Eventos Anais de eventos
DINAME 2017
XVII International Symposium on Dynamic Problems of Mechanics
NONLINEAR FREE VIBRATION ANALYSIS OF MULTI-ARTICULATED OFFSHORE TOWERS
Submission Author:
Gabriel Jung , PR
Co-Authors:
Gabriel Jung, Elvidio Gavassoni
Presenter: Gabriel Jung
doi://10.26678/ABCM.DINAME2017.DIN17-0061
Abstract
Articulated towers are compliant offshore structures economically attractive in the oil industry. As the water depth increases the tower could be built-up with multi-framed structures, generally modeled as rigid bars linked by articulated joints. The restoring forces are included by the introduction of rotational springs in the structural model. When subjected to sea environmental loads, articulated towers may exhibit large displacements. A nonlinear analysis is thus required. In this work the nonlinear normal modes (NNMs) technique is used to derive a reduced order model to study the free nonlinear vibration of a tri-articulated tower. The modal surfaces are obtained by the asymptotic method. The obtained modal oscillators present only odd nonlinearities, and the harmonic balance method is then applied to obtain the frequency-amplitude curves analytically. The results show hardening behavior for the first mode and softening behavior for both second and third modes. These curves are compared with results obtained by the numerical solution of the original equations of motion via Runge-Kutta integration on time domain, at chosen points near the equilibrium point – used as reference solution. These comparisons show a good agreement between the proposed analytical solution and the reference one, revealing also the validity region for the asymptotic method for each mode. The domain of validity of the expansion used is also checked by comparing the phase portrait obtained using the modal oscillators and the numerical integration of the system of equations of motion, confirming the effectiveness of the reduced order model derived from the nonlinear modal analysis from this study. A comparison between the free vibration analysis and analysis considering hydrostatic forces is also carried out, revealing the influences on the structure’s natural frequencies.
Keywords
offshore structures, nonlinear vibration, nonlinear normal modes, invariant manifolds, articulated towers
