© 2011 ANSYS, Inc. September 8, 2011 1
Installations of Subsea Equipment in Deep Waters: ANSYS CFD and AQWA Solution
Madhusuden Agrawal
Paul Schofield
ANSYS Inc
© 2011 ANSYS, Inc. September 8, 2011 2
• Introduction – Subsea Installation
• Modeling Challenges and Different Methodologies
• ANSYS CFD
• Transient simulations
• Moving mesh
• Free surface modeling
• ANSYS AQWA
• AQWA-FLUENT Coupling
Contents
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• Ultra Deep Water Deployment of Subsea Equipment – Upto 10,000 ft range, costly operation
• Subsea equipment include a variety of sizes and shapes – Manifolds, mudmats, trees, jumpers, piles, etc
– System response depends strongly on the package hydrodynamic properties
• Added-mass, drag, as well as its weight, and buoyancy
• Three of the most critical phases during the
installation – Overboarding procedure
• Sea-state, slamming
– Lowering equipment through splash zone
• large transient loads, difficult to estimate
– Landing of equipment on the sea floor
• Need precision, tight tolerance
Introduction
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• Large Domain, Complex Geometry, Mesh Resolution
• Transient Simulations – long duration
• Rigid Body Motion - Moving Mesh
• Fixed Motion
• 6DOF Motion
• Waves and Current Modeling
• Non-linear waves
• Turbulence, Compressibility Modeling
Challenges in CFD Modeling
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• Life Boat Launching
• Structure Landing near Seabed
• Platform Settling in the Mud
• Hollow Cylinder in Splash Zone
• Wave Slamming
CFD Examples
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CFD Example: Life Boat Launching
• The lifeboat launched from a ramp – Initial linear, vertical and angular
accelerations as it slides down the ramp.
– Then it is allowed to free fall through air before hitting into water.
• Free Motion, Moving Deforming Mesh, Free Surface Modeling, Compressibility…
• Predict the linear and angular velocities of the lifeboat
• 3DOF motion is considered. – Two linear motions (horizontal and
vertical) and the rotation around an axis
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Life Boat Launching
Free surface
25 m 4m/s
8m/s
Water
Air
Initial conditions
8.6 deg/s
Rigid body motion is given to the boat and the spherical region around boat to maintain the good quality mesh.
75m
15m
25m
air
water
95m
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Motion History of the Lifeboat
Vertical Position Angular Position
Horizontal Position
Vertical Velocity Horizontal Velocity • Positions are
measured in global coordinate system
• The center of this coordinate system is the CG of the boat at t=0 s
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CFD Example: Body Motion near Sea Floor
Dropped at constant velocity of 1m/s
Sea Floor
Fixed motion, Moving Deforming Mesh with Layering, Compressibility Damping, High velocity and pressure forces, Viscous effects…
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Force History on Bottom Surfaces of the Body
0
5000000
10000000
15000000
20000000
25000000
30000000
0 20 40 60 80 100 120 140 160 180 200
Forc
es
(N)
Distance from Sea Floor (inches)
Vertical Force vs Distance from Sea Floor
0
5000000
10000000
15000000
20000000
25000000
30000000
0 2 4 6 8 10
Forc
es
(N)
Distance from Sea Floor (inches)
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CFD Example: Hollow Cylinder Motion in Splash Zone
5m diameter and 25m long pipe, Open from one end and 1m diameter hole at other end
Body Dropped at Fixed velocity of 1 m/s
Mesh
Four Different Angle of Entry in Water
Fixed Motion, Moving Deforming Mesh, Free Surface with Waves, Trapped Air in Cylinder, Compressibility, Transient Forces on Structure
00 Entry Angle
200 Entry Angle
600 Entry Angle
900 Entry Angle
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Trapped Air and Forces History
0
100
200
300
400
500
600
0 10 20 30 40 50
Air
Vo
lum
e (
m3
)
Time (sec)
Trapped Air Volume
0 degree case
20 degree case
60 degree case
90 degree case
-1000000
0
1000000
2000000
3000000
4000000
5000000
6000000
7000000
0 10 20 30 40 50
Forc
e (
y-d
ir)
in N
Time (sec)
Force in Vertical Direction
0 degree case - net
20 degree case - net
60 degree case - net
90 degree case - net
-2000000
-1000000
0
1000000
2000000
3000000
4000000
0 10 20 30 40 50
Forc
e (
x-d
ir)
in N
Time (sec)
Force in Wave Direction
0 degree case - net
20 degree case - net
60 degree case - net
90 degree case - net
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CFD Example: Platform Settling in Mud
Mud (Bingham fluid)
Water
Immersed Body motion, Free Surface Modeling, Non-Newtonian Fluid
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inlet outlet
submarine
Top (pressure-outlet)
• 3-D unsteady solver
• Multiphase: VOF Model
• Open channel wave BC to generate 5th order waves
• Turbulence model: SST k-omega
CFD Example: Wave Slamming
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ANSYS AQWA
• AQWA is a hydrodynamic radiation/diffraction code that enables the modeling of cables and moorings.
• The lowering of equipment through the splash zone is difficult for such a code to simulate directly since the variable immersion cannot be accounted for in the diffraction/radiation loading (would require multiple diffraction analyses).
• F-K and dynamic hydrostatic loads can be included for the variable immersion scenario.
• Drag loading is not directly included in a diffraction based solution, neither is slamming.
• Because of this it is often simplified by modeling the equipment using Morison elements with appropriately computed drag and added mass coefficients (normally from CFD).
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ANSYS AQWA - Example
Lowering/raising of equipment onto floating vessels
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ANSYS AQWA - Example
The equipment is modeled using a Morison element
Incident and variable hydrostatic loads can be accounted for in the AQWA model
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Need for Coupling CFD with AQWA
• Steady state approximation ignores motion of body in drag calculations
• Drag force will depend on orientation and location of body
• Free surface / wave motion effects are not included in drag calculations
Need for Bi-directional connectivity with CFD and Diffraction codes
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Fully Coupled FLUENT-AQWA
• Automated coupling
• FLUENT and AQWA exchange information at each time step
• Multiphase VOF model with Open Channel Boundary Condition in FLUENT to include waves effects
• Moving Deforming Mesh Motion with non-conformal interfaces to avoid bad skewness after remeshing
Transient FLUENT simulation with Rigid Body motion
AQWA simulation for Global Analysis
Drag forces/moments
Linear and angular velocities
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AQWA-FLUENT Coupling – Proof of Concept
Geometry – Solid Cylinder (5m diameter and 25m long) Mass – 500 tons Payout rate – 1 m/s
Rope attached to top of the cylinder
•Transient CFD simulation •Airy waves •VOF Model •MDM with non-conformal interface •Time step size – 0.1 sec •About an hour of clock time to run 10 sec of simulation on single CPU
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Body Motion with Water VOF
Body Motion with Velocity Magnitude
65
70
75
80
85
90
95
15 20 25 30 35 40
Z-C
oo
rdin
ate
(m
)
X-Coordinate (m)
COG Trajectory
65
70
75
80
85
90
95
0 2 4 6 8 10 12 14 16
Z-C
oo
rdin
ate
(m
)
Time (sec)
COG-Z position History
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Forces and Velocities
-2000000
-1000000
0
1000000
2000000
3000000
4000000
0 5 10 15 20
Forc
es
(N)
Time (sec)
Forces History (Vertical Dir and Wave Dir)
Force-Wave-DirForce-Vertical-Dir
-5000000
0
5000000
10000000
15000000
20000000
25000000
0 2 4 6 8 10 12 14 16
Mo
me
nts
(N
-m)
Time (sec)
Time History of Moment (Y dir) -12
-10
-8
-6
-4
-2
0
2
4
6
0 2 4 6 8 10 12 14 16
Z-V
elo
city
(m
/s)
Time (sec)
Time History of Z-Velocity,
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0 2 4 6 8 10 12 14 16
An
gula
r V
elo
city
(ra
d/s
)
Time (sec)
Time History of Angular Velocity
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• Need for advanced modeling for Subsea Installation operations
• Role of CFD and Challenges in CFD Modeling
• More accurate and proven approach
• Complement with AQWA solution
• AQWA-FLUENT Coupling
• State of art, comprehensive and accurate modeling approach for installation process
• Proof of concept study to demonstrate the feasibility of this approach
Conclusion
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