WO2019123492A1

WO2019123492A1 - Method and system for non-destructive monitoring of the structural integrity of containers for storing compressed gas

Info

Publication number: WO2019123492A1
Application number: PCT/IT2017/000288
Authority: WO
Inventors: Alessandro GIRARDI
Original assignee: Fincantieri Oil & Gas S.P.A.; Mios Elettronica S.R.L.
Priority date: 2017-12-20
Filing date: 2017-12-20
Publication date: 2019-06-27

Abstract

The present invention regards a method for non-destructive monitoring of the structural integrity of a container for storing compressed gas, characterized for: - detecting over time, through a plurality of optical fiber Bragg grating extensometers firmly associated with the container, the deformations of the structure of said container induced by variations in the internal pressure of the gas; and - comparing, being the internal pressure of the gas equal, the deformations thus detected with corresponding reference deformations of the structure previously detected on the same container under conditions of established structural integrity through said same plurality of extensometers, any deviations from said reference deformations being assumed as indicators of anomalies in the structural integrity of the container, being the internal pressure equal. The invention also relates to a monitoring system and a compressed gas storage plant.

Description

DESCRIPTION

"Method and system for non-destructive monitoring of the structural integrity of containers for storing compressed gas"

SCOPE

[0001] The present invention relates to a method and system for non-destructive monitoring of the structural integrity of containers for storing compressed gas.

[0002] In particular, the method and the system according to the present invention are applied in the naval field in the continuous and real-time monitoring of the gas storage cylinders installed in CNG (Compressed Natural Gas) transport vessels.

PRIOR ART

[0003] Gas storage containers are generally made of metallic material, in particular steel, or composite materials. The normal use of these containers involves the pressurized filling and subsequently emptying of the same. The presence of impurities in the gas introduced under pressure, together with the condensing effects due to compression/decompression of the gas itself in the aforesaid cyclical working phases, involves the accumulation of such impurities in the "low" area inside the container. In such preferential accumulation zone, oxidation (rust) phenomena are more likely to occur, such phenomena leading to loss of structural integrity and therefore of containment capacity. For these reasons, a periodic (non-destructive) inspection of such containers is necessary.

[0004] Currently, the non-destructive periodic inspection of the containers is performed using the well-known ultrasonic detection method, which allows permanent deformations and/or yielding in the container structure to be detected. Such system uses one or more ultrasonic probes that must be physically applied on the surface of the container and moved from time to time to analyze different areas of the container. After detections, data analysis must be performed by qualified personnel to determine any presence of anomalies. The current procedures of non-destructive analysis using ultrasound are regulated by national and international laws and regulations, among which is cited as a reference: UNI EN 14127 (specific to the detection of thicknesses by ultrasound) ; UNI EN 473, which sets out the guiding principles for the qualification and certification of the personnel involved in non-destructive testing; and UNI EN 13018 for the guiding principles of non-destructive testing.

[0005] Monitoring through ultrasonic probes - while being effective presents a series of limitations, not yet overcome .

[0006] A first limitation is linked to the fact that this monitoring method may be used practically only for periodic monitoring and is very difficult to use to implement real-time and continuous monitoring.

[0007] A second limitation is linked to the fact that the use of ultrasonic probes requires direct accessibility to the total surface area of the containers. In some cases, this may constitute an operating constraint that is difficult to overcome. One considers, for example, the case of vessels for the transport of CNG, wherein normally the containers (in the form of cylinders) extend in height for several meters and are arranged in groups inside special racks. To minimize the floor space occupied, the containers are arranged very close to each other. This makes it difficult to access the surfaces of the containers (cylinders) that face the inside of the rack .

[0008] In the field of compressed gas storage containers (cylinders) , there is therefore a need to have a method for monitoring the structural integrity of the containers which allows continuous and real-time monitoring, and which is also operationally simple to implement irrespective of the availability of direct access to the containers. PRESENTATION OF THE INVENTION

[0009] Therefore, the object of the present invention is to eliminate in whole or in part the drawbacks of the prior art mentioned above by providing a method for non- destructive monitoring of the structural integrity of containers for storing compressed gas which allows continuous, real-time monitoring.

[0010] A further object of the present invention is to provide a method for non-destructive monitoring of the structural integrity of containers for storing compressed gas which is operationally simple to implement irrespective of the availability of direct access to the storage containers .

[0011] A further object of the present invention is to provide a method for non-destructive monitoring of the structural integrity of containers for storing compressed gas which is operationally reliable.

[0012] A further object of the present invention is to provide a system for non-destructive monitoring of the structural integrity of containers for storing compressed gas which allows continuous, real-time monitoring.

[0013] A further object of the present invention is to provide a system for non-destructive monitoring of the structural integrity of containers for storing compressed gas which is operationally simple to implement irrespective of the availability of direct access to the storage containers.

[0014] A further object of the present invention is to provide a system for non-destructive monitoring of the structural integrity of containers for storing compressed gas which is operationally reliable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The technical features of the invention, according to the above objects, are clearly apparent from the content of the claims below and the advantages thereof will become more apparent in the following detailed description, made with reference to the accompanying drawings, which represent one or more purely exemplifying and non-limiting embodiments, wherein:

[0016] - Figure 1 schematically shows an example of an optical fiber structure containing FBG sensors;

[0017] - Figure 2 schematically illustrates the reflection mode of a portion of the incident light spectrum by means of an FBG sensor;

[0018] - Figure 3 schematically illustrates the reflection mode of a portion of the incident light spectrum by means of an FBG array;

[0019] - Figure 4 schematically illustrates the behavior of an FBG sensor when it is mechanically stressed;

[0020] - Figure 5 shows an example of a characteristic deformation curve of a metallic element subjected to a force;

[0021] - Figure 6 schematically shows a system for nondestructive monitoring of the structural integrity of containers for storing compressed gas according to a preferred embodiment of the invention;

[0022] - Figure 7 schematically shows the deformation axis of the shell of a cylindrical gas storage container;

[0023] - Figure 8 shows a preferred mode of helical spiral wrapping of an optical fiber with FBG on a gas storage container according to the present invention;

[0024] - Figure 9 is an example of a punctual deformation curve detected by an FBG sensor of a gas storage container as the pressure varies;

[0025] - Figure 10 shows a comparison between a deformation curve on a container placed under pressure; without structural alterations (continuous line) and a deformation curve on a container placed under pressure with structural alterations (broken line) ; and

[0026] - Figure 11 schematically shows the monitoring system according to a preferred embodiment of the invention installed on a cylinder rack intended for installation on a CNG transport vessel.

DETAILED DESCRIPTION

[0027] The method for non-destructive monitoring of the structural integrity of a container for storing compressed gas according to the invention is based on the following two essential operational steps:

[0028] - detecting over time - by means of a plurality of optical fiber Bragg grating extensometers firmly associated with the container - the deformations of the container structure induced by variations in the internal pressure of the gas; and

[0029] - comparing, being the internal pressure of the gas equal, the deformations thus detected with corresponding reference deformations of the structure of the container, previously detected on the same container in conditions of established structural integrity through the aforesaid plurality of extensometers.

[0030] Operationally, any deviations from the aforesaid reference deformations are assumed as indicators of anomalies in the structural integrity of the container, being the internal pressure equal.

[0031] In fact, the method according to the present invention is based on the fact that any erosions or thinning of the inner walls of the container causes a change in the response of the container in terms of mechanical deformations to the stresses induced by the internal pressure of the gas.

[0032] As is known, an object subjected to the action of a force assumes a deformation, which depends on the type of the object's construction material and its geometric shape, being the direction and value of the force equal.

[0033] In general, the deformation assumed by the object may be identified with a deformation graph with four typical zones: elastic zone (a); plastic zone (b) ; yield zone (c) ; and breaking zone (d) . Each material, in particular metallic, has its own characteristic deformation curve. The graph in Figure 5 shows a typical example of the deformation curve of a metallic material.

[0034] Any change in the geometry of the object (reduction in thickness, drilling, curvature, etc.) changes this characteristic curve. By comparing the characteristic deformation curve of an intact object subjected to a known force with the characteristic curve, which the same object assumes after a change of geometry (for example, reduction of the thickness) being the force applied equal, a deviation of the deformation will be obtained. Such deviation is an index of a structural alteration and the difference between the curves is proportional to the extent of the alteration.

[0035] By exploiting this principle, it is therefore possible, being the geometric shape, the type of material and the force applied to the object equal, to identify a structural alteration and its magnitude. [0036] More specifically, in conditions of structural integrity, i.e., in the absence of structural changes, the compressed gas storage container - regardless of the material of which it is built and being the environmental conditions (typically, temperature) equal - will react univocally to the same tensile stress, i.e., it will always deform in the same way during the operating cycles of loading and unloading the gas. Such situation is easily identifiable even when the intensity of the tensile stresses increases (progressive increase and decrease in the internal pressure of the gas) .

[0037] In this regard, it should be noted that the containers for storing compressed gases are sized to operate in the field of elastic deformation within the normal operating conditions to which they will be subjected. Therefore, during the normal operating cycles of loading and unloading the gas inside, the container will be cyclically subjected to temporary expansion with microvariations of shape, returning cyclically and elastically to the resting condition. Therefore, if the container is made to work under project operating conditions and therefore in the field of elastic deformations, the detections of deformation will not be "polluted" by any plastic deformations.

[0038] Otherwise, in the presence of structural anomalies, i.e. in the presence of structural changes (erosion, thinning, plastic deformations) , the container - being the environmental conditions equal - will react differently to the same tensile stress, i.e., it will deform differently during the cycles of loading and unloading the gas. Such situation is immediately detectable by a comparison with the reference deformations. This allows the anomaly to be revealed in real time and the necessary actions to be implemented, which, depending on the severity, may involve the planning of a maintenance intervention including the immediate decommissioning of the container.

[0039] By virtue of the present invention, it is therefore possible to carry out real-time monitoring of the structural integrity of a container for storing compressed gas, since (beyond the technical periods of detection and processing of the signals coming from the extensometers) deformations are detected continuously and continuously compared with reference values.

[0040] Unlike the known methods of monitoring by ultrasonic probes, the method according to the invention is furthermore operationally simple to implement irrespective of the availability of direct access to the storage containers. The "physical" detection means on which the method according to the invention is based, i.e., the plurality of optical fiber Bragg grating extensometers, are in fact firmly associated with the container and remain firmly associated therewith for the whole of the operational life thereof. From an operational point of view, it is therefore not necessary for the monitoring personnel to have direct access to the surface of the container.

[0041] The monitoring method according to the invention may therefore also be implemented on containers that are not accessible or only partially accessible, as in the case of gas storage cylinders installed in CNG transport vessels .

Optical fiber Bragg grating extensometers (also known as FBG sensors) are known per se to a person skilled in the art. For such reason, a detailed description would be unnecessary. However, for reasons of complete disclosure, some general information is provided below.

[0042] FBG (Fiber Bragg Grating) sensors are diffractive optical elements with the property of reflecting an incident light beam in a certain wavelength. The behavior of FBG sensors is linear. In general, they are stressed by temperature changes or by mechanical deformations suffered by the optical fiber in the area of interest. They are not influenced by magnetic fields. [0043] Operatively, an FBG sensor is an optical extensometer obtained by photoengraving (generally by UV rays) a grating, characterized by a different refractive index, in the core of the optical fiber (i.e. the central part of an optical fiber through which the light is transmitted) . Figure 1 shows an example of a structure of an optical fiber 21 accommodating FBG 20 sensors, where C indicates the core of the optical fiber, A the fiber optic cladding, G the Bragg grating and A the grating pitch .

[0044] Such photoengraving, carried out on the optical fiber through a suitable phase mask, produces a periodic variation of the refractive index of the core of the fiber in the longitudinal direction (as already said, the FBG sensor is directional), characterizing the reference wavelength of the sensor. Any deformation that affects the grating causes a variation of the reflected wavelength from which one may ascertain the deformation itself. This phenomenon is illustrated schematically in Figure 2, where it is shown how an FBG sensor reflects a portion of the incident light spectrum.

[0045] Since the grating is also sensitive to temperature, it may be used as a sensor of such physical parameter; in such case, in order to correctly measure the temperature, it is essential that the sensor be free from any type of mechanical deformation.

[0046] When using the sensor as an extensometer, if the thermal excursion of the object is important, it is necessary to compensate the effect of temperature in order to ascertain the real deformation of the object.

[0047] In order for FBG sensors to monitor the deformation of any object, it is essential that the part of the optical fiber containing the FBG is perfectly adhered to the object itself. The type of adhesive must ensure that there is complete transmission of all dimensional variations from the object to the FBG sensor without altering the values thereof.

[0048] Operationally, the light beam needed to "stimulate" the FBGs is generally generated by a laser of suitable power. As illustrated schematically in Figure 2, a single

FBG will refract a portion of the incident light spectrum; such diffraction will be the information that the FBG control electronics will receive as data to be processed later.

[0049] Advantageously, a single optical fiber may accommodate a series of FBGs (FBG array) . Each grating will refract a portion of the different incident light spectrum so that the control electronics may have more information with a single luminous pulse of incident light. [0050] In an FBG array, a light incident on the optical fiber, by effect of all the individual refractions that occur on each plane of the various gratings, is composed by combining with each other (waves diffracted in phase) , creating a beam of retro-reflected light at ABragg wavelength in accordance with the AIncident light wave. Such phenomenon is illustrated schematically in Figure 3.

[0051] The spectral components that do not satisfy the reflection condition of the FBG sensors are instead transmitted through the fiber to the end (transmitted light) .

[0052] Operationally, as already mentioned, FBG sensors are directional sensors. This means that they perceive mechanical or thermal physical stresses only in the direction of the optical fiber itself.

[0053] Both stresses produce variations both in the fiber refractive index and the grating pitch A. This results in variations in the Bragg wavelength (ABragg) that the control electronics may monitor and translate into deformation. This situation is illustrated schematically in Figure 4.

k -k ~k

[0054] According to a preferred embodiment of the invention, each detection of deformation is associated with the position of the Bragg grating extensometer that detected the deformation.

[0055] In this way, operationally, the deformations detected and any deviations from the reference values are punctually referable to specific zones of the structure of the container. This makes it possible therefore to locate any anomaly on the container, facilitating any maintenance interventions.

[0056] Advantageously, the amplitude of the aforementioned deviations from the reference values may be taken as a measure of the thinning of the container wall in the detection zone and therefore as an index of the severity of the anomaly. This is useful for evaluating the type and timing of the intervention (scheduled maintenance or immediate decommissioning) .

[0057] Advantageously, the deformation detections obtained by means of the above FBG sensors may be accompanied by temperature measurements of the container structure and are compensated by a temperature dependent coefficient so as to obtain temperature-independent deformation detections.

[0058] The adoption or not of a method for compensating the thermal effects is linked to the environmental conditions wherein the container is made to operate.

[0059] Within certain limits (± 2°C), temperature variations are practically irrelevant and may be disregarded. Therefore, in the case wherein the container operates in a thermostated environment (as typically happens in CNG naval installations) the method of compensation of the thermal effects may be avoided. Otherwise, in the case wherein the container does not operate in a thermostated environment and the temperature variations are not negligible, it is appropriate to adopt a method of compensation for the thermal effects.

[0060] Preferably, each deformation detection is associated with:

[0061] - a specific point of the outer surface of the container, corresponding to the anchoring point of the extensometer which has generated said detection;

[0062] - an internal pressure value of the gas; and

[0063] - a temperature value of the container (optional) .

[0064] As already pointed out, for the application of the monitoring method according to the present invention, it is essential to know the response (in terms of deformations) of the container under conditions of established structural integrity to the stresses deriving from working situations. In other words, it is essential to dispose reference deformations on the structure of the container, previously detected on the same container in conditions of established structural integrity by means of the aforesaid plurality of extensometers . [0065] Operationally, such reference deformations may be obtained by subjecting the newly produced container (and therefore in conditions of established structural integrity) to a data acquisition campaign, simulating all the pressure conditions to which it will then be subjected during the operating life (cycles of loading and unloading the gas) . In this way, a set of reference values may be created.

[0066] Advantageously, each reference deformation (obtained on the container in conditions of established structural integrity) is associated with:

[0067] - a specific point on the outer surface of the container, corresponding to the anchoring point of the extensometer which has generated said detection;

[0068] - an internal pressure value of the gas; and

[0069] - a temperature value of the container (optional) .

[0070] In this way it is possible to compare the measurements with the reference values in a homogeneous way and therefore to correlate the deviations from the reference values with any anomalies.

[0071] Advantageously, each deformation detection is stored in mass storage means. In this way, it is possible to create a historical archive of data relating to the behavior of the container subjected to the cycles of loading and unloading the gas. [0072] According to a most preferred embodiment, the monitoring method is implemented automatically by means of an electronic data acquisition, storage and analysis unit which is connected to the plurality of optical fiber Bragg grating extensometers and to the pressure sensor, and optionally is connected to at least one temperature sensor .

[0073] Preferably, the electronic data collection, storage and analysis unit is able to interface with multiple optical fibers so that more than one container may be monitored at the same time. A typical application in this sense is the monitoring of a group of cylinders installed in a rack of a vessel for the transport of CNG, as shown in Figure 11, where the containment rack is indicated at 11.

[0074] Advantageously, such electronic unit is designed to be integrated into an automation system which heads the control of the filling of the containers so as to know at all times the state of the structural integrity of the same containers .

[0075] Preferably, the aforesaid electronic data acquisition, storage and analysis unit comprises:

[0076] - at least one optical interface for fiber optics with Bragg grating;

[0077] - at least one optical signal analyzer; [0078] - a microprocessor processing unit; and

[0079]- mass storage means.

[0080] Advantageously, the aforementioned electronic data acquisition, storage and analysis unit may further comprise one or more communication interfaces suitable for connecting said electronic unit to a system for loading and unloading gas in the container.

[0081] Advantageously, the optical interface comprises the laser for excitation of the FBG sensors, the optical sensor and all the components necessary to guarantee the physical interface of the optical fiber containing the FBG sensors.

[0082] Advantageously, the optical analyzer is a signal processing system (typically DSP - Digital Signal Processing algorithms, in FPGA - Field Programmable Gate Array) capable of analyzing the return signal coming from the FBGs in a fiber and then measuring the extension or the compression of the FBG grating.

[0083] The processing unit interfaces with the other functions and performs the necessary calculations to correlate the various measurements coming from the FBGs, the pressure sensor and the temperature sensor to determine the deformation "profiles" of the container and to create an archive of these measurements and analyses for a subsequent comparison. The difference between the profiles, being conditions (pressure and temperature) equal, determines the presence of an anomaly in the structure of the container and, when higher than a predetermined value, its exclusion from the work cycle.

[0084] The processed data are then "archived" by storing them in a mass memory and then made available for comparison operations.

[0085] Finally, the results obtained from time to time in real-time may be communicated to the regulation system that supervises the process of loading and unloading the gas containers by means of communication interfaces such as Ethernet, CAN, RS485 or other appropriate communication protocols. In this way, the automation system is able to know, moment by moment, the status of all the containers monitored by this system.

* * *

[0086] Advantageously, the monitoring method according to the present invention comprises the following preparatory operating steps:

[0087] - a) firmly associating with the outer surface of said container a plurality of said optical fiber Bragg grating extensometers, each of said extensometers being firmly anchored to the outer surface of the container at the portion wherein the Bragg grating is made; and

[0088] - b) equipping the container with a pressure sensor to detect the gas pressure inside the container.

[0089] Advantageously, the monitoring method according to the present invention may also comprise the further preparatory operating step c) of providing the container with at least one temperature sensor for detecting the temperature of the container, for the purpose of implementing a thermal effect compensation mode.

[0090] Advantageously, the aforementioned preparatory operating steps a) , b) and c) (if provided) are carried out during the construction of the container and/or during installation on the work site of the same container .

[0091] Advantageously, as shown schematically in Figure 6, the anchoring points of the Bragg grating extensometers to the outer surface of the container form a grating of detection points extending over the outer surface of the container .

[0092] Preferably, the aforementioned grating extends over the whole surface of the container, so that it is possible to monitor the entire container.

[0093] Operatively, the deformations occurring on the structure of the container in areas corresponding and/or adjacent to the detection points are measured directly by means of the extensometers there located. [0094] The deformations occurring on the structure of the container in areas not adjacent to the detection points and/or equidistant from more detection points may, on the other hand, be estimated by interpolating the measurements obtained from the detection points present in the vicinity of such areas.

[0095] Advantageously, the aforementioned point grating may have a density of points per surface unit variable according to the specific surface portion of the container.

[0096] In other words, it is possible to increase the density of the detection points in the areas of the container that are hypothesized to be more critical in terms of the onset of erosions (for example the condensation collection areas inside the container, typically the lower areas of the container in the installed condition) , whereas it is possible to decrease the density of the detection points in the container areas that are hypothesized to be less critical in terms of the onset of erosion.

[0097] Advantageously, the aforementioned extensometers are positioned on the outer surface of the container following a predefined distribution pattern, selected according to the shape of the container.

[0098] Operationally, as already pointed out, FBG sensors are directional sensors. This means that they perceive mechanical physical stresses (and therefore detect deformations) only in the direction of the optical fiber itself. Taking into account this functional characteristic, it is possible to provide two modes of positioning the FBG sensors.

[0099] More specifically, a first positioning mode provides for each extensometer to be arranged in such a way that its direction of sensitivity is aligned with a specific deformation axis of the outer surface portion of the container to which it is associated. In this case, it is necessary to provide an extensometer for each deformation axis. Such first method enhances the detection accuracy, but requires the use of a high number of extensometers .

[00100] A second positioning mode provides for each extensometer to be arranged in such a way that its direction of sensitivity is not aligned with any specific deformation axis of the outer surface portion of the container to which it is associated so as to be able to detect deformations on several axes and not only on one specific deformation axis. This second mode decreases detection accuracy (within certain limits) , but allows the number of extensometers necessary to detect deformations to be reduced.

[00101] Advantageously, combinations of these two positioning modes may be provided, in the sense that a part of the extensometers may be arranged according to the first mode, while another part may be arranged according to the second mode.

[00102] Preferably, the Bragg grating extensometers are associated with the container in such a way that two or more of them are made on a same optical fiber that wraps at least a portion of the container and is connected to the aforementioned electronic data acquisition, storage and analysis unit.

[00103] According to a preferred embodiment, the Bragg grating extensometers are associated with the container in such a way that the Bragg grating extensometers are all made on a single optical fiber which wraps the container externally and is connected to the aforementioned data electronic acquisition, storage and analysis unit. This configuration is preferred if it is necessary to monitor a plurality of compressed gas storage containers. With this configuration, it is possible to associate a single optical fiber with FBG arrays on each individual container, significantly simplifying the system.

[00104] Generally, compressed gas storage containers consist of cylinders having a cylindrical main body with a circular cross-section. The specific and preferred case of monitoring such a type of container is analyzed hereinafter .

[00105] Operationally, the normal use of the container provides that it is loaded by introducing a gas and consequently increasing the internal pressure and is subsequently unloaded thereby decreasing the internal pressure. During the loading phase, the pressure will gradually increase, and the container will deform in a characteristic way.

[00106] As already pointed out, the FBG sensors are directional: the dilatation/compression (and therefore the correlated deformation) is measured longitudinally to the fiber.

[00107] As shown in Figure 7, in a cylindrical compressed gas storage container, the shell may undergo deformations in the three main directions: circumferential (sΐ) , axial (s2) and radial (s3) .

[00108] As is known, the deformations depend on the following parameters: cylinder diameter, wall thickness and internal pressure. The relationship between these quantities are expressed by the following well-known formulas (where P = Pressure; R = Internal Radius; S = Thickness) , referring to the tensions of a thin-walled cylindrical structure subjected to pressure. PR

si = T <7'

5

[00109] The aforementioned formulas confirm the existence of a direct and univocal correlation between the gas pressure, structural deformation and geometric dimensions of the container and therefore the validity of the principle on which the method according to the invention is based.

[00110] To detect the aforesaid quantities ol, o2 and s3 by means of FBG sensors, the FBG sensors on the cylindrical container must be installed following a geometry that allows the deformations to be appreciated on all three axes.

[00111] Several solutions may be adopted.

[00112] A first solution consists in positioning the optical fibers with Bragg grating (FBG array) both longitudinally (along the whole length of the container) and transversely (around the circumference) . This solution certainly allows an accurate measurement on the three axes, but requires a considerable number of fibers to be installed and therefore read.

[00113] According to an embodiment of the method not shown in the attached Figures, if the container consists of a cylindrical tank, the FBG extensometers may thus be positioned as follows:

[00114] - a first part of the FBG extensometers are made on two or more optical fibers, each of which wraps the container circumferentially in a predefined position along the X axis of the cylindrical main body; and

[00115] - a second part of the FBG extensometers are made on two or more optical fibers, each of which is arranged parallel to the X axis of the cylindrical main body in a predefined circumferential position.

[00116] In this way, each extensometer is dedicated to detecting deformations on a specific deformation axis: the extensometers of the first part detect only the circumferential deformations (tangential and radial components) ; the extensometers of the first part detect only longitudinal deformations (axial components).

[00117] A second solution (more efficient than the first from a point of view of the burden of installation) is to install the fiber optics with Bragg grating (FBG array) by wrapping them in a helix around the container so as to cover the entire surface with a grating of Bragg sensors and obtain a deformation measurement that is the sum of all the vectors involved. This solution allows a quick and simpler installation, to the detriment of the accuracy of the detection that is not the same on all the axes. Accuracy may be increased by using multiple fibers to construct a multiple helix.

[00118] According to a preferred embodiment of the method, shown in Figures 6 and 8, in the case wherein the container is composed of a cylindrical tank, the FBG extensometers are made on one or more optical fibers, each of which wraps the aforementioned container in a spiral helix. In this case, each extensometer is able to detect both circumferential deformations (tangential and radial components) , and longitudinal deformations (axial components) .

[00119] However, it should be noted that for the application of this monitoring method, no precise measurement of deformations in absolute terms is necessary, since the aim is not to measure the size of the container, but to monitor the deviations of the measurements over time.

[00120] According to the invention, by comparison with historical data, on the other hand, the repeatability of the measurement and therefore the sensitivity of the individual optical fiber sensors in detecting the deformations, not only at the measurement point but also in the surroundings, is much more important.

[00121] As already highlighted above, the pressure of the gas contained, measured continuously by means of a specific pressure sensor, is correlated, in real-time, with the instantaneous deformation measurements coming from the FBG sensors. The particular helix positioning of the optical fiber containing the FBG sensors allows a good detection of the deformation of the surface of the container to be obtained by extending the sensitivity over all the axes. The possible positioning of several fibers allows the entire surface to be completely covered allowing one to trace, with good approximation, a map of the deformation characteristic of the container in relation to the different internal pressures.

[00122] The result is in fact a portion of the characteristic deformation curve of the container in the so-called elastic zone, a zone wherein the structure temporarily deforms under the effect of the compressed gas inside to then return to the original shape and size without any residual structural change when the relative internal pressure returns to 0 bar. Figure 9 shows an example of a punctual deformation curve detected by an FBG sensor of a gas storage container as the pressure varies.

[00123] A deviation of the structural deformation detected after a certain period of time, compared to that initially detected (under conditions of established structural integrity, for example, new) , being the pressure equal, indicates that the structure of the container has undergone an alteration.

[00124] The correlation of adjacent detection points also allows the detection of deformations whose center is distant (also equidistant) from the individual measuring points.

[00125] Figure 10 shows a comparison between a deformation curve on a container placed under pressure without structural alterations (continuous line) and a deformation curve on a container placed under pressure with structural alterations (broken line) . The deviation amplitude of detected deformation AS is proportional to the magnitude of structural alteration and precisely identifies the degree of structural damage to the container.

[00126] By correlating the measurements of all the points and comparing the result obtained with those stored previously, it is therefore possible to determine variations in the deformation even in the areas not adjacent or equidistant from the individual measurement points.

* * *

[00127] Also object of the present invention is a system for non-destructive monitoring of the structural integrity of containers for storing compressed gas.

[00128] According to an embodiment illustrated in particular in Figure 6, the monitoring system 1 comprises :

- at least one container 10 for storing compressed gas;

- a plurality of Bragg grating extensometers 20, each of which is anchored firmly to the outer surface of the container 10 at the portion wherein the Bragg grating is made and is suitable for detecting, over time, the deformations of the structure of said container induced by changes in the internal pressure of the gas at the anchoring point;

- a pressure sensor 30 associated with said container 10 to detect the gas pressure inside the container;

- optionally, a temperature sensor 40 associated with said container 10 for detecting the temperature of said container; and

- an electronic data acquisition, storage and analysis unit 50 which is connected to said plurality of optical fiber Bragg grating extensometers 20 and to said pressure sensor 30, and optionally connected to said temperature sensor 40.

The aforementioned electronic unit 50 is configured to compare, being the internal pressure of the gas equal, the deformations detected by said extensometers 20 with corresponding reference deformations of the structure previously detected on the same container 10 in conditions of established structural integrity through said same plurality of extensometers 20, any deviations from said reference deformations being assumed as indicators of anomalies in the structural integrity of the container, being the internal pressure equal.

[00129] Preferably, the aforementioned electronic unit 50 is configured to associate with each deformation detection: a specific point on the outer surface of the container corresponding to the anchoring point of the extensometer 20 which has generated said detection; an internal pressure value of the gas; and, optionally, a temperature value of the container.

[00130] Preferably, the aforementioned electronic unit 50 is configured to compensate deformation detections by means of a temperature-dependent coefficient so as to obtain deformation detections independent of temperature.

[00131] Advantageously, the aforementioned electronic unit 50 is configured to store the deformation detections over time.

[00132] Preferably, the aforementioned electronic unit 50 is configured to estimate the deformations occurring on the structure of the container in areas not adjacent to the detection points and/or equidistant from more detection points interpolating the detections obtained from the detection points present around said areas. [00133] Advantageously, the anchoring points of the FBG extensometers 20 to the outer surface of the container 10 form a grating of detection points extending over the outer surface of the container, preferably said grating extending over the entire surface of the container.

[00134] Advantageously, the aforementioned point grating may have a density of points per surface unit variable according to the specific surface portion of the container .

[00135] In particular, the aforementioned extensometers

20 are positioned on the outer surface of said container 10 following a predefined distribution pattern, selected according to the shape of the same container.

[00136] According to an embodiment not shown in the attached Figures, each extensometer 20 is arranged in such a way that its direction of sensitivity is aligned with a specific deformation axis of the outer surface portion of the container to which it is associated.

[00137] According to an embodiment illustrated in Figures 6 and 8, each extensometer 20 is arranged in such a way that its direction of sensitivity is not aligned with any specific deformation axis of the outer surface portion of the container to which it is associated so as to be able to detect deformations on several axes and not only on one specific deformation axis. [00138] Advantageously, two or more of said Bragg grating extensometers are made on a same optical fiber 21 that wraps at least a portion of said container and is connected to the aforementioned electronic unit 50 for data acquisition, storage and analysis.

[00139] Preferably, the aforementioned Bragg grating extensometers 20 are all made on a single optical fiber 21 which wraps the container externally and is connected to said electronic unit for data acquisition, storage and analysis.

[00140] According to an embodiment not illustrated in the attached Figures, the aforementioned container 10 has a cylindrical main body with a circular cross-section. A first part of said Bragg grating extensometers 20 are made on two or more optical fibers, each of which wraps circumferentially said container in a predefined position along the axis of said cylindrical main body, whereas a second part of said Bragg grating extensometers 20 are made on two or more optical fibers, each of which is arranged parallel to the axis of said cylindrical main body in a predefined circumferential position.

[00141] According to an embodiment illustrated in the Figures 6 and 8, the container 10 has a cylindrical main body with a circular cross-section. The aforementioned Bragg grating extensometers 20 are made on one or more optical fibers, each of which wraps the container in a helical spiral.

[00142] Preferably, the aforementioned electronic data acquisition, storage and analysis unit 50 comprises:

[00143] - at least one optical interface for fiber optics with Bragg grating;

[00144] - at least one optical signal analyzer;

[00145] - a microprocessor processing unit; and

[00146] - mass storage means.

[00147] Advantageously, the aforementioned electronic data acquisition, storage and analysis unit 50 may further comprise one or more communication interfaces suitable for connecting the aforementioned electronic unit to a system for loading and unloading gas in the container.

* * *

[00148] The object of the present invention is also a compressed gas storage plant.

[00149] Such compressed gas storage plant, comprises:

[00150] - one or more containers 10 for storing compressed gas; and

[00151] - a system for regulating gas loading and unloading operations in said one or more containers,

[00152] In particular, the gas loading and unloading regulation system comprises: [00153] - pumping means (not shown) of the gas to and from each individual container; and

[00154] - valve means 60 for regulating the inflow and outflow of gas from each individual container and possibly isolating it from the rest of the system.

[00155] According to the invention, such storage plant comprises a system for monitoring the structural integrity of said one or more containers according to the present invention and, in particular, as described previously.

* * *

[00156] The invention allows many advantages already partly described to be obtained.

[00157] The method and system for non-destructive monitoring of the structural integrity of containers for storing compressed gas allow a continuous monitoring in real-time .

[00158] The method and the system of monitoring according to the invention are operatively simple to implement irrespective of the availability of direct access to the storage containers.

[00159] The method for non-destructive monitoring of the structural integrity of a container for storing compressed gas according to the invention is operationally reliable. [00160] The structural inspection of the containers for storing compressed gas that may be implemented by virtue of the method according to the invention, is based on the use of optical fibers with FBG sensors managed by appropriately developed embedded electronics.

[00161] The processing of information on the deformation of the mechanical structure of the container detected through FBG sensors, appropriately correlated, provides an adequate diagnostic to monitor the occurrence of anomalous deformations such as to indicate the probable presence of corrosion, cracks, or more generally weakening of the structure of the container by allowing policies of exclusion of the container from the work cycle to be implemented and preventive maintenance thereof to be provided.

[00162] The method and the system of monitoring object of the present invention are able to detect even slight anomalies in the surface deformation (for example, just at the beginning of the formation of corrosive events inside the container) . This allows maintenance activities to be carried out to better preserve the structure of the container (washes and internal treatments) .

[00163] The invention allows in particular the following advantages to be obtained:

[00164] - continuous and real-time detection of deformation deviations of a container for gas storage without decommissioning the container;

[00165] - high reliability in detecting the occurrence of structural alterations in the long term due to the long-term stability of the optical fibers, the FBG sensors and the comparison method put in place;

[00166] - a system compatible with environments having risk of explosion and unalterable by harsh conditions of use related to temperature and/or magnetic fields;

[00167] - a system applicable to any type of gas storage container regardless of geometry;

[00168] - a system that does not need re-calibration other than the initial installation for plotting the deformation model of the container.

[00169] The invention thus conceived therefore achieves the foregoing objects.

[00170] Obviously, in its practical implementation, it may also be assumed to take forms and configurations other than those described above without, for this reason, departing from the present scope of protection.

[00171] In addition, all details may be replaced by technically equivalent elements and the dimensions, shapes and materials used may be of any kind according to the needs.

Claims

1. Method for non-destructive monitoring of the structural integrity of a container for storing compressed gas, characterized for: - detecting over time, through a plurality of optical fiber Bragg grating extensometers firmly associated with the container, the deformations of the structure of said container induced by variations in the internal pressure of the gas; and - comparing, being internal pressure of the gas equal, the deformations thus detected with corresponding reference deformations of the structure previously detected on the same container in conditions of established structural integrity through said same plurality of extensometers, any deviations from said reference deformations being assumed as indicators of anomalies in the structural integrity of the container, being the internal pressure equal.

2. Method according to claim 1, wherein each deformation detection is associated with the position of the extensometer which has detected it, such that the deformations detected and any deviations from the reference values are punctually referable to specific areas of the container structure and it is therefore possible to locate any anomaly on the container.

3. Method according to claim 1 or 2, wherein the amplitude of said possible deviations is taken as a measure of the thinning of the wall of the container in the detection zone and therefore as an index of the severity of the anomaly.

4. Method according to one or more of the preceding claims, wherein the deformation detections are accompanied by temperature measurements of the structure of the container and are compensated by a temperature- dependent coefficient so as to obtain deformation detections independent of the temperature.

5. Method according to one or more of the preceding claims, comprising the following preparatory operational steps :

- a) firmly associating with the outer surface of said container a plurality of said optical fiber Bragg grating extensometers, each of said extensometers being firmly anchored to the outer surface of the container at the portion wherein the Bragg grating is made; and

- b) equipping the container with a pressure sensor to detect the gas pressure inside the container.

6. Method according to claim 5, comprising the further preparatory operating step c) of providing the container with at least one temperature sensor for detecting the temperature of said container.

7. Method according to claim 5 or 6, wherein said preparatory operating steps are carried out during the construction of said container and/or during the installation in the work site thereof.

8. Method according to one or more of the preceding claims, wherein each detection of deformation is associated with: - a specific point on the outer surface of the container, corresponding to the anchoring point of the extensometer which has generated said detection; - an internal pressure value of the gas; and, optionally, - a temperature value of the container.

9. Method according to one or more of the preceding claims, wherein each detection of deformation is stored in mass storage means.

10. Method according to one or more of the preceding claims, characterized in that it is implemented automatically by means of an electronic data acquisition, storage and analysis unit which is connected to said plurality of optical fiber Bragg grating extensometers and to said pressure sensor, and optionally is connected to said at least one temperature sensor.

11. Method according to one or more of the preceding claims, wherein the anchoring points of the extensometers to the outer surface of the container form a grating of detection points which extends over the outer surface of the container, preferably said grating extending over the entire surface of the container.

12. Method according to claim 11, wherein the deformations occurring on the structure of the container in areas not adjacent to the detection points and/or equidistant from more detection points are estimated by interpolating the measurements obtained from the detection points present in the vicinity of said areas.

13. Method according to claim 11 or 12, wherein said point grating has a density of points per surface unit variable according to the specific surface portion of the container .

14. Method according to one or more of the preceding claims, wherein said extensometers are positioned on the outer surface of said container following a predefined distribution pattern, selected according to the shape of the container.

15. Method according to one or more of the preceding claims, wherein each extensometer is arranged in such a way that its direction of sensitivity is aligned with a specific deformation axis of the outer surface portion of the container with which it is associated.

16. Method according to one or more of the claims 1 to 14, wherein each extensometer is arranged in such a way that its sensitivity direction is not aligned with any specific deformation axis of the outer surface portion of the container to which it is associated so as to be able to detect deformations on several axes and not only on one specific deformation axis.

17. Method according to one or more of the preceding claims, wherein two or more of said Bragg grating extensometers are made on the same optical fiber that wraps at least a portion of said container and is connected to said electronic unit for data acquisition, storage and analysis.

18. Method according to one or more of the preceding claims, wherein said Bragg grating extensometers are all made on a single optical fiber which wraps the container externally and is connected to said electronic data acquisition, storage and analysis unit.

19. Method according to one or more of the preceding claims, wherein said container has a main body with a cylindrical shape with a circular cross-section and wherein a first part of said Bragg grating extensometers are made on two or more optical fibers, each of which wraps circumferentially said container in a predefined position along the axis of said cylindrical main body, and a second part of said Bragg grating extensometers are made on two or more optical fibers, each of which is arranged parallel to the cylindrical main body axis in a predefined circumferential position.

20. Method according to one or more of the claims from 1 to 18, wherein said container has a cylindrical main body with a circular cross-section and wherein said Bragg grating extensometers are made on one or more optical fibers, each of which wraps said container with a helical spiral .

21. Method according to one or more of the preceding claims, wherein the electronic data acquisition, storage and analysis unit comprises:

- at least one optical interface for optical fiber with Bragg grating;

- at least one optical signal analyzer;

- a microprocessor processing unit; and

- mass storage means.

22. Method according to claim 20, wherein said electronic data acquisition, storage and analysis unit further comprises one or more communication interfaces suitable for connecting said electronic unit to a system for loading and unloading gas into said container.

23. System for non-destructive monitoring of the structural integrity of at least one container for storing compressed gas, characterized in that it comprises :

- at least one container for storing compressed gas;

- a plurality of Bragg grating extensometers, each of which is anchored firmly to the outer surface of the container at the portion whereon the Bragg grating is made and is suitable for detecting, over time, the deformations of the structure of said container induced by changes in the internal pressure of the gas at the anchoring point;

- a pressure sensor associated to said container to detect the gas pressure inside the container.

- optionally, a temperature sensor associated with said container for detecting the temperature of said container; and

- an electronic data acquisition, storage and analysis unit which is connected to said plurality of optical fiber Bragg grating extensometers and to said pressure sensor, and optionally to said temperature sensor, and by the fact that said electronic unit is configured to compare, being internal pressure of the gas equal, the deformations detected by said extensometer with corresponding reference deformations of the structure previously detected on the same container in conditions of established structural integrity through said same plurality of extensometers, any deviations from said reference deformations being assumed as indicators of anomalies in the structural integrity of the container, being the internal pressure equal.

24. System according to claim 23, wherein said electronic unit is configured to associate with each deformation detection: a specific point on the outer surface of the container, corresponding to the anchoring point of the extensometer which has generated said detection; an internal pressure value of the gas; and, optionally, a temperature value of the container.

25. System according to claim 23 or 24, wherein said electronic unit is configured to compensate the deformation detections by means of a temperature- dependent coefficient so as to obtain deformation detections independent of temperature.

26. System according to one or more of the claims 23 to

25, wherein said electronic unit is configured to store the deformation detections over time.

27. System according to one or more of claims 23 to 26, wherein said electronic unit is configured to estimate the deformations occurring on the structure of the container in areas not adjacent to the detection points and/or equidistant from more detection points by interpolating the detections obtained from the detection points present in the vicinity of said areas.

28. System according to one or more of claims 23 to 27, wherein the anchoring points of the extensometers to the outer surface of the container form a grating of detection points extending over the outer surface of the container, preferably said grating extending over the entire surface of the container.

29. System according to one or more of the claims 23 to 28, wherein each extensometer is arranged in such a way that its sensitivity direction is not aligned with any specific deformation axis of the outer surface portion of the container to which it is associated so as to be able to detect deformations on several axes and not only on one specific deformation axis.

30. System according to one or more of claims 23 to 29, wherein two or more of said Bragg grating extensometers are made on the same optical fiber that wraps at least a portion of said container and is connected to said electronic unit for data acquisition, storage and analysis .

31. System according to one or more of claims 23 to 30, wherein said Bragg grating extensometers are all made on a single optical fiber that wraps the container externally and is connected to said electronic unit for data acquisition, storage and analysis.

32. System according to one or more of claims 23 to 31, wherein said container has a cylindrical main body with a circular cross-section and wherein said Bragg grating extensometers are made on one or more optical fibers, each of which wraps said container with a helical spiral.

33. Compressed gas storage plant, comprising:

- one or more containers for storing compressed gas; and

- a system for regulating gas loading and unloading operations in said one or more containers,

characterized in that it comprises a system for monitoring the structural integrity of said one or more containers according to one or more of the claims from 23 to 32, and wherein said monitoring system is connected to said regulating system.