SIMILAR SERVICE GUIDELINE

Transcription

1 Table of Contents OVERVIEW... 2 SCOPE... 2 WHEN TO USE SIMILAR SERVICE... 2 PERSONNEL... 2 DEFINITIONS... 3 ASSESSMENT METHODOLOGY... 3 PROCEDURE... 4 ASSUMPTIONS IF SIMILAR SERVICE DOES NOT APPLY... 10

2 Overview This document provides updated i guidance for conducting a similar service assessment to establish inspection intervals for tanks for which corrosion rates have not been directly measured or adequately documented in previous records. The intent is that if one tank is similar to another tank according to specific criteria, these two tanks can be considered to be in similar service and hence, come from a population of tanks which can be treated using statistical methods. The combination of matching criteria with expert knowledge and opinion allows the use of Bayesian analysis to assess the likelihood of potential tank failure. Justification for the use of similar service is based on combining expert opinion with data using Bayesian analysis. A similar service assessment offers an alternative to settling for a conservatively short interval due to lack of corrosion information for a tank. The frequency at which formal internal inspections (FII) occur is an important topic as there are competing benefits and consequences of performing too few or too many FIIs. It is important that excessive internal inspections (FIIs which are too short) not be performed. For example, by increasing the inspection frequency, the risk of tank failure is reduced a significant benefit. However, the probability is increased of having other kinds of incidents such as confined space accidents, fire, or significant emissions releases to the environment (both air and water). Scope This document may be used as a guideline on fuel distribution terminals or similar terminals such as breakout terminals where the conditions are relatively uniform and the population of tanks may be considered "similar". For other kinds of tank facilities the principles of these guidelines should be adopted and developed by the Owner/User applying appropriate statistical methods to determine how similar service may be applied. The development of similar service should be conducted with appropriate tank experts and with management approval. This document applies to petroleum storage tanks covered by API 653. When to Use Similar Service This document provides suggested guidelines for conducting a similar service assessment to establish the next formal internal inspection interval (FIII). These guidelines are most applicable in the following situations: A formal API 653 inspection report does NOT exist, A formal API 653 inspection report exists BUT it does NOT specify the next FIII, A formal API 653 inspection report exists and specifies the next FIII BUT it does NOT document the calculation of that interval or the corrosion rates used to determine it, When setting the FIII for a new tank. Personnel These guidelines are intended for use by an Authorized Inspector, Storage Tank Engineer, or other person(s) having knowledge and experience in performing similar service assessments and tank management. However, the final decision to accept its use is the responsibility of the Owner/Operator management. May 1, 2011 Page 2 of 10

3 Definitions Authorized Inspector A tank inspector who meets the certification requirements of API 653 or other acceptable qualification standard (see Section 3.1 of Standard ). The inspector must be technically qualified and certified/licensed for the purpose of conducting tank inspections. Local regulations may require that the inspector be from a government regulatory agency. Candidate Tank(s) The tank on which the similar service assessment is being performed by comparing it with the control tank(s). It does not have available corrosion rate information. Control Tank(s) The tank(s) for which service conditions and corrosion rates are known and well-documented. Reliable past inspection reports are available. Corrosion Rate The rate at which metal loss occurs in the steel plates that make up the tank s structure. Throughout this document, corrosion rates refer to those on both the stock side and soil side of a tank s bottom. Corrosion rates in other contexts could refer to rates of metal loss on any part of the tank. Double Bottom A tank which has an under bottom containment and leak detection. Examples include double steel bottom tanks with leak detection and single steel bottom tanks with HDPE liners and leak detection. Full Match Occurs when all specified internal and external criteria match for a control tank and a candidate tank. Ponding Occurs when standing water accumulates around the tank foundation. Owner/Operator Consists of Terminal Manager, Head Operator, Terminal OE/HES Specialist, and Terminal Engineer. Slope Configuration The type of slope of the tank bottom (e.g. cone down, cone up, shovel (single slope), etc.). Storage Tank Engineer An engineer, contractor, or other tank specialist with sufficient tank experience and knowledge. Water Bottom A layer of water that is maintained below the product inside the tank. Water remains in constant direct contact with the top side of the tank bottom plates. This can accelerate top side corrosion of these plates. Assessment Methodology This guideline presents step-by-step guidance to the user and highlights important assumptions and considerations that facilitate completing a similar service assessment. May 1, 2011 Page 3 of 10

4 It is important to note that the only tank component directly addressed by this similar service guideline is the tank bottom. The methodology only applies to the tank bottom because it is the component that typically governs the remaining life and integrity of a large aboveground storage tank. The tank shell, fixed roof, floating roof, and all appurtenances will NOT be directly evaluated through matching criteria as part of the following procedure. The Owner/Operator shall determine whether these other main tank components are fit for service for the duration of the calculated FIII based on specific knowledge of the tank and previous inspection reports. If this information cannot be reasonably verified, similar service should not be used. It is important to consider the integrity of ALL components including the shell, roofs, roof hose drains, and floating suctions, even though they are not covered by similar service, because it is possible for any of these to fail without warning and cause an unanticipated business interruption or major incident. The Owner/Operator shall exercise caution in this regard when applying the maximum interval allowed by a similar service assessment. Consider the following example: A candidate tank gets assigned an FIII of 15 years based on a similar service assessment. However, the floating roof primary seal in the tank is believed to be worn and is expected to last for only 2 more years. What FIII should be assigned to the tank? The answer is 2 years. However, it would behoove the Owner/Operator to explore alternatives to using the worn floating roof seal, if available. If it is possible to perform an in-service repair, the FIII could be much greater than 2 years, but no longer than the 15 years required based on similar service. Consider another example illustrating a similar point: A new tank is constructed at a terminal. A similar service assessment yields an FIII of 20 years based on the bottom and shell components. The next question should be, will all of the other new tank components be able to last that long? The answer will come from expert knowledge and calculated judgments from the Owner/Operator after evaluating the risk of failure of each component over that time span. No matter the answer, the Owner/Operator should be confident that their decision is the right one. Communication is essential to ensuring reliable results. Procedure Flowchart 1 shows the steps involved in completing a Similar Service Assessment: May 1, 2011 Page 4 of 10

5 1. Data Collection 2. Perform Assessment Flowchart 1 Similar Service Conceptual Overview 3. Determine Evaluation Method 4. Establish the Next Formal Internal Inspection Interval (FIII) for the Candidate Tank 5. Record Assessment Data 6. Obtain management approval and apply new interval. 8. Determine if the Additional Inspection Data Changes the Inspection Schedule for the Candidate Tank 7. Review with New Data Step 1: Data Collection. Gather reports and information on the candidate tank and the control tank(s). You cannot use similar service unless you have complete and reliable reports available for the control tank(s). A complete report shall include adequately documented corrosion rate information and a corresponding FIII calculated in the report. Start collecting data on tanks at the same site as the candidate tank. If necessary, continue to collect data from other tanks in the area. The more similar the total environment for the tanks, the more accurate the assessment will be. Step 2: Perform Assessment. There are two sets of matching criteria used in a similar service assessment: Examination of internal tank surfaces. Examination of external tank surfaces. See below for details on how to determine matches. Each assessment is independent of one another. In fact, two different sets of tanks may be used to perform each of the two parts of the assessment. Internal Surfaces Assessment: For internal surface assessments, the control and candidate tanks may or may not be at the same site. Either way, the candidate and the control tanks must satisfy the following internal criteria: May 1, 2011 Page 5 of 10

6 1. Both tanks must contain the same class of product as classified in Table 1 below. Each tank may have different service histories so specific evaluation periods must be compared to warrant a match for similar service. Table 1 Similar Service Product Classification System Class A B C D E F G H I Products Included Finished lube oil Finished fuels (e.g. mogas, diesel, jet) Avgas Slop & process water (e.g. transmix) Crude oil Ethanol Biodiesel Fire water Other (must be similar products) 2. Both tank bottoms must be coated with the same coating, both must be uncoated, or ONLY the candidate tank is coated. 3. Both tanks carry a water bottom, both tanks do NOT carry a water bottom, or ONLY the control tank carries a water bottom. For this criterion, if the candidate tank will have regular water draws to eliminate a water bottom, the control and candidate tanks match regardless of bottom slope configuration. If not, both tanks must also have the same bottom slope configuration. 4. Both tanks operate at the same temperature. When comparing control and candidate tank operating temperatures, use the average ambient temperatures of the two sites. If the candidate has a higher temperature than the control, then apply a doubling of corrosion rates from the control to the candidate. Apply this doubling factor for every 10 F (18 C) of temperature difference. ONLY when all 4 criteria above are satisfied can similar service be utilized for the candidate tank s internal surfaces. External Surfaces Assessment: For external surface assessments, the control and candidate tanks MUST be located on the same site. The candidate and the control tanks must also satisfy the following criteria: 1. Both tanks have ponding at the foundation, both tanks do NOT have ponding, or ONLY the control tank has ponding. 2. Both tanks have a double bottom or both tanks do NOT have a double bottom. If both tanks have a double bottom, they must also contain the same cushion or fill between the two bottoms. 3. Both tanks are UNheated or both tanks are heated. 4. Both tanks are NOT insulated or both tanks are insulated. 5. Both tanks have cathodic protection, both tanks do NOT have cathodic protection, or ONLY the candidate tank has cathodic protection. ONLY when all 5 criteria above are satisfied can similar service be utilized for the candidate tank s external surfaces. May 1, 2011 Page 6 of 10

7 Step 3: Determine Evaluation Method. Using findings from data collection and the results of the matching assessment (steps 1 and 2 above, respectively), determine which evaluation method applies based on the number of matching control tanks on the same site as the candidate tank. Two methods are available: Same Site Method Use if at least one control tank fully matches the candidate tank from the same site. That is, both portions of the assessment (internal and external) were able to be completed using only control tanks from the same site as the candidate tank. Multi-Site Method Use to evaluate control tanks from multiple sites in the event that there are no full matches from the same site. Note that even for the Multi-Site Method, at least one control tank must be on the same site as the candidate tank in order to complete the external assessment portion. For the internal assessment, tanks from different sites may be used. Only one method may be chosen. The Same Site Method is preferred if applicable. Note: A similar service assessment can be performed using only one control tank. However, utilizing multiple control tanks will improve the reliability of the data set and will permit a longer interval that can be applied to the candidate tank. The Owner/Operator shall take into account ALL applicable control tanks, within reason. Step 4: Establish the FIII for the Candidate Tank. Same Site Method First, gather the FIIIs listed in the inspection reports of all control tanks assume that these FIIIs were formulated correctly in the reports. Next, calculate an average FIII from the collection of control tank intervals. Lastly, to apply this average FIII to the candidate tank, it needs to be adjusted based on the number of control tanks that were used in the assessment. For example, if two control tanks were used, the candidate tank s new FIII will be 95% of the average of the two control tanks FIIIs. See maximum allowable percentages in Table 2. Table 2 Same Site Method, Guidelines for Adjusting FIII # of Control Tanks used in assessment Percentage Adjustment to Candidate Tank FIII 1 90% 2 95% Multi-Site Method First, gather the product side and soil side corrosion rates listed in the inspection reports of all control tanks assume that these rates were formulated correctly in the reports. Next, calculate the harmonic mean of the product side corrosion rate and soil side corrosion rate from the collection of control tank corrosion rates. The harmonic mean is calculated according to: May 1, 2011 Page 7 of 10

8 Where n=number of measurements cr1=corrosion rate 1 cr2=corrosion rate 2 crn=corrosion rate n Example: Suppose that 3 control tanks have corrosion rates of 10,15, and 20 mils per year respectively. The harmonic mean is h(stockside)=3/(1/10+1/15+1/20)=13.85 mils per year or inches per year. Suppose further that the soil side corrosion rates are 12,15, and 19 mils per year. Then h(soilside)=3/(1/12+1/15+1/19)=14.8 mils per year or.0148 inches per year. These corrosion rates are then used in the formula below. Then, use the following formula to calculate the total operating life of the candidate tank based on an original bottom nominal thickness (usually 0.25 inch) and the average corrosion rates just calculated: where O r = expected life of tank t o = original thickness of bottom plates( 0.25 inch is most common) StP r = stock side corrosion rate UP r = soil side corrosion rate MRT=minimum remaining thickness (see API 653 Table 4.4) Continuing the example, Or=( )/( )= years Next, subtract the elapsed time between the current date and the year the tank bottom was built or installed at original nominal thickness and denote this as T. Then subtract T from O r to get O r, the remaining operating life of the tank from the present time until the next required internal inspection: O r = O r T O r is now factored down by a certain percentage due to the use of similar service. O r can be at most 20 years before reduction. Note that if the number of tanks used for the internal assessment is not the same as the number of tanks used for the external assessment, the lesser of the two numbers shall be used in Table 3. Table 3 Multi-Site Method, Guidelines for Adjusting FIII # of Control Tanks used in assessment Percentage Adjustment to Candidate Tank FIII May 1, 2011 Page 8 of 10

9 1 80% 2 90% 3 95% Note: In Tables 2 and 3, the percentage adjustment to the calculated FIIIs from control tank(s) that can be used for the candidate tank is instituted as a factor of safety accounting for the variance of the actual corrosion rate distribution. A similar service assessment involves, by definition, extrapolating data from one tank to another. The matching checks that are in place provide guidance on when similar service is appropriate to apply, but in reality, the tanks are completely separate entities. Therefore, a certain percentage reduction of the extrapolated FIII is necessary and justified. The percentages used are conservative and are based on expert opinion and judgment. In any similar service assessment, the maximum FIII that can be applied to the candidate tank, after adjustment, is 20 years. In addition, based on the tank bottom configuration details, the limits as given in API 653 paragraph for maximum initial intervals shall apply as the maximum value of O r as calculated by similar service unless a risk based analysis shows that these limits do not apply. In no case shall the intervals exceed 20 years when using similar service assessment. Step 5: Record Assessment Data. Once a similar service assessment has been completed, the conclusion and analysis performed shall be properly documented using the Summary Sheet. This form MUST be filed at the terminal with inspection reports and other records for the candidate tank. If an inspection was just completed, attach the Summary Sheet to the Inspector s report. Step 6: Obtain management approval and apply new interval. Following the assessment, evaluation, and recording of the data, management approval shall be obtained before officially applying the new interval to the candidate tank. Management approval is necessary because in some cases, for example, local regulations may actually prohibit the use of similar service. Also, similar service relates only to the likelihood of a corrosion failure. As such, the component of risk due to consequences is not addressed. Together, the likelihood and consequence produce the risk, a measure of which management is highly interested and solely qualified to make the final decision. Steps 7 & 8: Review and Update with New Data. Review a completed similar service assessment when new information becomes available. For example, if an inspection is completed for another tank that could have been used as a control for this candidate tank, the Owner/Operator should review this new information to determine its impact on the FIII established during the similar service assessment. Another example is if one of the control tanks used has an FII after the similar service assessment was completed; this new information may impact the original candidate tank. If the FIII is significantly impacted by the new information and re-performing the similar service assessment would provide business value, the Owner/Operator should establish a more current and accurate FIII for the candidate tank and document it using the Summary Sheet. May 1, 2011 Page 9 of 10

10 Assumptions The use of this guideline is permitted given that there are no known or suspected issues with the tank at the time of the assessment. If the Owner/Operator has any reason to believe that the tank is leaking or has any outstanding issues, DO NOT perform a similar service assessment to extend the FIII. The tank should be inspected immediately. The similar service concept assumes that reports used from the control tank(s) have accurate corrosion information and corresponding inspection intervals. If the Owner/Operator is not confident in the information provided by previous reports for the control tank(s) for any reason, similar service should not be used. If the control tank and candidate tank are both insulated, determine if there are additional considerations to applied in the analysis. If additional information exists that would support using or not using similar service, document it and attach to the summary sheet if it is used in the decision process. For tanks with a cathodic protection system, assume that the bottom corrosion rate is zero as long as the system has been properly designed, maintained, and inspected. When comparing tanks located on the same site, assume that soil type, ph, and other soil characteristics are the same for all tanks on that site unless they are known to be significantly different. Tank Corrosion Rate Data Each tank owner/operator interested in using similar service should keep data from internal inspections that give corrosion rate data and any failures that have occurred as a result of corrosion. This can be used to develop distributions of corrosion rates which can then be applied more accurately than addressed by the approach in this Appendix. Such data can be developed by regional areas as well as by facility so that the probability that the similar service assessment approach is appropriate each time that it is used is even higher than as applied here. Standard statistical methods may be used to develop these distributions from inspection report data. i Updated after the API 653, 4 th edition, April 2009, Annex H, edition May 1, 2011 Page 10 of 10