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The response of pile supported Tension Leg Platforms during seismic loading and seabed liquefaction is analyzed numerically, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when liquefaction in the subsoil is extensive, leading to degradation of the pseudo-static factors of safety against pullout failure of the pile well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking due to subsoil liquefaction, but fully recovers during the following excess pore pressure dissipation phase and may even exceed the initial (pre-shaking) resistance value. Pile head displacements develop steadily during shaking and the following dissipation phase, but only during the finite time period when the static pullout factor of safety of the pile remains less than unity. Thus, final pile head displacements may increase considerably when this time period is elongated. Due to the generally high tensional stiffness of the tendons, relative to the buoyancy stiffness of the platform, the pile head pullout is mostly transmitted to the platform, with only a small percentage corresponding to reduction of tendon elongation. For the cases examined herein, the potential loss of platform buoyancy and tendon pretension may prove detrimental but is unlikely to threaten the safety and functionality of the platform. On the other hand, initially minor tilting of the platform (e.g. due to long period lateral loading and differential tendon pretension) may be magnified during seismic loading above allowable limits. |
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