CA2207563A1 - Method and apparatus for monitoring of tensioned cables - Google Patents

Abstract

Structural cables, for example those in suspension bridges, are monitored by acoustic sensors placed along their length. Breakage of a wire forming part of the cable or slippage of the cable in the clamps retaining it gives rise to an acoustic signal. The signal is recorded and its location is found from the relative times at which it arrives at two sensors (except in the case of slippage, where the signal may not be strong enough to reach two sensors.) Elements of the signal can be examined to distinguish between breakage of a wire forming part of the cable, breakage of a wire of a cable attached to the cable being monitored, or slippage of the cable in its clamps.

Description

CA 02207~63 1997-06-12 METIIOD AND APPARATUS FOR MONITORING OF TENSIONED CABLES

Field of the Invention The invention relates to the detection of breakage of wires forming part 5 of cables under tension, which wires form part of structural cables.

Background of the Invention Suspension cables are widely used as structural elements in bridges and the like. For example, a suspension bridge or support for carrying a pipeline across an 0 obstacle such as a river typically will have at least one suspension cable. This is anchored at both sides of the gap to be traversed, and crosses the gap in a catenary curve, usually between two pylons or the like. Vertical cables are attached to the main suspension cable, and these extend dowllwards to support the roadway or pipeline to be carried across the gap. A typical suspension bridge has two main catenary cables, but a pipeline may have only one.

Tensioned cables are also used in other forms of construction not involving catenary curves. For example, buildings are sometimes built with a central pylon, and tensioned cables run down to the ground to make a tent-like structure or else hang 20 suspended concrete slab floors from the pylon.

In many cases tensioned cables are exposed to the elements. This is particularly the case in suspension bridges and the like, where the suspension cables are totally unprotected (except for whatever paint or anti-rust coating is placed on them).
25 Such bridges are also are frequently in high corrosion environrnents, such as near salt water.

Cables of the type used in a large suspension bridge are bundles of smaller wires. A typical wire has a diameter of approximately 0.1 inch. A cable can be formed CA 02207~63 1997-06-12 of up to 10,000 of such wires clustered together. Typically, the cable is formed with a circular cross section.

The end of cables are typically clamped within a steel clamp. There are s also clamps known as hangers ~1tacl~ing vertical cables to the catenary cable. The vertical cable descend to hold the structure being supported. Depending on the nature of the structure being supported, there may also be T-clamps at the bottom ends of the vertical cables, so that horizontal cables supporting the road or pipeline can be attached. All of these types of clamps will be described herein by the general word clamps.

Frequently, where a cable terminates in a clamp, the cable end is filled with potting metal, so the clamp will have a better grip, and to prevent moisture and the like attacking the ends of the wires.

Individual wires in a cable may break from corrosion at any point along the cable. However, breakage is most frequent in or around the area of the clamps. There is also breakage due to fretting of wires together, which usually occurs within the clamps, especially if the wires have not been immobilized with respect to one another by the use of potting metal. Even when potting metal is present, some fretting may occur.
2() There is also another problem which occurs around the clamps, which is the gradual slippage of cable or individual wires that which from the cable out of the clamp.

2s Various methods have been used to attempt to inspect cables for breakage.
Visual inspection is of course possible, but this shows only the outer wires of the cable bundle. ~Ires inside the cable bundle cannot be seen, and therefore their state cannot be ascertained. It is of course possible, and has typically been done, to spread the cable for a short period of time in one or more selected locations using a sledge hammer and a CA 02207~63 1997-06-12 wedge, so as to be able to see inside the cable. However, this requires the inspector to approach the cable in the place where inspection is to be done. This can be very difficult, as many portions of the cable are suspended in mid air.

It is also possible to inspect the exterior of the cable by moving a television camera along it to view whether there is exterior corrosion, or moving along a magnet which will locate local magnetic anomalies which may be indicative of breakage.

None of these methods is very satisfactory, even for the portions of the o cable which can be accessed by them. They cannot access the portions of the cable within the clamps, so are not able at all to determine the state of the cable within such clamps.
Additionally, they cannot determine whether there is creep within the clamps, unless such creep occurs to such an extent that there is a visually observable movement of the entire cable relative to the clamp.

Summary of the Invention The present invention provides a system for monitoring of a cable which registers breakage of wires as it occurs, and gives the location of such breakage. In many cases, it registers creep of the cable through the clamp holding it in place. It can also register breakage of a wire in a cable which is attached to the cable being monitored. It is non-destructive, and does not require inspectors to go into in~ccessihle locations.

Dcsc. ;I,lion of the Invention According to the invention, acoustic sensors are placed at selected locations along the cable, and their output is continuously monitored. The output is recorded, at least temporarily, so that it can be retrieved if an apparent event of interest is determined to have happened. The acoustic sensors are preferably accelerometers, CA 02207~63 1997-06-12 which measure the deflection of the surface of the cable as an acoustic or other wave propagates through the cable.

s The sensors are preferably high impedance accelerometers. The use of high impedance devices provides some protection in case of, for example, a li~;htning strike on the cable, and also permits a large number of sensors to be attached to the same data collection line. The sensors are placed at known locations along the cable.
0 In a plt;r~llt;d embodiment ofthe invention, sensors are located at or near the nodes. A node is defined as where two cables are clamped to one another (as for ~x~llple by a hanger) or where a cable end is clamped to an anchor (as for example where a catenary cable is anchored at its two ends).

The speed of sound in the cable is usually a known quantity, but if desired it can be measured by determining the time taken for an acoustic wave to pass between two sensors a known distance apart.

When a wire breaks, it causes an acoustic event, and sends acoustic waves in both directions along the cable. These acoustic waves are recorded by the acoustic sensors. Thus, the location ofthe break can be determined by measuring the difference in the length of time that is taken for the signal from the breakage to reach the first sensor on either side of the breakage. It is pl~relled that there be a sensor at or near any node which is the termination of the cable being monitored, so that the location of breakages 2s near the cable end can be determined accurately. Where the cable passes through a node (for example if the cable is a catenary cable and the node is the point where a vertical cable is ~tt~ched to it along its length) the signal will propagate through the node. Thus, where monitoring a cable which continues through a node, it is not necessary to have a sensor at that node. However, it is pr~rell~d to have sensors at each node, as such CA 02207~63 1997-06-12 sensors will acquire useful information as discussed hereinafter about cable creep and the nature of the event causing the signal.

The use of at least one sensor to each node requires the use of a large 5 number of sensors on a large structure, such as a long suspension bridge. There must be means for the sensors to transmit signals which they have sensed to a recording device.
This can be done by joining the sensors to data ch~nnPl~, or by providing each sensor with a tr~n~ cer that transmits signals from it, together with some identifier for the particular sensor, to the recording device. Several sensors can be connected to a common data 0 ~h~nnP.l, provided that each sensor gives some unique i~entifiPr for its signals. Alternately, a multiplexing system can be used in which there are more than two data lines, and each sensor is connected to a pair of such lines which pair is not common to any other sensor.
Thus, when the same signal is received over both lines of a pair, the sensor which generated that signal is known.

In an especially pl~relled form ofthe invention, at least two accelerometers are located at the same location along the length of a cable, but are spaced from one another around the circumference of the cable. Preferably, they are oriented relative to the axis ofthe cable at some angle from one another other than 180~, most preferably, at 20 an angle of 90~.

Use of two sensors oriented in this way provides additional information about the cause of the acoustic event. It is observed that the two sensors pick up slightly different signals in some cases. Comparing these signals provides a ~C~ign~t~re~ of the 25 event, and this "c i~n~t--re" will be di~renl depending upon whether the event is caused by the breakage of a wire, or by cable creep, or by tr~n~mic~ on of an event from a cable which attaches at a node to the cable being monitored.

CA 02207~63 1997-06-12 While it is not desired to limit the invention by the following theoretical explanation, the applicant theorizes that a wire breakage creates both a pressure wave, arising from the fact that the wire was under tension and this tension and this tension is abruptly released, and also other, more complex waves. The pressure wave moves 5 e~nti~lly linearly along the axis ofthe cable, so reaches both of the accelerometers which are angularly spaced around the circumference at the same time. It is believed that he other complex patterns arrive slightly later, and may travel slightly di~erell~ paths to reach the two accelerometers. Thus, the signature of a wire breakage, at least with some types of cable, is a sharp peak which reaches both angularly separated accelerometers at the 0 same time, followed by several smaller peaks which may differ at the two accelerometers.

Where the breakage event is in another cable, and is tr~n~mitted to the cable being monitored through a node, the pressure wave is absent at least in most cases, and, the signature is several peaks, which may or may not differ in the two accelerometers.
This will be registered at the accelerometers at that node (if there are such accelerometers) or other accelerometers in both directions down the cable.

If the acoustic event is caused by creep, it will be much weaker than the acoustic event caused by breakage, and the pressure wave will be absent or of a dirrel en~
20 type than that of a break. Where there is a series of small peaks, and these register either only at one node, or else strongly at that node and only weakly at sensors spaced from that node, creep instead of breakage can be suspected. Signals attçn..~te with distance, and the signals given by creep are sufficiently small so that they register only poorly if at all at sensors removed from the node where the creep occurred.
It is not necessary to put a sensor or a group of sensors at every node where a hanger is connected to a catenary cable. Sensors on the catenary cable will identify wire breakage in vertical cables connected by hangers to that catenary cable, as the shock is tr~n~mitted to the catenary cable through the hanger. Such information will CA 02207~63 1997-06-12 tell where along the catenary cable the shock was transmitted. This permits determination of which of the vertical cables has had a wire break. It is useful to know how many wire breaks have occurred in a particular vertical cable, whether or not the precise locations of these wire breaks can be det~nnined. Afcer a certain number of wire breaks are known 5 to have occurred in a particular vertical cable, it may be decided to replace that vertical cable, even though the precise locations of the wire breaks are not known.

Where a cable is has sufficient sensors (such as for example at its nodes) to enable the location of wire breaks to be determined with some accuracy, it is possible o to use this information to determine the replacement schedule for that cable. The wires within a cable have what is called a "development length". This is defined as being the distance over which a cut in the wire has an effect. It will be recalled that each of the wires is tightly in contact with the other wires of the cable. Thus, when the wire is cut, it may not lose tension over its entire length, depending on the length. Instead, after a certain distance (which varies with the nature of the wire and the cable), its contact with other wires of the cable causes it to have as much tension as if it had not been cut. It is found for ~A~llplc, that in a particular cable of 1000 wires, and a length of 1000 feet, the deliberate cutting of one wire has no effect on the tension of that same wire 100 feet away. It appears that the tension imparted to the wire by its contact with other20 surrounding wires is enough to prevent tension at being lost. The development length for the particular cable is Lhelt;r~le defined as being less than 100 feet. Thus, if there are two breaks in the same wire 600 feet away from one another, the portion between the two breaks retains tension, even though it is severed from its anchor points at both ends.

When the precise locations of the breaks in wires in a cable are known, a determination can be made as to whether cable replacement is necessary by considering the total number of breaks and the number of breaks which are within the development length of each other. If breaks are scattered from one another by more than the development length, it may be acceptable not to replace the cable after a certain total CA 02207=,63 1997-06-12 number of breaks has occurred. However, if there are numerous breaks within one development length of each other, it may be prudent to replace the cable when the same total number of breaks has occurred.

s While the above description con~titutes a prerelled embodiment of the present invention, it is to be understood that the invention is not limited thereby and that in light of the present disclosure of the invention, various other alternative embodiments will be apparent to persons skilled in the art. Accordingly, it is to be understood that changes can be made without departing from the scope of the invention as particularly o pointed out and distinctly claimed in the claims set forth below.

Claims (9)

1. A method of monitoring a tensioned cable which comprises:

positioning a plurality of first acoustic sensors along the cable at known locations monitoring such sensors for signals noting the relative time of arrival of an acoustically-sensed signal at a at least two such sensors determining from the relative arrival times of such signal the location along the cable of the origin of the signal.

2. A method as claimed in claim 1, in which each said first acoustic sensor has in association with it a second acoustic sensor which is displaced from it a desired distance around the circumference of the cable.

3. A method as claimed in claim 2, in which the signals from the said first sensor and the said second sensor associated with it are compared, and the comparison is used to determine the probable nature of the cause of the signals.

4. A method as claimed in any of claims 1-3, in which at least one of said knownlocations is adjacent a location where such cable is attached to another cable.

5. A method as claimed in either of claims 3 or 4 in which said second acoustic sensors are accelerometers.

6. A method as claimed in any of claims 1-5, in which said first acoustic sensors are accelerometers.

7. Apparatus for monitoring a tensioned cable, comprising a plurality of acoustic sensor means positioned at known locations along the cable recording means for recording signals sensed by such sensor means timing means recording when such signals are recorded by the recording means, so that the relative arrival times of the signal of at least two such sensors can be determined.

8. Apparatus as claimed in claim 1, additionally comprising means for calculating the location of the event causing such signal along the cable from said relative arrival times.

9. Apparatus as claimed in either of claims 7 or 8, additionally comprising a common data channel for transmitting signals from at least two of said acoustic sensor means to said recording means, and means for distinguishing which of said two sensor means has sensed such transmitted signals.