Probabilistic Lateral Buckle Formation for Three Deepwater HPHT Pipeline Mitigations

A comparison assessment of lateral buckle (LB) formation among three typical buckle mitigation devices (sleeper, distributed buoyancy modules (DBM), and residual curvature method (RCM)) commonly considered in the design of high pressure and high temperature (HPHT) flowlines is carried out by adopting a probabilistic approach based on Virtual Anchor Spacing Finite-Element (VAS FE) modeling to assess buckle initiation reliability. The pros and cons of each device are detailed, the critical stochastic parameters and deterministic parameters for the buckle formation associated with each device are identified, and a probabilistic assessment via a case study is carried out using the three mitigation devices for a subsea deep-water HPHT production flowline. The probabilistic assessment of pipeline lateral buckling includes the following: (1) Calculation of the critical buckling force (CBF) and its probability density function for both engineered and rogue buckles based on deterministic and stochastic parameters via Monte-Carlo simulation and design of experiments (DOE); (2) Determination of the tolerable VAS by FE analyses for both engineered and rogue buckles to satisfy the design criteria; (3) Positioning of the engineered buckles along the flowline through a reliability analysis using BUCKFAST. The analysis results indicate that the stochastic parameters that control the buckling formation are different for each of the three mitigation methods. Comparison of the three methods showed that the RCM option had the most significant impact on reducing the CBF due to the influence of the pipe/soil lateral breakout friction factor while the DBM option had the least significant impact, with the highest CBF among the studied mitigation devices. Results also show that some of the stochastic parameters do not have significant effects on the CBF for each considered mitigation device and therefore could be treated as deterministic parameters in future analyses. The competition between the CBF of rogue buckles along the curve section and the trigger CBF distribution is the key parameter when comparing the different mitigation methods. Sleeper and DBM options may not satisfy the LB design criteria in some cases when using the deterministic approach, especially for a rough seabed profile along the route, but may lead to a solution when using a probabilistic assessment. RCM has the advantages of a lower CBF and the lowest cost among the three mitigation methods, which means the RCM is a cost-effective lateral buckle mitigation solution for subsea HPHT pipelines.