Subsea Wellhead System Design for Fatigue Performance · 11 March 2015 Subsea Drilling Services Subsea Wellhead System Design for Fatigue Performance ... Finite Element Analysis (FEA)

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


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


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

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Finite Element Analysis (FEA)

• “Global” model determines overall load transfer characteristics of the system


Finite Element Analysis (FEA)

• “Sub” models determine local stress response at each critical feature in the system








Cyclic Load Profile

Fatigue Life



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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



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



• 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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Example: Design for Fatigue Performance


Design for Fatigue Performance

Example:• Geometry transitions are

stress concentrations• Cause increased stress

response• Sharp features are worst


Design for Fatigue PerformanceStress vs. Load

Decreasing the slope of the stress vs. load line is the goal



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



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


Conclusions








Cyclic Load Profile

Fatigue Life



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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

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Subsea Wellhead System Design for Fatigue Performance · 11 March 2015 Subsea Drilling Services Subsea Wellhead System Design for Fatigue Performance ... Finite Element Analysis (FEA)