Measurement Of Roof Topping Effects As An Integral Part Of Routine In-Line Inspection
Figure 1: Outward peaking (top) and inward peaking (bottom) of steel plates along the long seam(illustration from Boothby, 2009)
Caused by improper crimping of the steel sheet in the production process, roof topping is a geometric anomaly that occurs along the longitudinal seam weld of the pipe. These incorrectly crimped sheets then result in a deviation from the perfectly circular geometry of the pipeline. The implications of roof topping for pipeline integrity are severe. They include risk of failure during hydro-testing or fatigue failure during pipeline operation.
Whereas the roof topping effect has so far typically been assessed using a profile gauge to take an angle measurement manually from the outside of the pipe, a high resolution geometry tool now enables automatic measurement of all joints from the inside as part of routine In-Line Inspection. Providing 100 % coverage, this tool measures and plots the radius at each point of the pipe, so that any deviation from the ideal circle is detected immediately. The maximum deviation is then used to calculate the roof topping angle. This paper discusses the nature of the measurement method, relevant experience gained in the field, and the degree of accuracy achieved with this high resolution geometry technology.
Roof topping (also referred to as pipe peaking) is defined as a non-circular geometric anomaly that occurs along the longitudinal seam weld of pipelines (Boothby, 2009). This effect is more commonly found in, but by no means limited to, older pipes. As part of the manufacturing process of longitudinal pipes, the plate edges are crimped before final welding. If crimping is done incorrectly, this can result in a departure from the perfectly circular geometry of the pipe when the edges of the plates are welded together. A distinction is made between positive (i.e., outward pipe peaking) and negative (i.e., inward pipe peaking) roof topping (see Figure 1).
Excessive peaking resulting from incorrectly crimped sheets can lead to cracking or splitting along the longitudinal weld in the cold expansion stage of the manufacturing process. It is not surprising, therefore, that roof topping can have profound and far-reaching effects on pipeline integrity. These notably include the risk of failure during hydro-testing and even fatigue failure during pipeline operation. As such, they pose a serious threat to the environment. Where the effects of roof topping coincide with ovality, the stress concentration factor is further exacerbated. The conventional approach to assessing the roof topping effect has been to use profile gauge to take an angle measurements manually from the outside of the pipe (Figure 1).
A fresh approach to the challenge of measuring roof topping
A resolution geometry tool is now available which takes a fresh approach to the challenge of gauging roof topping: it enables automatic measurement of all joints from the inside as part of routine In-Line Inspection. Combining the benefits of two sensor planes with the advantages of a mechanical caliper arm with touchless high resolution distance measurement, this high resolution geometry tool provides highly accurate geometry data on the internal contour of the pipeline (Figure 2).
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