Corrosion & Prevention 2014 Paper 54 - Page 1 NANOUT: ACCURATE MEASURING OF WALL THICKNESS CHANGES D. Kelly 1 , S. Taylor 2 , S. Yellapantula 3 1 ConocoPhillips, Darwin, Australia, 2 ConocoPhillips, Bartlesville, USA, 3 Green Country Petrophysics 3 , 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 r esolution permits the rapid determination of wall loss rates. This is a patented technology called NanoUT. The maximum precision of 30 nanometres was a chieved with 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 tr ial 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 t he 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 test ing, 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 sound with 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 po tential to assist in process and control applications by rapid ly 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 w ere 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 or der to predict the parameters supporting the corrosion mechanism, and to provide improved accuracy in predicting vessel retirement date. ’
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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
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
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
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.
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,