subsea wellhead system design for fatigue performance · 11 march 2015 subsea drilling services...
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Subsea Drilling Services11 March 2015
Subsea Wellhead System Design for Fatigue PerformanceF. Justin Rodriguez – 11 March 2015
Subsea Drilling Services11 March 2015 1
Introduction to Fatigue
Mechanics of the failure• Dislocations accumulate near
surface stress concentrations• Generates stress risers where
cracks initiate• Cracks join together and begin to
propagate through material• Crack propagates to critical point
when part can no longer sustain load
Source: http://ars.els-cdn.com/content/image/1-s2.0-S092150931001049X-gr4.jpg
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Introduction to Fatigue
Wellhead System Loading• Loads generated due to:
– Waves (vessel motion & offset)– Currents along water column– Vortex-Induced Vibration (VIV)
• Loads cyclic in nature• Transferred to wellhead system
through riser
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Fatigue Analysis Process
Finite Element Analysis (FEA)Input: GeometryOwner: Equipment Manufacturer
Component Stress Response
Analysis MethodologyInput: Industry Code or Internal SpecificationOwner: Operator
Calculation Methodology
Riser AnalysisInput: Rig, Weather DataOwner: Operator or Equipment Manufacturer
Cyclic Load Profile
Fatigue Life
Fatigue Damage CalculationOwner: Equipment Manufacturer / Operator
Accumulated Fatigue Damage
Inpu
tO
utpu
t
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Finite Element Analysis (FEA)
• “Global” model determines overall load transfer characteristics of the system
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Finite Element Analysis (FEA)
• “Sub” models determine local stress response at each critical feature in the system
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Fatigue Analysis Process
Finite Element Analysis (FEA)Input: GeometryOwner: Equipment Manufacturer
Component Stress Response
Analysis MethodologyInput: Industry Code or Internal SpecificationOwner: Operator
Calculation Methodology
Riser AnalysisInput: Rig, Weather DataOwner: Operator or Equipment Manufacturer
Cyclic Load Profile
Fatigue Life
Fatigue Damage CalculationOwner: Equipment Manufacturer / Operator
Accumulated Fatigue Damage
Inpu
tO
utpu
t
Subsea Drilling Services11 March 2015 7
Analysis Methodology
Methods of Fatigue Assessment• Stress-Life Approach (S-N Approach)
– Based on empirical material data– Calculates accumulated “damage” due to cyclic loading– Predicts life based on number of cycles to failure– Heavily impacted by geometry optimization
• Fracture Mechanics approach– Calculates crack growth in material due to cyclic loading– Assumes initial flaw size– Less sensitive to geometry optimization
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Analysis Methodology
Purpose of the analysis to determine the number of cycles in a given stress state a piece of material can endure before failure
is predicted to occur
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Analysis Methodology
• Accumulated damage– Stress response raised to m-power
• m is an empirical constant– Ranges from ~3 to 5
• Small changes in stress response lead to large effects on the accumulated damage
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1∙ ∆
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Example: Design for Fatigue Performance
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Design for Fatigue Performance
Example:• Geometry transitions are
stress concentrations• Cause increased stress
response• Sharp features are worst
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Design for Fatigue PerformanceStress vs. Load
Decreasing the slope of the stress vs. load line is the goal
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Design for Fatigue Performance
Stress vs. Load
A small decrease in stress response can have a substantial effect on the fatigue life
Radius = X Radius = 60X
σ = 1.00*L
σ = 0.65*L
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Design for Fatigue Performance
Final Outcome:• Fatigue life increased by factor
of 5 to 8 depending on other assumptions
• No impact on function of the system
• Geometry modification is the only way to improve fatigue life of the wellhead system without potentially impacting other areas of operation
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Conclusions
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Fatigue Analysis Process
Finite Element Analysis (FEA)Input: GeometryOwner: Equipment Manufacturer
Component Stress Response
Analysis MethodologyInput: Industry Code or Internal SpecificationOwner: Operator
Calculation Methodology
Riser AnalysisInput: Rig, Weather DataOwner: Operator or Equipment Manufacturer
Cyclic Load Profile
Fatigue Life
Fatigue Damage CalculationOwner: Equipment Manufacturer / Operator
Accumulated Fatigue Damage
Inpu
tO
utpu
t
Geometry Modification:- Optimization of wellhead
system geometry- No impact to operations
Improves fatigue performance of the wellhead
system in reality
Interpretation of ocean data:- Reduction of conservatism- Tightening of watch circles- Restriction of operational
envelopes
Does not actually improve the fatigue performance of
the wellhead system
Modification of methodology:- Modification of material
characteristics assumptions
Does not actually improve the performance of the
wellhead system