METHOD AND DEVICE BASED ON FIBRE-OPTIC INTERFEROMETER SENSORS, FOR ANALYZING THE DYNAMIC DEFORMATION OF A STRUCTURE OR ITS COMPONENTS

Abstract: A method and device based on fibre-optic interferometer sensors for analyzing the dynamic deformation of a structure, by determining the phase modulation which occurs in one or more increasing-length series of fibre-optic sensors which are fixed to the structure and are traversed by laser beams.

Description: This invention relates to a method and device as defined in claims 1 and 5 for measuring the dynamic deformation of a structure or parts thereof, for the purpose of determining the variation in this deformation when subjected to forces which vary in intensity and direction with time. More particularly, the method and device according to the invention relates to the determination of dynamic deformation in bodies which are of difficult accessibility and/or are in a hostile environment, so that the use of traditional measuring devices such as conventional extensometers or accelerometers is impracticable or ineffective. Determining the variation in the deformation of a body subjected to forces which vary with time is useful with a view to knowing its behaviour, such as its vibration modes and the distribution and type of stresses in its interior, knowing the stresses which it receives from the outside and which would otherwise be undeterminable with sufficient accuracy, knowing which suitable remedies to adopt in order to prevent damage to the body, and for diagnostic purposes in ascertaining the state of the structure. The present invention is described by way of non-limiting example with reference to a typical embodiment in the crude oil industry, and in particular to the upwardly extending pipes, commonly known as risers, which connect the well heads of underwater hydrocarbon deposits to platforms (such as TLPs, ie tension leg platforms) at the sea surface or to floating means retained in position by other connections to the sea bed which are kept under tension by the buoyancy thrust and/or by propellers. It is apparent that the device according to the invention can also be used to determine dynamic deformation in other technical fields, such as in lattice structures or their components subjected to variable loads or to periodic stresses, such as bridges of lattice structure, cranes, lattice structures for long-distance power lines, machine parts etc. In the described embodiment relating to risers, the riser pipe is one of the most delicate parts of the infrastructure concerned in the working of underwater hydrocarbon deposits, in that breakage of the riser would result both in environmental damage and in lengthy interruption to the working of the deposit, with considerable expense involved in its restarting. Such risers are kept as far as possible free from the anchoring forces involved in maintaining the platform or floating craft in its correct position relative to the sea bed, however because of the effect of wave motion, wind, tides and currents, the riser is also involved in periodic movements and stresses, which have to be continuously monitored. In measuring the deformation, the device according to the invention effects extensometric measurements with optical fibres. Basically, a fibre-optic transducer converts its undergone deformation into modulation of the main parameters of the conveyed light, ie intensity, polarization and phase. Fibre-optic extensometer transducers are known which separately utilize each of these types of modulation, either by direct measurement or by indirect measurement. In the case of direct measurement, the optical fibre is rigidly connected to that part of the structure under examination. In the case of indirect measurement, the structure deformation is made to act on a device which itself contains the optical fibre. When used to solve the technical problem towards which the present invention is directed, fibre-optic extensometer sensors for measuring intensity modulation are able to provide absolute deformation measurements but have numerous drawbacks, such as non-linearity of the sensitivity and measurement characteristics, limited measurement bases, limited frequency response, and poor system reliability. Likewise, extensometer sensors for measuring polarization modulation have drawbacks in terms of non-linearity of the sensor and high sensitivity to disturbances. Phase-modulation sensors of autointerferometer type are linear and simple, but have other drawbacks such as the need for access to both ends of the fibre, and frequency modulation problems. In contrast, the device of the invention is based on the use of fibre-optic sensors with interferometer measurement of phase modulation, using laser sources to measure vibration and slow deformation. These measurement sensors are described in European Patent 64789 and U.K. Patent 2144215 which relate to the effecting of measurements undisturbed by environmental disturbances, by suppressing the noises concerned in transmission via optical fibre. In these sensors the optical fibre, rigidly connected to the deforming mechanical structure, undergoes a modification in its geometrical and optical characteristics such as to induce a phase lag in the guided light. The relationship between the integral of the deformation impressed on the fibre and the measured phase displacement of the light beam is linear over the whole measurement range. Description of figures: Fig 1 General illustration of present invention Figs 2 & 3 Typical embodiment of present invention Fig 4 Electro-optical unit. The fibre-optic interferometer sensor used in the present invention is illustrated in its most general sense in Figure 1 in terms of its construction and operation. The beam 1 from the laser source 2 is split into two parts by the splitter BS to form the reference light beam 3 and the measurement beam 4. This latter proceeds to the connector C and is then fed via a suitable conventional optical focusing system, not shown, into the sensor optical fibre 5. This optical fibre is made specular at its end M by depositing a suitable thin reflecting film on its flat end face. To prevent fluctuation in the state of polarization of the light conveyed along the fibre by the beam 4 which may arise due to any small-radius curvatures in the fibre, it can be advantageous to use a polarization-maintaining fibre. In this case the delay plate HWP causes the polarization plane of the entering light to coincide with one of the two main birefringence planes of the optical fibre. The return light beam from the optical fibre 5, having passed through it in both directions after starting from HWP, undergoing reflection at M and returning in the opposite direction, conveys the information required for the measurement, ie the optical phase modulation. This optical phase modulation is proportional to the state of deformation of the fibre. The optical fibre 5 is used as a deformation sensor by rigidly connecting its end portion 7, between 8 and M, to a body 6 to be measured. More specifically, the sensor for measuring the deformation of the body 6 over that body portion between 8 and M consists of the optical fibre portion 7. The optical return signal 9 is made to interfere with the reference beam 3 in BS. At this point the reference beam 3 has already passed through the modulator UM twice, having been reflected in the mirror RM. The purpose of the modulator UM, fed with the modulation command U m , is to provide the interferometer system with suitable frequency translation to allow better reconstruction of the phase signal, including the sign of the mechanical deformation phenomenon. The interferometer signal generated in BS is detected by a photodetector PD and fed to the electronic unit for signal demodulation. The extensometer sensor of the type proposed herein is extremely advantageous for the following reasons: extreme sensitivity practically unlimited measurement base linearity of sensor response great flexibility of application, in terms of the system geometry large passband large dynamic acceptance in the measured deformation values. The device according to the invention uses a plurality of phase-modulation fibre-optic interferometer sensors. It is described with reference to Figures 2 and 3, which show a typical embodiment thereof by way of non-limiting example. As shown in Figure 2 the optical fibres of the sensor are bunched parallel to each other into a bundle in which each optical fibre starts at the same level but is of different length L, and is therefore able to measure the deformation occurring over the length L of that portion of the structure to which it is rigidly connected. The optical fibres can also be combined into a cable and fixed to the structure, but according to a preferred embodiment of the invention the bundle of different-length parallel fibres is inserted into a protective sheath, for example of stainless steel, which is then fixed to the structure. By way of example, the sheath can be fixed to the structure by either a continuous or spot weld using weld material and possibly with an interposed convex insert, or by adhesive, or by binding. Once this sheath has been fixed, it deforms in the same manner as the structure, as do the optical fibres contained in it. The various optical fibres 7 a .......7 j ....., of length L a .......L j ..... respectively, constitute the extensometer sensor applied to the structure, and measure its deformation over the lengths L a .......L j ..... through which they are rigidly fixed to it, this structure being shown as a pipe. To obtain proper information, the deformation reconstruction must be based on the parallel analysis of a large number of signals originating from different lengths of the structure under consideration, in order to obtain the variation in the deformation along the structure. It is also necessary to reconstruct the spatial deformation of the structure by determining it with respect to two mutually perpendicular planes, and further necessary to eliminate the axial deformation component due to the presence of the axial load acting on the riser to which the present embodiment relates, this load being represented at least by its own weight. These requirements are satisfied by a preferred embodiment of the invention, shown in Figure 3, in which the phase-modulation fibre-optic sensors are divided into a plurality of identical bundles and arranged on the structure as a double assembly of sensors placed in quadrature. An assembly of modular electro-optical units UEO converts the phase modulation induced by the deformation into an analog electrical output signal u. Each unit UEO is associated with two optical fibres arranged in differential configuration, so that only the deformation states due to flexure are measured. Over each portion of length L j a double pair of sensors j and jq are placed to allow the point-reconstruction of the deformation in two mutually perpendicular plains. For n measurement points there are 4n extensometer sensors and 2n electro-optical units. The signal u j from UEO j is processed in the electronic unit UE j and fed in analog form to the data collection system. The signal e j is directly proportional, with a known fixed proportionality constant, to the phase modulation value. The fibre portion CI is sensitive to the "non-common" noise due to deformation or vibration. This noise can be eliminated with the devices described in U.K. Patent No. 2 144 215 or can be satisfactorily reduced by making the portion CI as short as possible and maintaining it in a constant position, ie as immune from deformation as possible. The optical fibre portion IM represents the extensometer sensor and is allowed to deform rigidly with the tubular body 10. The schematic of the unit UEO is shown in Figure 4, this fibre-optic interferometer schematic being of Michelson type with frequency shifting or modulation. The arriving laser signal is split in BS into two signals and fed to a modulation unit UM which provides the frequency shift. This shift is very convenient for the purpose of completely reconstructing the progress of the deformation with the appropriate sign. The two laser beams from UM are fed into the optical fibres via the components indicated by PBS, FR and HWP. These components have the following functions: to prevent the return signal through the optical fibre again interfering in the direction of the component BS and the laser; to recover the entire signal originating from the optical fibre; to feed the laser beam into the optical measuring fibre with the correct polarization. The beams from the laser are fed into the polarization-maintaining fibres along the main birefringence axis and are reflected at the mirror-surfaced fibre end M. The polarization-maintaining fibre is a single-modal fibre with the characteristic property, given its high birefringence, of having two main propagation planes in which a linearly polarized light substantially maintains its polarization state. The use of such optical fibre prevents disturbances due to polarization fluctuations while maintaining good interferometer sensitivity characteristics. The return light collected by the separators PBS is recombined by interference in BS. The photodiodes PD arranged in differential configuration result in rejection of the common mode intensity noises. The signal u j from the amplifier A is fed to the electronic processor unit UE which also contains the programmable carrier generator feeding the unit Um with the signal U m , and the demodulation chain. The signal u j is fed to a frequency demodulation stage and integrator. The output e j is the required analog signal proportional to the difference in phase modulation between the optical fibres, and this is proportional to the difference in deformation between the two generators of the riser. The extensometer device according to the invention has numerous advantages, including the following. Its reliability is sufficiently high to allow its use in an unmanned hostile environment, because only the fibre-optic sensors, covered by a sheath or combined into a cable, are exposed to the hostile environment while the remainder of the device is located in a protected environment. The small transverse dimension of the optical fibre bundle plus its protection, just a few millimetres in total, means that there is no disturbance to the hydrodynamic characteristics of the riser and no need for modifications to be made to the riser itself. The device allows high-accuracy determinations to be made within a large range of measurement by linear transducing of complex quantities, and with ample dynamic acceptance. Furthermore, it is simple to form and install because of its modular construction, which also enables it to be easily adapted to widely differing structures. The device also has the advantageous characteristic of differential application in which both the measurement beam and reference beam are conveyed in different fibres, allowing rejection of all common-mode external stresses.

Claims: A method for analyzing the dynamic deformation of a structural component by comparing, in a fibre-optic interferometer having a reflecting surface at its distal end, the modulation difference between a first measuring laser beam which has flowed through a first optical fibre bundle in both directions, and a second measuring laser beam which has flowed in both directions through a second optical fibre bundle of a length equal to that of the first fibre bundle, each fibre bundle being comprised of a plurality of fibres of different lengths, the fibre bundles being arranged on opposite sides of said structural component, characterized by comprising the steps of: (a) detecting the phase modulation of the light; (b) carrying out the measurement of the deformation of the structure component concerned by measuring the length variation taking place in a set of structure portions of increasing lengths, each portion completely containing all the portions of the series of shorter length and having rigidly fastened thereto fibre-optic sensor portions of the same length, to reconstruct the structure deformation, and (c) measuring the length variations by detecting the phase delay between said first measuring laser beam and said second measuring laser beam on the basis of the slippage of the interference fringes produced by the impingement of said light beams onto a beam splitter which recombines said beams. Method according to Claim 1, wherein the determination of a single-plane deformation of each structural component is carried out by comparing the portional length variations along opposed faces or generating lines of the structure. Method according to Claim 1, wherein the measurement of the spatial deformation of the structure is made by measuring and comparing the length variations along two or more pairs of faces or generating lines lying in quadrature. Method according to Claim 1, wherein the frequencies of both the first and the second measuring laser beams are modulated. A fibre-optic interferometer system for analyzing dynamic deformations of a structure with the method according to Claim 1, comprising: optical fibre bundles each comprising (a) a plurality of optical fibres (7g, 7h, 7i, 7k) having various lengths which are bunched parallel and are rigidly connected to the structure (10) to be analyzed; (b) a modular electronic unit (UEO) which processes signals sent along the optical fibres (7g, 7h, 7i, 7k) and consists of: (A) means (LASER) for originating a measuring laser beam; (B) means (BS) for splitting said measuring laser beam into a first measuring laser beam and a second measuring laser beam; (C) means (UM) for modulating said first and second measuring laser beams; (D) means (PBS, FR and HWP) for passing said first measuring laser beam with the correct polarization through a first optical fibre bundle with a reflective distal end (M); (E) means (PBS, FR and HWP) for passing said second measuring laser beam with the correct polarization through a second optical fibre bundle with a reflective distal end (M);and for preventing a response-measuring laser beam created by said first or second measuring laser beam reflecting on the distal end of the optical fibre from interfering with said first or second measuring laser beam; (F) means (BS) for combining the first and the second response measuring laser beams (BS), (G) means (PD) for rejecting common mode intensity noise, and (H) means (A) for amplifying the combined signal from step (F), and (c) an electronic unit (UE) which processes the signal exiting the fibre-optic interferometric sensor (j, jq) into an analog signal. Fibre-optic interferometer system according to Claim 5, wherein said optical fibres (7g, 7h, 7i, 7k) are polarization-maintaining fibres. Fibre-optic interferometer system according to Claim 5, wherein the optical fibres (7g, 7h, 7i, 7k) are divided into a plurality of identical bundles having varying length optical fibres and are arranged on the structure (10) to be analyzed as a double assembly of sensors in quadrature. Fibre-optic interferometer system according to Claim 5, wherein the fibre bundle is combined into a cable core and inserted into a protective sheath. Fibre-optic interferometer system according to Claim 5, wherein the light beam input end of each optical fibre lies in a plane perpendicular to the fibre axis. Fibre-optic interferometer system according to Claim 5, wherein the fibre bundle has the same length as the structural component to be analyzed and is secured parallely thereto.

Inventors: CAMPELLI, PAOLO, MARTINELLI, MARIO
Applicants: AGIP S.P.A., C.I.S.E.-CENTRO INFORMAZIONI STUDI ED ESPERIENZE S.P.A.
Application year: 1990
Main classification: G01D5
IPC codes: G01L1, G01B11, G01D5