Evaluation of hydrodynamic coefficients on riser floaters using CFD Erico Santos, Pedro Mendes, Bruno Luna – PETROBRAS (CENPES/PDEP/TDUT)

Ricardo Damian – ESSS

AGENDA

• MOTIVATION

• PROCEADURE OVERVIEW

• RISER GLOBAL ANALYSIS

• CFX SIMULATION

• PARAMETER ESTIMATION PROCEDURE

• PRELIMINARY CONCLUSIONS

• CFD RESULTS

• HYDRODYNAMIC PARAMETERS RESULTS

• CONCLUSIONS & NEXT STEPS

MOTIVATION

• CFD – Drag coeficient

– Added mass

PROCEADURE OVERVIEW

Characterization

• Object

• Amplitude

• Period

• Current

CFD

• Domain

• Mesh

• Parametric Setup

• Export Forces

Estimation

• Morison Model

• Drag (Re, Direction)

• Added Mass (Re, Direction)

Global Analysis

RISER GLOBAL ANALYSIS

• Waves

• Current

• Offset

• Soil

• Floatings

• Riser Properties

• Fluid

– Internal

– external

FLUID FORCES

• Morison´s Equation

– Inertia: linear function of structural acceleration

– Added mass: proportional to the fluid acceleration

– Drag: proportional to the relative velocity between fluid and structure

FLUID FORCES

• Added Mass

• Drag Coefficient DNV-RP-H103

MOTION CHARACTERIZATION

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0,4

0 20 40 60 80 100 120 140 160

tempo [s]

Am

plitu

de

XL

YL

ZL

Z

X

CFD DOMAIN

Riser Sector:

Diameter 0.2755 m

Cylindrical Floaters:

Diameter 1.6 m

Length 1.8 m

Spacing 4.2 m

Corner Rounding 0.1 m

Cylindrical Domain:

Diameter 20 m

Length 22.2 m

CFD MESH

ANSYS Meshing (Workbench)

Global Sizing:

Min Size 0.05 m

Max Size 2.0 m

Growth Ration 1.1

Curvature Sensitive

Riser & Floaters:

Max Size 0.1 m

10 Prismatic Layers

Growth Ration 1.25

Transition Ratio 0.6

Turbulence:

SST Model

Y+ ≈ 100

CFD SETUP – BOUNDARY CONDITIONS

Transient Setup

Moving Domain

Rigid Mesh Motion

Farfield:

CFX Opening

Moving Boundary

Velocity

Current Conditions

Turbulence

5% Intensity

Length Scale => L Floater

Riser & Floaters:

Moving Wall

Stationary Setup

Farfield:

CFX Opening

Velocity

Current Conditions

Turbulence

5% Intensity

Length Scale => L Floater

Riser & Floaters:

Wall

CFD SETUP – RISER/FLOATER MOTION

Typical Motion Behavior

Period

Ocean Wave Motion

12.5 s chosen

Amplitude

≈ Floater Diameter / 10

0.16 m chosen

Harmonic Motion chosen

CFD SETUP – PARAMETERIZATION

Current Parameters

Direction (Horizontal Angle)

Reynolds Number

Characteristic Length

Floater Diameter

U, V, W, ρ & µ

Motion Parameters

Amplitude

Orientation (i, j , k)

Period

Rigid Mesh Motion

Monitor Points

Total Force (pressure + shear)

Central Floater

X, Y, Z directions

CFD SETUP – SOLVER SETTINGS

PARAMETER ESTIMATION PROCEDURE

CFD Model

Monitor Points

Forces along time

Position

Velocity

Acceleration

Morison Equation

Minimum Square Method

Optimization Algorithm

Minimize Sum of the Quadratic Error

• For the tested motion, added mass is the dominant force – The amplitude and period produces small velocities

• The estimation of drag and added mass coefficients in the same time isn't precise for the drag

• The added mass coefficient doesn't depend on the current (speed or direction)

• Drag coefficient can be easily obtained with stationary runs

• New strategy established:

PRELIMINARY CONCLUSIONS

Stationary CFD for all current direction

Drag coefficient estimation

Single Transient CFD w/ motion & no

current

Added Mass coefficient estimation

CFD RESULTS

N – 90° NE – 45°

N – 22.5° E – 0°

NNE – 67.5°

Vortex core isosurface colored with velocity

HYDRODYNAMIC PARAMETERS RESULTS

Transverse

Longitudinal

• A systematic CFD procedure for hydrodynamic coefficient estimation of moving submerged bodies was established

• Stationary runs are used for drag estimation according to the current direction

• A single transient run with body motion is used for added mass estimation

• Only the drag coefficient depends on current direction

• The CFD setup parameterization saves user´s time

• Next steps:

– To automate the parameter estimation step

– To perform similar analysis to complex bodies (manifold, anchor, subsea separator, etc)

CONCLUSIONS & NEXT STEPS