DNV GL © 2016
Ungraded
31 October 2016 SAFER, SMARTER, GREENER DNV GL © 2016
Ungraded
31 October 2016
OIL & GAS
DNV GL’s 16th Technology Week
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Advanced Simulation for Offshore Application:
Application of CFD for Computing VIM of Floating
Structures
DNV GL © 2016
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31 October 2016
OUTLINE
Introduction
Elements of Computational Fluid Dynamics
Solution Process & Quality Measures
VIM & VIV Problem Description
Case Studies: Model Scale VIM and Full Scale
Model Scale
Full Scale Single Column
Summary
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DNV GL © 2016
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31 October 2016
INTRODUCTION
Availability of fast computers &
robust software has enabled use of
CFD for complex problems like VIM
CFD of VIM is challenging (flow
separation, FSI, mooring damping
& stiffness effects and Re
dependence
Little published full-scale data for
validation
Questions remain concerning
spatial and temporal resolutions in
full-scale CFD simulations
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31 October 2016
Elements of Computational Fluid Dynamics
Basic Elements
Flow Classification
Flow Equations
Numerical Techniques
Errors & Uncertainties
Verification & Validation
Key Techniques
Turbulence Models
Rigid Body Motion
Fluid-Structure Interaction
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Solution Process & Quality Measures
Geometric Modeling
Grid Generation
Initial & Boundary Conditions
Computation
Post-Processing
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31 October 2016
VIM & VIV Problem Description
Unsteady
Strong fluid/body coupling
Flow separation
Larger turbulent scales must be
resolved
Re dependence
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CASE STUDY – Model Scale VIM
VIM of deep draft semi-
submersible platform
Comparison of CFD against
scaled model tow test data
Used OpenFOAMTM
Performed convergence study
Compared 3 turbulence models
(URANS, DES, SAS)
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HOE Paired Column Semi
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NUMERICAL METHODOLOGY
Single-phase flow
2nd order implicit scheme
for time derivatives
2nd order upwind scheme
for spatial derivatives
Coupled 6DoF solver
Arbitrary Lagrangian-
Eulerian (ALE) method to
handle rigid body motion
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Location of platform
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BASE MESH DETAILS
Three mesh resolutions tested (2, 5 & 10M cells)
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10 prism layers, y+ < 1
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Drag Test Validation
Heading 0°
U = 2 m/s
Three turbulence models:
URANS kOmegaSST
Hybrid URANS-LES
SA-IDDES agrees well
within 3
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31 October 2016
Drag Test – Q-Criterion
Drag Test – Q-Criterion
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SA-IDDES (LES) kOmegaSST (URANS)
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Drag Test – Eddy Viscosity
Eddy Viscosity = Turbulent Viscosity – transfer of momentum
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SA-IDDES (LES) kOmegaSST (URANS)
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31 October 2016
Decay Test– Natural Period
Comparison of estimated natural period in sway and yaw
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Sway Yaw
Experiment 15.5 s 9.3 s
OpenFOAM 15.3 s 9.4 s
Acusolve 15.2 s 9.3 s
Fluent 15.4 s 9.5 s
Star CCM+ 15.3 s 9.6 s
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VIM Q-criterion & Velocity Mag.
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Experimental & CFD Results
0°heading: Ur = 4, 6, 8, and 10
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CASE STUDY – Full Scale Single Column
Flow around full scale fixed
column
Single Column at
0°,8°,22.5°and 45°angle of
attacks
Comparison of CFD against
scaled model tow test data
Used OpenFOAM
Performed mesh sensitivity
Used Spalart-Allmaras IDDES
turbulence model
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FS vs MS Contour of Velocity Magnitude
0°angle of attack
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Re=3.3x104 (Model-scale) Re=14x106 (Full-scale)
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Drag and Lift Coefficients
0°,8°,22.5°and 45°headings: Comparison of time averaged drag
and lift coefficient for a fixed square column with rounded corners
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31 October 2016
SUMMARY
CFD is increasingly being used as an alternative to model tests
in the assessment of fluid dynamics aspects of offshore
structures.
This study demonstrates the effectiveness and accuracy of free,
open source CFD software, OpenFOAM™ to predict VIM
response.
DNV GL attempts to provide a best practice guideline for
applying CFD to the investigation of VIM and VIV problems of
offshore structures. DNV GL has as a long-term goal to evolve
these guidelines into a Recommended Practice.
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DNV GL © 2016
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31 October 2016
SAFER, SMARTER, GREENER
www.dnvgl.com
Thank You
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Mustafa Kara
+1-281-396-1635