System Identification of Nonlinear Coupled Ship/Offshore Platform Dynamics in Beam Seas

Abstract

The Mobile Offshore Base (MOB) is designed to transit to anywhere in the world in the required time frame. This means that the MOB must be able to transit in severe environmental conditions. In these extreme sea conditions, a primary cause for concern is the large accelerations that the vessel motions might experience due to the high static stability of the MOB at Transit Draft. Furthermore, since the vessel has minimum freeboard in this condition, it is exposed to green water over the pontoon tops. The submergence of the pontoon deck causes a considerable loss in the vessel’s restoring moment. These concerns have warranted a study by the Office of Naval Research into the Transit Draft Dynamics of the MOB. During the research of MOB dynamical properties, a nonlinear system modeling and optimization tool utilizing Reverse MI/SO (Multiple-Input / Single Output) techniques was developed and applied to different aspects of MOB dynamics analysis. MISO is based on statistical signal processing of the recorded time histories of the excitation and response of the non-linear multi-degree-of-freedom system. This method of analysis is functional and reliable in identifying an optimum representation of the linear and non-linear parameters of the system under consideration. In this paper, we analyze the model testing data in beam seas using the Reverse MISO technique. We expected to identify significant nonlinear roll damping for the nonlinear integro-differential equation as is the case with ships. Instead, a significant nonlinear heave damping related with the nonlinear relative heave velocity has been found during the analysis. This reminds us again that due to the strong nonlinearity of MOB motions in the severe sea ways, nonlinear analysis methods such as Reverse MISO are important and need to be applyed in order to fully identify the dynamics of the MOB motion.