3D Numerical Model for Fluid-Seabed Interactions Around Pipelines Using OpenFOAM

Liang, Zuo Dong (Griffith University) | Jeng, Dong-Sheng (Griffith University)

OnePetro 

ABSTRACT

A precise prediction of seabed stability involving the fluid-pipe-soil interaction can lead to significant cost reductions by optimising design. Unlike previous investigations, a three-dimensional numerical model for the wave-induced soil response around an offshore pipeline is proposed in this paper. The numerical model was first validated with 2-D experimental data available in the literature. Then, a parametric study will be carried out to examine the effects of wave, seabed characteristics and confirmation of pipeline. Numerical examples demonstrate significant influence of wave obliquity on the wave-induced pore pressures and the resultant seabed liquefaction around the pipeline, which cannot be observed in 2-D numerical simulation.

INTRODUCTION

Nowadays, many offshore structures have been commonly constructed over the last few decades due to the growing engineering resource in the ocean. Submarine pipelines, as one of the popular offshore infrastructures, have been extensively used for transportation of natural gas and oil from offshore platform, and disposal of industrial as well as municipal waste. To ensure the safety of usage of such submarine pipelines, the coastal engineers have to consider the unexpected loads including the wave, current, and anchor dropping/dredging, which might cause the its stability and decrease its life span. Thus, it is customary to bury the pipeline by trenching and refilling soil whose cost is relatively high and time-consuming (FredsØ e, 2016).

As reported in the literature, two well-known main mechanisms of dynamic wave-induced seabed liquefactions are the momentary liquefaction and residual liquefaction, based on in the field measurements and laboratory experiments (Zen and Yamazaki, 1991). The fist mechanism, momentary liquefaction, can occur beneath wave troughs when the great seepage flow is upward directly. Since this kind of liquefaction may be happen within a short duration as the passage of wave trough, it is also called instantaneous liquefaction. The other mechanism, residual liquefaction, takes place as a result from a compacted and cyclic shearing process that the build-up of excess pore pressure in the seabed (Seed and Rahman, 1978). As mention previously, the waves also can induce shear stress in the soil when the waves propagate, which has been analytically investigated by Yamamoto et al (1978). Whereas the wave-induced shear stress has less impact on seabed instability compared to that caused by the previous two mechanism above. This study only focuses on the wave-induced seabed liquefaction incorporating both instantaneous mechanism.