Installations of Subsea Equipment in Deep Waters: ANSYS ... · Installations of Subsea Equipment in Deep Waters: ANSYS CFD and AQWA Solution Madhusuden Agrawal Paul Schofield ANSYS

© 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


• Introduction – Subsea Installation

• Modeling Challenges and Different Methodologies

• ANSYS CFD

• Transient simulations

• Moving mesh

• Free surface modeling

• ANSYS AQWA

• AQWA-FLUENT Coupling

Contents


• 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


Splash Zone Hydrodynamics

4


Modeling Approaches

AQWA

AQWA Analytical Analytical


• 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


• Life Boat Launching

• Structure Landing near Seabed

• Platform Settling in the Mud

• Hollow Cylinder in Splash Zone

• Wave Slamming

CFD Examples


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


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


Lifeboat Launching Animation


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


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…


Animations: Body Motion near Sea Floor


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)


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


Animation - Hollow Cylinder in Splash Zone






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



-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





CFD Example: Platform Settling in Mud

Mud (Bingham fluid)

Water

Immersed Body motion, Free Surface Modeling, Non-Newtonian Fluid


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


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).


ANSYS AQWA - Example

Lowering/raising of equipment onto floating vessels


ANSYS AQWA - Example

The equipment is modeled using a Morison element

Incident and variable hydrostatic loads can be accounted for in the AQWA model


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


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


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


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


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


• 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

Documents

Installations of Subsea Equipment in Deep Waters: ANSYS ... · Installations of Subsea Equipment in Deep Waters: ANSYS CFD and AQWA Solution Madhusuden Agrawal Paul Schofield ANSYS