BASED on inspection results, advanced integrity assessments allow tank operators to postpone repairs, eliminate the need for repairs, or be exempt from hydrostatic testing.
In his presentation, Brian D Rose, a senior consulting engineer at Quest Integrity Group, said tank-integrity assessments have advanced because of improvements in inspection and computing technologies, along with the emergence of fitness-for-service standards. Rose spoke during the International Liquid Terminals Association's 31st Annual International Operating Conference June 6-8 in Houston, Texas.
Advanced inspection technologies, such as ultrasonic testing (UT) and laser scanning, contribute to detailed mappings of corrosion, settlement, bulging, and denting of tank shells and floors.
Rose said that according to API 653's 2010 addendum, a full hydrostatic test, held for 24 hours, should be performed on a reconstructed tank or any tank that has undergone major repairs or alterations, unless exempted for the applicable combination of materials, design, and construction. The owner-operator may utilize fitness-for-service or other appropriate methodology based on established principles and practices to be exempt from hydrostatic testing after repairs are made.
“The drawbacks to hydrostatic testing are that the tank is out of service for a long time, you need thousands of gallons of water — which is in short supply in some regions and is difficult where cold winters cause freezing — and disposal of the water can be very expensive. There are internal corrosion risks, particularly with ammonia tanks, so that's an addedheadache. The benefits of an assessment are that it greatly reduces the cost and provides a quick turnaround.”
He said a fitness-for-service assessment can demonstrate that the tank (with flaws) is fit for service or identify required remediation, which could provide the following options: fit for continued service (as is, in the newly repaired state); fit for service until the next shutdown; monitor or implement an inspection program; rerate (fit-for-service at a lower fill height); repair; or replace.
To inspect for flaws and detectability limits, the tank is assessed by incorporating measured flaw size or flaws sized at the detectability limit (assume the tank has a flaw the same size as the minimum detectable flaw). Detectability limits differ with the inspection technique: magnetic particle testing (MT), dye penetrant testing (PT), shear wave testing, and phased array ultrasonic testing (PA-UT).
“So what do we do with the detectability limit?” he said. “Typically, inspections show that repairs are free of major flaws, so you assume that the repair contains a flaw at the detectability limit. Also, the detectability limit is typically smaller than the critical crack size. So, for the typical case presented here, the assessment demonstrates that the tank is fit for continued service.”
Saving money and time
He gave the real-life example of a refiner with a 1960s asphalt tank with a 117-foot diameter, 42-foot, 4-inch height, and 80 bbl. Repairs performed included a new bottom and annular ring, nozzles with repads and inserts, support repad, 24-inch shell manway with insert, and stacked doorsheets.
The critical flaw size for bottom fillet weld, nozzles and other locations was determined. For this tank, the fracture toughness was over 170 ksi in 0.5, which indicated that the material was very ductile, and therefore had low susceptibility to crack instability.
“The assessment demonstrated that the repairs do not require a hydrostatic test and the tank is fit-for-service, because there were no defects of critical size in the region of the repairs,” he said. “The owner said, ‘You saved me an enormous amount of money. Hydrostatic testing of this tank would have required a minimum of four days to fill with water, six days to empty the water, so I saved a minimum of 10 days.'”
And what if the tank is not fit-for-service?
“Don't despair,” he said. “Obtain a better estimate of fracture toughness instead of relying on lower bound fracture toughness from API 579. First, identify the material of construction by determining the date of construction — older steels tend to contain a fair amount of sulfur, and that makes them more brittle. Find mill certificates or get a positive material identification.”
“Measure the fracture toughness if you can. Charpy V-notch impact test results, often listed in mill certificates, can also be converted to fracture toughness. You should measure fracture toughness on cut-out samples. Use measured strength properties, and decrease the detectability limit by using a higher resolution inspection technique.”
He said hydrostatic testing has traditionally been used to detect cracks. A simplified fracture mechanics analysis is typically used to estimate the largest flaw that survived the test. However, this analysis usually underestimates the critical flaw size. Thus, significantly larger than expected flaws survive the test.
The hydrostatic testing process includes purchasing or acquiring water; filling the tank with water; hydrostatic testing (hold for 24 hours); emptying the tank; cleaning through an on-site water treatment facility and disposing of the water (through the sewer with municipal approval or a heating procedure to remove water in asphalt tanks).
He provided examples of costs for past hydrostatic testing : $24,500 for a 25-foot diameter and $46,500 for an 80-foot diameter tank. That included the costs associated with closing the tank, filling and monitoring, water sampling and testing, approved discharge, opening the tank, squeegeeing out the water, closing the tank, and bolting up the valves and piping. There was no water treatment required for these examples, but for severe cases, water must be transported to a treatment facility following the test.
In comparison, hydrostatic test exemption analysis costs ranged between $2,500 and $5,000, and the average cost ranged from $3,500 and $4,500.
He said a tank settlement assessment is similar to a hydrostatic test exemption and provides a cost savings. API 653 says that “all indications should be repaired or evaluated for risk of fatigue prior to returning the tank to service. … Welds in the area of the high strains should be removed and replaced, or be subjected to a fitness-for-service evaluation by an engineer experienced in settlement evaluation. … In lieu of repairs, a detailed analysis of the settled area may be performed by an engineer experienced in tank design.”
“This allows us to do a fitness-for-service assessment instead of doing repairs,” he said.
Rose said that a hydrostatic test typically is required for major repairs, per API 653. One three-foot area in the tank bottom, away from the shell, could cost $1200 to $1400 due to excessive tank settlement, while an intersecting shell could cost $1400 to $1800. Tank jacking could cost up to $200,000.
As for fitness-for-service, past integrity assessment costs ranged from $10,000 to $20,000, and the associated fracture toughness testing cost around $1300.
For corroded tanks, local thinned areas (LTA) assessments are performed. Stresses in thin areas plastify and stresses redistribute, allowing the tank to carry more pressure than indicated by a “tmin” calculation. The LTA assessment accounts for this phenomenon, resulting in more accurate assessments and eliminating the over-conservatisms associated with a “tmin” calculation.
There are other types of fitness-for-service assessments, including internal localized depressions or bulges, shell bulging/denting, and fire-damage assessment.
“Planning ahead for an assessment and completing it along with an inspection can speed up the decision-making process,” he said. “Quantitative assessment assists in the decision making process and can save a lot of money. Prepare for an inspection. Having an assessment engineer on site or on call can significantly speed up the assessment so that important decisions can be made as soon as possible.”
“All documentation and material samples should be stored and labeled when repairs are made. Assessment results will be more accurate based on these past records.” ♦