Patent Publication Number: US-2010131210-A1

Title: Method and system for non-destructive inspection of a colony of stress corrosion cracks

Description:
The invention relates to a method for non-destructive inspection of a colony of stress corrosion cracks in a pipe or vessel. The invention also relates to an inspection system for carrying out a method according to the invention. 
     BACKGROUND OF THE INVENTION 
     Several pipeline failures around the world have been attributed to Stress Corrosion Cracking (SCC) since its discovery in pipelines in the 1960&#39;s including USA, Canada, Russia, France, Saudi Arabia, Australia and South America. While the number of incidents attributed to SCC is less than those attributed to other threats to pipelines such as corrosion or mechanical damage, it constitutes a challenge due to the following reasons: 
     no reliable, accurate and industry-accepted in-line-inspection tools or predictive modelling based tools exist that are capable of determining what locations along the pipeline are affected by SCC; 
     no reliable and widely accepted assessment tools exist for evaluation of SCC, once found; and 
     no reliable and widely accepted tools exist that are capable of measuring the depth of these cracks accurately. 
     Given these limitations, development of an effective SCC mitigation plan and assessment techniques has been slow. Recent developments in Inline Inspection (ILI) technology and increasing understanding of the phenomenon seem to show promise, but the lack of a reliable non-destructive means for measuring crack depths within an SCC colony makes it difficult to develop a comprehensive approach. 
     Currently, there are no standard practices for managing SCC. Most operators capitalize on available literature and their experiences to devise an appropriate SCC management and mitigation plan. These practices utilize hydrostatic testing, ILI or Direct Assessment. 
     In-line Inspection and Direct Assessment require some means of determining the impact of SCC on the integrity of the pipe affected. While several methods have been suggested for calculating the failure pressure, none of the methods have had extensive validation using full-scale burst tests and therefore, are not widely used. 
     Recent results from a joint initiatives program undertaken by Major North American Operators, accepted as part of the recent CEPA Stress Corrosion Cracking Recommended Practices, utilize a system of severity ranking of SCC into four categories. The categories are defined on the basis of predicted failure pressures. The implementation of such an approach, with the exception of hydrostatic testing, requires a means for the determination of the failure pressures and by extension, a means for accurate measurement of crack lengths, crack depths, and the interlinking crack lengths for an SCC colony. 
     Common practice in the industry is to use Magnetic Particle Inspection for the detection of SCC at excavation locations. It enables the measurements of colony dimensions with ease. However, due to the large number of cracks that may be present in a colony, crack specific measurements such as crack lengths, mutual separation and interlinking crack lengths are practical estimates. 
     The application of any of the Failure pressure calculation methods for SCC utilizes crack depth data as well. No widely acceptable, proven and reliable non-destructive technology exists that is capable of measuring the depth (sizing) of SCC. Incremental grinding or buffing remains the most accurate and widely applicable means for sizing SCC as found in the field. The lack of non-destructive sizing technology is also responsible for the lack of a validated SCC evaluation method. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method for non-destructive inspection of a colony of stress corrosion cracks. 
     Accordingly, the invention provides a method for non-destructive inspection of a colony of stress corrosion cracks in a pipe or a vessel, comprising mapping the colony of stress corrosion cracks, identifying at least one individual crack to be sized within the colony, and sizing the at least one individual crack to be sized. Herein “sizing” refers to determining a depth of the crack. This method provides the advantage that the depth of one individual crack within the colony of stress corrosion cracks can be determined without interference from surrounding cracks within the colony. This provides a more accurate representation of the depth profile of the crack(s) identified to be sized within the colony of stress corrosion cracks, thereby enabling the prediction of the remaining strength of the affected pipe or vessel, e.g. the pipe or vessel comprising carbon steel or stainless steel. Moreover, the measured value of the depth of the individual crack allows calculation of the failure pressure of the colony. 
     Preferably the at least one individual crack to be sized is identified on the basis of a predetermined criterion, such as crack length. For example the longest crack may be identified as the crack to be sized. Hence, an operator independent identification of the individual crack(s) to be sized is obtained. 
     Preferably, the predetermined criterion is based on fracture mechanics and/or simulation. Thus, the at least one individual crack to be sized may be chosen such that it is representative for predicting a failure pressure of the stress corrosion cracking affected section of the pipe or vessel, for instance a crack or a group of cracks with a high probability of leading to failure. 
     Preferably, mapping the colony of stress corrosion cracks, identifying at least one individual crack to be sized within the colony, and/or sizing of the at least one individual crack to be sized is performed automatically. Hence a partially or, preferably, fully automated method may be obtained. Thus, the method can be performed autonomously by an inspection system, and may be independent of an operator. 
     In an embodiment, the step of sizing is performed using laser ultrasonics. Laser ultrasonics proves to be highly suited for determining the depth of an individual crack within the colony of stress corrosion crack, due to its small footprint. 
     In an embodiment, the step of mapping is performed using one or more non-destructive examination techniques, such as eddy current, optical imaging, flash thermography and/or radiographic tomography. 
     It will be appreciated that it may be preferred to perform the step of sizing using a different technique from the technique used in the step of mapping. In this way the method for non-destructive inspection of a colony of stress corrosion cracks may be optimized, e.g. with respect to speed, cost, accuracy etc., by selecting the optimum technique for mapping and the optimum technique for sizing. 
     In an embodiment, the method is practised on a volume of solid material that comprises a metal, such as a volume of a metal pipeline, the mapping step comprises performing a method of non-destructive inspection along a surface of the volume of solid material (e.g. electromagnetic inspection), for determining the defect pattern and/or distribution along the surface, and the sizing step comprises performing a method of ultrasonic inspection determining a depth of an individual crack associated with a defect of the defect pattern. By using this method, a rather quick overview of defects can be obtained in the mapping step, whereafter an individual defect or set of defects can be sized with high accuracy in the sizing step. The method may thus integrate the advantages of electromagnetic inspection and ultrasonic inspection into a powerful inspection method. 
     Preferably, the sizing, e.g. the ultrasonic inspection, is performed only over part of the surface that is inspected by the mapping, e.g. the electromagnetic inspection. More preferably, the sizing is performed only adjacent to, close to, and/or near the individual crack(s) to be sized determined by the mapping and subsequent application of a predetermined criterion. In particular, the sizing is performed only for a selection of the defects of the defect pattern determined by the mapping. 
     Preferably, the mapping, e.g. the electromagnetic inspection, and the sizing, e.g. the ultrasonic inspection, are carried out as a combined scan step, for example substantially at the same time or substantially without waiting time between both methods. Each one of such spatial and temporal limitations on performing the sizing and/or the mapping reduce the inspection time of the method according to the invention. 
     In an embodiment, a frequency of the applied electromagnetic field in the solid material in the mapping step is varied at or adjacent to the inspection location. Preferably, a plurality of electromagnetic responses are received at or adjacent to the inspection location, influenced by one and the same part of the volume of solid material. This enables an estimate of the defect depth being larger than a predetermined value, thereby allowing a short listing of defects (cracks) that would not be significant enough to affect the strength of the structure 
     It is another object of the present invention to provide an improved system for carrying out the improved method. 
     Accordingly, the invention provides an inspection system for non-destructive inspection of a colony of stress corrosion cracks in a pipe or a vessel, comprising a mapping detector for mapping the colony of stress corrosion cracks and arranged for outputting mapping data representative of the colony, a processing unit for identifying at least one individual crack within the colony on the basis of the mapping data, and a sizing detector for sizing of the at least one individual crack. 
     Different parts of the inspection system may be separate from one another, for example the mapping detector may be separate from, and may be arranged to function independently from, the sizing detector. Preferably, the mapping detector and the sizing detector are integrated into a single inspection device. 
     In an embodiment, the volume of solid material is comprised by a pipe(line) or a vessel such as a storage tank, and the inspection system comprises positioning means for positioning the mapping detector and/or sizing detector with respect to the pipe or vessel, preferably in two mutually transverse directions with respect to the pipe or vessel. The two mutually transverse directions may be directed in a longitudinal direction of the pipe or vessel and in a circumferential direction of the pipe or vessel, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by, non-limiting, examples in reference to the accompanying drawing, wherein: 
         FIG. 1  shows a schematic representation of an embodiment of an inspection device according to the invention; 
         FIG. 2A  shows an example of a conductivity pattern; 
         FIG. 2B  shows an enlarged view of two other fractures and corresponding distances x and y; 
         FIG. 3  shows an example of bulk ultrasonic signals generated in a solid material by using an exciting laser beam; 
         FIG. 4A  shows schematically an example of a Time-of-Flight-Diffraction (ToFD) measurement method of a fracture that extends into the solid material from a surface of the solid material; 
         FIG. 4B  shows schematically an example of a Crack-Tip-Diffraction (CTD) measurement method of the fracture that extends into the solid material from the surface of the solid material; 
         FIG. 4C  shows a sub-surface fracture; 
         FIGS. 5A and 5B  show scan methods in a top plan view of a surface of the solid material; 
         FIG. 6  shows an example of a sizing detector in an embodiment of an inspection system according to the invention, a pipeline; and 
         FIG. 6A  shows a plot of fracture depth 
         FIG. 7  shows a schematic representation of an example of a method for non-destructive inspection of a colony of stress corrosion cracks according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic representation of an embodiment of an inspection system  1  for non-destructive inspection of a colony of stress corrosion cracks in an object. In this example the object is a carbon steel or stainless steel pipe  2 . The pipe may e.g. be part of a pipeline, such as a pipeline for natural gas or crude oil. The object may also be a vessel such as a column or storage tank, e.g. in a chemical plant. The object in this example thus has a steel wall  4 . 
     The inspection system  1  comprises a mapping detector  6  for mapping the colony of stress corrosion cracks. In this example, the mapping detector  6  comprises an electromagnetic mapping detector comprising a first electromagnetic transducer  8  for applying an electromagnetic field in the steel wall  4 , and a second electromagnetic transducer  10  for receiving a resulting electromagnetic response of the steel wall  4 . The electromagnetic mapping detector  6  is arranged for outputting mapping data representative of the detected electromagnetic response. More in general, the mapping detector  6  is arranged for outputting mapping data representative of a mapped colony of stress corrosion cracks. 
     The inspection system  1  in  FIG. 1  includes a scan frame  12 . In this example the scan frame  12  comprises two clamps  14 A,  14 B which are mounted, e.g. clamped, onto the pipe  2 . The scan frame  12  further comprises a beam  16 . In this example the scan frame  12  comprises first positioning means  18  for positioning the mapping detector  6  in an axial direction of the pipe  2 . In this example, the first positioning means  18  comprise a carriage movably connected to a guide rail of the beam  16 . Here, the first positioning means  18  also comprises a drive unit, such as an electric motor for positioning the first positioning means  18 . Preferably, the first positioning means  18  also comprise a position detector, such as a linear encoder, for determining an absolute or relative position of the carriage with respect to the clamps  14 A, 14 B. 
     The scan frame  12  further comprises second positioning means  20 A, 20 B for positioning the beam  16 , and hence the mapping detector  6  in a tangential direction of the pipe  2 . Thus the mapping detector  6  is positionably mounted to the scan frame  12  for scanning the mapping detector across a surface of the steel wall  4  of the pipe  2 . In this example, the second positioning means  20 A, 20 B each comprise a carriage movably mounted to a circumferential guide rail of the clamp  14 A, 14 B. Here, each of the second positioning means  20 A, 20 B also comprises a drive unit, such as an electric motor for positioning the second positioning means  20 A, 20 B. Preferably, the respective drive units of the second positioning means are mutually synchronised. Preferably, the second positioning means  20 A, 20 B also comprise a position detector, such as a linear encoder, for determining an absolute or relative position of the carriage with respect to a circumference of the pipe  2 . 
     The inspection system  1  further comprises a sizing detector  22 . The sizing detector is arranged for sizing an individual crack within a colony of stress corrosion cracks. In this example the sizing detector comprises an exciting laser  24  for generating a ultrasonic waves in the wall  4 . The exciting laser  24  can be formed by a pulsed laser, for example a pulsed Nd:YAG laser, for example with a wavelength of 1064 nm, a pulse length of 10 ns, a pulse energy of 50 mJ and a pulse repetition rate of 10 Hz. The sizing detector  22  in this example further comprises a detection laser  26  for detecting an ultrasonic response of the wall  4  to the ultrasonic wave generated by the exciting laser  24 . Unlike conventional ultrasonic testing, Laser Ultrasonics has a large frequency bandwidth and a tiny (˜0.5 mm) footprint. These characteristics make it ideally suited for application as a depth sizing tool for closely-spaced cracks in a colony. Laser Ultrasonics provides the ability to size an individual crack within the colony without interference from surrounding cracks within the colony. 
     In this example, the sizing detector  22  is also mounted to the first positioning means  18  and can hence also be scanned across the surface of the steel wall  4  of the pipe  2 . It will be appreciated that it is also possible that the scan frame  12  comprises separate positioning means for the mapping detector  6  and for the sizing detector  22  for independently scanning the mapping detector  6  and the sizing detector  22 . 
     In  FIG. 1  the inspection system  1  further comprises a processing unit  28 . The processing unit  28  is arranged for controlling the first and second positioning means  18 , 20 A, 20 B. Thus, the processing unit  28  can position the mapping detector  6  and/or sizing detector  22  between the clamps  14 A, 14 B. The processing unit is further arranged for receiving the mapping data from the mapping detector  6 . 
     It will be appreciated that the inspection system  1  shown in  FIG. 1  is designed as an in-line-inspection tool, wherein the mapping detector, sizing detector, positioning means and scan frame are integrated. It will be appreciated that the processing unit is communicatively connected to the remainder of the system  1  and can be physically connected to the remainder of the system if desired. 
     The inspection system  1  described thus far can be used in a method for non-destructive inspection of a colony of stress corrosion cracks in a pipe or vessel according to the following first example. 
     In this example, the method starts with the mapping detector  6  scanning at least a part of the wall  4  for mapping the colony of stress corrosion crack. The mapping detector  6  is moved by the first and second positioning means  18 , 20 A, 20 B. The mapping detector  6  may e.g. be moved in the axial direction for a number of consecutive tangential positions. In this example, the mapping detector only maps the wall  4  while moving in one direction for avoiding hysteresis effects. In this way, a defect pattern can be determined by the mapping detector  6 , wherein the defect pattern includes defect locations and/or defect geometries along the surface of the wall  4 . The defect geometry may include a length of the defect, a width of the defect, and/or a shape of the defect. The defects in the defect patter may e.g. be associated with locations where stress corrosion cracks are present in the wall  4 . The mapping data output by the mapping detector  6  may for instance comprise a table of coordinates at which defects have been detected. If the wall  4  comprises a colony of stress corrosion cracks, the mapping detector  6  will also output mapping data representative of the colony. An example of a defect pattern is described below with respect to  FIG. 2 . 
     Then, in this example, the processing unit  28 , analyses the mapping data and identifies the individual crack or cracks to be sized in the colony. The crack(s) to be sized may e.g. be determined on the basis of a length of a defect in the defect pattern determined by the mapping detector  6 . In this example, the processing unit identifies the longest defects in the defect pattern as the individual crack to be sized. Additionally, or alternatively, interaction rules between cracks may be used in determining which crack or cracks in the colony need be sized, or e.g. interlinking crack lengths (lengths between cracks prone to merge). It is also possible that the crack(s) to be sized is (are) determined on the basis of expected relevance of that crack on the local wall strength, e.g. on the basis of fracture mechanics and/or numerical simulation. It is possible to identify the crack(s) with a high probability of leading to failure as the crack(s) to be sized. Also, the processing unit  28  may be arranged to identify a predetermined number of individual cracks to be sized, e.g. the largest crack, or the largest five cracks of the colony. 
     Next, the inspection system  1  positions the sizing detector  22  at or adjacent the crack to be sized and sizes that crack. It will be appreciated that the sizing detector  22  may be moved towards the individual crack to be sized at a greater speed than at which the sizing detector  22  is moved while sizing the crack to be sized. Here sizing the individual crack indicates determining the depth of the individual crack. Examples of how the individual crack can be sized are described with respect to  FIGS. 4A-5B . If a plurality of individual cracks within the colony are to be sized, the inspection system positions the sizing detector adjacent these cracks consecutively. 
     Once the individual crack(s) to be sized has(have) been sized, data representative of the determined crack depth(s) can be stored into memory and/or further processed, e.g. for predicting, e.g. calculating, the failure pressure. It is also possible to measure crack length and/or interlinking crack length using the mapping detector and/or sizing detector, e.g. for calculating the failure pressure. The determined crack depth(s) and/or the calculated failure pressure may provide a quantitative indication of the safety risk posed by the inspected colony of stress corrosion cracks. The failure pressure may give a quantitative indication of the remaining strength of the affected wall  4 . 
       FIG. 7  shows a schematic representation of a second example of a method for non-destructive inspection of a colony of stress corrosion cracks, e.g. in a pipe or vessel, according to the invention. 
     In this example, the method starts with the mapping detector  6 , here an eddy current detector, scanning at least a part of the wall  4  for mapping ( 101 ) the colony of stress corrosion crack. In this way, again a defect pattern can be determined by the mapping detector  6 . Here the mapping detector  6  outputs a table of coordinates ( 102 ) at which defects have been detected. If the wall  4  comprises a colony of stress corrosion cracks, the mapping detector  6  will also output mapping data representative of the colony ( 103 ), e.g. in the form of an SCC colony map. 
     Then, in this example, the processing unit  28 , analyses the mapping data and identifies the individual crack or cracks to be sized in the colony. The crack(s) to be sized are in this example determined ( 104 ) on the basis of a depth of a defect in the defect pattern as determined by the mapping detector  6 . In this example, the processing unit identifies the deepest defects in the defect pattern as the individual crack to be sized. Additionally, or alternatively, interaction rules between cracks may be used in determining which crack or cracks in the colony need be sized, or e.g. interlinking crack lengths (lengths between cracks prone to merge). The processing unit filters ( 106 ) the SCC colony map to remove ( 105 ) less relevant cracks. 
     Next, the inspection system  1  positions the sizing detector  22 , here the laser ultrasonics sizing detector, at or adjacent the selected cracks to be sized and sizes these selected cracks ( 107 ). Once the selected individual cracks to be sized have been sized, data representative of the determined crack depths can be provided as crack depth profiles ( 108 ). Next, the data representative of the determined crack depths can be assessed ( 109 ) and a predicted failure pressure of the SCC colony can be calculated ( 110 ). 
     In general, the method comprises the steps of mapping a colony of stress corrosion cracks, identifying at least one crack within the colony of stress corrosion cracks, and sizing the at least one crack. It will be appreciated that the processing unit  28  may be arranged for identifying at least one individual crack within the colony of stress corrosion cracks which at least one individual crack should be sized using the sizing detector  22 . The processing unit  28  may identify such relevant crack to be sized on the basis of the mapping data received from the mapping detector  6 . It will be appreciated that the processing unit may also comprise a number of separate units, some of which may be integrated with the mapping detector  6  and/or the sizing detector  22 . 
     In the above examples, the inspection system  1  automatically performs the steps of mapping a colony of stress corrosion cracks, identifying at least one crack within the colony of stress corrosion cracks, and sizing the at least one crack. Thus, an automated measurement method is obtained. 
     It will be clear that such automated mapping and sizing provides the advantage that only individual cracks fulfilling objective criteria, i.e. cracks identified as cracks to be sized by the processing unit  28 , will be sized, which reduces inspection time. Also, automatically determining which individual cracks in the colony need to be sized reduces inspection time, as interpretation by an operator is not required, and enhances reproducibility of the inspection. 
     In these examples, the step of mapping the colony of stress corrosion cracks includes positioning the first electromagnetic transducer  8  at or adjacent to an inspection location of the surface of the wall  4 , and applying an electromagnetic field in the wall  4  by using the first electromagnetic transducer  8 . Applying the electromagnetic field, for example as an electromagnetic pulse, may result in an eddy current in the wall  4 . The eddy current will decay and diffuse away from the first electromagnetic transducer  8  into the wall  4 , and create a magnetic field as a resulting electromagnetic response of the wall  4 . 
     In addition, in these examples the step of mapping the colony of stress corrosion cracks includes positioning the second electromagnetic transducer  10  at or adjacent to the inspection location, and receiving the resulting electromagnetic response of the wall  4  using the second electromagnetic transducer  10 . The first electromagnetic transducer  8  may be integrated with the second electromagnetic transducer  10  into an integrated mapping detector  6  as shown in  FIG. 1 . The mapping detector  6  may include a meandering conducting structure integrated on a flexible foil. An example of such an integrated sensor is disclosed in U.S. Pat. No. 5,966,011. 
     Here, the step of identifying at least one crack within the colony of stress corrosion cracks includes inferring from the electromagnetic response an electromagnetic conductivity of the wall  4 . A, e.g. numerical, model for the electromagnetic response may be used wherein the electromagnetic conductivity is a parameter of the model. Such a model is known to the person skilled in the art. By adjusting the parameter that represents the electromagnetic conductivity to match a model response with a measured response, the electromagnetic conductivity can be inferred. 
     A frequency of the applied electromagnetic field in the solid material may be varied at or adjacent to the inspection location. The frequency may, for example, be in a range from 50 kHz to 250 kHz, for example approximately equal to 130 kHz, within a range of ±10%. 
     In these examples, the step of mapping the colony of stress corrosion crack comprises positioning the first electromagnetic transducer, applying the electromagnetic pulse, positioning the second electromagnetic transducer, receiving the electromagnetic response, and inferring the electromagnetic conductivity for a plurality of measuring locations along the surface of the wall  4 . In this way, a defect pattern can be inferred from the conductivity pattern, wherein the defect pattern includes at least defect locations and defect geometries along the surface. A defect geometry may include a length of the defect, a width of the defect, and/or a shape of the defect. The defects in the defect patter may e.g. be associated with locations where stress corrosion cracks are present in the wall  4 . 
       FIG. 2A  shows an example of a conductivity pattern  40 , as a function of a plurality of inspection locations represented by coordinates u, v along the surface of the wall  4 . Dark-coloured regions  42  in the conductivity pattern  40  refer to relatively low conductivity, which are associated with defects constituting the defect pattern  44 . In this example, the defects of the defect pattern approximately coincide with the regions  42  of low conductivity, so that the defect pattern  44  approximately coincides with the conductivity pattern  40 . 
     The step of identifying at least one crack to be sized may include applying an interaction criterium to at least two defects  46  of the defect pattern, for determining whether the at least two defects  46  are interacting. Thereto two distances x and y are defined, that form a box whose opposing corners are approximately coinciding with neighbouring ends of the defects  46 , for example neighbouring fracture tips  48  of two cracks, also termed fractures  46 . 
       FIG. 2B  shows an enlarged view of two other fractures  46  and the corresponding distances x and y. The fractures have a length indicated by l 1  and  12 , respectively. In general, an interaction criterium may be that the fractures  46  are interacting if y≦0.14(l 1 +l 2 )/2 and if x≦0.25(l 1 +i 2 )/2. Such interacting fractures  46  in general increase a risk for extension of the fractures  46  in the wall  4 . 
     In these examples, the step of sizing a crack in the colony of stress corrosion cracks includes performing ultrasonic inspection for determining a size of the defect of the defect pattern in a direction transverse to the surface of the wall  4 , i.e. determining the depth of that crack. The method of ultrasonic inspection may include selecting the defect from the defect pattern and determining the location of the defect from the defect pattern, and generating a bulk ultrasonic signal in the solid wall  4  at a first position adjacent to the location of the defect. Generating the bulk ultrasonic signal may include applying an excitation laser beam at the first position using the exciting laser  24 . 
       FIG. 3  shows an example of the bulk ultrasonic signals, in this example ultrasonic waves  50 A and  50 B, generated in the solid material wall  4  by using the excitation laser beam  52 . The excitation laser beam  52  is applied at a first position  54  on the surface  56  of the wall  4 . A diameter of the excitation laser beam at the first position  54  may be in a range of 0.02 mm to 5 mm, for example 0.1 mm. The bulk ultrasonic waves  50 A predominantly comprise longitudinal waves, while the bulk ultrasonic waves  50 B predominantly comprise transversal waves. In general, surface waves  58  will also be generated 
     In this example, the step of sizing the individual crack further includes measuring a bulk ultrasonic response signal at a second position adjacent to the location of the defect. The bulk ultrasonic response signal originates from the bulk ultrasonic signal by interaction with the defect in the volume of wall  4 . In addition, sizing may include determining at least a first time difference from a moment of generation of the bulk ultrasonic signal at the first position to a moment of arrival of the bulk ultrasonic response signal at the second position, and determining a size of the defect transverse to the surface using at least the first time difference. 
     In general, the sizing may include a Time-of-Flight-Diffraction (ToFD) measurement.  FIG. 4A  schematically shows an example of the ToFD measurement of the defect  46 , for example a fracture  46 , that extends into the wall  4  from the surface  56  of the wall  4 , transverse to the surface  56 . Ultrasonic waves, like the ultrasonic waves  50 A and  50 B in  FIG. 3 , are generated in the wall  4  for example by using the excitation laser beam  52 . The ultrasonic waves are diffracted at a tip  48  of the fracture  46 , and subsequently received at a second position  60 . At the second position  60 , measuring the bulk ultrasonic response signal, in this case the diffracted ultrasonic waves, can be carried out by using a sensing laser beam  62  generated by the detection laser  26 . A diameter of the sensing laser beam at the second position  60  may be in a range of 0.02 mm to 5 mm, for example 0.1 mm. The diffracted ultrasonic waves are an example of a bulk ultrasonic response signal that originates from the bulk ultrasonic signal, for example the transversal ultrasonic waves  50 B from  FIG. 3 , by interaction with the defect, in this example the fracture  46  that extends from the surface  56  into the wall  4 . 
     According to the ToFD method, the size of the fracture  46 , in this example the fracture depth d, can now be determined. In order to do this, a first distance c from the first position to the second position can be determined. In addition, or as an alternative, for example a second distance a is determined from the second position  60  to a fracture location  64 , being an example of the defect location, in this example the position of the fracture  46  at the surface  56 . The first distance c and/or the second distance a can for example be determined by visual inspection, for example by using a ruler. The first distance c and/or the second distance a can also be determined automatically by using ultrasonic waves, for example the surface waves  58 . Determining the second distance a in addition to determining the first distance c can also be omitted, for example by positioning the excitation laser beam  52  and the sensing laser beam  62  such that the first and second position  54  and  60  are at substantially equal distances from the fracture location  64 . Alternatively, the fracture location  64  can for example be assumed to be at substantially equal distances from the first and second position  54  and  60 , while substantially ignoring the actual fracture location  64  between the first and second position  54  and  60 . The depth d determined by the ToFD method is relatively insensitive for an error in fracture location  64  made in this way. 
     In addition, the first time difference from the moment of generation of the bulk ultrasonic signal at the first position  54  to the moment of arrival of the bulk ultrasonic response signal at the second position  60  can be determined. From the first time difference and known or predetermined velocities of the bulk ultrasonic waves  50 A and/or  50 B in the wall  4 , a summed length of a ray path  66 A and a ray path  66 B of respectively the generated bulk ultrasonic signal and the bulk ultrasonic response signal that originates from the bulk ultrasonic signal, can be determined. In order to do this, an orientation and shape of the fracture  46  is assumed. In first order, it is assumed that the fracture  46  is oriented perpendicularly to the surface  56  and has a planar shape. However, if a-priori information about the fracture orientation and shape is available, for example as a result of a dominant stress loading of the wall  4  in use, or as a result of analysing other fractures in the wall  4 , another orientation and/or shape can be assumed. By combination of the summed length of the ray path  66 A and  66 B, the first and second distance c and a, a size, in this case the depth d, of the fracture  46  can be determined. This can be done using mathematical methods known as such to the person skilled in the art. 
     The detection laser  26  is in this example a Nd:YAG Diode-Pumped Solid-State (DPSS) laser, for example with a wavelength of  532  nm and a power of 200 mW. Preferably, the bulk ultrasonic response signal is retrieved from the signal carried by the sensing laser beam  62  by demodulation. Alternatively, or additionally, the bulk ultrasonic response signal is retrieved by using an optical interferometer, that makes use of the sensing laser beam  62 . Alternatively or additionally, the bulk ultrasonic response signal may also be received by using a piezoelectric ultrasonic transducer. 
     In general, the sizing may include a Crack-Tip-Diffraction (CTD) measurement method.  FIG. 4B  schematically shows an example of the CTD measurement method of the fracture  46  that extends into the wall  4  from the surface  56  of the wall  4 , transverse to the surface  56 . In this method, the first position  54  and the second position  60  can substantially coincide. Also, the first distance c is substantially equal to zero. This CTD method can form an alternative or an addition to the ToFD method of  FIG. 4A . Determination of the fracture depth d is carried out analogously to determination of the fracture depth d in  FIG. 4A , with the difference that the excitation laser beam  52  and the sensing laser beam  62  are positioned at the same side with respect to the fracture  46 . 
     Determining the first and second distance c and a may also both be omitted. This can be done for example if the first distance c and/or the second distance a are known, for example by being predetermined. Also, this can be done for example when the fracture depth d is much larger than the first distance c. This can be relevant for example when a fracture  46  has a separation to surrounding defects of the defect pattern that is much smaller than the depth d of the fracture  46 , so that the first and second position are chosen relatively close to the fracture location  64 . 
     The sizing can also include determining a second time difference, from a moment of generation of the bulk ultrasonic signal at the first position  54  to a moment of arrival of another bulk ultrasonic response signal at the second position  60 . The first time difference in general is different from the second time difference if the other response signal related to the second time difference is caused by another part of the defect, for example another fracture tip, than the response signal related to the first time difference. 
       FIG. 4C  shows a sub-surface fracture  46 . The bulk ultrasonic signal travels along ray paths  66 A, and the response signal  66 B and the other response signal are caused by opposing fracture tips  48 . Information about a size of the sub-surface fracture  46 , for example a depth d′ of the sub-surface fracture  46  in a direction perpendicular to the surface  56 , can be obtained by determining a difference between the first time difference and the second time difference. In combination with a distance s from the sub-surface fracture  46  to the surface  56 , and the first distance c, the depth d′ of the sub-surface fracture may be determined or estimated using mathematical methods known as such to the person skilled in the art. 
     In general, methods for determining the depth d of the fracture  46  described with reference to  FIGS. 4A ,  4 B, and  4 C, may be applied for determining depths (s+d′ ) and s related respectively to the first time difference and the second time difference, after which for example the depth d′ of the sub-surface fracture can be determined by determining a difference of the depths related to the first time difference and the second time difference. 
     After determining the fracture depth d or d′, the fracture may be grinded out. The depth of the fracture determined by grinding out can be compared with the fracture depth d or d′ determined as described with reference to  FIGS. 4A ,  4 B, and/or  4 C. 
     When applying the ToFD or CTD method of  FIGS. 4A and 4B , information about the summed length of the ray path  66 A and  66 B, or about one or more of the individual ray paths  66 A and  66 B, may be obtained by combining information of a moment of arrival of a transversal wave and a longitudinal wave from the fracture tip  48 . In particular, by using known or predetermined transversal- and longitudinal wave velocities and assuming that the moment of diffraction of both waves is equal, the length of the ray path  66 B can be determined from another time difference from the moment of arrival of the longitudinal wave to the moment of arrival of the transversal wave. The fracture depth d can now be determined from the second distance a and the length of the ray path  66 B. In this way a determination or verification of the fracture depth d can be carried out. 
       FIGS. 5A and 5B  show scan methods in a top plan view of the surface  56  of the wall  4 , that may be included by the method of ultrasonic inspection. During the scan method of  FIG. 5A , the first and second position  54  and  60  are moved in a first scan direction, indicated by the arrows  68 , substantially parallel to and along the fracture  46 , which extends into the wall  4  from the fracture location  64 . The scan method of  FIG. 5A  is an example of a straddle B-scan. A beam separation equal to the first distance c from the first position  54  to the second position  60  remains substantially unchanged during scanning. By making a straddle B-scan according to  FIG. 5A , a length of the fracture in the direction of the arrows  68  can also be determined. It will be appreciated that if the fracture extends at an angle with respect to the axial direction of the pipe, and/or comprises a bend, this may already be identified in the defect pattern identified by the mapping detector  22 , so that the sizing detector  22  may be scanned along the fracture in the direction in which the fracture actually extends (locally). Thus, the laser may follow the specific path of the fracture. 
     During the scan method of  FIG. 5B , the excitation laser beam  52  and the sensing laser beam  62 , and as a result also the first and second position  54  and  60 , are moved in a second scan direction, indicated by the arrow  70 . This second scan direction is directed transverse to the fracture  46  that extends into the wall  4  from the fracture location  64 . The scan method of  FIG. 5B  is an example of a separation B-scan. The first distance c from the first position  54  to the second position  60  remains substantially unchanged during scanning. 
     During the scan methods of  FIGS. 5A and 5B , at regular positions along the first scan direction  68  respectively the second scan direction  70  an ultrasonic signal is generated and subsequently a bulk ultrasonic response signal that originates from the bulk ultrasonic signal by interaction with, for example by diffraction from, the defect, is measured. Subsequently measured bulk ultrasonic response signals can be plotted as a function of position along the first scan direction  68  and/or the second scan direction  70 , to obtain one or more stacked B-plots. Such a stacked B-plot enables accurate determination of travel times and interpretation of measured signals. 
       FIG. 6  shows an example of a sizing detector  22  in an embodiment of an inspection system  1  according to the invention. In this example, the wall  4  is included by the pipe  2  having the fracture  46 . The surface  56  of the pipeline can be scanned in one or two of the shown scan directions  68  and  70 , which are transverse to one another. In this example, the fracture  46  is one individual crack in a colony of stress corrosion cracks. The orientation of the fracture  46  in the pipe  2  in  FIG. 6  is parallel with a longitudinal direction of the pipe  2 . Alternatively, it can for example also have an orientation perpendicular to a longitudinal direction of the pipeline, or another orientation. The methods described with reference to  FIGS. 4A ,  4 B,  4 C,  5 A, and  5 B are illustrated using a flat surface  56 . However, it will be clear how a curvature of a surface can be taken into account when determining the depth d of the fracture using the methods described with reference to  FIGS. 4A ,  4 B,  4 C,  5 A, and  5 B in conjunction with a curved surface, such as the surface  56  of the pipe  2 . 
     The excitation laser beam  52  and the sensing laser beam  62  are applied at a mutual distance, being equal to the first distance c in  FIGS. 4A ,  4 C,  5 A, and  5 B. This mutual distance can be chosen based on one or more of an expected depth of the fracture  46 , a position of neighbouring fractures to the fracture  46 , and obstacles (not shown in  FIG. 6 ) that may hinder entrance of the laser beams  52 ,  62  to part of the surface  56  adjacent to the fracture  46 . Here the sizing detector  22  is arranged to determine the first distance, for example by using infrared distance measurement, or by ultrasonic means such as by measuring a travel time of an acoustic surface wave. The sizing detector  22  in this embodiment includes a fiber umbilical  72  including optical fibers  74  that guide the laser beams  52 ,  62  from a base station  76 , also being included by the sizing detector  22 . The base station  76  includes the exciting laser  24  and the sensing laser  26 . In this example, the base station  76  also includes a demodulator  78  that is arranged to demodulate a signal received from the sensing laser  26 , from which the bulk ultrasonic response signal can be retrieved. The base station  76  can also be provided with a signal processor for determining the first time difference and/or for determining the depth d of the fracture according to one of the methods described here above. 
       FIG. 6A  shows a plot  80  of fracture depth d against position along the surface along the first scan direction  68 , for example obtained by making a straddle B-scan. A length of the fracture can be also be determined from such a plot. The sizing detector  22  may for example be arranged to provide such a plot  80 . 
     In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. 
     In the examples, the mapping detector is arranged for inducing and detecting eddy currents, however, other techniques are suitable for the mapping detector. The mapping detector may e.g. be designed as an optical imaging apparatus, flash thermography apparatus and/or radiographic tomography apparatus. 
     Due to the small footprint of the exciting laser beam on the wall, laser ultrasonic detection is presently preferred for the sizing detector. Nevertheless, other sizing techniques may be suitable for sizing the individual crack within the colony of stress corrosion cracks. 
     However, other modifications, variations and alternatives are also possible. The specifications, drawings and examples are, accordingly, to be regarded in an illustrative rather than in a restrictive sense. 
     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 then 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 advantage.