OA18841A - Procédé de détermination des variations de contraintes au cours du temps d'une conduite sous-marine de transport de fluides. - Google Patents

Abstract

L'invention concerne un procédé de détermination des variations de contraintes au cours du temps d'une conduite sous-marine de transport de fluides, comprenant l'installation sur toute la longueur de la conduite (1) d'au moins un capteur à fibre optique réparti (2-1 à 2-4) utilisant la rétrodiffusion de Rayleigh, le capteur étant dédié à la mesure d'au moins un degré de liberté de variation de déplacement au cours du temps de la conduite en chaque section transversale de celleci, la mesure continue de la variation de déplacement du capteur à fibre optique au cours du temps, et la détermination des variations de contraintes au cours du temps en chaque point de la conduite par intégration temporelle de la variation de déplacement du capteur à fibre optique mesuré.

Description

Title of invention

Method for determining stress variations over time of an underwater fluid transport pipe

Invention background

The present invention relates to the general field of underwater fluid transport pipes which are subject to dynamic deformation. It relates more particularly to pipes for the transfer of hydrocarbons, for example oil and gas, ensuring a bottom-surface connection or a connection between two floating supports.

The bottom-surface connecting pipes or connecting pipes between two floating supports used for the transfer of hydrocarbons, called below "riser" in English, are typically produced by continuously raising pipes previously laid at the bottom of the sea, directly to a floating support, for example by giving them a chain configuration.

The pipes which are thus raised from the bottom to form the risers must be made using flexible pipes when the water depth is less than a few hundred meters. However, as soon as the water depth reaches and exceeds 800 to 1000 m, the flexible pipes are replaced by rigid pipes made up of elements of unit length of pipe which are welded together and made of a resistant material such as thick steel. Rigid risers made of resistant material, in chain configuration are commonly called by the English term “Steel Catenary Riser” or SCR (meaning “steel riser in the form of a chain”).

These bottom-surface connecting pipes or connecting pipes between two floating supports are subjected over time to variations in dynamic stresses which it is necessary to monitor in order to prevent any risk of significant damage to the pipes, or even breakage of those -this. In practice, operators of the offshore petroleum industry calculate the life of their subsea installations, and in particular of their pipes ensuring a bottom-surface connection or a connection between two floating supports, from oceanographic data taken from the location of the oil field. The determination of the stress variations of these submarine pipes makes it possible to calculate the deformations and displacements of the pipe, its state of fatigue and the measurement of the stress ripples due to the vibrations induced by the detachment of vortices (called "Vortex Induced Vibration" in English). or VIV).

These calculations are generally pessimistic in order to minimize risk-taking, and installations are most often declared to be at risk long before they are actually declared for safety reasons. In addition, these calculations are subject to verification of standards which contain safety coefficients. The use of these standards then allows the operator to submit his certification dossier to a control body in order to make a guarantee vis-à-vis the competent authorities.

Also known are the publication FR 3,047,308 which describes a method for monitoring the thermomechanical behavior of an underwater pipe for transporting fluids under pressure and the publication FR 3,047,309 which describes a process and device for monitoring the mechanical behavior of a pipe under pressure - marine transport of pressurized fluids, these two processes using fiber optic sensors.

We also know the publication US 2016/0161350 which describes a process for monitoring a pipe from an optical fiber arranged helically around the pipe and a plurality of Bragg grating sensors placed in the optical fiber. This installation makes it possible to measure, at each Bragg grating sensor, possible deformations of the pipe (bending, twisting, etc.). One of the drawbacks of this monitoring method is that it limits the measurements to the positions of the Bragg grating sensors and allows only a sampling of the deformations of the pipe to be obtained, not a measurement over the entire length of that. -this.

We also know the publication WO 2014/013244 which describes a method for monitoring the position of a structure using a distributed optical fiber sensor and a plurality of acoustic emitters deployed at known positions in the area of the structure to be monitored. This method determines an acoustic field, and from the known positions of the acoustic emitters, it deduces displacements of the structure. Such a method has the main drawback that it is complex to implement, in particular because the correspondence between the acoustic field obtained and the displacements of the structure is not direct.

Subject and summary of the invention

The main object of the present invention therefore is to propose a method for monitoring the dynamic deformation of an underwater pipe which does not have the aforementioned drawbacks.

According to the invention, this object is achieved by a method of determining the stress variations over time of an underwater pipe for transporting fluids ensuring a bottom surface connection or a connection between two floating supports with a view to deduce the displacements, deformations, fatigue, and vibrations induced by detachment of vortices from the pipe, comprising:

the installation over the entire length of the pipe of at least one distributed optical fiber sensor using Rayleigh backscattering, the sensor being dedicated to the measurement of at least one degree of freedom of variation of displacement over the time of the pipe in each cross section thereof;

continuous measurement of the variation in displacement of the optical fiber sensor over time; and the determination of the stress variations over time at each point of the pipe by temporal integration of the displacement variation of the optical fiber sensor measured from the following matrix relation: d _t e (s) = A (s) d _t x (s) in which:

s is the curvilinear abscissa of the fiber;

d _t e (s) is a vector of size 1 to 6 whose components correspond to the time derivatives of the respective local axial deformations of the optical fiber sensors;

d _t x (s) is a vector of the same size as d _t e (sj representing the time derivatives of the deformation reduction elements taken at the point of gravity of the structural section corresponding to the curvilinear abscissa s to be measured on the conduct; and

A (s) is a deformation matrix which is a function of the position and the angular orientation of the fiber optic sensors locally on the pipe, of the curvilinear abscissa s and of the mechanical and geometric properties of the structure, the sensors with optical fibers being installed so that the deformation matrix A (s) is invertible.

The method according to the invention provides for using fiber optic sensors as a multitude of strain gauges and resorting to distributed acoustic sensing (DAS) technology to measure the variation in deformation. axial of optical fibers. This technology makes it possible to analyze variations in optical indices along fiber optic sensors by analyzing the light reflected by Rayleigh reflection (these variations possibly being due to variations in temperature and in variations in stresses in the core of the optical fiber). From calculations similar to those of the beam theory which gives the relationship between the axial deformations of the fiber optic sensors and the deformations at each point of the pipe, it is then possible to determine in real time the displacements, the deformations, the state of fatigue and the vibrations induced by detachment of vortices around the dynamic pipe. In particular, the method according to the invention makes it possible to have a direct correspondence between the variations in optical indices along the fiber optic sensors and the deformations of the pipe. In addition, the measurement of the deformations of the pipe takes place over its entire length, and not only on certain portions thereof.

Thus, the invention is remarkable in that it combines the installation of fiber optic sensors directly on the pipe with the measurement of stress variations over time at each point of the pipe using DAS technology and with the calculation of displacements, deformations, state of fatigue and vibrations induced by detachment of vortices around the pipe at any point thereof.

In the offshore petroleum industry, measuring the dynamic deformations of pipes ensuring a bottom-surface connection or a connection between two floating supports, and therefore fatigue along them, allows operators to monitor the structural integrity and by Therefore significantly extend the life of these pipes with an acceptable risk, which allows significant savings. This measurement also makes it possible to locate the appearance due to the phenomenon of fatigue by dynamic stresses on cracks in the underwater structures in order to intervene in advance and thus to preserve the installations as long as possible.

Preferably, the distributed fiber optic sensors are installed in a helix around the pipe. This helical installation has the advantage of achieving better mechanical adhesion of the fiber optic sensors on the pipe.

Alternatively, distributed fiber optic sensors can be installed in straight lines around the pipe.

The method may further comprise installing over the entire length of the pipe at least two additional distributed fiber optic sensors to improve the accuracy of the determination of the stresses at each point of the pipe. The presence of these fiber optic sensors makes it possible to achieve a more precise and rigorous calculation of the deformations in driving dynamics.

The method can include installing over the entire length of the pipe at least four distributed fiber optic sensors dedicated to the simultaneous measurement of three degrees of freedom in rotation and one degree of freedom in movement of the pipe in each cross section thereof.

The method may further include installing over the entire length of the pipe a fiber optic pressure sensor for measuring the pressure in the pipe. In this case, the fiber optic pressure sensor can be arranged in a straight line parallel to a longitudinal axis of the pipe or in a helix around the pipe.

The method may further include installing over the entire length of the pipe a fiber optic temperature sensor for measuring the temperature in the pipe.

More preferably, the method further comprises determining the displacements over time at each point of the pipe by temporal and spatial integrations of the variation in measured displacement of the optical fiber sensor located on the corresponding section of the point of the pipe.

Brief description of the drawings

Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate exemplary embodiments thereof without any limiting character. In the figures:

- Figure 1 schematically shows a pipe equipped with optical fiber sensors for implementing the method according to a first embodiment of the invention;

- Figure 2 schematically shows a pipe equipped with fiber optic sensors for implementing the method according to a second embodiment of the invention;

- Figure 3 shows schematically a pipe equipped with fiber optic sensors for the implementation of the method according to a third embodiment of the invention; and

- Figure 4 schematically shows a pipe equipped with optical fiber sensors for the implementation of the method according to a fourth embodiment of the invention.

Detailed description of the invention

The invention applies to any underwater pipe (single or double jacket) for transporting fluids, in particular oil and gas, ensuring a bottom-surface connection or a connection between two floating supports, such as the pipe 1 shown partially and schematically in FIG. 1.

The method according to the invention proposes to determine the stress variations over time of such a pipe, that is to say to quantify the deformations undergone by the pipe at any point thereof and in real time.

From the obtaining of the stress variations, it is possible to deduce therefrom in a manner known per se the state of fatigue of the pipe over time, the deformations undergone by the pipe over time (by temporal integration stress variations), the displacements of the pipe over time (by double temporal and spatial integration of stress variations), and the vibrations induced by the detachment of vortices (called "Vortex Induced Vibration" in English or VIV).

To this end, according to the invention, provision is made to install around the pipe and over its entire length thereof, at least one distributed optical fiber sensor using Rayleigh backscattering.

The different distributed fiber optic sensors are each used as distributed acoustic detection sensors. They can be single-mode (preferably) or multimode fiber sensors.

Each distributed optical fiber sensor is dedicated to the measurement of a degree of freedom of variation in displacement over time of the pipe in each cross section of the latter among the following four degrees of freedom: two degrees in bending, one degree in torsion and a degree in axial displacement (i.e. compression or axial tension).

In most configurations, it is necessary to position four fiber optic sensors distributed around the pipe so as not to measure combinations (sum or difference) of different degrees of freedom. On the other hand, in the case where it is known in advance that the pipe will be subjected to only one degree of freedom, a single fiber optic sensor will be necessary.

Once the distributed fiber optic sensors are installed around the pipe, a continuous measurement is made of the variation in displacement of the fiber optic sensor over time using the known technology of distributed acoustic detection, this technology being applied here at low frequency (i.e. between 0.01 and 1Hz).

Distributed acoustic detection technology analyzes variations in optical indices along fiber optic sensors by analyzing the light reflected by Rayleigh reflection. These variations in optical indices may be due to variations in temperature and in variations in stresses in the core of the optical fibers. The very high speed of light thus makes it possible to detect very rapid stress variations even in the ultrasonic field and therefore to detect ambient sound waves. Within the framework of the present invention, one will be limited to the low frequencies which correspond to the frequency of the waves at sea and to the frequencies of the detachments of vortices (typically between 0.01 Hz and 1 Hz).

The variations in stresses over time at each point of the pipe are then determined from the following matrix relation:

d _t e (s) = A (s) 5 _t x (s) in which:

5 _t e (sj is a vector of size 1 to 6 whose components correspond to the time derivatives of the respective local axial deformations of the optical fiber sensors;

d _t x (s) is a vector of the same size as d _t e (sj representing the time derivatives of the elements of reduction of the deformation at the point of gravity G of the section of structure S corresponding to the curvilinear abscissa s of the fiber optics to be measured on the pipe; and

A (s) is a square matrix called "deformation" which is a function of the position and the angular orientation of the fiber optic sensors locally on the pipe, of the curvilinear abscissa s, of the geometry of the pipe and of its mechanical characteristics.

To obtain the stress variations, it is necessary that the optical fiber sensors are installed around the pipe so that the deformation matrix A (s) is invertible.

Different installation configurations of the optical fiber sensors can be envisaged in order to achieve that the deformation matrix A is invertible.

Figures 1 to 4 show non-limiting examples of installations for which the deformation matrix A (s) is invertible. Of course, a person skilled in the art can envisage other configurations than those described below in conjunction with FIGS. 1 to 4.

For a given configuration of a fiber optic sensor, there is a single strain matrix A (s) which connects the strains of the optical fiber to those of the structure. The positioning of a fiber optic sensor on the structure determines a line vector of the deformation matrix A (if x and e are column vectors). For tube type structures and for fiber optic sensors placed helically or straight on the tube, the line vectors of the corresponding deformation matrix A are given below. For a given fiber configuration and according to the reduction elements of the deformation torsor that it is desired to determine the deformation matrix A may or may not be invertible.

General case :

We consider a tube of external radius r and we have on it fiber optic sensors numbered 1,2, ... which are arranged either in straight lines or in helices around this tube. To describe the position of the different fibers on the tube we introduce the following parameters:

- a is given by the relation a = p / 2 π where p is the pitch of the helix of the optical fiber sensors. This parameter is + infinite in the case of a straight fiber.

- φ represents the angular position of each fiber optic sensor on the section of zero curvilinear abscissa

- ψ is a variable worth +/- 1 and describing the direction of rotation of the propeller of each fiber optic sensor around the pipe.

- s is the curvilinear abscissa linked to the fiber.

The position of a fiber in the Fresnel coordinate system (G, T, N, B) is then given by the following relation:

In the case of a straight fiber we find:

/ (s) = s \ rcos ((p)] XsinÇcp ') J γχΐβ

In order to describe the deformations of the tube and the fiber, the following parameters are introduced, all of which depend on the curvilinear abscissa:

- ... represent the axial deformations of the optical fibers 1,2, ... respectively.

- The 6 degrees of freedom of movement of the structure in each point itself will be described by the torsor of the deformations and its deformation reduction elements, a vector ίο deformation ë and a rotation gradient k. These elements are written as a function of a displacement vector of the point of gravity of the section S (s): ü of components u _{1 (} u ₂ and u ₃ and of a rotation vector of the section S applied to the point G of S: r of corn posa ntes r _lt r _2l r ₃ .

The derivatives with respect to the curvilinear abscissa of a quantity g will be noted g '.

It is established that the axial deformation of a helical fiber on a tube is written at each point of the tube as a function of the components of the displacement and rotation vectors of the tube defined above as the scalar product of two vectors. A vector x depending on the 6 degrees of freedom of movement of the tube and a vector A _t which depends on the geometric characteristics of the fiber as well as the Lamé coefficients of the tube λ and μ:

f ₂ r ^z A \ hTTF 2 (1 + 7)) (s -raisin ψ, —- + φ _ι r '+ a?

is ra ^ cos ψι r ² + a ² ay ^{2 rsin} (ψί - / = 7 = 7 ⁺ ^) / _Γ 2 _λ r ² + a ²

-rcos [ψ ₍ , / + Φι r ² + a ^{2 u} 2 ~ ^r 3 “3 + ^r 2 rl ^r 3 e _t = Atx

The vector x is thus equal to the spatial derivative of the vector displacement of the point of gravity of the section S (s), that is to say that we have: x - d _s u

Knowing d _t e (sj by measuring the time derivatives of the local axial deformations of the optical fiber sensors, and knowing A, by inverting the matrix A we obtain d _t x (s) which we integrate with respect to time. Then, by spatial integration (according to s), we obtain the displacement vector of the point of gravity in section S (s).

In the particular case of a straight fiber:

! ° \ rsin ^ i) x — rcos ^ i) /

This technique makes it possible to establish a matrix relationship at each curvilinear abscissa between the deformation of the tube and that of the fibers thereon.

e = A.x

The matrix A does not depend on time. We can therefore also write:

d _t e = Ad _t x

This relationship is important because d _t e is the quantity measured by the fibers.

The line vectors of the strain matrix A are the vectors A _t

By judiciously choosing the arrangement of the optical fiber sensors on the tube, it is possible to obtain the invertibility of the deformation matrix A.

In practice, on rigid structures we make the approximation of the rotations of sections according to which the components 2 and 3 of the vector x are zero. In this case the matrix is reduced to size 4 and the implementation gives an example for which the strain matrix A remains invertible. Conversely, if we choose 4 fibers with the same helix pitch, then the strain matrix A is not invertible.

In the particular case where it is known that the tube is subjected to only one type of force, for example traction, the strain matrix can be reduced to the corresponding scalar.

The measurement of the time derivative of axial deformation of the optical fibers therefore makes it possible at each point of the tube to determine all the components of the time derivative of deformation of the tube, then by integration, the displacements, positions or the time derivatives of the stress torsor, then by integration the torsor of the constraints. As the measurement is carried out at any point and in real time, it is also possible to access quantities such as damage to the structure and therefore fatigue.

Special case of implementation:

In a first embodiment illustrated by FIG. 1, the fiber optic sensors distributed 2-1 to 2-4 are advantageously arranged in helices around the longitudinal axis XX of the pipe 1, which improves their mechanical adhesion to the conduct.

In this embodiment, the fiber optic sensors 2-1 to 2-3 are positioned in a helix of the same pitch p but the respective initial angular positions of these propellers are offset from each other by 2tt / 3. As for the fourth fiber optic sensor 24, it is positioned in a helix having the same pitch p, any initial angular position but in the opposite direction relative to the fiber optic sensors 2-1 to 2-3.

In practice, with this helical configuration, to determine the state of the stresses and fatigue of the pipe at any point thereof, it is necessary to know the helical position of the fiber optic sensors 2-1 to 2-4, this position being described using the curvilinear abscissa given by the following equation (already mentioned above):

as / (s) = sir ² + a ² ( ^s rcos (ψ. + φ \ vr ^z + a ²

Τ.Ν, Β

Still with the helical configuration of FIG. 1, the link between the respective angular and axial deformations; e ₂ ; e ₃ ; ^e 4 of the fiber sensors 2-1 to 2-4 and the deformations at each point of the pipe is given, as indicated above, by the matrix relationship:

e = Ax in which:

/ Γ ² Λ \ ^{ail = -} 2 (2 + m) J _ Ψί “ίΓ ^{2 a} ' ² r ² + a?

(Ψί ⁵ 1 ^ 1, - + φί I < ²⁺ h 2 ^τ2λ \ ^ai3 “7 ^ + ÔÎ C 2 (λ + μ))

-rcos I. ^ ^lS + <pi j \ ^ ² + "i // r ² A \ ^a " ~ r ² + af V ^É 2 (λ + μ))

Thus, for the configuration of the first embodiment of FIG. 1, from the knowledge of the time derivative measurements of axial deformation of the optical fibers, it is possible to determine the desired components of deformation variations d _t x and the forces . In addition, the integration of these moving vectors makes it possible to obtain step by step the position of each element of the pipe, and therefore the state of stress and fatigue of the pipe in real time.

It should be noted that here we make the approximation according to which the state of displacement of the pipe along the axes perpendicular to the generator of the pipe are negligible, which makes it possible to start from the assumption that u ₂ - r ₃ = 0 and u ₃ + r ₂ - 0 (and to simplify the calculations because of lower order). Only the knowledge of component 1 of the displacement vector and of the three components of the rotation vector thus makes it possible to obtain the stress variations.

In a second embodiment represented in FIG. 2 also making it possible to obtain an invertible deformation matrix A, three optical fiber sensors 2-1 to 2-3 are positioned in a helix of the same pitch ρ but the respective initial angular positions of these propellers are offset from each other by 2π / 3 as for the first embodiment. In addition, the direction of these propellers is changed at regular intervals (for example every L meters of the pipe). For the determination of the stress and fatigue state of the pipe, we consider here that the measurements are the same on two consecutive pipe sections, which makes it possible to obtain a sufficiently good approximation for very long pipes (several km ) when L is chosen small compared to the characteristic length of variation of the stress in the structure.

Compared to the first embodiment, this installation has the advantage of avoiding any overlapping of fiber optic sensors. In contrast, the spatial resolution is limited to 2L (instead of L for the first embodiment). Of course, here too the deformation matrix A is invertible, which makes it possible to determine the state of the stresses and fatigue of the pipe in real time.

In a third embodiment, it is possible to measure the deformation elements u ₂ -r ₃ = 0 and u ₃ + r ₂ = 0 by adding two additional fiber optic sensors 2-5 and 2-6.

The configuration shown in Figure 3 is an example for measuring the deformation elements u ₂ - r ₃ and u ₃ + r ₂ .

In this embodiment, the three fiber optic sensors 2-1 to 2-3 are positioned around the longitudinal axis XX of the pipe 1 in a helix of the same pitch p but the respective initial angular positions of these propellers are offset by compared to others by 2π /

3. As for the three fiber optic sensors 2-4 to 2-6, iis are also positioned in a helix with the same pitch p and the same initial angular position offset but in the opposite direction compared to the fiber optic sensors 2-1 at 2-3.

In this embodiment, the deformation matrix A is a 6X6 matrix which is invertible, which makes it possible to determine by means of the abovementioned calculations the state of stresses and fatigue of driving in real time.

FIG. 4 represents a fourth embodiment of the invention also making it possible to obtain an invertible deformation matrix A. Here, the fiber optic sensors are seven in number and are positioned identically to those of the third embodiment with an additional straight fiber optic sensor (i.e. the three fiber optic sensors 2-1 to 2-3 are positioned around the longitudinal axis XX of the pipe 1 helically in one direction and the three fiber optic sensors 2-4 to 2-6 are positioned helically in the opposite direction).

This fourth embodiment differs from the previous one in that the installation includes a fiber optic pressure sensor 2-7 intended for measuring the pressure in the pipe. Here, the fiber optic pressure sensor 2-7 is positioned in a straight line, that is to say parallel to the longitudinal axis X-X of the pipe. Thus, the fiber optic pressure sensor is insensitive to swelling of the pipe under pressure and only undergoes the elongation of the pipe under pressure. It therefore makes it possible to obtain by difference the dynamic pressure inside the pipe and therefore the share of the deformations due to the internal pressure.

It will be noted that this fiber optic pressure sensor can alternatively be arranged in a helix around the pipe with a pitch different from that of the fiber optic sensors 2-1 to 2-6.

It will also be noted that this fiber optic pressure sensor can be added to the first embodiment described in connection with FIG. 1.

It will also be noted that it is possible in each of the previously described embodiments to measure the temperature along the pipe to determine the deformation due to temperature variations along the pipe. To this end, provision may be made to add a fiber optic temperature sensor next to one of the distributed fiber optic sensors, the fiber optic temperature sensor being interrogated by a distributed temperature detection system (or DTS for “Distributed Temperature Sensing”) and the distributed fiber optic sensor being interrogated by a distributed acoustic detection system.

It will also be noted that it is possible in each of the embodiments previously described to add an additional optical fiber sensor next to each of the optical fiber sensors to obtain the static component of the state of deformation of the pipe at l using a BOTDR (for “Brillouin Optical Time Domain Reflectometry”) or BOTDA (for “Brillouin Optical Time Domain Analysis”) system. Such systems make it possible to obtain an average state of deformation over a period greater than one minute compared to a given reference state.

It will also be noted that in order to overcome numerous distributed acoustic detection systems (namely a system with a distributed optical fiber sensor), it is possible to position the distributed optical fiber sensors by connecting them to each other (the optical fiber 2-1 being connected at one end of the pipe to the distributed acoustic detection system and at the opposite end to the optical fiber sensor 2-2 which is itself connected at the other end to the optical fiber sensor 2 -3, etc.). Thus, a single distributed acoustic detection system is necessary here for the interrogation of all the optical fiber sensors.

Finally, it should be noted that the distributed optical fiber sensors could be installed in straight lines around the pipe (that is to say parallel to the longitudinal axis X-X thereof).

Claims (8)

1. Method for determining the variations in stresses over time of an underwater fluid transport pipe ensuring a bottom-surface connection or a connection between two floating supports in order to deduce the displacements, the deformations, the fatigue, and vibrations induced by detachment of vortices from the pipe, comprising: the installation over the entire length of the pipe (1) of at least one distributed optical fiber sensor (2-1 to 2-4) using Rayleigh backscattering, the sensor being dedicated to the measurement of at least one degree of freedom of variation in displacement over time of the pipe in each cross section thereof; continuous measurement of the variation in displacement of the optical fiber sensor over time; and the determination of the stress variations over time at each point of the pipe by temporal integration of the displacement variation of the optical fiber sensor measured from the following matrix relation: d _t e (s) = A (s) ô _t x (s) in which: s is the curvilinear abscissa of the fiber; d _t ë (s) is a vector of size 1 to 6, the components of which correspond to the time derivatives of the respective local axial deformations of the optical fiber sensors; ô _t x (s) is a vector of the same size as ô _t ë (s) representing the time derivatives of the deformation reduction elements taken at the point of gravity of the structural section corresponding to the curvilinear abscissa s to be measured on the driving ; and A (s) is a deformation matrix which is a function of the position and the angular orientation of the fiber optic sensors locally on the pipe, of the curvilinear abscissa s and of the mechanical and geometric properties of the structure, the sensors with optical fibers being installed so that the deformation matrix A (s) is invertible. 2. Method according to claim 1, in which the distributed optical fiber sensors are installed in a helix around the pipe. 3. Method according to claim 1, in which the distributed optical fiber sensors are installed in straight lines around the pipe. 4. Method according to any one of claims 1 to 3, comprising the installation over the entire length of the pipe of at least four distributed optical fiber sensors dedicated to the simultaneous measurement of three degrees of freedom in rotation and a degree of freedom in movement of the pipe in each cross section thereof. 5. Method according to any one of claims 1 to 4, further comprising the installation over the entire length of the pipe of a fiber optic pressure sensor (2-7) intended for measuring the pressure in the conduct. 6. The method of claim 5, wherein the fiber optic pressure sensor is arranged in a straight line parallel to a longitudinal axis (X-X) of the pipe or in a helix around the pipe. 7. Method according to any one of claims 1 to 6, further comprising the installation over the entire length of the pipe of a fiber optic temperature sensor intended for measuring the temperature in the pipe. 8. Method according to any one of claims 1 to 7, further comprising determining the displacements over time at each point of the pipe by temporal and spatial integrations of the variation in measured displacement of the fiber optic sensor located on the corresponding section of the pipe point.