UT gauging by drone inside stainless steel storage tanks

Highlights

• 5 tanks inspected in 1.5 days
• Significantly reduce the time in a confined space entry for personnel
• Full inspection coverage of the tank walls and roof
• Cut inspection costs by up to 50%
• Ensure the integrity of your storage tanks by adhering to all applicable local regulations, standards, and codes.

Problem Description

Usually, insulated stainless steel storage tanks are inspected from the inside to avoid removing the insulation. Following draining, flushing and cleaning, oxygen levels are measured prior to sending personnel inside. Sending personnel inside a confined space carries a certain amount of risk, and if not mitigated, can cause loss of life. 

Typically, several inspections are required to return the tank back to service. These can include a tank floor scan by MFL, a visual inspection of the welds and an ultrasonic thickness inspection of the tank walls, roof and floor. To inspect the tank walls and roof, there are a number of existing options to allow personnel to work at height: 

• Scaffolding would have been impractical and inefficient as access inside these tanks is only through the manholes. 
• Rope access from the roof wasn’t feasible as the roof was deemed unsafe for supporting a person. 
• An ultrasonic sensor on the end of a long rod. The rod is limited to a height of 12 metres, meaning in this case only limited coverage would have been possible of the 20-metre tank wall and nothing of the roof. Pressure is needed for ultrasonic sensors to get accurate readings – another reason why long poles are a challenging option.

Solution Description

The Voliro T, which was fitted with the UT payload, was used by a drone inspection service provider to carry out ultrasonic thickness measurements inside a total of five insulated stainless steel tanks. These tanks were 20m in height and 18m diameter. As there is no GPS signal inside the tank, the Voliro T can switch to use the 6 optical flow sensors for positioning and control. The LED’s mounted on the prop guards give illumination of up to 20000 Lm for a close visual inspection. The other advantage is that due to the unique force vectoring capability of the Voliro you can push up to 3kg of pressure on the surface at any height and orientation, needed for accurate sensor readings.

The inspection scope was to take 3 point measurements per plate on the walls and roofs of the tanks. In total, across the 5 tanks, 700 measurement points were taken. The measurements were also validated by a large multinational notified body

The inspection carried out using the Voliro T significantly reduced the time personnel were in a confined space to just 3 hours per tank, while also offering full inspection coverage.