This paper investigates effects of nonlinearities which arise from the drag force term of the Morison equation on response values of an example monopod tower. In the investigation, nonlinearities with stochastic character as well as due to large deformations are taken into account. A time domain analysis is carried out by using time dependent hydrodynamic damping which arises from the fluid-structure interaction. A traditional linearized spectral analysis is also carried out and the results are compared with those obtained from the time domain analysis. Since the hydrodynamic damping and the applied force are coupled with the response velocity an iterative procedure is used to solve the dynamic equilibrium equation. Discrete Fourier transform is used to calculate response spectra from the results of the time domain analysis.


Deep water offshore structures are dynamic sensitive and drag force dominant due to small diameters of members. Response characteristics of these structures under random waves and current are usually estimated in practice by using a spectral analysis procedure which is carried out in the frequency domain for its computational efficiency. Such an analysis is applied only to a linear system so that nonlinearities, in the system equation, in general, must be linearized by using a stochastic linearization process, see e.g. Atalik and Utku (1976), Roberts and Spanos (1990). A general survey of nonlinearities for offshore structures is presented in Chakrabarti (1990). There are various origins of nonlinearities, "see e.g. Barltrop and Adams (1991) and Chakrabartil (1990), among which the drag force term of the Morison equation is well known. The drag force term is nonlinear in terms of the water velocity, or the relative velocity if structural velocities are considerable in comparison with the water velocity. This nonlinear term is linearized mainly to be able to apply a frequency domain spectral analysis in practice (Karadeniz, 1993).

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