Post on 23-Oct-2020
Risers, Pipelines & Subsea Systems
Reducing Uncertainty & Gaining Confidence by Monitoring
Tze King Lim, Hugh Howells
AOG 2015, Perth
12th March 2015
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Fatigue in Action
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Agenda
Fatigue Sources
Uncertainties and Need for Monitoring
Selecting Correct Instrumentation
Getting Best Value from Measurements
Screening
Filtering
Conversion to Useful Parameters
Correlation with Environment
Benefits
Conclusions
Applicable to all subsea systems subjected to cyclic loads 4 of 28 Learn more at www.2hoffshore.com
Fatigue Sources
Wave action
Wave-induced vessel motions
VIV (Vortex Induced Vibrations) due to steady current flow
Internal flow e.g. slugging
Transportation and installation
Vessel motions
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Fatigue Sensitive Structures
Subsea jumpers: VIV, slugging
Pipeline spans: VIV, slugging
Jacket risers/flowlines and conductors: waves, VIV
Mid-water flow bundles: towing, installation, in-place
waves & VIV Learn more at www.2hoffshore.com
Sources of Uncertainties in Fatigue Predictions
Metocean conditions
Vessel motions
Hydrodynamic properties
Structural damping
Soil strength – range of strengths specified
Internal fluids – density variations, flow regimes
Fatigue details – S-N curves and SCFs
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Calculated Fatigue Damage vs Fatigue Resistance
A < B
FSF
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Approach to Fatigue Design
Conservative parameters considered in analysis
Large safety factors (e.g. 10)
Design code objective to obtain target probability of failure (~10-5)
Conservatism may lead to unfeasible design
Monitoring is performed to address conservatisms and operational concerns
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Structure Fatigue Design
Instrument Specification
Execute Monitoring Campaign
Data Processing
Correlation with
Environment
Monitoring System Implementation
*not in this presentation
No Monitoring Required
Feedback to Future Design
Feasible, not novel
Not feasible
Feedback to present design
Structure Fatigue Design
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Instrument Specification
Understand expected behaviour based on analysis predictions
What parameters to monitor – acceleration, angular rates, strain?
Define minimum motions/strains to be measured (set threshold)
What accuracy is required?
What uncertainties will be introduced from monitoring system: calibration error, resolution, noise
Testing to verify calibration and noise
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Instrument Selection Case Study
Sensor noise level
Is instrument precision sufficient?
Threshold accelerations, more precision required for worse fatigue detail
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Data Processing Steps
Data Management
Screening Data
Correction
Review Frequency Spectra
Filtering Conversion to Useful
Parameters
Data Processing
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Data Management Challenges
Large volumes of data are collected:
1 motion measurement device, 3 accelerometer, 2 angular rate, 1 temperature for 1 year = 2.4 Gb
How and where to store?
Timestamp and file naming conventions
Providing reliable access to data and results
Handover responsibility with change in personnel
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Screening
High level review of data
Checks that instrument is working as expected
Data collected is in line with expectations
Identify events with significant motions to be investigated further
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Screening Case Study
Events to investigate
further
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Data Correction
Gravity correction:
Component of gravity is measured by accelerometers when inclined
Results in over or under-prediction of fatigue depending on deflected shape
Unexpected Responses – remove measurements of installation/retrieval of instruments, drilling vibration, impacts
Clock Drift – needed if data from multiple devices are combined
Temperature Drift – calibration changes with temperature
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Inspect Frequency Spectra
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Angula
r ra
te (
deg/s
)
Filtering
High pass – remove drift
Low pass: remove noise
Noise affects magnitude of measurements and introduce errors
Integration amplifies error at low frequencies
Uncertainty in fatigue life is ^3 or ^4 uncertainty in stress
Noise can be minimised by correct filtering
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Conversion to Useful Parameters
Measured parameters: accelerations, angular rates, curvature
Stress range & number of cycles
Accumulated fatigue damage & remaining fatigue life
Is it safe to continue operations?
Is the component performance up to spec?
Are remedial measures needed?
Can service life be extended?
Fatigue details
Transfer functions
Feed into operations
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Example Conversion to Accumulated Fatigue
Most fatigue accumulated during few events with large waves 21 of 28 Learn more at www.2hoffshore.com
Correlation with Environment
Compare wave and VIV motion measurements with environmental conditions
Compare slugging motion measurements with flow conditions
Allows calibration of analysis models and reduces conservatisms
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Example Calibration of VIV Parameters
Calculated VIV fatigue is conservative compared to calculated VIV
Adjusted input parameters which are less conservative can be justified
Ref: M. Tognarelli, S. Taggart (BP), M. Campbell (2H) – “Actual VIV Fatigue Response of Full Scale Drilling Risers: With and Without Suppression Devices”, OMAE 2008
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Assessing Conservatism
Probability of fatigue failure revised
Bias in measurement mean vs design
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Effects of Reducing Uncertainty
Probability of fatigue failure reduced
Variability reduced
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Feedback into Present and Future Design
Final step is to implement findings from monitoring:
Refined fatigue lives for present system
Optimised design for future systems
Justified reduction in safety factors
Use calibrated analysis models for future systems
Enables cost savings
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Conclusions
Monitoring can address uncertainties
For best value from monitoring, we need:
Good instrument specification
Accurate data processing methods
Correlation with environment
Feedback into present and future design
Benefits:
Justify less demanding safety factors
Reduce over-design
Avoid unnecessary remedial work/assess effectiveness
Reduce costs
Better predictions for future design
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Questions?
Further information:
2H Offshore Engineering
www.2hoffshore.com
+61 8 9222 5000
Learn more at www.2hoffshore.com
http://www.2hoffshore.com/
Learn more at www.2hoffshore.com
http://www.2hoffshore.com/