Transit Draft Heave and Pitch Motion Analysis of the Mobile Offshore Base (MOB) Using Reverse MI/SO Techniques

Abstract

A major design feature of the Mobile Offshore Base (MOB) is its ability to transit anywhere in the world in the required time frame. This means that the Single Base Units (SBU’s) of 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. A part of the research in progress involves the development of a nonlinear system modeling and optimization tool utilizing Reverse MI/SO (Multiple-Input/Single Output) techniques. Reverse MI/SO is based on the statistical signal processing of the recorded time histories of the excitation and response of the nonlinear multi-degree-of-freedom system. This method of analysis is functional and reliable in identifying an ideal representation of the linear and nonlinear terms of the system under consideration. Reverse MI/SO is a frequency domain analysis technique that also provides coherence functions for each of the terms in the model enabling an evaluation of the correctness of the proposed integro-diffrential equation of motion representing the system. In this paper we analyze the large amplitude heave and pitch motion of the MOB. It is a well-known fact in linear ship motions theory that for a symmetric ship with zero forward speed the cross-coupling added mass and damping coefficients are zero [1]. However, for large amplitude (nonlinear) motions of the MOB, we find these linear coefficients to be non-zero.