When the pipeline operation temperature increases, pipe material will expand and the expansion will accumulate resulting in an extension of the pipeline. The increase in pipe length will be accommodated by pipe bending deformation in some part of the pipeline, which is often accompanied with pipe displacement. Due to the ratcheting behaviour of the ground soil, pipe displacement will not fully recover by moving to its original position when the temperature drops. This phenomenon is common in pipelines, particularly for those with frequent operation temperature changes and with unstable ground conditions such as muskeg and frost heaves.

Continued changes in thermal expansion of a pipeline will make the pipeline operating in a condition with a high additional pipe stress that is not included in the original pipeline design stress analysis. Pipeline failures due to thermal expansion have been reported in the past decades, most cases have been in the northern areas of Alberta. To ensure a safe operation, pipeline thermal expansion assessment needs to be conducted, especially for scenarios where an operation temperature increase is proposed where ILI data shows pipeline profile changes or where an above ground pipeline facility displacement is observed.

The controlling mechanism for pipeline thermal expansion failures is the pipe stress, and thus the focus of the assessment should be in stress analysis. Depending on the complexity of the given case, different levels of FEA can be used. For high level pipe displacement and stress screening, AutoPIPE can be used. For detailed local stress analyses, 3D FEA packages such as ABAQUS and ANSYS are normally used at CCPGE.

Stress analysis was conducted by using commercial FEA software for a case study. The pipeline is NPS42, and the valve weight is about 12,000 kg. The valve station portion of the pipeline is isolated for FEA modeling. The images below show the 3D FEA model and the output. Contact elements are used between the pipe elements and the soil elements. According to the stress results, the above ground bends are the assessment focus of integrity where the stress level is a function of the lift amount.

The main focus for thermal expansion analyses is on the ovality and stress level at bends and elbows. The assessment may involve wall thickness verification and threat feature assessment, such as SCC, cracks and dents.

At CCPGE, we typically use AutoPIPE for high level stress screening and use ABAQUS for local detailed analyses. The figures below show a case of operation temperature profile change for a pipeline having muskeg ground condition.

The figures below show an example of pipeline operation temperature increase. The ground partially has muskeg. The high level FEA model was built using AutoPIPE. The first figure shows the pipeline geometry profile change due to thermal expansion. Detailed local bending analyses were carried out using ABAQUS. The second figure shows an image of elbow ovality and the stress contour.

Depending on the type and the scale of a geohazard, the magnitude of the external loading to the pipeline varies. To ensure that the pipeline is operating in a safe condition, stress analyses are required. Based on the stress levels and the criticality of the system, integrity management plans can be made for system remediation, such as stress relieving and operation pressure changes.

At a high level, pipeline stress analyses using FEA for geohazard problems are not different from stress analyses of other loading conditions. Due to the time dependent nature of geohazard loading, however, special geotechnical data types and assessment scheduling are required. For example, slope sliding can happen either in a short time or gradually during a long term period. The soil property data has to be location and season-specific. Pipe soil interactions must be considered and proper FEA element types need to be selected.

Frost Heave and/or Thaw Settlement

Frost heaving and thaw settlement are a threat to pipelines and station piping. By using finite element analysis, stress conditions can be assessment for a given case and stress relieving plan can be made for mitigation. The pictures below show a case of meter station piping there the skid lifting due to frost heaving caused high stress in the piping. By identfying the key loading areas, significant stress relief was achieved through hydro excavation (hydrovac).

Slope Sliding

Slope stability is typically considered in the pipeline design stage. Detailed ground condition information, however, can only be obtained when a geotechnical survey is conducted during the pipeline operation. Depending on the soil types, slope sliding can cause various levels of stress in the pipe and the highest stress may not be right at the soil fracture locations. The figures below are from the analysis of a river band slope sliding. The left figure is an as-built drawing and the right figure shows a stress contour in a 3D FEA model that used site-specific data.

Flood Caused Exposure of Pipe

Flood can cause changes in pipeline support conditions where the pipe is exposed in the curent. Vortex will be generated in the current and it in turn will induce pipeline vibration. This phenomenon is called vortex induced vibration (VIV). Mechanically the pipe will experience cyclic deformation in VIV, and depending on the case specific parameters the pipe may be at a risk of fatigue failure. At CCPGE, we use DNV-RP-105 as analytical guidelines and FatFree tool for critical span calculations. Figures below show a case of river crossing, for that the critical span calculation is shown in the right figure.