Evaluating thin-wall butt welds – industrial radiography alternatives
13 February 2014
Author: Mark Cassidy, technical manager and director, Advanced NDT Solutions
Industrial radiography is a widely used inspection method for testing the integrity of welds. Advanced NDT Solutions (ANS) was recently approached and asked to look at the feasibility of replacing radiography with ultrasonics for thin-wall austenitic materials.
The project was an upgrade to existing pipe work on an offshore Installation in the North Sea (Norwegian sector). All welding was performed in-situ offshore inside habitat enclosures to ensure safety for hot-work operations. Typically, these welds would be evaluated using industrial radiography. However, this requires closing off sections of platform to all other activities and leads to increased time and cost for the project.
There are also other incidental safety implications that need to be addressed when using radiography. There are transportation and storage requirements that have to be met; there is also the use and the disposal of the chemicals that have to be used for processing the film.
ULTRASONICS AND RADIOGRAPHY
Ultrasonics can and has been used to replace radiography. However, there are restrictions with regard to material thickness and there is no permanent record of the inspection. Several international specifications such as Norsok (Norwegian Standards) require a minimum thickness of 8.0 mm before ultrasonics can be used as the inspection method. Where third-party validation of the inspection results is a requirement, a permanent record of the inspection results is mandatory.
With recent advances in design of inspection equipment technology, there now exists the capability to allow the use of ultrasonic phased array accurately evaluate pipe butt welds, even in thinner wall piping with thickness above 3mm. Phased array also allows for the storage of the inspection data creating the permanent record of the inspection.
Ultrasonic phased array would be commonly associated with the ultrasound images of an unborn child obtained during pregnancy examinations. This is an ultrasonic technique that allows electronic sweep through an array of angles typically 45-70 degrees from a fixed location.
ANS collaborating with some of their global partners looked to deliver an inspection solution to be employed for weld quality acceptance for the piping systems. The materials and the piping sizes to be inspected were as follows:
- Material groups: 316L and Duplex;
- Piping diameter range: 1” to 12” diameters;
- Material thickness range: 3mm-8mm.
However, the next stumbling block was that currently there is no provision in Norsok standards for ultrasonic assessment of welds in materials less than 8mm thickness. This meant that the client owner/end user would have to take responsibility for use of the test method and inspection findings.
DNV was requested to review proposed inspection procedures and witness validation of technology.
Once general agreement of the proposal was established with the client we set about fabrication of a range of samples with welded joints. All variations of material; thickness and relevant pipe diameter in an exact replica of production welds were fabricated. Weld-cap material was ground flush on the pipe outside diameter to facilitate placement of the ultrasonic phased array probe over the weld.
EDM (electric discharge machining) notches were used as reference reflectors (artificial defects) in each of the individual sample pipes 0.5mm deep and 20mm long on both external and internal surfaces at toe of welds for thin wall samples.
A high-frequency phased array probe suitable to evaluate thin wall materials in combination with a selection of curved wedges that accurately fit each specific diameter of pipe to be tested were purchased for the project. This probe is then attached to a miniature bracelet scanner which is manually moved around the pipe circumference with an encoder tracking in millimetre intervals and all ultrasonic signals are recorded by the imaging computer that can later been analysed in detail.
Due to the thin wall thickness it was required to perform two separate offset scans to acquire full coverage of Weld and HAZ (heat effected zone) regions. Ultrasonic scans are evaluated using custom-designed software that displays an outline of weld profile. The location of any defects found is shown in relation to depth below the surface and circumferential position around the weld.
From these initial trials, it became apparent that we could clearly demonstrate the ability of this technique to identify and size 0.5mm notches connected to both ID (internal diameter) and OD (outside diameter) as shown in the actual screen images shown below.
WELDED SAMPLES EVALUATED
2” Ø x 3mm Wall Duplex – 0.5mm Deep x 20mm Long ID & OD Notches
2” Ø x 4.6mm Wall Duplex – 0.5mm Deep x 20mm Long ID & OD Notches
2” Ø x 3mm Wall – 0.5/0.8mm Deep x 20mm Long ID & OD Notches
3” Ø x 3mm Wall Duplex – 0.5mm Deep x 20mm Long ID & OD Notches
4” Ø x 3mm Wall 316L – 0.5mm Deep x 20mm Long ID & OD Notches
4” Ø x 3mm Wall Duplex – 0.5mm Deep x 20mm Long ID & OD Notches
12” Ø x 3mm Wall Duplex – 1mm Deep x 20mm Long ID & OD Notches
System calibrations for phased-array technique were performed using the relevant 15mm calibration standards for the 6Mo; 316L and Duplex materials. Ultrasonic codes require sensitivity to a known reflector to make a comparison with actual defects so in this case we used a 1.5mm diameter side drilled hole. (SDH) set to 80% of full screen height (FSH).
The phased array ultrasonic system utilized for these trials and subsequent field inspections was the Olympus Omniscan MX2 unit. System calibrations were performed using a IIW (International Institute of Welding) Type 2 Reference Block and material specific calibrations performed with custom designed 316L (15mm) calibration standard and a Duplex (15mm) calibration standard.
The owner/end user realised significant improved results in the execution of a hassle-free inspection programme through innovation and a willingness to invest in developing an alternative means for weld evaluation. It is proposed to proceed in developing a Norsok standard for the phased array ultrasonic inspection of material thickness less than 8mm to allow this technology to be utilised as the preferred testing method in the future.
The general weld rejection levels were found to be comparable with levels typically anticipated on similar projects evaluated utilising industrial radiography.
Advanced NDT Solutions (ANS), based in Athlone, was founded by Mark Cassidy and David Finegan. ANS is the leading provider of advanced non-destructive testing services in Ireland. Their staff of 16 provide these services in Ireland and globally for – among others – the pharmaceutical, chemical, petrochemical, power generation and construction industries. See www.ansndt.com.