From WO 2008/10306 it is known to inspect pipelines for corrosion damage using ultrasonic signals. A transmitter is used to excite an ultrasound wave in the wall of a pipe and a detector is used to detect arrival of the ultrasound wave after it has travelled through the wall.
The information that is obtained in this way can be used to form an image of damage in the pipe wall as a function of position along the wall surface. Properties of the wall are determined from the travel time, i.e. the delay between transmission and arrival. The wall acts as an ultrasound waveguide for waves confined between its inner and outer surface. One can picture wave propagation in terms of a ray that bounces back and forth at an angle to the surfaces, with a net speed of propagation parallel to the surface that depends on the angle. A detector at a specific location on the pipe receives only rays that bounce back and forth at certain discrete angles. At large wavelengths compared to wall thickness, the angle selectivity can be viewed as an effect of constructive interference. At small wavelengths the view is that the rays skip a detector position except at discrete angles.
When pulses of sufficient ultrasound frequency are used, net propagation parallel to the surface gives rise to ray paths that spiral around the pipe, with net axial and circumferential direction components (propagation straight along the axial direction or circularly around the circumference will be considered as special cases of a spiral, with zero circumferential direction component and axial direction component respectively). Typically a detector at a specific location on the pipe will receive rays that reach it along different spiral ray paths, which differ by an integer number of revolutions around the pipe. When the pipe wall has a uniform thickness, the spiral ratio (the ratio between the circumferential and axial direction components) remains constant along such ray paths. The travel time between the time of transmission of a ray and its time of arrival depends on the ray path followed by the ray and the net speed of sound along the ray, which varies with pipe wall thickness.
Local damage to the pipe, which leads to local variation in the pipe wall thickness, results in modulation of the net speed of sound and/or scattering between rays with different spiral ratios. Propagation speed modulation gives rise to modified travel times for combinations of transmitter/detector positions that are connected by ray paths through the location of the local damage. From these travel times, the axial and circumferential location of the damage can be determined. In fact, with arrays of transmitters and detectors, an image of wall thickness variation as a function of position can be obtained. This is called time of flight (TOF) tomography.
WO 2008/10306 discloses a measurement system with ultrasound transducers on a pipe that are used to transmit ultrasound pulses along the wall of the pipe, ultrasound transducers that are used to detect the time of arrival of the ultrasound pulses and a computer that is configured to compute the location of damage from the travel times between the transmitting and receiving transducers. The document mentions that wave dispersion, due to wavelength dependent ultrasound propagation speed, can make the detection of the time of arrival inaccurate. This problem is solved by applying a frequency dependent phase correction to the Fourier transform of the detected ultrasound signal. The result is a sharp pulse, from which the travel time can be determined. Using the resulting travel times for different pairs of transmitting and receiving ultrasound transducers ray paths along the pipe are identified wherein the travel time has changed due to damage.
For the purpose of detecting corrosion damage in the wall of a pipe, high resolution imaging is desirable. At the early stages of pipeline wear corrosion results in small pits that threaten to pierce the wall. High resolution is needed to form images that show such pits. To realize such a high resolution, narrow rays due to ultrasound pulses with frequency content at a relatively high frequency such as 1 MHz are desirable. However, it has been found that at such frequencies it is difficult to compute a reliable tomographic image. The images often show artefacts. It has been found that these artefacts are due to convergence of the tomographic reconstruction to local maxima.
WO2009139627 also discloses modelling of the surface of an object, such as a pipe, using ultrasonic waves. Propagation delay times obtained from ultrasonic measurements are compared with predictions based on a model of the surface. Both surface height and temperature parameters of the model are iteratively adapted. Inclusion of temperature as a parameter makes it possible to account for refraction, where the time delay corresponds to a bent ray path that deviates from the ray path at homogeneous temperatures. Non dispersive waves are used, such as waves that are concentrated in a single narrow frequency band. The propagation delays are modelled by treating ultrasound propagation as propagation along a ray path. No wave vector summation or wave interference computations are considered. A two level model is used, wherein the height and temperature of a limited number of points is adapted and values for other points are obtained by interpolation.