Pipeline integrity management comprises of all the activities required to anticipate and prevent a pipeline failure. Among the various operations and maintenance (O&M) measures, in-line inspection (ILI) plays an important role. ILI tools can detect metal loss defects, cracks and geometry features.
Whilst completing regular ILI surveys, in recent years it has become routine practice for many operators to map the position of the pipeline by including an Inertial Mapping Unit (IMU) during the ILI survey. The IMU provides a synchronized stream of mapping information (coordinates) which, aligned to the ILI data, gives the means to accurately map the pipeline and easily locate pipeline anomalies, features and fittings.
The data collected from the IMU has a further use which is less widely realised. Specialized assessment of the IMU data can be performed to determine where sections of the pipeline have potentially deviated in relation to the original position. The use of data from repeat IMU surveys can identify even small changes in the shape of the pipeline that have occurred between the inspection surveys.
Calculations can be performed using the IMU data to determine the curvature (or change in curvature from two surveys) at these locations and to evaluate the associated consequential bending strains.
The analysis of the data from a combined MagneScan Magnetic Flux Leakage (MFL) and IMU ILI survey was instrumental in identifying and quantifying the extent of an offshore pipeline displacement. The primary purpose of running this combined tool had been to detect and size the metal loss features, detect geometry features and assist in physically locating anomalies and features. However, in this case study, the IMU data was also utilized to quantify the displaced length and consequential bending strain of the subsea pipeline. This resulted in considerable saving of costs which would have been incurred should an underwater inspection spread have been needed to measure the extent of the displacement.
During the analysis of the MFL data an unusual signal pattern (Figure 1) alerted the analyst of potential damage to the concrete coating in a section of the pipeline; this damage had not been evident in the previous ILI run.
An alert was raised for further investigation since the offshore pipeline is susceptible to ship anchor pull over damage and other displacements due to high environmental stresses.
A review of the magnetic inspection survey data from the previous ILI in 2010 (Figure 2) showed that the concrete coating is intact, i.e. 0.4m upstream/downstream of the girth weld. However, comparison of the same girth weld in the new 2016 magnetic inspection survey (Figure 3) shows that the concrete coating has been damaged, exposing a circa. 5.2m section of the pipeline. i.e. 3.8m upstream and 1.4m downstream. No metal loss defects were identified at this location in either inspections.
The pipeline is routinely surveyed using Remotely Operated Vehicles (ROV). The ROV survey conducted at the time of the previous ILI did not report any concrete damage coincident with this location. The next routine underwater survey of this pipeline was conducted in 2015, approximately one year before the 2016 ILI survey. The KP data from this ROV survey was aligned with the absolute distance from ILI data. The ROV survey data was reviewed in the area of interest for any concrete coating damage of a similar length as identified from the MFL signal data analysis. Concrete coating damage of circa. 5m in length was observed at the same position as the damage found from the ILI data.
The 2015 ROV findings are illustrated in Figure 4. As expected, the 2015 ROV survey reported 25% anode depletion near to the weight coat damage (anode depletion is expected where bare metal is exposed). However, the cathodic protection (CP) potential readings were still reported to be adequate to protect the bare metal area from external corrosion. This agreed with the ILI finding of no external metal loss at this location.
Although, external corrosion was not an immediate threat at this location so long as the CP system was protecting the exposed pipe, this section of pipe had no protection against external interference and furthermore the pipeline had been displaced from its original position and consequently was subjected to additional external bending loads due to its displacement.
The pipeline had been surveyed multiple times using combined MFL and IMU tools, and comparison of the repeat IMU data using BHGE’s proprietary StrainCom™ software was used to identify and quantify changes to the pipeline shape (i.e., curvature) and the consequential bending strains.
By directly overlaying both sets of IMU survey data (Figure 5) change in the shape of the pipeline is evident, resulting in a change in both the horizontal and vertical bending strains at the location. The IMU data comparison revealed that a 280m section of the pipeline had been laterally displaced by up to 8m. The reported magnitude of the horizontal bending strain associated with the global curvature in the displaced pipe was measured at 0.12%; however, it is possible the strain in the area is, or has previously been, higher. At the point of maximum displacement, which coincided with a girth weld, a localised change in the curvature was observed (possibly due to a local deformation in the pipe shape) causing a sudden peak in the strain magnitude.
This location will be investigated further during the forthcoming underwater survey campaign and a caliper and IMU survey is planned to be conducted at the time of the next metal loss ILI to monitor the condition of this pipe section.
A full assessment of the IMU survey data along the entire pipeline identified a further four other areas of bending strain and an additional four areas where a change in the bending strain has occurred.
None of these locations have any metal loss or mill/manufacturing anomalies reported by the ILI tool. If any anomalies were reported in locations of bending strain, all stresses due to bending, thermal elongation/contraction, Poisson’s effects (due to internal pressure in a pipeline that is restrained by anchors or soil interaction), axial forces due to free spanning or buoyancy would need to be considered for the assessment. The bending stress can be derived from the bending strain obtained using BHGE’s StrainCom software.
The use of IMU data, BHGE’s Bending Strain assessment methodology and StrainCom software resulted in considerable cost savings which would have been incurred should a subsea inspection spread had been needed to measure the extent of the displacement.
Recent Pipeline Bending Strain papers published by BHGE include:
Managing the Threat from Weather and Outside Force Using In-line Inspection. Jane Dawson, Ian Murray, PPIM Conference, Houston, Texas, February 2017.
Achieving critical assessments of pipelines through accurate and reliable inspection information. J. Sutherland, A. Caley, J. Dawson, M. Bluck. Ageing Pipelines Conference, Belgium, October 2015.
Assessment of ILI Data for the Effective Integrity Management of Pipeline Threats. D. Adler (Columbia Pipeline Group), J. Dawson, PPIM Conference, Houston, Texas, February 2015.