Technical Insight: Guided Wave Long Range Ultrasonic Testing

Also known as Guided Wave Testing (GWT), Guided Wave Long Range Ultrasonic Testing (GWLRUT) is a long-range, non-destructive testing method that uses low-frequency, guided ultrasonic waves to inspect long, in-service, or inaccessible pipes, rails, plates and other structures for corrosion, erosion, and cracks.

Sound waves are generated by transducers from where they travel along the surface of the structure until they encounter a defect that has caused a change in the thickness of the structure, such as cracking or metal loss from corrosion. The defect causes a pulse echo to be sent back that can be analysed to characterise, identify and locate the defect. The benefits of this process include being able to screen large and hard-to-access areas from a single test point, which has seen it used in industries including oil and gas, where it can be used to inspect sleeved or buried pipes as well as those that are ‘unpiggable’ or hard to inspect by other means.

TWI has been instrumental in developing this process for industry through a range of different projects over the decades. These include core research programme (CRP) projects that are created for the wider benefit of our Industrial Members (who can all access the outcomes of the research), as joint industry projects (JIPs) which allow interested parties to act as sponsors in return for the project outcomes, during public-funded project work where we typically work as part of a consortium made up of other industrial or academic organisations for the wider benefit of industry and society, or directly on behalf of individual Industrial Member companies.

This combined research has afforded TWI a level of expertise that is trusted by many of the largest names in industry, including in the use of guided wave long range ultrasonic testing. Some examples of our work in this area include…

Core Research Programme

The core research programme (CRP) allows our experts to conduct research and development activities for the benefit of our Industrial Members. The outcomes of our CRP projects are available to our Industrial Members and typically involve advancing a process, innovating new solutions, and addressing industry challenges. CRP projects cover a range of industries and capabilities, including GWLRUT.

- Electromagnetic Transducers to Generate/Detect Guided Waves

This 1999 project was created to assess the use of electromagnetic transducers to generate long range low frequency guided waves for detecting environmental damage in ferritic steel pipes. This foundational work investigated the replacement of piezoelectric transducers, which required the pipe surface to be exposed and any excess corrosion to be removed, with electromagnetic transducers (EMATs), which are not adversely affected by poor surface conditions, operate through coatings, and are relatively cheap to manufacture.

- Finite Element Analysis of Guided Waves in Pipe/Rail Flaws

Having shown that GWLRUT could be used to screen large areas of pipe from a single location, this 2003 project investigated the potential for the technique to be applied to the detection of flaws in rails. In addition, our experts worked to validate a new wave propagation modelling technique against experimentally measured transmission data in a steel plate and to quantify the reflection characteristics of guided waves from flaws in pipes.

- Improved Guided Wave Technology for Pipe Defect Detection

This 2007 project sought to advance long range ultrasonic inspection by finding an effective technique for focusing guided waves into one region of steel pipe, which would increase the sensitivity to defects, making it easier to size and position them. This would reduce the time and cost required for inspection as well as open up the possibility of steering the beam around bends in pipes. The focussing technique was tested against a range of pipe geometries as our experts also assessed the potential for sizing corrosion damage using techniques for modelling the focusing of guided waves.

- Sizing Locally Thinned Areas and Guided Wave Pipe Inspection

Although guided waves had found use for the corrosion screening of straight sections of industrial pipeline, there remained challenges around its use on pipelines with bends as well as for distinguishing between uniform circumferential thinning and a severe patch of localised corrosion at one circumferential position. This 2008 project addressed both of these challenges through the quantification of pipe bends on guided waves as well as creating a technique to size locally thinned area defects in straight pipes using guided waves.

- Long-Range Guided Wave Inspection Beyond Pipe Bends

This project built upon the findings of the previous one through the use of insight modelling to quantify the effects of pipe bends on guided waves for a range of pipe bend angles. In addition the TWI project team developed signal reconstruction techniques to overcome the effects of pipe bends on the propagation of guided waves.

- Long-Range Guided Wave Pipe Modelling and Inspection

Continuing the investigation into the effect of pipe bends on ultrasonic signals, TWI’s experts used finite element modelling to assess the behaviour of guided wave propagation around pipe bends. The results of this finite element assessment were then experimentally validated before a technique was created for the correction of the signal distortion caused by propagation around a bend.

- Guided Wave Inspection of Plate-like Structures

This 2019 project sought to develop and evaluate a new transducer array design suitable for performing guided wave testing on plate-like components such as bridges, storage tank floors and large diameter cylinders like storage tanks walls, pressure vessels and wind turbine support structures. The aim of the technology was to open up a route to reduced cost structural health monitoring for a range of different industries. The TWI team created a prototype ‘omnidirectional SH0 transducer,’ incorporating a localised circular array of eight commercially available piezoelectric thickness-shear monolithic transducers (Figure 1).

- Signal Post Processing to Improve Guided Wave Tests

Experts at TWI undertook a project in 2019 to develop post-processing methods based on split-spectrum processing (SSP) in order to reduce the noise on the guided wave signals caused by scattering. Scattering can occur where a pipeline is buried or covered with a protective coating, reducing signal strength and increasing background noise. To maintain sensitivity, it is necessary to identify small signals that may be within the noise floor. As such, TWI developed

post-processing methods based on split-spectrum processing (SSP) in order to reduce the noise on the guided wave signals caused by such scattering. This project presented the findings of a validation exercise that took data from earlier guided wave tests on highly attenuating pipes, containing deliberately introduced defects. This allowed us to assess the ability of SSP to identify defects in coated and buried pipes which are not detectable using standard guided wave techniques.

- Quantitative Guided Wave Inspection of Pipes

Although guided waves are capable of screening straight pipe for patches of corrosion (Figure 2) and can provide the axial distance of the defect from the tool location and an estimate of cross sectional area loss, the sensitivity is such that a defect must be around 5% of the cross section to be reliability detected. At the time of this 2018 CRP project, available systems couldn’t easily distinguish between a general wall thinning around an entire pipe circumference and a deep corrosion pit. This project addressed these issues by introducing wave modes that had been largely ignored in the past, with data being collected to capture information from these wave modes and using them to detect smaller defects and provide a more quantitative assessment of them (Figure 3).

- Guided Wave Focusing for Pipeline Inspection in the Field

Improvements to the guided wave inspection procedures for pipes helped deliver improved sensitivity for defect location and improved methods for size and shape prediction. However, these advances were only applied to straight pipes with regular wall thicknesses, rather than more complex tubular structures that may have changes in thickness, attachments or weld cap geometry. TWI’s experts addressed this during a 2020 CRP project that investigated guided wave focusing techniques alongside finite element analysis to identify, develop and validate an improved focusing capability (Figures 4-5).

- Guided Wave Flaw Sizing for Pipe Inspection in the Field

Also in 2020, TWI turned its attention to the creation of a robust flaw sizing technique for use with flaws in otherwise inaccessible areas. This would not only allow for better decision making for intervention activities such as excavation but also offer important guidance for guided wave tooling design. This project determined that it was possible to accurately determine the size of a 1.4% cross-section area loss flaw with only 10 degree circumferential extent if at least 12 flexural wave modes are included in the formulation, although this requires improvements to the hardware, such as increased numbers of transmit-receive channels and transducers around the circumference of the pipe. Finite element models found that it was possible to measure the size of flaws lying beyond welds without any modifications to the technique (Figures 6-7). However, for flaws occurring at a weld or lying beyond a pipe support, a correction procedure was required that needs knowledge of the geometry of the weld or pipe support. A correction procedure was successfully developed for a flaw lying at a weld so that it was possible to measure the sizes of flaws at welds with a comparable level of accuracy. However, this still required prior knowledge of the geometry of the weld.

- Signal Processing Techniques for Guided Wave Inspection of Buried Pipelines

Continuing our research into guided wave testing of buried pipelines, this CRP project assessed a variety of improved signal processing methods for data collected on coated pipes. The aim was to increase the capacity of guided wave through an enhancement of the signal-to-noise ratio (SNR). Among the methods tested were the spatial variances method, an adaptive filtering technique, a spectral matching technique, a low frequency testing technique, and the coded excitation technique.

Joint Industry Projects

TWI joint industry projects (JIPs) allow us to focus on areas that may be of particular interest to specific Industrial Members. Interested parties are invited to sponsor these projects in return for access to the outcomes and the opportunity to provide input into the project direction. This not only ensures that the outcomes are best suited to the needs of the sponsors but also makes sure that only those Members with a vested interest in a particular project need to get involved. These projects cross a wide variety of industries and capabilities, including guided wave long range ultrasonic testing, as shown below…

- Use of Long-Range Guided Wave Ultrasonic Testing for Fitness-for-Service Determination of Pipelines

This project, completed in 2009, sought to integrate flaw sizing with fitness-for-service procedures, while also extending flaw sizing capability to a wider range of pipe diameters. The accuracy of these assessments were validated though testing with a focus on difficult to inspect areas before evidence to support the use of the technology was prepared for presentation to regulatory bodies.

- Performance Assessment of Non-Invasive Monitoring of Above Ground Storage Tank Floors Using Guided Wave Ultrasonic Monitoring

This project ran from 2013 until 2018 with the aim of monitoring selected in-service tanks to provide long-term data on floor condition. This data was compared with results from internal examinations to demonstrate the reliability of the technique with the ultimate goal of assisting the liquid storage and distribution industry in reducing the costs involved in ensuring that leaks do not occur from aboveground storage tanks.

Public-Funded Projects

Our experts have also been called upon to participate in a number of public-funded projects involving guided wave long range ultrasonic testing. These projects typically see us partnering with different industrial and academic organisations to solve specific industry challenges.

The PIGWaves project aimed to develop an enabling technology to allow existing pipe-crawling robots (known as ‘pigs’) to provide internal inspection of pipe walls using long-range ultrasound guided waves. These would be deployed in pipe segments of around 50 metres in length to provide a total volume inspection far more rapidly, accurately and cheaply than was possible with magnetic and ultrasonic pigs. The solution developed a circumferential collar of sensors and novel time reversal focussing to produce a map of the circumferential and axial pipe corrosion and cracks (Figures 8-9). Since the sensors can detect all significant corrosion in oil and gas pipelines within typically 50 metre of the sensors in a few milliseconds, long lengths of pipe can be scanned very rapidly with sample wave echo patterns obtained only every 50 metres, thereby making data storage requirements orders of magnitude less than the existing inspection methods.

The European Commission-funded MONITORAIL project was created to develop a cost-effective, long range ultrasonic inspection solution and wireless condition monitoring system to improve the maintaining of railway systems for better efficiency and safety. The MoorInspect project, which was also funded by the European Commission, was created to develop a medium-range in-water ultrasonic testing technology using ultrasonic guided waves to identify cracks and fatigue in a large, closed circular/elliptical chain links in deep-water offshore facilities.

The DeICE-UT project took ultrasonic guided wave technology and combined it with low frequency vibrations to perform de-icing on wind turbine blades. Ice build-up on turbine blades can lead to losses of up to 20% as a result of reduced efficiencies as well as causing wear in structural and mechanical components, creating potential incidents of falling ice, and ultimately the failure of the turbine operation altogether. The combination of ultrasonic guided waves and low frequency vibrations both prevent further ice accumulation and remove already formed ice (Figures 10-11).

Also concerned with the wind power industry was the Innovate UK-funded BIOClean-CMS project, which investigated the inspection of offshore wind turbine foundations using guided wave technology. The technical objective was to maintain the performance of the turbines by monitoring structural degradation such as corrosion and fatigue defects.

The CROSS-IT project shifted our focus to the inspection of both the concrete structure and the steel reinforcements of bridges to assess for dangerous levels of age-related degradation caused by water ingress into surface cracks, which leads to the corrosion of internal steel re-bars and reinforcement.

Vessel Clean was an Innovate UK-funded project that addressed the problem of hull fouling, which was the largest contributor to excess fuel consumption and carbon emissions from shipping. At the time of the project, global merchant fleet hull fouling was costing £8 billion in additional fuel costs and produced 70 million tonnes of additional carbon dioxide. The aim of the project was to develop an in-service, permanent prevention, detection, and removal solution for vessel hull fouling using a sparse network of low frequency (~40kHz) active ultrasonic compressional wave sensors embedded in a ship’s hull.

CRANEInspect was a two-year, European Commission-funded, collaborative project that developed an advanced structural health monitoring system for cranes used in industrial, logistics, construction, and shipbuilding sites. This was a multi-disciplinary industrial problem that required a cost-effective, real-time solution that reduces the need for human intervention and subjective decision-making. Critical areas of a crane’s structure were identified, with the reinforcing plates being shown to be submitted to the highest equivalent level of stress compared to the crane’s other components (Figure 12). A dual-capability transmission-reception algorithm using acoustic emission (AE) and guided wave testing (GWT) techniques was developed to monitor, localise and focus on defects. Acoustic emission, as a passive method, was used for defect detection and localisation. Once a defect was detected, GWT was used to assess its size. As part of the project, control software for acquisition and processing was developed to analyse the monitoring data collected. A complete prototype system was created and assessed both in the laboratory/factory and in the field as part of the capability validation process of the developed system (Figure 13), before experiments were performed on an in-service crane in Rotterdam.

The SWAK project (‘Sealed Without A Kiss: Non-Destructive Testing of Bonded Assemblies’) was created to investigate different non-destructive testing technologies and models to determine the quality of adhesive bonds in aerospace composites, with a focus on kiss bonding. The project team tested a range of NDT techniques to locate discontinuities in kissing bonds, including computed tomography, thermography, high frequency C scans, ultrasonic phased array inspections, laser shock testing, and guided wave NDT. Each technique was evaluated for its effectiveness in locating kissing bond defects in a range of assemblies to deliver safety improvements through zero defect NDT and mechanical testing techniques for bonded structures and geometries in aircraft structures, as well as reducing labour costs for composite products due to there being fewer, stronger assemblies with fewer parts.

The S4CE project (Science for Clean Energy) allowed our experts to turn their attention to the inspection of geothermal well casings. The project aimed to identify the most appropriate technique of techniques for monitoring these casings, which are in effect large diameter pipes. Tests were conducted for acoustic emission (AE), guided wave (GW), and vibration analysis (VA) (See figures 14-15). AE and GW both showed positive attenuation rates allowing us to finalise a system that featured both techniques.

We also conducted the European Commission-funded iPerm project, which was concerned with the development of a guided wave monitoring tool for industry.

Dedicated Industrial Member Support and Other Projects

Much of the work conducted at TWI is undertaken on behalf and for the benefit of our Industrial Members. Understandably, much of this work is confidential in nature but there are some example projects that we are able to mention here, as follows…

- Improving Performance of Guided Wave Testing on Coated Pipes

Our experts worked on a combination of modelled procedural enhancements, improved instrumentation functions and a novel signal post-processing technique to address the challenge of attenuation rates for guided waves in pipes where coatings are present or the pipes are sufficiently buried so as to cause a major reduction in guided wave test capability. The objectives of this project included increasing the inspection length of buried pipe, enhance the sensitivity to defects in highly attenuating pipes, and demonstrate the practical implementation of such a system by employing a combination of modelled procedural enhancements, improved hardware functions and novel post processing technique. Our research found that it is possible to reduce the attenuation effects by understanding the acoustic properties of the coating and selecting procedural parameters to reduce the attenuation effects.

- Guided Wave Testing on Pipelines in the Algerian Desert

A team from TWI was called to use guided wave testing to inspect over 5,300 metres of pipeline in the Algerian desert, resulting in corrosion being detected at two different locations and the provision of data around the remaining asset life for the pipeline. The client had identified specific areas of high corrosion risk ahead of our experts arriving to inspect the pipeline. 25 different pipelines were inspected over a 30-day period (Figures 16-17). The pipeline posed challenges due to the range of diameters (6-18 inches) and the location of the pipeline, including where it was buried within shifting sand dunes or under road crossings. Two test locations with substantial pipe wall reduction were subjected to phased array ultrasonic testing (PAUT) and visual examination to gain further insights.

These are just some examples of the work undertaken at TWI using our expertise in guided wave long range ultrasonic testing. You can find out more about the process and our services here:

https://www.twi-global.com/what-we-do/services-and-support/asset-management/non-destructive-testing/ndt-techniques/guided-wave-technology