Comparative study on global motion of rectangular and circular column semisubmersibles

Abstract

A comparative study on the global motion characteristics of two semisubmersible units having similar hull arrangement, but different hull column geometry is carried out. All comparisons are considered at an operational draft. Each unit has four column ring pontoon hull. One of the units has quadratic hull column while the other unit has circular hull column. The hull of each unit has the same column center to center distance and pontoon dimension. At the considered operation draft, each of the two units is set to have similar initial stability in terms of metacentric height, similar water plane area and nearly similar displacement. Deck load arrangement is not discussed in detail but the overall mass distribution is also kept the same for the two units by employing the same radius of gyration about the global coordinates. Both of the units that are set for comparison are considered to be unmoored and also, riser stiffness, damping and inertia effects are not included for analysis. The sesame suite of software is used for modeling of each unit, running global hydrodynamic analysis and for the post processing of the results from the analysis. The global first order motion transfer functions of the two units are compared. Statistical post processing is performed by applying sea states from a selected environmental contour line. Then, comparison between the two units is also carried out in terms of global short term maximum responses, short term minimum air gap requirement, slamming load from breaking waves and also in terms of second order horizontal mean drift loads.