WO2024099556A1

WO2024099556A1 - Pipeline inspection device and methods for detecting a defect in a pipeline wall

Info

Publication number: WO2024099556A1
Application number: PCT/EP2022/081336
Authority: WO
Inventors: Magne AANES; Michael Haas
Original assignee: NDT Global Corporate Ltd.
Priority date: 2022-11-09
Filing date: 2022-11-09
Publication date: 2024-05-16

Abstract

An inspection device for inspecting a pipeline wall (30) is provided. The inspection device comprises a plurality of ultrasonic transducers (11, 12, 13, 14, 15), each of the ultrasonic transducers (11-15) being configured to emit a respective ultrasonic signal (100) along a respective pre-determined ultrasonic propagation direction (111, 121, 131, 141) in operation of the inspection device. The ultrasonic transducers (11-15) are arranged and/or adjusted such that, in operation of the inspection device a propagation direction angle (β12, β23, β34, β41, β15) between a first ultrasonic transducer (11, 13) of the plurality of ultrasonic transducers (11-15) and a second ultrasonic transducer (12, 14, 15) of the plurality of ultrasonic transducers (11-15) is less than 180°, the propagation direction angle (β12-β15) being the angle enclosed by an ultrasonic propagation direction (111, 131) of an ultrasonic signal (100) emitted by the first ultrasonic transducer (11, 13) and an ultrasonic propagation direction (121, 141) of an ultrasonic signal (100) emitted by the second ultrasonic transducer (12, 14, 15). Furthermore, methods for detecting a defect (31) in a pipeline wall (30) are provided.

Description

PIPELINE INSPECTION DEVICE AND METHODS FOR DETECTING A DEFECT IN A PIPELINE WALL

TECHNICAL FIELD

The disclosure relates to a pipeline inspection device and methods for detecting a defect in a pipeline wall.

BACKGROUND

Pipelines can be inspected non-destructively and in situ using so-called pipeline pigs carrying the inspection equipment including, e.g., (piezoelectric) ultrasonic transducers. The pressure- driven flow of the fluid or gas in the pipeline to be inspected is used to push the pig along down the pipeline. As the pig travels through the pipeline, selected ones of the ultrasonic transducers pick up the inspection signals as a function of the distance covered. After or even during inspection, the inspection signals can be read and analyzed.

In some detection methods, detecting defects in the pipeline wall using Lamb waves is based on the use of Lamb waves of a fundamental mode (e.g., SO and/or AO) and/or of higher order mode (e.g., S1 and/or A1 and higher) that were excited and propagate along and/or in the pipeline wall. The Lamb waves may be reflected by a defect in the pipeline wall and the reflected signal may be measured by the original transducer (so-called pulse-echo scheme). This detection scheme, however, may suffer from the so-called pulse-echo dead zone, where the defect is located rather close to the point of incidence of the ultrasonic signal emitted by the ultrasonic transducer. The pulse-echo dead zone may, for example, be reduced by increasing a stand-off distance of the ultrasonic transducers to the pipeline wall. An increased stand-off distance, however, may result in a decreased signal-to-noise ratio. Furthermore, an increased stand-off distance may limit the design choices for the inspection device. There is therefore a need for an improved inspection device that avoids the disadvantages of the so- called pulse-echo dead zone. SUMMARY

An aspect of the disclosure relates to an inspection device for inspecting a pipeline wall of a pipeline. The inspection device comprises a plurality of ultrasonic transducers, each of the ultrasonic transducers being configured to emit a respective ultrasonic signal along a respective pre-determined ultrasonic propagation direction in operation of the inspection device. The ultrasonic transducers are arranged and/or adjusted such that, in operation of the inspection device: each of the ultrasonic transducers may have a pre-determined finite standoff distance to the pipeline wall of the pipeline; the ultrasonic signal emitted by each of the ultrasonic transducers impinges on the pipeline wall at a different point of incidence; an angle of incidence of the ultrasonic signal emitted by each ultrasonic transducer may be larger than 0°, the angle of incidence of the ultrasonic signal being the angle enclosed by the ultrasonic propagation direction of the ultrasonic signal emitted by the ultrasonic transducer and the surface normal of the pipeline wall at the point of incidence of the ultrasonic signal emitted by said ultrasonic transducer; and a propagation direction angle between a first ultrasonic transducer of the plurality of ultrasonic transducers and a second ultrasonic transducer of the plurality of ultrasonic transducers is less than 180°, the propagation direction angle being the angle enclosed by the ultrasonic propagation direction of the ultrasonic signal emitted by the first ultrasonic transducer and the ultrasonic propagation direction of the ultrasonic signal emitted by the second ultrasonic transducer.

Another aspect of the disclosure relates to a method for detecting a defect in a pipeline wall of a pipeline. The method may be performed with an inspection device as disclosed herein. The method comprises emitting, with the first ultrasonic transducer, an ultrasonic signal towards the pipeline wall, wherein the ultrasonic signal emitted by the first ultrasonic transducer excites at least one Lamb wave of a fundamental Lamb mode in the pipeline wall and wherein at least a part of the excited Lamb wave is reflected by the defect in the pipeline wall and at least partly coupled out of the pipeline wall as a reflected signal. The method further comprises receiving, with the second ultrasonic transducer, at least a part of the reflected signal originating from the first ultrasonic transducer. Another aspect of the disclosure relates to a method for detecting a defect in the pipeline wall. The method may comprise arranging a plurality of ultrasonic transducers in the pipeline. The method may further comprise emitting, with at least a first ultrasonic transducer of the plurality of ultrasonic transducers, an ultrasonic signal towards the pipeline wall, wherein the ultrasonic signal excites at least one Lamb wave of a fundamental Lamb mode in the pipeline wall. At least a part of the excited Lamb wave is reflected by the defect in the pipeline wall at a deflection angle that is larger than 0°, thereby creating a reflected signal. The ultrasonic transducers are arranged in the pipeline such that the reflected signal is received by a second ultrasonic transducer of the plurality of ultrasonic transducers different from the first ultrasonic transducer, and an angle of incidence of the ultrasonic signal emitted by the first ultrasonic transducer is larger than 0°, the angle of incidence of an ultrasonic signal being the angle enclosed by an ultrasonic propagation direction of the ultrasonic signal emitted by the ultrasonic transducer and the surface normal of the pipeline wall at the point of incidence of the ultrasonic signal emitted by said ultrasonic transducer.

Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar or identical elements. The elements of the drawings are not necessarily to scale relative to each other. The features of the various illustrated examples can be combined unless they exclude each other. Examples are depicted in the drawings and detailed in the following description.

Fig. 1A and 1B schematically depict an inspection device and a method according to aspects of the disclosure.

Fig. 2 schematically depicts the excitation of a Lamb wave with an inspection device using a method according to aspects of the disclosure. Fig. 3 schematically depicts creating a reflected signal according to aspects of the disclosure.

Fig. 4 schematically depicts creating a transmitted signal according to aspects of the disclosure.

Fig. 5 and Fig. 6 schematically depict creating an echo signal and measurement of the echo signal according to aspects of the disclosure.

Figs. 7 A, 7B, 7C and 7D schematically depict an inspection device and a method according to aspects of the disclosure.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings, which form part of the disclosure and in which specific examples of an inspection device and/or a method are shown for illustration purposes. The existence of further examples will be self-evident.

An aspect of the disclosure relates to an inspection device for inspecting a pipeline wall. The pipeline comprising the pipeline wall may be based on a metal (e.g., may consist of a metal or a metal alloy, such as steel, except for impurities). The pipeline may have a pre-known fixed diameter. For example, the pipeline may approximate the shape of a cylinder and may have an axial direction along an axis of the cylinder and a circumferential direction perpendicular to the axis and along the pipeline wall. The pipeline wall may be an inner pipeline wall of the pipeline. The pipeline may be filled with a pressurized gas, e.g. at least one of hydrogen or a natural gas. That is to say, it may be possible that the inspection device may be inserted in a (pressurized) gas environment and/or that the method described herein can be performed in a (pressurized) gas environment. The pipeline wall may comprise a defect, e.g. a crack, a notch. The defect may have a non-uniform shape. However, a uniform defect may also be measured with the inspection device. For example, the defect has an elongated shape and extends along a main extension direction. For example, the defect may be an axial defect that mainly extends along the axial direction. In other examples, the defect may be a circumferential defect that mainly extends along the circumferential direction. The inspection device comprises a plurality of ultrasonic transducers. Each of the ultrasonic transducers may be configured to emit a respective ultrasonic signal along a respective predetermined ultrasonic propagation direction in operation of the inspection device. The ultrasonic propagation direction of an ultrasonic transducer may be taken along the maximum intensity (e.g., the central part) of the ultrasonic field of the ultrasonic signal emitted by the ultrasonic transducer. “In operation of the pipeline inspection device” may refer to the case of the pipeline inspection device being inserted into a pipeline. The operation does not necessarily require the pipeline inspection device and/or each of the ultrasonic transducers to be turned on (so-called “on-state”), but it can also comprise this on-state of the pipeline inspection device. The inspection device may comprise a signal controller with electronics components for switching the ultrasonic transducers on or off, for example individually. Each of the ultrasonic transducers may further be configured to detect and/or measure an ultrasonic signal emitted by any one of the other ultrasonic transducers. Hereinafter, if the terms “emit” or “receive”, etc., are used in connection with the inspection device and/or the ultrasonic transducers, this refers to the operation, in particular the on-state, of the pipeline inspection device.

The ultrasonic signal emitted by each of the ultrasonic transducers in operation can be an ultrasonic pulse with a finite pulse length. The ultrasonic pulse may have a narrow-banded frequency width with a center frequency or may even be a single-frequency pulse (the single frequency then corresponding to a center frequency). The ultrasonic signal may be a single pulse or may comprise a repetition of several ultrasonic pulses. The pulse length and the frequency of the ultrasonic signal may be independently tunable and/or adjustable. For example, the ultrasonic signal may be a so-called tone burst. A tone burst is an ultrasonic sine pulse with a finite length in the time-domain with a center frequency. The repetition rate of the ultrasonic pulse of the ultrasonic signal may be chosen according to the signal processing capabilities of the used electronics. A higher repetition rate may enable to perform the method in a faster manner, e.g. to move the pipeline inspection device faster through the pipeline. For example, the repetition rate of the ultrasonic pulse of the ultrasonic signal may be at most 500 Hz (e.g. at most 400 Hz or at most 300 Hz or at most 200 Hz). It may be possible that the repetition rate is at least 80 Hz. The pulse length of the ultrasonic pulse of the ultrasonic signal may be in the range of at least 1 ps and at most 30 ps.

The ultrasonic transducers may be arranged and/or adjusted such that, in operation of the inspection device each of the ultrasonic transducers has a pre-determined finite stand-off distance to the pipeline wall of the pipeline. For example, the stand-off distance may be equal for all of the ultrasonic transducers. In other examples, a first sub-group of the ultrasonic transducers may have a first stand-off distance to the pipeline wall and a second sub-group of the ultrasonic transducers may have a second stand-off distance to the pipeline wall. Further sub-groups (e.g., with a third, fourth, etc. stand-off distance) may be present. The stand-off distance may be in the range of at least 3 cm and at most 20 cm, e.g. at least 6 cm and at most 16 cm.

The ultrasonic transducers may be arranged and/or adjusted such that, in operation of the inspection device the ultrasonic signal emitted by each of the ultrasonic transducers impinges on the pipeline wall at a different point of incidence. In other examples, the point of incidence may be identical for at least two ultrasonic transducers.

The ultrasonic transducers may be arranged and/or adjusted such that, in operation of the inspection device an angle of incidence of the ultrasonic signal emitted by each ultrasonic transducer is larger than 0°. The angle of incidence of the ultrasonic signal is the angle enclosed by the ultrasonic propagation direction of the ultrasonic signal emitted by the ultrasonic transducer and the surface normal of the pipeline wall at the point of incidence of the ultrasonic signal emitted by said ultrasonic transducer. Using a finite angle of incidence may enable exciting Lamb waves of the fundamental Lamb mode SO and/or AO by the central part of the ultrasonic field of the ultrasonic signal, in particular with a higher signal-to-noise ratio compared to the excitation using a vanishing angle of incidence. For example, the angle of incidence may be in the range of 3° to 15°, or 5° to 12°. This angular range may be particularly useful in a (pressurized) gas environment, where exciting the fundamental Lamb mode may be more angular-dependent. The angle of incidence of the ultrasonic signal emitted by each ultrasonic transducer may deviate by at most ±5° (or at most ±3° or at most ±1°) from the angle of incidence of the ultrasonic signal emitted by another one of the ultrasonic transducers. Having approximately the same angle of incidence for different ultrasonic transducers (within mechanical tolerances) may allow for exciting similar or identical Lamb modes.

The ultrasonic transducers may be arranged and/or adjusted such that, in operation of the inspection device the propagation direction angle between a first ultrasonic transducer of the plurality of transducers and a second ultrasonic transducer of the plurality of transducers is less than 180°. The propagation direction angle between two ultrasonic transducers is the angle enclosed by the propagation directions of said two ultrasonic transducers. For example, the propagation direction angle between the first ultrasonic transducer and the second ultrasonic transducer is the angle enclosed by the propagation direction of the first ultrasonic transducer and the propagation direction of the second ultrasonic transducer. In other words: a connecting line between the first ultrasonic transducer and the second ultrasonic transducer is not parallel to the ultrasonic signal emitted by any of the first ultrasonic transducer and the second ultrasonic transducer. The propagation direction angle may, for example, be at least 1° and at most 179°, or at least 5° and at most 175°, or at least 20° and at most 160°, or at least 40° and at most 140°.

Placing the ultrasonic transducers such that the propagation direction angle is below 180° allows for measuring a reflected signal with an ultrasonic transducer that is different from the ultrasonic transducer that emitted the ultrasonic signal from which the reflected signal originates. For example, the ultrasonic signal emitted by the first ultrasonic transducer is reflected at a defect and received (as the reflected signal) by the second ultrasonic transducer, or vice versa. By this, a further detection signal is provided that avoids the entry echo signal and therefore the so-called pulse-echo dead zone. This allows for significantly lower stand-off distances in comparison to state-of-the-art inspection devices. Furthermore, using the reflected signal measured by a different ultrasonic transducer than the emitting ultrasonic transducer may allow detection of both circumferential and axial defects. In comparison, using only an echo signal and/or a transmitted signal for defect detection may reduce defect detection to axial or circumferential defects only. For example, for each of the ultrasonic transducers, the stand-off distance, the ultrasonic signal (e.g., the intensity and/or the center frequency and/or the beam angle of the ultrasonic signal), and the angle of incidence of the ultrasonic signal emitted by the ultrasonic transducer are chosen such that, in operation of the inspection device, at least one fundamental Lamb mode is excited in the pipeline wall by the ultrasonic signal emitted by the ultrasonic transducer. For example, the AO and/or the SO Lamb mode are/is excited.

According to some aspects, in operation of the inspection device, the Lamb wave of the excited fundamental Lamb mode travels along and/or in the pipeline wall along a Lamb wave propagation direction. The Lamb wave propagation direction is parallel to a projection of the ultrasonic propagation direction onto the pipeline wall. In other words: The Lamb wave propagates along the direction of the ultrasonic signal along and/or in the pipeline wall. Typically, the Lamb waves are guided waves and the wave front of the Lamb waves has a finite curvature. The Lamb waves therefore have a beam spread and an opening angle. The Lamb wave propagation direction may be measured along the maximum intensity of the Lamb wave and/or along the center of the Lamb wave.

According to some aspects, each of the ultrasonic transducers is configured to receive at least a part of a reflected signal and/or at least part of a transmitted signal and/or at least part of an echo signal. For example, each of the ultrasonic transducers is configured to receive a part of a reflected signal and at least part of a transmitted signal and at least part of an echo signal. Each of the ultrasonic transducers may be configured to determine an arrival time and/or a time duration and/or an intensity and/or a frequency (e.g., a center frequency) of the signal received by said ultrasonic transducer. For example, from these parameters, the origin of the ultrasonic signal (i.e. , the ultrasonic transducer from which it was emitted) and the position of a defect in the pipeline wall may be determined.

The reflected signal is generated by reflecting at least a part of the Lamb wave corresponding to the excited Lamb mode at a defect in the pipeline wall, wherein the Lamb wave was excited by the ultrasonic signal emitted by at least one of the other ultrasonic transducers. The transmitted signal is generated by transmitting at least a part of the excited Lamb wave through a defect in the pipeline wall, wherein the excited Lamb wave was excited by the ultrasonic signal emitted by at least one of the other ultrasonic transducers. Compared to the case without a defect in the pipeline wall, an intensity of the transmitted signal may drop if transmission through a defect occurred. The echo signal is generated by reflecting at least a part of the excited Lamb wave at a defect in the pipeline wall, wherein the excited Lamb wave was excited by the ultrasonic signal emitted by the same ultrasonic transducer.

The reflected signal, the transmitted signal and the echo signal originate from different ultrasonic transducers. In the case of the echo signal, the ultrasonic signal originates from the same ultrasonic transducer that receives the echo signal. Hereinafter, if a signal (e.g., the reflected signal, the transmitted signal and/or the echo signal) “originates from an ultrasonic transducer”, this refers to the case that the ultrasonic signal emitted from said ultrasonic transducer excites a Lamb wave of a (fundamental) Lamb mode in the pipeline wall, which Lamb wave is then reflected at and/or transmitted through a defect in the pipeline wall. In the case of the transmitted signal, the ultrasonic propagation direction of the ultrasonic transducer that emitted the ultrasonic signal and the ultrasonic propagation direction of the ultrasonic transducer that received the ultrasonic signal may be opposite to one another (e.g., the propagation direction angle between these two ultrasonic transducers is roughly 180°). In the case of the reflected signal, the propagation direction angle between the ultrasonic transducer that emitted the ultrasonic signal and the ultrasonic transducer that received the ultrasonic signal may be below 180°. The propagation direction angle may then correspond to a deflection angle.

For example, in operation of the inspection device, if a first ultrasonic transducer emits the ultrasonic signal, the first ultrasonic transducer may receive the echo signal, while a second ultrasonic transducer may receive the reflected signal and a third ultrasonic transducer may receive the transmitted signal. The propagation direction angle between the first ultrasonic transducer and the second ultrasonic transducer may be below 180°, e.g. at least 20° and at most 160°. The angle between the ultrasonic propagation directions of the first ultrasonic transducer and the third ultrasonic transducer may be 180° (within the mechanical tolerances of e.g. ±2°). For example, the ultrasonic propagation direction of the ultrasonic signal emitted by a third ultrasonic transducer of the plurality of ultrasonic transducers may be opposite (e.g., may enclose an angle of at least 178° and at most 182°, for example 180°) to the ultrasonic propagation direction of the ultrasonic signal emitted by the first ultrasonic transducer. The propagation direction angle between the first ultrasonic transducer and the third ultrasonic transducer may therefore be roughly 180°. Separately or in combination, the ultrasonic propagation direction of the ultrasonic signal emitted by a fourth ultrasonic transducer of the plurality of ultrasonic transducers may be opposite to the ultrasonic propagation direction of the ultrasonic signal emitted by the second ultrasonic transducer. The propagation direction angle between the second ultrasonic transducer and the fourth ultrasonic transducer may therefore be roughly 180°.

For example, if the first ultrasonic transducer emits an ultrasonic signal, the second ultrasonic transducer may receive the reflected signal originating from said emitted ultrasonic signal, the third ultrasonic transducer may receive the transmitted signal originating from said emitted ultrasonic signal, and the first ultrasonic transducer may receive the echo signal.

According to some aspects, the inspection device may comprise a mounting means. The ultrasonic transducers may be mounted to the mounting means such that the stand-off distance and the ultrasonic propagation direction of each of the ultrasonic transducers are predetermined and fixed. For example, the mounting means may comprise a spacer for determining the stand-off distance. The mounting means may comprise holding means that are adjusted such that the propagation direction may be set.

According to some aspects, a method for detecting a defect in the pipeline wall of a pipeline with an inspection device comprises emitting, with the first ultrasonic transducer, an ultrasonic signal towards the pipeline wall. For example, for emitting the ultrasonic signal, the signal controller is activated. The signal controller may either cause all ultrasonic transducers, or only some ultrasonic transducers, or only one ultrasonic transducer (e.g., the first ultrasonic transducer) to emit an ultrasonic signal at the same time. The ultrasonic signal emitted by the first ultrasonic transducer excites at least one Lamb wave in the pipeline wall. At least a part of the excited Lamb wave is reflected by the defect in the pipeline wall and at least partly coupled out of the pipeline wall as a reflected signal. It may further be possible that a part of the excited Lamb wave is reflected at the defect to create an echo signal and/or that a part of the excited Lamb wave is transmitted through the defect to create a transmitted signal.

The method may further comprise receiving, with the second ultrasonic transducer, at least part of the reflected signal originating from the first ultrasonic transducer. In some examples, the method may also comprise receiving, with the fist ultrasonic transducer, at least part of the echo signal originating from the first ultrasonic transducer, wherein the echo signal is generated by reflecting at least a part of the excited Lamb wave by the defect in the pipeline wall. Separately or in combination, the method may also comprise receiving, with the third ultrasonic transducer, at least part of the transmitted signal originating from the first ultrasonic transducer, wherein the transmitted signal is generated by transmitting at least a part of the excited Lamb wave through the defect in the pipeline wall.

According to some aspects of the disclosure, an incoming angle may deviate by at most ±20° (e.g., at most ±10° or at most ±5° or at most ±1°) from an outcoming angle. The incoming angle is enclosed by a main orientation direction of the defect and the ultrasonic propagation direction of the first ultrasonic transducer and the outcoming angle is enclosed by the main orientation direction of the defect and the ultrasonic propagation direction of the second ultrasonic transducer. The excited Lamb wave may travel along a Lamb wave propagation direction. The reflected signal may travel along a reflected Lamb wave propagation direction. The Lamb wave propagation direction and the reflected Lamb wave propagation direction may enclose the same angle with a main orientation direction of the defect. The main orientation direction of the defect may be the direction along which the defect has its largest extension (e.g., the defect’s main extension direction). The Lamb wave propagation direction and the reflected Lamb wave propagation direction both represent the propagation direction of the maximum intensity of the respective Lamb wave. There may, however, also be a reduced signal with a reduced intensity with a propagation direction deviating from the (reflected) Lamb wave propagation direction due to the spread (i.e., the divergence) of the Lamb wave (e.g. with a Lamb wave opening angle). This reduced signal may be measured by the second ultrasonic transducer. For example, in the ideal case, the reflected Lamb wave propagation direction may be identical to the projection of the propagation direction of the second ultrasonic transducer onto the pipeline wall. In this case, the incoming angle and the outcoming angle would be identical. In general, the reflected Lamb wave propagation direction of the reflected signal may be parallel to a Lamb wave propagation direction of an excited Lamb wave that is excited by an ultrasonic signal emitted by the second ultrasonic signal.

In some examples, the incoming angle may be at least 20° and at most 70°, for example at least 30° and at most 60°, for example at least 40° and at most 50°. Such an angle may allow for an improved reflection to the second ultrasonic transducer. Less intensity may be transmitted through the defect and/or reflected back to the emitting transducer.

According to some aspects, the method may further comprise receiving, with a fifth ultrasonic transducer, at least a part of the reflected signal originating from the first ultrasonic transducer. The part of the reflected signal received by the fifth ultrasonic transducer differs from the part of the reflected signal received by the second ultrasonic transducer. The propagation direction angle between the first ultrasonic transducer and the fifth ultrasonic transducer of the plurality of ultrasonic transducers differs from the propagation direction angle between the first ultrasonic transducer and the second ultrasonic transducer. The part of the reflected signal received by the second ultrasonic transducer may have a different intensity than the part of the reflected signal received by the fifth ultrasonic transducer. This may be caused by a beam spread (also referred to as beam divergence) of the reflected signal. The beam spread may stem from either one of: a beam spread of the ultrasonic signal emitted by the first ultrasonic transducer, a beam spread of the excited Lamb wave, or scattering in addition to the reflection at the defect. The opening angle of the reflected signal may be large enough so that the reflected signal is received by several ultrasonic transducers.

It may be possible to determine a projection profile of the defect by evaluating the reflected signal received by the second ultrasonic transducer and at least one of: the echo signal received by the first ultrasonic transducer, the transmitted signal received by the third ultrasonic transducer, or the reflected signal received by the fifth ultrasonic transducer. For each of the signals (if evaluated), the intensity and the time-of-f light of the signal may be determined for determining the projection profile. For example, this may allow to determine the shape of the defect. According to at least some aspects, the method may comprise emitting, with the second ultrasonic transducer, an ultrasonic signal towards the pipeline wall, wherein the ultrasonic signal emitted by the second ultrasonic transducer excites at least one Lamb wave of a fundamental Lamb mode in the pipeline wall and wherein at least a part of the excited Lamb wave is reflected by the defect in the pipeline wall and at least partly coupled out of the pipeline wall as a reflected signal and receiving, with the first ultrasonic transducer, at least a part of the reflected signal originating from the second ultrasonic transducer. In other words: the method may be reversed and the first ultrasonic transducer may be used as a receiver for the reflected signal originating from the second ultrasonic transducer. By this, an improved knowledge of the defect position and orientation may be gained.

Another aspect of the disclosure relates to a method for detecting a defect in the pipeline wall. The method may comprise arranging a plurality of ultrasonic transducers in the pipeline. The method may further comprise emitting, with at least a first ultrasonic transducer of the plurality of ultrasonic transducers, an ultrasonic signal towards the pipeline wall, wherein the ultrasonic signal excites at least one Lamb wave of a fundamental Lamb mode in the pipeline wall. At least a part of the excited Lamb wave is reflected by the defect in the pipeline wall at a deflection angle above 0°, thereby creating a reflected signal. The deflection angle corresponds to the sum of the incoming reflection angle (which is the angle the Lamb wave propagation direction of the Lamb wave encloses with the normal of the main orientation direction of the defect) and the outcoming reflection angle (which is the angle the Lamb wave propagation direction of the reflected Lamb wave encloses with the normal of the main orientation direction of the defect). The incoming reflection angle corresponds to 90° minus the incoming angle and the outcoming reflection angle corresponds to 90° minus the outcoming angle. In the ideal case, the incoming reflection angle corresponds to the outcoming reflection angle and the deflection angle is twice the incoming reflection angle and/or twice the outcoming reflection angle.

For example, the deflection angle may be at least 40° and at most 140° or at least 60° and at most 120° or at least 80° and at most 100°. The ultrasonic transducers are arranged in the pipeline such that the reflected signal is received by a second ultrasonic transducer of the plurality of ultrasonic transducers different from the first ultrasonic transducer, and an angle of incidence of the ultrasonic signal emitted by the first ultrasonic transducer is larger than 0°, the angle of incidence of an ultrasonic signal being the angle enclosed by an ultrasonic propagation direction of the ultrasonic signal emitted by the ultrasonic transducer and the surface normal of the pipeline wall at the point of incidence of the ultrasonic signal emitted by said ultrasonic transducer. The deflection angle may correspond to a propagation direction angle between the first ultrasonic transducer and the second ultrasonic transducer.

In some examples, at least a part of the excited Lamb wave is transmitted through the defect in the pipeline wall, thereby creating a transmitted signal. The ultrasonic transducers may be arranged in the pipeline such that the transmitted signal is received by a third ultrasonic transducer of the plurality of ultrasonic transducers different from the first ultrasonic transducer and the second ultrasonic transducer.

Separately or in combination, at least a part of the excited Lamb wave is reflected by the defect in the pipeline wall, thereby creating an echo signal. The ultrasonic transducers are arranged in the pipeline such that the echo signal is received by the first ultrasonic transducer.

According some aspects, the ultrasonic signal received by any one of the ultrasonic transducers (e.g. the reflected signal received by the second ultrasonic transducer and/or the echo signal received by the first ultrasonic transducer and/or the transmitted signal received by the third ultrasonic transducer) is further processed with a signal processing unit. The inspection device may comprise several signal processing units or only one signal processing unit. In the case of several signal processing units, different ultrasonic transducers may be coupled to different signal processing units. In the case of a single signal processing unit, each ultrasonic transducer may be coupled to the signal processing unit. The signal processing unit may convert the ultrasonic signal into a storable data format, e.g. into digital bits. The data may then be stored in a memory device as stored data. It may be possible that no further signal processing is performed and only the signal conversion into a storable data format is performed while the ultrasonic signal is measured by the ultrasonic transducer. The memory device may then be evaluated in a later method step, which may be performed outside of the pipeline wall and/or while there is no signal measured by the ultrasonic transducer. It may be possible that signal processing unit and/or the signal processing units decide, depending on the received ultrasonic signal and/or the stored data, whether or not a defect is present in the pipeline wall. It may also be possible that a further signal processing unit decides whether or not a defect is present, depending on the received ultrasonic signal and/or the stored data.

Referring to the schematic illustration of Figs. 1A and 1 B, further aspects of the disclosure are explained in detail. Fig. 1A depicts a schematic side view of an inspection device according to the disclosure while Fig. 1 B depicts a schematic on-top view of the inspection device. The inspection device is shown in operation. The inspection device comprises a first ultrasonic transducer 11 , a second ultrasonic transducer 12, a third ultrasonic transducer 13, and a fourth ultrasonic transducer 14. The inspection device may comprise further ultrasonic transducers (e.g., a fifth, sixth, etc., ultrasonic transducer), not shown in the drawings. The ultrasonic transducers 11 , 12, 13, 14 are arranged with respect to each other and with respect to a pipeline wall 30. Each of the ultrasonic transducers 11 , 12, 13, 14 may have a respective standoff distance d1 , d2, d3, d4 to the pipeline wall 30 that is pre-determined and fixed. Each of the ultrasonic transducers may emit a respective ultrasonic signal along a respective ultrasonic propagation direction 111 , 121 , 131 , 141 in operation of the inspection device. The ultrasonic transducers 11 , 12, 13, 14 may be similar or even identical. For example, the ultrasonic signal emitted by each of the ultrasonic transducers 11 , 12, 13, 14 may have the same center frequency and/or the same intensity and/or the same beam divergence. It may be possible to control the ultrasonic transducers 11 , 12, 13, 14 individually so that each ultrasonic transducer 11 , 12, 13, 14 may emit an ultrasonic signal independent of the other ultrasonic transducers 11 , 12, 13, 14.

In operation, the ultrasonic signal emitted by each of the ultrasonic transducers 11 , 12, 13, 14 may impinge on the pipeline wall 30 at a different point of incidence POI. The ultrasonic propagation direction 11 , 12, 13, 14 and the surface normal of the pipeline wall 30 at the point of incidence POI of the ultrasonic signal may enclose an angle of incidence a1 , a2, a2, a4 of the ultrasonic signal emitted by the respective ultrasonic transducer 11 , 12, 13, 14. The angle of incidence a1 , a2, a2, a3 for each ultrasonic transducer 11 , 12, 13, 14 is above zero. This may allow for exciting a fundamental AO and/or SO Lamb mode within the pipeline wall 30.

The ultrasonic transducers 11 , 12, 13, 14 are tilted with respect to one another. As can be seen in Fig. 1 B, the ultrasonic propagation direction 111 of the first ultrasonic transducer 11 encloses a propagation direction angle (312 with the ultrasonic propagation direction 121 of the second ultrasonic transducer 12, which is below 180°, e.g. below 90°. Likewise, the ultrasonic propagation direction 121 (131 / 141) of the second ultrasonic transducer 12 (third ultrasonic transducer 13 / fourth ultrasonic transducer 14) encloses a propagation direction angle P23 (P34 / P41) with the ultrasonic propagation direction 131 (141 / 111) of the third ultrasonic transducer 13 (fourth ultrasonic transducer 14 / first ultrasonic transducer 11), which is below 180°, e.g. below 90°. This may allow for the second ultrasonic transducer 12 receiving a reflected signal originating from the first ultrasonic transducer 11 , the third ultrasonic transducer 13 receiving a reflected signal originating from the second ultrasonic transducer 12, the fourth ultrasonic transducer 14 receiving a reflected signal originating from the third ultrasonic transducer 13, the first ultrasonic transducer 11 receiving a reflected signal originating from the fourth ultrasonic transducer 14, and vice versa.

The first ultrasonic transducer 11 is opposite to the third ultrasonic transducer 13 and the second ultrasonic transducer 12 is opposite to the fourth ultrasonic transducer 14. This may allow for the third ultrasonic transducer 13 receiving a transmitted signal originating from the first ultrasonic transducer 11 , the fourth ultrasonic transducer 14 receiving a transmitted signal originating from the second ultrasonic transducer 12, and vice versa.

In the following, in order to simplify the explanation, it is assumed that an ultrasonic signal is emitted by the first ultrasonic transducer 11. It should, however, be appreciated that the ultrasonic signal can also be emitted by any one of the other ultrasonic transducers 12, 13, 14. The following explanation is valid, mutatis mutandis, for this scenario.

Referring to the schematic illustrations depicted in Fig. 2, an inspection device and a method according to aspects of the disclosure are explained in more detail. Fig. 2 depicts a first ultrasonic transducer 11 of an inspection device emitting an ultrasonic signal 100 along an ultrasonic propagation direction 111. The ultrasonic signal 100 impinges on the pipeline wall 30 with a finite angle of incidence a1 . The ultrasonic signal 100 excites a Lamb wave 20 in the pipeline wall 30. The Lamb wave 20 travels along and/or in the pipeline wall 30 along a Lamb wave propagation direction 201. The Lamb wave propagation direction 201 is parallel to a projection of the ultrasonic propagation direction 111 onto the pipeline wall 30. In other words: The Lamb wave propagation direction 201 extends the ultrasonic propagation direction 111 within the pipeline wall.

Fig. 3 depicts the case where the first ultrasonic transducer 11 emits an ultrasonic signal 100 with an angle of incidence a1 above zero onto the pipeline wall 30. The pipeline wall 30 comprises a defect 31. The ultrasonic signal 100 excites a Lamb wave 20 within the pipeline wall 30 along the Lamb wave propagation direction 201. A part of the Lamb wave 20 is reflected at the defect 31. The reflected Lamb wave 20’ travels in the pipeline wall 30 along a reflected Lamb wave propagation direction 20T for a Lamb wave travel distance LR. At least a part of the reflected Lamb wave 20’ is coupled out as a reflected signal 101. The reflected signal 101 propagates along the ultrasonic propagation direction 121 of the second ultrasonic transducer 12 and is received by the second ultrasonic transducer 12, that is positioned with a distance D12 to the first ultrasonic transducer 11 . The reflection changes the direction of the propagation of the Lamb wave 20. The ultrasonic propagation direction 111 of the first ultrasonic transducer 11 and the ultrasonic propagation direction 121 of the second ultrasonic transducer 12 enclose a propagation direction angle (312. At least a part of the reflected Lamb wave 20’ is parallel to a projection of the ultrasonic propagation direction 121 of the second ultrasonic transducer 12 onto the pipeline wall 30.

Fig. 4, again, depicts the first ultrasonic transducer 11 emitting an ultrasonic signal 100 towards the pipeline wall 30. Different to the schematic illustration of Fig. 3, the Lamb wave 20 is now transmitted through the defect 31 and the transmitted Lamb wave 20” basically propagates along the (original) Lamb wave propagation direction 201 for a Lamb wave travel distance LT. The outcoupled transmitted signal 102 travels along the ultrasonic propagation direction 131 of the third ultrasonic transducer 13 and is received and measured by the third ultrasonic transducer 13. In the schematic illustration depicted in Fig. 5, the Lamb wave 20 originating from the ultrasonic signal 100 emitted by the first ultrasonic transducer 11 is reflected at the defect 31 , but now travels back to the first ultrasonic transducer 11 and is measured as an echo signal 103. The Lamb wave 20 travels within the pipeline wall 30 for a Lamb wave travel distance LE, which is twice the distance between the point of incidence POI and the position of the reflection at the defect 31.

Fig. 6 shows an exemplary measurement signal as derived from such an echo signal 103. In particular, the upper and the lower insert of Fig. 5 both show an amplitude A of an ultrasonic signal measured over a time-of-flight T. The upper insert of Fig. 5 shows the case where no echo signal 103 is detected by the first ultrasonic transducer 11 , while the lower insert of Fig. 5 shows the case where an echo signal 103 is detected (as a detected echo signal 502) by the first ultrasonic transducer 11.

Even without a defect 31 in the pipeline wall 30, a part of the ultrasonic signal 100 is reflected by the surface of the pipeline wall 30 and measured by the ultrasonic transducer (so-called surface echo, measured as the detected surface echo signal 501). If the defect 31 is very close to the point of incidence POI, the time-of-flight T of the echo signal 103 is usually very small and the detected echo signal 502 and the detected surface echo signal 501 may overlap. In this case, the signal-to-noise ratio of the detected echo signal 502 may be too small to be measured. The time-of-flight region that hinders detection of the detection echo signal 502 corresponds to a certain spatial region on the pipeline wall 30, which is referred to as the pulseecho dead zone 40 (see Fig. 5). A defect 31 within this dead zone 40 cannot be detected or may require very complicated electronics to allow for detection.

It may be possible to minimize the dead zone 40 by increasing the stand-off distance d1. However, increasing the stand-off distance d1 may result in a decrease of the amplitude of the echo signal 103. To overcome this problem, the disclosure suggests to measure the reflection signal 101 with one of the other ultrasonic transducers 12, 13, 14. This can be implemented by arranging the ultrasonic transducers 11 , 12, 13, 14 such that at least two ultrasonic propagation directions 111 , 121 , 131 , 141 enclose an angle of below 180°. Referring to the schematic illustrations of Figs. 7A, 7B, 7C and 7D further aspects of the disclosure are explained in detail. Each of Figs. 7A, 7B, 7C and 7D shows a plurality of ultrasonic transducers 11 , 12, 13, 14, 15 of an inspection device, wherein a first ultrasonic transducer 11 of the plurality of ultrasonic transducers 11 , 12, 13, 14, 15 emits an ultrasonic signal onto a pipeline wall 30 of a pipeline. The ultrasonic signal excites a Lamb wave in the pipeline wall 30, wherein the Lamb wave travels along a Lamb wave propagation direction 201 . The Lamb wave propagation direction 201 runs parallel to an ultrasonic propagation direction (not shown in Figs. 7A-7D) of the first ultrasonic transducer 11. At least part of the Lamb wave is reflected at a defect 31 in the pipeline wall 30. The defect 31 extends along a main orientation direction 311. The reflected Lamb wave propagates along a reflected Lamb wave propagation direction 20T. The reflected Lamb wave is coupled out of the pipeline as a reflected signal, which is measured an ultrasonic transducer 12, 14, 15 different from the first ultrasonic transducer 11. Depending on the orientation of the defect 31 and/or the reflection and/or the opening angle of the reflected Lamb wave, the reflected signal is measured by the second ultrasonic transducer 12 and/or the fourth ultrasonic transducer 14 and/or a fifth ultrasonic transducer 15.

The Lamb wave propagation direction 201 encloses an incoming angle e1 with the main orientation direction 311 of the defect 31. The reflected Lamb wave propagation direction 20T encloses an outcoming angle e2 with the main orientation direction 311 of the defect 31. A deflection angle 5 between the Lamb wave propagation direction 201 and the reflected Lamb wave propagation direction 20T may correspond to 90° minus e1 and/or 90° minus e2. In the ideal case, where a projection of the ultrasonic propagation direction 121 of the second ultrasonic transducer 12 (or the ultrasonic propagation direction 141 of the fourth ultrasonic transducer 14, Fig. 7B, or the ultrasonic propagation direction of the fifth ultrasonic transducer 15, Fig. 7C) onto the pipeline wall 30 is parallel to the maximum intensity of the reflected Lamb wave, the incoming angle e1 is essentially identical to the outcoming angle e2. Nevertheless, scattering reflection and/or a beam divergence of the ultrasonic signal (and therefore the excited Lamb wave) may result in a spread of the reflected Lamb wave. Therefore, the incoming angle e1 may deviate by at most ±20° or at most ±10° from the outcoming angle e2. The incoming angle e1 (and/or the outcoming angle e2) may be at least 20° and at most 70°, for example at least 40° and at most 50°, for example 45°±2°. It may be possible that a part of the Lamb wave is reflected back to the first ultrasonic transducer 11 by the defect 31 and the outcoupled echo signal is then received by the first ultrasonic transducer 11. Separately or in combination, a part of the Lamb wave may be transmitted through the defect 31 and the outcoupled transmitted signal may be received by the third ultrasonic transducer 13. It may further be possible that the defect 31 has a more complex shape so that a reflected signal is received by both the second ultrasonic transducer 12 and the fourth ultrasonic transducer 14. Intermediate scenarios may be possible - for example, a larger part of the Lamb wave may be reflected to the second ultrasonic transducer 12 and a smaller part of the Lamb wave may be reflected to the fourth ultrasonic transducer 14 or vice versa.

In Fig. 7A, the main orientation direction 311 is essentially parallel to an axial direction L of the pipeline. In Fig. 7B, the main orientation direction 311 is essentially parallel to a circumferential direction C of the pipeline. Depending on the main orientation direction 311 of the defect 31 , the reflected signal is primarily measured by the second ultrasonic transducer 12 (Fig. 7A) or the fourth ultrasonic transducer 14 (Fig. 7B). Therefore, the main orientation direction 311 may be derived from the ultrasonic transducer that received the maximum intensity of the reflected signal. Evaluating also the echo signal and the transmitted signal may allow for a more detailed knowledge of the defect 31 . For example, a projection profile of the defect 31 may be derived from the intensity distribution and/or the time-of-flight distribution of the echo signal, the transmitted signal and/or the reflected signal.

In Fig. 7C, the main orientation direction 311 is oblique to both the axial direction L and the circumferential direction C. For example, a part of the Lamb wave may be reflected towards the second ultrasonic transducer 12 (outcoming angle e2 and reflected Lamb wave propagation direction 20T) and another part of the Lamb wave may be reflected towards a fifth ultrasonic transducer 15 (outcoming angle e2’ and reflected Lamb wave propagation direction 201”). A propagation direction angle (315 (corresponding to the deflection angle S’) between the first ultrasonic transducer 11 and the fifth ultrasonic transducer 15 may differ from the propagation direction angle (312 (corresponding to the deflection angle 5) between the first ultrasonic transducer 11 and the second ultrasonic transducer 12. The intensity received by the fifth ultrasonic transducer 15 may be significantly smaller than the intensity received by the second ultrasonic transducer 12. A projection profile of the defect 31 may be derived from the intensity profile and/or the time-of-flight profile detected by the second ultrasonic transducer 12 and the fifth ultrasonic transducer 15. The scenario depicted in Fig. 7C may be independent of the main orientation direction 311 and may also occur for defects 31 that are not oblique to, but aligned with the circumferential direction C and/or the axial direction L.

In Fig. 7D, the main orientation direction 311 is essentially parallel to the axial direction L, but the first ultrasonic transducer 11 is now closer to the second ultrasonic transducer 12 as compared to Fig. 7A. The incoming angle e1 (and the outcoming angle e2) in Fig. 7D is therefore larger than the incoming angle in Fig. 7A. The closer the incoming angle e1 and the outcoming angle e2 are to 45°, the higher the intensity of the reflected Lamb wave (and therefore the reflected signal) may be. Fig. 7D also depicts different reflection scenarios for Lamb wave reflection at the defect 31 , depending on which of the ultrasonic transducers emitted the ultrasonic signal for exciting the Lamb wave (indicated by arrows). For example, the ultrasonic signal for exciting the Lamb wave may be emitted by the fourth ultrasonic transducer 13 and the reflected signal may be received by the fourth ultrasonic transducer 14. The third ultrasonic transducer 13 may emit the ultrasonic signal at the same time as the first ultrasonic transducer 11 or at a different time. It will be appreciated by the person skilled in the art that different combinations of emitting and receiving ultrasonic transducer are possible.

Although specific examples have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific examples shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific examples discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.

It should be noted that the examples of an inspection device and/or a method as outlined in the present document may be used stand-alone or in combination with the other examples disclosed in this document. In addition, the features outlined in the context of an inspection device are also applicable to a corresponding method, and vice versa. Furthermore, all aspects of the examples of an inspection device and/or a method outlined in the present document may be arbitrarily combined. In particular, the features of the claims may be combined with one another in an arbitrary manner.

It should be noted that the description and drawings merely illustrate the principles of the proposed methods and systems. Those skilled in the art will be able to implement various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and embodiments outlined in the present document are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the proposed methods and systems. Furthermore, all statements herein providing principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

Claims

1. An inspection device for inspecting a pipeline wall (30) of a pipeline, wherein the inspection device comprises a plurality of ultrasonic transducers (11 , 12, 13, 14, 15), each of the ultrasonic transducers (11-15) being configured to emit a respective ultrasonic signal (100) along a respective pre-determined ultrasonic propagation direction (111 , 121 , 131 , 141) in operation of the inspection device; wherein the ultrasonic transducers (11-15) are arranged and/or adjusted such that, in operation of the inspection device: each of the ultrasonic transducers (11-15) has a pre-determined finite stand-off distance (d1 , d2, d3, d4) to the pipeline wall of the pipeline, the ultrasonic signal (100) emitted by each of the ultrasonic transducers (11-15) impinges on the pipeline wall (30) at a different point of incidence (POI), an angle of incidence (a1 , a2, a3, a4) of the ultrasonic signal (100) emitted by each ultrasonic transducer (11-15) is larger than 0°, the angle of incidence (cd , a2, a3, a4) of the ultrasonic signal (100) being the angle enclosed by the ultrasonic propagation direction (111-141) of the ultrasonic signal (100) emitted by the ultrasonic transducer (11-15) and the surface normal of the pipeline wall (30) at the point of incidence (POI) of the ultrasonic signal (100) emitted by said ultrasonic transducer (11-15), and a propagation direction angle (P12, P23, P34, P41 , P15) between a first ultrasonic transducer (11 , 13) of the plurality of ultrasonic transducers (11-15) and a second ultrasonic transducer (12, 14, 15) of the plurality of ultrasonic transducers (11- 15) is less than 180°, the propagation direction angle (P12-P15) being the angle enclosed by the ultrasonic propagation direction (111 , 131) of the ultrasonic signal (100) emitted by the first ultrasonic transducer (11 , 13) and the ultrasonic propagation direction (121 , 141) of the ultrasonic signal (100) emitted by the second ultrasonic transducer (12, 14, 15).

2. The inspection device according to the preceding claim, wherein the propagation direction angle (pi 2-p 15) is at least 40° and at most 140°.

3. The inspection device according to any of the preceding claims, wherein for each of the ultrasonic transducers (11-15), the stand-off distance (d1-d4), the ultrasonic signal (100) and the angle of incidence (a1-a4) of the ultrasonic signal (100) emitted by the ultrasonic transducer (11-15) are chosen such that, in operation of the inspection device, at least one Lamb wave (20) of a fundamental Lamb mode is excited in the pipeline wall (30) by the ultrasonic signal (100) emitted by the ultrasonic transducer (100); wherein, in operation of the inspection device, the excited Lamb wave (20) travels along and/or in the pipeline wall (30) along a Lamb wave propagation direction (201), wherein the Lamb wave propagation direction (201) is parallel to a projection of the ultrasonic propagation direction (111-141) onto the pipeline wall (30).

4. The inspection device according to the preceding claim, wherein each of the ultrasonic transducers (11-15) is configured to receive at least a part of a reflected signal (101), wherein the reflected signal (101) is generated by reflecting at least a part of the excited Lamb wave (20) at a defect (31) in the pipeline wall (30), wherein the excited Lamb wave (20) was excited by the ultrasonic signal (100) emitted by at least one of the other ultrasonic transducers (11-15) and/or wherein each of the ultrasonic transducers (11-15) is configured to receive at least a part of a transmitted signal (102), wherein the transmitted signal (102) is generated by transmitting at least a part of the excited Lamb wave (20) through a defect (31) in the pipeline wall (30), wherein the excited Lamb wave (20) was excited by the ultrasonic signal (100) emitted by at least one of the other ultrasonic transducers (11-15) and/or wherein each of the ultrasonic transducers (11-15) is configured to receive at least a part of an echo signal (103), wherein the echo signal (103) is generated by reflecting at least a part of the excited Lamb wave (20) at a defect (31) in the pipeline wall (30), wherein the excited Lamb wave (20) was excited by the ultrasonic signal (100) emitted by the same ultrasonic transducer (11-15).

5. The inspection device according to any of the preceding claims, wherein the ultrasonic propagation direction (131) of the ultrasonic signal (100) emitted by a third ultrasonic transducer (13) of the plurality of ultrasonic transducers (11-15) is opposite to the ultrasonic propagation direction (111) of the ultrasonic signal (100) emitted by the first ultrasonic transducer (11) and/or wherein the ultrasonic propagation direction (141) of the ultrasonic signal (100) emitted by a fourth ultrasonic transducer (14) of the plurality of ultrasonic transducers (11-15) is opposite to the ultrasonic propagation direction (121) of the ultrasonic signal (100) emitted by the second ultrasonic transducer (12).

6. The inspection device according to any of the preceding claims, further comprising a mounting means, wherein the ultrasonic transducers are mounted to the mounting means such that the stand-off distance (d1-d4) and the ultrasonic propagation direction (111-141) of each of the ultrasonic transducers (11-15) are pre-determined and fixed.

7. A method for detecting a defect (31) in a pipeline wall (30) of a pipeline with an inspection device according to any of the preceding claims, comprising: emitting, with the first ultrasonic transducer (11 , 13), an ultrasonic signal (100) towards the pipeline wall (30), wherein the ultrasonic signal (100) emitted by the first ultrasonic transducer (11 , 13) excites at least one Lamb wave (20) of a fundamental Lamb mode in the pipeline wall (30) and wherein at least a part (20’) of the excited Lamb wave (20) is reflected by the defect (31) in the pipeline wall (30) and at least partly coupled out of the pipeline wall

(30) as a reflected signal (102); receiving, with the second ultrasonic transducer (12,14), at least a part of the reflected signal (102) originating from the first ultrasonic transducer (11 , 13).

8. The method according to the preceding claim, further comprising: receiving, with the first ultrasonic transducer (11 , 13) at least a part of an echo signal (103) originating from the first ultrasonic transducer (11 , 13), wherein the echo signal (103) is generated by reflecting at least a part of the excited Lamb wave (20) at the defect (31) in the pipeline wall (30) and/or receiving, with a third ultrasonic transducer (13), at least a part of a transmitted signal (102) originating from the first ultrasonic transducer (11), wherein the transmitted signal (102) is generated by transmitting at least a part of the excited Lamb wave (20) through the defect

(31) in the pipeline wall (30).

9. The method according to any of the two preceding claims, wherein an incoming angle (e1) deviates by at most ±20° from an outcoming angle (e2), wherein the incoming angle is enclosed by a main orientation direction (311) of the defect (31) and the ultrasonic propagation direction (111) of the first ultrasonic transducer (11) and wherein the outcoming angle (e2) is enclosed by the main orientation direction (311) of the defect (31) and the ultrasonic propagation direction (121) of the second ultrasonic transducer (12).

10. The method according to the preceding claim, wherein the incoming angle (e1) is at least 20° and at most 70°and/or wherein the outcoming angle (e2) is at least 20° and at most 70°.

11. The method according to any of the four preceding claims, further comprising receiving, with a fifth ultrasonic transducer (15) of the plurality of ultrasonic transducers (11-15) at least a part of the reflected signal (102) originating from the first ultrasonic transducer (11), wherein the propagation direction angle (p15) between the first ultrasonic transducer (11 , 13) and the fifth ultrasonic transducer (15) of the plurality of ultrasonic transducers (11 ,

12. 13, 14, 15) differs from the propagation direction angle (P12, P34) between the first ultrasonic transducer (11 , 13) and the second ultrasonic transducer (12, 14).

12. The method according to any of the five preceding claims, further comprising determining a projection profile of the defect (31) by evaluating the reflected signal (101) received by the second ultrasonic transducer (12) and at least one of: the echo signal (103) received by the first ultrasonic transducer (11), the transmitted signal (102) received by the third ultrasonic transducer (13), or the reflected signal (101) received by the fifth ultrasonic transducer (15).

13. The method according to any of the six preceding claims, further comprising: emitting, with the second ultrasonic transducer (12, 14), an ultrasonic signal (100) towards the pipeline wall (30), wherein the ultrasonic signal (100) emitted by the second ultrasonic transducer (12, 14) excites at least one Lamb wave (20) of a fundamental Lamb mode in the pipeline wall (30) and wherein at least a part (20’) of the excited Lamb wave (20) is reflected by the defect (31) in the pipeline wall (30) and at least partly coupled out of the pipeline wall (30) as a reflected signal (102); receiving, with the first ultrasonic transducer (11 , 13), at least a part of the reflected signal (102) originating from the second ultrasonic transducer (12, 14).

14. A method for detecting a defect (31) in a pipeline wall (30) of a pipeline, comprising: arranging a plurality of ultrasonic transducers (11-15) in the pipeline; emitting, with at least a first ultrasonic transducer (11 ,13) of the plurality of ultrasonic transducers (11-15), an ultrasonic signal (100) towards the pipeline wall (30), wherein the ultrasonic signal (10) excites at least one Lamb wave (20) of a fundamental Lamb mode in the pipeline wall (30), wherein at least a part of the excited Lamb wave (20) is reflected by the defect (31) in the pipeline wall (30) at a deflection angle (5) that is larger than 0°, thereby creating a reflected signal (101); wherein the ultrasonic transducers (11-15) are arranged in the pipeline such that: the reflected signal is received by a second ultrasonic transducer (12, 14, 15) of the plurality of ultrasonic transducers (11-15) different from the first ultrasonic transducer (11 , 13), and an angle of incidence (a1 , a2, a3, a4) of the ultrasonic signal (100) emitted by the first ultrasonic transducer (11) is larger than 0°, the angle of incidence (a1-a4) of an ultrasonic signal (100) being the angle enclosed by an ultrasonic propagation direction (111-151) of the ultrasonic signal (100) emitted by the ultrasonic transducer (11-15) and the surface normal of the pipeline wall (30) at the point of incidence (POI) of the ultrasonic signal (100) emitted by said ultrasonic transducer (11-15).

15. The method according to the preceding claim, wherein at least a part of the excited Lamb wave (20) is transmitted through the defect (31) in the pipeline wall (30), thereby creating a transmitted signal (102), wherein the ultrasonic transducers (11-15) are arranged in the pipeline such that the transmitted signal (102) is received by a third ultrasonic transducer (13) of the plurality of ultrasonic transducers (11-15) different from the first ultrasonic transducer (11) and the second ultrasonic transducer (12) and/or wherein at least a part of the excited Lamb wave (20) is reflected by the defect (31) in the pipeline wall (30), thereby creating an echo signal (103), wherein the ultrasonic transducers (11-15) are arranged in the pipeline such that the echo signal (103) is received by the first ultrasonic transducer (11).