As part of a joint Industry research program, model tests have been performed using a six-column "generic' semi-submersible model. Data has been collected to provide Input for: calibration and correlation studies for analytic prediction techniques, studies of nonlinear phenomena, and qualitative assessment of realistic environment modeling. Test conditions at operating and survival drafts included moderate to extreme waves, damage conditions, stead/unsteady wind, current, mooring line failure and comparison of mooring systems. Loading conditions were consistent with current stability criteria with variations included to address extremes. A brief summary of the associated correlation study is presented at the end of the paper.
Over the past several years the American Bureau of Shipping has been conducting a Joint Industry Project (JIP) to Investigate and assess Mobile Offshore Drilling Unit (MODU) stability with respect to both the current stability criteria and a newly developed, more rational criteria [1]. The JIP has been funded by seventeen participating organizations representing all aspects of the offshore Industry, Including owners, contractors, builders, oil Companies and regulatory bodies. ABS Research and Development and Offshore Engineering Divisions are acting as principal investigator and project administrators of the project.
One of the tasks of the JIP was the performance of a model test program conducted by Arctec Offshore Corporation during December 1986 and January 1987 [2]. The tests were performed using a "generic" semi-submersible design. The simplified six-column unit was subjected to a wide range of environmental conditions and Intensities. Conditions included waves, current and steady and turbulent wind. The primary focus was on the extreme environmental conditions critical to the objectives of the JIP. The unique measured data collected from the test program Is being utilized directly in the JIP work as: support for the analytic results of other-tasks, Input to the evaluation of non-linear phenomena, and qualitative analyses of more realistic test conditions. In particular the data has been critical In improving and demonstrating, through calibration and correlation studies, the capabilities and Viability of the dynamic motion prediction computer programs used in the JIP for much broader based parametric studies of factors affecting MODU stability.
This paper Is comprised of summaries of the development of the generic test model, the scope of work of the three phase test program, pertinent observations made during the model tests and how the test results are being used in the ABS JIP MODU Stability Study.
Semi-submersible dynamic behavior is governed by the geometry of the unit, particularly In terms of size and configuration. The latter can be defined In terms of the number of columns and pontoons and by a series of non-dimensional coefficients defining various proportions or relationships of hydrostatic properties. Three coefficients were ultimately chosen which respectively define the ratio of pontoon volume to total enclosed volume (i.e., volume up to column top), waterplane area as a function of total volume, and a ratio defining the distribution of the waterplane area based on the second moment of the waterplane area.
