Mechanised Ultrasonic Inspection

Contact: Guy Cotterill Manager - Specialised NDT Applications

General

A typical mechanised ultrasonic system is a portable, multi-channel ultrasonic imaging system that incorporates the following functions:

• Monitor probe position
• Manage multiple probes
• Digitise ultrasonic signals
• Store digitised information
• Perform multiple functions with a single probe using different range, delay and gain settings

Portable mechanised ultrasonic systems generally consist of a notebook style computer, ultrasonic data box and a scanning mechanism that holds the probes and a position measuring encoder.

Ultrasonic time of flight diffraction (ToFD) technique

Flyer: Time of Flight Diffraction

The ToFD technique is a relatively new inspection method that is being widely accepted internationally as one of the most reliable technologies for the detection and accurate sizing of flaws.
The item to be tested is straddled by a scanning mechanism that holds a pair of ultrasonic probes and a position measuring encoder. One of the probes floods the test area with pulses of ultrasonic energy and the other is used to detect diffracted and reflected energy from geometric features and any flaws that may occur.
Signal processing is used to convert this information into an image similar to that shown in Figure 1 below. This image is created in real time as the item is being tested and is interpreted by skilled operators to define item condition.

Figure 1: ToFD image of a flaw embedded within a weld

Mechanised pulse-echo technique

Flyer: Flaw Mapping

The mechanised pulse-echo technique refers to the mechanisation of conventional ultrasonic testing methods. This technique produces high-resolution /images of the item being tested. Examples of mechanised pulse-echo techniques include C-Scan, B-Scan and P-Scan imaging. With all of these techniques, the test area is fully scanned with one, or multiple, ultrasonic probes. A video camera tracking a target on top of the probe, or an X-Y encoder, is used to plot the movement of the probe. The ultrasonic signals from geometric features and any flaws that may occur are recorded with each movement of the transducer. Signal processing is used to convert this information into a coloured image similar to that shown within Figure 2 below. This image is created in real time as the item is being tested and is interpreted by skilled operators to define item condition. The various colours of the image represent a range of thickness values resulting in a topographic map of the test item

Figure 2: Pulse echo C-Scan image (coloured insert) of an area inspected on a pressure vessel. This technique is particularly useful for the mapping of flaws in welds, castings and for mapping corrosion in pipes and vessels.

CCI Pope Capability

CCI Pope has been providing mechanised ultrasonic inspection services since 1996. We have the equipment, personnel, procedures and experience to provide ToFD and pulse-echo mechanised ultrasonic services.

CCI Pope was the first company in Australia to gain accreditation for ToFD by NATA (Australia's inspection and testing regulatory association).

Our personnel, equipment and procedures have also been assessed, and accredited, by Chicago Bridge & Iron (CB&I), Bechtel, Duke Energy and Shell Global Solutions.

Mechanised ultrasonics inspection techniques are rapidly becoming the preferred testing option for inspecting critical welded joints in pressure vessels and pipes for both process and in-service inspections.

Australian Standard AS1210 and International Standards, such as the ASME Boiler and Pressure Vessel Code, accept mechanised ultrasonics as an alternative to radiography for testing such welds.

As well as these standards, major international corporations (such as Shell, Bechtel, CB&I) have produced extensive technical specifications for the use of mechanised ultrasonics.

Recent evidence of the acceptance of mechanised ultrasonics in Australia includes major projects such as:

• The use of mechanised ultrasonics as the prime test method during the construction of a large double-walled ammonia tank by CB&I for Orica.
• The use of mechanised ultrasonics for the inspection of main steam pipe welds during the construction of Millmerran Power Station.
• The use of mechanised ultrasonics for the testing of all longitudinal and girth welds in the off-shore section of the Tasmania natural gas pipeline.
• The exclusive use of mechanised ultrasonics to examine welds in large pressure vessels for Shells Geelong refinery.
• The use of mechanised ultrasonics to inspect critical pipe welds during construction of the Comalco Aluminium Refinery in Gladstone.

Corporations such as Shell, BHP, Mobil, Orica, Incitec, QENOS, Kimberly Clark, SANTOS and Woodside are regularly using mechanised ultrasonics techniques for the in-service inspection of pressure vessels and pipes.
It is also becoming more common for insurance sub-underwriters to recommend the use of mechanised ultrasonics to assess the condition, and therefore better estimate remnant life, of large pressure vessels.

Safety

Mechanised ultrasonics removes the hazard of radiation exposure associated with radiography. It also lessens the risk of slip, trip and fall injuries that can be incurred by ultrasonic technicians that are totally focussed on viewing the oscilloscope trace of an ultrasonic flaw detector while performing a manual ultrasonic test along the length of a weld.

Reduced inspection times

The full volume of welds can generally be examined in a single line scan. Under ideal conditions scan rates of 8 to 10 metres per minute are regularly achieved. And, unlike radiography, the inspections can be done in the immediate vicinity of other workers.

Immediate confirmation of testing integrity

The system displays processed images from the test in real time as the scanner is moved along the weld. Since an encoder is used, the technician can stop on, or backtrack over, an area at any time during the scan. These features allow for immediate confirmation of the inspection integrity and weld quality during the scan.

High probability of detection with accurate flaw locating and sizing

The mechanised ultrasonic ToFD technique is internationally recognised as having a higher probability of flaw detection than radiography and manual ultrasonics. and is able to detect flaws irrespective of their type and orientation.

Data Storage

The unprocessed test data, processed images and inspection parameters (including text comments) from each test are all saved as a computer file at the conclusion of each test. These files are then stored on CD to be archived in the job file.

The advantages of being able to permanently store data are:

• Third party audits. The file is positive proof that a weld was tested and there is enough information stored in the file for a third party to be able to accurately repeat the test. Files can also be electronically mailed for a second opinion assessment if necessary.
• Fast repeat inspections. The inspection software has the facility to quickly reload set-up parameters stored with previous tests. This enables fast set-up times when testing many welds of the same geometry and thickness.
• Accurate in-service inspections. Welds can be accurately monitored through their service life after an initial test has been performed.

The repeatability of the test, and the ability to accurately size flaws for length and height means that any in-service cracks that develop can be readily detected and monitored for growth.
This information can be used for accurate fracture mechanics calculations to determine service life.