Cyclically loaded risers and pipelines in cohesive soil

Abstract

Lateral resistance and lateral displacement are important design factors of risers/pipelines. This work presents the behavior of the riser/pipeline-soil interaction due to the “zig-zag” cyclic movement (combined of vertical and lateral movements). This study has been based on a series of monotonic, lateral cyclic, and T-bar tests using a single gravity model test. This study aims to evaluate (1) the fundamental mechanism of riser/pipeline-soil interaction due to the zig-zag cyclic loading, (2) large monotonic deformation at different embedment depths of 0.5 and 1.0 D (where D = pipe diameter), (3) zig-zag cyclic loading for cyclic displacement amplitudes of 0.15–0.5 D, (3) lateral resistance degradation, and (4) trench formation. The findings from this paper are that (1) the pipe tends to rise due to the behavior of light pipes during the first few cycles. Continuing towards more cycles, the pipe tends to move downward, (2) the riser/pipeline lateral resistance drops after trench formation, continuing towards more cycles, the resistance starts to lightly increase, (3) lateral soil resistance on a shallowly embedded pipe (w/D = 0.3) drops significantly after forming the trench, and it is less pronounced at increased embedment depths, and (4) the depth and width of the trench depend on the zig-zag displacement amplitude.