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IEEE Robotics and Automation Magazine

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NASA/Jet Propulsion Laboratory

Putting 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 report.

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 explosive."
   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.

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.

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 Maverick Robot

   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.

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.

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.

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 tank.

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.

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.

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.

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 hurricane.

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 be reliable.
   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.

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.

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 proceeded uninterrupted.


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: drh@solexrobotics.com.

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