WO2009011595A1

WO2009011595A1 - Chain load measurement

Info

Publication number: WO2009011595A1
Application number: PCT/NO2008/000270
Authority: WO
Inventors: Audun Andersen
Original assignee: Scan-Sense As
Priority date: 2007-07-17
Filing date: 2008-07-17
Publication date: 2009-01-22
Also published as: NO332193B1; NO20073676L

Abstract

A method for measuring the tensile force in a studless chain (6), such as an anchor chain for a floating offshore structure, wherein a link (7) of the chain is held securely in a chain stopper (10) in a jack winch while a chain link (11) above is lowered by means of the lifting device (14) of the jack winch so that an opening is created between the upper bend (18) of the link (7) held securely and the lower bend (15) of the link (11) above. Next, a load measuring device (1) comprising a compression load cell (2) is inserted into the opening, whereupon the lifting device (14) tensions the chain so that the force measuring device (1) is jammed in place between said bends (18, 15) and takes up the entire tensile force (F) during further raising of the lifting device (14). The tensile force may be read in a portable device (17) connected to the load cell (2). The tensile force may also be read during the following lowering of the lifting device (14) in order to release the force measuring device (1). A force measuring device (1) suitable for carrying out the method is also shown.

Description

CHAIN LOAD MEASUREMENT

The present invention relates to a method for measuring the tensile force in a studless chain, such as an anchor chain for a floating offshore structure, wherein the chain extends through a jack winch or the like having a chain stopper, above which a lifting device is arranged which has a gripping device for the chain and which can lift the chain at least one link at a time.

A jack winch of this type is e.g. shown in US 6655661. Here, the gripping and lifting devices cooperate with a chain wheel, the gripping device acting on a standing chain link in the chain wheel and rotating together with the lifting device and the chain wheel about the rotational axis of the latter during the lifting operation of the jack winch. The jack winch has a chain stopper located in the chain path a distance below the lifting and gripping devices, said chain stopper being brought in operation in order to hold the chain in the chain link there above, so that the lifting and gripping devices can be rotated back to engage the next standing link in the chain wheel, in order to perform a new lifting operation while the chain stopper is in a released position.

The chains in question can have very large dimensions. For instance, each chain link may have a length exceeding 90 cm, a thickness of about 15 cm, weigh about 300 kg, and have a working load of 300 tons. The strength of the anchor chain is of decisive importance for the ability of offshore structures to survive extreme weather conditions, and it is therefore necessary to know what strains the chain has been subjected to in order to prevent fatigue failures. The chain tension is therefore monitored by means of load cells arranged in one or more of the supporting points for the chains on the offshore structure, but such load cells can normally not measure the chain load directly. Besides, they require periodic maintenance and calibration, which is very difficult, if not impossible, while the chain is in operation.

The present invention aims at alleviating this problem. This is obtained according to the invention by means of a method as recited in claim 1 and a force measuring device as recited in claim 6.

The method according to the invention includes placing a compression load cell temporarily between two links of the chain which are located in unloaded condition between the chain stopper and lifting device of the jack which, whereupon the lifting device is brought to load the two links with the chain force, thus permitting it to be read by means of the load cell. Following the reading, the chains may be unloaded anew, in order for the load cell to be removed.

According to an advantageous embodiment of the method, the chain is moved in the measuring phase up and down a distance sufficiently large to provide sliding friction in the guiding devices of the chain below the chain stopper, while concurrently reading and recording continuously the signal from the load cell. The difference between measurements at static and sliding friction, and the difference between the movement in and out, may be useful in order to determine the condition of the guiding devices and the actual tension in the chain extending from the floating structure.

The method and force measuring device according to the invention will be described more closely with reference to the exemplifying embodiment illustrated schematically in the appended drawings, where

Figure 1 is a perspective sketch of an example of a force measuring device according to the invention;

Figure 2 is a schematic vertical section through a part of a chain which extends through a jack winch, with the chain in a first position;

Figure 3 is a section similar to Figure 2 with the chain in a second position; and

Figure 4 is a section similar to Figure 2 with the chain in a third position.

The force measuring device illustrated in Figure 1 is generally designated 1 and comprises a compression load cell 2, which is provided with axial extensions in the form of end pieces 3, 4, which at their free end are shaped according to the internal shape of the rounded portions of the chain links for which the force measuring device is intended to be used. Thus, the end piece 3 has an end surface 5 of double curvature, and the opposite end piece 4 has a corresponding end surface rotated 90 degrees about the longitudinal axis of the force measuring device. The force measuring device has a generally cylindrical form and preferably has somewhat smaller diameter than the thickness of the chain links for which it is to be used. The end surface 5 may have the approximate form of a saddle surface.

The compression load cell 2 may be based on strain gauges, but the skilled person will understand that also other types may be used. The end pieces 3 and 4 are preferably made of steel, and the end surfaces 5 may have a surface treatment or a coating which provides a lowest possible friction in contact with the chain links. The load cell 2 may have a capacity in the order of 300 tons. The length of the force measuring device may vary according to the shape and size of the chain links, but will normally be somewhat s below half the length of the opening of the chain links.

It is referred to Figure 2, which shows a vertical section through a portion of a chain which is generally designated 6 and which is subjected to a substantial tensile force F in the downwards direction. The chain has a first link 7 which is held securely by the pivotable pawls 8, 9 of a chain stopper generally designated 10 and not shown in further io detail.

The first link 7 is connected to a second link 11, which in turn is connected to a third link 12. For the purpose of illustration, a schematically shown gripping device 13 which can hold securely the third link 12 in a likewise schematically suggested lifting device 14 on the jack winch. In practice, the gripping device can engage a chain link I₅ higher up, and this may be decisive for the length of the chain that may be moved during the measurement of the chain load.

In Figure 2, arrows below the lifting device 14 suggest that the latter and the gripping device 13 are on their way downwards. Figure 3 illustrates the situation at the end of this movement, where the third link 12 has gone into contact with the first link 7 and the

20 lower bend 15 of the second link 11 is located at the bottom of the opening 16 in the first link 7. Figure 3 shows that the force measuring device 1 according to the invention is inserted in the opening 16 above the lower bend 15. The load cell 2 of the force measuring device is shown to be connected to a portable reading device, which preferably also will be able to continuously register the indicated force, e.g. a portable

25 computer.

Once the force measuring device 1 is brought in place in the opening 16 as shown in Figure 3, the lifting device 14 is moved slowly as shown by the arrows in Figure 4 until the force measuring device 1 comes into contact with the upper bend 18 of the first link 7. When it has been established that the force measuring device is located in the correct ₃₀ position, the lifting device 14 is once more activated in order to overcome the tensile force F in the chain and lift the first link 7 clear of the pawls 8, 9 of the chain stopper 10, in order for the pawls to be pivoted outwards as shown in Figure 4. The force measuring device 1 now transmits the entire force F between the first link 7 and the second link 11, in order for the force F to be measured quite accurately. If desirable, the lifting device continues its upwards movement as long as there is no noticeable pivoting of the second link 11. This distance may vary dependent upon the construction of the jack winch and lifting device, but in most cases it will be at least equal to the length of the force measuring device 1. It may therefore be advantageous to make this device as long as possible, although it will be understood that it will also be able to function with substantially shorter end pieces than those shown in Figure 1.

After the lifting device having reached its upper permissible position, the movement is again reversed and the lifting device permitted to lower the chain slowly until the first link 7 can come to rest on the pawls 8, 9 of the chain stopper, whereupon the second link 11 is lowered further in order to make it possible to take out the force measuring device 1. During the lowering movement, it will also be possible to register the measurement signal from the load cell, so that by comparing the variations of the chain force F useful information can be obtained of the type mentioned previously.

It will be understood that the invention is not limited to the exemplifying embodiment described above, but may be modified and varied by a skilled person within the scope of the following claims.

Furthermore, it will be understood that it may be necessary to make minor changes in some jack winches in order to ascertain that the lifting device can pull the chain back in order to create the necessary space for the force measuring device. However, such changes will be within the competence of the skilled person and therefore need no further description here.

The relatively large length of the force measuring device is of importance for the even transmission of the forces into the load cell. The three parts 2, 3, 4 of the force measuring device may be approximately of the same length and preferably be detachable, so that the parts may be brought in place between the chain links, one on top of the other. The parts may be held in place laterally by means of a central tap (not shown) on the end pieces 3, 5 extending into a corresponding recess in the end surfaces of the load cell 2. Hereby the parts may be mutually rotatable, which permits the end pieces to be self-aligning if the chain links 7, 11 are not positioned at exactly 90° with respect to each other. A lubricant on the contact surfaces between the parts can make the self-alignment easier.

Claims

C l a i m s

1. A method for measuring the tensile force (F) in a studless chain (6), such as an anchor chain for a floating offshore structure, wherein the chain extends through a jack winch or the like which has a chain stopper (10) and a lifting device (4) arranged above

5 the chain stopper, the lifting device having a gripping device (13) for the chain and being able to lift the chain at least one chain link at a time, the chain stopper (10) being able to hold the chain securely in at least one of its links (7) while the lifting device (14) is lowered to engage the chain anew by means of its gripping device (13), c h a r a c t e r i z e d i n that while the chain stopper (10) holds a chain link (7) io securely, the lifting device (14) is used to lower the link (11) above so that a free space is created between the top of the opening (16) in the link (7) held securely and the bottom of the opening in the link (11) above, in that a force measuring device (1) is placed in said free space, in that the chain (6) is tensioned by means of the lifting device (14) such that the chain

I₅ stopper (10) can be released, whereupon the force measuring device (1) comes into engagement between said top and bottom and transmits the entire tensile force (F) in the chain between the previously securely held link (7) and the link (11) above, and in that the tensile force (F) is determined by means of the force measuring device (1).

2. A method according to claim 1,

20 wherein the previously securely held link (7) is lowered and anew is held securely by the chain stopper (10), whereupon the lifting device (14) is lowered further so that the force measuring device (1) may be removed.

3. A method according to claim 1 or 2, wherein the determined tensile force (F) is used to calibrate a further force measuring is device permanently arranged in the j ack winch or in another place on the floating structure.

4. A method according to claim 1, 2 or 3, wherein during the determination of the tensile force (F), the lifting device (14) is lifted sufficiently for sliding friction to occur in guiding devices for the chain (6) below the 30 chain stopper (10).

5. A method according to claim 4, wherein the tensile force (F) is also determined during the return movement of the lifting device (14) until the chain stopper (10) is activated once more.

6. A force measuring device (1) for measuring the tensile force (F) in a chain (6) having studless links (7, 11, 12), comprising a compression load cell (2) for arrangement between two of the links (7, 11) of the chain, c h a r a c t e r i z e d i n that the force measuring device (1) is a portable unit which is separate from the chain links, and in that the compression load cell (2) of the force measuring device is provided with axial extensions (3, 4) which at their free end (5) are shaped at least partly to the internal shape of at least parts of the rounded portions (18, 15) of the chain links (7, 11).

7. A force measuring device according to claim 6, wherein the force measuring device (1) has a generally cylindrical form between the shaped free ends, the latter preferably having the approximate form of a saddle surface.

8. A force measuring device according to claim 6 or 7, wherein the compression load cell (2) is based on strain gauges.

9. A force measuring device according to claim 6, 7 or 8, wherein the compression load cell (2) has a capacity in the order of 300 tons.

10. A force measuring device according to any one of claims 6-9, wherein the compression load cell (2) is connectable to a reading device (17) for continuously registering the chain force (F) during the use of the force measuring device

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