A New Holistic Approach for Subsea Pipeline Upheaval Buckling Design

Abstract

For a trenched and buried pipeline, the propensity to upheaval buckling (UHB) is a major design concern. Predictive UHB design is typically required at the outset to determine both trenching and backfilling requirements. Additional rockdump schedule can be established by analyzing post pipelay out of straightness (OOS) survey data incorporating appropriate safety factors based on a structural reliability analysis (SRA). The normal approach is to examine the as-laid pipeline imperfection survey statistics and data accuracy. The structural reliability analysis and load factor calculation are typically performed a priori based on the assumed initial imperfections using the universal design curve methodology. A new pseudo-energy method for UHB and OOS is proposed and discussed in this paper based on the variational principle and modal analysis. The approach takes into account the effects of varying effective axial force, trench imperfections, and vertical uplift resistance, by combining both axial friction and lateral resistance methods into a unified model. A new concept, effective uplift resistance and associated load, is also introduced to deal with nonuniform backfill cover. Adjacent imperfections and backfill profiles are considered in detail. A finite element (FE) model is developed to consist of three-noded quadratic pipe elements using abaqus Ver 6.12, and iterations of FE analyses are performed to demonstrate the tangible benefits of the approach specifically for UHB OOS design in relation to target trenching and backfilling, leading to improved reliability and potential cost saving in UHB OOS design and rockdump installation.