Laser Scanning Aids in Tank Inspections
In the U.S. alone, storage tank (or shell) capacity totals more than 1.1 billion barrels of crude oil and refined oil products. The number is increasing as a result of generally rising crude oil inventories since the end of 2014.
Large petroleum storage tanks can be more than 400 feet in diameter and contain 1.5 million barrels, and they can change over time. The large volume of fluid puts enormous stress on tank walls and floors, which can result in deformation. Other issues may be induced by corrosion, subsidence and weather. In extreme cases, a tank could fail and release hazardous or toxic material into the environment.
To keep storage tanks working safely and efficiently, tank owners need to undertake a program of regular inspection and maintenance. A key component of American Petroleum Institute (API) published standards for inspection, repair, alteration, and reconstruction of steel aboveground storage tanks (API 653) is accurate physical measurement of tank shells, floors and roofs. The API 653 standards call for measured data on verticality of tank walls, the roundness of tank shells, flatness of floors and any subsidence.
Today, the measurements are typically gathered using total stations. Depending on the size of the tank, crews of two or three people use a total station to capture data on 12 locations around an 120-foot tank.
According to Mike Mutina — a storage tank subject matter expert and manager of the Above Ground Storage Tank Center of Excellence at MISTRAS Group, Inc., a nondestructive testing (NDT) and asset protection solution provider — a skilled crew can complete a survey of an in-service 120-foot tank exterior in about four hours.
Laser scanning using LiDAR technology and improved LiDAR data processing software can improve this by capturing millions of individual points on a tank’s shell, floor and roof. Laser scanning also collects information on tank appurtenances (nozzles, access points, stairs and ladders) and nearby structures and terrain. Built-in cameras capture digital images that provide additional documentation of tank conditions. The 3D laser scan data can also be used for fitness-for-service assessments (FFS).
Thanks to advances in software technology, the additional data that laser scanning provides over total stations opens the door for a more detailed look at a tank and the analysis process itself is far less cumbersome. It also improves the “visualization” delivered to the client.
“When I was first adopting laser scanning, LiDAR data-processing software wasn’t at the level it is today,” Mutina says. “After I got the raw data, I would spend hours loading it into CAD and extracting the verticality information, and then have to manually produce the API reports.”
Today, when he receives data from an inspection, Mutina loads it into Trimble RealWorks software. The software can automatically merge multiple point clouds, including data from non-Trimble scanners, into a single cohesive dataset. The resulting point cloud provides a comprehensive, precise picture of the entire tank.
With today’s laser scanning data and software, MISTRAS can produce API reports and gather even more information in under two hours. “We can do the standard API requirement with 12 stations, but with laser scanning, we can also look at the whole tank,” Mutina explains. “We can see inward and outward deformation and give clients a much better evaluation.”
The visualization tools do more than help clients understand the measurements and results. Mutina explains that some clients also use them to explain any issues to other stakeholders. “For municipal agencies, such as a water district, they need to go through budget authorization before making repairs or improvements. The graphics and extra reporting help them to justify the needed expenditures.”