasing its tension. In a similar way, the rope tension can be reduced by lowering the second sheave 6 by means of the drive unit.