Articles


Making the Most of Storage Tank Inspection Data

By: Devon Brendecke, Quest Integrity Group  

Download the PDF version as it appeared in the Jan/Feb 2012 issue of Inspectioneering Journal

Thanks to constantly improving technology developments, inspection of atmospheric storage tanks has yielded better data which, when used as input, improves the accuracy of advanced assessment techniques. Coupling the improved inspection data with an advanced engineering assessment often means that tank operators are able to postpone repairs until the next shutdown, eliminate the need for repairs or be exempt from hydrostatic testing.

The emergence of fitness-for-service standards has advanced the state of the art in tank integrity assessment. The application of API 579/ASME FFS-1 [1] Fitness-for-Service rules means that common damage mechanisms such as corrosion, shell bulging,cracking and edge settlement no longer automatically require costly repairs. The fitness-for-service methods and advanced engineering analysis make the most of the standard data collected during required inspections and help define new inspection requirements when additional inspection becomes necessary to “fine-tune” the assessment. 

Several examples of advanced engineering analyses are presented here. Each highlights the use of valuable data collected during standard out-of-service inspections as well as new inspection techniques for in-service and out-of-service inspections. The case histories demonstrate how the engineering analysis can often be used to avoid costly repairs and downtime.

Hydrostatic Test Exemption

The purpose of a hydrostatic test exemption analysis is to establish that the repaired tank is fit for continued service without the need for a hydrostatic test by application of detailed stress analysis and fracture mechanics technology per API 653[2]. Repairs requiring hydrostatic testing include floor replacement, shell repairs for removing nozzles and new nozzle installations. The application of advanced fitness-for-service methodology allows for many tanks to be exempt from hydro testing.

A hydrostatic test exemption analysis has many benefits.  It eliminates the cost and time of the hydrostatictest itself as well as costs of treatment and disposal of the water used for the test. The exemption is particularly well suited for tanks repaired during the winter, when cold temperatures make hydrostatic testing extremely difficult. Furthermore, eliminating a hydrostatic test allows an operator to return the tank to service earlier.

Much of the input data required to complete the hydro static test exemption is standard data collected during a thorough API 653 internal inspection. Minimum measurements of each shell course thickness, general tank dimensions, materials of construction, service product specific gravity, etc. are all pieces of information available in an internal inspection report which are used for the fitness-for-service assessment.

To determine if the repaired tank is fit for service without the need for a hydrostatic test, a fracture mechanics analysis is performed to establish the critical sizes of surface connected defects located in the repaired regions. The tank is considered safe if all flaws are reliably detected and eliminated before the flaws grow to critical size. Otherwise, flaws may grow to critical size and cause catastrophic failure prior to being detectable.

In order to perform a fracture mechanics analysis (FMA), the stresses imposed on the region of interest must be established.  A simple finite element analysis (FEA) model can be used to simulate the storage tank hydrostatic test when the tank is filled with water (which has a specific gravity that may exceed that of the contents during operation). An example of an FEA model is shown in Figure 1. Shell thickness obtained from inspectiondata can be incorporated in the finite element model.

Figure 1. Shell FEA model of tank.

Stress results from the FEA are examined for two stress directions: the hoop stress on the outside surface of the tank and the bending stress at the shell to floor fillet weld on the inside of the tank. Figure 2 shows an example of hoop stress results on the outside surface of the tank and the inside surface of the tank at the same location. The locations of maximum hoop stress typically occur at the edge of an insert plate or reinforcing pad in the first shell course. Figure 3 illustrates another example, showing that the bending stress at the bottom fillet weld connecting the tank wall to the tank bottom is of concern.

Figure 2. Hoop stress on the outside and inside of the tank at manway.

Figure 3. Bending stress on the inside of the tank near the manway.

 

The material fracture toughness is required to perform an accurate fracture mechanics analysis. When repairs to the tank do not yield any samples for material testing, the material toughness is determined from lower bound estimates described in API 579/ASME FFS-1. Lower bound estimates are designed to be conservative.

 

Once the applied stress and material toughness are established, critical defect sizes for regions of interest can be calculated. For this analysis, Quest Integrity Group’s commercial software Signal™ Fitness-for-Service is used[3]. This is a Windows®-based program that implements methodologies described in the British Standards BS-7910 as well as API 579/ASME FFS-1 Part 9 Level 3. Signal Fitness-for-Service is used to establish critical defect sizes. For this work, the flaw is assumed to be a semielliptical surface crack in a flat plate withv a surface length, “2c” and a depth, “a”. For a semi-elliptical flaw, there is no unique critical flaw size (2c and a). Instead combinations of 2c and a resulting in a critical flaw are plotted as a line on a graph of surface crack length versus crack depth. It is important to select inspection methods that are sensitive enough to detect flaws/defects prior to reaching maximum tolerable sizes.

 

Figure 4 shows an example critical flaw size curve (shown in blue). In the modified tank, the new welds must be inspected visually and by magnetic particle, dye penetrant or radiographic testing. Assuming that ASME standard procedures and acceptance limits are used, linear indications are only considered relevant if the length is greater than three times the width.  Relevant rounded indications greater than 3/16 inches or 0.1875 inches are cause for rejection. A rounded indication is one of circular or elliptical shape with a length equal to or less than three times the width. For this example, using these criteria for acceptance, it is observed that the critical defect size curves established by fracture mechanics analysis methods lie entirely above the relevant rounded indication size. The finite element analysis and fracture mechanics calculations have shown for this example that the repairs made to the tank do not require a hydrostatic test. The tank is considered fit for service so long as no defects of the critical size exist in the region of the new repairs. This statement can be made since the critical defect sizes exceed the defect sizes that would be cause for rejection during inspection of the new welds in the repaired regions.

Figure 4. Critical flaw size curve.

Leak Before Break

The fitness-for-service rules in API 579/ASME FFS-1 include a procedure for a leak-before-break evaluation which is particularly applicable to ammonia storage tanks. Ammonia tanks are ideally suited for leak detection systems due to the cold nature of the ammonia. The leak-before-break evaluation is intended to determine if an existing flaw will likely cause a catastrophic failure or if a leak will be detected prior to failure. In the leak before break evaluation where a small initial flaw is detected during an inspection, fracture mechanics methodology is used to predict the nature of the flaw. Two scenarios are possible: that an existing flaw is detected or that hypothetical flaws exist in the material but are too small to be detected. In either case, a leak-before-break scenario is evaluated.

Like the hydrostatic test exemption, a finite element analysis provides detailed stress results in the tank. Peak stresses exist in several typical regions of a storage tank. These peak stress regions are those most likely to develop cracks. Axial cracks would tend to propagate along the vertical seam welds between shell plates due to the applied hoop stress. Circumferential cracks would tend to propagate cracks along the bottom fillet weld due to the applied bending stress. Cracks of varying orientations may develop around stress risers like nozzles, repads and manways.

Using the stress results at the peak stress locations in the leak before-break evaluation may provide confidence that a leak before- break scenario is achieved and narrows the scope of inspection by defining the most critical regions of the tank.

 

Edge Settlement

 

Tank bottoms and shells can settle for various reasons. API 653-2009 suggests periodically measuring settlement. Figure 5 shows an example of elevation measurements for a tank bottom recorded during a typical internal inspection. For edge settlement exceeding permissible values, API 653-2009 stipulates “…all shell-to-bottom welds and bottom welds should be inspected visually and with magnetic particle examination or liquid penetrant examination. All indications should be repaired or evaluated for risk of brittle fracture and/or fatigue failure prior to returning the tank to service.”

Figure 5. Elevation measurements for a tank bottom.

Often, assessments demonstrate that the repairs are unnecessary, thereby allowing the operator to return the tank to service sooner than would have been possible with repair. The fitness-for-service assessment for tank edge settlement follows procedures that are nearly identical to the procedures previously described for the hydrostatic test exemption. Stresses in the bottom and shell are computed using a finite element model (FEM) incorporating an axisymmetric profile of the tank, settled soil and ring wall (if present). Modeling the contact and separation between the tank bottom and soil as well as the yielding of steel is necessary for an accurate analysis.  Several fill cycles are simulated to assess if the tank is susceptible to fatigue or plastic ratcheting. Through-thickness stress profiles for key locations are obtained from the FEM.  This stress profile is used to establish critical defect sizes of varying dimensions. The data is plotted on a graph of surface crack length versus crack depth. The dimensions for measured flaws and the minimum detectable flaws are plotted on the same graph to determine if these flaws are stable. Appropriate factors of safety should be incorporated to ensure a conservative assessment.

Figure 6 shows radial stress for a tank with edge settlement. In such cases, the critical location is often the underside of the tank bottom opposite the chime weld. Magnetic particle and liquid penetrant testing cannot identify flaws in this location. Measuring flaws in this location requires inspection techniques such as phased array UT along with verification that such small flaws can be detected with this technique. The FEA combined with the FEM narrows the inspection scope for any additional inspection that may be required.

Figure 6. Radial stress in a tank with edge settlement.

This demonstrated capability saves tank owners the cost of jacking and re-leveling the tank, repairing the bottom or other costly repair options and reduces out-of-service time.

References

  1. The American Petroleum Institute, API Standard 653 Tank Inspection, Repair, Alteration & Reconstruction Addendum 2 © American Petroleum Institute April, 2009.
  2. The American Petroleum Institute and The American Society of Mechanical Engineers, Fitness-for-Service API 579/ASME FFS-1 (API 579 Second Edition). © API Publishing Services June 5, 2007.
  3. Signal Fitness-for-Service commercial software, Quest Integrity Group LLC. 2465 Central Ave, Suite 110 Boulder, CO, www.questintegrity.com.

 

 

 

 

 

 

 

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