Dynamic Behaviors Of Subsea Foundations

With the present fast increasing need of energy, more and more countries are exploring how best to tap into the energy sources from the sea, such as putting wind turbines over the ocean, and offshore drilling platforms to extract petroleum and gas hydrate below the seafloor. However, it is not easy to install the necessary physical facilities to extract these energies, as the environment in the ocean is quite different from that on land. Compared to the forces on the land, the forces in the ocean are almost all dynamic in nature in the form of waves, winds and currents, besides earthquakes. To anchor the wind turbines or drilling platforms to the seabed, foundations like the gravity foundations, plate anchors, piles and bucket foundations or suction-type foundations have attracted a lot of interest as in the case of offshore foundations. Once it is installed, however, the foundation, may it be any one of the above mentioned foundations, will need to deal with various dynamic forces. So, it will be very important and meaningful to study the dynamic characteristics of the oceanic foundations.As all foundations in the ocean engineering are installed in the seabed covered by seawater, and the influence of the seawater on the dynamic characteristic of the foundations are not clear, this study propose a coupled seawater-seabed half-space model to study the oceanic foundations vibration problem. The gravity type foundations are treated as a surface rigid disc on the seabed covered by seawater, the plate anchors and the top ending of the bucket foundations are simplified as a buried rigid disc in the seabed covered by seawater, and the pipe piles and the shell part of the bucket foundations are treated as elastic shells or rigid shells buried in seabed covered by seawater. The results can be applicated to gravity foundations, plate anchors, pipe piles and suction type foundations. The details are as follows:(1) The dynamic Green’s functions of the coupled seawater-poroelastic seabed half-spaces to a time-harmonic vertical point force, horizontal point force, vertical pore pressure discontinuity or a point source are investigated. These Green’s functions can be used to solve the dynamic interaction problem of oceanic foundations with the coupled seawater-seabed half-spaces directly, and also can be used as basic solutions in Boundar Element Method;(2) The vertical vibration of a rigid disk on the surface or embedded in a poroelastic seabed half-space incontact with a seawater half-space is studied. The results for the vertical dynamic compliance coefficient and dynamic responses of the seawater-seabed half-spaces are examined based on different poroelastic materials, embedment depths, permeability of disc and excitation frequencies; furthermore, the results are analyzed for the cases in which there is and is no seawater overlying the poroelastic medium to examine the effect of seawater.(3) The horizontal vibration of a rigid disk on the surface or embedded in a poroelastic seabed half-space incontact with a seawater half-space is addressed. The results for the horizontal dynamic impedance coefficient are examined based on different poroelastic materials, embedment depths, and excitation frequencies; furthermore, the results are analyzed for the cases in which there is and is no fluid overlying the poroelastic medium to examine the effect of fluid.(4) The rocking vibration of a rigid disk on the surface or embedded in a poroelastic seabed half-space incontact with a seawater half-space is addressed. The results for the horizontal dynamic compliance coefficient are examined based on different poroelastic materials, embedment depths, permeability of disc and excitation frequencies; furthermore, the results are analyzed for the cases in which there is and is no seawater overlying the poroelastic medium to examine the effect of seawater.(5) The vertical vibration of bucket foundations embedded in a fully saturated porous seabed in contact with a seawater half-space is studied. Results for the dynamic contact load distributions, displacements, and dynamic compliance coefficient are examined based on different shell length-radius ratio, poroelastic materials, shell properties and frequencies of excitation. Moreover, the results are analyzed for the cases in which there is and is no seawater overlying the poroelastic medium to examine the effect of seawater.(6) The coupled horizontal and rocking vibration of rigid shells embedded in a fully saturated porous seabed in contact with a seawater half-space is studied. Results for the dynamic contact load distributions, displacements, and dynamic impedance coefficients are examined based on different shell length-radius ratio, poroelastic materials and frequencies of excitation. Moreover, the results are analyzed for the cases in which there is and is no seawater overlying the poroelastic medium to examine the effect of seawater.(7) The influence of the depth of the seawater is studied based on the vertical vibration of a disc on the seabed covered by a seawater layer.