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Very Large Floating Structures Hydroelasticity

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Very Large Floating Structures Hydroelasticity

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    1. Very Large Floating Structures Hydroelasticity Reza Taghipour reza@ntnu.no Wednesday, September 05, 2012 1

    2. Wednesday, September 05, 2012 2 The Blue Planet Earth! 70% H2O

    3. Buy land. They've stopped making it. Mark Twain, XIX century. N.B. Production is resumed. VLFS researchers, XXI century. Wanted! A comfortably-appointed, well-drained desert island, neighborhood of South Pacific Ocean preferred. Jerome K. Jerome, Three Men in a Boat. Wednesday, September 05, 2012 3 The need for space!!!

    4. Very Large Floating Structures (or, as some literature refers to them, very large floating platforms, VLFP for short) can be constructed to create floating airports, bridges, breakwaters, piers and docks, storage facilities (for instance for oil), wind and solar power plants, for military purposes, to create industrial space, emergency bases, entertainment facilities, recreation parks, mobile onshore structures and even for habitation. Researches on VLFS have made significant progress in the last decade. A VLFS is a unique concept of ocean structures primarily because of its unprecedented length, displacement and associated hydroelastic response, analysis and design. Wednesday, September 05, 2012 4 VLFS, What is it?

    5. Wednesday, September 05, 2012 5 VLFS as Mega-Projects

    6. Wednesday, September 05, 2012 6 VLFS as Mega-Projects VLFS are megaprojects characterized by largest-ever construction of their type, massive costs, labor and resources, technology that is risk averse, modularity or flexible configuration, and long design lives (50-100 years).

    7. Pontoon-Type (Box-type structure) Designed for sheltered sea env. Characteristic dimensions: 5km length 1km width 5m draft Semisubmersible-Type Designed for open ocean env. Characteristic dimensions: 1.5-2km length 300-500 m width 10-20m draft Wednesday, September 05, 2012 7 Classification of VLFS

    8. is a branch of science which is concerned with the motion of deformable bodies through liquids (Wikipedia). Study of marine structures when Fluid flow and the structural elastic reactions must be considered simultaneously and that we have mutual interactions (Faltinsen, 2005). For a hydrodynamicist, the term hydroelasticity refers to the satisfaction of the deformable body surface boundary condition of the boundary value problem for the velocity potential mathematical model (P. Paulo 2000) Wednesday, September 05, 2012 8 Hydroelasticity

    9. Wednesday, September 05, 2012 9 Hydrolasticity (Cont.)

    10. Frequency Domain models Time Domain models Hybrid Frequency-time domain models Dynamic models solving directly for fluid structure interactions. (in time-domain) Dynamic models using mode superposition technique (mode expansion methods) Wednesday, September 05, 2012 10 Methods for dynamic analysis

    11. Advantage: It’s fast and requires less computational burden. Disadvantage: Can be utilized to study the linear structural behavior (the load—stress, response relation is linear) Wednesday, September 05, 2012 11 Dynamic Structural Analysis using Mode Expansion Method

    12. A FEA commercial software package suitable for various types of structural analyses. ABAQUS can be utilized conveniently to carry out modal analysis. Wednesday, September 05, 2012 12 Finite Elements- ABAQUS

    13. A powerful software utilizing BEM to perform hydrodynamic analysis for Marine Structures. It can be utilized to perform sophisticated hydro-dynamic calculations in the Frequency Domain. The results of WAMIT analysis are e.g. wave loads and structure responses. Wednesday, September 05, 2012 13 Boundary Elements- WAMIT

    14. Newton’s Second Law: Fi=SF=Fh+Fc Fi is the inertia. Fh is the hydrodynamic load. (It is the main goal of hydrodynamic analysis, e.g. WAMIT to evaluate this component.) Fc is the restoring force. Wednesday, September 05, 2012 14 Response in regular sea waves Part I-Rigid Body

    15. Potential theory is assumed meaning no viscosity and no compressibility for the fluid and that the flow is irrotational. Total Potential is defined as: Wednesday, September 05, 2012 15 Hydrodynamic Loads using Potential Theory

    16. Wednesday, September 05, 2012 16 Boundary conditions! Each potential must satisfy the following conditions: – Continuity (Laplace) equation. i.e. conservation of mass. – Free surface conditions (kinematic and dynamic). No fluid particle on the water surface flies away. The dynamic pressure on the free surface is atmosphere pressure. – Radiation condition .i.e. the waves radiate away from the body. – Body boundary conditions. For Radiation:The velocity of the water particle adjacent to the body is equal to the body’s velocity. For diffraction: The water particle does not penetrate the body. – Sea bottom condition. No fluid particle penetrates sea bottom.

    17. Software solves for the potential??rad, ?diff. When the potential is known, then the pressure is known by using Bernoulli's equation: Integrating the pressure over the mean wetted surface of the body gives the force and moment due to diffraction and radiation potentials: Wednesday, September 05, 2012 17

    18. Radiation potential: Added mass and hydrodynamic potential damping forces: Diffraction potential: Wave exciting forces, X. Total hydrodynamic force after all is: Wednesday, September 05, 2012 18 Contributions from each potential:

    19. Dynamic equilibrium states Fi=SF=Fh+Fc Wednesday, September 05, 2012 19 Back to Newton’s second law:

    20. The radiation potential must be extended in this case. fj , j=7~(6+N) is due to excitation of the body itself at its jth natural frequency ?j in calm water. The body BC regarding the radiation potential due to the elastic modes needs to be corrected, correspondingly. Wednesday, September 05, 2012 20 Hydroelasticity in Equations

    21. Wednesday, September 05, 2012 21 Hydroelastic Equations

    22. Case Study The Flexible Barge, Scaled model of Mega-Float

    23. Structural Data from ABAQUS Shell Elements for 3DWLO

    24. Methodology Use of standard software Structural Analysis: ABAQUS (FEM) Hydrodynamic Analysis: WAMIT (BEM)

    25. WAMIT Results (RAOs)

    26. WAMIT Results (RAOs)

    27. Convergence Study 1

    28. Convergence Study 2

    29. Post Processing: Vertical Displacements

    30. Post Processing: Vertical Displacements

    31. VLFS HE (background) Description of ABAQUS (FEM) Description of WAMIT (BEM) HE in more detail Potential Theory Boundary conditions Pressures and Loads e.g. added mass, excitation forces Dynamic equations of motion Results for an example case study Figures from the codes in the paper with further discussion on their physical meanings and explanations. Wednesday, September 05, 2012 31 What have we learned in this lecture?

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