On the consideration of wave and structural nonlinearities in the design of compliant towers through dynamic analysis

Abstract

The purpose of this work is to highlight the importance of considering the wave and structural nonlinearities in the design through dynamic analysis of offshore platforms located in deep water. The paper studies the necessity of applying dynamic analysis in combination with the accurate calculation of the water particle kinematics. The example treated in this context is a compliant tower, set up in deep water. Nonlinearities are considered both in the calculation of the wave loading and in the structural analysis. The wave loading is calculated with the water particle kinematics drawn from a fully nonlinear 3D model, able to incorporate a spread of energy in the frequency spectrum and in the spatial directions. Comparisons are provided with the calculations associated with linear random wave theory and the steady wave theories of Airy and Stokes 5th. The unsteady calculations are based on the most probable shape of a large wave on a given sea state: the auto-correlation function of the underlying spectrum. In addition, the effect of directionality in the wave loading calculations is examined. The outcomes are quantified for all scenarios that are investigated. As far as the structural analysis is concerned, the effect of geometric nonlinearity and the effect of the nonlinear behavior of the foundation soil are examined. To this end, three cases are considered and compared. A geometrically linear dynamic analysis with linear soil properties, a geometrically linear dynamic analysis with nonlinear soil properties and a fully nonlinear dynamic analysis that takes into account both the geometrical nonlinearity and the nonlinear properties of the soil. The structural calculations are performed using the well-known structural analysis software SAP2000, enhanced by a special software developed to generate and directly apply the wave loading on the structural members. The results indicate that the consideration of the water particle kinematics associated with a fully nonlinear unsteady wave model for the evaluation of the wave loading leads to notable differences with respect to the linear, unsteady model. Significant are also the differences with respect to steady wave theories in terms of the structural response parameters that are examined. These differences are additionally affected by the applied directional spreading. Irrespectively of the adopted wave theory, the nonlinear dynamic analyses lead to significant discrepancies with respect to the linear ones. Ultimately, these types of structures respond dynamically under the effect of the wave loading. Therefore, the wave field energy distribution within the spectrum is of significant importance, as well as the choice of the corresponding period of the steady wave theory (Ttr−tr, TC, TP).