Leakage of fluids from above ground atmospheric storage tanks is a major issue. Most countries have stringent regulatory requirements for the avoidance of leaks of potential pollutants from tanks, so that there is considerable pressure on tank and terminal operators to ensure that leaks do not occur. Other than cases where catastrophic failures of tanks have occurred, the main site for leakage is the tank floor, which may degrade in service owing to corrosion or, in some cases, cracking at welds. Well-founded procedures have been developed for the inspection and repair of tank floors, for example those in API standard 653 (API 2007), but the operations required to empty, clean and make tanks safe for regular internal examinations are costly and time consuming. Further, the examinations are usually scheduled on an equal time basis and do not take account of the varying rates of degradation occurring in different tanks. Tanks which require the highest priority action may therefore not be examined or rectified first.
The monitoring technology, which has been under development at Plant Integrity for a number of years, involves the use of low frequency ultrasound to examine the whole of the tank floor from a number of permanently attached sensors around the perimeter on the outside. The long distance propagation characteristics of the ultrasonic waves used allow signals from one side of the tank to be picked up by a sensor on the other side (up to 30m diameter). By using combinations of transmit and receive amongst the set of sensors, it is possible to cover the whole floor area. By taking readings from the sensors regularly, long term trends in the condition of the floor can be determined. In this way, short term variations, for example from temperature fluctuations or changes in the fill levels in the tank, can be separated from changes in the physical condition of the floor. In this way, tanks needing priority attention may be identified and the more rigorous internal examinations may be concentrated on these.
The main goal of the project is to provide performance data to demonstrate the capability of a non-invasive ultrasonic guided wave monitoring system to detect time dependent degradation of the Atmospheric Storage Tank (AST) floors that may give rise to leaks.
Specific objectives are:
• Instrumentation and remote monitoring of specified tanks at identified refineries;
• Generation of condition reports on tank floors monitored;
• Review of detection and assessment performance of the non-invasive system in comparison with internal inspection (MFL, ultrasonic and visual) results;
• Production of a report detailing capabilities and limitations of the system
Tank floor monitoring system data may be gathered at pre-determined intervals while the tank is in operation so that monitoring without service interruption can be achieved. The test results are stored in the unit itself and external hard disks and also are copied on Pi servers at intervals for archiving and analysis.
