At an LNG terminal, an unexpected settlement of 12 inches of soil, containing several sections of Vacuum Insulated Piping (VIP), occurred. Because the above-ground pipe supports were unaffected by the settlement, the differential displacement between the above-ground supports and the underground soil resulted in plastic deformation of some of the connected above-ground piping.
Dynaflow Research Group (DRG) was requested to assess the consequences of this settlement. In addition to analyzing pipe stress, the assessment considered in-line components such as flanges, bellows, and the VIP internals. Based on the outcome of this analysis, the client was able to decide if remedial actions were necessary. DRG proposed a mitigating strategy to reduce the impact of the settlement on the piping components.
Analysis
The current state of the pipeline was based on accurate measurements of the above-ground pipe deformation. Stresses, plastic deformation, and fatigue damage were computed for the present condition, as well as the cyclic capability for continued use. To enhance the accuracy of the analysis model and obtain a more precise picture of the current situation, the model was improved in several ways. The torsion resistance of the soil on the pipe was included in the model, the plastic pipe deformation was artificially modeled in the otherwise elastic model, and the ASME code yield stress values were replaced with an accurate estimation of the true values.
Although the analysis results indicated that part of the total deformation was indeed plastic, the plastic strain was still small concerning the ultimate strain of the carbon steel pipe material. A Finite Element Analysis of the highest loaded flange showed that it exceeded the yield stress in a limited region at the hub. However, the gasket pressure was largely maintained, and no leakage was predicted from the calculation. The internals, consisting of cone-to-pipe connections, spacers, and expansion joints, were also assessed. Using realistic material values with a sufficient safety margin, no problematic loads or stresses were found for these components.
Results
Based on the analysis results, it was concluded that leaving the pipe in its current settled position was an option, provided that future settlement would be monitored. To prevent uncertainties due to future settlement, a mitigation procedure involving the excavation of the pipe and lifting the pipe section was also examined and put into a detailed step-by-step lift plan. Recoil of the excavated pipe reduces the stress levels because this redistributes the imposed differential displacement due to settlement over a longer pipe length. Correct lifting prevents high weight stresses and loads in the excavated pipe, which would otherwise develop due to the lack of support from the underlying soil.
This incident analysis demonstrates the importance of a thorough assessment when dealing with soil settlement impacts on piping systems. The comprehensive approach by DRG, which included accurate modeling and stress analysis, ensured that the client had a clear understanding of the situation and the potential risks involved. By proposing effective mitigation strategies, DRG provided a practical solution to manage the impact of soil settlement on the LNG terminal’s piping infrastructure.
