Method and device for measuring the thickness of any deposit of material on an inner wall of a structure

"METHODS AND DEVICES TO MEASURE THE THICKNESS OF A DEPOSIT OF MATERIAL ON AN INTERNAL WALL OF A STRUCTURE, AND TO DETERMINE THE STIFFNESS OF A MATERIAL DEPOSITED ON AN INTERNAL WALL OF A STRUCTURE"

Field of invention

The present invention relates to a method of measuring the thickness of any deposit of material on an internal wall of a structure, for example, an oil pipeline. The present invention also relates to a corresponding device or system.

Fundamentals of the invention

Wax deposition on the inner wall of oil pipes is a serious problem in the current oil production infrastructure. When hot oil flows through a cold-walled pipe, wax will precipitate and adhere to the walls. This, in turn, will reduce the cross-sectional area of ​​the pipeline, which, without countermeasures, will lead to a loss of pressure and, ultimately, to a total blockage of the pipeline.

To know when remediation techniques (for example, scraping, heating, etc.) should be applied, it is essential to know the current thickness of the wax layer. Known techniques for determining or measuring the thickness of the current wax layer include the use of oil pipe inspection gauges (scrapers), pressure pulse techniques, and pressure drop measurement (throughout the pipe). However, each of these known techniques has several drawbacks. For example, scrapers and pressure pulse techniques do not provide any continuous measurement, which can disrupt operating procedures, and are expensive. The pressure drop measurement approach gives only a comprehensive measurement over the entire length of the pipe, and the measured pressure drop is influenced by a series of parameters, in addition to wax thickness (for example, the roughness of the inside of the pipe) , so there is really no direct correlation with wax thickness.

Summary of the invention

It is an object of the present invention to overcome, at least in part, the above problems, and to provide an improved deposit thickness measurement method. This object and others that will be apparent from the following description are achieved by a method and device according to the appended independent claims. Advantageous embodiments are set forth in the appended dependent claims.

In accordance with one aspect of the present invention, there is provided a method of measuring the thickness of a deposit of material on an internal wall of a structure, the method comprising:

(a) heat a portion of the structure;

(b) detecting vibrations in the heated portion;

(c) detecting vibrations in an unheated portion of the structure;

(d) determining a resonance frequency or frequencies of the structure based on the vibrations detected in (c); and (e) determining the thickness of a material deposit on the inner wall of the structure in said unheated portion using the determined resonant frequency or frequencies, this step using the vibrations detected in (b) as calibration data.

Resonance, or resonance frequency, here means the lowest characteristic frequency of the structure and/or any of its implications or harmonics. The lowest characteristic frequency of the structure can also be referred to as the peculiar frequency of the system. The resonance frequency can also be approximately equal to the natural frequency of the structure.

The present invention is based on the understanding that the resonance frequency of the structure will change as soon as, for example, a layer of wax begins to form inside it. This change in the resonance frequency is due to the changed total elasticity coefficient (vibration damping due to the visco-elastic wax layer). In this way, the measured resonance frequency can be correlated to the thickness of the wax layer. The present method advantageously provides low-cost continuous measurement of wax thickness, which in turn allows wax remediation techniques to be applied (just in time).

Vibrations in the structure can be caused, for example, by means of a device adapted to give a mechanical impulse to the structure, for example, as a hammer hitting the structure. Alternatively, vibrations in the structure can be caused by a medium flowing inside the structure. For example, two-phase lagging irregular flow can exchange mechanical energy with the structure, setting the structure into vibration. In addition, vibrations in the structure can be caused by changes in the flow of a medium flowing inside the structure, in the event that the flow is not irregular or sufficient.

In addition, the vibrations in the structure can be detected by means of a sensor mechanically linked to the structure, and also to a fixed point (reference), to measure the variation of distance between the structure and the fixed point. Alternatively, vibrations in the structure can be detected by means of an optical detector. In this way, no mechanical contact is necessary. Alternatively, an accelerometer fixed on the outside of the structure can be used.

In one embodiment, the resonant frequency used to determine the thickness of any material deposit on the inner wall of the structure is the lowest characteristic frequency thereof. This will give the greatest accuracy.

In one embodiment, the method further comprises (e) predicting the thickness of any material deposit in one or more portions of the structure, remote from where the vibrations were detected, using a material deposition model of the structure. Therefore, although the present method basically provides a spot measurement, the exact prediction of the thickness of the deposit in other portions or parts of the structure can be beneficially provided.

In one embodiment, the method further comprises performing steps (a) - (d) for different portions of the structure, for example, at various locations along the structure. In addition to providing information on the exact thickness of the deposit in the aforementioned locations, the information can be used beneficially to update, in real time, the material deposition model mentioned above, to increase the accuracy of the produced model.

In one embodiment, a resonance frequency of the structure is determined by determining the frequency at which the structure vibrates at a maximum amplitude.

In one embodiment, the thickness of any material deposit on the inner wall of the structure is determined by comparing the (currently) determined resonant frequency with a previously determined resonant frequency of the structure, for which previously determined resonant frequency, the thickness of any material deposit on the internal wall of the structure is known. The previous resonance frequency can, for example, be determined for a clean structure.

In one embodiment, said structure is a pipe, for example, an oil pipe.

In one embodiment, said material is wax. Wax can refer to solids that precipitate from fluids due to thermodynamic changes. These solids typically include solids dissolved in crude oil under well conditions, such as asphaltenes, higher paraffins, hydrates, and inorganic and organic salts. The composition of the wax will depend on the source of the fluid flow.

In a tube-shaped structure, the heating must keep the inner tube wall above the deposit-appearing temperature, thus avoiding deposition in the heated portion. Heating can, for example, be obtained through heating cables installed locally around the pipe. An advantage of this embodiment is that the determination of the resonance frequency can be more accurate, since unwanted vibrations caused by pipe flow, especially multiphase pipe flow with flow patterns such as wavy flow or slow flow, exert significant dynamic forces on the pipe wall, can be taken into account. This, however, does not deny that the internal flow, in fact, can be used as a source to cause vibrations, as is evident to one skilled in the art.

The steps performed in relation to the heated and unheated portions of the structure can be performed using the respective different measuring devices, so that the calibration can be performed in real time, which increases the accuracy of the measurement.

The present method may additionally comprise determining at least one of the following: (i) the amount of vibration damping at the resonance frequency, that is, how the amplitude of the resonance frequency decreases with time, (ii) the change of frequencies of the structure's harmonics, and (iii) the amount of vibration damping in the structure's harmonics based on the detected vibrations. Data resulting from at least one of (i) - (iii) can then be used to determine the modulus of elasticity (both the real and the imaginary component) of any deposit of material on the internal wall of the structure. For example, the amount of damping for each harmonic frequency will be different, depending on the modulus of elasticity. The determination of the modulus of elasticity of the deposit of material on the internal wall of the structure is beneficial to the extent that it allows to establish what type of material the deposit is made of.

According to a second aspect of the present invention, a method of determining the stiffness of a material deposited on an internal wall of a structure is provided, the method comprising:

detect vibrations in the structure;

determine a resonance frequency or frequencies of the structure based on the detected vibrations; and determine a stiffness of a deposited material, observing changes in resonance frequency/frequencies or resonance amplitude/s over time

According to a third aspect of the present invention, an apparatus is provided for measuring the thickness of a deposit of material on the internal wall of a structure, the apparatus comprising:

a sensor that can be placed on the outside of the structure to detect vibrations in it;

a signal processor for determining a resonance frequency of the structure based on the vibration detected by means of the sensor; and an analyzer to determine the thickness of a deposit of material on the inner wall of the structure based on the resonance frequency determined by said signal processor.

According to a fourth aspect of the present invention, an apparatus is provided for determining the stiffness of a material deposited on an internal wall of a structure, the apparatus comprising:

a sensor to detect vibrations in the structure;

a signal processor to determine a resonance frequency or frequencies of the structure based on the sensed vibration; and an analyzer to determine the stiffness of a deposited material by observing changes in resonance frequency/frequencies or resonance amplitude/s over time.

The apparatus of the third or fourth aspect above of the invention may additionally comprise a device adapted to provide a mechanical impulse to the structure.

Brief description of the drawings

The Fig. 1 is a schematic view of a device, according to an embodiment of the present invention.

The Fig. 2 is a flowchart of a method, according to an embodiment of the present invention.

The Fig. 3 is an exemplary graph of type xy (reservoir thickness versus resonance frequency).

The Fig. 4 is an exemplary graph of type xy (Young's modulus versus resonance frequency).

The Fig. 5 is a schematic view of a device, according to another embodiment of the present invention.

The Fig. 6 is a schematic view of an arrangement comprising a pipe and various measuring devices of the present invention.

The Fig. 7 is a schematic view of an arrangement comprising a pipe and two measuring devices, according to another embodiment of the present invention.

The Fig. 8 is a flowchart illustrating a method of determining the thickness of a deposit and determining the stiffness of the deposit material.

Detailed description

The Fig. 1 is a schematic view of a device for measuring the thickness of a deposit of material on an internal wall of a structure according to an embodiment of the present invention. Specifically, the device of Fig. 1. is a measuring device 10 adapted to measure the thickness of the layer of wax on the inside of the wall 12 of a pipe or pipe 14 for transporting oil 16. The pipe 14 can be made, for example, of steel pipes.

The measuring device 10 comprises a device 18 adapted to provide a mechanical impulse to the pipe 14. The device 18 can function, for example, as a hammer. The device 18 can be placed on the outside of the pipe 14.

The measuring device 10 additionally comprises a sensor or detector 20 that can also be placed on the outside of the pipe 14. The sensor 20 is adapted to detect vibrations and convert the vibrations into corresponding electrical energy. To convert vibrations into electrical energy, the sensor 20 may, for example, comprise a piezoelectric transducer (not shown). In use, the device 18 and the sensor 20 are both mechanically coupled to an external surface 22 of the pipe 14, in a particular portion thereof, directly or through some displacement means (not shown). In addition, the generator 18 and the sensor 20 are placed next to or close to each other, preferably on the same side of the tube 14, as illustrated.

The measuring device 10 additionally comprises a first determiner 24 connected to the sensor 20. The first determiner 24 is adapted to determine a resonance frequency of the pipe 14 based on vibrations detected by the sensor 20.

The measuring device 10 additionally comprises a second determiner 26 connected to the first determiner 24. The second determiner 26 is adapted to determine the thickness of any wax layer 28 deposited on the interior 12 of the pipe 14 based on the resonance frequency determined by first determinant 24.

Although illustrated as separate elements, the functions of the first and second determinants 24, 26 may be performed by a single unit 30, for example, a computing device. This unit can also be used to control device 18.

An exemplary operation of the present measuring device 10 will be described with reference to figs. 1-3 In stage (a), the device 18 is initially excited in order to provide a mechanical impulse to the pipe 14, causing vibrations in it. When the pipe 14 is excited with an impulsive function such as a blow by the device 18, it initially vibrates at all frequencies present in the impulse (an impulsive function, theoretically, contains all frequencies). However, all frequencies except the peculiar frequency and its implications will be quickly damped, so that after a very short period of time after the impulse, the vibration will be composed mainly of resonance frequencies.

The vibrations generated in the pipe are then detected by the sensor 20 in step (b). The sensor 20 converts the detected vibrations into corresponding electrical energy and records the vibration signal over a certain period of time.

From the electrical energy that represents the vibrations detected by the sensor 20, in step (c), the first determiner 24 then determines a resonance frequency of the pipe 14, including any wax deposits. The first determiner 24 can, for example, transform the detected vibrations via FFT (Fast Fourier Transform) to the frequency domain and plot the output on an xy graph (frequency versus amplitude), and observe the peak(s) s) that happens(em). Each peak is a resonance or resonance frequency of pipe 14.

Then, in step (d), based on the resonance frequency determined by the first determiner 24, the thickness of any layer of wax deposited on the inside of the pipe is determined by the second determiner 26. The second determiner 26 may, for example, use an xy plot (deposit thickness versus resonance frequency) for the particular portion of the tubing 14 and enter the current resonance frequency (eg, the first implication) to determine the thickness tank current. An example of this graph is shown in fig. 3. The plot can be prepared by first using FEM (Finite Element Method) to determine the peculiar frequencies of a clean pipe for the pipe geometry in the particular pipe portion 14. Then a layer (deposit) is added inside of the clean pipe and the peculiar frequencies are recalculated. Preferably, these FEM calculations can also take into account the environment surrounding the pipe, meaning if the pipe 14 is suspended in free water, or if it is seated on the seabed, or if it is semi-buried in the bed of the sea

The determined thickness may be presented to an operator in a variety of ways (e.g., via a display, not shown), as evident to someone skilled in the art, and/or be entered into some other system for further processing, etc.

Preferably, the method described above is repeated continuously, as indicated by the optional dashed line 32 in FIG. 2, in order to provide a real-time measurement of any wax deposition layer thickness.

In a modification of the present device and method, the type of deposition can also be determined, for example, by means of the second determinant 26. That is, the change in the Young's modulus of the deposit (that is, its stiffness) also changes the frequency peculiar, but the change is different for the different implications, see fig. 4. For example, the change of the third implication in relation to the stiffness of the deposit is, in fact, greater than the corresponding change of, for example, the first implication. In truth, the first implication does not vary significantly in relation to the stiffness of the deposit. Therefore, preferably, the first implication or, more preferably, the lowest resonant frequency characteristic of the structure, is used to determine the thickness of the deposit. On the other hand, the change in the frequency of a major implication (for example, the third implication), in combination with the determined thickness, can preferably be used to determine the Young's modulus or the hardness of the present deposit. In particular, the change due to thickness, as determined from the first implication, can be deduced from the change from the third implication, thus, the hardness of the deposit can be determined from the remaining change from the third implication. Hardness can then be used to determine whether the deposit consists primarily of hard crust, or of wax, which is elastic. Graphs (like the graph in fig. 4) can be prepared from FEM, making calculations for several cases with the same thickness of deposits, but changing the modulus of elasticity of the deposit, each time .

In addition, the first determinant 24 may also be adapted to determine at least one of the following: (i) the amount of vibration damping at the resonance frequency, that is, how the amplitude of the resonance frequency decreases with time, (ii) ) change in the frequencies of the harmonics of the structure as the thickness of the deposit increases, and (iii) the amount of vibration damping in the harmonics of the structure, based on the detected vibrations. Additionally, the second determinant 26 can also be adapted to determine the modulus of elasticity, both the real and the imaginary component, of any deposit of material on the interior of the structure based on data resulting from at least one of (i) - ( iii). For example, the amount of damping for each harmonic frequency will be different depending on the modulus of elasticity. To this end, the determiner 26 can use a prepared look-up table including different amounts of damping of each harmonic frequency for a given pipe geometry by a set of elastic moduli. Based on the currently detected amount of damping of each harmonic frequency, the current modulus of elasticity of the reservoir can be retrieved. And, based on the current modulus of elasticity, the type of deposit can then be determined, at least approximately, as above. For example, wax is a visco-elastic medium, while crust (precipitated salt) is a relatively rigid and hard medium. A deposit of equal thickness of these two will show a completely different amount of damping of vibrations in the harmonics.

It will now be described measuring device according to another embodiment of the present invention, as illustrated in fig. 5. The measuring device 10 of fig. 5 is similar to that of fig. 1, but device 18 may be omitted. Instead, the vibrations in the pipe 14 may be caused by the medium 16 flowing inside the pipe 14. If the flow is irregular, as in most real production flows, for example a slow two-phase flow, it will exchange mechanical energy with the piping structure 14, setting it in vibration. Alternatively, if the flow is not regular enough, shock waves can be introduced into the flow, for example, increasing the flow, suddenly, by a certain percentage. These shock waves will move through the pipe 14 and will introduce vibrations in it, which can be detected.

Basically, the present method using a single measuring device, as described above, provides a spot measurement. However, the exact knowledge of the wax thickness at a point makes it possible to adjust, in real time, an existing wax deposition model of the pipe so that a forecast (optional step (e) in Fig. 2) of layer of wax, also at a great distance from the measuring point, is possible with very high precision. Generally, it should be sufficient to have a measurement at a few critical points (eg connecting new wells, junctions, etc.) to cover the entire pipeline. Exemplary wax deposition models that can be used in conjunction with the present invention are presented in the publication Simulating Wax Deposition in Plumbing for Flow Assurance; Edmonds Beryl, Tony Moorwood, Richard Szczepanski, and Xiaohong Zhang; Energy Fuels, 2008, 22 (2), 729-741.

Therefore, in an advantageous arrangement of the present invention, as illustrated in fig. 6, several measuring devices 10 of the type described above are arranged at different locations along the pipeline 14. In addition, a central processing means 34 (eg, a computer device) is provided, which central processing means 34 is adapted to receive local wax layer thickness data from measuring devices 10. The received local data can be used by the processing means 34 to predict the thickness of the wax layer at the remote location where the measuring devices 10 are located, using a wax deposition model of the pipe 14, as well as to update the present model of pipe wax deposition. Certainly, the arrangement of fig. 6 também can be used without the wax deposition model. In this case, the output is a plurality of point measurements, one at the position of each measuring device.

The Fig. 7 is a schematic view of an arrangement, according to yet another embodiment of the present invention, where two measuring devices 10a, 10b are arranged at different locations along the pipe 14. The devices 10a, 10b are basically of the same type that the device 10 described above, although the determinants 24 and 26 may be omitted in the device 10a. A heater 36 is also provided in the portion of the pipe 14 where the device 10 is arranged. The heater 36 is adapted to heat the inner wall of the aforementioned portion of the pipe 14 to a temperature above the wax appearance temperature, thus preventing deposition in the heated portion. The heater 36 may be, for example, electric heating cables installed locally around the outside of the pipe 14. The device 10b is, on the other hand, arranged in an unheated portion of the pipe 14, as illustrated. The device 10a and the heater 36 can be placed upstream or downstream of the device 10b.

In operation, the device 10a performs the above steps (a) (b) for the heated portion of the pipe 14. At the same time, the device 10b performs the above steps (a) - (d) for the unheated portions, but using , additionally, data resulting from the operation of the device 10a as calibration data. For example, the vibrations detected by the device 10a can be subtracted from the vibrations detected by the device 10b by determining the resonance frequency, optionally already in the time domain, before an FFT is applied to determine the dominant frequencies of the vibrations. In this way, the device 10b can determine the thickness of the deposit considering the vibrations in the pipe 14 caused by the flow in it.

Figure 8 illustrates, in general terms, a method for determining the thickness of a material deposited on an internal surface of a structure, such as a pipe. In step 100, heat is applied to a portion of the structure. In step 200, vibrations are detected in that heated portion. This data provides calibration data indicative of the vibrations present where no deposit exists. In step 300, vibrations are detected in a portion without heating, that is, a portion of the structure where a deposit is formed. In step 400, these vibrations are analyzed and a resonant frequency (or frequencies) determined. In this step, changes in the resonance frequency (or frequencies) with time can be monitored and used to determine material thickness, steps 800 and 900. After the determination of the 5 resonance frequency, in step 500, the thickness of the deposit is determined using the vibrations detected in step 300, the result is calibrated in step 600. The result is generated in step 700.

One skilled in the art will appreciate that the present invention is in no way limited to the preferred embodiment(s) described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the present invention is applicable to all types of structures or containers carrying hydrocarbon streams, whose hydrocarbon streams comprise components that possibly deposit on the wall of the container, for example, wax. In addition, in the arrangement of fig. 6, the first and second determinants 24, 26 of each device 10 can be centralized in the processing means 34. Also, instead of using the sensor 20, the vibrations in the pipe 14 can be detected by means of an optical detector or an accelerometer.