This paper reports based on a numerical study of multiphase fluid flow through a channel with a semi-submersible platform using computational fluid dynamics (CFD). The present study will be designing and analysing semisubmersible platform with pencil column and without pencil column. The computational fluid dynamics which solves simple differential equations and finite volume method (FVM) will be used. A turbulence model is considered i.e. large eddy simulation (LES). The semi-submersible model is designed as pontoons, columns, horizontal brace, pencil column and deck. The pontoons are horizontal placed stadium shaped structures which are submerged into the water. The columns are structures which connects the deck and pontoons. The horizontal braces are structures which connects the two or more columns which increases the rigidity of the columns. The pencil columns are columns with lesser diameter of about 0.25D which are placed between the columns. The deck are flat surfaces which provides workable area. This paper is a comparison of fluid flow by varying the model i.e. with and without pencil column in the semi-submersible platform. The velocity contours, pressure contours and streamline contours are plotted. The difference in pressure, velocity and streamline flow are tabulated and graphically represented. The percentage difference in pressure and velocity are calculated for structural design for various offshore platforms. © 2021 Institute of Physics Publishing. All rights reserved.