Multi-phase simulation of semi-submersible platform with pencil column using CFD

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.


Introduction
A semi-submersible platform is an offshore platform or a marine vessel which are used in oil mines and other applications that are done in seas and oceans. These platforms play a major role offshore infrastructure like drilling rigs, heavy lift cranes etc. These semi-submersibles were found in 1960s for oil mines. Mostly the semi-submersibles are used in water depths higher than 520meters. The semisubmersibles are usually compared with drill ships. Compared to the drill ships, the semi-submersible provides for a better stability. There are many factors that may affect these structures such as lift and drag. This paper is an experimental study which compares the semi-submersible design with and without a semi-submersible using computational fluid dynamics.
Sadeghi et al, [1] performed experiments in designing semisubmersible platforms with pencil columns and analyzing the effect of fluid flow. Ma et al, [2] performed analyzing the structural dynamic responses of the semisubmersible platform in sea states under the combined wind/wave loads using Computational Fluid Dynamics (CFD). Raed et al, [3] performed Weibull distribution for wave height and log-normal distribution in the zero-up crossing period and inverse first order reliability Method and also the direct Monte Carlo Simulation. Alimuddina et al, [4] performed experiments in predicting the motion of a Semi-submersible and analyzing the motion of the semi-submersible platform. Liu et al, [5] performed experiments for semi-submersible floating offshore wind turbines. Zhong et al, [6] performed experiments in analysis of motion of semisubmersible in sea waves. R. T. Goncalves et al, [7] [8] performed experiments on vortex-induced motions with four square columns in a semi-submersible platform. Emami and Gharabaghi [9] performed experiments using poroelastic layers using motion response reduction method in a semi-submersible platform. Choi et al, [10] performed coupled motion analysis of a semi-submersible system in tension leg platform.
Ghafari and Dardel [11] study on catenary mooring system of the semi-submersible platform with a dynamic response. Karimirad and Michailides [12] performed alternate concept for wind technology based on v -shaped semisubmersible offshore for wind turbine. Liu et al, [13] performed various experiments on designing loads for semisubmersible platform to support wind turbines. Mao and Yang [14] investigated on deep draft for parametric pitch instability on an irregular wave. Mas-Soler [15] performed the motions of a semi-submersible to estimate by on-site wave spectra. Qiu [16] used algorithm based on surrogate model for multiple objective optimizations for a semi-submersible platform. Servan-Camas et al, [17] validation on semi-submersible platform using second order time depended FEM model. Tie-bing et al, [18] experimentally investigated on semisubmersible platform for wave run-up characteristics which was conducted along the column and air gap response. Tran and Kim [19] [20] used coupled dynamic response computation for a floating wind turbine placed on a semisubmersible platform. Travanca and Hao [21] performed on floating production systems to control wave induced vibrations. Wang et al, [22] performed experiments based on hydrodynamic performance on semisubmersible platforms having a non symmetrical pontoon.
The objective of the paper is investigating the multi-phase turbulent flow around a semisubmersible platform. The model is placed inside a channel and multi-phase fluid flow air and water is simulated. The model chose for the simulation is large eddy simulation model. The velocity magnitude for the phases is given as 10m/s. The numerical simulations are performed by using a simple differential equation and Finite Volume Method (FVM). The enclosure is considered to be fluid medium and also two phases are used air and water. The results for pressure and velocity are simulated and found at specific locations.
Two models are considered for the experiment one is with pencil column and other without pencil column. The percentage difference between pressure and velocity for both the models are tabulated. The average percentage difference is calculated for both the models. The various contour of velocity, pressure and the streamline flow are found and represented in figures. Thus, several governing parameters are investigated and the flow characteristics are analysed.

Methodology
The semi-submersible model consists of pontoons, columns, horizontal brace, pencil column and deck as shown in the figure 1. The pontoons are horizontal structures which placed at the bottom which is totally submerged into the waters. The columns are vertical structures which withstand the semisubmersible platform and also connects the deck to the pontoons. The horizontal braces are structures which connects the two or more columns. The pencil column [1] is similar to the columns but dimensional different with about 0.25 diameter of the columns. The deck are surfaces upon which works takes place and where all the humans, machinery and other equipment's are placed. The model used in the present study is of 160m long, 90m wide and 40m high semi-submersible and two models are designed with and without pencil column. The model is centrally placed inside a channel through which multi-phase fluid is simulated along the x-direction. The type of solver used pressure-based and absolute velocity formulation is used. A turbulence model is used and large eddy simulation with Smagorinsky-Lilly is considered. The boundary conditions along x -axis at the inlet is Velocity-inlet and the outlet is pressure-outlet. The residual for continuity, x-velocity, y-velocity and z-velocity is considered to be 0.001. The velocity magnitude in the x-direction is set to be 10m/s. The numerical simulations are performed by using a Finite Volume Method (FVM). The turbulent flow is considered for the model by solving the large eddy simulation (LES) equation for different velocity.

Governing equations
The governing equations following are used to model the semi-submersible: Where 'ρ' indicates.the density of fluid and 'u' represents.the velocity and 'k' & 'ε' indicates the kinetic energy and dissipation.fields

Geometry and Specifications
The

Results and Discussions
The velocity, pressure, streamline is resulted. The variation in the model is simulated with velocity as 10m/s. The velocity contour of semi-submersible without pencil is shown in fig 6 and fig 7. The velocity contour of semi-submersible with pencil is shown in the figure 8 and figure 9.

Conclusion
The fluid flow characteristics across a semi-submersible platform through a channel are numerically studying various model of semisubmersible. The turbulent flow approach using LES is modelled and also the governing equations are discretized by using FVM. The stream line contours, velocity contours and pressure contours are plotted for different models. The stream line and vectors indicated the flow direction of the fluid through open channel. The pressure and velocity behind the columns comparatively less and further increase along the x-direction without a pencil column. To reduce this effect pencil columns are modelled which improve significantly. The velocity difference slightly reduces at the vicinity of the column when pencil columns are used. The results obtained from the present study will provide a suitable data for designing semi-submersible platforms with pencil column which can be used in offshore applications.