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31 October 2016 SAFER, SMARTER, GREENER DNV GL © 2016

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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

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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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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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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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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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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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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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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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SAFER, SMARTER, GREENER

www.dnvgl.com

Thank You

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Mustafa Kara

Mustafa.Kara@dnvgl.com

+1-281-396-1635