QMT Features: May 2013
Wrinkles and buckles
Verifying the quality of CRA-lined pipes by detecting the presence of wrinkles or buckling using high precision non contact metrology technologies. By Hugh Davies of Optical Metrology Services (OMS)

For deepwater offshore pipelines, risers and flowlines, as well as pipes for onshore applications in oil, gas and petrochemicals, the demand for corrosion-resistant alloy (CRA) clad pipes is expected to grow, with more than 80% of developing oil and gas fields demonstrating corrosive properties.

CRA clad pipes are protected from the production fluid by a liner that is mechanically bonded to the inside of the pipe. Production fluids (oil and gas) often contain a high sulphur or CO2 content that requires the use of materials that are suitable for corrosive environments. When the sulphur and CO2 content are too high for the corrosion resistance properties of standard carbon-steel pipes, a CRA liner is often employed.

Once oil and gas pipes have been manufactured and lined, they are often subjected to bending or reeling trials in order to verify whether the CRA liner is likely to buckle or wrinkle (and the extent of this) due to the high bending stresses involved. These trials normally take place during the bending process itself after the pipes have been lined. This simulates the pipe being continuously bent and straightened whilst reeled and unreeled from the spool onboard an ocean pipelaying vessel.

By inspecting the inside profile of these pipes, companies can verify whether the liner has wrinkled or buckled during the bending trials (and the extent of this), or whether the liner is likely to wrinkle under repeat bending and straightening aboard the pipelaying vessel.

Traditionally, inspecting the inside of lined pipes has been done manually or by projecting a laser ring around the inside of the pipe and then estimating measurements based on a camera view of the ring. These systems are relatively crude and much more sophisticated, reliable measurement systems are now available.

For example, OMS has developed various systems for inspecting the inside profile of CRA lined pipe. Since 2002, OMS has been performing pipe bending trial measurement surveys for the companies such as BP, Technip, Subsea 7 and Exxon Mobil. These surveys range from relatively simple ovality measurements of pipes, through to much more detailed  3D profile surveys.

OMS’ system for pipe bending trials – the Automatic Pipe Checker system – uses a high precision laser profile measurement probe, which is part of a fully calibrated measurement module, which in turn is mounted to a rotary arm. The system uses a precision linear slide, which moves down the inside of the pipe, stopping at regular intervals to measure the circular profiles (inside diameter cross-sections). In this way, entire sections of pipe can be profiled within a consistent coordinate frame. System accuracy varies from 0.5mm to 0.05mm, depending on the pipe and customer requirements.

The internally-calibrated laser profile measurement module is able to record up to 2,048 measurements per ID cross-section (which are typically smoothed out and reduced to 600 points). Smoothing of data points is critical, since this will remove any outliers caused by features such as debris inside the pipe. Any unwanted features can also be manually edited such as debris on the wall of the pipe.
For typical bending trials, pipe sizes range from 125mm up to 1,200mm. Often, measurements are confined to a small area of the pipe, but at other times, these can include the complete length of a pipe. Each profile takes about 15 seconds to measure. It takes another 15 to 30 seconds to review the profile and move on to the next measuring position. For example, full measurement of an 8m long pipe at 20mm intervals will require 400 separate measurements. These will take 400 x 45 seconds to complete or around 5 hours of continuous work. Accounting for breaks and data calibration checks, one 8m long pipe normally takes one day to arrive, set up and measure.

These overlapping profile measurements enable pipe sections to be “stitched together” or mapped in the Automatic Pipe Checker software to form a consistent 3D model of the interior of the pipe. Comprehensive suveying of the internal profile of pipes can be carried out every 1mm through the pipe – at every stage of a project if the customer requires. Measured data can then be mapped into accurate 3D CAD models in a format (e.g. SolidWorks) that the customer requires in order to drive further analysis, for example, Finite Element Modelling. This, in turn, can facilitate part of a life prediction for the pipe.

The Automatic Pipe Checker system is fully calibrated and measurements are traceable to National Standards. OMS staff carrying out the inspections will typically be responsible for all stages of the measurement survey: setting the tool, calibrating the tool, measuring the pipes, verifying the tool (calibration checks), and then checking the measurement data itself.

In addition to laser profile measurement tools, OMS also uses other inspection techniques to supplement this. Video inspection tools can be used to visually inspect the internal profile of pipes. This equipment typically comprises a tractor and camera systems, together with LED lighting (end and side lighting) and digital video recording capability.

Using several techniques to inspect the lined pipes enables OMS to supply sophisticated geometrical analysis of the measurement data, including detection of features, analysis of wrinkles or buckling of the liner, 3D modelling of features, and differential analysis (i.e. comparisons between different measurement surveys). Colour visualisations of radial deviation can also be provided.l


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