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|NDT.net Issue - 2009-05 - NEWS |
|A girth weld pipeline in Australia is the first pipeline in the world to be completely inspected using computed radiography. Global Technical Services Group (GTSG) performed the inspection, using GE Sensing & Inspection Technologies CR50P portable scanner. The team inspected girth welds on the clad pipe, which consisted of 128 km of 18 inch pipe, during laybarge operation on the northwest shelf in western Australia.
The 18-inch diameter pipeline, which handles sour gas, is clad in stainless steel, has a wall thickness of around 1-inch and a surface temperature of 220°C. A particularly important requirement of the girth weld inspection was a very fast cycle time. GTSG immediately ruled out automated ultrasonics as an inspection technique, because of the cladding. Wet film radiography was deemed too slow and too difficult to configure on the barge. The company had experienced some previous success with a direct digital X-ray system but unfortunately this could not meet the cycle time specifications as direct radiography array needs to rotate around the pipe while the X-ray source is operating.
Ultimately, Computed Radiography was chosen for its fast exposure and scanning times to easily meet the cycle constraints. The only problem was that the technique did not carry DNV approval for this kind of work. Consequently, GTSG arranged for inspectors from DNV, a global risk management and certification agency, to fly out to the site, to carry out the required approval procedures. As a result, the technique was approved to DNV OS-F101 and it has proved possible to achieve better than 1.2% sensitivity at 100µm resolution, a quality equal to that achievable by film radiography.
In operation, GE Sensing & Inspection Technologies IPS and IPC2 phosphor imaging plates were placed around the weld on top of an insulating layer to protect them from the very high pipe surface temperatures. They were then exposed to the radiation of a source carried by an X-ray crawler inside the pipe. Exposure times were very short, a particular feature of these latest imaging plates, which also offer a wide dynamic range and exposure latitude, minimizing the need for time-consuming retakes. The exposed plates were fed into an adjacent CR50P portable CR scanner.
GEs CR50P weighs just 22 kg (48.5 lb) and offers excellent image quality through its optimized optics and a scan resolution which can be user variable from 50 to 130µm pixel pitch. Plates can be fed on a continuous basis, with one plate being fed into the unit while another plate is being scanned. Results are displayed on a black and white monitor with a resolution of 3 million pixels and 65000 grey scale. The data is managed and processed using GEs Rhythm software platform, which offers advanced image review tools and storage cabalilty. Data is stored in the DICONDE protocol, which has been specifically developed for non-destructive testing inspection applications and allows images to be stored with text to facilitate future database searches. Currently, GTSG is storing data on CDs and DVDs, although this can be converted to computer hard drive if required. Moreover, the Rhythm suite also includes Rhythm Archive, an elegant and comprehensive long-term data storage solution.
Since obtaining the DNV approval, the technique has also been approved to AS 2885.2 and as Dave Ryan, managing director at GTSG, says, We believe that Computed Radiography has a lot to offer the oil and gas industry and we are very pleased to have played a part in ensuring that the technique has gained the approvals it merits. We view it as an important addition to our existing range of NDT technologies.