A multi-modality approach to NDT in the Oil&Gas segment

Inspection, and especially non-destructive testing (NDT), is vital to establish and ensure asset integrity in the oil and gas segment, whether at the pipework manufacturing construction stage or in service. However, such is the complexity of the modern oil and gas industry that there is no single technique that can be universally applied in every inspection application. Fortunately, GE Inspection Technologies can offer, from a single source, equipment and systems employing a variety of modalities to ensure accurate and reliable inspection, both to maintain a safe working environment and to help maximise the productivity of new and ageing assets.

The four major modalities used to carry out inspection and NDT in the oil sector are: ultrasonics; radiography; remote visual inspection; and eddy current.

Ultrasonics

Ultrasonics is arguably the most versatile of all the NDT modalities and great strides have been made in the discipline over the past 25 years. Developments in the ways that data is presented and displayed and the introduction of multiplexing and flaw-sizing techniques, such as time of flight diffraction, now significantly improve the probability of detection and analysis and interpretation of results. These benefits are being realized in the pipe mill and the fabrication yard, as well as in offshore and in overland pipe laying operations.
 
For example, mechanized or “automatic” welding is taking over from manual techniques on larger diameter pipe. As a consequence, this has paved the way for improved inspection methodologies, such as Automated Ultrasonic Inspection (AUT).

AUT systems typically employ an array of individual ultrasonic probes, which are mechanically driven to scan the circumferential length of the weld to provide a comprehensive volumetric ultrasonic picture of the weld or pipe wall. 

AUT provides a wide range of benefits over traditional wet film radiography. For example, it does not present a radiation hazard to personnel and there is no need for processing chemicals and darkroom facilities. Furthermore, AUT cycle times per weld, including acquisition and interpretation of data, are very fast. Girth welds can be inspected as soon as the weld is appropriately quenched, providing near real- time process feedback to the weld crew and dramatically reducing the cost of rework. AUT also provides a tremendous amount of data, allowing accurate sizing and location of defects, facilitating the use of alternative acceptance criteria. 

Naturally, AUT systems, like all technologies, have undergone continuous development since their introduction. For example, Weldstar, the state-of-the-art system from GE, has all the ultrasonic electronics located on the scanner head, minimising any possible degradation of inspection data caused by outside electrical noise from sources such as power lines and welding equipment from being picked up on long umbilical cables. Weldstar also uses phased array probes to provide fast, accurate inspection and combines these with conventional ultrasonic probes to allow those critical ultrasonic shots required for techniques such as Time of Flight Diffraction (TOFD) and transverse defect detection. Furthermore, all 16 conventional ultrasonic channels and 128 phased array ultrasonic channels are packaged within the scanner head itself and the umbilical cable contains only the wiring necessary to provide power, control, and communication with the scanner head. The net result is umbilical simplicity and reliability and enhanced system performance.

Naturally, advances in ultrasonics have not been restricted to girth weld inspection and recent developments have seen phased array technology made available to portable flaw detection equipment and the evolution of ultrasonic wall thickness measurement systems which allow corrosion monitoring in remote and hostile environments.

Radiography

Radiography is the most mature of the inspection modalities and is widely used for weld inspections and corrosion detection in piping, refinery pressure vessels and storage tankers, LNG tankers, offshore rigs and platforms, pipelines and sub-sea risers.

However, radiography has now undergone a step change with the development of digital radiography in various forms. Currently, computed radiography (CR), which uses an imaging plate instead of film, is fast replacing conventional radiography in many applications, which require volumetric inspection.

Much lower costs of consumables and the absence of darkroom or processing chemicals are just some of many benefits offered by digital radiography. There is greater tolerance for varying exposure conditions reducing the need for retakes, which also means substantial dose reduction. Imaging plates require no development time so images are available for viewing in real time and inspection productivity can be twice that of film systems. However, probably the greatest benefit of a digital is the ability to enhance images electronically, to store them on a computer or removable storage device and to transmit them to other computers. Image enhancement allows operators to alter the contrast of images to compensate for poor original shooting conditions and to zoom in on areas of interest

A significant development, which has been driven by the conversion to digital radiography, has been the creation of software to acquire, display, process and store the images produced. Now a new software architecture is available, which addresses previous industry problems and complies with an image-sharing standard which has emerged as the most stable and widely accepted of all alternatives.

This is the DICONDE standard, a non-proprietary format, developed from the DICOM system used throughout the medical sector, but incorporating many features which are purely NDE-focused. It is a system that allows images to be saved with context, in that all the technique information and information on location, date and time and inspector is saved with the image. Such information can then be included in any report generated, while its inclusion with the image into databases means that database searches can be carried out on a variety of criteria. This powerful, global dictionary has now been used as the basis of a new software suite, Rhythm, to realise the enormous potential of digital radiography. Rhythm is currently available for radiographic and remote visual images but it will be extended to other modalities in the near future.

Remote Visual Inspection (RVI)

Remote visual inspection (RVI) is playing an increasingly important role as a vital inspection tool throughout the process and oil and gas sector, in much the same way that endoscopy and “key hole” surgery is now such an important procedure in the medical field.

Classically, RVI has been used to perform routine or emergency inspection in inaccessible and hazardous areas. Inaccessible areas include small diameter pipes and tubes in boilers, product transfer lines etc and small tanks and vessels to check for corrosion, scale blockage or mechanical damage. Hazardous areas can include storage vessels, fractionation towers and steam generators, where checks are carried out for corrosion, nozzle weld condition and product residue, for instance.

Today’s range of RVI equipment extends from flexible borescopes, which are suitable for tube of down to 0.5mm diameter, through pan, tilt and zoom (PTZ) cameras for viewing in large areas, to robotic crawlers which can be used in vessels and pipes up to 1500mm diameter and can be operated under water. Data can be provided as still image or video motion. With additional features such as stereo and shadow measurement capabilities for defect sizing, RVI is often used as a stand-alone inspection technique. Its results can also complement conventional test data, such as vibration, pressure and flow, or can support the data from other NDT modalities.

A recent development has seen the introduction of Menu Directed Inspection (MDI) software, a tool that guides VideoProbe users in the oil and gas sector through the inspection process and intelligently auto-generates a report.  MDI software saves time, improves quality and increases productivity for the end user.  It provides a 35 to 70 percent timesavings.

Eddy current

Eddy current is by far the most used technology for the in-service inspection of boiler and heat exchanger tubes for corrosion, erosion, fatigue and cracks. Tubes can be tested at high speed (up to 1m/s with computerised data analysis) and by using phase analysis, defects such as pitting can be assessed with an accuracy of around 5% of tube wall thickness. This allows accurate estimation of remaining tube life and informed decisions can be taken as to the appropriate action to be taken.

Eddy current is fast and it offers high probability of detection and good resolution. Consequently, it finds ready application in other areas, including crack and flaw detection in pipework and welds, where the fact that it can detect through paint coatings is often of great advantage. However, eddy current application is currently undergoing a major step-change with the introduction of techniques such as flexible eddy current arrays and pulsed eddy current array probes. These can provide the same kind of easily-understood display imagery as ultrasonics and significantly improve both the speed and reliability of data acquisition.

Conclusions

In the implementation of any inspection regime it is important that suitable inspection techniques are selected. In some instances, a single technique will prove capable of reliably detecting and accurately reporting any defects. In some instances, successful inspection may rely on the application of a number of techniques. It is of major benefit if all the inspection equipment can be provided from one source. This source then becomes a solution provider rather than merely an equipment supplier. It is also of major benefit if that solution provider can also provide the necessary servicing and training facilities, within the relevant geographical location. This combination of the very latest in proven inspection technology with a comprehensive and focused back-up service ensures that everyone involved in the inspection function, from the NDT technician to the plant owner/operator, has access to the expertise, experience and equipment which can help to ensure the safe and productive operation of valuable assets.