Semisim is a six degree-of-freedom wave-frequency motion package for use with semi-submersible vessels in the non-transit, hulls-submerged mode. Computed response characteristics reflect the highly nonlinear nature of motion damping and excitation for this type of vessel. Features include efficient input of column and hull forms based on the use of simple geometrical shapes, permitting quick specification and easy modification of the underwater configuration for comparative performance analysis and evaluation. A wide range of environmental conditions is accommodated, including extensive built-in wave spectral types, azimuthal spreading of wave directions and an optional independent background swell. The program provides for calculation of air gap clearances between deck and water surface in addition to absolute displacements, velocities and accelerations at any point on the vessel. Effects of finite water depth are fully modeled and either English or metric units may be selected for vessel specification and data output. Output is in the form of amplitude and phase of vessel Response Amplitude Operators (RAOs) and/or statistical characterizations of vessel response to irregular wave conditions. Execution is carried out in the frequency domain, resulting in short run times and unambiguous predictions of statistical response values.
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