Numerical Analysis Of Dynamic Response Of Submarine Pipeline And Seabed Under Wave Loading

Pipelines are presently the most efficient and economical means of transporting petroleum and natural gas from offshore sources to onshore facilities. They will undoubtedly continue to be used in a number of widespread applications. Proper pipeline design will continue to be an important consideration in the future. One of the major problems encountered with the use of pipelines in the seabed is that of the instability due to the wave effects. When gravitational waves propagate over the ocean, they cause fluctuating pressure upon the seabed, which will further induce excess pore pressure and effective stresses within the seabed soil. A wave-induced excess pore pressure cyclically generated in the vicinity of a buried submarine pipeline constitutes one of the main factors that has to be considered in the pipeline stability analysis. Therefore, a proper evaluation of the wave-induced excess pore pressure and effective stresses around pipeline is particularly important for the design of submarine pipelines.Wave-induced excess pore pressure and effective stresses in the saturated porous seabed under wave loading are the main factors that govern the overall stability of submarine pipelines. In this paper, the governing equations of the seabed and pipeline are formulated based on the Biot's theory of consolidation and elastic dynamics theory respectively. The seabed-pipeline interaction problem is investigated by using the friction contact theory and the finite elements method. Based on the numerical results presented, we found that the soil-pipeline contact effect plays an important role in the estimation of the internal stresses, but not the excess pore pressure. On the other hand, inclusion of inertial terms will affect the excess pore pressure, but not the internal stresses. It is also noted that the geometry of the trench layer significantly affect the internal stresses. A simple comparison for different types of trench shape is conducted in this paper. Thus, pipeline engineers need to pay attention on the shape of the trench when they design of trench layer for the protection of a buried pipeline.In most previous investigations for the seabed-pipeline interaction, the seabed has been taken as to be elastic and generally the influences of the non-linearity of progressive wave loading and stand wave loading have been overlooked. The effect of the non-linearity of wave loading on excess pore pressure in the seabed and internal stresses within the pipeline has not been well understood. In this paper, the governing equations of the seabed and pipeline are formulated respectively based on the Biot's theory of dynamic consolidation of two phase media and principle of elastic dynamics. We investigate the pipeline-seabed interaction under non-linear wave loading using friction contact theory. And numerical formulations based on FEM are established. Numerical results demonstrate that if the effect of non-linear wave components is neglected, the internal stresses in the pipeline and excess pore pressure in the seabed will be under-estimated or over-estimated probably. For the stand wave, neglecting the non-linear wave components we may obtain the opposite results with the real situation.In engineering practice, a cover layer of coarser material has been used to protect a buried marine pipeline from wave-induced seabed instability. This paper proposes a two-dimensional finite element model by employing the principle of repeatability to investigate the wave-induced responses of pipeline and seabed. The porous elastic soil behavior is included in the present model and the pipeline is considered to be an elastic medium. The seabed-pipeline interaction problem is investigated by using the friction contact theory. This study focuses on the effects of a cover layer width, depth and slope angle on the wave-induced internal stresses in the pipeline and excess pore pressure in the vicinity of the buried pipeline. It is found that the wave-induced excess pore pressure increases as the width of a cover layer increases in the vicinity of a buried pipeline. Shear stress in the pipeline increases as the width of a cover layer increases. Radial normal stresses and circumferential normal stresses along the pipeline decreases as the width of a cover layer increases. Wave-induced excess pore pressure increases as the depth and slope angle of a cover layer decreases in the vicinity of a buried pipeline. Radial normal stresses and circumferential normal stresses along the pipeline increases as the depth and slope angle of a cover layer increases. Shear stress in the pipeline decreases as the depth and slope angle of a cover layer increases.By introducing the experimentally-achieved empirical model of vibration-induced excess pore pressure increment into the 2 dimensional equation of consolidation to establish the generalized form of 2 dimensional consolidation equation. A numerical procedure based on FEM is developed to assess the accumulative excess pore pressure. Based on numerical computations, the accumulation process of excess pore pressure and liquefaction potential of seabed soil during wave loading can be predicted. Based on the numerical model presented, the effects of soil characteristics, wave characteristics and pipeline geometry on the wave-induced accumulative excess pore pressure and accumulative excess pore pressure ratio around the pipeline will be discussed in detail. Numerical results shows that the accumulative excess pore pressure on the pipeline surface increases as soil modulus of deformation, permeability and Poisson's ratio decreases. The accumulative excess pore pressure on the pipeline surface increases as seawater depth and seabed thickness decreases, but as wave height increases. Pipeline radius and pipeline burial depth also affect the accumulative excess pore pressure in the seabed around the pipeline, but the effects of the pipeline radius and pipeline burial depth can be neglected at the seabed region that far away from the pipeline.Two mechanisms for the wave-induced excess pore pressures in a porous seabed, i.e. oscillatory and residual excess excess pore pressures, have been observed in laboratory experiments and field measurements. Most previous investigations have focused on one of the mechanisms individually. The excess pore pressure induced by waves has been treated closely with the seabed deformation and consolidation in this paper. The empirical pattern of excess pore pressure generation of sand under undrained cyclic shearing is incorporated with Biot's coupling equations which governing vibration and consolidation coupling effects of seabed. A new 2-D effective stress method is proposed which is applicable to liquefaction analysis of seabed. A numerical procedure based on FEM is developed to assess the excess pore pressure. Based on numerical computations, the accumulation process of excess pore pressure and liquefaction potential of seabed soil around pipeline during wave loading can be predicted. Based on numerical computations, momentary excess pore pressure may be the key issue to induce the liquefaction of the seabed at the shallow seabed region. Liquefaction is easyer to occour at the shallower seabed region than at the deeper seabed region. It is also noted that the soil characteristics, wave characteristics and pipeline geometry significantly affect the excess pore pressure in the seabed around the submarine pipeline.