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Nortek: bringing navigation accuracy to tank inspection robots deployed in a harsh environment

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27 October 2021

A pioneering autonomous robot with a high-performing DVL is revolutionizing fuel tank inspection – while eliminating potential operational hazards.

Nortek’s Doppler Velocity Logs (DVLs) are already established as industry-leading instruments for velocity calculations in subsea applications. But pioneering Boston-based company Square Robot has now successfully deployed them in a very different and challenging environment.  

Square Robot is helping to change the face of the oil and gas industry through its development of an innovative robot that can provide detailed assessments of the conditions in above-ground petroleum storage tanks.

Using the robot eliminates the need to empty the tanks of liquid so that humans can enter to carry out detailed inspections – a costly, time-consuming and potentially hazardous procedure still used in most tank assessments.  

Founded in 2016, the company took just two years to design and build a highly robust autonomous robot certified to operate in these hazardous environments. Five versions of the robot are now being deployed by leading firms in the US petrochemical storage sector.

Robust autonomous robot for fuel storage tank inspections
The "Square Robot”, from which the company takes its name, is essentially a highly robust, autonomous robot laden with sensors, able to fit through the 24-inch "manway”, the standard-size portal fitted to the top of storage tanks.

The robot is then able to navigate through the liquid-filled tank – typically gasoline or diesel – to detect damage such as corrosion on the tank floor. The tanks, a familiar sight for many, are typically 10–30 m tall and 15–80 m in diameter.

"The robot essentially performs a floor inspection of the tank that would otherwise have to be done by a human,” says Amy Underwood, Senior Roboticist at Square Robot.

"During a traditional out-of-service inspection, the tank is emptied, cleaned to make it safe for entry, then inspected by personnel. All of this downtime means a significant loss of revenue for the tank owner. Our robot gets rid of all that – your tank is still in service and full of product,” she says.

The vehicle is innovative in that it carries out tests in a way that won’t damage the tank – so-called non-destructive testing (NDT) – using technologies such as ultrasonic testing (UT), which can characterize the thickness and internal structure of a surface using high-frequency sound waves. Additionally, the robot is capable of hovering, unlike "crawler” robots sometimes used for basic inspections.

Highly accurate velocity measurement for precise positioning of robot

But in order to precisely examine and map areas of concern, the robot needs to know exactly where it is in the tank. That requires highly accurate velocity measurement to aid navigation in a critical way – neither of which is straightforward in an enclosed tank full of liquid product.

To solve the problem, the company looked to its roots in the subsea industry. Square Robot’s founders and several other team members previously worked at a leading AUV developer and have had long careers in the subsea industry. So, they already knew what worked well for velocity measurement in the ocean and wondered whether it could be made to work in the harsh environment of a storage tank.

"We needed high navigational accuracy, and knew DVLs could provide this from our background in subsea,” says Underwood.

DVLs use the Doppler effect to calculate velocity by measuring the shift in wavelength between acoustic pulses transmitted to and reflected back from particles in a liquid or a surface such as a seabed.

The team were familiar with Nortek’s Doppler velocity logs, which had a reputation for being highly accurate, reliable and straightforward to use for underwater navigation, as well as offering the compact form needed to fit into the restricted space of the robot.

Crucially, Nortek’s 1 MHz DVL1000, the instrument Square Robot decided on, could operate accurately at a distance of only around 20 cm from a given surface – often the seabed, but in this case the tank floor. That is closer to the floor than many other DVLs on the market are capable of operating accurately, and a prerequisite for the inspection vehicle to do its job properly.

The fact that this DVL performs so well in such a difficult environment can be attributed to Nortek’s proprietary bottom-tracking algorithm.

A DVL operating accurately at even smaller distances from the floor

While the Nortek DVL already works closer to a surface than many other instruments on the market, Square Robot is looking forward to technological advances now being rolled out by Nortek, which will enable it to operate accurately at even smaller distances from the floor.

Meanwhile, Square Robot is seeking to expand its operations both in the US market, where it currently operates, and internationally. The robot is currently deployed through Square Robot’s subsidiary inspection services company Veritank.   

The company’s five models of the robot have differing, customizable payloads, which could be adapted for a number of uses, including subsea inspection and in the shipping industry.

"We’re focused on tank inspections right now, but we’re looking at alternatives and we’re always open-minded. Our goal is to be the best in whatever space we’re in,” says Underwood.

The successful use of the DVL in this novel environment is further evidence of the accuracy, reliability and versatility of one of Nortek’s most popular and recognised technologies.

The Square Robot vehicle with the four-transducer Nortek DVL1000 mounted at the extreme left. The vehicle deploys ultrasonic testing, HD cameras and other non-destructive methods to assess tank wear and tear; the assessment depends on the DVL for pinpoint positioning. 
Nortek: bringing navigation accuracy to tank inspection robots deployed in a harsh environment