Modeling of Risers Using Hybrid FEA

Hugh Liu

Jan. 2015

Overview

1. Background

2. Codes and Buckling Criteria

3. Hybrid FEA Modeling Details

4. SCR Touchdown Area Simulation

5. TTR Dynamic Buckling Check

6. Q & A

1. Background

• Automobile vs Offshore Oil Industry

• Compression, Buckling & FEA

• Beam & Shell Elements

3. Codes and Buckling Criteria

• API 2RD: compression likely unacceptable for single string riser systems

• DNV OS-F101: εp ≤ 0.3%, otherwise ECA needed

• ISO WD 13628-12 (Petro. & Nat. Gas Ind.-Des. & Opr.

of Subsea Prod. Sys. –Pt12: Dyn. Risers for Float. Prod. Instl.): bending buckling strain limit εp = t/2D

• JFE ‘08 test (30”x0.61” pipe), εp ≈ (0.5~1) t/D

• Local ε < material elongation

Definition of Plastic strain εp:εp ≈ ε - 0.2%

: True stress/strain curve: Engineering stress/strain curve

Ultimate strength

Yield strength

Rupture

Hardening Necking

elongation

4. Hybrid FEA Modeling Details

• Shell elements used to model pipe portion of interest and capture features such as ovality, defects, local buckling etc

• External/internal pressure effects can be modeled by applying pressures on shell elements

• Beam elements used to model other portions of riser to reduce model size

• Shell- and beam-element portions connected by rigid elements

• Non-linear material stress-strain curve used

• Hydrodynamics and riser-seabed interaction (SCR) considered for both shell and beam portions

• Correlations made validated the methodology

5. Touchdown Area Simulation for a SCR

Export SCR Information

Description Value

OD (in) 20

WT (in) 1.21

Water Depth (ft) 7900

Departure Angle (°) 15

Strake Coverage 80%

Internal Pressure (psi) 3250

Material API X-65

Independence Hub with SCRs

Nonlinear Stress/Strain Curve

“Compression Assessment of Deepwater Steel Catenary Risers at Touch Down Zone”, Paper no. OMAE2007-29332 pp. 345-353

5. Touchdown Area Simulation for a SCR

ABAQUS Model with Shell Elements at Touchdown Area of a 300 ft Portion

5. Touchdown Area Simulation for a SCR

Shell Element Mesh-size Sensitivity Study for the 300 ft SCR TDA Section

Mesh 1: 12x200 = 2400aspect ratio = 2.9 ~ 4.6

Mesh 2: 16x600 = 9600aspect ratio = 1.2 ~ 2.1

5. Touchdown Area Simulation for a SCR

Sensitivity Study Results

Description

Case 1 Case 2

MaxDisplacement

Max StressMax

DisplacementMax Stress

Theory 100% 100% 100% 100%

FEA by Mesh 1 104.8% 115.4% 105.9% 113.3%

FEA by Mesh 2 102.5% 97.0% 103.5% 98.2%

P

L

L/2

Test Cases with Theoretical Results

P

L

Case 1 Case 2

-3.0E+05

-2.0E+05

-1.0E+05

0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

6.0E+05

7.0E+05

8.0E+05

1250 1300 1350 1400 1450 1500 1550

Time (s)

SF

1 (

lb)

-10

-8

-6

-4

-2

0

2

4

6

8

10

Ve

rt V

el

(ft/

s)

Axial section force (SF1) Heave velocity at hangoff point

5. Touchdown Area Simulation for a SCR

Tension Time-trace at Touchdown Point, 100-yr Hurricane

Simulation Results for 100-yr Hurricane @ Near Condition

Plastic StrainStress

5. Touchdown Area Simulation for a SCR

Hybrid (Beam + Shell) modelBeam model

Mesh 1 Mesh 2

Maximum VM stress (ksi) 69.3 62.4 58.3

Maximum equivalent plastic strain

PEEQ = 2

3𝜀𝑝𝐿

2+ 𝜀𝑝𝐻

2+𝜀𝑝𝑅

2 0.0328% 0.000% 0

5. Touchdown Area Simulation for a SCR

Animation Example 1

Stress

5. Touchdown Area Simulation for a SCR

Animation Example 2

Plastic Strain

Stress Joint

Transition Joint

Standard Joints

Standard Joints

Connectors

FEA Model with 7200 Total Elements (6550 Shell)

6. Dynamic Buckling Check of a Prod TTR

“Production TTR Modeling and Dynamic Buckling Analysis”, Engineering Sciences, Chinese Academy of Engineering, Vol. 11 No.4，Aug. 2013

w/o current w/ current

Von Mises Stress on Riser Outer

6. Dynamic Buckling Check of a Prod TTR

Analysis Results

w/o current w/ current

Plastic strain on Inner Tubing

6. Dynamic Buckling Check of a Prod TTR

Analysis Results

w/o current w/ current

Tension Variation on Tensioner Cylinders

(One Tensioner Cylinder Damaged)

6. Dynamic Buckling Check of a Prod TTR

Analysis Results

w/o current w/ current

Tension and Moment Variations at SJ Top

(One Tensioner Cylinder Damaged)

6. Dynamic Buckling Check of a Prod TTR

Analysis Results

w/o current w/ current

Centralizer Force Variations

(One Tensioner Cylinder Damaged, Centralizer Position: 1,2-top, 3,4-btm)

6. Dynamic Buckling Check of a Prod TTR

Analysis Results

6. Dynamic Buckling Check of a Prod TTR

Analysis Results

Stress

6. Dynamic Buckling Check of a Prod TTR

Analysis Results

Plastic Strain

7. Q & A