Integrated Hazards Mitigation and Fitness-for-Service Assessment for Effective Landslide Management

When a landslide event was identified in 2019 in the eastern US, an integrated approach between hazard mitigation and fitness-for-service (FFS) assessment was taken to develop the most effective approach to address both immediate and potentially long-term integrity concerns. A quick-turnaround preliminary FFS analysis determined that the line could be operated at normal pressure and that immediate site work was not necessary. A more refined analysis later determined that a stress relief excavation was unnecessary. Site work focusing on water management was performed to reduce the possibility of future movement. Strain gages were installed at strategically selected locations. These strain gages are monitored and compared regularly with threshold strains established through FFS analysis. Further evaluation and site mitigation would occur if the strains were to approach the intervention threshold.    

A geotechnical investigation suggested that a pipeline might have been impacted by previous and current landslides. The owner engaged CRES to conduct an FFS assessment and to work with Golder Associates to develop necessary mitigation and monitoring measures. The FFS assessment was to estimate the strain demand imposed on the pipeline by the landslide, assess the strain capacity of the pipeline, and determine the current integrity status and tolerance to further ground movement.

In February 2019, when the current ground movement was identified, information on the landslide and the pipeline properties was limited and still being collected. With the limited information, including the span and maximum magnitude of the landslide, CRES performed a preliminary assessment of the strain demand with simplified analytical models. Combined with strain capacity estimates from previous work for the same pipeline, CRES concluded that the strain demand was lower than the lowest strain capacity of all girth welds in the affected segment by a comfortable margin. Consequently, immediate field work that might have involved strain/stress relief was deemed unnecessary and the line was allowed to operate at its normal pressure. In addition, working on the saturated soil in early spring carried a risk of further destabilizing the ground and causing additional movement, especially considering the pipe at the location was under over 10 feet of cover and the pipeline was on a side slope.

In March and April 2019, detailed information became available, including the landslide characteristics from a field survey by Golder and site-specific pipeline properties collected by the Owner. Based on the updated information, CRES performed a refined assessment to obtain the strain demand and site-specific strain capacity. Pipe-soil interaction modelling performed as a part of the assessment indicated a lower strain demand than that found in the preliminary analysis. After a comprehensive review of the mill test reports (MTRs) and the welding procedure specifications (WPS) for the affected pipe joints and girth welds, the case-specific strain capacity analysis focused on two manual tie-in girth welds, as other types of welds in the affected segment were expected to have a higher strain capacity than those two tie-in welds. The analysis produced a slightly higher strain capacity than the preliminary strain capacity estimates. CRES concluded that strain/stress relief of the affected segment was unnecessary due to the large margin that existed between the strain capacity and the strain demand. Concurrently, site stabilization work involving the diversion of ground water was performed by Golder. In addition, strain gages were installed at strategically selected locations based on strain demand and strain capacity analysis to detect additional strains at those critical locations. Golder and CRES worked together to develop bell-hole locations so the same locations could be used for the work on the ground water management and strain gage installation, thus minimizing the amount of excavation. CRES provided future intervention strain thresholds for possible mitigation actions should the strains from the strain gages were to approach the thresholds.

The integrated approach for hazard management and FFS assessment considerably reduced the amount of field work, while the line was kept in service with a reasonable assurance of integrity. The site work simultaneously addressed the immediate threat and provided a path forward for future evaluation through the use of strain gage monitoring and intervention strain thresholds.