Advances in Fibre Optic Condition Monitoringof Flexible PipesIntegrity Management of Unbonded Flexible Pipelines and RisersNick Weppenaar, NKT Flexibles2008-11-27

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Condition Monitoring Techniques for Flexible Pipelinesand Risers

• Reasons for riser condition monitoring• Requirements for a monitoring system• The NKTF optical monitoring system (OMS)

• S-OMS: Strain• T-OMS: Temperature• G-OMS: Gas• P-OMS: Pressure

• Data mining• OMS status and future developments

The NKTF OMS, September 2008

Reasons for monitoring flexible pipes

Flexible pipes have several characteristics thatmake it important to monitor them:

• Sophisticated and complex product• Possibly catastrophic consequenses of failure:

• Lethal danger to crew and platform• Pollution• Very bad PR

• Critical component, lost production can be 10 m$ per hour

• Difficult to replace logistically• Long lead times for new pipes

…and they have to last 20 years or more.

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Advantages of riser condition monitoring

• Greater operational safety:• Detect pipe degradation before failure or danger to

crew• Better information during emergencies from external

causes• Natural disasters• Collisions• Freak accidents

• Lifetime extension• Pipe designs based on worst-case load history• Comparison between design load and actual loads can

yield years of extra life• Predictive maintenance

• No unscheduled shutdowns• Better pipe models and designs

• Better theoretical understanding of pipe behaviour inreal operating conditions

The NKTF OMS, September 2008

Why measure internally?

• Pipe models are very conservativewith high safety factors

• Measuring externally and calculatingbackwards requires the sameconservatism

• Some parameters are very sensitiveto changes in the assumptions

Better to measure the criticalparameters directly

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Requirements for a condition monitoring system

• No electrical leads• Pipe vicinity is an explosion-hazard area• One spark can kill crew, damage platform

• No signal drift over time• Sensors are usually embedded into the pipe structure• Maintenance work near the pipes is preferably avoided

• Lifetime as long as the pipe lifetime• No way of changing the sensors

• Chemical resistance to the annulus environment• Heat, humidity, CO2, CH4, H2S, H2

Conclusion: Only fiberoptics will work

The NKTF OMS, September 2008

What do we want to measure and why?

Strain• Metal fatigue usually determines pipe lifetime• Model calibration

Temperature• Overheating of polymer layers• Flow assurance• Detection of outer sheath breaches• Detection of trenching and free spans

Gas (in annulus)• Corrosion fatigue

Pressure• Useful in gas sensing calibration• Detection of pressure build-up due to faulty valves

The NKTF OMS, September 2008

S-OMS – Strain sensing

Built-in fiberoptical sensors in selected steel armour wires• Fiberoptic wires embedded in the inner tensile armouring

layer• Direct measurement in the most fatigue-critical layer• Measurement in the region with greatest fatigue loads• Updated service life calculation• Up to 192 separate sensing points distributed as needed• Well-established monitoring technology in NKTF

Current uses:• Two pipes in service, two more in production• Increasing use in full-scale tests

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S-OMS – Integration in the pipe structure

The NKTF OMS, September 2008

FBG strain sensing

• Fiber has pattern with specific grating length Λ (figure a).• Light with wavelength λ equal to Λ is reflected, all other light is transmitted.• Strain in fiber causes change in grating length Λ which gives a change in the reflected

wavelength λ (figure b).• Wavelength change directly proportional to strain change, and completely hysteresis-free.

The NKTF OMS, September 2008

S-OMS specifications

• Sampling of 16 fibers, each with up to 12 sensors• Up to 192 sensing points over one or more risers

• Sampling rates up to 1 kHz• Measures 0 - 5000 microstrain• Accuracy ~10 microstrain• Outstanding fatigue life

• 10 million cycles at 0-3000 microstrain with no breakage• Continuous peek coating protects fiber during production

The NKTF OMS, September 2008

P-OMS: Fiberoptic pressure sensing

• Diaphragm presses on fiber end with Bragg grating• Compression causes change in reflected wavelength• Early commercial models available• Useful as warning system for pressure build-up• Useful with gas sensing

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T-OMS: Distributed fiberoptic temperature sensing

Sensor fiber placed under the outer sheath• Scattering of light from the fiber itself.• Wavelength peaks can be observed which shift

with temperature.• Allows measurement of temperature along entire

fiber length• Can measure more than 10 km of fibre• Systems commercially available• Has been used in full-scale thermal tests (Moho

Bilondo)• Two instrumented pipes ordered

PC

Controlroom

Sensor fiberTransmissiontube

Connectorbox

BendingstiffenerPipeEndfitting

Forward path outsideendfitting

Interrogationunit

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T-OMS advantages

Advantages:• Outer sheath temperature profiling under bend stiffener

• Warning system against polymer degradation• Monitor water filling of riser annulus• Detect trenching and free spans in flowlines

• Flow assurance during shut-down• Longer time till injection based on precise data rather than

conservative estimates

The NKTF OMS, September 2008

Photoacoustic absorption spectroscopy (PAS)

Accuracy proportional to:• Laser power• Sampling time

Accuracy can therefore easily beincreased.

The NKTF OMS, September 2008

QEPAS

Quartz-Enhanced Photoacoustic AbsorptionSpectroscopy

• The microphone is a resonant quartz tuningfork

• Sound is amplified by resonant tubes• ~100 times better resolution than normal PAS• Immune to noise• Simple and robust unit• All gas types measured with one sensor• Small!

Developed by the Laser Science Group at RiceUniversity (Houston).

Has previously developed early fire warningsensors using QEPAS for the InternationalSpace Station

The NKTF OMS, September 2008

G-OMS: QEPAS advantages

• Compact sensor, size of a matchbox• Useful for corrosion fatigue calculations• External mounting possible → retrofitting• Real-time sensing of all gas concentrations simultaneously:

• CO2 , H2S, CH4, H2O• Accuracy (preliminary results, no optimisation):

• CO2: σ = 28 ppm (9 s sampling time, dependent on humidity)• H2S: σ = 2.2 ppm (36 s sampling time)• CH4: σ = 350 ppm (9 s sampling time, dependent on humidity)• H2O: σ = 90 ppb (3 s sampling time, dependent on H2O levels)

• Patent pending

The NKTF OMS, September 2008

G-OMS, internal mounting

Bore

Annulus

End fitting

Outer sheath

ValveEmitter fiber Receiver fiberGas sensor

Innersheath

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G-OMS schedule

Q3 2008: Construction of lab model

Q4 2008 – Q1 2009: Lab test series• Accuracies• Cross-sensitivities• Temperature dependence• Pressure dependence• Humidity dependence

Q2-Q4 2009: Development and test of EX-proof prototype on platform

2010: Commercial G-OMS unit

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Data mining

• One data source is good• Several data sources are better• Several data sources working

together are best

Several correlated data sources give farmore knowledge than the individualdata.

Analogies:• Doctor with patient• Oil field exploration

Example:• Outer sheath breach in water

Temperature(localised)

H2O

Pressure

Time

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Status and outlook

Current status:• Strain end temperature monitoring

becoming an 'off-the-shelf' system• Two strain OMS pipes in the water,• Two pipes with both strain and

temperature OMS being produced.

Future developments:• Gas sensing tests beginning in 2008• Gas sensing to become commercially

available in 2010

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