Ian Murray reveals how mapping can be used to add long-term value to in-line inspection
The remote nature of long-distance pipelines can expose them to a range of external loads. Earthquakes, landslides, sea bed movement, ship anchor drags, permafrost, flooding, third party damage, construction and backfill all have the potential to locally deflect a pipeline from its ‘as built’ position. Although steel pipelines exhibit small amounts of inherent ductility, any deflection from the design centre-line will impart an increased level of strain into the material. Too much deflection can cause buckling, wrinkles, damage at weld or defect locations and ultimately, failure. Although the incidence of natural force damage is comparatively low, the consequences are far greater. Natural force damage only equates to 8% of significant incidents but causes 34% of all property damage. This is because these incidents tend to result in pipeline rupture rather than leakage, hence greater spill volumes, longer downtime and increased property and environmental damage.
It is possible to identify ground conditions and locations where pipelines may be at risk of damage from bending deformation and where mapping should be considered as a matter of course. As well as any pipeline that has known or suspected pipeline movement or failure history, these include pipelines in areas of known or suspected soil settlement, wash-out or flooding, known or suspected subsidence or landslide, known or suspected earthquake or seismic activity, known or suspected sand dune migration and areas experiencing large seasonal swings in ground conditions (dry to wet, frozen to thawed). Subsea pipelines in areas of known or suspected seabed movement or areas of spanning, as well as subsea pipelines in areas of known marine activity where there is a chance of anchor drag or trawling should also be assessed routinely. There is also a major long term benefit of mapping newly laid pipelines in order to validate ‘as laid’ straightness and provide a baseline for future integrity surveys.
As it is not practical to directly measure the strain in the pipeline material along sections that might extend to hundreds of kilometres in length, the strain is calculated by considering the curvature of the pipeline. Curvature is a numerical measure of how ‘bent’ a pipeline is and it is defined as the angle a pipe turns through over distance. To assess the additional strain in a pipeline due to natural force damage or other external loading, the exact position is surveyed using an in-line inspection tool or ‘pig’ equipped with an inertial measurement unit (IMU) module. The IMU measures the pig’s movement in 3D, using three gyroscopes to measure rotation and three accelerometers measure acceleration plus gravity. The resulting data is used to determine pipeline coordinates in 3D so that pipeline curvature and resultant strain can be calculated.
Integrity monitoring
Pipeline mapping can be carried out as part of a wider integrity monitoring programme where defects and metal loss can also be identified in a single run using smart pigs. PII Pipeline Solutions’ (PII) MagneScan high-resolution metal-loss inspection for advanced length and width sizing of pitting and narrow axial external corrosion (NAEC) or CalScan EP (Caliper) tools can locate and measure dents and other geometric deviations. Inclusion of the mapping function makes little change to the overall logistics of an inspection run. The main additional activity is the provision of surveyed reference points prior to inspection, approximately every 3km (2 miles) along the pipeline. These points can be features such as block valves or temporary above-ground markers. Applying multiple inspection techniques in a single run helps to make the best possible use of a time-limited inspection window.
The mapping data provided by the IMU is used to create a 3D model of the pipeline’s actual centreline co-ordinates so that any areas of significant curvature and the associated bending strain magnitude can be identified and investigated. When repairs are required for defects reported by an inspection, highly accurate IMU coordinates enable the pipeline operator to quickly and reliably locate them via a precise GPS location prior to excavation, significantly reducing digging costs and in-field time. With a GPS accuracy of ± 1.5m the IMU mapping technology helps pipeline operators plan the most effective and efficient repair methodology by taking into account local geography and third-party constraints that may impede site access. If bending strain is found, remedial action can include exposing the pipe and replacing backfill or rock dumping. In extreme cases, extended environmental loading can lead to buckles, which need to be cut out and repaired.
Various existing industry codes consider the effect of excessive bending strain and offer guidance on limits. The presence of an axial bending stress can reduce the failure pressure of a circumferentially orientated defect including cracks and corrosion. Several fracture mechanics based methods can be used to estimate the axial failure stress for a circumferential flaw in a pipeline. The total stress due to internal pressure and axial bending load can then be compared to the estimated axial failure stress.
Reporting bending strain allows consistency of results between different pipe diameters as well as highlighting areas that may be a potential integrity threat. When a single run analysis is carried out without any historical data, the strain on the line is calculated from the measured curvature. Considering a pipeline subjected to a maximum radius bend of 400 x diameter (400D), over a 12m length the strain will be 0.125%. The 400D curvature threshold is roughly equivalent to the strain at yield for Grade B line-pipe. When historical data is available, the comparison with a previous inspection greatly improves confidence in the identification of low-level deformations. Changes in strain as low as 0.02% (equivalent to a 2500D bend, over a 12m length of pipeline) can be detected when new IMU data is compared with a benchmark dataset. During field testing the performance of PII’s IMU mapping system has been confirmed by blind tests in a client’s pipeline. In one particular test, the client exposed a 60m length of pipe and displaced the centre by 200mm. By running an IMU tool before and after the deformation, PII successfully located and sized the deformation feature in 29km of 30” pipeline. Other run-to-run comparisons have confirmed the repeatability of PII’s bending strain data, both onshore and offshore.
PII has first-hand experience of inspecting undersea pipelines that have been subjected to considerable external force. The company was recently engaged by a European customer with a number of large diameter offshore lines in its infrastructure portfolio. A single IMU inspection run was undertaken as part of a strain screening investigation to produce a baseline assessment. When the data was analysed it identified areas of deflection from the design centreline by up to 90m. Further investigation indicated that previous repairs to the pipeline had been carried out in the area that had suffered the most severe deflection, potentially causing the movement. In other areas it appeared that the damage and pipeline movement was consistent with impact from an anchor and subsequent dragging.
As well as helping to assess bending strain, IMU mapping can help pipeline operators to satisfy regulatory demands. Increasingly, regulations demand that pipeline operators document the precise location of pipeline assets. In some cases, however, records are old and of unknown accuracy, or may not include details of centreline location. Pipeline mapping can also benefit operators by determining the precise location of each girth weld and pipe feature.
Weld ruptures
Another example of where mapping has been fundamental to an operator’s integrity monitoring programme came during PII’s inspection of a spirally welded crude oil pipeline. The pipeline had been built during the 1970s in a geologically unstable area with additional ground condition variations. A number of the spiral welds had suffered ruptures due to the combined loading from internal pressure, cyclic pressure loading and axial stress from ground movement. PII’s initial inspection established that the pipeline was subject to multiple threats including internal and external corrosion, spiral weld anomalies/cracks, girth weld anomalies/cracks, dents and ovalities. Triax magnetic flux leakage (MFL), calliper and IMU ILI tools were deployed into the line to detect and quantify threats.
More than 0.5 million defects were detected by the ILI tools together with over 1,000 strain events. With such a wide range of combined threats, PII created an assessment matrix to govern assessment rules and criteria.
Increasingly, strain-based designs are being considered for new pipelines. These designs can use modern pipe material such as x80, x100 or x120. With strain-based designs it is even more important to confirm that the strain capacity of the pipeline has not been fully utilised during pipe laying. An IMU strain inspection can provide pipeline operators with this confirmation.
Strain measurement is an excellent indicator of where unknown or unexpected pipeline movement may have occurred. By identifying change of shape of a pipeline and any potential movement since the last inspection run it offers enhanced integrity monitoring and early warning of ground instability. Strain measurement also helps prevention of failures through identification of strain events and coincident features throughout the pipeline. Combined with PII’s IMU technology, it provides an invaluable integrity-monitoring tool for oil and gas pipeline operators.
Ian Murray is with PII Pipeline Solutions.
