Study on Slamming Pressure on the Bottom of Upper Hull of Semi-Submersible Type Floater

Abstract

Unlike a commercial vessel, offshore platforms operate at a specific spot over a long period time, which makes them more exposed to harsh environments. Especially when slamming occurs, a structure is subjected to high levels of pressure in a very short time

if this is repeated over a long time span, slamming can pose a serious threat to eh integrity of the structure. However, in reality, there are no clear criteria on how to apply slamming pressure in the design stage. For this reason, a procedure of calculating a new equivalent static pressure which can be used in the design stage is proposed in this paper. As the start of this procedure, to select design parameters which should be considered in calculating slamming pressure, air-gap analyses in the time domain are conducted. As an analysis condition, sea states with the two highest significant wave height, along the 100year-return period contour, are selected with two wave directions, 90°and 180°where the largest roll and pitch motion occur. Air-gap analyses are performed for 3 hours under irregular waves generated by those specified conditions and 20 random seeds are applied to each analysis condition to consider wave randomness. Based on the tendency of each parameter obtained from air-gap analysis, parameters to be considered in the design stage are selected. In order to investigate the slamming pressure on the bottom of a semi-submersible rig, a simplified slamming model is proposed with those parameters applied. Various parametric studies are carried out to examine the effects of structural flexibility, coupling stiffness, mesh size, velocity, stiffener size, and air cushion. The pressure response on the plate by the coupling of fluid and structure is studied and the hydroelastic effect of each parameter is discussed. Then, equivalent loads defined as transient and static loads that result in the same maximum or permanent deformation as FSI are evaluated for a design purpose through a series of parametric studies.