BP-CUI.pps

04/08/23 1

CORROSION UNDER INSULATION (CUI)

• CUI poses a major threat to plant operability– Particularly older plant

• On most sites CUI tends to be a medium to long-term problem– Risk increasing significantly after 5 to 10 year “holiday

period”– General pattern conspires to give false sense of security– Routine inspection and maintenance of thermal insulation

systems tends to be avoided or deferred until a process leak actually occurs

– Resulting failures can have serious H, S & E implications

• Thermal insulation refurbishment projects carried out at two major BP sites in past 15 years have cost ~ $450MM

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CUI – Not a pretty sight!

04/08/23 3

CUI – Will find you out!

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Greater Prudhoe Bay

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GPB – CUI Process

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GPB - History

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GPB – Incident Rate

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GPB – Corrosion Rates

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GPB - Outcome

• Failure Rate and ForecastActual failure rate increasingForecast trend to continue

• Current Program Scope/ScaleCost $ 2 million per annumScope/scale 10,000 #/yearFull field cycle 25-30 years

• Recommended Program Scope/ScaleCost $ 10-12 million per annumScope/scale 50-60,000 #/yearFull field cycle 5 years

• Move to Mainstream O&M ProgramFull time program management from part-time

• Actively Pursue Technology Advances

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CUI – The cause• Normally associated with ingress of water from external environment

into/beneath insulation• Some insulating material can contain measurable amounts of aggressive

ions (eg Cl-), but …– Water composition is more likely to reflect environmental location– Highly polluted industrial environments can result in water pH of 4 to 5– Coastal locations will usually result in significant Cl- content– If steel substrate above ambient then can get concentration of aggressive ions

as water vapour is driven off, eg: • Tests at Sullom Voe found a 20x higher Cl- level in water removed from insulation than

local rain water

• CUI problems more common where local climate comprises:– Frequent rain fall– High winds– Coast location

• Local microclimates can also represent a high risk, eg:– Cooling towers– Emissions of acidic vapours– Frequent testing of water deluge systems– Enthusiastic water jetting activities

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CUI –The effect

• Carbon and Low Alloy Steels– Manifest as localised wastage at areas in contact with water

either held within the insulation or between steel and insulation

– All insulated equipment operating between –5oC and 200oC at risk

• Highest corrosion rates found between 60oC and 120oC• 1.5 mm/y typical corrosion rate; but 3 mm/y has been reported

– For operating temperatures below ambient the differential vapour pressure across the insulation draws water towards steel substrate

• Corrosion rate at sub zero temperatures low, but …• Severe corrosion can occur at locations where transition to

ambient temperature or at protrusions through insulation

– All thermally insulated surfaces operating between –5oC and 200oC and service life > 10 years should receive adequate corrosion protection before application of thermal insulation

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CUI – The effect

• Austenitic and Duplex Stainless Steels– 300 series stainless steels used widely but …

• Prone to high rates of localised attack and stress corrosion cracking under certain conditions

• Critical temperature, Cl- level, pH

– Pitting threshold temperature, eg:• 304L ~25oC; 316L ~36oC; 2204 duplex ~90oC

– Stress corrosion cracking threshold temperature, eg:• 50oC – 60oC for 304L/316L• >100oC for 2507 duplex

– It is prudent to require corrosion protection on process equipment operating above the following threshold temperatures

• Austenitic and low Mo-duplex stainless steels >25oC• Duplex, super duplex and super austenitic stainless steels

>80oC

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CUI – The effect

Rainwater Fog Concentrated Water

Critical Pitting Temperature

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CUI – Design to minimise risk

• Complex geometric shapes, usually associated with ancillary attachments, supports, instrumentation etc, are where water ingress/accumulation most likely– Little can be done to alter design of “off the shelf” items,

valves and instruments– Attention to design of vessel/pipe supports, nozzles,

stiffeners, insulation supports, piping layouts can greatly assist insulation contractor in reducing opportunity for water ingress

An overhead pipe support detail which is impossible to seal from

water ingress

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CUI – Insulation Specification

• Should be sufficiently prescriptive to ensure that only quality materials from an established supplier are used– Clearly state client’s expectations wrt materials and design

details– For new build, the spec. should be made available to process

equipment designer in time to influence design/layouts– Contractor should provide certificate of conformance for

materials used

• The extent of insulation should be challenged– Only use that required for process reasons

• Where not required but already installed, remove

– Use metal mesh guards to provide personnel protection

• CUI has been reported under the entire range of commonly used thermal insulating materials

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CUI – Insulating Material

• Three key properties affect CUI:– Moisture absorbency or ability to transport aqueous fluids– Ability of material to contribute to the chemical composition

of the moisture– Ability to withstand mechanical abuse

• In practice water penetration to steel substrate occurs through gaps between– Adjacent sections of installed insulation– Adjacent pieces of broken insulation

• Where rate of water ingress is slow, absorbent materials may draw a significant amount of water away from steel surface, but …!

• Prevailing weather conditions and general physical conditions of insulation are most significant factors

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CUI – Insulating Material

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CUI – Weather Barriers

• Metallic sheet provides optimum resistance to mechanical damage, but …– Poor track record of preventing water ingress– Need to select sheet material according to design life of

plant and prevailing weather conditions

• Non-metallic weather barriers have greater potential for limiting water ingress as more flexible and accommodating of complex geometries etc. but …– If water ingress occurs there is less likelihood of draining

away of evaporating off– Success of application highly dependent on skills and

experience of work force– Insulation contractors are reluctant to use them

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CUI – Metal Sheeting

• Galvanised steel– Life expectancy related to Zn coating thickness and environment

• Typically 80 microns

– Unlikely to give life > 10 years in polluted and/or coastal atmosphere

• Aluminised steel– Typically contains ~10% Si which is detrimental to corrosion performance

• Rust breakthrough can occur within 1 year of exposure to aggressive environment – unsightly but does not cause rapid perforation

– Life > 10 years unlikely to be realised unless use 99% Al

• 55% Al - 43.4% Zn alloy coated steel– Combines sacrificial properties of Zn with passive properties of Al– Numerous reports of giving life > 20 years

• Aluminium– Extremely good resistance to environmental corrosion

• thicker gauge to galvanised or aluminised steel for improved mechanical properties

– Poor performance in fires – should not be used on flammable processing units

• Austenitic stainless steels– Significant cost penalty, therefore only considered for harshest environments– Type 316 stainless steel gives superior pitting/crevice corrosion resistance

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CUI – Non-metallic Barriers

• Reinforced membranes– Wet applied weather/fire resistant mastic – usually water based – with glass

cloth reinforcement• Certain products can be obtained pre-applied

– Flexibility and ease of repair mean they are good for complex geometries etc– Success dependent on skill of applicator; but exhibit poor impact resistance

and are susceptible to environmental degradation

• Elastomeric sheet– Typically 1mm sheet cut to size and sealed with both adhesive and sealant– Not such a big departure from metal sheeting – more acceptable to contractors– Optimum performance dependent on skill of applicator; sensitive to

environmental degradation under extreme weather conditions

• Glass fibre reinforced epoxy– Generally confined to specialist insulation systems eg. cryogenic service– Potential to provided greater resistance to mechanical damage and lends itself

best to low maintenance systems– Examples of still being in good condition after 20 years in service

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Elastomeric Sheet

Water being expelled

It is comparatively easy to achieve a completely water tight system initially. However, if the barrier is breached and water gains access it

is most like to collect and stay within the insulation – no loss due evaporation or natural drainage.

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Recent development of mouldable fibre glass reinforce plastic sheet, cured by UV light, which forms a firm bond to itself and range of substrates and accommodates complex geometries, is showing

great potential

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CUI – Protective Coatings

• Can be extremely effective method of preventing or delaying the onset of CUI– Same levels of quality control during surface prep. and painting to

that for non-insulated surfaces must be applied– Thermally sprayed Al has potential to provide protection for full

service life (> 20 years)• Consider as viable option for new build!

– Aluminium and stainless steel overlapping spiral wrap foils are alternatives to liquid applied coating systems for protecting stainless steels from pitting and/or SCC

• New build provides best opportunity to minimise risk of CUI– External condition of process equipment and access most

favourable– For many cases it is the only time steel substrate is accessible

when at ambient temperature– Extremes of temperature during coating application reduce

dramatically coating performance

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• As for all coating systems good surface preparation is the key factor to achieving optimum coating performance/service life

• For carbon steel surfaces:– Hand or power tool cleaning is not a good base – widely

demonstrated that this will typically give 18 – 24 mths service life for coating system

– Dry abrasive blasting provides the optimum surface condition – 8 to 10 years protective performance from coating

• In maintenance situation dry blasting often considered unsafe or inappropriate

– Slurry blasting, which uses small amounts of entrained water in abrasive stream, is capable of producing similar degree of surface cleanliness to dry blasting

• Minor rust staining considered to have minimal effect when surface tolerant primers are applied as the first coat

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• For stainless steels:– Should always be given some form of mechanical preparation to

ensure good coating adhesion

– Abrasive blasting (dry or slurry) or power sanding can be used

– Wire brushing using stainless steel bristle brushed only should be used on weldments

• Maintenance painting:– The need to maximise plant availability means much of the

insulation maintenance and CUI mitigation activities are carried out while the plant is operating

– Applying protective coating to live plant is fraught with difficulties due to the influence of substrate temperature, eg:

Substrate Temperature (oC) Lifespan (years)

<60

60 to 100

100 to 150

12

6

3

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CUI – Maintenance PracticesPoorly maintained insulation

showing many years of neglect

Mechanically damaged cladding – note the absence of mastic sealant at joints

Open cladding terminations allowing free water ingress

04/08/23 28

CUI – Maintenance Practices

• Many of the worst incidences of CUI are “self inflicted”– Due to general lack of awareness of consequences of day to

day routine plant maintenance and operational activities by many site personnel

• Responsibilities for the integrity of the thermal insulation system need to be clearly assigned– A full risk assessment should drive the CUI inspection and

maintenance schedule– Routine inspections of insulated plant and equipment

should be carried out to identify displaced, missing or damaged sections of the weather barrier, and potential points of water entry

– A strict quality control regime should be enforced during all insulation maintenance activities

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CUI – Inspection

Where and how to start?

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CUI – Inspection Techniques

• No single inspection technique available today is capable of determining economically the extent of CUI– The optimum approach is to use a combination of

techniques linked to a risk assessment and visual surveys of the condition of insulation

• Isolated “strip and search” visual inspection alone is uneconomic and carries a low probability of finding all CUI affected areas– However, practical experience would dictate that for high

risk areas complete removal of insulation plus visual inspection is the most reliable method of determining the extent of the problem

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• Non-intrusive and semi non-intrusive inspection methods fall into three man categories:– Indirect …. provide evidence that corrosion could be taking

place• Thermography; Neutron Backscatter

– Screening ….. provide evidence that corrosion is taking place, but unable to measure unequivocally

• Long Range Ultrasonics; Local and Long Range Electromagnetics

– Direct …. permit quantitative measure of the extent of CUI• Conventional or Tangential Radiography; Removal and Visual /

Physical / Conventional Ultrasonic Inspection

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• Thermography– Infra red thermovison camera capable of detecting ~ 0.2oC

temperature variation• Looking for loss in thermal efficiency of insulation• Conduct survey after heavy rainfall – worst conditions!• Run first survey after commissioning

– Advantages• Quick to set up and use• No hazards to equipment operator• Insulated plant and equipment inspected remotely

– Disadvantages• Only detects deficiencies in insulation• For optimum results need ~30oC gradient across insulation• Need to exercise care where insulation subject to wet/dry cycles• Vicinity of very hot bare objects or reflective metal cladding can

obscure defects• Interpretation of results requires a skilled and experienced operator

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Heat loss from a section of insulated pipe at the bottom of a vertical run

due to wet insulation

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• Neutron Backscatter– Been field trialed at a number of sites around the world

• Water in insulation increases density of hydrogen atoms which cause scatter of fast neutrons to lower energies – detected on slow neutron monitor

• The amount of water in insulation is related to rate of counting of slow neutrons

– Use involves• Positioning a small probe close insulation for ~20 secs while the

backscattered neutrons are counted

– Trials and practical experience has demonstrated• Can unambiguously detect the presence of water• Distinction between wet and dry insulation is greater for some

insulating materials than others• Proximity of concrete foundations can affect interpretation of results• Results can be affected by liquid content of equipment if insulation

thickness < 5cm• Cannot be used when raining

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• Tangential Radiography– Has emerged as most commonly applied inspection method

for reliable detection of corrosion under insulation on pipework

– Uses low level radiation sources (X-rays or gamma rays)• Work area does not need to be roped off

• The radiation only passes through insulation and weather barrier

– Capable of detecting uniform and pitting corrosion

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The current preference amongst plant operators is for Real-Time Radiography (RTR).

This utilises a visual display unit to show a “real time” view of the silhouette of the outside

surface of the pipe.

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PORTABLE PROFILE RADIOGRAPHY

Uses low energy radiation source Radiation signals converted into “linear equivalent” wall thickness Cannot distinguish between internal and external wall loss Handle wall thickness of between 1” and 1.5” “C-arm” lengths currently available are 8”, 13” and 18”

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• Long Range Ultrasonics– Use of ultrasonics for the detection of CUI has been focussed on

the possibility of introducing low frequency ultrasonic waves into an exposed section of the insulated plant remote from the sites of interest

• Guided Wave Technology– Pulsed (~70kHz) guided waves emitted from single ring of

transducers encircling pipe– Returning echoes are detected and analysed– Response from metal loss feature is function of its depth and

circumferential extent– Orientation of defect determined by sensor array– Claimed limit of detection 9% but 2% - 3% has been achieved in

practice– Under favourable circumstances detection of defects 150m+ from

sensor array is considered possible– Temperatures up to 100oC are claimed to have no adverse effect on

performance of equipment

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CUI – Inspection TechniquesTELETEST GUIDED WAVE Typically need to remove ~1 m of insulation

Can be used on both horizontal and vertical pipeGives approximation of % wall loss at remote location, but

can reproducibly locate same defectRange of detection determined by signal attenuation:

• High viscosity products in pipe, including water, limit interrogation lengths to 1 – 18 m

• Light oils, gasoline or diesel, enable interrogation lengths of 20 – 100 m

• The insulation if tightly fitted can affect attenuation• Runs of pipe with numerous welded bends limit ease of

interrogation of signal

CHIME - Creeping Head-Wave Method

Similar in principle to Teletest – uses surface-skimming waves which become out of phase if a defect is detected

Claims to provide more detailed information but inspection distance ~ metres

Under favourable circumstances, including good weather, it may be possible to inspect 1000 m per day

In current form best viewed as screening tool for focusing more detailed (visual) examination

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• Electromagnetic Methods– Pulsed Eddy Current

• Measures decaying eddy currents induced into carbon steel after switching off applied electromagnetic field

• Rate of decaying eddy currents enables wall thickness to be determined

• Measurement of wall thickness for pipe ID 75 mm and operating temperature range –1000C to 550oC claimed

– Long Range Electromagnetics• Analogous to Long Range Ultrasonic but using electromagnetic

pulses over broad frequency spectrum• Dual pulse method introduces two synchronised pulses to

extremities of pipe section with imposed relative time delay in order to pinpoint the intersection of pulses within pipe wall via a receiver located adjacent to either pulse source

• Single pulse can be used with receiver repeatedly located at several point along interval to be inspected

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Can measure pipe wall thickness through insulation of thickness ~ 200 mm max.Only applicable to carbon & low alloy steelsTypical smallest defect detected is 50% of stand-off distanceThickness readings require calibration against measured defects If steel is not electro-magnetically homogeneous the tool will give spurious resultsA minimum clearance of 2” between measurement point and any adjacent detail eg.

welds, supportsElectrical distortions and mechanical vibrations affect the tool’s performancePerformance is significantly reduced by galvanised or aluminised weather-proofing Individual readings take typically 2 to 5 secs but the method can not be regarded as

scanningMeasurements can be made at remote locations from stairways and ladders by

mounting probe on a pole

PULSED EDDY CURRENT

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LONG RANGE ELECTROMAGNETICS

Ranking

A No damage

B ~15% wall loss

C ~10 to 30% wall loss

D > ~25% wall loss

Tendency to provide a high frequency of false indications, but rarely fails to locate external corrosion

Unable to quantify degree of severity of damage, with typically only C and D corresponding to actual CUI

Should be limited to interrogating straight pipe – max. length ~150mMin. pipe OD is 4.5”, with max. ~60” Insulation removal is minimal compared to Long Range Ultrasonics resulting

typically ~50% cost savingsTypical tolerance for positioning of anomalies is 1.5mUnable to determine the specific orientation on pipe of an anomaly

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CUI – Risk Based Inspection

• The shear scale of insulated plant and equipment on many sites renders full inspection impractical

• Need to develop a risk based inspection scheme for cost effective management of CUI– Risk = Probability of Failure x Consequence of Failure

• Probability of failure– Normally apply a scoring type assessment based on list of

key factors

• Consequence of failure– The impact upon safety of personnel– Effect on the environment of a product leak– Cost of lost production and/or repairs to damaged

equipment

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CUI – RBI

PROBABILITY

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CUI – RBI

RISK RATING

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CUI - RBI

Hydrocarbon Service

Are Lines Insulated

Insulation required for process

Exposed location or subject to deluge testing

Insulation in poor location

Gas Service

High pressure design orow wall thickness (Schedule 40)

CUI Risk Pyramid

(Remove insulation where not required)

Recent bare pipeinspection

No

1

3

5

6

7

8

Order for lines to be stripped

No

No

No

No

No

No

Yes

Out

No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Insulation Removal

2

4

04/08/23 47

Corrosion Under Insulation

• CUI is a latent problem with a long half life, where its impact can all too easily be underestimated and addressed too late

– It was born with the industry and continues to take its toll given an opportunity

– Many of the problems even today are arguably “self inflicted”

• Avoidance requires proactive management throughout the life of a plant, equipment, etc starting with informed design and installation practice

– eg. Application of a sound coating system to the insulated steel substrate, where possible, can really pay dividends

• Cost effective management throughout operational life requires a risk based assessment approach and clear line of responsibility

– Rigorously applied and regularly reviewed

• There is a growing range of inspection techniques, of increasing sophistication, available to support an RBI approach, but ….

– No single technique practically and/or economically holds all the answers

• Never rest on your laurels

04/08/23 48

CUI – Never sleeps!

• NGL Roof, Forties Alpha

• The 4” recycle line on the K01 gas

compressor recycle line ruptured releasing

the contents of the compression system to

atmosphere.

• The released gas was detected by an

infrared beam detector located on the NGL

roof, which initiated automatic production

shutdown and depressurisation.

• The gas did not ignite and was quickly

dispersed from the roof.

• The platform went to muster and was stood

down 2 hrs later once the failed line had

been made safe.

Documents

BP-CUI.pps