Scope of Work:
Flexible offshore structures in deep water can undergo significant amount of dynamic responses at the resonant frequency under nonlinear wave loading conditions even though these resonant frequencies are well outside of range of frequencies of the excitation wave. A quadratic transfer function model based on Volterra series representation provides a convenient means to model these nonlinear behavior of the flexible offshore structures. K.I. Kim et al (1987) successfully modeled nonlinear responses of moored vessels subjected to random seas from experimental data of a scalded model wave-basin test. S.B. Kim et al (1989) also used the quadratic transfer function t model nonlinear surge motion obtained from the experimental data of a scaled TLP model. The objective of this research is to determine the nonlinear transfer functions of offshore structures such as risers of floating cylinders using the computed nonlinear response obtained from dynamic analysis.
The main objective of this research is to develop a better understanding of the various nonlinearities and the importance of nonlinear effects on the dynamic response of flexible offshore structures. The nonlinearities to be considered are the drag term in Morison’s equation, the effect of current, the calculation of wave forces in the displace position of the structure, and the consideration of the surface wave elevation in the calculation of the forces. In investigating the nonlinear effects the goal is to understand if these nonlinearities are of a quadratic or a cubic nature and what type of structures, in the terms of natural frequencies, are affected by them.
A second objective is to investigate the applicability of quadratic or cubic transfer functions under different loading conditions. The goal is to determine if these transfer functions, in addition to their ability to illustrate the orders of nonlinearity for the response of a given system, can be used for general analyses or if they are only valid for the conditions from which they were derived. By pursuing this research it is expected that a better understanding of the important nonlinear aspects which must be considered in dynamic analysis of offshore structures will be obtained.
Paik, I. and Roesset, J.M. “Use of Quadratic Transfer Function to Predict Response of
Tension Leg Platforms,” Journal of Engineering Mechanics, ASCE, September
Paik, I. and Roesset, J.M. “Applications of Higher Order Transfer Functions in Modeling Nonlinear Dynamic Behavior of Offshore Structures,” International Journal of Offshore and Polar Engineering, Vol.7, No.4, Dec. 1997