| Abstract: |
- 1 -<Unlike onshore foundations, offshore foundations are generally constructed in anunfavorable conditions including low strength soils, wave and storm effects, and strongearthquakes. Piles in these foundations are of vital importance as they are the key elementstransmitting the load to the sea floor soil and any failure in these piles means failure of theoffshore platforms as a whole.During a seismic event, ground shaking usually results in compaction III the soilparticles, especially loose sands. This soil densification results in an increase in the pore waterpressure and consequently a decrease in the effective stress which may result in liquefaction,which is a very disastrous phenomena and should be prevented by any means.Assuming the soil as a two phase material, i.e., solid phase which is the soil and fluidphase which is the water gives a clear insight into the problem with respect to the effectivestresses and pore water pressures. Dynamics of porous media, which is the very heart of thisthesis, is studying the soil as a coupled two phase material under the effect of dynamic loads .. ~In this thesis, the behavior of an offshore tension loaded pile, usually used in tensionloaded platforms (TLP), in a porous soil media subjected to seismic excitation is studiednumerically using the finite element technique. A computer program is developed by the authorto analyze a tension loaded pile under earthquake loading. Special composite finite elements areused to represent the soil as a two phase material. Elasto-plastic Mohr-Coulomb model is usedto model the nonlinear soil behavior. Many factors affecting the amount of pore water pressuregenerated and vertical pile head displacement are studied. These factors include; soilpermeability, earthquake magnitude, bias load, pile-soil stiffness ratio, soil damping, and fluidcompressibility along with other factors.While earthquakes of magnitudes up to 7.5 on the Richter scale have a moderate effecton pore water pressure generated and the vertical pile head displacement occurred during theseismic event, earthquakes of magnitude 8.0 have a destructive action on the pile. Althoughearthquakes of magnitude 7.5 and 8.0 are close to each other on the Richter scale, largedifferences on the pile behavior are observed between these magnitudes. Using pile and soilproperties close to the actual field conditions, piles are going to survive earthquakes ofmagnitudes up to 7.5 and will fail under earthquakes of magnitude 8.0 or higher.
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