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Corrosion & Prevention 2014 Paper 54 -

NANOUT: ACCURATE MEASURING OF WALL

THICKNESS CHANGES

D. Kelly

1

, S. Taylor

2

, S. Yellapantula

3

1ConocoPhillips, Darwin, Australia,

2ConocoPhillips, Bartlesville, USA,

3Green

Country Petrophysics3, Bartlesville, USA

SUMMARY: Ultrasonic thickness (UT) testing has historically been accurate to within 1mm, and in

recent times the general accepted accuracy of manual UT testing is 0.1 - 0.5mm. Although this is

sufficient for most applications; recent technological advancements in time-of-flight determination and

signal processing have allowed resolutions of UT testing to approach 30 nanometers. This resolution

permits the rapid determination of wall loss rates.

This is a patented technology called NanoUT. The maximum precision of 30 nanometres was achievedwith a 10MHz dual element transducer in the lab, with short cable lengths – 1.8m (6ft). There is a

decrease in resolution with lower frequency, and increased cable lengths. In a field trial recently

conducted in Darwin, Australia, on a non-corroding channel, over 13 months of monitoring, the relative

standard deviation for the wall thickness was 0.03% using 2.25 MHz single element transducers and 91m

(300 ft) long cable.

The NanoUT technology was applied to the Carbon Dioxide Absorber at the Darwin Liquefied Natural

Gas Plant. Evidence of an internal corrosion, erosion cycle was discovered in the 2012 plant shutdown. In

order to justify the continued operation of the vessel a thickness monitoring program was put in place.

This program included manual spot UT testing, UT thickness scanning and applying NanoUT.

Sixteen probes were installed on the vessel using rare earth magnets and measured the wall thickness over

the course of 13 months. The data was temperature compensated - due to the change in speed of soundwith varying temperatures. The change in wall thickness was measured by a patented algorithm that

identifies inflection points within back wall echoes to precisely determine the ultrasonics pulse transit

time through the wall.

NanoUT has a great potential to assist in process and control applications by rapidly determining wall loss

rates for pipes and vessels. Beyond this, NanoUT has the potential to measure steel wall thicknesses

changes with great precision.

Keywords: Steel, Ultrasonic Thickness, Precise, Accurate.

1. INTRODUCTION

It is a regular occurrence in the first few years of a petrochemical plant’s life to discover various corrosion modes which were

not predicted or not effectively controlled by the design or operations team [1]. Once this mode is discovered it becomes the

challenge of the owner/user to maintain the item in order to ensure it is safe to operate and economically viable. Corrosion

monitoring applied to vessels with internal corrosion has been in place around the globe for many years [2], [3]. However,

recent advances in the use of highly precise ultrasonics has allowed for very short term corrosion rates to be analysed in order

to predict the parameters supporting the corrosion mechanism, and to provide improved accuracy in predicting vessel

retirement date.’




2. THE GAS LIFECYCLE

Darwin Liquefied Natural Gas (LNG) is a processing facility where natural gas is processed into LNG for shipment from

Darwin to Japan. The gas is sourced from the Bayu-Undan reservoir where along with condensate, water and liquefied

petroleum gases (LPGs) it is produced through 12 wells. On the offshore facilities, the natural gas, LPGs condensate and water

are separated and the gas is then re-injected either back into the reservoir or into the 502km long subsea pipeline which

connects Darwin LNG to the offshore facilities.

On arrival at Darwin LNG the pressure of the gas is dropped, and the temperature increased to provide the ideal conditions toenter the cleaning process. The gas is stripped of its trace contaminants of carbon dioxide, hydrogen sulphide, water and

mercury and then it enters the liquefaction process. Optimized Cascade

Technology is utilised at Darwin LNG, this is a patented technology for the

production of LNG.

Once the gas is liquefied, it is stored in above ground, cryogenic, storage

tank. Each week, an LNG tanker arrives at Darwin LNG and is loaded with

approximately 140 000m3 of LNG. Once loaded, it sails to Tokyo where

the gas is regasified and used for power generation and heating

requirements.

3. THE CARBON DIOXIDE ABSORBER

As a part of the cleaning process, the gas travels through a large vessel

known as the carbon dioxide absorber. This is a 30m tall packed column

with two beds. See Figure 1: The Carbon Dioxide Absorber.

The feed gas travels in through the side wall approximately one quarter of

the way up the vessel. This gas exits through the top of the vessel.

Travelling in the opposite direction is a liquid, amine solution which is

sprayed into the top of the vessel, and then exits through the bottom. The

purpose of the packing and spray function is to encourage contact between

the gas and the amine solution. As the two mix, the carbon dioxide and

hydrogen sulphide adsorb onto the amine, and are carried away in solution.

The methane does not react with the amine, and continues on, uninterrupted

through the vessel.

The absorber is 30m tall, 4m in diameter, nominally 100mm thick, and its

material of construction is American Standard for Testing and Materials

516 grade 70 Normalised steel [4], [5].

Figure 2: The Carbon Dioxide Absorber (with scaffolding) Figure 1: The Carbon Dioxide Absorber




4. FULL PLANT SHUTDOWN – 2012

In May 2012 Darwin LNG was scheduled for a full plant shutdown for maintenance and inspection purposes. One of the

scheduled items was the carbon dioxide absorber. This vessel had previously been internally inspected, but only via video

boroscope. This was to be the first confined space entry internal inspection since the beginning of the vessel’s operational life.

The internal inspection found several locations of corrosion in the bottom half of the vessel. The corrosion type was determined

to be a combination of two types of corrosion:

1. Amine corrosion – which is a group term referring to corrosion caused by dissolved acid gases (CO2 and H2S), amine

degradation products, Heat Stable Amine Salts (HSAS) and other contaminants. [6] And:

2. Erosion Corrosion – which is a combination of erosion a physical damage mechanism – usually due to higher than

anticipated velocities in a fluid and some form of corrosion a chemical damage mechanism.

In some cases this corrosion was found to have consumed up to 7mm of the vessel wall. The vessel had been in service 7 years

– and this high corrosion rate was not anticipated to occur. See Figure 3: Carbon Dioxide Absorber Internal Corrosion Map.

Figure 3: Carbon Dioxide Absorber Internal Corrosion Map




4.1 Fitness for Service Assessment

Once the corrosion was discovered, the Engineering team conducted an extensive level three fitness for service assessment to

determine the maximum overall corrosion allowance permitted for the vessel [7], [8], [9]. The assessment assumed that the

vessel was in its fully corroded state, and, in addition to this, that a 2m high band around the nozzle of concern was reduced to

88mm thickness (from 93.5mm) [10]. The assessment was conducted using the allowable stresses specified in ASME Section

VIII Division 2, and the result was that the vessel was approved for continuous service until the next scheduled shutdown.

4.2 Repair or Replace

During the plant turnaround discussions began about how to rectify the problem. Three possibilities were put forward: 1.

Internal Weld Overlay – this would be probably the least expensive option from a capital point of view. 2. Bottom Half

Replacement – this would address the issue, and could theoretically be completed in a shorter time frame than the weld overlay.

3. Full Replacement – this would allow other defects in the vessel’s design to be addressed, and would place the least pressure

on the facility for a timely execution.

After several months of planning and comparison, the third option was chosen as the preferred. It is important to note that

pressure vessels of this size were considered an extra-long lead time item, and so any rectification required careful planning and

execution in order to satisfactorily control costs.

4.3 Monitoring Program

Based on the inspection report provided to ConocoPhillips by Bureau Veritas (the third party independent inspector), to justify

continued use of the vessel, a monitoring program was required. In the petrochemical industry, and in the case of a vessel of

this size and nature, this usually consists of a map being drawn on the outside of the vessel, and individual UT readings taken

from different areas. After a baseline reading is taken, future readings can be compared to this to determine indicative corrosion

rates. From this the retirement date of the vessel can be predicted. In this particular case this style of monitoring would be

incredibly labour intensive. The vessel stands 30m tall with a diameter of 4m this gives a surface area of approximately 377m2!

Through a risk based decision making process it was decided that the corrosion rate on this vessel would be monitored via a

combination of existing and new technologies. Firstly a baseline thickness scan would be completed, as well as follow up scans

when required, and in addition to this a new technology: NanoUT would be deployed on the vessel.

4.3.1 Initial Manual Ultrasonic Thickness Scan

Instead of scanning the entire vessel, only the areas of interest (i.e. those with internal corrosion identified) were scanned

dropping the scanned surface area to approximately 38m2. Nonetheless, it still took 3 Non-Destructive Testing (NDT)

technicians 4 weeks to complete this work. This work was completed via a purpose built cyclone coded scaffold – refer to

Figure 2: The Carbon Dioxide Absorber (with scaffolding).

As a result of the initial scan a graphical representation of the thickness of the vessel was created:




Figure 4: Thickness map around the gas inlet area - blue = nominal, yellow = thin, green = thinner, red = thinnest, white = no

data taken.

This was the baseline reading. See Figure 4 for an extract from the results of this baseline.

4.4 NanoUT Technology Chosen

A research division of ConocoPhillips: Production Technologies had recently developed a new thickness monitoring

technology called NanoUT. In principle this technology is very similar to typical manual UT, in that a probe is placed on the

surface of the item to be measured, a sound wave is sent through the item, and then is received back by the probe, and through a

calculation of the speed of sound in the material in question a thickness is returned. The major differences between NanoUT

and manual UT are these:

Accuracy: NanoUT is theoretically accurate to a nanometre – manual UT is only accurate to 0.1 of a millimetre – at best.

Temperature Compensation: The speed of sound in metals is a function of temperature, as the temperature increases, the speed

decreases and needs to be accounted for in order to preserve the accuracy of the reading.

Mobility: NanoUT is currently only a fixed technology. Manual UT is quite mobile and allows the user to take a reading in any

location with access to the surface being measured.

NanoUT was the technology chosen to monitor this vessel.




5. NANOUT INSTALLATION DETAILS

The system chosen consisted of 16 UT probes of 25 mm diameter and operating at a frequency of 2.25 MHz. These were

positioned on the outside of the vessel in the locations of concern (lowest wall thickness according to the manual UT scan).

These probes were attached to the outside of the vessel by way of aluminum housing with thumb screw. The aluminium housing

held two rare earth magnets which attached to the outside of the carbon steel vessel with a force equivalent to approximately

100N. This fixation method struck a good balance between straightforward adjustment of the location of the probe and

permanency. The wall temperature sensor was attached directly adjacent to the probe to give the most accurate correlation forthat UT reading. The temperature sensing cable and the UT cable then made their way back to a central data gathering station at

the base of the vessel. See Figure 5

Figure 5: Aluminium housing for UT probe and temperature sensor

At each channel, the vessel’s surface temperature was measured by a four wire Resistive Temperature Device (RTD). The

RTDs employ a platinum resistor and measure directly the voltage across the resistor and were rated from -50 C to 200 C. The

temperature is measured by noting the change in resistance of the platinum windings as the wall temperature changes. The RTD

sensor was placed on the vessel surface using small rare earth button magnets as shown in Figure 5. The RTD heads were then

attached to 96 meter long cables that make their way down the scaffolding to the data gathering station.

Paired with the RTD cables using cable ties were 96 meter long transducer cables. The vessel’s surface temperature is warm ~50 to 80 C and direct contact by the cable on the vessel wall needed to be avoided. Placing the RTD/transducer cable pair in a

temperature resistant split conduit and securing the conduit to the scaffolding surrounding the vessel protected the wires from

the elements and the heat from the vessel wall.

The data gathering station was a 10 foot shipping container fitted out as an air-conditioned office, and inside resided the

computer which collected all of the UT and temperature information required for processing. All of the cables terminated in the

back end of the computer. The station and its contents were not considered intrinsically safe or explosion proof, and since this

installation was inside a live hydrocarbon processing facility it was a regulatory requirement that this station be positioned in a

non-hazardous area [11]. This meant that the longest cable length – from the highest probe on the vessel to the data station –

was approximately 96m. This created challenges from a signal point of view. As the cable length increases, reflections,

attenuation, noise pickup, and cable delay, become factors in accurate signal processing [12]. These phenomena become even

more of an issue as the transducer frequency increases. However, these issues can be minimized by using a lower frequency

transducer while preserving ultrasound beam qualities sufficient to probe the back wall to the needed level.

The temperature sensor has an uncertainty of ± 0.3 °C at 70 °C. However since the temperature reading was taken from the

outside of the vessel, it had the possibility of being affected by wind, rain and sunlight. This challenge was overcome by

covering both the aluminium housing and the temperature sensor with roof flashing and sealing around the edges. This

effectively insulated the temperature sensor from the external elements.

The UT probes themselves were calibrated on a 100mm thick block of carbon steel – the same material used to fabricate the

vessel.




5.1 Signal Initiation and Reception

Thickness of the material is calculated using the pulse/echo method for ultrasonic transducers. The pulser/receiver transmits an

excitation pulse (up to -300V, 50ns pulse width) to a single element transducer. The piezoelectric crystal within the transducer

vibrates at its natural frequency (2.25MHz in this case) and generates a longitudinal wave that progresses through the material

and reflects off the back wall as shown in Figure 6.

Figure 6: Ultrasonic Pulse/Echo Wave Progression14

The wave then returns back to the transducer, which converts the mechanical vibration back to an electric pulse. This is

captured by the digitizer. The time taken to traverse twice the wall thickness is measured very precisely by the instrument, and

knowing the velocity of sound (compensated with temperature), the wall thickness is determined.

5.2

Signal Processing

Modern ultrasonic flaw detectors typically use a gate or windowed section of the signal to isolate echoes. The distance to the

flaw is measured when the signal intersects a predetermined threshold in time. This is likely the most common approach with

the advantage of simpler algorithms decreasing the computational demand on the processor. Other methods exist for measuring

the time-of-flight, but the one employed in the NanoUT system uses a more mathematical approach to glean more information

from the wave form.

A typical echo generated by an ultrasonic transducer has smooth curves with local minima and maxima, also known as extrema

points. These extrema and inflection points (see Figure 7) can be found mathematically with a high degree of accuracy and

repeatability using the patented time-of-flight algorithm [13].

Figure 7: Typical back wall echo landmarks

These landmarks are found for each back wall echo, and are averaged to precisely determine when an echo occurred in time

[14]. Knowing the time difference between two echoes and the speed of sound in carbon steel, the thickness of the metal can

now be calculated.

The temperature of the wall is measured using 100Ω platinum RTDs. The relationship between temperatu re and wall thickness

is highly linear; in fact, in a non-corroding environment, the wall thickness and temperature profiles are identical. A tenfold

increase in resolution is obtained by temperature compensation, which is performed using a Nelder-Meed Simplex algorithm.




5.3 Technical Specifications of Installed System

The NanoUT instrument used at Darwin was a custom built, single board computer which consisted of the following

components:

1. 100MS/sec digitizer PCI 5122 from National Instruments

2. High voltage pulser-receiver operating at 2300Hz

3. High speed 16 channel multiplexer

4. National Instruments 4 slot cDAQ to read 16 temperature probe readings5. Computer with 3 PCI slots, Intel 8 core processor, 8GB RAM , 256GB of flash memory and Windows 7.

6. LabVIEW 2009 SP1 installed on the computer

7. High precision 100Ohm 4-wire Pt element surface RTD’s for measuring temperature.

8. Sixteen 2.25 MHz single element transducers

6. PRELIMINARY RESULTS

The preliminary results of the monitoring program showed a long term corrosion rate of 1 mm/year in the worst affected areas

(areas subject to erosion as well as corrosion). In other wall areas the corrosion varied anywhere from 0mm/year up to this rate.

In this application it appears that the technology can be accurate up to 1 micrometer; however this could be increased through

an optimization process and further research and development.

7. DISCUSSION

This new technology is an example of how corrosion monitoring techniques are further advancing in order to ensure that safety

and asset integrity are not compromised while still ensuring economic viability of an asset [15].

8. CONCLUSION

The results of this program suggest that NanoUT can be used to accurately predict wall thicknesses changes over a prolonged

period and under adverse environmental conditions. It further shows that the aluminium housing and rare earth magnet design

was successful in holding the probe stationary during the testing period. What remains, is to confirm the results of this program

via internal, close visual inspection and pit depth measurement through CSE. This entry will occur in the upcoming 2014 full

plant shutdown. It is also intended to install a replacement vessel during this turnaround.




9. REFERENCES

1 Melchers, R. E. Jeffrey, R. J. On Predicting Long-Term Corrosion Behaviour From Short-Term Tests (2010)

2 Boulton, L. H. Wallace, G.

Life Extension of a Critical Pressure Vessel in a Paper Mill Through Managed Condition Assessment Corrosion and

Prevention(1998)

3 Groysman, A. Hiram, A. Corrosion Monitoring in the Oil Refinery (1996) 13th International Corrosion Council

4 Lee, H. S. 1V-1201 CO2 Absorber General Assembly Darwin LNG Proprietary Limited (2004)

5 Standard Specification for Pressure Vessel Plates, Carbon Steel, for Moderate and Lower-Temperature Service American

Society for Testing and Materials A516/A516M – 10 (2010)

6 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry API RECOMMENDED PRACTICE 571 SECOND

EDITION, (2011), American Petroleum Institute

7 Rules for Construction of Pressure Vessels Division 2 Alternative Rules ASME Section VIII (2012) The American Society of

Mechanical Engineers

8 V1201 Corrosion Finding – Root Cause Analysis Meenakshisundaram, S. (2012), ConocoPhillips Company

9

Fitness-For-Service API 579-1/ASME FFS-1 (2007) The American Society of Mechanical Engineers / American PetroleumInstitute

10 DLNG Amine Contactor V-1201, 2012, ConocoPhillips: Global Production Excellence Edwards, D. R.

11 Explosive Atmospheres Parts 0-31 Australia/New Zealand Standard 60079:2012 (2012) Standards Australia / Standards New

Zealand

12 Olympus (2014) www.olympus-ims.com/en/applications/effects-long-cables-ultrasonic-testing-single-element-transducers

last accessed 30-APR-2014

13 High Precision Ultrasonic Corrosion Rate Monitoring, US patent App# 20110067497,Grubb,S.A., Blumer,J.D.

14 Development and use of a Multi-Point Temperature Compensated High Precision Permanent Mounted Ultrasonic

Instrument to Measure the Slug Flow Erosion Pattern in a Horizontal Standard Elbow, 2011, Ph.D. Thesis, Scott A.

Grubb,The University of Tulsa, Tulsa, OK.15

Scala, C. M. Innovations in Corrosion Monitoring Corrosion Control and NDT (2003)

http://www.olympus-ims.com/en/applications/effects-long-cables-ultrasonic-testing-single-element-transducers






10. AUTHOR DETAILS

Daly Kelly is the Asset Integrity Engineer for the Darwin LNG Plant. His duties

include: managing and planning vibration, coating and corrosion surveys. Planning

and executing remedial work to vessels, piping and valves. Assessing and

monitoring corrosion, setting and managing risk based inspection (RBI) strategiesand performing fitness for service assessments.

Scott Taylor is a Staff Scientist with the Global Production and Excellence group

for ConocoPhillips located in Bartlesville, OK. He received his PhD from the

University of Utah in Physical Chemistry in 1989 and has been involved in

inspection and inspection technology since 2008. His role in the NanoUT projectwas specifying the transducers and their mounts, the field calibration and

installation of the system. He also processed the data presented in this article.

Sudha Yellapantula was an Automation and Control Engineer in the Global

Production and Excellence group at ConocoPhillips in Bartlesville, OK from

March 2010 - March 2014. Her roles included building the NanoUT instrument,

writing the field ready software to run the instrument, and calibrating it for the field

trial in Darwin. In addition, she also provided instrumentation and LabVIEW

support to the group. She has a Master of Science degree in Electrical Engineering

from Texas A&M University, College Station,TX and is also a Certified LabVIEW

Developer. She is presently working as a Research Engineer at Rice University,

Houston, TX, USA

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