GB2205948A

GB2205948A - Improved torsional exciter for non-destructive tests of structural members

Info

Publication number: GB2205948A
Application number: GB08813633A
Authority: GB
Inventors: Luciano Dossi; Gerlando Antonino Claudio Di
Original assignee: Tecnomare SpA
Current assignee: Tecnomare SpA
Priority date: 1987-06-12
Filing date: 1988-06-09
Publication date: 1988-12-21
Anticipated expiration: 2008-06-09
Also published as: GB2205948B; IT1216934B; GB8813633D0; IT8720885A0; NO882575D0; NO882575L

Abstract

A torsional exciter for non-destructive tests on ferromagnetic tubes 1 of metal trestlework structures, comprises a set of moulded elements 3-7 with a "U"-shaped cross-section, of a flexible material, evenly positioned along the circumference of the tube 1 to induce stressing of the tube by magnetostriction. Each one of said moulded elements 3-7 has a coil 8, 9 embedded inside it in near contact with the outer wall of the tube 1 via a thin base support 12. These elements are inclined to the direction of the axis 14 of the tube at such an angle that the perpendicular to them forms an angle of 54.7 DEG with said tube axis direction, and they are connected at their ends by means of two reverse-"U"-shaped connection heads. All the coils 8, 9 are connected in series with one another, and also with an a.c. generator. In another arrangement, all the coils are encapsulated within a single moulded element encircling the tube. <IMAGE>

Description

"IMPROVED TORSIONAL EXCITER FOR NON-DESTRUCTIVE TESTS OF STRUCTURAL DIAGNOSTICS" The present invention relates to a novel type of torsional exciter without a magnetic circuit of its own, and adaptable to a wide range of tube diameters, which, by making it possible the structure under test to be made vibrate within a very wide range of frequencies with practically pure torsional waves obtained by a magnetostriction effect, allows the diagnosis of same structure to be carried out more efficaciously and reliably.

It is known that the metal structures require periodical checks directed to verify the integrity of their structural elements, and the Larger the dimensions of the same structure, the more important such a need.

The whole of the techniques and of the methods for verifying the integrity of a structure is calted "structural diagnostics".

An important class of metal structures is the class of the trestlework structures built by assemblying steel tubes of suitable diameters and thicknesses. Meaningful examples of such structures are the offshore trestlework platforms for drilling oil wells and for the production of crude oil.

Now then, the diagnostics of such structures composed by metal tubes is generally carried out by means of non-destructive tests by means of exciters which make it possible said structures to undergo mechanical stresses, in order to examine the vibrationaL modes of their parts. In other terms, the individual parts of the structure are subjected to elastic waves of different frequencies, and possible changes in the vibration mode of the tested part are then detected, which changes are synonyms of the presence of faults, or of damages. On the other hand, inasmuch as each existing crack or damage has a preferential effect on one or more specific vibration frequency(ies), it is clear that, in order to secure that any damages of a certain magnitude are detected, one should have the possibility of performing the test within a very wide range of frequencies.Therefore, as the torsional vibrations can be produced within a frequency range much wider than of other types of vibrations, the greater importance arises of the torsional exciters as compared to other exciter types.

From the priot art, torsional exciters suitable for such tests are already known, with said exciters being substantially constituted by electromechanical vibrators fastened to the parts to be excited by means of suitable mechanical fastening elements.

However, such torsional exciters known from the prior art suffer from a number of drawbacks, which seriously limit the information which can be obtained on the modes of vibrating of the structure under test.

In fact, due to the presence of parts of ferromagnetic material, they have a not negligible mass which, by being concentrated in one point, besides rendering them not very handeable and not easily constrainable to the structural part to be excited, considerably alters the dynamic properties of the structure under test, and furthermore strongly reduces the band of the frequencies which can be used for the excitation, in that the torsional waves are generated in the electromechanical vibrator and must be then transmitted to the structure through the mechanicaL connection elements.

The purpose of the present invention is namely to obviate the above said drawbacks, and hence provide a torsional exciter for non-destructive tests of structural diagnostics which, by being without heavy parts of a ferromagnetic material, enables only torsional vibrations to be generated within a very wide range of frequencies, to that an as large as possible number of information can be obtained for diagnostic purposes.

The above purpose is substantially achieved by exploiting the well-known magnetostrictive properties of the ferromagnetic material, due to which the magnetic field generated by the current flowing through an active conductor alters the dimensions of the area of a ferromagnetic meta structure, such as a tube of a trestlework structure, opposite to said active conductor positioned parallel to, and at a short distance from, the surface of said structure.

Hence, if through an active conductor an alternating current is flown, so as to create a variable magnetic field, the consequent alternating expansions and retractions of said area undergoing the alternating induction generate elastic waves which propagate along the structure according to known laws, wherein the acceleration of the produced vibration is a function of the induction. More precisely, in the general case that one has a stationary field of induction B* and an alternating field of induction B at frequency f, an acceleration of frequency f is produced, and such an acceleration is proportional to the product of the above said inductions. Furthermore, an acceleration component at frequency 2f is generated, and such a component is only proportional to the square of the variable induction B.Consequently, in order to obtain elastic waves of frequency 2f, it is enough to feed the actiye conductor with an AC generator giving â frequency f, while, in order to obtain elastic waves of frequency f, also a direct current generator should be used.

On the other hand, inasmuch as said area wherein the magnetostriction phenomena take place practically has dimensions of the same order of magnitude of the transversal overall dimensions of the active conductor which generates the magnetic field, clearly having available active conductors uniformly distributed parallelly to one another along the circumference of the tube under test will be necessary, in order to be able to keep controlled each point of the same tube.

Summing up, a magnetostrictive exciter results to be constituted by a set of active conductors positioned parallel to one another at a short distance from the surface of the tube being tested, and connected in series with a generator of sinusoidal alternating current.

Actually, such a generator, even if shows the advantages of not requiring a magnetic circuit of its own, and of generating the elastic waves directly on the part to be excited, suffers from the drawback that it propagates elastic waves of both longitudinal and torsional types.

But the present Applicant has surpsisingly observed now on an experimental basis that by causing the lines of force of the magnetic induction field generated by said active conductors of a magnetostrictive exciter to have an inclined direction, such as to form an angle of 54.7 with the axis of the tube under test, or, which is the same, by causing said active excitation conductors to be parallelly inclined to said axis of the tube at the complementary angle of the above said angle of 54.'0 elastic waves are obtained, which are substantially onLy torsional, and said torsional waves can have frequencies ranging within a very wide frequency range, e.g., from 1 kHz to 20 kHz.

Now then, the torsional exciter according to the present invention is precisely substantially realized as a magnetostrictive exciter, but with its active excitation conductors being parallelly inclined in such a way that the perpendicular to them forms with the direction of the axis of the tube an angle of 54.7 .

Furthermore, according to another characteristic of the present invention, said active conductors parallelly positioned at equal distances along the whole circumference of the tested tube in order to obtain a homogeneous torsional effect on the same tube, are grouped to form coils, each of which is constituted by two active sides positioned parallelly inclined as above said into contact with the outer surface of the tube through a thin base support, which are connected to one another at their ends by two connection heads positioned perpendicularly to the axis of the tube, and having a substantially reverse-"U" shape, in such a way that the connection leads result far away from the surface of the tube, so as to minimize their magnet-ic effect, which is disturbing, in that would generate a magnetic field with lines of force parallel to the axis of the tube.

Indeed, from the above the possibility derives of very simply producing also an efficacious longitudinat exciter, by means of a set of coils as above-said, positioned circumferentially around the tested tube, but with their connecting heads being opposite to the surface of the tube, and with said two active sides being at an upper level. In fact, in such case it is the connection heads which become active and, by generating a direct induction along the axis of the tube, generate longitudinal waves by magnetostrictive effect.

Still according to the present invention, each coil is furthermore embedded inside a corresponding moulded element with an "U"-shaped cross-section, made from a flexible material (such as a polyurethane or a rubber), obtained by casting a compound of said flexible material inside a mould inside which the coil is inserted; from the moulded element only the leads come out, which are required for connecting in series said coils.

In such a way, the torsional excited results to be modularly constituted by a set of moulded elements containing, embedded inside them, coils with the proper angle, and connected in series with one another, which moulded elements are kept wound and pressed against the tube to be tested by means of a belt of nylon, or the like, running through the hollows of the "U"-crosssection of the moulded elements, and is tightened by means of a suitable tightening means, all these elements being easily available on the market.

Such a modular realization, by making it possible exciters to be composed, which contain various numbers of moulded elements, computed according to the diameter of the tube to be stressed, evidently enables the same exciter to be adapted to a wide range of tube diameters.

According to a different form of practical embodiment of the present invention, all the coils with the proper angle, and already connected in series with one another, are embedded inside one single moulded element with a "U"-shaped cross-section made from a flexible material, to be wound and pressed against the tube to be tested.

Such a form of practical embodiment makes it possible a stronger, more reliable and efficacious exciter to be obtained, which has a very good dimensional stability and is easier to apply to the tube to be stressed, but, on the other hand, suffers from the limitation that it can be applied to one specific tube diameter only.

On the other hand, according to a preferred practical embodiment of the present invention, in case tubes have to be stressed, which have a small diameter of less than 500 mm, the magnetostrictive torsional exciter is produced by placing the coils with the proper angle and connected in series with one another on the flexible base support, inside a mould having the proper banding radius, and not straight, as it generally occurs, inside which the flexible material constituting the mouLded element is cast. In such a way, said base support can be pre-curved according to the outer diameter of the small tube to be stressed, rendering more perfect the coupling of the exciter with the tube, and, hence, more efficacious the magnetostriction effect.

It is clear then that the above said pre-bending of the base support, with the consequent advantages in case of small-diameter tubes, can be adopted also for the individual moulded elements of the modular, multi-element torsional exciter provided according to the teachings of the present invention.

Then, due to the fact that, as hereinabove mentioned, for the controlled generation by magnetostriction effect of elastic waves of frequency f equal to the frequency of the applied alternating magnetic field of induction B, also having a stationary magnetic field of constant induction B* is necessary, the above result can be obtained in two different ways and, more precisely, either simultaneously feeding to the coils in series, and in the same direction, a constant current which generates the stationary field of induction B*, and a sinusoidal alternating current of the desired frequency f, or adopting, as according to a preferred form of practical embodiment of the present invention, couples of coils superimposed parallelly to the base support and respectively connected in series, with the coil nearer to the above said support being fed with an alternating current, and the other one being fed with a direct current.

Then, according to a further preferred form of practical embodiment of the present invention, each coil of said coil couple is realized with a plurality of turns of an electrical lead of copper in strand form, which makes it possible a larger induction of the magnetic field, with the electrical power being the same, as well as a greater mechanical flexibility, to be obtained in the exciter.

The present invention is now better explained by referring to the hereto attached drawings, which illustrate preferred forms of practical embodiment supplied for merely exemplifying, and non-limitative, purposes, in that technical, technologic or structural changes may be made without departing from the scope of the present invention. So, e.g., instead of moulded elements containing one single coil, or a single couple of coils, moulded elements with two, three or more, coils, or coil couples, may be used.

Figure 1 shows a perspective view of a magnetostrictive torsional exciter according to the present invention applied to a length of tube to be stressed; Figure 2 shows an enlarged-scale perspective view of one of the moulded elements constituting the torsional exciter of Figure 1; Figure 3 shows a top view of the moulded element of Figure 2; Figure 4 shows a sectional view, on a different scale, of the moulded element, made along path AA of Figure 3; Figure 5 shows a perspective view of a trifilar coil used in the exciter according to the invention; Figure 6 shows a perspective view of a variant, still according to the invention, of a magnetostrictive torsional exciter applied to a length of tube to be stressed; Figure 7 is a diagram schematically showing the connections in series of the various exciting coils according to the invention;; Figure 8 is a diagram schematically showing a variant of connection in series of the various coils of an exciter according to the invention, but only applicable in case of exciters of the type of Figure 6; Figure 9 shows an enlarged-scale perspective view of one of the moulded elements constituting the torsional exciter of Figure 1, pre-curved according to the invention, for application to small-diameter tubes; Referring to the figures, by the reference numeral 1 a generic tube of a metal trestlework structure, not shown in the figures, is indicated, which must be checked for integrity, and, for the intended purpose, is stressed with torsional elastic waves generated by an exciter 2.

Said torsional exciter 2, according to the invention, is of magnetostrictive type, and is modularly constituted by a set of moulded elements with. a "U"shaped cross section, 3 to 7, visible in Figure 1, made from such a flexible material as, e.g., polyurethane or rubber, obtained by casting a compound of said material inside a mould.

Inside each one of said moulded elements, a couple of mutually superimposed coils are embedded, wherein the lower coil 8 (see specifically Figure 4) is to be fed by alternating current in order to generate an alternating magnetic field of frequency f and induction B, and the upper coil 9 is to be fed by direct current in order to generate a stationary magnetic field of induction B*.

Each coil 8, 9 is then constituted by two paraLlel active sides, respectively 10 and 11, which, inasmuch as must generate the above said magnetic fields for generating elastic waves by magnetostrictive effect in the areas of the tube which- are opposite to said sides, must be as close as possible to the surface of the tube, and, anyway, at a distance from it not greater than 1 mm, in order to obtain a gnod level of iron magnetization.

For the intened purpose, said active sides 10, 11 rest on a thin base support 12 (see still Figure 4), which is preferably formed by the same material as of the moulded elements from 3 to 7, and has a thickness of the order of 0.5 mm, so that the active sides of the coil for the alternating current are at a distance of approximateLy 0.7 mm from the surface of the tube, with the thickness of the insulating material being made due allowance for.

On the other hand, said parallel active sides are positioned inclined in such a way that the perpendicular 13 to them (see Figure 3) forms, with the direction of the axis 14 of the tube to be stressed 1, an angle out of 54.7 , or, which is the same, that the above said active sides form, relatively to the direction 14 of the axis of the tube 1, a complementary angle of the above said angLe /. In fact, the present Applicant was able to observe, by way of experiments, that such a specific inclination of the active sides makes it possible substantially pure torsional elastic waves to be generated.

The above said active sides 10, 11 of each coil 8, 9 are then connected to one another, at their ends, by two connection heads, respectively 15 and 16, perpendicular to the axis 14 of the tube being tested 1, and having a reverse-"U"-shape, so that the connection leads 1 5 and 16 result raised from the surface of the tube (see Figures 2 and 5), and hence their magnetic effect, which would generate longitudinal elastic waves, results minimized.

Indeed, in order to make sure that such disturbances do not occur, the distance of the connection leads from the surface of the tube should never be smaller than 10 times the distance between the active sides and said tube surface.

The above said coils 8, 9 can be furthermore realized by means of a single copper lead, or by means of a number of copper leads, e.g., three copper leads as shown in Figure 5, and furthermore, strand conductors may be used, in order to obtain a larger flexibility in the same coil, and, therefore, a more efficacious adaptation thereof to the diameter of the tube to be stressed.

The coils 8, 9 of the various moulded elements, from 3 to 7, are finally respectively connected with one another by connecting in series their ends, respectively 17, 18, 19, 20, which come out from said moulded elements, according to the diagram of Figure 7.

Said moulded elements 3 to 7 of flexible material with said coil couples positioned to form the proper angle and connected in series, are evenly positioned, at equal distances, along a whole circumference of the tube 1 to be stressed, with their lower faces laying on said tube, in order to obtain a high enough homogeneity in the generated torsional elastic wave front, and, hence a homogeneous torsional effect on the same tube to be stressed, and are kept pressed against said tube by means of different means, selected according to the envisaged application, and, preferably, of a non-ferromagnetic material.In the drawings, a nylon belt 21 was used, which is provided with slots 22 for a rapid coupling with the hoies 23 of the moulded elements from 3 to 7 (see specifically Figure 3), and the fastening of these latter by bolts and nuts 24 (see Figure 2), which belt is then inserted inside the hollows of the same "U"-shaped moulded elements and is tightened by means of a suitable tightening means not shown in the figures, but easily available from the market.

In Figure 6, a different form of practical embodiment of the invention is then depicted in which, instead of a large number of "U"-shaped moulded elements, one single moulded element 25 with a "U"-shaped crosssection is used; in other terms, couples of superimposed coils 8 and 9 positioned with the proper angle on a relevant thin base support and respectively connected in series to one another are positioned in a straight mould, along the whole length of this latter, which is approximatively equal to the circumference of the tube under test to be stressed, the mould is then filled by a casting of a flexible material compound, such as polyurethane or rubber, with a straight, single-body exciter with an "U"-shaped section being thus obtained which, thanks to its flexibility, can be easily wound around the tube to be tested, as shown in Figure 6.

For this latter type of single-body torsional exciter, besides a connection in series of the colts, as schematically shown in the diagram of Figure 7, also the variant schematically shown in Figure 8 can be adopted, wherein the active sides 10, 11 of adjacent coils are close to each other, and the coils are connected with one another in such a way that through said active sides all equidirectional currents flow, in order that their induction effect adds up.

Finally, in case the tubes to be stressed have a diameter smaller than 500 mm, the coils should be preferably inserted inside moulds with the suitable bending, so as to obtain pre-curved moulded elements, as shown in Figure 9.

Then, it is then that, in order to use the magnetostriction torsional exciter of the invention, it is necessary to connect the free ends 26 and 27 of the winding to be fed with alternating current to a generator of sinusoidal alternating current with the suitable f frequency, which may be constant or variable, and the free ends 28 and 29 of the winding to be fed with direct current to a continuous current generator; said generators are not shown in the figures.

As hereinabove already said, in order to obtain elastic torsional waves of frequency 2f, it is enough to use the alternating current generator, with the circuit of the continuous current winding being left open, but if one desires to obtain elastic torsional waves of frequency f, both generators should be used. In this latter case, the output of the direct current generator is preferably characterized by a high impedance, and by a no-charge voltage not lower than the voltage applied by the alternatig current generator to the ends of the relevant winding of the exciter.

In fact, the windings of the exciter are coupled by mutual induction, like in a transformer, and it is preferable to prevent a reverse current from being induced in the direct current generator. In the same way, it is necessary that the alternating current generator be adequately protected from the effects of the even accidental transients, due to the switching-on of the direct current generator, which would be transmitted due to the mutual induction. Such a generator must be of the type suitable for piloting inductive loads, and it, as well as the direct current generator, is easily available from the market, with a very wide range of output powers.

Claims

1. A torsional exciter particularly suitable for non-destructive tests on tubes of metal trestlework structures such as offshore steel platforms, comprising a plurality of active excitation conductors positioned substantially parallel to one another and at substantially equal distances along the circumference of the tube to be stressed by magnetostrictive effect, said conductors being connected in series to one another and with a generator of sinusoidal alternating current of adjustable frequency, wherein said active conductors are positioned inclined to the direction of the axis of said tube to be stressed in such a way that the perpendicular to them forms with the said direction of the axis of the tube an angle of about 490 to about 600 and are furthermore grouped to form coils, with each one of said coils comprising two of said active conductors positioned at a suitable angle, and into contact with the outer surface of the tube through a base support, which are connected with each other, at their ends, by two connecting heads positioned perpendicularly to the said direction of the axis of the tube, and with a substantially reverse-"U" shape.

2. Torsional exciter according to claim 1, characterised in that each one of the above said coils is embedded inside a corresponding moulded element with an "U"-shaped cross-section made from a flexible material, manufactured by casting a compound of said flexible material inside a corresponding mould, inside which the coil is contained, which only comes out from the moulded element with the leads for the connection in series of the same coils, said moulded elements being kept wound and pressed against the tube to be stressed by means of a belt running through the hollows of the "U"-shaped crosssection of the same moulded elements.

3. Torsional exciter according to claim 1, characterized in that all the above-said coils are embedded inside one single moulded element with an "U"shaped cross-section made from a flexible materiaL, obtained by casting a compound of said flexible material inside a corresponding mould inside which the coils are inserted already connected in series with one another, said moulded element being kept wound and pressed against the tube to be stressed by means of a belt running through the hollows of the "U"-shaped cross-section of the same moulded element.

4. Torsional exciter according to claim 1, characterized in that instead of coils, couples of superimposed coils positioned parallelly to said base support and respectively connected in series are used, wherein the coils closer to said support are fed with alternating current, and the other coils are fed with continuous current.

5. Torsional exciter according to claim 1, characterized in that each one of the above said coils is realized with a plurality of turns of an electrical copper lead having a strand structure.

6. Torsional exciter according to claim 2 or 3, characterized in that each moulded element with "U"shaped cross-section, made from a flexible material, is realized pre-curved, by casting said compound of said flexible material into a mould having a bent base.

7. A torsional exciter according to any of claims 1 to 6, wherein said active conductors are inclined to the direction of the axis of the tube such that the perpendicular to them forms with the direction of the axis of the tube an angle of 54.7 + 50.

8. A torsional according to claim 7, wherein said angle is 54.7 + 3 .

9. A torsional exciter according to claim 8, wherein said angle is 54.7+10.

10. A torsional according to claim 9, wherein said angle is 547O

11. A torsional exciter according to claim 1, substantially as hereinbefore described with reference to the drawings.

Torsional exciter particularly suitable for nondestructive tests on tubes of metal trestlework structures, such as offshore steel platforms, constituted by a set of active excitation conductors positioned parallely to one another, and at equal distances along the whole circumference of the tube to be stressed by magnetostrictive effect, said conductors being connected in series to one another and with a generator of sinusoidal alternating current of adjustable frequency, characterized in that said active conductors as positioned- inclined to the direction of the axis of said tube to be stressed, in such a way that the perpendicular to them forms with the above said direction of the axis of the tube an angle of 54 , and are furthermore grouped to form coils, with each one of said coils comprising two of the above said active conductors positioned at the suitable angle, and into contact with the the outer surface of the tube through a thin base support, which are connected with each other, at their ends, by two connecting heads positioned perpendicularly to the above said direction of the axis of the tube, and with a substantially reverse-"U" shape.