OPCOM Publishable Final Reportv1 - CORDIS · shore structures such as oil production platforms and...

Nanotechnologies and nanosciences, knowledge based multifunctional materials, new production processes and devices

(NMP)

Specific Targeted Research Projects (STREP)

NMP2-CT-2005-516993

OPCOM

DEVELOPMENT OF ULTRASONIC GUIDED WAVE INSPECTION TECHNOLOGY FOR THE CONDITION MONITORING OF OFFSHORE

STRUCTURES

Publishable Final Activity Report

Document: OPCOM/FIN/TWI/PJM/201109/0 Period covered: June 2005 - December 2008 Date of preparation: November 2009 Start date of project: 1 June 2005 Duration: 43 months Project co-ordinator: Peter Mudge Revision No: 0 Project co-ordinator organisation: TWI Ltd

OPCOM Project Final Publishable Report _________________________________________________________________________________

_____________________________________________________________________________________________________ OPCOM/FIN/TWI/PJM/201109/0 Contract No. NMP2-CT-2005-516993 Copyright © OPCOM Consortium 2009

Executive Summary

The objective of the OPCOM project is to design and build a prototype system, using Ultrasonic Guided Wave (UGW) Technology to continuously monitor the integrity of off-shore structures such as oil production platforms and wind turbine towers. The majority of the offshore wind turbine towers are at the start of their life cycle (currently being installed in their thousands) whilst the offshore oil and gas platforms are at the end of their life cycles (between 25-40 years old). Both are in urgent need of inspection. The development of Ultrasonic Guided Wave (UGW) Technology for inspecting offshore structures has the potential to impact upon the life cycle cost of vital components of the infrastructure through: 1. Continuous monitoring of structural integrity from fabrication to scrapping. 2. Extending the life of offshore structures, therefore reducing depletion of resources for new builds. Such structures are located in very hazardous sea conditions, so, to inspect them, a fully ruggedised system is to be developed. Moreover, several parameters had to be investigated, not only for the design of the equipment, but also for the property of the wave to be generated. The size and the complexity of these structures mean that none of the UGW systems available on the market could be used for condition monitoring. Results The main project results in this report include:- The determination of the issues of relevance to the operators of offshore wind farms

and oil and gas production facilities.

The development of techniques for examination of offshore wind turbine and oil production structures.

Development and production of the prototype equipment.

Results of the field tests carried out.

The dissemination activities.


_____________________________________________________________________________________________________ OPCOM/FIN/TWI/PJM/201109/0 Contract No. NMP2-CT-2005-516993 Copyright © OPCOM Consortium 2009

Contents

1 Project execution 1

1.1 Objectives 1

1.2 Project Partners 1

1.3 Approach 2

1.4 Specifications 2 1.4.1 Objectives 2 1.4.2 Structures for study 2

1.5 Technique development 4 1.5.1 Objectives 4 1.5.2 Techniques 4

1.6 Apparatus 5

1.7 Offshore trials 8

2 Dissemination and use 9

2.1 List of Publications 11 Appendix A - Conference Paper


_____________________________________________________________________________________________________ OPCOM/FIN/TWI/PJM/201109/0 1 Contract No. NMP2-CT-2005-516993 Copyright © OPCOM Consortium 2009

1 Project execution

1.1 Objectives

The scientific and technical objectives are: To develop Ultrasonic Guided Wave (UGW) technology as a qualitative and quantitative

tool for detection of fatigue cracks and corrosion/defects in tubular members. To develop permanently mounted marinised UGW Sensors and Systems that can be

deployed and fixed permanently onto the tubular members of offshore structures. To develop UGW sensors and systems that will remain permanently in place for

decades needing only occasional maintenance (say every 5 years). To develop the above techniques with automated defect recognition capabilities so that

the operator subjectivity that is present in all current NDT techniques is eliminated.

The economic objectives are: A significant reduction in the cost of underwater inspection of tubular support structures

due to the elimination of the need for removal of marine growth and the removal and reinstatement of coatings and elimination of the use of divers.

A reduction in the number of spillages caused by leaks from offshore oil and gas platforms due to cracking and corrosion and subsequent product losses and clean-up costs.

A reduction in inspection time and thus reduction in loss of production during inspection.

The social objectives are: Elimination of hazard due to diving accidents while inspecting offshore structures. Elimination of labour intensive and monotonous underwater inspection tasks. Reduction in operator stress and error caused by the need for great attention to detail

and NDT process variability. Reduction in exposure of contract workers to danger.

The environmental objectives are: Elimination/reduction of spillages of hazardous fluids from offshore oil and gas platforms

and contamination of seas and pollution of natural habitats. The project will also encourage the use of renewable wind power by developing safe

methods of wind turbine tower inspection.

The EC policy objectives This project supports the high priority given to the prevention of oil pollution from offshore platforms and facilities by the EC. In addition to the Petroleum Act 1998, other legislation, for example the Prevention of Oil Pollution Act 1971 puts additional controls on the discharge of oil or contaminated water. Prevention and control is required by EU Directive 96/61/EC. The offshore Petroleum Production and Pipelines (Assessment of Environmental Effects). Regulations 1999 came into force on March 1999. These regulations implement the European Council Directive on the Assessment of the Effects of Certain Public and Private Projects on the Environment (85/337/EEC as amended by Council Directive 97/11/EC) in so far as it relates to the effects of the environment of certain offshore oil and gas projects. The new EU Liability Directive makes companies and individuals liable for damage they cause to the environment. This project supports the above EC policies.

1.2 Project Partners

TWI Ltd TWI United Kingdom Advanced Technology Group ATG Czech Republic Coaxial Power Systems Ltd CPS United Kingdom DONG Energy A/S DONG Denmark Health and Safety Executive HSE United Kingdom Instituto de Soldadura e Qualidade ISQ Portugal



Kaunas University of Technology KTU Lithuania Kingston Computer Consultancy Ltd KCC United Kingdom London South Bank University LSBU United Kingdom NDT Consultants Ltd NDTC United Kingdom Statoil AS STA Norway Zenon SA ZNN Greece This Specific Targeted Research Project (STREP) is co-ordinated and managed by TWI Ltd. and is partly funded by the EC under the Nanotechnologies and Nanosciences, Knowledge Based Multifunctional Materials, New Production Processes and Devices (NMP) programme, Contract Number: NMP2-CT-2005-516993 The lead partner is TWI Ltd Granta Park Great Abington Cambridge, CB21 6AL UK Tel: +44 1223 899000 Project Coordinator: Peter Mudge [email protected] Project web site: www.opcom.eu.com

1.3 Approach

The project was divided into a number of work packages, as described below: Title Lead Partner WP A Specifications TWI WP B Technique Development KTU WP C Development of lab equipment KCC WP D System integration CPS WP E Prototype system build ZNN WP F Oil platform field trials STA WP G Wind turbine field trials DONG WP H Project Management TWI

1.4 Specifications

1.4.1 Objectives

To determine needs of end users in offshore oil production and wind energy and of safety authorities.

To provide essential input parameters, including inspection objectives, range of tube diameters and wall thicknesses and discontinuities for development of test techniques.

To design mock-ups for parametric studies in the laboratory.

To procure structures for field trials offshore and in a diver training tank.

1.4.2 Structures for study

The following were selected as appropriate components for study: ‘Monopod’ tubular foundation piles for offshore wind turbines, Figure 1. Secondary structure on steel offshore oil and gas platforms, such as conductor

guide frames or caissons, Figure 2. Drilling riser sections, Figure 3.



Figure 1. 4.7m diameter by 30m long wind turbine foundation pile

Figure 2. Caissons inspected on the Sleipner ‘B’ platform

Caisson 2

Caisson 1



Figure 3. Example section of drilling riser

1.5 Technique development

1.5.1 Objectives

To determine the essential procedure parameters influenced by the given input parameters.

To investigate, both with numerical models and experimentally, the propagation of guided waves through tubular frames above and below water.

To investigate, both with numerical models and experimentally, the propagation of guided waves along large diameter, thick-walled cylinders above and below water.

To investigate the interaction of guided waves with cracks, corrosion and flooded members.

To determine the environmental parameters that will affect system performance throughout its life-cycle.

1.5.2 Techniques

Both modelling and experimental techniques were used to determine appropriate conditions for examination of the selected components. These were backed up by controlled field tests to establish the applicability of the equipment and procedures developed. Figure 4 shows an early test on a full scale wind turbine foundation pile using the laboratory array of ultrasonic transducers. This work was used to identify the basic requirements for the final tools used for the field tests. Figure 5 shows a basic CAD design for the field test system based on the initial full scale tests.



Figure 4. Initial tests of the low frequency ultrasonic array on a full scale wind turbine pile

Figure 5. Initial concept for the field tool, based on tests as shown in Figure 4.

1.6 Apparatus

This was of two kinds; a ruggedised version of the curved linear array, of the kind shown in Figure 5, and a marinised device suitable for deployment on oil and gas installations either in the splash zone or sub-sea. The linear array, designed by partner Zenon, was developed specifically to be attached to the top edge of the large tubular piles used as the foundation of the offshore wind turbines. The prototype system is shown in Figure 6.



Figure 6. Final Wind turbine pile testing tool (mounted on test block) For the oil and gas applications, a combined tool was designed, in which an encircling collar could be placed around the tubular components to be tested to attach the test transducers, with the controlling electronics in an integrated watertight compartment. This is shown (with outer protective cover removed) in Figure 7.

Figure 7. CAD representation of the marinised collar with integrated electronics (cover removed)

To assess the longevity of the tool in a marine environment, the tool was subjected to a 2000 hour salt spray test. The chamber and tubular specimen is shown in Figure 8. The



chamber provided a constant mist of salt laden spray over the specimen for the duration of the test.

Figure 8. Salt spray chamber with the test specimen in situ. Note the lead for the ultrasonic signals emerging from the chamber, so that signals could be monitored without interrupting

the test

A key factor in long term monitoring is the stability of the test system. This salt spray test was designed to establish if the ultrasonic tool would produce a stable output under the simulated marine conditions so that changes due to the presence and growth of defects in the structure could be detected. The results are shown in Figure 9.

Tool 002 60kHz L F axi-sym with additional data

0

100

200

300

400

500

600

0.02

0.35

0.67 1

1.32

1.65

1.97 2.3

2.62

2.95

3.27 3.6

3.92

4.25

4.57 4.9

5.22

5.55

5.87 6.2

6.52

6.85

7.17 7.5

7.82

8.15

8.47 8.8

9.12

9.45

9.77

10.1

m

mV

Week 0

Week 1

Week 2

Week 3Week 4

Week 5

Week 6

Week 9

Week 10

Week 11

Week 12

Figure 9. Data from 2000 hours salt spray test



The high repeatability of the signals is encouraging, in that a straightforward gain adjustment may be made to align successive signals to enable them to be directly compared. This indicates that the ability to detect degradation in the structure is maintained. However, there will be a point when the signal level becomes sufficiently low that sensitivity will be impaired.

1.7 Offshore trials

A key element of this project was to demonstrate the performance of the test system offshore. Tests were carried out on the Statoil Hydro Sleipner ‘B’ production platform in the Norwegian sector of the North Sea (Figure 2). The Sleipner ‘B’ production platform was selected for these tests as it was undergoing major modifications and consequently there were both an accommodation barge for personnel alongside the platform and access via scaffolding to parts of the platform which would normally be inaccessible. The platform is usually unmanned. The components tested were two large tubular caissons which were part of the fire water supply system for the platform. Each was 26” (660mm) in diameter, 17.48 mm thick and approximately 63m long, see Figure 2. These were selected for study as they were both known to contain corrosion, which allowed the capability of the system to detect this damage to be assessed. The top of each caisson was a flange at the underside of the main platform deck, where it was bolted to the platform pipework. The tubulars ran vertically downwards into the sea and were supported by guide brackets at the ‘spider deck’ level. Sea level was approximately 34m below the flange, so that the lower 29m of each caisson was submerged. The bottom end of each caisson was open, so that the caissons were free-flooding. The test tool was placed around the each tubular at the location shown in Figure 2. Access was from a scaffolding platform under the main deck. The results are shown in Figures 11 and 12 for caissons 1 and 2 respectively.

a)

b)

Figure 11 Caisson 1: a) longitudinal schematic and b) area plot of indications reported It may be seen on the area plot in Figure 11b that the weld signals may be easily distinguished from more localised corrosion by the fact that the responses extend around the full circumference. The top part of the line, above the sea level, shows some minor indications of corrosion. All these signals are relatively small and all these indication seem to be smaller than 5% cross section area (CSA) as they are below the dotted DAC line. The splash zone area is more corroded and actually presents a strong indication around 34m but further analysis is required as the pipe is apparently thicker in this area than in the others.



Finally, the bottom part of the line has number of indications of various amplitudes. Nevertheless, it is possible to tell that this caisson looks corroded all the way down but that none of these indications exceed 15% CSA loss.

a)

b)

Figure 12. Caisson 2: a) longitudinal schematic and b) area plot of indications reported The general pattern of the responses from this caisson is very similar to caisson 1. There are only minor indications (less than 9% CSA loss) down to the splash zone. Again, the splash zone has more corroded areas. There were more high priority category 3 indications on this caisson, and the lower amplitude of response from the far end indicated that there was more metal loss (causing more losses of the transmitted signal) than for caisson 1.

The deepest part of the caisson seems to be more corroded as several indications appear as category 3 responses (shown in red on Figure 12). The noise level on this caisson is also higher than caisson 1 which is a sign of greater corrosion.

2 Dissemination and use

A demonstration of the capability of long range ultrasonic inspection and condition monitoring for the Norwegian oil and gas industry was carried out at the Statoil Mongstad Refinery, Norway. The demonstrations were carried out on 12” and 10” Pipework. Representatives from Aker Kvaerner, Statoil and others were present. The purpose was to demonstrate how these tests can be applied and to show the performance attained. An invited paper was given to the 5th International Conference on Condition Monitoring and Machinery Failure Prevention Technologies, held in Edinburgh, Scotland on 15-18 July 2008. A session was arranged which was devoted to long range ultrasonic testing and the paper ‘Long term monitoring of offshore installations using ultrasonic guided waves’1 was central to this. The Paper itself is included as Appendix A. Second, a seminar was organised by TWI on 28 October 2008 to present the findings of this and related projects. There were 43 delegates present.

1 Mudge PJ, Chan S, Kayous M (TWI Ltd), Andersen E, and Andersen JKF (DONG Energy)



Table 1 Overview of the dissemination of knowledge

Planned/Actual Dates

Type Type of Audience

Countries Addressed

Size of Audience

Partner Responsible/ Involved

December 2005 onwards

Project website1 Operators

Maintenance contractors

SME Inspection Companies

Worldwide 5,000 TWI

January 2007 Anglo-French Physical Acoustics Conference2

Physical acoustics professionals

European/ Worldwide

200 LSBU

June 2007 World Conference on Engineering Asset management3,4

Owners & operators of major plant.

NDT industry professionals

Worldwide 100+ TWI

LSBU

June 2007 OMAE conference2 5



Worldwide 200 TWI

November 2007 16th SIMoNET Meeting, SIM in Civil Infrastructure6

NDT and structural integrity professionals, civil engineers

Predominantly UK

50 LSBU

January 2008 Anglo French Physical Acoustics Conference7

Physical acoustics professionals

European/ Worldwide

200 LSBU

July 2008 CM 2008 and MFPT 2008 8,9



Worldwide 100+ LSBU

TWI

DONG Energy

July 2008 Review of Progress in Quantitative NDE 200810


Worldwide 100+ LSBU

October 2008 TWI Technical seminar11



Predominantly UK

43 TWI

DONG Energy

2 Offshore Mechanics in Arctic Engineering - now one of the premier conferences for offshore engineering.



July 2009 CM 2009 and MFPT 2009 12



Worldwide 100+ LSBU

July 2009 Review of Progress in Quantitative NDE 200913


Worldwide 100+ LSBU

It is planned that publications and presentations of this type will continue in the future. In addition to the conference presentations listed above, there have also been a number of journal papers arising from the modelling carried out by Kaunas Technical University 14-16.

2.1 List of Publications

1. www.opcom.eu.com 2. V. Zernov, L.J. Fradkin, ‘Modelling losses in the layered steel/grout/steel structures,

with applications to modelling inspection of the wind turbine monopod’ Anglo-French Physical Acoustics Conference 2007 (AFPAC 2007) 17 - 19 January 2007, Fréjus, France

3. P. Mudge, ‘Condition monitoring of engineering assets using ultrasonic guided waves’ World Conferernce on Engineering Asset Management,11-14 June 2007, Harrogate, UK

4. V. Zernov, L.J. Fradkin, ‘Modelling losses in the layered steel/grout/steel structures, with applications to modelling inspection of the wind turbine monopod’, World Conferernce on Engineering Asset Management,11-14 June 2007, Harrogate, UK

5. GR Edwards, T-H Gan, ‘Detection of corrosion in offshore risers using guided ultrasonic waves’, OMAE Conference, San Diego, 10-15 June 2007.

6. V. Zernov, L. J. Fradkin, ‘Modelling losses in the layered steel/grout/steel structures, with applications to modelling inspection of the wind turbine monopod’, 16th SIMoNET Meeting: Structural Integrity Monitoring in Civil Infrastructure, 14 November 2007, NDE Centre, Dept of Mechanical Engineering, UCL, London WC1E 7JE

7. V. Zernov, L. J. Fradkin ‘Guided waves in overlapping grouted plates immersed in water’, Anglo French Physical Acoustics Conference 2008 (AFPAC 2008) 16 - 18 January 2008, Ashford, Kent

8. V. Zernov, L. J. Fradkin ‘Guided waves in overlapping grouted plates immersed in water’, Fifth International Conference on Condition Monitoring and Machinery Failure Prevention Technologies, 15–18 July 2009, Edinburgh, Scotland

9. P. Mudge, S. Chan, M. Kayous, E. Andersen, JKF Andersen, ‘Long term monitoring of offshore installations using ultrasonic guided waves’, Fifth International Conference on Condition Monitoring and Machinery Failure Prevention Technologies, 15–18 July 2009, Edinburgh, Scotland

10. L. J. Fradkin, V. Zernov, ‘Modelling Propagation of Ultrasonic Guided Waves in the Layered Steel/Grout/Steel Structures’ Review of Progress in Quantitative NDE 2008, 20-25 July 2008, University of Illinois - Chicago (UIC), Chicago, Illinois

11. P. Mudge, S. Chan, M. Kayous, E. Andersen, JKF Andersen, ‘Long term monitoring of offshore installations using ultrasonic guided waves’, TWI Technical seminar, Abington Cambridge, 28 October 2008

12. L.J. Fradkin, V. Zernov, ‘Modelling Propagation of Ultrasonic Guided Waves in the Layered Steel/Grout/Steel Structures’, Sixth International Conference on Condition Monitoring and Machinery Failure Prevention Technologies, 23–25 June 2009, Dublin, Ireland.



13. V. Zernov, L.J. Fradkin, ‘Modelling Propagation of Ultrasonic Guided Waves in the Asymmetric Layered Steel/Grout/Steel Structures’, Review of Progress in Quantitative NDE, July 26-31 2009, University of Rhode Island, Kingston, RI

14. G. Butėnas, R. Kažys, ‘Application of time reversal approach to focusing of Lamb waves’ Ultragarsas (Ultrasound) - the Journal of the Lithuanian Society of Non-Destructive Testing, 62 (2007), No 2, pp 38-43

15. Maciulevičius, R. Kažys, L. Mažeika ‘Investigation of phased arrays for guided waves applications’, Ultragarsas (Ultrasound) - the Journal of the Lithuanian Society of Non-Destructive Testing, 62 (2007), No 2, pp 51-55

16. Maciulevičius, R. Kažys, L. Mažeika ‘Influence of the object edge on efficiency of Lamb wave excitiation’, Ultragarsas (Ultrasound) - the Journal of the Lithuanian Society of Non-Destructive Testing, 62 (2007), No 4, pp 28-31



Acknowledgement

OPCOM is a collaboration between the following organisations:- TWI Ltd Advanced Technology Group Coaxial Power Systems Ltd DONG Energy A/S Health and Safety Executive Instituto de Soldadura e Qualidade Kaunas University of Technology Kingston Computer Consultancy Ltd London South Bank University NDT Consultants Ltd Statoil AS Zenon SA

This Specific Targeted Research Project (STRP) is co-ordinated and managed by TWI Ltd. and is partly funded by the EC under the Nanotechnologies and Nanosciences, Knowledge Based Multifunctional Materials, New Production Processes and Devices (NMP) programme, Contract Number: NMP2-CT-2005-516993



APPENDIX A

Conference Paper



LONG TERM MONITORING OF OFFSHORE INSTALLATIONS USING ULTRASONIC GUIDED WAVES

PJ Mudge, S Chan TWI Ltd

Granta Park, Cambridge, UK [email protected]

M Kayous

Plant Integrity Ltd Granta Park, Cambridge, UK

E Andersen, JKF Andersen

DONG Energy Fredericia, DK

Abstract The potential for low frequency ultrasonic guided waves to monitor large areas of structures offers considerable scope for long term monitoring of a structure’s condition. This paper reports on the work carried out within the European Commission funded OPCOM project which has examined the application of this technology for monitoring both oil and gas production installations and offshore wind turbine towers. Developments have included design of sensor arrays capable of withstanding marine environments, long term environmental influences on the test data and modelling of the influence of component geometry and condition on the test.

1. Introduction This work is concerned with the development of tools and techniques for the 100% examination and monitoring of very large diameter tubular steel structural components using ultrasonic guided waves. Such tubulars exist in large numbers in offshore installations and their structural integrity may be affected by both corrosion and fatigue cracking over the service life of the installation, which is normally in excess of 20 years. This work has been carried out under the European Union funded OPCOM project. The low frequency (i.e. approximately 20 to 100kHz) ultrasonic guided wave technique was developed in the 1990s for the rapid survey of pipes, for the detection of both internal and external corrosion(1). The propagation of the so-called guided waves is affected by changes in thickness of the component, so that they are sensitive to metal loss defects, notably corrosion. The principal advantage is that long lengths, 30m (~100ft) or more in each direction, may be examined from a single test point. The main area of application has been to pipes and pipelines and, in the decade since this technique was introduced commercially, it has been enhanced and refined and has become widely accepted as a method for rapid screening of pipes for corrosion(2). The properties of these guided waves in cylinders, i.e. pipes, are well understood and this knowledge has governed the development of commercial guided wave testing systems. For these an encircling bracelet tool is placed around the outside of the pipe to be tested, with the aim of generating an axially-symmetric wave, to send a circular wave along the pipe. An example of such a tool is shown in Figure 1.



Figure 1. Bracelet tool for long range guided wave testing on a 10” (273mm) diameter pipe.

The guided waves propagating in pipes are a special case of the Lamb waves which can exist in plates. One of the parameters which affects the propagation characteristics is the pipe diameter. It may be appreciated that, as the pipe diameter increases, the behaviour becomes more like that of the Lamb or plate waves. Equally, as pipe diameters become very large the practicality of mounting an external bracelet tool becomes difficult owing to the size and weight of the tool itself. The aim of this work was to develop tools and techniques applicable to large diameter steel structural tubulars, with particular reference to the 4m diameter foundation piles for ‘Monopile’ offshore wind turbines.

2. Structure to be examined The Monopile design for offshore wind turbines, developed by DONG Energy, relies on a large diameter (4.7m) tubular pile driven vertically into the sea bed to support the tower (which may be up to 80m high) and a turbo-generator of up to 3.5 MW. Figure 2 shows the pile after driving into the sea bed and the installation of the superstructure.

Figure 2. Left, the monopile after driving and, right, installation of the superstructure

The 4.7m diameter piles are substantial structures, being 30m long and with a wall thickness varying between 40 and 60mm. The weight of each is in excess of 150 tonnes. They are fabricated from ten 3m long tubular cans welded together. Examples of these piles at the fabrication yard are shown in Figure 3.



Figure 3. 4.7m diameter x 30m long piles. The circumferential welds may be seen Of principal concern is the fatigue loading on the pile caused by the bending stresses resulting from wind loading on the tower and, to some extent, wave loading. Fatigue cracks, should they occur, are likely to be at the circumferential welds, but the initiation of cracking from localised corrosion cannot be ruled out. Consequently, the requirements of an inspection/monitoring system are for detection of both corrosion and cracking. As may be seen from Figure 2, most, if not all, of the pile is below sea level, so that it is hard to access for inspection. Further, the section below the sea bed is also required to be examined as the bending stresses continue affect the region in the sea floor, at least for part of its length. The scale of the inspection task is enormous. Whilst the welds are at known locations, so that inspections may be concentrated at those points, the total length of circumferential weld in the pile is 118m. Further, corrosion may potentially occur at any position on the outer or inner surface of the pile. This total surface (inside and out) is 886m2. Thus to make the examination of this component effective it is necessary to employ a method which is capable of covering such a large test area rapidly and one that does not require a sensor to be scanned over the whole surface.

3. Inspection development 3.1 Approach adopted The ultrasonic guided wave technique is highly suited for the rapid inspection and monitoring of such a large volume of material. However, it is impractical to consider an encircling tool, such as is shown in Figure 1, for the 14.8m long circumference. Consequently it was decided that a linear array of finite length (curved to fit the shape of the pile) would be used to test a strip running down the length of the pile. This tool would then be moved around the circumference in steps to cover the whole of the circumference. The top edge of the pile, clearly visible in Figure 2, is accessible from the inside of the column, as the superstructure is fitted over the pile. It was decided to use an array of compression wave transducers mounted on this top edge to generate the waves of interest. 3.2 Initial experiments The properties of linear arrays for ultrasonic testing are well known from both phased array applications and SONAR. The following principles were used to develop an array design. Initial studies had shown that the optimum test conditions were achieved when exciting the S0 plate wave mode at a frequency of around 30kHz. When a number of transducers are assembled together in an array and driven electrically, the velocity of motion of the individual elements is not constant, but varies from element to element in a complex manner due to acoustic interactions between them. These interactive effects may be reduced by: Separating the elements of the array,



Changing the number of element and so the length of the array, Making the individual elements large so their self radiation overwhelms the mutual radiation

between elements, Using individual amplifiers to drive each element at the correct amplitude and phase to obtain a

uniform velocity of motion across the array. 3.3 Distance between elements The distance between each element will affect the number and the dimension of side-lobes from the ultrasound beam. These are not wanted and could produce noise. Ideally, the transducer elements should be infinitely close, in which case there will be few side-lobes with low amplitude. Increasing the distance between elements up to λ/2 will increase the number of side-lobes but not their amplitude. For a separation distance beyond λ/2, the amplitude of the side-lobes perpendicular to the main beam direction will increase. 3.4 Length of the array The length of the array is directly linked to the beam width of the main beam of the emitted signal. Indeed, a good approximation of the beam divergence is given by:

L

1042 20

dB

Where: λ is the wavelength of the signal, L the length of the array,

dB20 the divergence angle of the –20dB profile of the sound beam.

Using this equation it occurs that, using a distance between elements of λ/2, a reasonable length would be 4λ, which mean using an array of 8 elements.

3.5 Size of the transducer As explained previously, the bigger the diameter of the transducers, the lower the acoustic interactions, but this reduces the steering ability of the array. It was decided therefore to use a 40mm diameter transducer as a good compromise.

3.6 Testing These design principles were tested first by modelling the beam profile of the array using Civa and Quicksonic software. These both indicated that the array had good directionality and acceptable side lobe properties. Second, a series of experiments was carried out on heavy (50mm thick) plates in the TWI laboratory. The set up is shown in Figure 4.

Figure 4. Initial laboratory test on the array. The well defined responses from the far end of the plate may be clearly seen on the screen



Rayleigh wave

Welds

The array was driven by a multi-channel Plant Integrity Teletest® unit which allowed the signal supplied to each element to be controlled independently. This enabled beam focusing and steering to be implemented. Following the successful laboratory tests, the array was tested on the full size piles at the fabrication yard. An example of the test set up is shown in Figure 5. The aims of this test were:

To demonstrate that the whole length of the pile could be tested from the top end,

To establish the sensitivity by examining two 200mm diameter holes present in the pile (these are access holes for electrical cables - one of these holes may be seen in the top left corner of figure 5),

To test the effect of focusing the beam,

To test the effect of beam steering.

Figure 5. Linear array mounted on one of the piles shown in Figure 3 The results were very encouraging. The far end of the pile was clearly seen and the holes were readily detected. Clear signals were also observed from each of the nine circumferential welds. A typical result is shown in Figure 6.

Figure 6. Result from a test on a full scale pile. The far end signal may be seen on the right hand side

The pile end signal may be seen on the right hand side of the trace. However, the most significant feature of these tests was the presence of high amplitude Rayleigh waves which were propagating around the circumference of the pile at the top end. These waves appeared to be generated from



Figure 7. Experimental set up

each end of the array and were virtually impossible to dampen. Rayleigh waves at megahertz frequencies are readily damped by placing a layer of a substance such as grease on the surface. However, this was ineffective in this case. There were also issues of coupling of the individual transducer elements. Differences in efficiency between the elements produced non-optimum conditions for array performance and affected the focusing and steering behaviour. Consequently, it was decided to conduct a thorough investigation of the array characteristics and coupling effects. It was clearly impractical to do a long-term study on a full size pile, so a small-scale mock up was required. To do this, advantage was taken of a useful property of guided waves, that their behaviour scales with size. An important parameter which describes guided wave behaviour is the frequency x thickness product, expressed in MHz.mm. For the 50mm thick pile, testing at 30 kHz gives an f x t value of 1.5 MHz.mm. A length of 22” (559mm) diameter schedule 20 line pipe, with wall thickness 9.53mm was used for the studies. By using smaller ultrasonic transducers, operating at 157 kHz the equivalent f x t value was obtained to that on the full scale pile. Diameter to wall thickness ratio is also an important factor. For the full size pile this is 94:1. For the 22” pipe this ratio is 62:1, not as large as for the full scale pile, but big enough that a realistic approximation to the geometry of the full size pile is achieved.

4. Small scale array optimisation 4.1 Set up and hardware (properties of the array) The array consisted of 8 independent compression transducers (see Figure 7) designed to provide waves at frequencies identified as optimum for the inspection for this pipe size (in the range 80 to 200 kHz). Using the array on the edge of the structure, as shown, the probes will generate longitudinal waves propagating along the pipe. For the experimental set up, the data was collected via a Teletest Focus system. Unlike the initial design, the transducers were mounted on a rigid frame, which allowed the spacing between the elements to be varied. The frame also ensured that a uniform load was applied to each probe. Even using the mounting frame ensure a uniform load, the coupling efficiency was not the same for each transducer. The coupling therefore needed to be optimised. 4.2 Improving coupling consistency In order to improve the consistency of transmission of the wave into the component, some coupling medium has to be used. The most suitable for this application is to use dry coupling by adding a soft material such as neoprene rubber to the face of the transducers (see Figure 8). This reduces the difference of acoustic impedance across the interface and thus allows a better transmission of the wave between the probes and the material under test. A study was carried out into the transmission of the ultrasonic energy for different surface roughnesses. It was shown that for a roughness above an

x

y



Rubber 6mm 150kHz G=30

-400

-200

0

200

400

0 1000 2000 3000 4000s

mV

Without rubber 150kHz G=30

-400

-200

0

200

400

0 1000 2000 3000 4000s

mV

3mm rubber 150 kHz G=30

-400

-200

0

200

400

0 1000 2000 3000 4000s

mV

1mm rubber 150 kHz G=30

-400

-200

0

200

400

0 1000 2000 3000 4000s

mV

Ra value of 3 m, the transmission coefficient is low, and that the transmission is better at lower frequency. In order to see the effect of the rubber on the output signal, some experiments using different thickness of rubber were carried out (see Figure 9):

Figure 9. A-scans, different thickness of rubber

For each A-scan, the Signal-to-Noise ratio (SNR) was calculated, see Table 1:

Table 1. Signal to noise ratio for different thicknesses of rubber

Thickness of the rubber

(mm)

Frequency (kHz)

Ringing zone duration

(s)

Amplitude of the signal

(mV)

Amplitude of the noise

(mV)

Signal to Noise ratio

0 150 1236 47.96 17.39 2.76 1 150 1097 81.56 11.95 6.83 3 150 968 235.53 24.11 9.77 6 150 961 286.92 33.22 8.63

It may be seen from Figure 9 and Table 1 that the presence of rubber decreases the transducer ringing and thus improves the quality of the signal. Also, the amplitude of the signal increases considerably with the thickness of rubber. However, the amplitude of the noise behaves in the same

Probe

Rubber

Figure 8: Rubber coupling layer



way. It may be seen that the signal to noise ratio is maximum when 3mm rubber was used. Therefore, this thickness of rubber was adopted for the experiments. 4.3 Rayleigh wave suppression (spacing of the array) As found in the field tests on the full scale piles, the linear array also generates Rayleigh waves propagating around the end of the pipe The Rayleigh wave is a non dispersive surface wave propagating with a velocity slightly less than the shear wave (about 2970 m.s-1) in a steel material. This wave is of high amplitude and can thus hide the other peaks which may appear. One way to cancel this Rayleigh wave is to space the transducers in the array at half the wavelength of the Rayleigh wave, see Figure 10. The Rayleigh wave component generated by the first transducer is cancelled by the out of phase wave from the second transducer when the spacing is λ/2. Thus, for a given frequency, the spacing between the transducers (in order to cancel the Rayleigh wave) may be calculated, Table 2 presents the optimum spacing between the transducers, depending on the frequency:

Table 2. Optimum transducer spacing with frequency Frequency

(kHz) 50 60 70 80 90 100 110 120 130 140 150 175 200

(mm) 59.4 49.5 42.43 37.13 33 29.7 27 24.75 22.8

5 21.21 19.8 16.97 14.85

/2 (mm) 29.7 24.75 21.21 18.56 16.5 14.85 13.5 12.38 11.4

2 10.61 9.9 8.49 7.43

The minimum spacing between two transducers is 25 mm, as at this point adjacent elements touch each other. It may be seen from Table 2, this distance is optimum at 60 kHz. Above this frequency, it is impossible to reach the optimum spacing, so it is not possible to suppress the Rayleigh wave by this method at the frequency of interest for the small scale mock-up, which is at around 150kHz. However, the observed results ware not as bad as the predictions in Table 2 would suggest. This may be explained by the fact that most of the energy of the Rayleigh wave is concentrated within one wavelength of the surface. Using a high excitation frequency increases the concentration of the energy in the wave near the surface, so that the Rayleigh wave component is more highly attenuated than at lower frequencies. The effect of this is shown in Figure 11. At 100 kHz the amplitude of the Rayleigh wave is high, with repeat echoes being observed which correspond to the wave travelling around the pipe circumference at the end. The first reflection of the pipe end is hidden behind the peak of the Rayleigh wave at approximately 1300 μs. At 170 kHz, the Rayleigh wave is not present and the reflections from the pipe ends may be clearly seen.

Figure 10. Cancellation of the Rayleigh wave



Divergent beam Ideal beam

Figure 11. A-scans, influence of the frequency on the Rayleigh wave 4.4. Optimising transducer array design One of the characteristics of a linear array is that there will be a spreading of the beam from the origin with increasing range. This is illustrated in Figure 12. One of the aims of this study was to reduce the natural divergence of the beam to enable the inspection to be carried out in a series of strips around the pile circumference with reasonable resolution. The circumferential resolution is important as this enables the position of any reflectors in the pile to be located. A study was made of the key parameters of the array which affect divergence of the beam. These are: the number of transducers, the spacing between them (see Figure 13) between their centre (Pitch) and their edge (Aperture), the frequency, the velocity of the wave, the material under testing, the couplant and the focus distance.

A number of different arrangements of transducer elements were considered. Table 3 presents the results for different spacing between the probes and for a different numbers of elements. The width of the beam was calculated at a range of 2 metres from the transducer.

Figure 12. Effect of beam divergence

Figure 13. Spacing between transducers

100 kHz G=50

-150

-75

0

75

150

0 1000 2000 3000 4000s

mV

5mm spacing

No spacing

170 kHz G=40

-150

-75

0

75

150

0 1000 2000 3000 4000s

mV

5mm spacing

No spacing

Rayleigh wave

Pipe end



200 kHz

-20

-10

0

10

20

0 1000 2000 3000 4000 5000 6000 7000 8000

Focus

UnfocusPipe end

Table 3. Influence of the spacing and the number of probes on beam width at 2m.

These results show that the principal influence on the divergence is the width of the array. The number of transducers does not affect the beam geometry, provided that they are close enough to act as a single array. Nevertheless the beam width is large for all cases studied and over the whole length of the pile there would be insufficient lateral resolution. It was therefore decided to use a delay profile in the signals applied to each of the transducers in order to focus the beam. In fact a number of measures were used to achieve enhanced beam characteristics. These were:

Increase in frequency to 200 kHz (f x t = 2 MHz.mm) to reduce the Rayleigh wave component, and to reduce the divergent angle (which is proportional to frequency)

Application of the delay profile to concentrate the energy,

Use of the smallest array length (200mm) to start with the narrowest beam. The results are shown in Figure 14. The dark trace is the unfocused signal. The reflection from the far end of the 8.18m long 22” pipe may be seen at approximately 6300μs. The pale trace is the focused result (the focus was at the pipe end). It may be seen that there is an increase in amplitude of 14.5dB (a factor of 5) when the beam is focused.

Figure 14. A-scan, focusing test



These design principles are now being applied to the array geometry for the full scale tests and the procedures are being based on the focusing regime demonstrated on the small scale specimen. Acknowledgement This work was carried out under the European Union funded OPCOM project, which is partly funded by the EC under the Nanotechnologies and Nanosciences, Knowledge Based Multifunctional Materials, New Production Processes and Devices (NMP) programme, Contract Number: NMP2-CT-2005-516993. References 1. Cawley P & Alleyne DN, ‘The use of Lamb Waves for the Long-Range Inspection of Large

Structures’, Proceedings of Ultrasonics International 95, published in Ultrasonics, Vol. 34, pp287-290, 1996.

2. Mudge PJ, ‘Practical Enhancements Achievable in Long Range Ultrasonic Testing by Exploiting the Properties of Guided Waves’, proceedings of the 16th World Conference on NDT, Montreal, Canada, September 2004.

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