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NUMERICAL ANALYSIS OF PILES IN OFFSHORE REGIONPresented by ATUL KUMAR SINGH

In Association with1

Department of Civil Engineering, MIT puneUnder the Guidance ofProf Dr. P.K.Maiti (Dept. of Civil Engg., IIT(BHU))Prof. Dr. M.S.Kulkarni (Dept. of Civil Engg., MIT, Pune)Prof G.S.Ingle (Dept. of Civil Engg., MIT, Pune)

1DEPARTMENT OF CIVIL ENGINEERING, MIT PUNEINDEX1)Finite Element Analysis

2)Fluid Structure Interaction

2Department of Civil Engineering, MIT puneDEPARTMENT OF CIVIL ENGINEERING, MIT PUNE2FINITE ELEMENT ANALYSISDefinitionFEM is a numerical method for solving a system of governing equations over the domain of a continuous physical system, which is discretized into simple geometric shapes called finite element.

3Continuous systemTime-independent PDETime-dependent PDEDiscrete systemLinear algebraic eq.ODE

Department of Civil Engineering, MIT pune4Inmathematics, thefinite element method(FEM) is anumerical techniquefor finding approximate solutions toboundary value problemsforpartial differential equations. It usesvariational methods to minimize an error function and produce a stable solution. FEM is best understood from its practical application, known asfinite element analysis (FEA). FEA as applied inengineering is a computational tool for performingengineering analysis. It includes the use of mesh generationtechniques for dividing acomplex probleminto small elements, as well as the use ofsoftwareprogram coded with FEM algorithm.

Department of Civil Engineering, MIT puneFLUID STRUCTURE INTERACTION

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Department of Civil Engineering, MIT pune6Fluid structure interaction (FSI) is a multi-physics phenomenon which occurs in a system where flow of a fluid causes a solid structure to deform which, in turn, changes the boundary condition of a fluid system. This can also happen the other way around where the structure makes the fluid flow properties to change. This kind of interaction occurs in many natural phenomena and man-made engineering systems.

It becomes a crucial consideration in the design and analysis of various engineering systems. For instance, FSI simulations are conducted to avoid flutter on aircraft and turbo-machines , to evaluate the environmental loads and dynamic response of offshore structures and in many bio medical applications.

Offshore platform, also referred as oil platform, is a large structure used for exploration of oil and gas from beneath the seabed. This kind of structure contains all facilities needed to locate and extract oil and natural gas below the earth surface.

Most of these platforms are placed in a rough sea environment; these structures are subjected to different types of environmental loads like waves, winds and ocean currents. So, it is essential for an offshore engineer to estimate the forces generated by wave loading for the existing and future platforms to ensure safe and robust design.Department of Civil Engineering, MIT pune7

Oil RigJacketDepartment of Civil Engineering, MIT pune8

Figure 6:- Structural member

Length=20mDiameter=.8mFigure 7 Computational mesh of the fluid domainPile of diameter 0.8m in the fluid domainPROBLEM STATEMENTDepartment of Civil Engineering, MIT pune9

Figure 8:- Different types of meshing done in the ANSYS softwareDepartment of Civil Engineering, MIT pune10BOUNDARY CONDITION AND GOVERNING EQUATIONSDepartment of Civil Engineering, MIT puneDEPARTMENT OF CIVIL ENGINEERING, MIT PUNE10

3) Hydrodynamic Loading (Wind Generated Waves)Hydrodynamic loading is the load that is applied by the Oscillatory flow of Oceanic waves. Influid dynamics,Airy wave theory(often referred to aslinear wave theory) gives alinearizeddescription of thepropagationofgravity waveson the surface of a homogeneous fluidlayer.This linear theory is often used to get a quick and rough estimate of wave characteristics and their effects. This approximation is accurate for small ratios of thewave heightto water depth (for waves inshallow water), and wave height to wavelength (for waves in deep water).

12EulerBernoulli beam theory(also known asengineer's beam theoryorclassical beam theory)is a simplification of the lineartheory of elasticitywhich provides a means of calculating the beam that is subjected to lateral loads only.

The EulerBernoulli equation describes the relationship between the beam'sdeflectionsand the applied load:

4) EulerBernoulli beam theory

Department of Civil Engineering, MIT puneDepartment of Civil Engineering, MIT pune13Inertial ForceDrag Force5) Morison Equation14

Free surface flow around cylinder at t=19.5 Free surface flow around cylinder at t=21.5 Free surface flow around cylinder at t=20.0 Free surface flow around cylinder at t=22.0 Free surface flow around cylinder at t=20.5 Free surface flow around cylinder at t=22.5Department of Civil Engineering, MIT puneDEPARTMENT OF CIVIL ENGINEERING, MIT PUNE1415

123456Natural Frequency= 2.77Natural Frequency= 7.43Natural Frequency= 13.87Natural Frequency= 7.43Natural Frequency= 13.87Natural Frequency= 2.77Department of Civil Engineering, MIT pune16Graph shows the time history of CFD horizontal force (wave loads) and the reaction force (dynamic response) of the structural member for the one-way coupling simulation of case1. At the time of 0.112s approximately, free surface of the water touches the bottom side of the cylinder which gradually tends the CFD force to increase. The maximum horizontal force is attained by the time of around 0.330s.

Department of Civil Engineering, MIT pune17

The peak vertical force (slamming force) is attained at the time of around 0.115s with the magnitude of 480kN approximately. The magnitude of the CFD force starts to decline in a steady way after it achieves the peak value.Vertical (CFD & Reaction) forces Department of Civil Engineering, MIT pune18

Horizontal and vertical deformations of the structural member Graph shows the horizontal and vertical deformation of the structural member. This is taken with respect to a single point located at the center of the structure using CFD Post.Department of Civil Engineering, MIT puneDepartment of Civil Engineering, MIT pune19Validation and Conclusion

Some very limited validation work of this project have been done by comparing the horizontal fluid force with the result of Morison equation and vertical CFD force with the experimental data presented by Isaacson and Prasad (1994)

Above figure shows the comparison of horizontal fluid force attained by the numerical method with result of Morison equation. The formula mentioned in above slide with CM and CD as 2.0 and 0.65 respectively is used to find horizontal force value. The difference between peak values of two curves is observed as 100 kN approximately.DEPARTMENT OF CIVIL ENGINEERING, MIT PUNE19REFRENCES1. B.Iwanowski, R.Gladso, M.Lefranc, Wave-in-deck load on a jacket the platform, CFD derived pressures and non- linear structural response 28 OMAE International conference, Hawaii, USA. May 31- June 5, 2009. 2. A.K.Pozarlik, J.B.W.Kok, Numerical investigation of one-way and two-way fluid structure interaction in combustion systems International conference on computational methods for coupled problems in Science and Engineering, Barcelona.2007. 3. ANSA 13.2.1 BETA CAE Systems S.A. P.O. Box 18623, GR-54005 Thessaloniki, Greece 4. Fluent 14.0. ANSYS, Inc., Southpointe, 275 Technology Drive, Canonsburg, PA15317, USA. 5. Z.Yun, Y.Hui, Coupled fluid structure flutter analysis of a transonic fan Chinese Journal of Aeronautics, vol.24, 2011, 258-264. 6. S.A.Anagnostopoulos, Dynamic response of offshore platforms to extreme waves including fluid structure interaction Eng.Struct.,vol. 4, July, 1982. 20Department of Civil Engineering, MIT pune21THANK YOUDepartment of Civil Engineering, MIT pune