A note on effect of geometric dimensions, shape and arrangement of columns and pontoons on heave and pitch response of semi-submersible

Abstract

The study of motion response of offshore structures is important in design and operation of offshore structures. Primarily, the motion response of a semi-submersible depends upon centre of gravity, meta-centric height, geometric dimensions, arrangement and shape of the structural elements such as columns and pontoons, and operating draft of the structure. This paper reports a numerical study on the effect of geometric dimensions, shape and arrangement of columns and pontoons on the hydrodynamic response of a semi-submersible. In this paper five design configurations of a semi-submersibles with different dimensions are studied numerically using a commercially available software (i.e. ANSYS-AWQA™) and in semi-submersible design the geometric dimensions, shape and arrangement of columns and pontoons are varied and a `time domain analysis' is performed for head and beam sea conditions with an eight point moorings for 1000 m depth with an angle of 45° between each set of mooring lines for regular waves and also for different sea state conditions. In the numerical study, the potential flow diffraction analysis is used and the viscous damping is suitably assumed from the available results from literature and that is given as an `input' in the numerical simulation. In this paper, the heave and pitch responses are studied and in the first step frequency domain analysis is carried out for obtaining response amplitude operators of heave and pitch responses for head and beam sea, and these gives an idea about the geometry which has the minimum response. In second step three hour time domain analysis is performed to study heave, pitch, surge and sway responses with the two sea states (i.e. 9 and 10). The responses are analyzed for a better geometry between the five designs. Two types of mooring system are studied by varying the length of mooring lines to vary the tension. The results obtained are analyzed for minimum motion response for surge, sway, heave and pitch degrees of freedom for the design configurations and compared with each other. The shape of column is changed for the design to study the effect of shape and compute the minimum response for further study. All the results are analyzed in detail and are used to derive favourable range of column and pontoon dimensions for achieving low motion response. The presented results show the effect of geometric dimensions, shape and arrangement of columns and pontoons on heave and pitch response of semi-submersible and they can be used to arrive at the suitable shape, dimension and arrangement of columns and pontoons for minimum response.