1. The document discusses fatigue analysis of a subsea cladded vessel using the fe-safe software. Finite element models of the vessel were created to analyze residual stresses from welding and hydrotesting. 2. Fatigue life predictions using fe-safe were compared to ASME calculations and showed good agreement. Residual stresses from hydrotesting increased the predicted high cycle fatigue life. 3. Extending the analysis to model the cladding process using Abaqus Weld Interface was proposed to provide more accurate residual stress inputs for fatigue analysis.

1. FATIGUE ANALYSIS OF SUBSEA CLADDED
VESSEL USING FE-SAFE
10/17/2017
N. Kumar, Dr. A. Chakraborty
Virtual Integrated Analytics Solutions (VIAS)
Dr. K. Karpanan
TechnipFMC

2. © 2017 Virtual Integrated Analytics Solutions Inc.
Agenda
1 VIAS: Company Overview
2 Background of Current Topic
3 Problem Statement
5 Finite Element Modelling and Results
6 fe-safe - Introduction
7 Fatigue Model Set Up and Analysis
8 Discussion
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3. © 2017 Virtual Integrated Analytics Solutions Inc.
Who We Are
Engineering
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Software
• Multiple Industry Experience – Oil & Gas, Machinery &
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• Team consists of Ph.D. and Masters in Solid Mechanics,
Fluid Mechanics, Materials and Corrosion, Numerical
Analysis, Optimization and Reliability, Data Analytics
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Abaqus, Isight, fe-safe, Tosca; CATIA, and DELMIA
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3

4. © 2017 Virtual Integrated Analytics Solutions Inc.
Technical Capabilities
© 2017 Virtual Integrated Analytics Solutions Inc.
Design Analysis and
Validation using Simulation
Fatigue / FEA based Fracture
/ Damage Mechanics
Optimization and Reliability
Multi-physics Simulations
(CFD, Thermal Analysis)
FFS and Code Based Design
by Analysis
Composite Structures
Modelling
Structural Analysis / Plant
Engineering
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Simulation Automation
FEA Model Validation Testing
Digital Industrial Operation
(DELMIA)
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Background
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• HPHT equipment: API Technical Report 17TR8
• Mean pressures greater than 15,000 psi [103 MPa]
• Temperatures above 350 °F [177 °C]
• Mean pressure greater than 20,000 psi
• Need for Elastic-Plastic Analysis since functional
pressures exceed 20 ksi
• ASME Div. 3 : KD-231
Base (low-carbon steel)
Clad (Alloy 625)
HPHT Gate Valve

6. © 2017 Virtual Integrated Analytics Solutions Inc.
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• Cladding Process: Thin layer of Corrosion
Resistant Alloy (CRA) material integrally bonded
to the surface
• Low-carbon steel is cladded with CRA such as
Alloy 625
• Any crack in the cladded layer will expose the
underlying base material
• Rapid corrosion
Numerical Simulation of Cladding Process
Background
Cladding Process

7. © 2017 Virtual Integrated Analytics Solutions Inc.
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Cladding process
• High thermal tensile stress on the clad layer and HAZ
• Shortens Fatigue Life
• More likely to damage
Background
Cladded Layer (ID)
Cladded Layer (OD)
Base Metal (OD)

8. © 2017 Virtual Integrated Analytics Solutions Inc.
Background
8
Analysis of residual stress relief mechanisms in post-weld heat
treatment
• Post Weld Heat Treatment (PWHT)
• Reduced Tensile Stress
• Heating relaxes the weld
stresses
• Hydrostatic Test: Effect similar to
Autofrettage
• Operating Pressure Cycles (startup and
shutdown):
• The mean stress on the Fatigue
Sensitive Region (FSR) reduces
significantly
• Fatigue life is affected

9. © 2017 Virtual Integrated Analytics Solutions Inc.
Problem Statement
9
Objective
• Numerical Investigation of the effect of PWHT
and hydrotest on the fatigue life of cladded
vessel
• Compare and validate ASME VIII-3 fatigue
analysis method with results from fe-safe
Important Input Parameters Range of Values
PWHT Temperature 12000F – 4000F
Hydrostatic Test Pressure 30 and 40 ksi
Low Cycle Amplitude 0-20 ksi
High Cycle Amplitude 10-16 ksi

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FEA Modelling (Geometry & Mesh)
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Clad
Base Metal
Clad
Base
Metal
FSR
• Valve Body
• Height = 10”
• Outer Radius = 8”
• Cladding
• Thickness = 0.15”
• Thickness at FSR = 0.2”
• An axisymmetric FE model of a valve inner
body.
• CAXA elements (~8000 elements)
• Finer mesh at the FSR zone

11. © 2017 Virtual Integrated Analytics Solutions Inc.
FEA Modelling (Material)
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F22
Alloy 625
Cladding Layer Valve
Material Alloy 625 F22
Young’s Modulus (ksi) 28203.85 29692.22
Poison’s Ratio 0.3 0.3
Expansion Coefficient 8.52E-6 6.42E-6
Yield Stress (ksi) 69.62 59.28
• Both Linear Elastic and Elastic Plastic
methods are analyzed
• The true stress-strain curves used in the
Elastic-Plastic FEA are generated using
ASME VIII-3, KD-231.4 method
• Temperature dependant material properties
are not considered

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FEA Modelling (Loads and BCs)
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Fixed in vertical direction
Steps Load
PWHT 12000F – 4000F
Hydrotest Pressure 30 & 40 ksi
Fatigue Life Cycles 0-20 ksi
Fatigue Life Cycles 10-16 ksi
• Simplified Method
• No residual stress at PWHT (12000F)
• Residual stresses arise as a result of
cooling to Operational Temperature
(4000F)

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FEA Modelling (Loads)
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Scenarios 1 2 3 4 5 6
Material Model LE EP EP EP EP EP
PWHT No No Yes No Yes Yes
Hydrotest Pressure, ksi No No No 30 30 40
Fatigue Life Cycles (0-20 ksi) Yes Yes Yes Yes Yes Yes
Fatigue Life Cycles (10-16 ksi) Yes Yes Yes Yes Yes Yes
• Realistic Simulation
• Multiple Scenarios
• Faster and at very low cost.

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LE Model
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• Max and Min Principal Stress at 20 ksi internal pressure using Linear Elastic model
• High Stresses on the fillet

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EP Model: WRS – End of PWHT
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• Clad layer is mostly in tension
• Difference in expansion coefficients

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EP Model: End of Hydrotest
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• No tensile stresses in the clad layer
• Even the maximum principal stresses are
compressive in the FSR zone
1st principal
stress is
negative

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EP Model: 20 ksi Working Pressure Cycle
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• High stresses region is not on the surface

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Residual Stresses
after Hydrotest
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Path for mapping
residual stresses
Clad-Base interface
Residual Stresses measured in the radial
direction show expected behaviour

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Residual Stresses: Effect on WP Cycle
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Residual Stresses: Varying Hydrotest Pressure
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21. © 2017 Virtual Integrated Analytics Solutions Inc.
fe-safe: A Comprehensive Tool for Fatigue
Modelling
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• Leader in fatigue analysis tools (Critical
Distance Methods, Multi-axial Loading,
Infinite Life Models)
• Large Database of Materials (350+
materials)
• Metals, Composites, Rubber, Weld, High
Temperature
• Seamless User-friendly Workflow
• Reads leading FEA software output files
and produces results in the same format

22. © 2017 Virtual Integrated Analytics Solutions Inc.
fe-safe: A Comprehensive Tool for Fatigue
Modelling
23
• Fatigue of welded joints
• Seam or Spot Welds
• Thermo-Mechanical Fatigue
• Creep Fatigue
• Fatigue of Elastomers

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Fatigue Modelling
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• Biaxial Strain fatigue algorithm:
• Brown Miller (Critical Plane) Algorithm with
Morrow mean stress correction
• Other popular Strain and Stress based fatigue
algorithms are available
• Infinite life algorithms.

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Fatigue Modelling
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• Material Parameters for Strain based calculations
σ'f, psi 150,000
b -0.14
ε'f 0.45
c -0.8

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Fatigue Modelling
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• SN curve for stress based calculations
Input in Fe-Safe

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Fatigue Modelling
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• Fatigue life is minimum at
the cladded surface,
which is not the region of
maximum stresses.
• Values and locations for
both high and low cycle
fatigue compares well
with ASME calculations

27. © 2017 Virtual Integrated Analytics Solutions Inc.
Results
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• Multiple analyses cases were run
• Both stress and strain based fatigue analysis results are presented
• Stress based analysis use Linear Elastic (LE) model and strain based analysis uses
Elastic Plastic (EP) model
• No significant change in the fatigue life in 0-20 ksi case
• Fatigue life shows improvement in 10-16 ksi when stress from hydrotest are
considered
Case A* 1 2 3 4 5
Material Model LE EP EP EP EP EP
PWHT NA NA Yes NA Yes Yes
Hydrotest Pressure, ksi NA NA NA 30 30 40
Fatigue Life Cycles (0-20 ksi) 531 556 525 636 617 660
Fatigue Life Cycles (10-16 ksi) 1,500 16,520 12,829 31,044 26,627 37,132
* ASME VIII-3 Fatigue analysis method

28. © 2017 Virtual Integrated Analytics Solutions Inc.
Discussion (Results and Tools)
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• Fatigue life of a cladded subsea component is evaluated by considering the residual
stress from the weld cladding process and the hydrotest to predict the fatigue life more
accurately
• In LCF, predicted fatigue lives for both elastic and multiple E-P cases are slightly
different because mean stress effects are less significant and the shear stress ranges
are same in all cases
• In HCF, the means stress effects are very pronounced and this resulted in higher
fatigue life for the E-P model compared to the L-E model
• Results from fe-safe and ASME fatigue calculation methods are very close

29. © 2017 Virtual Integrated Analytics Solutions Inc.
Discussion (Extension)
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• Abaqus Weld Interface (AWI) seems to very promising to simulate the cladding process
• A sequential stress analysis based on the temperature history from cladding simulation
will give more insights into the residual stresses and subsequently fatigue life
estimation
• Validated temperature dependent properties for thermal-stress simulation and fatigue
parameters of the clad material will be needed.

30. © 2017 Virtual Integrated Analytics Solutions Inc.
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Thank you