Background & Introduction
API 650 Standard for Welded Tanks for Oil Storage is a standard practice document used in industries for the construction (meaning design, materials selection, fabrication, inspection, testing ) of atmospheric storage tankage (AST) and their appurtenances. Hydrocarbon processing (refining, oil sands), petrochemical, oil and hydrocarbon product storage facilities all make use of AST equipment. The tanks operate at ambient conditions of temperature and pressure; although, these conditions are not as severe as required by much on-site equipment, i.e., compared to, say, FCC reactors and regenerators, fractioning towers, steam generating equipment or fired heaters, nonetheless, AST's must be able to withstand relatively severe atmospheric conditions.
AST's can be very large, over 250 feet [76 metres] in diameter and thin wall, O(1 inch) / O[25.4 mm]. This equipment must be able to withstand temperature extremes from -40 oF to 140 oF / -40 oC to 60 oC. In addition, AST's must be able to withstand climatic and meterological conditions such as rain, snow, wind, and seismic loads. To provide structural integrity and reliability, this necessitates the selection of qualified steels and design of tank appurtenances (wind girders) to resist these external loads.
A recent recognition in the asset integrity management of tanks is the incidence of increasing numbers of AST's experiencing buckling type faiures. Since tanks operate at atmospheric conditions and are thin wall, they must be reinforced to prevent failure induced by high wind loads occassioned by hurricanes, tornadoes, and typhoons. These failures are evidenced by a "squashing" of the tank face which faces the wind load. In addition, wind-indueced debris strikes may also cause permanent, localized deformation of the tank wall. Interestingly, atmospheric anomalies may also induce a full vacuum load on the tank, resulting in general ring buckling failure.
Ring Buckling of Atmospheric Storage Tanks
In Figure 1, we present a laser scan VT examination of a tank that has been damaged by a debris strike. However, during the course of the FFS evaluation it was found that the tank also exhibited a number of symmetric indentations and apparent bulges.
Figure 1 Laser Scan VT Inspection of Damaged Atmospheric Storage Tank
The initial motivation for the laser scan inspection was an apparent
debris strike on one side of the large diameter tank, ostensibly, from a hurricane with wind speeds up to 150 mph.
However, on analyzing, the graphics revealed that a number of dents and bulges were symmetrically
distributed about the circumference.
The issues are whether this amount of damage precludes continued use of the tank
and the resulting quantification of limitations to tank fill height and cyclic use.
In Figure 2, a plot from another damaged tank is presented that clearly exhibits 10 cyclic lobes. These deformations
approach 24" [600 mm] and are a result of external pressure / vacuum loading of the tank.
Figure 2 Tank Deformation Profile -
This is a plan view of laser scan results for a large diameter
tank located in a tank farm of multiple tanks; a detailed root cause failure analysis (RCFA) was performed
to determine the reason that one tank suffered this amount of ring buckling while the remaining tanks were unaffected.
The FFS evaluation of the tank suggests that the API 650 rules are inadequate in guarding against ring buckling failure.
The possibility of extensive failures in the industry are likely. Note that the tank in Figure 1 had been exposed to two
hurricanes within 1 year in which wind speeds exceeded 140 mph. The API 650 standard at the time of original
tank construction indicated the design wind speed to be 100 mph.
Both "push" and "pull" repair procedures were specified to remediate the tank.
Alternatively, the tank shell plates could be replaced; of course, this amounts to a complete rebuild of the tank.
This strategy may be reasonable for specific instances where crack-like indications are found in the "kinks" of the deformations suggesting the plates are not sufficiently ductile.
An accompanying problem is that many facilities do not have adequate equipment
records to adequately perform FFS assessments. In that case, a risk assessment of the remediation strategy is necessary.
Similar situations have been encountered globally from which the accumulating evidence indicates both the increased severity
of climate / weather and seismic events. Consider that most design data contained in Codes and standards are from short term
data (i.e., less than 150 years).
If you are interested in these real examples illustrating where API 650 is not up to the task, contact us for more details.
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