Deterministic and stochastic control of nonlinear oscillations in ocean structural systems

Graduation date: 2006Complex oscillations including chaotic motions have been identified in\ud off-shore and submerged mooring systems characterized by nonlinear fluid-structure\ud interactions and restoring forces. In this paper, a means of controlling\ud these nonlinear oscillations is addressed. When applied, the controller is able to\ud drive the system to periodic oscillations of arbitrary periodicity. The controller\ud applies a perturbation to the nonlinear system at prescribed time intervals to guide\ud a trajectory towards a stable, periodic oscillatory state. The controller utilizes the\ud pole placement method, a state feedback rule designed to render the system\ud asymptotically stable. An outline of the proposed method is presented and\ud applied to the fluid-structure interaction system and several examples of the\ud controlled system are given. The effects of random noise in the excitation force\ud are also investigated and the subsequent influence on the controller identified. A\ud means of extending the controller design is explored to provide adequate control\ud in the presence of moderate noise levels. Meanwhile, in the presence of over\ud powering noise or system measurements that are not well defined, certain filtering\ud and estimation techniques are investigated for their applicability. In particular,\ud the Iterated Kalman Filter is investigated as a nonlinear state estimator of the\ud nonlinear oscillations in these off-shore compliant structures. It is seen that\ud although the inclusion of the nonlinearities is theoretically problematic, in\ud practice, by applying the estimator in a judicious manner and then implementing\ud the linear controllers outlined above, the system is able to estimate and control the\ud nonlinear systems over a wide area of pseudo-stochastic regimes