Radiographic Testing (RT) solution for Alt-i-nova assembly site

Abbreviations used

• RT = Radiographic Testing
• UT = Ultrasonic Testing
• NDE = Non-Destructive Examination
• IQI = {Radiographic} Image Quality Indicator
• s-o-d = {Radiographic} source to {imaged} object distance

Collaborating Company - DQE of Kartal (Istanbul), Turkey

In April 2015 I formed a favourable opinion of DQE colleagues Nuvit Sahinturk, Melike Ozbekoglu and Erhan Turgut as they answered questions on ultrasonic testing in a project meeting of the 3rd Bosphorus Bridge project.

I therefore spoke with them after this meeting about a concept I had formed regarding Radiographic Testing (radiography), and what follows is the outcome.

DQE, otherwise known as "Dizayn", provide a range of Non-Destructive Examination (Non-Destructive Testing) and engineering services, is based in Kartal in Istanbul, Turkey, and operates in several countries.
Their website is http://www.dqe.com.tr

Benefits of RT & comparison between RT and UT

A separate page provides a general comparison between RT and UT, plus some comments specific to the application of NDE to the 3rd Bosphorus Bridge / Yavuz Sultan Selim Bridge.

Description of the Radiography (RT) solution offered

A quote (price) has already been offered for the RT solution as described following.

To meet the needs of the Project, it is clear that an RT method applied the the bridge deck assembly activity needs to have these characteristics:

• radiographs produced have good image quality - high contrast and good resolution of small detail
• available within 24 hours of weld completion
• radiographs to be in digital image format to enable easy simultaneous access to all interested parties, regardless of their location
• radiography is rapid per-radiograph and low-cost per-radiograph
• no exclusion zone necessary during radiography (therefore no interruption of fabrication / assembly work)

The following excellent solution, which meets all requirements elegantly, is made possible because the RT needed, for the highly-stressed deck-plate seam welds making coplanar butt joints between flat plates of equal thickness, is the simplest case of RT.

• the geometry of the RT arrangement is simple
• there is no significant restriction to the access to the seam weld area, so a large item / assembly of equipment can approach the weld needing RT

Practicality of the solution is vastly assisted because the large moderately heavy equipment will rest on the flat or slightly-sloping steel deck plates held-down by Gravity.

Given the 14mm and 16mm thickness of the deck-plates and therefore the thickness of the seam-welds butting them together; citing the specific example of 14mm deck plate thickness, the following solution fulfils requirements:

• illuminating / imaging radiation source is X-ray, at 160kV tube-voltage
• the beam-path is fully-shielded by steel plate at least 16mm thickness - hence no exclusion-zone around the RT activity is needed
• the X-ray solution obtains its electricity supply from the site supply used for welding and other activities which are continuing uninterrupted given absence of exclusion zone during radiography
• the X-ray tube and beam-path shield are mounted upon / within a tubular steel frame
  • the space left for the U-rib splices to enable welding is sufficient to grant access to the assembly
  • the X-ray tube and the beam-path shield are held inseparable together, guaranteeing safety
  • the frame can include electrical interlock switches preventing the X-ray tube being activated unless the frame and radiation generation equipment are firmly pressed against the steel deck-plate
  • the frame enables the equipment to be easily carried around inside the bridge-deck
• exposure ("shot") time is 1 minute
• source-to-object distance is 350mm
• the frame mounting the equipment is about 550mm tall (quite small)
• the image collection uses computed radiography, with the image collected by a scanner
• the scanned RT image is obtained within a few minutes of the radiograph, as the scanner is on-site next to location of the RT activity
• the radiograph image produced is inherently a digital image, so can be passed to a server (computer database) and viewed by all interested parties immediately
• the interested parties, simultaneously viewing the same radiograph, could discuss the radiographs by telephone, avoiding delays in evaluation and decision

The radiographs produced by this radiographic solution are shown alongside a radiograph using Ir192 Gamma source. All images are of the same weld sample, in the same location (unfortunately, the IQI does not remain in identically the same position).

It must be explained;
the imaging "optics" are identical to the offered solution - same source, same geometry of "shot".
However; because of the expense of obtaining the scanning equipment, which the NDE Contractor does possess and use in another country, the imaging was done using conventional "wet film" radiography.
Furthermore, the images are not properly scanned from the radiographic film but are obtained by photographing the radiograph as seen on a viewer a human uses to view and radiographically interpret the radiographs.
This is declared for appropriate disclosure; however it is believed that the image qualities are representative and the comparison of image quality is valid.

160kV; 350mm s-o-d

160kV; 500mm s-o-d (this image has better density but little better resolution than the "shot" at 350mm s-o-d (see IQI resolution))

Representative best obtainable Ir192 "shot" (radiograph quality is limited by low image contrast)

The X-ray radiographs are vastly superior.
Indicatively: 3 IQI wires can be seen in the Ir192 radiograph, whereas 5 IQI wires can be seen in both X-ray radiographs.
The quantitative 2-IQI-wires improvement in radiograph resolution accords with the qualititative impression of the X-ray solution giving a much higher radiographic quality than the Ir192 technique.

The quote and proposed contract is for as many images in a day as the team on-site can reasonably take; for a fixed Contract duration of several months.

Noting 1 minute per shot, this amounts to a lot of radiographs and the seams can be quite extensively radiographed.

The structure of the Contract offered is explained as follows. The fixed cost of the equipment is high; however the variable cost per "shot" is low, given the re-usable imaging plates.
Therefore: the NDE Contractor requires a guaranteed minimum Contract remuneration to cover the capital costs of the equipment committed to the contract. However, awarded this Contract, the Contractor is then happy to apply the equipment to take as many radiographs as the Customer wants.

(R. Smith, 02Jun2015 22Jun2015, 12Apr2017)