In this paper, a nonlinear foundation-hull finite element model, which consists of foundation, spudcan, leg and main hull four parts, is proposed for the vibration property analysis of jack-up platform. In this model, nonlinear constitutive relation is used to define the seabed soil foundation, which is modeled as nonlinear solid elements. The spudcan is modeled according to its actual size and scantling in order to take its elasticity into consideration accurately. With a transient analysis approach, the vibration property under surge, sway and yaw motion is acquired, and nonlinear factors such as p-delta effect and fluid resistance damping are taken into account. This numerical model is applied into the vibration analysis of the Santa Fe Magellan jack-up platform. By comparing the numerical result with the measured data, it proves that the proposed method is more accurate than the traditional methods. With this numerical model, the following influence factors on natural frequency of jack-up are analyzed in details, which are different types of foundation, different range of seabed soil foundation in the finite element model and different spudcan diameter. The proposed method and conclusion of this paper could be helpful in both general design and safety analysis for the jack-up platform.
Because the natural period of a jack-up platform is typically in the range of 5 to 15 seconds, there will be a dynamic amplification with waves in this period range. It is therefore often desirable to account for the dynamic effects of the jack-up in the elevated condition due to waves (ABS, 2014). The dynamic effect can come down to the vibration property analysis under different environmental loads, so the vibration property analysis is of great importance to both structural design and safety analysis of jack-up.
Generally, there are three kinds of vibration property analysis methods, which are modal analysis method, transient analysis method and practical measurement. The modal analysis method is mainly used to solve the natural frequency and vibration modes of structures, and its results can be used in other dynamic analysis methods, such as transient dynamic analysis, harmonic response analysis and spectral analysis etc. (B. Bienen and M.J. Cassidy, 2006). The modal analysis method is widely used in the frequency domain-spectral analysis, which could capture the random nature of the environmental loads (E. J. Greeves et. al., 1996). The modal analysis method requires that the model should be completely linear. There are nonlinear factors that should be taken into account, such as nonlinear foundation constraints. In the modal analysis method, the nonlinear factors should be linearized by introducing certain assumptions.
