October 2014, Vol. 241, No. 10
Features
Add Longer-Term Value To Inline Inspection Through Mapping
Natural force damage from earth movement and heavy rains or floods accounts for only 8% of all pipeline failure incidents. However, these type of incidents account for 34% of all property damage.
Natural force damage tends to result in rupture failure rather than leaks, hence greater spill volumes, longer downtime, and increased property and environmental damage.
This article outlines how reliable data gathering and the use of inertial measurement unit (IMU) technology during inline inspections (ILI) can help prevent failures of this kind.
The remote nature of long-distance pipelines can expose them to a range of external loads. Earthquakes, landslides, seabed movement, ship anchor drags, permafrost, flooding and third-party damage all have the potential to deflect a pipeline from its as-built position.
While steel pipelines exhibit small amounts of inherent ductility, any deflection from the design centerline will increase the level of strain to the material. Too much deflection can cause buckling, wrinkles, damage at weld points or defect locations – ultimately causing failure.
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 in addition to any pipeline with a history of movement or failure. These include pipelines in areas of potential soil settlement, flooding or the potential seismic activity.
Subsea pipelines in areas known for seabed movement or marine activity, including anchor dragging or trawling should also be assessed routinely. There is also a major long-term benefit in 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 extend hundreds of kilometers 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 is defined as the angle a pipe turns through over distance.
In order to assess the additional strain in a pipeline due to natural force damage or other external loading, the exact position is surveyed using an inline 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 to 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.
Pipeline mapping can be carried out as part of a wider integrity monitoring program in which defects and metal loss can also be identified in a single run using smart pigs.
Inclusion of the mapping function changes the overall logistics of an inspection run. The main additional activity is providing surveyed reference points prior to inspection – about every two 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 inertial mapping unit (IMU) is used to provide a 3D model of the pipeline’s actual centerline coordinates 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 locate them via GPS prior to excavation, significantly reducing digging costs and in-field time.
With a GPS accuracy of ±1.5 m the IMU mapping technology helps pipeline operators plan the most effective repair methodology, taking into account local geography and third-party constraints that may impede 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 that 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 axial-bending stress can reduce the failure pressure of circumferentially orientated defects, 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 consistent results for different pipe diameters as well as highlighting areas that may be potential integrity threats.
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 2,500D 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 test, the client exposed a 60m length of pipe and displaced the center by 200mm. By running an IMU tool before and after the deformation, the company located and sized the deformation in 29 kilometers of 30-inch pipeline. Other run-to-run comparisons have confirmed the repeatability of the bending strain data, both onshore and offshore.
PII was 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 analyzed, it identified areas of deflection from the design centerline by up to 90m.
Further investigation indicated that previous repairs to the pipeline had been carried out in the area suffering the most severe deflection, possibly causing the movement. In other areas it appeared the damage and pipeline movement was consistent with contact from an anchor and subsequent dragging.
As well as helping to assess bending strain, IMU mapping can help pipeline operators satisfy regulatory requirements. Increasingly, regulations demand that operators document the precise location of pipeline assets. In some cases, however, records are old and of questionable accuracy – some may not include centerline location details. Pipeline mapping can also benefit operators by determining the precise location of each girth weld and pipe feature.
An example of mapping being fundamental to an operator’s integrity monitoring program came during inspection of a spirally welded crude oil pipeline. The pipeline was built during the 1970s in a geologically unstable area with additional ground condition variations. A number of the spiral welds had ruptured due to loading from internal pressure, cyclic pressure loading and axial stress from ground movement.
The initial inspection showed the pipeline was subject to several threats, including internal and external corrosion, spiral weld anomalies/cracks, girth weld anomalies/cracks, dents and ovalities.
Triax magnetic flux leakage (MFL), caliper and IMU ILI tools were placed in the line to detect and quantify threats. More than a half million defects were found by the ILI tools together with over 1,000 strain events. With such a wide range of combined threats, an assessment matrix to govern assessment rules and criteria was created.
It was noted that crack defects identified as acceptable under pressure load may be unacceptable when bending strain is taken into account.
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 used during pipe laying. An IMU strain inspection can provide pipeline operators with this confirmation.
Strain measurement is an excellent indicator of where pipeline movement may have occurred. By identifying change of shape and any potential movement since the last inspection run, it offers enhanced integrity monitoring and an 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 IMU technology, it provides an invaluable integrity monitoring tool for oil and gas pipeline operators.
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