Method of determining stress variations over time in an undersea pipe for transporting fluids

1. A method of determining stress variations over time in a fluid transport undersea pipe and providing a bottom-to-surface connection or a connection between two floating supports, the stress variations being suitable for deducing the movements, the deformations, the fatigue, and the vortex induced vibration of the pipe, the method comprising:

installing along the entire length of the pipe at least one distributed optical fiber sensor using Rayleigh backscattering, the sensor being dedicated to measuring at least one degree of freedom of movement variation over time of the pipe in each cross section of the pipe;

continuously measuring the movement variation of the optical fiber sensor over time; and

determining stress variations over time at each point of the pipe by time integration of the measured movement variation of the optical fiber sensor on the basis of the following matrix relationship:

∂ t {right arrow over (∈( s ))}= A ( s )∂ t {right arrow over ( x ( s ))}

in which:

s is the curvilinear abscissa of the fiber;

∂ t {right arrow over (∈(s))} is a vector of dimension 1 to 6, in which the components correspond to the time derivatives of the respective axial local deformations of the optical fiber sensors;

∂ t {right arrow over (x(s))} is a vector having the same dimension as ∂ t {right arrow over (∈(s))} representing the time derivatives of the deformation reduction elements at the center of gravity of the structure section corresponding to the curvilinear abscissa s for measuring on the pipe; and

A(s) is a deformation matrix that is a function of the local positions and angular orientations of the optical fiber sensors on the pipe, of the curvilinear abscissa s , and of the mechanical and geometrical properties of the structure, the optical fiber sensors being installed in such a manner that the deformation matrix A(s) is invertible.

2. The method according to claim 1 , wherein the distributed optical fiber sensors are installed helically around the pipe.

3. The method according to claim 1 , wherein the distributed optical fiber sensors are installed in straight lines around the pipe.

4. The method according to claim 1 , comprising installing along the entire length of the pipe at least four distributed optical fiber sensors dedicated to measuring three degrees of freedom in rotation simultaneously with measuring one degree of freedom in movement of the pipe at each cross section of the pipe.

5. The method according to claim 1 , further comprising installing along the entire length of the pipe an optical fiber pressure sensor for measuring pressure in the pipe.

6. The method according to claim 5 , wherein the optical fiber pressure sensor is arranged in a straight line parallel to the longitudinal axis (X-X) of the pipe or helically around the pipe.

7. The method according to claim 1 , further comprising installing along the entire length of the pipe an optical fiber temperature sensor for measuring temperature in the pipe.

8. The method according to claim 1 , further comprising determining the movements over time at each point of the pipe by time and spatial integration of the measured movement variation of the optical fiber sensor situated on the section corresponding to the point of the pipe.

9. The method according to claim 2 , comprising installing along the entire length of the pipe at least four distributed optical fiber sensors dedicated to measuring three degrees of freedom in rotation simultaneously with measuring one degree of freedom in movement of the pipe at each cross section of the pipe.

10. The method according to claim 3 , comprising installing along the entire length of the pipe at least four distributed optical fiber sensors dedicated to measuring three degrees of freedom in rotation simultaneously with measuring one degree of freedom in movement of the pipe at each cross section of the pipe.

11. The method according to claim 4 , further comprising installing along the entire length of the pipe an optical fiber pressure sensor for measuring pressure in the pipe.

12. The method according to claim 6 , further comprising installing along the entire length of the pipe an optical fiber temperature sensor for measuring temperature in the pipe.

13. The method according to claim 7 , further comprising determining the movements over time at each point of the pipe by time and spatial integration of the measured movement variation of the optical fiber sensor situated on the section corresponding to the point of the pipe.