Large amplitude nonlinear dynamics of the Mobile Offshore Base (MOB) at transit draft in a random seaway

The Mobile Offshore Base (MOB) is an approximately one mile long structure made up of from 3-5 individual large semi-submersible Single Base Units (SBU's). Transit draft has been identified as a particular area of concern from a dynamics and stability standpoint. Although while at operating draft, semi-submersible are relatively transparent to wave excitation due to the majority of the hull volume being submerged far below the water surface, while at transit draft, an individual unconnected SBU operates as essentially a catamaran with a relatively high metacentric height. With the lower hulls penetrating the water surface, wave excitation can be important. Moreover, due to the reduced freeboard of the lower hulls, wetting of their tops may also occur. This will result in parametric excitation not explicitly considered herein. In this work, our recently developed approach (Vishnubhotla, Falzarano and Vakakis, 1998) is utilized which makes use of a closed form analytic solution which is exact up to the first order of randomness, and takes into account the unperturbed (no forcing or damping) global dynamics. The result of this is that very large amplitude nonlinear vessel motion in a random seaway can be analyzed with similar techniques used to analyze nonlinear vessel motions in a regular (periodic) seaway. The practical result is that dynamic capsizing studies can be undertaken considering the true randomness of the design seaway. The capsize risk associated with operation in given sea spectra can be evaluated during the design stage or when an operating area change is being considered. Moreover, this technique can also be used to guide physical model tests or computer simulation studies to focus on critical vessel and environmental conditions which may result in dangerously large motion amplitudes.