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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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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


Calculated Fatigue Damage vs Fatigue Resistance

A < B

FSF


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


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


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


Instrument Selection Case Study

Sensor noise level

Is instrument precision sufficient?

Threshold accelerations, more precision required for worse fatigue detail


Data Processing Steps

Data Management

Screening Data

Correction

Review Frequency Spectra

Filtering Conversion to Useful

Parameters

Data Processing


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


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


Screening Case Study

Events to investigate

further


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


Inspect Frequency Spectra


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


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


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


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


Assessing Conservatism

Probability of fatigue failure revised

Bias in measurement mean vs design


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


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


Questions?

Further information:

2H Offshore Engineering

www.2hoffshore.com

+61 8 9222 5000


http://www.2hoffshore.com/


http://www.2hoffshore.com/