Abstract:
A sensor device for registering voltage drops on corrosion exposed structures and coupled to a surface area thereof to which a voltage is supplied by electrodes causing an excitation current in that area and having a plurality of cables connected to a plurality of sensors arranged in a matrix defining measurement points with defined distances.

Description:
The invention concerns a sensor device for registering voltage levels on corrosion exposed structures. These structures may be pipes and supports in offshore environments, to which a voltage is supplied so that an excitation current flows in the structure. The invention also concerns a method for performing measurements with such a sensor device. The device may be utilized for detection of internal and external corrosion, crack formation, erosion and other defects. 
     BACKGROUND OF THE INVENTION 
     NO patent No. 150 136 teaches a method and a device for monitoring steel constructions in order to detect defects, e.g. cracks. By this system fixed measuring points are utilized to register voltage drops when an excitation current is supplied. By comparing the voltage drop at a certain site and a certain time with previously registered measurements at the same site, any development of defects to both sides of the measuring area can be detected. 
     However, in some cases, it is not convenient to perform measurements with such fixed measuring points, both due to the measuring points themselves, which need to be attached at several places, and due to their connections. The attachment of fixed measuring points may involve welding, which is undesirable in areas where there is a danger of explosion. 
     SUMMARY OF THE INVENTION 
     It is therefore a main objective of the present invention to provide a convenient sensor device for mobile or periodical (portable or semi portable) use at a measurement site, so that welded measurement points are not required. 
     It is a further objective to provide a sensor device which can be installed and used without safety risks in explosion hazard areas. 
     It is a particular objective to provide a sensor device with an elongated shape, a low production cost, a low height, and which can easily be assembled. 
     It is a still further objective to provide a measurement method which does not require a continuos connection of sensors as prescribed in NO 150 136, but which can rely on short time measurements. Singular measurements should permit calculation of wall thickness. 
     A pliable or jointed band configuration allows for easy adaption of this sensor device to various structures. One embodiment is especially convenient for pipes, while another embodiment is easily adapted for various other structures. Both are easy to set tip and remove. 
     These structures allow “snapshot” measurements. This gives the particular advantage that it does not require continuous operation, it may be performed in minutes with a free choice of intervals and without any need for measurement equipment and/or connections at the measurement site between each measurement. The intervals may be quite long, e.g. weeks or months. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The invention is described in further detail below with reference to the accompanying drawings where 
     FIG. 1 shows a view in perspective of two sensor chains according to a first embodiment of the invention, 
     FIG. 2 shows an exploded view in perspective of a contact joint for the sensor chain of FIG. 1, 
     FIG. 3 shows a section through the contact joint of FIG. 2 in partially assembled condition, 
     FIG. 4 shows a view in perspective of a sensor band according to another embodiment of the invention, seen from the contact side, while 
     FIG. 5 shows a segment of the sensor band according to FIG. 4, illustrating one contact point. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 a section of a steel pipe  11  which may be part of an offshore installation structure is shown. On the pipe  11  two sensor chains  12  and  13  are arranged which extend in band shape around the circumference of pipe  11  at a defined axial distance from each other. Each sensor chain constitutes a number of contact joints  14  with two extending ear members  15 ,  16  on one side and one extending ear member  17  on another side, the ear members each being provided with a hole for receiving a linkage pin  18  in order to form a chain. A tensioning device  50  is provided in order to hold the sensor chain tightly fit to the pipe  11 . 
     For each sensor chain a connection housing  19  is provided for connecting pairs of cables  20  from each contact joint  14  to a common cable clamp, for connection of a cable  21 . The connection housing  19  also contains a printed circuit board that inter alia provides identity signals for each of the measuring sites. 
     Iinterface circuits are provided that amplify the voltage signals measured between the pins and convert the analogue signals to digital signals that are transmitted to a data logger. In this way a computer can recognize data from the individual measuring points in order to process data from the measured signals. In addition the analogue-to-digital conversion allows the computer equipment needed to analyse the measured signals to be placed in significant distance from the measurement points with no deterioration of the signals. The measured signals may be transmitted by multiplexing. 
     Cables  21  from several sensor chains are collected in a collecting box  22  for further transmittal by means of a cable  23  to a computer (not shown). This implies few connections and easy assembly of the sensor device, while allowing the computer to be placed at an easily accessible location. 
     The measurement assembly according to this example also includes two contact chains  24 ,  25  which are to be fastened around the pipe  11  axially outside the sensor chains  12 ,  13 . Each of the chains  24 ,  25  comprises an electrode for the supply of an electric voltage, e.g. 10 mV, which gives a current (excitation current) through the pipe  11 . Each chain  24 ,  25  has a connecting joint  26  with a supply terminal  27  for supply of voltage and a clamp member  28  for tightening the contact chains around the pipe. 
     FIG. 2 shows a preferred embodiment of the design of each contact joint (chain element)  14 . In a block element  31  of an electric insulating material, e.g. plastic, with ear members  15 - 17 , two holes  32 ,  33  are provided for contacting and turning units  34 ,  35 . The contacting and rotating units  34 ,  35  are identical, with a slide bushing  36 , a contact pin  37 , and a rotary plug  38  which are covered by a cover  39 . The assembly of these parts is shown in FIG.  3 . 
     Each slide bushing  36  is arranged in a cylindrical bore in the block element  31 . The contact pins  37  have a stem  40  with a lower edge  41 , a ring shaped flange  42  at its middle section and an axial slit  43  downwards from the upper end. Under the ring shaped flange  42  a coil spring  44  is arranged, the lower end of which rests on the slide bushing  36 . Over the ring shaped flange  42  an upper coil spring  45  is arranged, the upper end of which is constrained by the cover  39 . 
     The rotary plug  38  has a lower tongue  46  which fits into the slit  43  and a free end  48  which will extend through opening  44  of the cover  39 . Free end  48  has an hexagonal shape to fit a spanner. The lower coil spring  46  rests against the slide bushing  36 , while the upper coil spring  47  rests against the flange  42 . The upper coil spring  47  is harder than the lower coil spring  46 , so that the edge  41  of the contact pin  37  is pressed downwards against the measurement object when the cover  39  is lowered by the aid of a bolt  49 . Supply cables (not shown) are soldered to the metallic slide bushings  36 . 
     In FIG. 3 a contact joint  14  is shown during assembly, before the cover  39  is fully tightened. 
     On assembly the contact pins are turned by turning the ends  48  with a spanner, so that an edge of the edge  41  will penetrate any surface coating on the object to be measured, forming a metal to metal contact with no significant transition resistance. 
     FIG. 4 shows an alternative embodiment of a sensor device according to the invention. Here the pliable support constitutes a band  51  of a plastic material being provided with printed longitudinal conductive paths  52  from a contact area  53  at one end to a series of contact points  54  distributed along the sheet band  51 . The conductive paths  52  are covered by a sheet, so that they are inaccessible with the exception of openings  55  (FIG. 5) which expose each contact point  54 , i.e. the free end of each conductive path  52 . 
     Such a sensor band can be manufactured efficiently and relatively inexpensivly by use of known materials and techniques. It can be assembled by pressurised gluing so that the paths are fixed to a support and covered under a sheet with the exception of a pressure area at each contact point  54 , where an electrically conducting area is formed. 
     An alternative embodiments a sensor band may be provided with two rows of contact points, e.g. for arrangement over a welding seam or another line where there is enhanced risk of impairment. The same effect can be obtained by arranging two sensor bands with a single row of sensors side by side. 
     According to the invention, measurements may be performed quite rapidly, in a few minutes, and with intervals in the magnitude of weeks or months between subsequent samples. Such “snapshot” measurements constitute a new method for inspecting the internal condition of pipes. Methods have previously been used for monitoring the condition of a pipe or a structure , by studying the development over time. The condition of the object is registered in a computer at the time of installation, and subsequent measurements is compared with the initial measurement so it becomes possible to calculate e.g. the remaining wall thickness of a pipe at any time in the surveillance period. Such measurements must be based on a series of measurements with duration from a week to some months, in order to enable the system to provide a specified accuracy. 
     In accordance with the invention the method is now utilised as an inspection tool using short time measurements. For inspection purposes it is of interest to be able to measure at new sites and to obtain a result immediately. The present invention makes this possible, due to use of moveable pin holders that are easy to install and where the distances between the various pairs of pins in the pin matrix are well defined and constant. The measurements provide differential voltages for different pairs of pins in the pin matrix, and these values are used to calculate the wall thickness for each pair of pins, thereby obtaining a picture of the internal corrosion in the area covered by the pin matrix. The wall thickness is calculated as follows: 
     The resistance for each pair of pins is calculated from measured voltage between the pins with a known excitation current. 
     The external pipe dimeter is known, 
     Specific resistance for the relevant material is known or measured (separate measurement), 
     The temperature of the pipe is measured, its temperature coefficient needs to be known. 
     The formula used for the calculation is principally: 
     
       
           R =ρ(1 −ΔTk ) pd /( OD′wt ) 
       
     
     where 
     R is resistance for the pair of pins in question, (=diff. pin voltage/ excitation current), 
     ρ is specific resistance for the pipe steel, 
     ΔT is temperature difference, 
     k is temperature coefficient, 
     OD′ is modified external pipe diameter or width of the object 
     wt is wall thickness at the pin pair in question, 
     pd is pin distance 
     Thus the wall thickness can be calculated. 
     The accuracy of the method is not as good as for the traditional method with measurements in time series, but it has its obvious advantages as a method for inspection and for this purpose the accuracy is sufficient.