Physical model tests were conducted on a large semi-~ubmersible model, representing a 1-to-70 scale model of a single module of the Mobile Offshore Base (MOB). Tests were conducted in head seas in two random sea states, with the model ballasted to a deep operating draft. This paper discusses the characteristics of the MOB model, the experimental setup and test conditions, the observed platform motions, and the degree of wave amplification under the hull leading to reductions in the effective air gap. These last measurements are based on nine wave gages located under the model, from which basic statistics, spectra, and probability distributions have been computed.


The Mobile Offshore Base (MOB) has been the subject of an extensive research program funded by the U.S. Office of Naval Research (see Zueck et al., 2000). Several different MOB concepts have been proposed which would link three to five semi-submersible modules to create a floating air base nearly 1.6 kilometers (1 mile) long. As part of the research program, a considerable effort has been made to develop and validate numerical hydrodynamic codes which could be used to predict the wave interaction with, and resulting motions of, the MOB structure. As part of that effort, the U.S. Naval Academy constructed a scale-model of a single MOB module and tested this in a large wave tank in order to provide verification data to the numerical modeling teams. The present paper seeks to review one set of model tests in which wave elevations under and around the MOB module were measured to document the reduction in air gap due to nonlinear wave interactions around the hull. Wave amplification, and the associated loss of air gap or under-deck clearance, is an important design consideration for the MOB or for any semi-submersible.

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