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  • Risers, Pipelines & Subsea Systems

    Reducing Uncertainty & Gaining Confidence by Monitoring

    Tze King Lim, Hugh Howells

    AOG 2015, Perth

    12th March 2015

    Learn more at www.2hoffshore.com

  • Fatigue in Action

    3 of 28 Learn more at www.2hoffshore.com

  • 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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  • 6 of 28

    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

    9 of 28 Learn more at www.2hoffshore.com

  • 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

    25 of 28 Reliability analysis can be used to justify less conservative design Learn more at www.2hoffshore.com

  • 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/