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ISSN 1310-3946

“NDT days 2013”/“Дни на безразрушителния контрол 2013”

Year /Година ХXI � Number/ Брой 2 (139) June/Юни 2013

INSPECTION OF PIPES USING LOW FREQUENCY GUIDED WAVE S

КОНТРОЛ НА ТРЪБОПРОВОДИ С ИЗПОЛЗВАНЕ НА НИСКО ЧЕСТОТНИ УЛТРАЗВУКОВИ ВЪЛНИ

КОНТРОЛЬ ТРУБОПРОВОДОВ С ПРИМЕНЕНИЕМ НИЗКОЧАСТОТНЫХ

УЛЬТРАЗВУКОВЫХ ВОЛН

M. Eng. Mirchev Y.N., M. Eng. Alexiev A.R., Institute of Mechanics- BAS (Bulgaria), M. Eng. Shekero A.L., E. O. PATON electric welding institute (Ukraine),

Assoc. Prof. D-r Bukharov S., M. Eng. Yakimovich N., V. A. Belyi Metal Polymer Research Institute of NAS (Belarus).

Introduction A major problem in the petrochemical industry is

corrosion under insulation of pipelines used for transport oil and chemical products. To ensure safe operation of the pipelines it is necessary to constantly monitoring of their wall thickness, as controlled 100% from object volume.

The investigation of pipelines for its integrity by traditional ultrasound is performed in a series of dots on the outside surface of the pipe. Implemented of such control on tube isolated, coated and located underground, as often happens, require access to the outer surface. Accesses to the outer surface are performed by revealing the tube and remove the cover. Then re-placed coverage pipe is laid in the ground and covered. Such an approach is unprofitable, requires a lot of time and money.

Emerging technologies and industries impose 100% volume control and a short time to implement it. Covering these requirements make the application of non-destructive testing more economic and more reliable (if properly implemented control). These requirements define the drawbacks of existing (traditional) ultrasonic methods for control of pipelines and require their development. Of the total volume of the control plants in the oil-chemical industry 70% control of pipelines. This fact, together with the time for control, cost of control, and other factors identified as an important task development and application of new methods for ultrasonic control of pipelines. Purpose of the work: Present of capabilities of the guided waves method for control of pipelines with its advantages, disadvantages, limitations, applications and choice of work mode. 1. Basic principles of guided waves (GW).

A new method for testing the pipeline is through action of low-frequency ultrasonic guided waves acting over large distances in the test object. In this method of control a number of transducers are applied to the outer surface of the control tube in place by approximately one meter over its length. Transducers are arranged successively in a circle and placed on the circumference of the pipe. This method checks about 150m of the length of the tube in either direction from the point of contact of the transducers [1-4]. GW offers an alternative to traditional ultrasound wall thickness of pipes. Figure 1 schematically illustrates different approaches to ultrasonic nondestructive testing of pipelines. Traditional approach (Figure 1, as shown above) is standardized

in EN 14127:2009, approach to implementing GW (Fig. 1, scheme below) is developed at this stage there is no European standard for its application. Individual companies have developed documented procedures for control with GW.

Fig.1 Tradition ultrasonic and GW of pipeline.

In fact, the classic control of materials with normal and

angular transducers according to the requirements of standards EN 581-3 and EN 14127 apply only transverse and longitudinal ultrasonic bulk waves. The propagation of ultrasonic waves with a wide range of action in plates and pipes haves many dispersive modes (their speed is a function of frequency). This can make the GW control of the pipeline quite complex to interpret the outcome results. Relevant to this, it is important for pipeline known geometric dimensions must be selected properly transducers, and their location on the tube and recording equipment. The aim is to obtaining a stable signal (S-image), which can be interpreted correctly. Receipt of stable signal required to select the range of the dispersion curves that meet the following requirements: to propagate ultrasonic waves to a single mode of the test volume, this mode to be with the smallest possible dispersion and to have a directivity of the ultrasonic waves in the desired direction of the controlled volume.

Using GW for control of pipeline at a low frequency, typically less than 100 KHz, the ultrasonic waves are distributed in the following modes: axial symmetrical or flexural non-axially symmetric. Axially symmetric modes are divided into longitudinal and torsional. In Figure 2 are shown three types of ultrasonic waves distributed in pipelines of different modes for control with GW.

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Fig.2 Type of modes for ultrasonic waves propagated in

pipes using GW.

To explain the differences between wave modes of in tubes were adopted the record of the following nomenclature X (n, m). Where X represents one of the three types of modes: L-longitudinal and T-torsional axially symmetric or F-flexural non-axial symmetry. The value of the symbol n represents the harmonic distribution of the particle displacements around the circumference of the pipe and the symbol m gives another number of modes as for Lamb waves in a plate. Axially symmetric modes are always zero value of n characters and no axial symmetrical flexural modes are different than zero values of this character. Schematically in Figure 3 presents the difference between the longitudinal axial symmetrical mode and not axially symmetric flexural modes with n = 1, 2 and 3.

Fig.3 Schemes of wave modes deforms the pipe in various manners depending on mode order.

Longitudinal modes (Figure 2 and Figure 3 above

scheme left diagram) have displacements Ur and Uz, without angular displacement Uθ and are similar to Lamb waves in a plate. They were labeled with L(0, m). Modes L(0,1) and L(0,2) overlap by modes of the A0 and S0 of Lamb waves in a plate. Torsional axially symmetric mode (Fig. 2 average scheme) is only angular displacement Uθ and is similar to a shear horizontal mode (SH) in the plates. This mode is marked with T(0,m). Mode T(0,1) overlaps with mode SH0 of Lamb waves in the plate. The other group is not axially symmetric modes existing in the tube are flexural modes (Fig. 2 below scheme and Fig.3 three right schemes), labeled with F(n,m). Flexural modes pass into longitudinal or torsional modes at higher frequencies. For example, in a high frequency mode F(n,1) passes into the L(0,1), F(n,2) to T(0,1), and F(n,3) in L(0,2). They has displacements of all three components r, z and θ. For visualization of said displacements in the tube of figure 4 is shown the geometry of a tube in a cylindrical coordinate system in which the oscillations of the particles in the material distributed in the ultrasonic wave tube for different modes.

Fig.4 Scheme of pipe geometry in cylindrical coordinate system. 2.Choice and excitation of suitable mode.

In this study we considered the excitation of ultrasonic waves acting over large distances in the pipeline through used piezo transducers. Each of piezo transducers is installed in the phased array ring. They can be managed separately through a multiplexer controlled by software via laptop. This allows in depending on the orientation of the polarization of the ultrasonic waves, and the position of piezo transducers in phased array ring and there number to excite various modes in the piping. Figure 5 presents two possible polarizations of the sound waves (in the axis and perpendicular to it) in depending on the oscillation of piezo plates, located in the antenna array [5].

a)

b)

Fig.5 Polarizations of ultrasonic waves from oscillation of piezo transducers in the axis a) and perpendicular to its b) direction.

When the polarization of the ultrasound waves is axial

direction (particles vibrate along the axis of the tube) and propagation them in the same direction is excite longitudinal axial symmetrical modes. Their excitation can be performed with only one ring phased array of simultaneous operation of all piezo transducers. Then the distribution of the ultrasonic waves will be in both directions along the axis of the placement of the antenna array on the tube. To be directed only in one or other direction using at least two rings phased arrays. Using more than two ring arrays allows eliminating unwanted modes. Often the case when used mode L(0,2) after reflection to transform part of it into a mode L(0,1). It has a lower velocity of propagation of ultrasonic waves than mode L(0,2) in the working range of the dispersion curves above 35 KHz. Not to accept and display along with working mode L (0,2) use more than two rings arrays. This increases the cost of the ultrasound system and made it more difficult, which tends increased with the increase diameter of the test pipe.

Another alternative to using longitudinal axial symmetrical mode are torsional axially symmetric mode T(0,1). Advantage is that almost no dispersion over the entire frequency range and no other axial symmetrical torsional mode. Excites only a one single T(0,1). Excitation and directing torsional axially symmetric mode T is performed by at least two rings arrays with polarization of emitted from piezo transducers ultrasonic wave perpendicular to the axis of the tube. For excitation of the axially symmetric modes is requiring the distance between piezo transducers in phased array antenna to be less than half length of the ultrasonic wave in the respective mode.

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To excite flexural non-axial symmetrical mode in the tube the ring from phased array is divided into sectors. Each sector is activated separately in order to obtain desired component n of the flexural mode. The number of used piezo transducers in the array ring should be larger than the value of n. It is defined by the dispersion curve for the specific pipeline. Considered the mode F(n,m) with a large value of n, which corresponds to the highest frequency at 6db of bandwidth for used piezo transducers.

The spaces between the two arrays and between piezo transducers are selected such that when sequentially activating the individual sectors of arrays to obtain the desired focusing and gain in a particular direction along the axis of the tube. In the other direction the sound waves are cleared (falling in anti phase). Figure 6 presents the amplitude dependence of the frequency of mode L(0,2) propagating in steel tube with a diameter of 305mm and a wall thickness of 9.53mm. Three arrays are used. In one case, the distance between them is 30mm and the other case is 46mm [6].

Fig.6 Dependence of output amplitude from frequency and from distance between array ring form mode L(0,2).

From the results shown in Fig.6 can be optimized

distance between the arrays, depending on the operating frequency of pizo transducers. The aim is to obtain greater amplitude of the signal for the emitted mode. 3. Application of GW.

The main application of the method GW is for control of pipeline. Carry out research in order to apply the method in shipbuilding for control of large sheets of ship's hull. Studies made by the project SHIPINSPECTOR with consortium coordinated by the Institute of Welding UK (TWI). The consortium of the project involved experts from the Bulgarian Society for Non Destructive Testing (BGSNDT). Specialists from BGSNDT are involved in a project INNOPIPES with seven other beneficiary to develop methods for control of pipelines used GW. Other studies have been conducted on the possibilities to apply the method in the control of the rails from the project MONITORAIL with consortium coordinated by TWI.

The application of the control method of the pipe is performed by an ultrasonic system, consisting of: phased array antenna in the form of a ring with a number of transducers, equipment for controlling the phased array antenna (multiplexer) and notebook with program product for displaying the signal in the scan image and control the multiplexer.

Four mass-produced systems for GW are developed and presented to the market at present. These are systems: Plant Integrity Ltd (www.plantintegity.com) of TWI, Guided Ultrasonics Ltd (www.guided-ultrasonics.com), MKC Korea (www.mkckorea.com/english.html) and the new systems from company OLYMPUS only for work with torsional GW.

Subject of discussion in this work are more common systems working with piezo transducers. Piezo plates used in phased arrays are with transverse polarization (Y-cut), so as to

transmit the tangential movement in the pipe. Arrays are in the form of rings arranged in the circumferential direction of the tube and are fixed thereto without contact layer with the pressure. Transmission of the ultrasonic waves from piezo transducers in the pipe without contact is facilitated by the use of low frequencies, which applied in this method. Figure 7 is an array attached to the pipe with the other two components of the ultrasound system from a company Plant Integrity Ltd.

Fig.7. Ultrasonic systems for GW on Plant Integrity Ltd.

4. Advantages and limitations of GW.

The sensitivity of the method to a small discontinuity is reduced and the probability of their detection is smaller than the conventional approaches to measuring deviations in the wall thickness of the pipe. However, this method is more cost effective. For control requires less time and cost is reduced compared to conventional approaches. The profitability of the process is expressed as its advantages compared to other ultrasonic methods for control of pipelines.

One of its advantages is that cover 100% of the controlled volume of the pipelines. When controlled pipelines are not passed section from it’s, which can occur in conventional approaches to the control of the thickness and this will lead to the destruction and fatal consequences.

Another advantage is that it does not require removal of the entire coating on the pipeline or its disclosure if it is below ground. Only need part of it to reveal and prepare to put arrays on its outer surface.

An advantage of the method is also that the control may be performed during operation without the need to interrupt the operation for removal of the product through the pipelines. Control takes place at the surface of the pipeline within from minus 400C to plus 2000C.

Limitations of the method are related to the following characteristics of the manufacture, installation and operation of controlled pipeline: geometry and components, insulation, content of fluid in the pipeline, the pipeline status, noise around the pipeline and others. Figure 8 is presented the influence of some of the features of construction and operation of the pipeline over the distance of the propagation of ultrasonic waves along the length of the pipelines [7].

Fig.8 Influence over the distance of the propagation of ultrasonic waves along the length of the pipelines in dependence from features of construction and operation of pipeline.

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The geometry of the pipe when controlled with GW is significantly affects on distance of the propagation of ultrasonic waves along the pipeline. The method is more effective when the component parts of the pipeline are at a greater distance from each other. The flanges of the conduit determine the end of the controlled section. Other components such as valves, elbows, welded supports and welded pipe sections strongly reduce the scope of control along the length of pipeline. Much of the energy spread of the ultrasonic waves in the pipeline is reflected by them. Flexural mode has more significantly reflection from the components of the pipeline from longitudinal axially symmetrical.

Coverage as bitumen, polyethylene and others is a factor of increasing attenuation of ultrasonic waves distributed in such a pipeline. In such case are uses transducers with possibly lower operating frequencies. Another case is folded under the ground or concrete. Then the sound waves of some mode are passing in soil or concrete and distributed energy ultrasound waves significantly weakened. In such case are selected the appropriate working mode for the specific conditions of the closure of the pipeline.

Consists of gas or non-viscoelastic fluids in controlled pipeline do not affect on distributed ultrasonic waves. Viscoelastic fluids or corrosive deposits substantially reduce the size of the range of propagation ultrasonic wave distributed in the walls of the pipeline. Depending on the polarization of the ultrasonic waves effect on the viscoelastic fluid can be neglected. For this purpose is used mode with transverse polarization of the ultrasound waves in the direction of their propagation. Such is torsional axially symmetric mode T(0,1).

The noise from the surrounding working machinery is usually in low frequency range. On this low frequency range work the transducers for GW. This could compromise the test results and to obtain false statements about controlled pipeline.

The status of the controlled pipelines also has an influence on the size of the length which is controlled by one position of the arrays put on the surface of pipeline. Condition of pipeline by 10% -20% reduction of the wall thickness of the general corrosion substantially reduces the size of the length of the controlled distance. When general corrosion is stronger and reduces of wall thickness is about 40% and it is impossible to continue monitoring the section of the pipeline after the corrosion damage. In this case, the arrays are placed after the corroded surface of the pipeline in order to further control in area after corrosion.

Depending on the characteristics of the manufacturing, construction and operation of the pipeline will be selected appropriate work mode and frequency, the selection takes into account the sensitivity of mode to the type, orientation and size of demand discontinuity. 5. Sensitivity of the GW to the discontinuity and components of the pipeline.

Effect on the sensitivity of the detection method and the subsequent identification of a discontinuity or a component of a pipeline has the transformation of emitted mode to other mode. For example, if torsional axially symmetrical mode and is reflected by an axially symmetrical components of the pipeline, such as flanges, welded joints or end of a tube, then only the axially symmetric modes are reflected.

When the component parts of the pipeline mounted thereon are not axially symmetrical and/or have a region on it with developed general corrosion, then in reflection of them is generated flexural not axially symmetric modes F(n,m). They are received from transducers. When emitted mode L(0,2) and reflected from not axially symmetrical sectors damaged from general corrosion most common transformed modes are F(1,3) and F(2,3). They have similar ultrasonic velocities with mode L(0,2). Possible transform modes depend on the degree of asymmetry, or the location of the discontinuity in the

circumferential direction of the pipe. On figure 9 are shows the reflection coefficient of the discontinuity type through-thickness notch placed in sector of the circumference of the pipeline of the emitted mode L(0,2) and transformed modes F(1,3), and F(2,3). Such discontinuity is used to adjust the sensitivity of the ultrasound system. The sector of the circumference of the pipe is shown in %. Other direction of the discontinuity along the axis of the pipeline does not affect at the result of the reflected coefficient for this type of reflector. The results in fig.9 are for pipe with diameter 76mm [8-10].

Fig.9 Measured and predicted reflection coefficients for a through-thickness notch in a 76mm, schedule 40 steel pipe at 70 kHz as a function of the circumferential extent of the notch. L(0,2) mode input.

The location of discontinuity to circumferentially to a

quarter of the circumference of the pipe gives almost the same values of the amplitudes of the reflected modes L(0,2), and F(1,3). This can be used to conclude that the discontinuity cover under ¼ of the size of the circumference of the pipe. The same effect was observed when working with mode T(0,1), as there transformed reflected mode is F(1,2).

The sensitivity of the method to discontinuity type notch is a function of frequency. It decreases with a decrease in the frequency, but not as Rayleigh attenuation. Figure 10 presents the calculation of coefficient of reflection of mode T(0,1) of the full circle notch with different depth for pipe with diameter 152mm and 610mm [8-10].

Fig.10 Finite element prediction of torsional mode reflection coefficient from axially symmetric crack in 152mm schedule 40 steel pipe as a function of crack depth. Also shown are results for 610mm pipe at 10 kHz and 50 kHz.

From fig.10 is seen that when the frequency decreases,

the coefficient of reflection for the smaller depths of the notch also reduces. Depth of the notch below then 10% of the wall thickness is almost impossible to be registered.

Used frequency has affects also on the length of distance in a controlled pipeline. For greater sensitivity using higher

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operating frequency, but this reduces the length of controlled distance in the pipe. For example, control pipeline to a distance of 5m in a single placement of the arrays, may be used an ultrasonic system with an operating frequency up to 1MHz. The high frequency gives better resolution and a probability of detection of discontinuity, but reduces the size of the action of ultrasonic waves in a control pipe. Depending on the requirements and purpose of control are selecting an operating frequency for control. Conclusions - Submitted possibilities of the GW with its advantages and limitations; - Have been considered directed excitation on some fundamental modes, most often used to control the pipelines; - An analysis of selection of mode depending on the controlled discontinuity and characteristics of manufacturing, installation and operation of the pipeline; - Analyzing the sensitivity of the modes L(0,2) and T(0,1) to the discontinuity type notch. This work was supported by Marie Curie Actions, International Research Staff Exchange Scheme, grants number: PIRSES-GA-2012-318874 of the FP7, project titles: INNOVATIVE NONDESTRUCTIVE TESTING AND ADVANCED COMPOSITE REPAIR OF PIPELINES WITH VOLUMETRIC SURFACE DEFECTS. References: 1. Alleyne, D.N. and Cawley, P. (1997) 'Long range propagation of Lamb waves in chemical plant pipework', Materials Evaluation, Vol 55, pp504-508. 2. Alleyne, D.N., Pavlakovic, B.N., Lowe, M.J.S. and Cawley, P. (2001) 'Rapid, long range inspection of chemical plant pipework using guided waves', Insight, Vol 43, pp93-96.

3. Paton B. E., Troitsky V. A., Gorbik V. M., Shvydkiy S. A, Works on low-frequency ultrasonic testing of pipelines of the E. O. PATON electric welding institute, The 2nd South-East European IIW International CongressWelding, High-tech Technology in 21st century, Pipeline welding: current topic of the regionSofia, Bulgaria, October 21st-24th 2010, p. 154-159. 4. Jackson P., Mudge P., Daniel I., Pipeline Corrosion Control: a historical perspective and a Guided Wave approach to the future, The 2nd South-East European IIW International CongressWelding, High-tech Technology in 21st century, Pipeline welding: current topic of the regionSofia, Bulgaria, October 21st-24th 2010, p. 118-12. 5. Alleyne, D.N. and Cawley, P. (1997) 'Long range propagation of Lamb waves in chemical plant pipework', Materials Evaluation, Vol 55, pp504-508. 6. Alleyne, D.N., Cawley, P., Lank, A.M. and Mudge, P.J. (1997) 'The Lamb Wave Inspection of Chemical Plant Pipework', Review of Progress in Quantitative NDE, Vol 16, DO Thompson and DE Chimenti (eds), Plenum Press, New York, pp1269-1276. 7. Trimborn, N., Heerings,J. and Herder, den Adriaan, "Inspection effectiveness and its effect on the integrity of pipework ", 4th ME NDT conference, 2007. 8. Alleyne, D.N., Lowe, M.J.S. and Cawley, P. (1998) 'The reflection of guided waves from circumferential notches in pipes', ASME J Applied Mechanics, Vol 65, pp635 641. 9. Lowe, M.J.S., D.N. Alleyne and P. Cawley, “The mode conversion of a guided wave by a part circumferential notch in a pipe”, J. Appl. Mech., Vol. 65, 1998, pp.649-656. 10. Demma, A., P. Cawley, M.J.S. Lowe, A.G. Roosenbrand and B. Pavlakovic, "The reflection of guided waves from notches in pipes: a guide for interpreting corrosion measurements", NDT & E International, Vol. 37, 2004, pp.167-180.