Step 1: Records
Proper maintenance is dependent on certain criteria being met, and this is the first step in EEMUA's maintenance loop. The tank's original data must be available as built files, and inspection, modification, product change records must be well maintained and accessible to maintenance and inspection personnel. Of course, following the out-of-service period, a description of all repairs, inspection reports and NDT reports must be added to these files so they are properly updated. This step actually represents the loop’s final step.
Step 2: Cleaning
The second step is the cleaning – and gas freeing – activity.
Step 3: Inspection
When the tank has been cleaned and declared gas-free, internal and external inspections must be performed to assess the actual status of all tank components. These activities are addressed in step three.
Step 4: Compliance
The fourth step is the most important step in tank maintenance. A compliance check on each tank component has to be performed to assess whether the component is still fit-for-purpose or whether it should be replaced.
EEMUA’s publication No. 159 addresses the method to assess the rejection limit of main tank components. The rejection limit is the most important degradation parameter as it determines whether a component, such as tank bottom plates (annular ring plates and normal bottom plates), tank shell plates, tank roof plates, and floating roof seals, should be replaced. This document can be used in the calculation of a main tank component’s remaining lifetime. When the remaining lifetime is calculated prior to the next planned out-of-service period, evaluations should be performed to replace such items in the current out-of-service window.
Step 5: Engineering
When it is evaluated that tank components need to be repaired or even replaced, engineering tasks are needed to assess where, how and when these repairs will be performed.
Step 6: Repair
The next step involves the actual repairs at the site.
Step 7: Fit for purpose
When all repairs have been executed, the tank must be declared fit-for-purpose by a responsible staff member of the tank owner before the tank is allowed back in service.
Life cycle costs
All of the above activities within a tank’s lifetime can now be expressed in a tank operating cost profile, which shows expenditures during the entire tank lifetime.
The diagram below represents possible costs of a tank during its technical lifetime
Figure 4: Typical life cycle costs of a particular tank. The illustration is not to scale
The initial costs (red bar) represent tank construction costs. During the initial in-service period, expenditures arise from routine (preventive) maintenance and inspection.
After between seven and fifteen years, depending on local legislation and product type, the tank must be opened for out-of-service inspection and repairs. Following this, when the tank is declared fit-for-purpose again, the second operational period will start.
Indirect costs may amount to as much as 10,000 USD per month
The light grey box in figure 4 shows expenditures due to indirect costs. These costs result from:
(i) Non-availability of the tank during its out-of-service period (during which time additional tank capacity may need to be hired),
(ii) Lost rental earnings for depots and terminals, and
(iii) Additional cleaning costs of tanks used to temporarily store the product removed from the out-of-service tank.
Benchmark figures were obtained from most European refineries and other major depots and terminals. Results show that the indirect tank maintenance costs are typically 1 US$ per m3 per month. For a medium sized tank with a capacity of approximately 10,000 m3, where the repair period lasts about 4 months, this amounts to costs of US$ 40,000.
It should be recognised that the non-availability costs may vary according to location. However, they need to be included when an overall cost calculation is made on the “shut-down time”, the out-of-service period of a storage tank.
Main goals for the industry
The main goals for the industry therefore are to:
- Reduce out-of-service time (non-availability cost)
- Reduce total maintenance and inspection costs
- Extend in-service time
These goals will optimise the total life cycle costs of a storage tank. Possible actions to implement such goals will be addressed in a separate article to be published later this year.
EEMUA Publication No. 159, “User’s guide to the inspection, maintenance and repair of above-ground vertical steel storage tanks”, Volume 1 and 2, 3rd edition, 2003.
API standard 653, “Tank inspection, repair, alteration and reconstruction, 3rd edition, December 2001.
API Recommended Practice 575, “Inspection of atmospheric and low-pressure storage tanks, 1st edition, November 1995.
NDT (Non-destructive testing) involves inspection using only sensors. Examples are ultrasonic thickness measuring, X-ray inspection and Pulsed Eddy Current inspection (for detection of corrosion).
Boxing-up is the process of connecting a tank to the plant's utilities, control room, pipeline systems etc., after the out-of service inspection and maintenance period, during which time it was disconnected from these systems.
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