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  • Final Report Summary - TANK-INSPECT (Condition monitoring of large oil and chemical storage tanks using ultrasonic guided wave tomography without the need to empty and clean the tanks)

Final Report Summary - TANK-INSPECT (Condition monitoring of large oil and chemical storage tanks using ultrasonic guided wave tomography without the need to empty and clean the tanks)

Large above ground storage tanks filled with hazardous liquids such as oil, oil-derived products, chemicals and food processing liquids are in widespread use in Europe and throughout the rest of the world. The TANKINSPECT project aimed to develop a condition monitoring system for large storage tanks based on ultrasonic guided wave tomography.

To overcome the drawbacks of current inspection practices, the consortium proposed to develop novel long range ultrasonic testing (LRUT) techniques for monitoring corrosion in tank floor plates, without the need to empty and clean the tank. The advantage of the proposed method was the ability to detect corrosion on tank floors without the need to empty and clean the tank and without the need for operator entry inside the tank.

The project was divided into several workpackages:
- A survey was undertaken of the problem to identify what hard and software capabilities would be needed.
- Representative defect samples were acquired and small scale model tanks for laboratory development work were manufactured.
- Numerical modelling studies were carried out of guided wave propagation in lap-welded plates to identify the best wave modes to use.
- Novel guided wave transducers were studied to generate these wave modes from the outside of a tank.
- Signal processing routines were developed to improve the signal to noise ratios.
- Tomographic reconstruction routines were developed to produce maps of tank floor attenuation.
- A prototype multi-channel guided wave ultrasonic pulser-receiver and control and data collection software was developed and field measurements were carried out on storage tanks.

Several test tanks were constructed, these were used for experimental verification of the numerical modelling.

Numerical modelling was used to study wave propagation through lap welded plates including attenuation losses into the liquid (diesel) and the tar sand. The results showed that the fast (non-dispersive) S0 wave was best suited as it has low attenuation. The losses into the diesel and tar sand were comparatively low with the main attenuation being due to reflection and mode conversion in the lap welds.

Two types of boundary conditions between the steel plate and the sand were modelled:
- perfect bonding - plate rigidly coupled to the sand;
- slip bonding- plate allowed to slide over the sand.

If the plate was rigidly coupled to the sand the attenuation losses would be extremely high (~10 dB/m) but experimental measurements with the test tanks confirmed the slip bonding boundary conditions where the attenuation was only ~0.5 dB/m in the low kHz range. The results demonstrated that the transmitted signal amplitude was not affected by liquid level.

The data was processed using tomographic reconstruction techniques to give a colour-coded map of the tank floor. Areas of high attenuation indicated potential areas of corrosion.

Dedicated man-machine interface software was written to control the prototype TANKINSPECT instrument and record:
- tank parameters
- client details
- transducer locations and collected data.

The completed TANKINSPECT system had the following functions:
- prototype multi-channel long-range ultrasonic pulser receiver suitable for the inspection of storage tanks;
- the TANKINSPECT man-machine interface for controlling the ultrasonic pulser receiver and storing data tank details and transducer locations;
- a set of post processing routines for tomographic reconstruction capable of producing an attenuation map of a tank floor.

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