Centrifuge Modelling of Pipe Penetration Due to Dynamic Lay Effects

Abstract

On-bottom pipelines are subjected to small-amplitude oscillations during installation due to dynamic motion of the pipe within the touchdown zone. This motion is driven by the movement of the lay vessel and hydrodynamic excitation of the suspended length of pipe. The increase in as-laid embedment caused by these dynamic lay effects has a significant influence on various aspects of the performance of the pipeline. A series of centrifuge model tests was conducted to assess the change in embedment of an unburied pipeline subjected to small-amplitude horizontal oscillations. Two clay samples were used — kaolin clay and a soft high plasticity clay recovered from offshore Angola. Dynamic lay effects were simulated by applying a constant vertical load to the model pipeline then imposing packets of cycles of horizontal motion, representing the pipe movement within the touchdown zone. The results demonstrate that a few (<5) small amplitude cycles (±0.05D) can double or triple the pipe embedment, and a number of cycles that might represent the entire lay process (∼100) could increase the pipe embedment by up to 8 times relative to the static embedment under the same vertical load. The amount of penetration is governed by the vertical load level relative to the bearing capacity, and the oscillation amplitude. Significant differences were observed between the two clays. The high plasticity clay was much more susceptible to softening and increased embedment under dynamic loading.