Depth of Cover

The ROSEN Group recently completed an innovation project, in partnership with National Grid Gas Transmission (United Kingdom), which successfully demonstrated a new methodology to identify depth of cover over an entire pipeline. This methodology takes into consideration and combines ground elevation data and high-resolution data collected during an inline inspection (ILI).

A 36” diameter, 43 km high pressure pipeline was chosen for the project. High pressure pipelines such as this are required to be inspected to support continued safe operation. In this case, an ILI was performed using the ROSEN RoCombo inspection tool which included ROSEN’s RoGeo XYZ service. Not only does this service aid in the identification of complex pipeline features, such as dents, it also contains an optimized inertial measurement unit (IMU), which accurately measures the position of the tool inside the pipeline.
The inspection data is combined with known reference locations placed at ground level along the pipeline at a spacing if up to 500 m to produce an accurate pipe centerline.

Related presentation at ptc 2018

Ground elevation data collected using light detection and ranging (LiDAR) techniques, available in the UK from the Environment Agency, was combined with the accurate pipe centerline to calculate the depth of cover for the entire pipeline. LiDAR is a remote sensing method which uses laser light to measure distance to a target and is commonly used to map terrain and surface objects. The advantage of this method is that a large amount of highly-accurate data can be collected allowing large areas to be surveyed efficiently. Other methods to capture ground elevation data, such as photogrammetry, can be used for smaller areas.

With the pipe centerline data derived from the IMU data and the ground elevation data, the depth of cover was calculated. The results of the calculation were validated against over 2000 infield depth of cover measurements obtained with a pipe and cable locator. This demonstrated a depth of cover accuracy of ±0.15 m root mean square (RMS) error.  The image below shows a sample report for a pipeline where the pipe centerline is colored based on pre-determined threshold values. This provided the operator with a visual record of the results and allows for further investigation to take place.

This project successfully demonstrated ROSEN’s methodology to calculate the depth of cover over pipelines. This methodology enables operators to review the depth of cover on the entire pipeline and identify locations not meeting the minimum requirements. These locations may have an increased likelihood of damage occurring from third parties, therefore the operator can implement mitigation measures. Previously, this work would have been undertaken by pipeline technicians performing time consuming survey activities in the field. The new methodology enables accurate estimates of depth of cover to be delivered as an additional service alongside a traditional in-line inspection.