Fatigue Monitoring of Deepwater Drilling Risers
Mateusz Podskarbi, Ricky Thethi, Hugh Howells; 2HOffshore Inc
IADC Conference, RIO 2005
How Far the Eye Reaches?
Slide No. 2
Drilling Riser Fatigue ? Fatigue loading from: – – – – current flow induced vortex induced vibrations (VIV); wave action; vessel motions; operational sources (i.e. pressure changes).
? VIV responsible for the majority of the fatigue loading.
Slide No. 3
Drilling Riser Fatigue Life Distribution
VIV LONG TERM FATIGUE LIFE ALONG RISER LENGTH
1.00E+10 1.00E+09 1.00E+08 1.00E+07 1.00E+06 1.00E+05 1.00E+04 1.00E+03 1.00E+02 1.00E+01 1.00E+00 1.00E-01 1.00E-02 1.00E-03 -200
Length along Riser From Mudline (ft)
13.7 ppg, 2.0 Mlbs TT, 1.0% damping Min Allowable Life (40 years Conductor, 60 years Riser)
Slide No. 4
13.7 ppg, 1.75 Mlbs TT, 0.3% damping 8.56 ppg, 1.5 Mlbs TT, 1.0% damping
Background of VIV Fatigue ? Uncertainties in VIV predictions. ? Impact of the predictions on the operations. ? Non-scalability of the tank test results. ? Risers are run in deepwater and high currents all the time. ? Opportunity to obtain full scale measurements. ? Riser dynamic response monitoring can be used to assure structural integrity and improve operational envelopes.
Slide No. 5
Monitoring Objectives ? Ensure the fatigue structural integrity of the riser. ? Reduce risk – expensive subsea architecture below vessel. ? Improve operational guidelines: – Guide operational decisions in response to high current events. – Calibrate analytical VIV predictions and improve riser VIV design practice and operating windows. ? Ensure appropriate maintenance and inspection strategies.
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? Stand-alone vs… ? … Real-time
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Monitoring Approach – Pros and Cons
Pros Simple project interface No umbilical or subsea connections Low CAPEX Readily available Cons Synchronization No real time data May require ROV retrieval – higher OPEX Battery and memory limitations Low power instrument options
? vs. Real-time
Pros Real time data access and decision Best data quality No need for retrieval Can accommodate high power instruments Cons High cost Complicated interface Longer lead time Need for umbilical and subsea connections
Slide No. 8
? ...Strain ? Motions vs...
Fiber-Optic Bragg Gratings
Foil Strain Gauges
? ? ?
3D Acceleration 2D Angular Rates 2D Inclination
Induction (LVDT, DVRT) Sensors
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Monitoring Instrumentation – Pros and Cons
Pros Simple structure interface Low cost Measure global response Proven track record in deepwater Cons Data processing to calculate stresses
Pros Direct reading of strain Little processing to get fatigue Cons High cost Complicated interface with pipe No response measurements in uninstrumented regions Little track record in deepwater Low subsea reliability of certain instruments
Slide No. 10
Case Study 1
? Objectives: – Monitor fatigue at Lower Stress Joint of the CWOR. – Real time to assist riser management.
? Solution: – Fiber-optic strain monitoring system connected to riser controls.
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Case Study 2
? Objectives: – Assess conductor fatigue accumulation.
? Solution: – Stand-alone motion logger to monitor stack vibrations.
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Case Study 2 cont
Lower Stack Excitation LMRP Vibrations
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Case Study 3
? Objectives: – Global response of the riser to assess the fatigue damage accumulation and calibrate VIV analysis.
? Solution: – Set of stand-alone motion sensors.
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Case Study 4
? Objectives: – Measure VIV real-time to optimize vessel drift to mitigate VIV during riser running. – Use measured data to set inspection intervals and avoid unnecessary costly inspections. – Obtain global VIV response of riser to determine fatigue response over entire riser length.
? Solution: – Combination of real time strain and motion sensors connected to a dedicated umbilical.
Slide No. 15
Case Study 4 cont
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Case Study 5
? Objectives: – Assess the amount and nature of axial movement between adjacent auxiliary lines.
? Solution: – Stand alone LVDT float sensor.
Slide No. 17
? VIV fatigue damage is a major issue – potentially very dangerous. ? No such thing as standard monitoring system. ? Matching riser response with monitoring system capabilities. ? KEY DELIVERABLES: – Ensuring structural integrity and reduce risk of operations. – Assisting maintenance and inspection scheduling. – Improving operational guidelines. – Improving design techniques.
Slide No. 18