In a nutshell
Sufficient depth of cover for pipelines is a key means of mitigating pipeline incidents due to third-party interference; a UK Onshore Pipeline Operators Association¹ report states that 20% of all product loss incurred between 1962 and 2014 was due to external interference. However, proper and accurate measurements of these values can be hard to come by, especially for the entire length of a buried pipeline. In a research and development project together with National Grid Gas Transmission, the ROSEN Group has begun validating a two-element approach to produce precise results. By combining specialized in-line inspection tools and ground elevation data, these calculations can be made to an accuracy of ±0.15 m root mean square error. Read the details of the project below.
Third-party interference is widely documented as being a major cause of damage to buried pipelines. This means damage caused not by material failures, equipment malfunction, or structural failure but by human interference, such as construction. Pipeline depth of cover can change over time due to natural erosion of soils, natural shrinkage of soils, human activity, or failure of anti-buoyancy systems. In addition to routine surveillance, maintaining a minimum depth of cover is recognized as a key means of mitigation against third-party interference. Current techniques available for measuring depth of cover on buried pipes require significant effort to produce a detailed survey for an entire pipeline.
Regulations require pipelines to have minimum depth of cover depending on their location.
The ROSEN Group recently completed an innovation project funded through the Network Innovation Allowance scheme, which is provided by the UK gas regulator OFGEM. Conducted in partnership with National Grid Gas Transmission, it successfully demonstrated a new methodology to identify reduced depth of cover over an entire pipeline. This methodology takes into consideration and combines ground elevation data and high-resolution data from an inertial measurement unit (IMU).
Table 1 provides a summary of minimum depth of cover stated in relevant UK standards.
Table 1: UK Standard Requirements for Minimum Depth of Cover²
Note 1: There were no specific cover requirements for road, rail and water course crossings prior to Edition 5 of IGEM/TD/1 other than a requirement in Edition 2 and later editions for greater than 300 mm above the concrete slab that was required when crossing under a ditch.
A 36”, 43 km natural gas pipeline was chosen for the trial. Prior to the in-line inspection, above-ground markers (AGM) were deployed at a nominal interval of 500 meters between markers. AGMs are devices placed directly over the buried pipeline being inspected to ensure that the in-line inspection tool provides accurate geographical data. The smaller the distance between each marker, the more accurate the results. For that reason, the 500 meters marker spacing was chosen. At each AGM, the pipeline position was recorded using a high-accuracy GPS system and a pipe and cable locator.
Following deployment of the AGMs, the in-line inspection was completed and inertial measurement unit (IMU) results were processed by the ROSEN data analysis team, resulting in an accurate pipe centerline. These units contain gyroscopes and accelerometers and are used to calculate the position of the inspection device. The IMU data is linked to known reference locations along a pipeline route to provide an accurate pipe centerline as a series of X, Y, and Z coordinates.
Ground-elevation data collected using light detection and ranging (LiDAR) techniques, available from the Environment Agency in the UK, was combined with the accurate pipe centerline to calculate the depth of cover for the entire pipeline. LiDAR is a remote sensing method that 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.
To conclude the project and authenticate the results, ROSEN engineers undertook in-field GPS and pipe depth measurements, which demonstrated a depth of cover accuracy of ±0.15 m root mean square error.
All in all, this research project has shown that the methodology can accurately conduct depth of cover measurement. This has enabled National Grid Gas Transmission 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, National Grid Gas Transmission 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.
The figures below show an example of ground and pipe elevation plotted against distance along the pipeline (left) where it crosses a series of embankments, and where ditches are in close proximity. The figure on the right shows the ground elevation with hill-shade rendering to aid visualization. The example clearly demonstrates how the inspection tool has measured the change in pipe elevation as the pipe passes beneath the ditch crossing. The increase in depth of cover associated with the two embankments is also evident.
The image below shows a sample report for a pipeline where the pipe centerline is colored based on pre-determined threshold values. This provided NGGT a visual record of the results and allows for further investigation to take place.
¹UKOPA/15/003. UKOPA Pipeline Product Loss Incidents and Faults Report (1962-2014). December 2015
²UKOPA/GP/001. Managing Pipelines with Reduced Depth of Cover. Edition 1, January 2016.