The invention relates to a probe holder system for at least one ultrasonic testing probe unit.

Ultrasonic testing probe units are widely used in non- destructive testing techniques in various fields, as for example in medical imaging or in industrial testing of pipelines, i.e. weld inspection, bond testing, thickness profiling, in-service crack detection. Ultrasonic testing can be done using a single ultrasonic testing probe unit, or using a pair of probes, which is typically the case in weld examination, when one testing probe unit is placed on either side of the weld to be examined. Ultrasonic testing probe units, in particular phased array probes, can typically comprise a

transducer assembly including a plurality of transducer elements, for example in a linear array, that can be controlled electronically and pulsed separately. For example, one of the transducer elements can transmit an ultrasonic wave, whereas the other transducer elements can function as a receiver of said ultrasonic wave that has been reflected by a surface of an object to be examined. Then, a next transducer element can function as a transmitter, and so on. Using all collected data, in particular travel time and beam angle of the ultrasonic wave, a reconstruction can be made of the object under examination, showing possible defects in the structure or material of the object, for which advanced algorithms are available, as for example the Inverse Wave Field Extrapolation algorithm, known as IWEX (US 7 650 789 B2 or EP 1709418 Bl). In case of a pair of ultrasonic testing probe units, an IWEX reconstruction image can for example be created for each array separately, or data of both arrays may be superposed to combine the information. In addition, an IWEX image may be created for ultrasonic waves travelling from one array via the object to the other array on the other side of the weld, a so-called cross-mode. As is the case with

conventional transducers, phased array probes may be designed for direct contact use, as part of an angle beam assembly with a wedge, or for immersion use with sound coupling through a water path.

Probe holder systems for at least one ultrasonic testing probe unit are generally known from the prior art. For example DE2945586 discloses an ultrasonic probe holder for two ultrasonic probes, which are suspended on a cardanic subframe, thus fixing the distance between the probes. Both probes are pushed onto the surface of an object to be tested by a single spring-loaded arm.

JP62-209355 discloses an ultrasonic probe holder to accurately position a pair of ultrasonic probes on a tubular object to be inspected. One of the ultrasonic probes is configured to be a transmission probe, the other is configured to be a receiving probe. The distance between the ultrasonic probes can be adjusted via an adjusting screw.

In order for the reconstruction algorithm to render correct reconstruction images, it is important to know the exact distance of each of the testing probe units with respect to the weld and/or with respect to each other. If one of these distances changes during operation, for example due to variations in surface geometry and/or surface location, then the resulting ultrasonic information will be incorrectly positioned by the reconstruction algorithm. Prior art devices, such as those mentioned above, suffer from a lack of accuracy with respect to the exact position of the testing probe unit relative to the object under examination due to variations in surface geometry and/or surface location. Variations of the surface on the side of one probe may also affect the position of a second probe. Prior art probe holders often have substantial play and mechanical tolerance. AH moving and rotating parts of the probe holder are also susceptible to dirt, which deteriorates performance and accuracy. More importantly, when the probe holder comprises a spring-loaded pivotable arm to keep one or more testing probe units in close contact with the surface of the object under

examination, for example the tubular structure, an irregularity in the surface of the object will have the probe move on a circular path around the pivot axis of the arm, causing the distance from the probe to the object under examination to change. Especially in case of examination of seam welds of tubular structures (i.e. a weld in parallel with a longitudinal direction of the tube), this problem becomes important.

It is an aim of the present invention to solve or alleviate one or more of the above-mentioned problems. Particularly, the invention aims at providing an improved probe holder system configured to provide ultrasonic measurements with a relatively high accuracy. The invention also intends to provide a relatively robust probe holder system, comprising as few parts as possible. A further aim of the invention is providing an adjustable probe holder system.

To this aim, there is provided a probe holder system characterized by the features of claim 1. In particular, the probe holder system for holding at least one ultrasonic testing probe unit comprises at least one ultrasonic testing probe unit including a scanning surface arranged to examine an object along a scanning direction. The system further comprises a probe holder frame to which said at least one ultrasonic testing probe unit is mountable, and at least one spring assembly attached to said probe holder frame and configured to push said at least one testing probe unit towards said object to be examined. In an inventive way, said at least one spring assembly comprises a first spring element, a second spring element, and a connecting element connecting said first spring element and said second spring element and arranged to keep said first spring element and said second spring element spaced-apart, wherein said connecting element is pivotally attached to said ultrasonic testing probe unit. Such a construction of the spring assembly allows for an efficient pushing of the at least one testing probe unit towards the object under examination, in spite of irregularities in the object's surface with which the testing probe unit's scanning surface is in contact during use of the testing probe unit. The spring assembly is able to compensate these irregularities without changing the position of the testing probe unit on the object to be examined thanks to the pivotal attachment of the spring assembly to the testing probe unit.

In a preferred embodiment, said first spring element and said second spring element can be fixedly attached to said probe holder frame. In particular, a first end of said first spring element can be fixedly attached to the probe holder frame, whereas a second end of said first spring element can be connected to a first end of said connecting element. Similarly, a first end of said second spring element can be fixedly attached to the probe holder frame, whereas a second end of said second spring element can be connected to a second end of said connecting element. In this way, the connecting element can connect the first spring element to the second spring element, which are each fixedly connected to the probe holder frame. As a consequence, a movement of a middle point of the connecting element is constrained to a substantially straight hne, assuming that the middle point is defined as the middle of the length of the connecting element measured between the connection points to said first and second spring elements.

Said ultrasonic testing probe unit is preferably pivotally attached to a middle point of said connecting element. In this way, the testing probe unit can follow the movement of said middle point of said connecting element, which can be constrained to a substantially straight line. In case the testing probe unit is connected with a different point of the connecting element and the connecting element is moved, the testing probe unit will follow the path of that connection point on the connecting element, which will depart further from a straight hne the further the connection point is situated from said middle point.

In a more preferred embodiment, said connecting element can be arranged to be movable towards and away from said scanning surface. In this way, the connecting element does not hinder the spring function of the first and second spring elements, and contributes in compensating an object's surface irregularities the scanning surface of the testing probe unit may engage.

Advantageously, said connecting element can be arranged substantially perpendicularly to said scanning direction. More

advantageously, said first spring element and said second spring element can also be arranged substantially perpendicularly to said scanning direction. Consequently, said first spring element, said second spring element and said connecting element can be arranged in a plane that is transverse to said scanning direction. In this way, irregularities in a surface of an object to be examined can be compensated by a movement of the at least one spring assembly in a plane that is transverse to the scanning direction of the probe holder system in use.

It is also preferred that said connecting element may be arranged substantially perpendicularly to said scanning surface. However, said connecting element need not always be in a position substantially

perpendicularly to said scanning surface, as the connecting element is movable. For example, when the probe holder system is not in use, the connecting element may take any possible position. When the probe holder system is for example placed on a surface of an object to be examined, the connecting element may take up a position such that connecting element is substantially perpendicular to said scanning surface.

In an advantageous embodiment, said connecting element may be arranged to be inflexible. An inflexible arrangement of the connecting element ensures a better control of the movement of the connecting element between the first and second spring elements. The connecting element can be made intrinsically inflexible, for example by providing an rigid

connecting element. Alternatively, and preferably, the probe holder system can further comprise a blocking element configured to prevent the

connecting element from bending. An important advantage of this second solution over the first solution is that the connecting element can be made of the same material as the first and second spring element, which provide some flexibility in order to exert the spring function. The flexibility of the connecting element can then preferably be obstructed by the blocking element.

More advantageously, said blocking element can be rotatably mountable in the at least one ultrasonic testing probe unit. The blocking element can for example be mounted in a wedge of the testing probe unit, or in a wedge's housing or in any other part of the testing probe unit. A rotatable mounting ensures a good engagement of the testing probe unit's scanning surface to a surface of an object to be examined in spite of irregularities in said object's surface.

In a preferred embodiment of the invention, said first spring- element, said second spring element and said connecting element can be a single piece. This avoids the use of connecting parts, as bolts, hinges or other parts, to hold the elements together, which connecting parts may attract dirt and hamper free movement of the spring assembly.

In a more preferred embodiment, said spring assembly may comprise a leaf spring. A leaf spring can provide flexibility in one plane, for example in a plane transverse to the scanning direction of the testing probe unit, while keeping a fixed length in a longitudinal direction of the leaf spring. At the same time, due to a certain width of the leaf spring, a leaf spring can provide additional solidity to the probe holder system and the testing probe unit.

In a still more preferred embodiment, said first spring element, said second spring element and said connecting element can form a single- piece leaf spring. In this way, the previously mentioned advantages of not having connection parts between the different elements, i.e. the first and second spring elements and the connecting element, and the advantages of a leaf spring, as mentioned above, can be combined. In an alternative embodiment, said spring assembly may also comprise a wire spring. A wire spring can provide similar advantages as a leaf spring, except that a wire spring provides less lateral stability than a leaf spring. However, lateral stability can be compensated in many other different ways. Moreover, a wire spring may be less expensive than a leaf spring in that it comprises less material than a leaf spring.

In another alternative embodiment, said first spring element, said second spring element and said connecting element may form a single-piece wire spring, which embodiment can again combine the advantages of not having connection parts between the different elements, i.e. the first and second spring elements and the connecting element, and the advantages of a wire spring, as mentioned above.

It is preferred that the at least one ultrasonic testing probe unit may include a plurality of transducer elements configured to transmit and receive ultrasonic waves. Such a plurality of transducer elements may for example be arranged in a linear array, and is generally known as a phased array. Such a configuration has the advantage over conventional

transducers that an object can be examined under more than one angle without moving the testing probe unit, by using electronic scanning along the length of the linear array.

The at least one ultrasonic testing probe unit may preferably include a wedge including said scanning surface of the testing probe unit configured to engage a surface of said object to be examined. Such a wedge may be designed for direct contact use, or for immersion use with a coupling through for example water.

In an advantageous embodiment, said probe holder frame is substantially U-shaped. Due to its simple symmetrical form, a U-shaped probe holder frame can advantageously combine stability and solidity of the probe holder system while minimizing weight, and thus costs. Other shapes are possible as well, according to the needs of the probe holder system. In a more advantageous embodiment, the probe holder system can comprise at least two spring assemblies arranged on either side of the at least one ultrasonic testing probe unit in said scanning direction. On top of providing an improved stability to the testing probe unit, a second spring assembly also improves the hold of the probe unit's scanning surface on the surface of an object under examination, in particular for irregularities in said surface under examination that are smaller than the scanning surface, in that the pushing force exerted by the first spring assembly may be different from the pushing force exerted by the second spring assembly.

It is preferred that the connecting elements of the at least two spring assemblies may be pivotably attached to said ultrasonic testing probe unit by a single pivot, providing an increased stability to the probe holder system. Alternatively, each of said at least two spring assemblies may be pivotably attached to said testing probe unit by a separate pivot, which preferably connects the testing probe unit to a middle point of the

connecting element as defined above. In both cases, the middle points of the connecting elements of the at least two spring assemblies on either side of the testing probe unit are preferably aligned in the scanning direction.

A probe holder system according to a very advantageous

embodiment of the invention may comprise at least two ultrasonic testing probe units arranged to examine a same object, each testing probe unit comprising a wedge including said scanning surface configured to engage a surface of said object to be examined. Two or more testing probe units can enlarge the viewpoint for scanning the object to be examined, however, the distance between said testing probe units must remain unchanged. It should be noted that the distance between said two testing probe units is defined along a plane in parallel with a plane tangent to the object to be examined at a point in the middle between said two testing probe units, for example a plane tangent to a pipeline weld. It is preferred that said at least two ultrasonic testing probe units can be spaced apart in a direction transverse to the scanning direction. In this way, the probe holder system can be placed such that an object to be examined is situated in between said at least two testing probe units, for example a weld. The testing probe units can be used in the same way, i.e. each of them is arranged to transmit and receive ultrasound waves, or the testing probes can have a separate task, i.e. one testing probe unit is arranged for transmitting ultrasonic waves, and the other for receiving said ultrasonic waves. Any other combination is possible as well.

It is very advantageous that a distance between said at least two ultrasonic testing probe units can be adjustable in a direction transverse to the scanning direction. This feature can provide a relatively flexible probe holder system that can ensure a optimized hold of the testing probe's scanning surface to the surface of the object to be examined, independently of the object's size. The testing probe units can preferably be adjustable independently. The distance between two testing probe units can for example be decreased to substantially zero

In a preferred embodiment, the probe holder system can comprise at least two probe holder frames, wherein each probe holder frame is provided with at least one spring assembly to which at least one ultrasonic testing probe unit is attached. In this way, the construction of various different probe holder systems is simplified and thus made cheaper, and can be independent of the number of testing probe units desired. When more testing probe units are needed, more probe holder frames can be used instead of needing an enlargement of the holder frame or a different design.

The connecting element of each of the at least two spring assemblies can preferably be oriented in substantially a same direction. This feature can provide a similar functioning of said at least two spring assemblies of the at least two testing probe units, which can simplify the combination of data provided by each of the at least two testing probe units. It is preferred that said at least two probe holder frames can be substantially U-shaped, wherein a pair of legs of one of said probe holder frames is oriented towards a pair of legs of another of said probe holder frames, thus simplifying a connection to be made between said two probe holder frames.

It is more preferred that said at least two probe holder frames can be positioned in a staggered way in the scanning direction. Such a

positioning allows a sliding of the legs of one of the U-shaped probe holder frames along the legs of the other U-shaped probe holder frame, leading to a relatively great freedom in the adjustment of the distance between the at least two testing probe units, which can even be brought to substantially touch each other.

It is also desirable that the ultrasonic testing probe unit is positioned asymmetrically in the scanning direction between the legs of the probe holder frame. On the one hand, the asymmetric positioning of the testing probe unit in the U-shaped probe holder frame can ensure an alignment with a second testing probe unit when said second testing probe is similarly positioned in a probe holder frame at a distance in a direction transverse to the scanning direction. On the other hand, said asymmetric positioning of the testing probe unit in the U-shaped probe holder frame allows an identical mounting for a testing probe unit in a U-shaped probe holder frame, as a second unit comprising a testing probe unit mounted in a U-shaped probe holder frame can be obtained only by rotation over 180° of a first unit, such that the fabrication and mounting process of the probe holder frame is identical for both frames in a probe holder system

comprising two probe holder frames and two testing probe units facing each other. In other words, there is not a different "left-handed" or "right-handed" frame.

According to a second aspect of the invention, there is provided a scanning system for examination of pipeline welds, in particular longitudinal welds, characterized by the features of claims 30-32. Such a scanning system can provide one or more of the above-mentioned

advantages.

The present invention will be further elucidated with reference to figures of exemplary embodiments. Corresponding elements are designated with corresponding reference signs.

Figure 1 shows a perspective side view of a preferred embodiment of a scanning system for examination of pipeline welds according to an aspect of the invention;

Figures 2a - 2f show a schematic representation of part of a transverse and a longitudinal cross-section of a pipeline under examination by two ultrasonic testing probe units;

Figures 3a, 3b and 3c show a side, top, and bottom view, respectively, of a preferred embodiment of a probe holder system according to an aspect of the invention, which is mountable in the scanning system of Figure 1;

Figure 4 shows a top view of a second embodiment of a probe holder system according to the invention;

Figures 5a and 5b show a top and bottom view respectively of the probe holder frame of the probe holder system of Figure 4;

Figure 6 shows a side view of the probe holder frame of Figure 5a; Figures 7a and 7b show a top view and a side view of the spring assembly which is attachable to the probe holder frame of Figure 6;

Figure 8 shows a schematic representation of the working principle of the spring assembly in Figure 6.

Figure 1 shows a perspective side view of a preferred embodiment of a scanning system 1 for examination of pipeline welds, in particular longitudinal pipeline welds, sometimes called seam welds, according to an aspect of the invention. The scanning system of Figure 1 comprises a curved frame 2, preferably following the curvature of the pipeline to be examined. A curved frame 2 can for example be adapted to a pipeline having a diameter of 20 cm (8 inch) or more, or for a diameter between 10 - 20 cm (4" - 8"). The frame 2 can also be adapted for a flat surface. To a first end of said curved frame 2, a positioning device 3 is mounted. Said positioning device 3 is configured to keep the curved frame 2 in position while the scanning system 1 can be moved along the pipeline to be examined. The positioning device 3 can for example comprise one or more magnetic wheels 4 ensuring a good hold on the pipeline and an easy moving along that pipeline. The magnetic wheels 4 thus define a scanning direction of the scanning system 1, and in particular of the probe holder system 6. The scanning system 1 further comprises an ultrasonic scanning device (not shown) mountable to said frame in parallel with a weld to be examined, for example to a second end 5 of said curved frame 2. The scanning system 1 also comprises a probe holder system 6 according to the invention. The probe holder system 6 of Figure 1 is configured to hold two ultrasonic testing probe units 10, each comprising a wedge 11 including a scanning surface 12 configured to engage a surface of said object to be examined. The probe holder system 6, in particular the testing probe unit 10, is connectable to said ultrasonic scanning device via at least one probe connector 9. The probe holder system 6 has been mounted to said frame 2 via a mounting structure 7 and an adjustment element 8. The adjustment element 8 can for example be a screw or a bolt or any other adjustment means known to the person skilled in the art, and is arranged such that the probe holder system can be lowered and then fixed to a position in which the probe holder system's spring assembly (see Figures 3a - 3c) is partly compressed providing a good hold of the probe holder system 6 on the pipeline to be examined. The ultrasonic testing probe unit 10 can for example include a conventional transducer configured to transmit and receive ultrasonic waves, but it is preferred that the testing probe 10 includes a plurality of transducer elements, preferably arranged in a linear phased array, which is commonly mounted in a direction transverse to the scanning direction. Other shapes of phased arrays are also possible according to the object to be examined. Such a phased array can typically comprise a transducer assembly with from 16 to as many as 256 small individual elements that can each be pulsed

separately. Transducer frequencies are most commonly in the range from 2 MHz to 10 MHz. A phased array system will also include a sophisticated computer-based instrument (not shown) that is capable of driving the multielement probe, receiving and digitizing the returning echoes, and plotting that echo information in various standard formats. Unhke conventional flaw detectors, phased array systems can sweep a sound beam through a range of refracted angles or along a linear path, or dynamically focus at a number of different depths, thus increasing both flexibility and capability in inspection setups. A phased array system can also use electronic scanning along the length of a linear array probe to similarly create a cross-sectional profile without moving the transducer. As is the case with conventional

transducers, phased array probes may be designed for direct contact use, or for immersion use with coupling through for example water. The preferred embodiment of the probe holder system 6 in Figure 3a includes for example a water inlet 18 to provide water through the wedge 11 to the scanning surface 12 engaging the surface of an object to be examined.

Figures 2a - 2f show a schematic representation of part of a transverse (Figures 2a - 2c) and a longitudinal (Figures 2d - 2f) cross- section of a pipeline 13 under examination by two ultrasonic testing probe units 10. Figures 2a - 2c show part of a transverse cross-section of a pipeline 13 including a longitudinal weld 14. An ultrasonic testing probe 10 is placed on either side of the weld 14. When data from both probes 10 need to be combined, the distance from each probe 10 to the weld 14, as well as the distance between both probes 10 must be accurately known at every instant of measurement. This is possible on a pipeline having a perfectly circular cross-section as in Figure 2a. However, due to surface irregularities of the pipeline 13 under examination, such as for example misalignment (see Fig. 2b), non-circular cross-section (see Fig. 2c), surface waviness, etc., the distance between one of the testing probe units 10 and the weld 14, or between both testing probe units 10 may vary while moving said testing probe units 10 along the pipeline 13 and/or weld 14, rendering difficult or impossible the combination of data from both probes. In case of a

circumferential weld 15, as shown in Figures 2d - 2f, the longitudinal cross- section of the pipeline 13 is normally flat. Therefore, distances between both testing probe units 10 or between the testing probe unit 10 and the weld 15 may still change slightly due to certain surface irregularities, such as in

Figure 2f. For the situation shown in Figure 2e, these distances may or may not remain unchanged depending on the mechanical design of the probe holder system .

Figure 3a, 3b and 3c show a side, top, and bottom view,

respectively, of a preferred embodiment of a probe holder system 6 according to an aspect of the invention, which is mountable in the scanning system 1 of Figure 1. The probe holder system according to the invention intends, among others, to solve or at least alleviate the problems described above with Figure 2. The probe holder system 6 of the preferred embodiment of Figure 3 comprises two ultrasonic testing probe units 10 arranged to examine a same object, each testing probe unit 10 comprising a wedge 11 including the scanning surface 12 configured to engage a surface of said object to be examined. The probe holder system 6 also comprises two probe holder frames 16, wherein each probe holder frame 16 is provided with a spring assembly 17 to which an ultrasonic testing probe unit 10 is attached. The two ultrasonic testing probe units 10 are spaced apart in a direction transverse to the scanning direction 19. A distance between said two ultrasonic testing probe units 10 is adjustable in a direction transverse to the scanning direction 19, via telescopically sliding bars 20. The sliding bars 20 can be fixed in a desired position so that the distance between said two testing probe units 10 is accurately known and does not change during examination of an object. The two probe holder frames are substantially U- shaped, which can also be clearly seen in Figures 4 and 5a - 5b.

Figure 4 shows a top view of a second embodiment of a probe holder system 6' according to the invention comprising one ultrasonic testing probe unit 10 including a scanning surface 12 arranged to examine an object along a scanning direction, a probe holder frame 16 to which said ultrasonic testing probe unit 10 is mounted, and at least one spring- assembly 17 attached to said probe holder frame 16 and configured to push said at least one testing probe unit 10 towards said object to be examined.

Figures 5a and 5b show a top and bottom view respectively of the probe holder frame 16 of the probe holder system 6' of Figure 4. Figures 3b, 3c and 4 show that the ultrasonic testing probe unit 10 is positioned asymmetrically in the scanning direction 19 between the legs 16a, 16b of the probe holder frame 16, i.e. closer to one leg 16b than to the other leg 16a of the U-shaped frame. Especially in case of a probe holder system with a single testing probe unit, as in the second embodiment of Figure 4, the testing probe unit 10 could also be placed centrally between the legs 16a, 16b of the U-shaped frame 16. This asymmetrical positioning of the testing probe unit 10 in the U-shaped frame 16 is particularly advantageous in the embodiment of Figures 3a - 3c, in which a pair of legs 16a, 16b of one of said probe holder frames 16 is oriented towards a pair of legs 16a, 16b of the other of the two probe holder frames 16. In order to provide for a probe holder system in which a distance between the two testing probe units 10 can be minimized, the two probe holder frames 16 are positioned in a staggered way in the scanning direction 19. An advantage of the

combination of an asymmetrically positioned testing probe unit and a staggered positioning of two probe holder frames 16 is the fact that the two testing probe units are aligned in a plane perpendicular to the scanning direction, hence examining the same part of the object. Another important advantage of the combination of an asymmetrically positioned testing probe unit and a staggered positioning of two probe holder frames 16 is the fact that a single design of the probe holder frame 16 can be used for a left-hand probe holder frame and a right-hand probe holder frame. Alternatively, the legs 16a, 16b of the U-shaped frame 16 could also telescopically slide into each other, in which case the advantage of a single design for probe holder frames to be combined would be lost.

Figure 6 shows a side view of the probe holder frame of Figure 5a. At least one spring assembly 17 is attached to said probe holder framelG. The spring assembly is configured to push said at least one testing probe unit 10 towards said object to be examined. The probe holder systems in the preferred embodiments of Figures 3c and 5b comprise two spring assemblies 17 arranged on either side of the at least one ultrasonic testing probe unit 10 in scanning direction 19. The spring assemblyl7 comprises a first spring element 17a, a second spring element 17b, and a connecting element 17c connecting said first spring element 17a and said second spring element 17b and arranged to keep said first spring element 17a and said second spring element 17b spaced-apart. A length of the connecting element 17c can for example be one third of the length of the spring assembly 17 defined as a sum of the lengths of the first and second spring elements 17a, 17b. The connecting element 17c can also be shorter, but preferably have a length of at least one fifth of said length of the spring assembly 17 as defined above. Said connecting element 17c is pivotally attached to said ultrasonic testing probe unit 10. The connecting elements 17c of the at least two spring assemblies, as for example in Figure 5b, can be pivotably attached to said ultrasonic testing probe unit 10 by a single pivot, or each by their own pivot. The ultrasonic testing probe unit 10 is pivotally attached preferably to a middle point 24 (see Figure 8) of said connecting element 17c. The first spring element 17a and the second spring element 17b are fixedly attached to said probe holder frame 16, for example using bolts 21 or screws or any other fixing means passing through a first end of the spring elements 17a, 17b. A second end of the first spring element 17a is connected to a first end of the connecting element 17c. Similarly, a second end of the second spring element 17b can be connected to a second end of said connecting element 17c. Said connecting element 17c is arranged to be movable towards and away from said scanning surface 12. Said connecting element 17c is arranged substantially perpendicularly to the scanning direction 19. Also the first spring element 17a and the second spring element 17b are arranged substantially perpendicularly to said scanning direction 19. The first spring element 17a, the second spring element 17b and the connecting element 17c are arranged in a plane that is transverse to said scanning direction 19. When the probe holder system 6 is placed on an object to be examined, for example on a pipeline, the spring assembly is pre-tensioned such that the connecting element 17c is arranged substantially

perpendicularly to the scanning surface 12. In case of a probe holder system comprising at least two testing probe units 10, as in the embodiment of Figures 3a - 3c, the connecting element 17c of each of the at least two, in casu four, spring assemblies 17 are oriented in substantially a same direction, at least before use. During use of the probe holder system 6, for example in a scanning system as in Figure 1, surface irregularities of the object under examination may cause the connecting elements 17c of the spring assemblies 17 to take up a different direction, as they are mounted and arranged to function in a completely independent way from each other.

Figures 7a and 7b show a top view and a side view of the spring assembly which is attachable to the probe holder frame of Figure 6. In this preferred embodiment of the spring assembly 17, said first spring element 17a, said second spring element 17b and said connecting element 17c are a single piece, in particular, the first spring element 17a, the second spring element 17b and said connecting element 17c form a single-piece leaf spring. Alternatively, the spring assembly 17 is made of two stacked single-piece leaf springs, to meet the requirements of spring force and maximum stroke. Alternatively, said first spring element 17a, said second spring element 17b and said connecting element 17c could also form a single-piece wire spring. In another alternative, said spring assembly 17 could comprise a leaf spring, for example only the first spring element 17a and/or the second spring element 17b could be a leaf spring, connected to the connecting element 17c. Similarly, the spring assembly 17 could comprise a wire spring, for example, only the first spring element 17a and/or the second spring element 17b could be a wire spring, connected to the connecting element 17c. Said connecting element 17c is arranged to be inflexible. In the preferred embodiments shown here, in which the connecting element 17c is part of a leaf spring, and thus intrinsically flexible, this is obtained by adding a blocking element 23 configured to prevent the connecting element 17c from bending. The connecting element 17c can for example be clamped in the blocking element 23, or attached to it in any other way. Said blocking element 23 is rotatably mounted in the at least one ultrasonic testing probe unit 10, for example in a wedge 11 of said testing probe unit 10, or partly or entirely in a housing of said wedge 11. The rotatable mounting of the blocking element 23 in the testing probe unit 10 preferably allows a smooth rotation minimizing play, which is easier with a blocking element 23 having a rather large diameter. The diameter of the blocking element 23 is preferably, but need not be, the same as the length of the connecting element 17c, assuming said length to be measured between the connection points to said first and second spring elements 17a, 17b.

Figure 8 shows a schematic representation of the working principle of the spring assembly in Figure 6. Thanks to the spring assembly's construction, the movement of the connecting element 17c, and more precisely of a middle point 24 of said connecting element 17c, is constrained to a substantially straight line, even when the first and second spring elements 17a, 17b, are pushed to a different positioned. As a consequence, when the testing probe unit 10 is pivotally attached to said spring assembly, preferably to a middle point 24 of said connecting element 17c, an

irregularity in the surface of an object under examination, which may exert a force on the scanning surface 12 of the probe unit's wedge 11, will be compensated by a pivoting movement of the pivot connection 24 and by a rotation of the connecting element 17c while keeping the middle point 24 on a same direction substantially perpendicular to said scanning surface 12 as before. This imphes that the distance to for example a weld under

examination, or to a second testing probe unit remains unchanged, in particular the distance projected on a plane in parallel with a plane tangent to the weld under examination. In the context of this invention, it is noted that the term "substantially perpendicular to said scanning surface" is considered to be equivalent to "parallel to a normal on a plane tangent to the surface of a pipeline at a pipeline weld". This is especially important in case of weld examinations with a probe holder system comprising two testing probe units on pipelines with small diameters.

For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include

embodiments having combinations of all or some of the features described. It may be understood that the embodiments shown have the same or similar components, apart from where they are described as being different.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'comprising' does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words 'a' and 'an' shall not be construed as limited to 'only one', but instead are used to mean 'at least one', and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage. Many variants will be apparent to the person skilled in the art. For example, the first and the second spring elements can also be coil springs, placed in a direction that is substantially perpendicular to the scanning surface. All variants are understood to be comprised within the scope of the invention defined in the following claims.