The on-bottom stability of a submarine pipeline is a flow-pipe-soil coupling problem. For the integrated pipe-soil system under the action of steady current, two correlated instability modes could be involved, i.e., the lateral-instability of the pipeline and the tunnel-erosion of the underlying soil. In the previous studies, the aforementioned two instability modes were investigated separately. In this paper, a finite element (FE) model for the flow-seepage-elastoplasticity sequential coupling is established to reveal the multi-physics coupling effects of such integrated system. This numerical model can realize the simultaneous and coupled simulation of the flow field over the partially-embedded pipe, the seepage-flow field and the elastoplastic stress-strain field of the soil. The proposed model is compared and well verified with the existing experimental results. Numerical simulation shows that both the non-uniform pressure distribution together with the viscous stress along the pipe periphery and the pressure-drop along the seabed are formed synchronously due to the existence of the pipe. The former will generate the hydrodynamic forces on the pipe, which may trigger the lateral-instability; while the latter will induce seepage flow in the underlying soil, which may result in the occurrence of tunnel-erosion. Numerical results also indicate that not only the hydrodynamic forces but the maximum upward hydraulic gradient increase nonlinearly with increasing inflow velocity. Instability mode and mechanism for the pipe-soil coupling system can be finally determined with the instability criteria.
As the offshore exploitation of gas and oil moves into deeper waters, the on bottom instability of a submarine pipeline induced by ocean currents becomes one of the main causes for the structural failure (see Drumond et al, 2018). For such a flow-pipe-soil interaction issue, two physical processes within the integrated pipe-soil system, i.e. lateral-instability of the pipe and tunnel-erosion of the underlying soil, are involved. The physical mechanisms and criteria for the two different instability modes have been explored intensively in previous studies (e.g., Fredsø e, 2016; Zhang et al., 2016; Gao, 2017; Drumond et al, 2018).
In ocean currents, the flow over a pipeline and the seepage-flow within the underlying soil can be generated synchronously. On the one hand, when the lateral soil resistance provided by seabed soil is insufficient to balance the hydrodynamic forces exerted on the pipeline by the flow, the pipeline would breakout laterally from its original position. On the other hand, if soil seepage failure due to the pressure drop between the upstream and downstream of a partially-embedded pipeline is triggered, the tunnel-erosion underneath the pipeline will be initiated immediately (see Chiew, 1990; Sumer et al. 2001; Yang et al., 2014). Note that, the lateral-instability of pipeline and the tunnel-erosion of soil were always investigated separately. Moreover, in the tunnel-erosion numerical simulation, the pipeline was often set as a fixed rigid boundary with assumption of a rigid and permeable seabed (e.g., Zang et al., 2009); and in the lateral-instability studies, the elasto-plastic behaviors of soil and the interfacial characteristics for the pipe-soil interaction were the main focus of attention (e.g., Gao, et al., 2012; Bai et al., 2015).
