the Maverick Fuel-Tank Inspection Robot to the Test
By D.R. Hartsell
Solex's MaverickTM mobile robot operates
in above-ground storage tanks to meet the requirements of performing periodic
inspections for corrosion and structural flaws, as established by the American
Petroleum Institute (API) Standard 653. The API has endorsed the use of
robotics as a means to provide 653 tank floor inspection surveys. The Maverick
Mobile Fuel Tank Inspection robot is a mobile, remote-controlled, purged
and pressurized, submersible inspection platform. It performs floor inspections
from inside the tank while completely submerged in midgrade petroleum distillates,
ranging from gasoline to light crude oil. Maverick travels on the interior
tank floors using traction wheels, and the instrumentation payload includes
a multichannel ultrasonic sensor system to capture and correlate metal
thickness data, an onboard video system to record inspections, and position-tracking
sensors. Maverick and its complete supporting systems have been independently
and verifiably certified for safe operations in Class I, Division 1, Group
D environments. This article describes the testing and engineering involved
in bringing the prototype into commercial use.
At the job site, the equipment is deployed
by raising the robot to the top of the tank with a crane and then it is
lowered through the open manway. An umbilical cable connects the robot
to the computers in a nearby trailer, where all functions of the robot
are controlled in comfort. A technician inside the trailer drives the robot
around the tank floor.
Maverick travels on the interior
tank floors using traction wheels. The instrumentation payload includes
a multichannel ultrasonic sensor system to correlate and survey metal thickness
data, an onboard video system to record inspections, and position-tracking
sensors. The ultrasonic inspection system onboard Maverick provides the
tank owner with a detailed view of the tank bottom. The thickness data
can be presented in A, B, or C-scan formats and is summarized in a final
In addition to eliminating the exposure
of employees to hazardous substances, robotic inspection reduces direct
and indirect costs of API 653 compliance by inspecting tanks while they
are still in service. This saves the operator the expense of draining and
cleaning the tank and lost revenues from downtime, which can add up to
$50,000 to $500,000 per tank. Costs and liabilities associated with disposing
of the cleaning agents and heavy metal sludge are also eliminated. Since
tank operations are not interrupted, it is no longer necessary to shift
the product to other tanks or to shut down operations if standby tanks
are not available.
Development and Testing
Moving a new technology from laboratory
models through prototypes to commercial systems is an arduous and expensive
task, particularly when the field conditions are "hazardous and potentially
The development and testing
of Maverick was sponsored in part by the US Department of Energy "National
Industrial Competitiveness through Energy, Environment, and Economics
(NICE3)" program, Texas Natural Resource Conservation Commission,
Colonial Pipeline, Mobil, Amoco, Shell, and Exxon. Much of the design of
Maverick was based on research performed by Thor Zollinger and others at
the US National Laboratories at the Savannah River Plant and Idaho Falls.
The Safety Certification process was led by Kerry Klingler of Solex.
Testing and refinement of
Maverick and achieving requisite safety certifications took place over
more than a year and included demonstrations at several sites and critiques
by industry and government experts. Four demonstrations were scheduled
and two were added. As the system moved into commercial application, government
and industry experts were also invited to witness and critique the demonstrations.
DEMO 1: PROTOTYPE DEMONSTRATION
The prototype Maverick robot was demonstrated
at the Mobil Oil refinery in Paulsboro, New Jersey, on November 13, 1997,
for representatives from the US Department of Energy, Exxon, Mobil, General
Electric, Thermo-Electron, and other organizations.
OPERATIONAL ISSUES: CABLES, SLUDGE,
The test-site location revealed several
operational issues. For instance, because access to the base of the tanks
was physically restricted by piping runs and the berms surrounding the
tanks, it was clear that longer control and data cables would be necessary.
Figure 1. The
The first day of testing leading
up to the demonstration required changes to the vehicle. The tank bottom
was covered with a heavy layer of sludge, consisting of metal filings and
rust held together in clumps by a black tacky oil residue.
Within the first 3 meters
of movement, the drive mechanisms began to jam and the vehicle was pulled
out of the tank. The robot had been fitted with magnetic wheels, since
traction was anticipated to be a problem due to large rivets protruding
upward from the tank floor plate seams. The magnetic wheels had picked
up about a 4.5-cm-thick coating of the sludge off the tank bottom, increasing
the wheel diameter and jamming the mechanism. The magnetic wheels were
removed and replaced with nonmagnetic wheels fitted with toothed rubber
treads to provide increased traction.
In addition, the sludge also
partially blocked the ultrasonic probes from getting thickness readings
on the tank bottom and was stirred up while driving, reducing visibility
to a few centimeters. A narrow broom attachment was fitted to the front
of the vehicle to dear a section of the tank floor prior to scanning it
by driving back and forth across the section. The broom worked well for
prepping the tank floor for inspection, but it kicked up so much material
that the cameras were completely obscured.
The sonar-based tracking
system was the last subsystem to be tested within the tank. The acoustics
in the tank were very poor. Because of the high amount of ringing, the
system was initially inoperable. When the system failed to come up even
after all appropriate tests had been performed, the equipment designer
was flown out overnight from California to troubleshoot the system. Modifications
to the software and alternative settings out of the normal ranges corrected
the problem caused by the acoustics, and the system was up for the demonstration
operating from two baseline stations. Testing was also conducted to determine
whether the sonar transducers could hear and transmit through the tank
wall, since deployment into the interior of the tank proved to be difficult.
The tests demonstrated that the system could operate from the exterior
of the tank, greatly simplifying operations of the subsystem.
All equipment and systems
were operating properly on the day of the demonstration. Throughout the
week, all of the problems that cropped up were corrected on-site. As part
of this series of tests, the complete Maverick deployment system was evaluated
for operational modifications, including the control trailer, which was
pulled over 5000 miles, testing all packing, including computers and instrumentation,
against road vibration.
DEMO 2: DEPLOYMENT THROUGH A FLOATING ROOF
After the Paulsboro demo, Solex engineers
spent four months upgrading all of the safety systems and correcting deficiencies
noted in the previous set of tests. On March 20, 1998, Maverick was demonstrated
in a diesel storage tank at the Mobil refinery in Torrance, California.
Personnel with the DOE NICE3 program (the group that sponsored
the demonstration), Mobil and Shell Oil personnel, and refinery directors
from Spain and Hungary witnessed the demonstration.
The goals of this demonstration
were to test the equipment modifications and pursue safety approvals for
use in product tanks. An extensive safety review was conducted and procedures
to allow safe deployment of the equipment into a diesel tank were jointly
written to control operations, along with all of the required permitting.
PURGE AND PRESSURIZATION SYSTEM FOR
THE ROBOT AND UMBILICAL
The most major upgrade to the equipment
was complete redesign of the purge. and pressurization system for the robot
and umbilical. The cabling was extended to simplify operations under varying
conditions, The rugged pressurized umbilical was long enough to allow inspection
of adjacent tanks without having to move the control trailer. The purge
and pressurization controls operated as designed and provided continuous
safe operation of the robot during the week of testing with only minor,
easily corrected, system leaks.
INSPECTION PROBE ARRAY
The second major modification to the equipment
was modifying the housing to protect the ultrasonic transducer (UT) inspection
probes. Previously, the UT inspection probes were mounted on the bottom
front of the robot housing. The first demonstration had shown that the
exposed UT probe faces posed a safety problem since they were exposed to
the liquid and also were vulnerable to being struck by obstructions inside
The demonstration tank had been in continuous
operation for 19 years without a floor inspection. The tank bottom was
coated with a layer of sludge, consisting of a stringy black jelly mixed
with sandy grit. As in the last test, the sludge totally obscured the cameras
as the vehicle moved through the fluid, and the diesel fuel was less transparent
than the water in the previous tank. The brush mounted on the front of
the vehicle was not sufficient to clear away enough sludge to get consistent
UT readings. A squeegee was added in front of the brush to improve sludge
removal on the second day in the tank. It worked well and greatly improved
the ability to read the thickness of the tank bottom. The new UT system
including the delays and a new type of transducer worked well in scanning
the steel plates of the tank bottom. A deficiency was noted with the new
multiplexor and it was replaced with the backup spare.
SONAR-BASED TRACKING SYSTEM
The sonar-based tracking system was the
last subsystem operational within the tank. The beacons were modified to
provide better accuracy. Extensive testing was conducted on the tank acoustics
and a problem was found in transmission of sound inside the tank. Sound
was echoing due to the wall curvature, thus creating false position points.
Despite the extensive corrective efforts, consistent positioning operation
could not be obtained. The inability to track the robot's position within
the tank restricted the ability to scan the tank floor to areas near the
entry point, preventing an inspection of the entire tank floor.
All of the equipment, with
the exception of the tracking system, was operating properly on the day
of the demonstration.
DEMO 3: EARNING SAFETY CERTIFICATION
The third demo was conducted in a large
fuel-oil storage tank (120,000 bbls, 50 meters diameter, 15 meter high,
1985 sq. m. bottom, dome with internal floating roof) at the Colonial Pipeline
facility in Francisville, Louisiana, August 3-4, 1998. Specific testing
goals targeted refinements to the sonar tracking system and deployment
into a tank with an internal floating roof Prior to the demo, an extensive
safety review was conducted and procedures to allow safe deployment of
the equipment into confined spaces were jointly written with Colonial personnel
to control operations, along with all of the required API permitting.
The tracking-system software was tested
to verify programming changes. Major portions of the code had been re-written
to support hardware modifications, and many operational upgrades were in
need of real-world testing. A minor bug was located and corrected in the
software relating to the display of the tank-bottom drawings. The software
and system hardware operated well during the remainder of the tests. A
number of other upgrades to the software were also tested successfully,
including prefiltering of the gyro data for stabilization, and rework of
the position determination math, and dynamic registration of the positioning
data to the tank map to allow for easier initial setup. Additional software
upgrades to improve the usability of the system have been scoped as a result
of the testing.
Deployment into an internal
floating-roof tank was also a major factor in the testing as this was the
first time any robotic inspection equipment had ever been deployed into
a full internal floating-roof tank. To enable this insertion, Solex engineers
prepared a Job Safety Analysis package and obtained approvals from the
safety personnel at Colonial Pipeline.
Solex personnel also received
training in Houston for entry into confined spaces, since the enclosed
area under the dome above the floating roof is considered a hazardous confined
space due to the potential for the concentration of fumes. Colonial was
concerned about damage to the top surface of the floating roof a very thin
sheet (6-7mm) of aluminum sheet metal over a honeycomb internal structure.
Solex placed plywood sheets and a long strip of carpet on the roof to avoid
potential damage. The internal roof doubled the amount of time needed to
deploy the robot into the tank compared to the previous demonstration.
All of the equipment and
software was operating properly on the second day of testing, and an abbreviated
inspection of a portion of the tank bottom was conducted for comparison
to the last inspection of the tank. The 49-meter diameter tank had 208
steel floor plates in its bottom construction. The previous inspection
of the same tank took over two weeks to conduct.
Figure 2. Maverick
is suspended from a cable
at the end of the crane.
DEMO 4: ALLSYSTEMS REHEARSAL
The fourth demo in the series of Maverick
tests was September 1-4, 1998, at Atlanta Junction, Georgia, in a fixed-roof
jet-fuel storage tank containing 100,000 bbls of jet fuel. The tank, which
serves as a primary source of jet fuel for Hartsfield International Airport,
remained in operation during the inspection. In two hours, more than 31,000
discrete ultrasonic data points were collected. Rather than producing simple
spreadsheets of raw numbers, Solex provided 3-D, color-enhanced representations
of the thickness data on the control trailer computer screens in real time.
Testing targeted refinements to the control software, UT transducer frequencies,
sonar blockage, and other operational issues, including dealing with a
TANK INSPECTION RESULTS
The tank inspection provided several significant
operational challenges. The drain piping was connected to pontoon floats
and would pivot up and down with the fuel level. During the inspection,
jet fuel was drawn and filled, with Maverick having no impact on the operations.
As with many storage tanks that have been in operation since as early as
the 1950s, no internal drawings of the tank were available. Maintenance
personnel believed there were five fixed columns with the possibility of
six, which made the calculation of the internal geometry difficult. At
a later date, when the tank was eventually opened, 15 columns were found
inside. The sump was open, without any guard. A quick visual inspection
conducted in June 1998 had revealed heavy top-side corrosion. Hurricane
Earl moved through Atlanta during the demonstration, which caused the Solex
team to put into effect emergency shutdown of operations.
The first item in the demonstration plan
for testing was the control system. A new driving system, including software
and the integration of a sophisticated joystick, was installed into the
system and tested for operability. Minor adjustments to the software provided
the operator with greater maneuverability, which later proved valuable
in extracting the robot from several tight situations.
The second major test was the UT system,
which encountered difficulties in getting thickness readings from corroded
tank floor plates. Initially, no thickness readings could be obtained with
the 5 MHz transducers. The robot was pulled from the tank, and every portion
of its UT system was checked and recalibrated. All portions of the system
were operating properly. The problem appeared to be that the plates directly
below the entry hatch were heavily pitted on the top surface, and the sound
was scattering too much to obtain thickness measurements. The UT analysis
software was reconfigured to measure the surface pitting and to generate
profiles of the top surface of the plates. This revealed the extent of
the top-side corrosion and yielded a survey analogous to a rough visual
inspection. Other areas of the tank floor were less corroded and were fully
mapped using the 5 MHz transducers. To improve the system's performance
on heavily corroded regions, a set of 2.25 MHz transducers was installed
and tested. The lower-frequency transducers were able to penetrate the
rough surfaces, allowing the system to map the roughest regions of the
tank floor successfully. Future inspection operations will include till
sets of transducer arrays to best match the surface condition encountered.
The UT data software can
be set up to directly measure two out of three characteristics: top-side
surface profile; bottom-side profile; and metal thickness. The third characteristic
can then be developed computationally by direct export to Excel. Scanning
and data acquisition is in real time and continuous. Within a discrete-time
or distance-traveled interval, Maverick records the worst data point observed.
Currently, Maverick makes a record of the data acquired during 2.2 cm intervals.
The recording system is infinitely adjustable. In two hours of actual scanning,
31,500 UT data points were collected. This amounted to 36 square meters
(3.5%) of the tank floor. The inspection company that had provided the
previous visual inspection considered the data acquired by Maverick to
The sonar tracking system
was also challenged by the internal geometry of the tank. The roof was
supported by several pillars, which occasionally blocked sound from reaching
the listening transducers. As a result, there were blind spots within the
tank where the positioning system was unable to function. The operators
were able to work effectively around this deficiency in most situations.
On one occasion, the robot was driven too close to the sump due to positioning
error and fell in, but it was pulled out with minimal effort. The sound-blockage
problem had been previously anticipated. To solve the problem, Solex planned
to use additional listening transducers to eliminate the blind spots.
The tank roof pillars also
posed a substantial navigational problem inside the tank. Both the location
and the number of pillars within the tank were in question, since drawings
of the tank were unavailable. Throughout the week, visual surveys of the
inside of the tank were conducted to identify obstacles like the roof pillars
and to demonstrate the visual capabilities of the robot. The pillars are
constructed from large back-to-back welded steel channels, with support
angles along the tank floor at the base, as seen clearly in the robot's
cameras. The cable tended to catch on the edges of the channels when the
robot was driven around a pillar. In one instance Maverick was caught in
an unreachable snarl, beneath 100,000 bbls of jet fuel. Using "on the spot
engineering," the robot was retrieved. The Atlanta demo was a success,
but it was clear that future surveys should be conducted in a way to avoid
wrapping the cable around roof pillars.
Figure 3. Diagram
produced during the Atlanta Station 352 demonstration.
DEMO 5: RETRIEVING TRAPPED ROBOTS
In the following weeks, Solex engineers
worked on developing emergency retrieval devices and procedures to avoid
wrapping the cables around pillars. A final test was performed on
October 2, 1998, at NASA’s Neutral Buoyancy Laboratory (NBL), which tests
and certifies personnel and equipment for mankind’s pinnacle engineering
project—building our outpost space station on the next frontier.
For this mission, the NBL has one of the world’s largest pools, holding
27.5 million liters (62m x 37m x 12m).
The goal of the test at the
NBL site was simple, but the task was not. How do you retrieve
Maverick when it is entangled in an unknown structure such as a sump or
a column at the bottom of a deep large tank full of jet fuel? Solex
Robotics engineers, working with NASA NBL divers, simulated trapping Maverick
and the umbilical at the bottom of the sump in the pool and successfully
tested a variety of new proprietary procedures and retrieval devices.
Maverick was now certified and ready for
full-scale demonstration. In the following applications, Solex engineers
continued to monitor the robot’s performance to demonstrate the efficacy
of the optical systems, Maverick’s generalized applicability for mapping
and inspection while immersed in a variety of middle distillates (ranging
from gasoline to light crude oil), and to determine how much data could
be accumulated in a short time.
PORT HUDSON, LOUISIANA
On February 15-17, 1999, Maverick, in
a full-scale demonstration, successfully inspected a large floating-roof
storage tank containing light crude oil at BP Amoco’s Port Hudson, Louisiana
facilities. The 206,000 bbl floating-roof tank had been in continuous
use since its construction in 1979. This tank is used for gathering
light crude for pipeline transport to barge terminal facilities on the
Mississippi River. In order to complete a conventional inspection,
it would have been necessary to hire barges for 35 days to provide alternate
storage capacity. The tank would have to be drained, the sludge removed,
the bottom cleaned, and the shell degassed prior to inspection. Inspection
was a complete API 653 consisting of an engineering evaluation of the foundation
and edge settlement; roof, roof vents, and seal inspection; wall and roof
UT examination; and featuring an in-service robotic UT floor survey. Based
on prior bids, Solex Robotics demonstrated to BP AMOCO a savings of more
than $185,000 over a conventional inspection.
The robotic demonstration
inspection was made difficult by several factors. Based on samples, the
bottom sludge was estimated to be less than 4 cm. In the initial insertion,
Maverick encountered sludge in excess of 55 cm deep and was completely
submerged below the sludge line.
Countermeasures were used
to dissipate the sludge and enabled the inspection to proceed. Due to the
beveled construction of the chine ring, Maverick was not able to survey
the ring. Collecting ultrasonic data through the bottom floor's fiberglass
coating was considered to be a major obstacle. Solex was able to match
the UT transducers with the substrates and gate the fiberglass readings.
This allowed the system to take back wall readings off the carbon steel
floor. Veritank Inspection, an independent inspection firm hired by BP
AMOCO, verified Solex's equipment and operating procedures, the UT data
reliability, and collection repeatability.
The sonar mapping and data provided more
than 100,000 discrete UT data points for analysis. The estimates on the
set-up and take-down time were accurate. Even though there was a torrential
downpour on the second and third days of operations, the Maverick inspection
DEFENSE ENERGY SUPPLY CENTER, MCDILL
The last inspection, at a jet-fuel tank
at the Defense Energy Supply Center, McDill AFB, Tampa Florida, on March
25, 1999, was "satisfyingly uneventful." Over approximately three days
of inspection, part of which was devoted to staff training, Maverick accumulated
200,000 separate data points, compared to about 2000 data points accumulated
in a visual inspection. While inspecting the back side of a pipe and valve,
Maverick sent back accurate readings of several serial numbers. Of greatest
importance in terms of developing a robotic service company, the inspection
was for commercial hire.
Solex's experience has demonstrated that
robotic inspection of fuel tanks is safer, more cost effective. and more
comprehensive than conventional inspection techniques. The demonstrations
revealed real-world engineering problems unique to the inspection environment,
such as acoustical problems, sludge, and physical impediments. More information
about Maverick can be found at http://www.solexrobotics.com.
D.R. Hartsell earned his Doctorate
of Jurisprudence at the University of Houston and has practiced as a Certified
Public Accountant in the State of Texas. Mr. Hartsell is Chairman of the
Board for Solex Environmental Systems, Inc., and he is the CEO for the
Solex Robotics engineering, manufacturing, and inspection subsidiaries
operating in North America, Europe, and Middle East. The US Department
of Energy honored Solex with the NICE3 award in recognition
of the company's pioneering environmental work based on the cost-effective
industrial application of robotics. Mr. Hartsell is a former Chairman of
the Board of Directors for the University of Houston Center of Applied
Technology. He can be reached at Solex Robotics, P.O. Box 460242, Houston,
Texas 77056. Tel: (713) 963-8600; Fax (713)
461-5877; E-mail: email@example.com.
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