Modeling of the Unsteady Separated Flow over Bilge Keels of FPSO Hulls under Heave or Roll Motions

1. Modeling of the Unsteady Separated Flow over Bilge Keels of FPSO Hulls under Heave or Roll Motions Yi-Hsiang Yu 09/23/04 Copies of movies/papers and today’s presentations may be downloaded from Http://cavity.ce.utexas.edu/kinnas/fpso/

2. Introduction • FPSOs are tanker like floating hulls which are used for production, storage and offloading of oil. • FPSO hulls have often been found to be subject to excessive roll motions, and the installation of bilge keels has been widely used as an effective and economic way of mitigating the roll motions of hulls. • The main focus of this research is to model the unsteady separated viscous flow over the bilge keels of a FPSO hull subject to roll motions and to determine its effect on hull forces.

3. Overview of the Presentation • Numerical Formulations • Governing Equations • Numerical Methods • Non-linear Term Treatment • The Effect of the Moving Grid • Results • An Oscillating Flow over a Vertical Plate • Submerged Body Undergoes Heave or Roll motions • FPSO Hull Subjects to Roll Motions • Conclusions and Future Work

4. Numerical Formulation • Governing Equation Non-Dimensional Governing Equation (Navier-Stokes Equation & Continuity Equation) where U represents the velocity; Q is the force term; and R indicates the viscous term. The definitions of the column matrices for the Navier-Stokes equation are given as where the Reynolds number is define as Re = Umh/ν; and the length scale, h, is a representative length in the problem being solved.

5. Cell Based Finite Volume Method (Collocated variable, non-staggered grid arrangement) According to the integral formulation of the Navier-Stokes equation and to the Gauss divergence theorem, a semi-discrete integral formulation of the momentum equation can be given as where Sijis the area of the cell; and ds represents the length of each cell face. A cell center based scheme is applied where “i,j” is the center of the cell (non-staggered grid: unknown value u, v, p are located at the cell center). when calculating the flux, the value on the cell face (at D) is needed. It can be obtained from Taylor series expansion.

6. Crank-Nicolson Method for Time Marching where f represents the summation of the convective terms, the viscous terms and the pressure terms at the present time step n and the next time step n+1. • Pressure-correction Method • SIMPLE method (Patankar 1980) where p’ is the pressure correction, V’faceis the velocity correction term, әp’ /әnis the pressure correction derivative with respect to the normal direction of the cell face, V*face= (u*; v*) is the predicted velocity vector obtained from the momentum equation.

7. Appropriate Pressure-Correction Equation Since our 2-D Navier-Stokes solver uses non-staggered grid, the scheme has to be modified somewhat to avoid the checkerboard oscillation problem. where aijis the coefficient of the unknown velocity in the momentum equation; "av" indicates the average value obtained from the cell center value; and "d" represents the value calculated directly at the face center. • Non-linear Terms Treatment The momentum equation can be rearranged as where u* is the unknown velocity at T = n + 1; the coefficient “a” is also a function of the velocity at T = n+1 which can be obtained from the previous iteration; and dijis the coefficient of pressure in the momentum equation.

8. Moving Grid When the grid is moving, additional terms need to be taken into account. where (ugrid, vgrid) is the velocity of the moving grid; and represents the total change in the value of u with both increment in time and the corresponding change in the location of the point. When the above equation is substituted into the momentum equation

9. Problem Description • The main focus of this research is to model the unsteady separated viscous flow over the bilge keels of a FPSO hull subject to roll motions and to determine its effect on hull forces. • It can be simplified as three different problems: • Oscillating flow over a vertical plate. • Free surface effect (linear and non-linear). • Submerged body with or without the bilge keels. • Consider the effect of the bilge keels and the effect of the free surface

10. Results • Oscillating Flow Past a 2-D Vertical Plate

12. Drag & Inertia Coefficient for a Range of Kc=UmT/h (0.5 < Kc < 5)

13. Submerged Body Motions

14. Potential Flow Results of the Hull Undergoing the Heave Motion at t/T=0.25

15. FVM FVM • Potential Flow Results of the Hull Subject to the Roll Motion The pressure distribution along the submerged hull without bilge keels

16. Viscous Flow Results of the Submerged Hull with Bilge Keels Subject to the Roll Motion

18. Results of the Unsteady Separated Viscous Flow over the Bilge Keels of a FPSO Hull Subject to Roll Motions

19. Previous Results • More details can be found in Kacham [2004], and Kakar [2002]

22. Conclusion • A numerical scheme for solving the Navier-Stokes equation has been developed. • It is well validated with experimental results in the case of an oscillating flow past a vertical plate. • The method is also applied to the case of submerged bodies which are subject to forced heave or roll motions. The numerical results have shown good agreement with the potential solver (boundary element method). • Then, the method is applied to the case of a FPSO hull undergoing roll motions. The effects of the bilge keels and of the free surface are also taken into account. The numerical results is improved after including the terms for a moving grid in a fixed inertial coordinate system.

23. Future Work • More convergence studies in time and space are necessary. • The capability of the solver to handle the non-orthogonal grid geometry still needs to be improved (Some small oscillating behaviors exist around the bilge keels area). • More investigations on the nonlinear free surface effect are needed. • Extend the model in 3-D and compare with experiments and other numerical results.