In historical background of oil and gas exploration industry, over years, fixed offshore platforms are widely used. In the design of fixed offshore structures, as known, they are always attacked by severe environmental loads especially dynamic wave loads which govern the design factor of the platform. In addition, they are found sufficiently rigid to withstand environmental loads in the open sea. However, if soil stiffness under seabed is taken into account, we know that soil layers are not acting as perfectly stiff foundation. Nonlinearity of soil stiffness with its property may affect the global structural stiffness and it is a time-dependent function of total forces at pile head. Therefore, natural frequency of structure becomes nonlinear and it may increase structural response or induce resonance. For design purpose, this book will give distinctively useful ideas to professional design engineers or students who are interested in methodology for modeling of non-linear foundation system of offshore structure. It is also recommended to researchers who would like to extend the scope beyond this book as recommended in the last section.