581_O - Aboveground Storage Tanks

7/22/2019 581_O - Aboveground Storage Tanks

1/61

TECHNICAL MODULE

ABOVEGROUND STORAGE TANKS

DNV BASE RESOURCE DOCUMENT

API 581, CHAPTER O

DETNORSKE VERITAS


2/61

American Petroleum Institute i

Aboveground Storage Tanks RBI Module

API 581 Appendix O.doc 11 October 2001

CONTENTS

1. SCOPE ..........................................................................................................................................1

1.1 Overview of Frequency Analysis ............................................................................................1

1.2 Overview of Consequence Analysis .......................................................................................1

1.3 Objective and Overview of Risk Analysis..............................................................................2

1.3.1 Quantitative Risk..............................................................................................................2

1.3.2 Qualitative Risk................................................................................................................2

2. REQUIRED DATA AND LIMITATIONS................................................................................5

2.1 Limitations................................................................................................................................6

3. FREQUENCY ANALYSIS METHODOLOGY.......................................................................7

3.1 Base Failure Frequency............................................................................................................7

3.2 Basic Assumptions ...................................................................................................................9

3.3 Soil Side Corrosion Rate........................................................................................................11

3.4 Product Side Corrosion Rate .................................................................................................14

3.5 Determination of Tank Bottom Leak Frequency .................................................................17

3.6 Summary Leak Frequency Calculation..............................................................................20

3.7 Rapid Bottom Failures ...........................................................................................................24

4. TANK SHELL LEAK FREQUENCY......................................................................................25

4.1 Failure Frequency...................................................................................................................25

4.2 Tank Shell Excluded ..............................................................................................................26

5. APPLICATION AND EXAMPLES.........................................................................................27

5.1 Similar Service........................................................................................................................27

5.2 Measured Corrosion...............................................................................................................27

5.3 Repair and Replacement ........................................................................................................28

5.4 Examples Likelihood Calculation......................................................................................28

5.5 Likelihood of Failure Calculation Flow Chart ..................................................................32

6. CONSEQUENCE ANALYSIS METHODOLOGY................................................................34

6.1 Bottom Leaks..........................................................................................................................35

6.1.1 Foundation Conditions................................................................................................. 35

6.1.2 Three-Dimensional Flow.............................................................................................. 38

6.1.3 Consequence Analysis Methodology........................................................................... 38


3/61

American Petroleum Institute ii



6.2 Shell and Fitting Leaks...........................................................................................................41

6.3 Rapid Shell and Floor-to-Shell Failures................................................................................41

7. REPAIR, REPLACEMENT, AND BUSINESS INTERRUPTION COSTS.........................42

7.1 Downtime Consequence Costs..............................................................................................42

8. CONSEQUENCE CALCULATION SUMMARY..................................................................43

9. RISK ANALYSIS METHODOLOGY.....................................................................................46

9.1 The Risk Scoring System.......................................................................................................46

9.2 The Risk Matrix......................................................................................................................48

9.3 Risk Calculations....................................................................................................................49

9.4 Steps in Conducting an AST Risk Assessment....................................................................50

9.5 Risk Results ............................................................................................................................51

9.6 Risk Assessment.....................................................................................................................51

10. INSPECTION PLANNING.......................................................................................................53

10.1 Objective.............................................................................................................................53

10.2 Inspection Planning Criteria ..............................................................................................53

10.3 Manual Inspection Planning ..............................................................................................53

10.4 Automated Inspection Planning ........................................................................................53

10.4.1 Inspection (Equipment Level only): ............................................................................. 53

10.4.2 Target (Batch and Equipment Level):.......................................................................... 54

10.4.3 Automated Inspection Planning In the ABPI RBI Software.................................... 54

11. REFERENCES ...........................................................................................................................57


4/61

American Petroleum Institute 1



1. SCOPE

The Aboveground Storage Tank (AST) Module consists of three parts: (1) FailureFrequency Analysis, (2) Consequence Analysis, and (3) Risk Analysis. The basicapproach used in the Aboveground Storage Tank Module is to modify a generic failure

frequency for tank bottom failures by a factor related to both the potential degradationoccurring in the particular service and the type of inspection performed.

1.1 Overview of Frequency Analysis

The estimation of a components leak frequency is found, for most items, using amodifying factor to adjust a base (generic) failure frequency. This modifier is referred toas the modifying factor. In mathematical terms, the leak frequency is found using thefollowing expression:

FactorModifyingFrequencyFailureBaseFrequencyLeak =

Equation 1-1

When necessary, leak frequencies are combined to produce an overall equipment spillfrequency. The scenarios in the risk model dictate how to combine componentfrequencies for an item. For each component, the likelihood of various hole sizes isrequired as input to the risk analysis for each scenario. As a result, the fraction of leaksof a given size are also derived as part of the frequency analysis.

1.2 Overview of Consequence Analysis

The consequence of a spill is measured in dollars and consists of environmental clean-upcosts, environmental penalties, repair costs, and lost opportunity costs.

Total Cost = Envi ronmental Clean-up Costs + Environmental Penalti es

+ Repair Costs + Lost Opportuni ty Costs

Equation 1-2

The basic approach to estimating the environmental clean-up costs of a scenario is to addthe cost for the various clean-up methods needed to remediate a spill. For instance, if aspill leads to groundwater contamination, the components of the cleanup may consist ofsoil remediation onsite, soil remediation offsite, and groundwater clean up. Eachcomponent has a Clean-Up Factor (CUF) that is based on the location of the spill and thetype of material spilled. The unit of measure for the CUF is dollars per barrel ($/bbl).

In mathematical terms, the cost for each component of the environmental clean-upoperation is expressed as follows:

Envir onmental Clean-Up Cost = Volume CUF

Equation 13


5/61




The costs for environmental penalties, repair costs, and lost opportunity costs are input bythe user.

1.3 Objective and Overview of Risk Analysis

The overall objective of this technical module is to develop a practical risk assessmentprocess applicable to Aboveground Storage Tanks (ASTs) to assist in the selection ofcontrol measures to prevent liquid releases. To satisfy this objective, both a quantitativescoring system and a risk matrix were developed to estimate and display risks and toassist the user in selecting control measures. Some typical control measures mightinclude inspections, internal lining, and repair/replacement of the tank bottom.

1.3.1 Quantitative Risk

One way to portray risk quantitatively is to produce a point-estimate of risk from theconsequence-frequency data pair. This is usually done by multiplying the likelihood andconsequence data points together to produce a measure with units of consequence per

year. The mathematical expression for this score is as follows:

Equation 14

Multiplying likelihood and consequence together is convenient because it reduces the riskmeasure to a single point. The single risk point is often referred to as the expected valueof risk for a scenario, and it can be thought of as a probability-weighted consequenceestimate.

1.3.2 Qualitative Risk

The above method portrays risk in quantitative terms. As an alternative, risk could berepresented in qualitative terms, such as a low, medium or high risk. The qualitativeassessments of likelihood and consequence can be assigned to categories. For instance, alow probability might be placed in Category 1, and a medium consequence might beassigned Category C. These values can then be displayed in a matrix. Figure 1-1 showsa risk matrix displaying five levels of likelihood and five levels of consequence.

eConsequencLikelihoodRisk =


6/61




5

4

3

2

1

A B C D ECONSEQUENCE

Plot of a scenario

with a rating of C-1

Figure 1-1: The Five-By-Five Risk Matrix

Risk increases from the lower left corner to the upper-right corner of the matrix. So, E-5would be the highest risk point on the matrix, and A-1 would be the lowest.

Levels of risk can be expressed in a matrix by assigning risk-levels to the various squaresin the matrix. It is important to note that assigning risk-levels to squares on the matrix isa reflection of the companys policies and attitudes about risk acceptability. Manycompanies choose not to assign levels of risk within a matrix. If a company assigns doesso, then decisions can be made regarding the disposition of various scenarios. Figure 1-2provides an example of risk-levels assigned in a five-by-five matrix.

5

4

3

2

1

A B C D ECONSEQUENCE

medium risklow risk

medium-high risk high risk

Figure 1-2: Risk Matrix Showing Levels of Risk


7/61




The matrix shown in Figure 1-2 portrays risk as neutral to likelihood or consequence.For instance, risk point C-1 has the same level of risk as A-3. To reflect aversion to oneof the two elements of risk, the risk levels represented by the shaded areas are shifted, asshown in the figure below. In Figure 1-3, an aversion to consequence is shown byassigning a higher risk level to higher consequences for some levels of likelihood.

5

4

3

2

1

A B C D E

CONSEQUENCE

medium risklow risk

medium-high risk high risk

Figure 1-3: A Risk Matrix Showing Consequence-Aversion

When compared to the unbiased matrix in Figure 1-2, note that risk point C-1 is assigned

a risk level of medium, rather than low. Other blocks on the matrix are changed toreflect an aversion to consequence in Figure 1-3.


8/61




2. REQUIRED DATA AND LIMITATIONS

The basic data listed in Table 2-1 is the minimum required to determine a modifying factorfor thinning when a corrosion rate has not been established by one or more effectiveinspections.

Table 2-1: Basic Data Required for Bottom Leak Analysis

Basic Data Comments

Age (years) The number of years that the equipment has been exposed to the current process conditionsthat produced the corrosion rate used below. The default is the equipment age. However, ifthe corrosion rate changed significantly, perhaps as a result of changes in processconditions, the time period and the thickness should be adjusted accordingly. The time

period will be from the time of the change, and the thickness will be the minimum wallthickness at the time of the change (which may be different from the original wallthickness).

Bottom External CorrosionRate (mpy)

The expected or observed corrosion rate for a typical tank under average conditions, i.e.neither highly susceptible to corrosion nor especially resistant to corrosion.

Bottom Internal CorrosionRate (mpy)

The expected or observed internal corrosion rate of the tank bottom.

Bottom Thinning Type(Widespread or Localized)

Determine whether the thinning is widespread or localized for inspection results of effectiveinspections. Widespread corrosion is defined as affecting more than 10% of the surfacearea and the wall thickness variation is less than 50 mils. Localized corrosion is defined asaffecting less than 10% of the surface area or a wall thickness variation greater than 50 mils.

Bottom Type Single or Release Prevention Barrier (RPB).

Cathodic Protection The existence of a cathodic protection system for the tank bottom, and the properinstallation and operation of such a system, based on API 651.

Inspection RatingCategory

The rating category of each inspection that has been performed on the equipment during thetime period (specified above).

Internal Lining Age

(years)

Based on the installation date, or the last date of lining rehabilitation.

Internal Lining Needed Yes or No. Is a lining needed to protect the tank bottom and shell from the corrosive natureof the product?

Number of Inspections The number of inspections in each rating category that have been performed during the timeperiod (specified above).

Operating Temperature(F)

The highest operating temperature expected during operation (considering both normal andunusual operating conditions).

Soil Resistivity (ohmcm) Soil resistivity under the tank or dike field. (A common method of measuring soilresistivity is described in ASTM G 57.)

Tank Drainage The effectiveness with which rain water is drained away from the tank, and prevented from

collecting under the tank bottom.

Tank Pad The type of material upon which the tank rests. In the case of a tank supported on a ringwall, the material used for fill inside the wall.

Tank Steam Coil Heater Yes or No. If a steam coil heater is utilized, the internal corrosion is adjusted upwardsslightly due to extra heat, and the possibility of steam leaks.

Thickness (mils) The actual measured thickness upon being placed in the current service, orthe minimumconstruction thickness. The thickness used must be the thickness at the beginning of thetime in service reported below.

Water Draws Water draws when consistently used can greatly reduce the damaging effects of water at thebottom of the tank.


9/61




2.1 Limitations

The following limitations apply to this technical module.

The module is only applicable for aboveground atmospheric storage tanks with

carbon steel floors.

Product is assumed to be hydrocarbons (gasoline, diesel oil, crude oil, fuel oil, etc.).The representative fluids that can be assigned are:

Table 2-2 Available Representative Fluids

ABI RBI Fluid Group AST fluid description API RBI Type fluids included

C6-C8 Gasoline Gasoline, Naphtha, Heavy Naphtha, LightStraight Run, Heptane

*EE; HF; PO; EEA; Methanol; Styrene;

AromaticsC9-C12 and C13-C16 Diesel Oil Diesel, Kerosene

C17-C25 Fuel and Crude Oil Jet Fuel, Atmospheric Gas Oil, TypicalCrude, Vacuum Column Top, Light Vacuum

Gas Oil

*Acid (Low, Med and High)

C25+ Asphalt Residuum, Heavy Crude, Heavy VacuumGas Oil

*EG; EO

(*) Denoted items are liquid groups that are not specified under any of the default hydrocarbongroups in the API RBI Software (November 2001), but based on the viscosity @75F can beassociate to the listed group the most conservative (lowest) viscosity value is used (gives thehighest consequence). Reference GPSA and Perries Handbook of Chemical Eng. for Viscosityvalues.

Failure mechanism is generally assumed to be corrosion thinning from product andsoil side. The one exception is that brittle fracture of the shell, or shell to floor joint,is included. Failure mechanisms such as cracking and bulging are not considered.

Vapor space corrosion is not specifically addressed since it does not generally lead toa loss of containment. Repair costs and lost opportunity costs can be included byadding a separate line in the AST Risk Scoring Table (Table 9-1).

Consequence does not consider Toxicity and Fatality issues. The consequence andrisk are expressed in US$. The cost contributors to the risk are Financial Risk andEnvironmental Clean-Up. The Financial Risk is the accumulated cost related toEquipment Damage, Outage time (lost business opportunities), Repairs andEnvironmental Penalties.


10/61




3. FREQUENCY ANALYSIS METHODOLOGY

3.1 Base Failure Frequency

The base failure frequency for the leak of a tank bottom was derived primarily from ananalysis of the American Petroleum Institute publication A Survey of API Members

Aboveground Storage Tank Facilities, Health and Environmental Affairs Department,July 1994. The analysis focused on the ASTs that were operated at refineries across theUnited States during 1983-1993. Sixty-one refineries provided data on over 10,000storage tanks which represents over 80% of all such tanks operated by refineries in theUnited States.

Figure 3-1 shows the number of tanks of each size included in the survey.

0

500

1000

1500

2000

2500

3000

1,000 10,000 50,000 100,000 > 100,000

CAPACITY (BARRELS)

NUMBER OF TANKS

Figure 3-1: Survey of Storage Tanks


11/61




One of the most significant findings of the survey was that tank bottom leaks contributingto soil contamination had been cut in half in the last five years compared to the first fiveyears covered by the survey. This was attributed to an increased awareness of theseriousness of the problem, and to the issuance of the API 653 standard for abovegroundstorage tank inspection. Table 3-1 shows the highlights of the survey results.

Table 3-1: Summary of Survey Results

Population Description Number of

tanks

Percent with

leaks in bottom

in last five years

Number with

leaks in bottom in

last five years

Tank Years* Bottom leak frequency

(1988 1993)

Tanks < 5 years old 466 0.9% 4 2330 1.7 10-3

Tanks 6 15 years old 628 3.8% 24 3140 7.6 10-3

Tanks > 15 years old 9204 3.8% 345 46020 7.5 10-3

All tanks in survey 10298 3.6% 373 51490 7.2 10-3

* Tank years = number of tanks average number of years in service

A bottom leak frequency of 7.210-3 leaks per year was chosen as the base leakfrequency. Although the leak frequency data in Table 3.1 indicates that tanks less than 5years old have a much lower leak frequency, it was decided to use the whole surveypopulation in setting the base leak frequency. The age of the tank is elsewhere accountedfor in the model since the percent wall loss is a function of the tank age, corrosion rate,and original wall thickness. The percent wall loss is the basis of the modifier on the baseleak frequency. Thus a very young tank with minimal corrosion will have a frequencymodifier less than one which will lower its leak frequency accordingly.

The survey did not report the size of leaks, but an informal survey of sponsors for the APIRBI project indicated that leak sizes of up to " in diameter would adequately describethe vast majority of tank bottom leaks. Rapid bottom failures (or failures at thebottom/shell interface) although rare, do occur. Based on DNVs experience and theexperience of the committee members, an expected frequency distribution of each leaksize is presented in Table 3-2.

Table 3-2: Base Leak Frequencies for Tank Bottom

Hole sizes Percentage Frequency

(per year)

Small Bottom Leak () 99.72% 7.20 10-3

Rapid Bottom Failure 0.28% 2.00 10-5

Total 100% 7.22 10-3


12/61


13/61




ProductSide

Establish BaseCorrosion Rate forSoil Side Corrosion(5 mpy)

- Average time toleakage

-Thickness- "Default" = 5 mpy

Adjust for SoilConditions (0.66-1.5)

Resistivity, SeeTable 3.4

Adjust for Tank PadMaterial (0.7-1.5)

Tank Pad Type,See Table 3.5

Adjust for Drainage(1.0-1.4)Drainage, SeeTable 3.6

Adjust for CathodicProtection (0.33-1.0)

Cathodic Protection,See Table 3.7

Adjust for Bottom

Type (1.0-1.4)

Establish BaseCorrosion Rate forInternal BottomCorrosion (2-5 mpy)

- Inspection Data

-BS&W-pH, etc.-See Table 3.10

Adjust for LiningAge (0.66-2.5)

Adjust for OperatingTemperature(1.0-1.4)

Adjust for SteamCoil Heater(1.0-1.15)

Is Product Side BottomCorrosion Widespread

or Localized?

Sum Corrosion Rates

Product SideCR Type isWidespread

Use the Greater ofCorrosion Rates

Calculate Modified Soil SideCorrosion Rate(Always Localized)

Adjust for OperatingTemperature(1.0-1.4)

Calculate Modified ProductSide Corrosion Rate(Widespread or Localized)

Adjust for InternalLining (0.3-1.75)

Lining needed? Applied

according to API 652?

See Table 3.12

Lining age? Applied

according to API 652?

See Table 3.13

Bulk FluidTemperature, See

Table 3.14

Use of Steam CoilHeater, See

Table 3.15Bottom Design,See Table 3.8

Bulk FluidTemperature, SeeTable 3.9

Adjust for WaterDraws (0.6-1.0)

Water Draw, SeeTable 3.16

Calculate ar/t for lookingup Modifying Factor in

Table (3.18)

Product SideCR Type isLocalized

SoilSide

Figure 3-2: Flow Chart to Determine Modifying Factor for Tank Bottoms


14/61


15/61


16/61




Table 37: Adjustment for Cathodic Protection

Functional Cathodic Protection in Place? Adjustment Factor

NO 1

YES (not per API 651) 0.66

YES (installed and maintained per API 651) 0.33

Adjust for Bottom Type, Single or RPB

Tanks with properly installed release prevention barriers (RPBs) tend to have bottomcorrosion rates comparable to those with a single bottom. Both tanks with a singlebottom and those with RPBs installed according to API 650 have an adjustment factor of1 while a tank with a non-API 650 RPB is given an adjustment factor of 1.4.Adjustments for bottom type are provided in Table 3.8.

Table 38: Adjustment for Bottom Type

Bottom Type Adjustment Factor

RPB (not per API 650) 1.4

RPB (designed and maintained per API 650) 1

Single bottom 1

Operating Temperature Adjustment

The operating temperature of the tank may influence external corrosion. Below 75F, thefactor is neutral (1). For temperatures between 75F and 150F, the factor is 1.1. If theaverage operating temperature is between 150F and 200F, the factor is 1.3. Fortemperatures between 200F and 250F, the factor is 1.4. Above 250F, the factorreturns to 1. Table 3.9 gives corrosion rate adjustment factors for bulk fluidtemperatures.

Table 39: Adjustment for Fluid Temperature

Bulk Fluid Temperature (F) Adjustment Factor

75 1

76 150 1.1

151 200 1.3

201 250 1.4

>250 1


17/61




3.4 Product Side Corrosion Rate

Establish Base Corr osion Rate for Product Side (I nternal) Corrosion

Tank bottoms can corrode from the inside of the tank as well as the outside. Basecorrosion rates for product side corrosion can be obtained from previous internalinspection data, or may be assumed to approximate the corrosion in the lower inch or twoof the shell, if significant bottom sediments and water (BS&W) are present. For dryproduct tanks, the internal corrosion can be insignificant. Table 3.10 shows the suggestedbase corrosion rates.

Table 310: Product Side Base Corrosion Rates

Product Condition Base Corrosion

Rate (mpy)

Dry 2

Wet 5

A summary of the conditions assumed for the product side base corrosion rate is given inTable 3.11 below.

Table 311: Summary of Conditions for Base Product Side Corrosion Rate

Factor Base Corrosion Rate Conditions

Internal lining Internal lining not needed for corrosion protection andnone applied

Bulk fluid temperature Below 75F

Steam coil heater No

Water draws No (Water draws conducted neither weekly nor after every receipt)

Adjust for I nternal Li ning (Coating)

To protect the tank bottom from internal corrosion, a lining may be needed. A lining is acoating bonded to the internal surfaces of a tank to serve as a barrier to corrosion by thecontained fluids. If an internal lining is needed, the adjustment factor is 1.15, if not, thefactor is 1. If the required lining is applied in accordance with API 652 then there is a

further reduction to 0.5 as shown in Table 3.12. The table also shows the benefit ofapplying an internal lining when none is required (0.3 0.6) and the demerit of failing toapply a lining when needed (1.75). Further adjustment is made based on the age of thelining, as illustrated in Table 3.13a. If there is no lining, then Table 3.13a is ignored andonly one adjustment factor is used either 1 or 1.75 from Table 3.12.

If a Liner is applied, the lining factor is set to 1 and the Liner adjustment factor is derivedform table 3.13b


18/61


19/61


20/61




3.5 Determination of Tank Bottom Leak Frequency

Estimate I nternal and External Cor rosion Rates

The internal and external corrosion rates are estimated by multiplying the base corrosion

rate by the respective adjustment factors. This will produce two separate corrosion ratesthat are combined as described below. It is assumed that the soil side corrosion will belocalized in nature while the product side corrosion will be either widespread orlocalized.

Combine Corr osion Rates

If the internal corrosion is widespread in nature, the corrosion areas will likely overlapsuch that the bottom thickness is simultaneously reduced by both internal and externalinfluences. In this case, the internal and external rates are additive.

For pitting, the chances are low that internal and external rates can combine to produce anadditive effect on wall loss. In this case, the user chooses the greater of the two corrosionrates as the governing rate for the proceeding step.

I nspection Rating Category

Inspections are rated according to their expected effectiveness at detecting corrosion andcorrectly predicting the rate of corrosion.

Table 3.17 provides inspection ratings for different inspection activities for the soil side andproduct side of the tank bottom. The guidelines are to be applied twice, once for the soilside, and once for the product side.


21/61


22/61




Determination of Number and Eff ectiveness of I nspections

The rating of each inspection performed within the designated time period must becharacterized in accordance with Table 3.17. The number and effectiveness of theinspections is used for deriving the appropriate modification factor in Table 3.18. The

highest rated inspections will be used to determine the modifying factor. If multipleinspections of a lower rating have been conducted during the designated time period, theycan be equated to an equivalent higher rated inspection effectiveness in accordance withthe following relationships:

1Ainspection = 2Binspections

1B inspection = 2 Cinspections

1 C inspection = 2Dinspections

An example of how to apply this rule is as follows. Suppose you conducted threeinspections for a given tank. Suppose that one of the three inspections was rated as A,while the second and third inspections were rated as B and C respectively. This wouldresult in a combined number and effectiveness of 1.75A inspections. The modificationfactor from Table 3.18 will be interpolated between the 1A and 2A columns.

Determination of M odif ying Factor

To determine the final modifying factor for the tank bottom, a dimensionless quantity,known as the ar/t value is estimated, and a table is consulted to look up the modifyingfactor for the generic failure frequency.

The ar/t is found as follows:

thickness

rateagetar

=/

Equation 3-1

where ais the age of the equipment, in years; ris the maximum corrosion rate in mpy;and tis the original thickness of the tank bottom, in mils. The ar/t method assumes thatthe corrosion rate r is constant over the life of the tank. The value, ar/t, is actually the

fraction of the original tank bottom that has been lost due to corrosion.

The calculated ar/t, and the combined number and rating of inspections, are used todetermine the modifying factorMFar/tfrom Table 3.18.

I nternal L iners

Internal liners (e.g. fiberglass liners) can prevent leaks even if there is a hole in the flooritself. A properly installed liner within its warranted life can be assumed to reduce the


23/61




leak frequency by a factor of 5, i.e. MFFiberglass Liner= 0.2, if FRP type liner is installed.The adjustment factor varies with liner condition or age, if condition is unknown. Thefactors are given in Table 3.13b.

LinerFiberg lasstar MFMFyearFrequencyLeakBottomSmall = /3 )/102.7(

Determination of Tank Specif ic L eak F requencies

The leak frequency for a specific tank is obtained by multiplying the base leak frequencyfor tank bottoms (Table 3.1) by the modifying factor obtained from Table 3.18 and the

Internal Lining factor from Table 3.13b.

3.6 Summary Leak Frequency Calculation

A summary of the steps required to determine the tank bottom leak frequency is

presented below:

(1) Determine the base leak frequency for tank bottoms.

(2) If the maximum corrosion rate is known from an A or B Level inspection, then usethat corrosion rate and skip to step 6.

(3) Start with an estimate of the base corrosion rate for the soil side of the tank bottom andmultiply that rate by the following factors:

(a) Soil conditions

(b) Tank pad

(c) Drainage

(d) Cathodic protection(e) Bottom type

(f) Operating (fluid) temperature

(4) Start with an estimate of the base corrosion rate for the product side of the tank bottomand multiply that rate by the following factors:

(a) Existence of internal lining

(b) Age of internal lining

(c) Operating (fluid) temperature

(d) Steam coil heater

(e) Water draws(5) If the corrosion is widespread in nature, add the two corrosion rates (one for product

side and one for soil side). If product side corrosion is localized, use the greater of thetwo corrosion rates.

(6) Look up the modifying factor in Table 3.18. Use the ar/t value, number of inspections,and rating of inspections to determine the modifying factor. In consulting the ar/t tablethe number and rating of the soil side inspections take precedence in those caseswhere the corrosion is additive.


24/61


25/61




Table 318a: Tank Bottom Modifying Factors

Number of Inspections

0 1 2 3 4

ar/t E D C B A D C B A D C B A D C B A

0.05 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.10 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.150.98 0.3 0.07 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.20 2.62 1.05 0.34 0.06 0.01 0.07 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.25 5.3 2.56 1.05 0.27 0.02 0.32 0.07 0.01 0.01 0.04 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.30 9.08 5.03 2.48 0.84 0.09 0.99 0.29 0.04 0.01 0.2 0.03 0.01 0.01 0.04 0.01 0.01 0.01

0.35 13.9 8.57 4.85 2.04 0.36 2.38 0.9 0.19 0.01 0.68 0.16 0.02 0.01 0.18 0.03 0.01 0.01

0.40 19.8 13.3 8.35 4.18 1.03 4.78 2.21 0.66 0.05 1.79 0.58 0.09 0.01 0.62 0.13 0.01 0.01

0.45 26.6 19.1 13.1 7.51 2.43 8.45 4.58 1.76 0.24 3.89 1.59 0.38 0.02 1.67 0.5 0.07 0.01

0.50 34.3 26 19.2 12.2 4.93 13.6 8.35 3.92 0.82 7.36 3.64 1.18 0.13 3.78 1.45 0.3 0.02

0.55 42.7 34 26.5 18.5 8.93 20.3 13.8 7.63 2.22 12.5 7.22 2.99 0.52 7.43 3.49 1.02 0.11

0.60 51.8 43 35.2 26.4 14.8 28.6 21.1 13.3 5.07 19.7 12.8 6.46 1.67 13.1 7.26 2.8 0.49

0.65 61.5 52.8 45 36 22.7 38.4 30.4 21.3 10.1 28.8 20.7 12.3 4.37 21 13.4 6.45 1.71

0.70 71.6 63.4 56 47.1 33 49.7 41.5 31.8 18 40 31.1 21.1 9.69 31.5 22.4 13 4.81

0.75 82.2 74.8 68 59.6 45.6 62.4 54.5 44.8 29.5 53.1 44.1 33.2 18.8 44.5 34.6 23.2 11.3

0.80 93.1 86.7 80.8 73.5 60.4 76.1 69.1 60.2 44.7 67.9 59.6 48.8 32.7 59.9 50.2 37.9 23

0.85 104 99.2 94.5 88.5 77.5 90.8 85.1 77.7 63.6 84.2 77.2 67.7 52 77.4 68.9 57.3 41.3

0.90 116 112 109 105 96.4 106 102 96.9 86.1 102 96.6 89.5 76.7 96.8 90.3 81 67

0.95 127 125 124 121 117 122 120 117 111 120 117 113 106 117 114 109 100

1.00 139 139 139 139 139 139 139 139 139 139 139 139 139 139 139 139 139

A, B, C, andDrefer to the rating (effectiveness) of the inspections. Eindicates that there have been no inspections. The range of the factors is derived

from the generic failure frequency (GFF=7.210-3), such that 1/GFF = 139, which equates to hole-through of the tank floor. The lowest factor of 0.01indicates that even under the best conditions, a tank can not be considered better than 1% of the average probability of failure.


26/61


27/61


28/61




4. TANK SHELL LEAK FREQUENCY

4.1 Failure Frequency

The leak frequency for tank shells is based on the experience of one of the majoroperating companies. All of their shell leaks are of the variety that wet the outside of the

tank; however, the vast majority of the leaks do not reach the ground before they arecleaned up and the tank repaired. Thus, for shell leaks only two categories will beconsidered: (1) small shell leaks of 1/8" or larger that reach the ground, and (2) rapid shellfailures. These leak frequencies are shown in Table 4-1. A failure rate for rapid shellfailures was determined separately based on actual incidents, as noted below.

A review of literature produced reports of two rapid shell failures in the petroleum industryin the United States in the last thirty years:

1. 1971 (location unknown), brittle fracture caused loss of 66,000 bbl crude oil

2. 1988 Ashland Oil, PA, brittle fracture caused loss of 96,000 bbl diesel

One could argue that this set of data may be incomplete. It is difficult to imagine,however, that a catastrophic failure of a large storage tank could escape the attention ofthe national news media.

The number of tanks that provided the basis for the two failures was estimated fromliterature to be about 33,300 large storage tanks. This value was based on a 1989 studycarried out for API by Entropy Ltd. Large, in this case, is defined as having a capacitygreater than 10,000 barrels. The number of tanks represents the total in the Unites Statesfor the refining, marketing, transportation, and production sectors. Thus the total numberof tank years was found to be approximately 1,000,000.

Dividing the number of failures by the number of tank years yields a rapid shell failurefrequency of 210-6 per tank year. However, similar to the tank floor to shell rapidfailure, the shell rapid failure probability is adjusted for whether or not the tank ismaintained in accordance to API 653. This causes the frequency factor to skew slightlytowards rapid failures.

A summary of the base shell leak frequencies is provided in Table 4-1.

Table 4-1: Leak Frequencies for Tank Shell

Hole Sizes Percentage Frequency

(per year)

Small Shell Leak 96.1% 1.00 10-4

Rapid Shell Failure Maintained to API 653 0.1% 1.00 10-7

Rapid Shell Failure Not Maintained to API

653

3.8% 4.00 10-6

Total 100% 1.04 10-4


29/61


30/61


31/61


32/61




Base Bottom Leak Frequency: 0.0072/year

Base Corrosion Rate (Soil Side): 5 mpy (localized)

Base Corrosion Rate (Product Side): 5 mpy (widespread)

Inspections: None

Tank Bottom Soil Side Corrosion Rate

The base corrosion rate for the soil side of the tank bottom is modified by six adjustmentfactors:

Base corrosion rate (Table 3-3): Default CR Base= 5mpy

Soil Conditions (Table 3-4): 600 ohm-cm AFSoil Cond.= 1.25

Tank Pad Material (Table 3-5): Construction grade sand AFTank Pad= 1.15

Drainage (Table 3-6): Storm water does not collect at tank base AFDrainage= 1.0

Cathodic Protection (Table 3-7): Yes; but not per API 651 AFCath. Prot.= 0.66

Bottom Type (Table 3-8): Single bottom AFBottom= 1.0

Operating Temperature (Table 3-9): 100oF AFOper. Temp.= 1.1

The adjusted corrosion rate (r) is calculated as follows:

r = CRBaseAFSoil Cond.AFTank PadAFDrainageAFCath. Prot.AFBottomAFOper. Temp.

Thus:

mpyr 22.51.10.166.00.115.125.10.5 ==

Tank Bottom Product Side Corrosion Rate

The base corrosion rate for the product side of the tank bottom is modified by fiveadjustment factors:

Product condition (Table 3-10): Wet CR Base= 5mpy

Internal Lining Needed (Table 3-12): Yes, applied per API 652 AFLining= 0.5

Lining Age (Table 3-13): 10 years AFLining Age= 1.0

Operating Temperature (Table 3-14): 100oF AFOper. Temp.= 1.1

Steam Coil Heater (Table 3-15): No AFCoil Heater= 1.0


33/61




Water Draws (Table 3-16): Yes, after every receipt AFWater Draws= 0.6

The adjusted corrosion rate (r) is calculated as follows:

r = CRBaseAFLiningAFLining AgeAFOper. Temp.AFCoil HeaterAFWater Draws

Thus:

mpyr 65.16.00.11.10.15.00.5 ==

Combine Corrosion Rates

Product side corrosion is widespread (generalized), i.e. the combined corrosion rate is:

mpympympyr 87.665.122.5 =+=

Calculate ar/t for use in ar/t lookup table:

( )( )275.0

250

87.610/ ==

mils

mpyyearstar

Then using the ar/t table for tank bottoms (Table 3.18) for an E inspection (i.e. no

inspection) the modifying factor can be determined by interpolation.

19.7)25.0275.0()25.030.0(

)3.508.9(3.5/ =

+=tarMF

Tank Bottom Leak Frequency = 7.19 0.0072 = 5.2 10-2/year

Rapid Bottom Failure Frequency

Designed according to API 650 and maintained according to API 653: MFDesign= 0.5

Corrosion modification factors is calculated as36.0)2.0,20/19.7( ==MAXMF

Corrosion

Thus,

Rapid Bottom Failure Frequency = 2 10-5/ year 0.5 0.36 = 3.6 10-6/ year


34/61




Similar to Table 3-2, the hole size distribution for bottom leaks would be:

Leak Frequencies for Tank Bottom

Hole sizes Percentage Frequency

(per year)

Small Bottom Leak 99.993% 5.2 10-2

Rapid Bottom Failure 0.007% 2.8 10-6

Total 100% 5.2 10-2

Example 2 Using the Measured Corrosion Rate

Tank Characteristics:

Tank Bottom Thickness: 250 mils

Age: 10 years

Base Bottom Leak Frequency: 0.0072/year

Inspections: 1Arated inspection at 10 years

Inspection Result: Maximum corrosion rate of 5.5 mpy

Assume that based on the findings in Example 1 that anA rated inspectionwas conductedand found a maximum corrosion rate of 5.5 mpy. The maximum corrosion rate found bythe inspection will supersede that predicted by the model (6.87 mpy). Thus:

( )( )220.0

250

5.510/ ==

mils

mpyyearstar

By interpolation from the ar/t table (Table 3.18):

MFar/t= 0.014

Tank Bottom Leak Frequency = 0.0072 0.014= 1.0 10-4/ year

Rapid Bottom Failure Frequency

Designed according to API 650 and maintained according to API 653: MFDesign= 0.5

Corrosion modification factors is calculated as


35/61


36/61




Figure 5-1 : Likelihood of Failure Calculation

Calculate

LoF

Calculate CR oruse measured CR

Figure 3-2

Calculate MFar/tfor Leak failure

(CR, Age, WT)

Change in CR Calc- Inspection,

measured CR- Similar tank data

- CP- Lining/Liner- Repairs (revised

Age and thickness

- New floor- etc.

Calculate MFLiner- Type- Age

- Condition

Changes sinceoriginal service

start date

Yes

No

Yes

No

Liner ?

LoFLeak= GFFLeak MFar/t

MFLiner

Calculate MFar/tfor Rupture failure

(CR, Age, WT)

Designed &Maintained to

API 653

Yes

No

Calculate RatioLoFLeak: LoFRupture

MFDesign= 0.5

MFDesign= 5 LoFRupture= GFFRupture MFDesign

MFCorrosion

MFCorrosion= MAX(MFar/t/20, 0.2)


37/61


38/61


39/61


40/61


41/61


42/61


43/61




As can be seen in Tables 6.2 and 6.3 above, the selection of a soil type greatly influencesthe release rate from the tank and the downward velocity through the soil. A relativelyinexpensive soil sample can be taken by a soils engineer to determine the hydraulicconductivity of the soil.

Only small bottom leaks from tanks without an RPB have the potential to reachgroundwater. The volume that affects groundwater is found by estimating the total spillusing the leak rate and detection time, and then multiplying this by the fraction of therelease that reaches groundwater (see Equation 6-4).

Fraction Contaminating Groundwater = (Detect. Time Time to Groundwater)/Detect. Time

Equation 6-4

Release Preventi on Barr iers

In the event of a release from a tank with a Release Prevention Barrier (RPB), it can beassumed that there is at least one small leak in the RPB. The head pressure will beassumed to be 4 the depth of the sand pad. Tanks with RPBs typically have at leastfour 1 drain pipes to detect the leak. The drain pipes also serve to evacuate the sand padbetween the tank bottom and the RPB thus relieving head pressure. The release from theRPB will be of short duration (see Table 6.4). The release rate from a single small holein an RPB is shown in Table 6.5. The user may specify the number of small holesassumed to be in the RPB and scale up the release rate accordingly.

Table 65: Release Rates from Small Hole in RPB (bbl/hr) on 1/8 hole

Soil Type

Hydraulic

Conductivity of Soil

(cm/sec)

GasolineDiesel Oil

Light Fuel Oil

Heavy Fuel Oil

Crude Oil

Coarse Sand 110-1 110-2 0.1 0.04 0.002

Fine Sand 110-2 110-3 0.02 0.006 0.0004

Very Fine Sand 110-3 110-5 0.001 0.0005 Negligible

Silt 110-5 110-6 0.0001 Negligible Negligible

Sandy Clay 110-6 110-7 Negligible Negligible Negligible

Clay 110-7 110-8 Negligible Negligible Negligible

Asphal t Releases

Asphalt releases are assumed to be similar to liquid releases contained in a dike. Theenvironmental clean-up cost for such releases is $10/bbl. In case of leakage, the asphalt isassumed to solidify within a short distance into the ground. Hence, asphalt releases areonly considered for rupture failures, and repair and outage costs in relation to leakage.


44/61




6.2 Shell and Fitting Leaks

Shell and fitting leaks are typically small and inconsequential. These leaks wet theoutside of the tank and are generally dealt with before any product reaches the ground.For the roughly 5 or 10 percent of these leaks that do reach the ground it will be assumed

that the spill size is 10 barrels.

6.3 Rapid Shell and Floor-to-Shell Failures

A storage tank is often surrounded by a dike that is designed such that the volume of thedike will hold 110% of the volume of the largest tank in the dike. In the event of a rapidshell failure a portion of the product will spill over the dike wall. A model has beendeveloped that provides a rough estimate of the fraction of product that overflows thedike. The model is based on predictions made by computational fluid dynamics (CFD)models used in previous studies. These studies were done for different materials (ACN,styrene, wastewater) and model spills from ASTs. The liquid height to radius ratios forthe tanks ranged from 0.5 to 2.5.

The model correlates the fraction of material that overflows the dike to the dimensions ofthe tank and the dike. Table 6.6 shows the fraction of the tank contents that can beexpected to overflow the dike in the event of a rapid shell failure. RTankis the radius ofthe tank, and RDike is the average distance from the center of the tank to the dike walls.VTank Contentsis the volume of the product in the tank, and VDikeis the total volume of thedike (assuming the tank is not there). This correlation is valid for dike walls that are 3 to6 feet high and should not be used for dike walls outside that range.

If the user does not want to calculate the dike overflow using Table 6.6, a reasonable

assumption for a typical diked tank is that 50% of the tank contents would overflow

the dike subsequent to a rapid shell or floor-to-shell failure.

Table 66: Rapid Shell Failure (Fraction Overflowing Dike)

VTank Contents/ VDikeRTank/ RDike


45/61




7. REPAIR, REPLACEMENT, AND BUSINESS INTERRUPTION COSTS

In addition to the environmental costs of a spill, there are also costs involved in repairingthe tank or replacing the tank bottom. In addition, there are costs associated with the tankbeing out of service. This could involve a production rate cut or possibly an unplanned

shutdown of a unit. Thus there are both tangible costs to repair the tank and the lostopportunity costs.

The cost and duration of tank repairs can vary drastically from tank to tank. It isdependent on the size and age of the tank, type of foundation, type of product (crude oilvs. refined product), etc. It is thus recommended that the user input the repair costs

and lost opportunity costs for the various repair/replace options for the tank underconsideration. A set of default repair/replacement costs are listed in Table 7-1.

Table 7-1: Default AST Repair and Replacement Costs

Repair Cost Downtime (Lost

opportunity cost)

Minor bottom repairs Small leak $5,000 5 days

Major bottom repairs Several small leaks $10/ sq. ft. 15 days

Bottom replacement (*) $20/ sq. ft 37 days

Rapid bottom failure (*) $15/ sq. ft. 37 days

Hydro-test -- 7 days

Install internal lining Epoxy type $4/ sq. ft. 7 days

Install internal lining FRP type $7/ sq. ft. 14 days

Rapid Shell failure (cost based on floor size) $15/ sq. ft. 50 days

Shell leak repair Negligible 0 days

*Hydro-test is normally required in relation to repair work. This has been included in downtimefor the denoted items

For small leaks events, the probability for a bottom leak repair being Small is 70%, Majoris 25% and Floor Replacement is 5%. These ratios are based on feed back and operatorsexperience gathered by DNV. The percentages are used to calculate the relativecontribution the total potential consequence.

For the same events, the probability of having to install floor lining or liner is 30% if onewas not installed before the leak and lining is not assessed to be needed, and 100% iflining is assessed to be needed and no lining or liner was previously applied. If lining or

liner was installed prior to the leak, there is a 100% probability of replacement.

7.1 Downtime Consequence Costs

The downtime and associated cost per day, takes into account cost factors such as tankblinding and preparation for inspection, tank cleaning, waste or sludge disposal, blasting,actual inspection work and bringing the tank back into service. Although these costs willvary with the tank size, stored fluid, etc. and as such will have differing contribution tothe overall risk, these have not been individually quantified in this AST module.


46/61




8. CONSEQUENCE CALCULATION SUMMARY

The consequence calculations are in effect split into two types of associated costs. InChapter 6, the elements of the Environmental Consequence are described as costrelated to environmental impact, and in Chapter 7, the elements of the Financial

Consequence are described as cost related to equipment damage and outage. In Figure8-1, the steps are illustrated on a flow chart for calculating the EnvironmentalConsequence of Failure for both Leak and Rupture failure scenarios. In Figure 8-2, thesteps are illustrated on a flow chart for calculating the Financial Consequence of Failure,also for both Leak and Rupture failure scenarios.

Once the consequence elements are calculated, the total Probability Weight Consequenceof Failure (CoF) is calculated as a function of Environmental and Financial Consequence,and expressed as follow:

)..(0028.0)..(9972.0RuptureRuptureLeakLeak

CoFFinCoFEnvCoFFinCoFEnvCoF +++=

Equation 8-1

The CoF is a fixed number and a characteristic for the specific tank. This CoF is theConsequence of Failure ($) factor that determines the Consequence Category for theTank, and as such largely discriminates the High risk tanks from the Low risk Tanks.

As shown in Equation 8-1 above and illustrated in the example in Table 9-2 and Table9-3, the Consequence of Failure ratios can be based on generic failure frequencies in lieuof an actual calculated ratios. The difference is the total CoF cost will in every case benegligible. Thus, the failure ratios illustrated in Table 3-2 are used for this module.


47/61




Figure 8-1: Environmental Consequence Calculations

EnvironmentalConsequence

Bottom Leak

Yes

No

Asphalt?

Yes

No

Leak Detection Time (t)Calculate time to

Ground Water (tgr)

Soil Type

Fluid

RPB?

Flow velocity.

See Table 6.2

CUF = $10;

Rupture failure and

Financial Costs only

Total volume

released

Flow velocity.

See Table 6.5

Distance to Ground Water (D)

Fraction that contaminatesGround Water (Fgr)

(Fgr) = (t tgr)/D

Calculate fluid Volume

contaminating Soil and

Ground Water respectively

Calculate Total Environmental

Cost caused by Leak: Soilclean-up cost + Ground Water

clean-up cost =Env.CoFLeak

Environmental Sensitivity

CUF

CUF = $10

Percent of Volumereleased outside

Dike area: Volesc

Volesc= 100%

EnvironmentalConsequence

Bottom

Rupture

Tank Volume

Asphalt?

Dike?

YesNo

No

Yes

% ofVolesc Contam.

Onsite Soil CUF of $50

% of Volesc Contam.

Offsite Soil

% ofVolescRetained

in DikeCUF of $10

% of Volesc Contam.

Surface Water

Calculate Total EnvironmentalCost caused by Leak: Soil

clean-up cost + Ground Water

clean-up cost = Env.CoFRupture

Environmental Sensitivity

CUF


48/61




Figure 8-2: Financial Consequence Calculations

FinancialConsequence

Leak &Rupture

Calculate EquipmentDamage and Outage Cost

(See Table 7-1)

- Floor Diameter

- Liner (Y/N)

- Outage Cost/Day

- etc.

70% : Repair Small Leak

25% : Repair several leaks

5%: Floor replacement

Total Cost related to Leak

(Fin,CoFLeak)

Total Cost related to Leak

(Fin,CoFRupture)


49/61




9. RISK ANALYSIS METHODOLOGY

All risk assessment systems analyze risk by estimating both the likelihood andconsequence of a postulated scenario. For ASTs, likelihood is measured in terms ofspills

per year, and consequence is expressed in terms ofDollars ($) per spill. When expressedas a single value, risk has the units ofDollars per year.

9.1 The Risk Scoring System

The risk scoring system has been employed for several beneficial reasons:

1. It derives a single measure that can be used to rank the relative risk of competingoptions

2. The results can be directly applied to a cost-benefit analysis

3. It has the ability to aggregate risks for an entire system

4. It can be used to evaluate single changes that may have multiple beneficial effects

The risk scoring system is quantitative in nature, since it produces a numeric score for anAST. To determine the score, the frequency and the consequence of a pre-defined leakscenario are multiplied together. The risks for all scenarios are then summed to find thecombined risk for the AST. An example of how the scoring system would be applied to atank is shown in Table 9-1 below. This example is based on a 120 diameter ASTcontaining gasoline, with an average fill level of 30 feet (i.e. 60,430 barrels). The tankdoes not have an RPB and is leaking into Very Fine Sand subsoil (0.08 bbl/hr at 1 ft/dayvertical velocity) in an area of low environmental sensitivity. Expected leak detection

time is 6 months = total leak of 346 barrels. The groundwater is 30 feet below thesurface, and is reached in 30 days. All releases are contained in a dike, with theexception of a rapid bottom or shell failure in which case 35% of the tank contents wouldoverflow the dike but stay onsite. No previous inspections are considered, and aLikelihood modification factor of 80 for the floor. This equates to a predicted floorthinning of ca. 75% of the original wall thickness. Outage cost per day is set to $20,000.

Table 9-1: Scoring System Example

Scenario

Calculated

Failure

Frequency

(per year)

Estimated

Environmental

Clean-up Cost

($)

Environ-

mental

Penalties

($)

Estimated

Repair

Costs ($)

Lost

Opportunity

Costs ($)

Risk Score

($/year)

Bottom leak 5.8 10-1 147,000 30,000 10,000 100,000 166,460

Rapid bottom failure 8.0 10-5 1,450,000 100,000 225,000 740,000 201

Small shell/fittingleak

1.0 10-4 100 -- 5,000 -- 1

Rapid shell failure 2.0 10-6 1,450,000 100,000 225,000 740,000 5

TOTAL RISK (Dollars per year) 166,667


50/61


51/61




9.2 The Risk Matrix

The risk matrix provides an alternate approach to risk assessment that fills some of thegaps in the scoring approach:

1) It is easy to interpret the risk of each scenario1) Each component of risk (likelihood vs. consequence) is displayed

2) High-risk scenarios are highlighted

3) Levels of risk can be assigned to each scenario

The sample data from Table 9-1 is displayed in the risk matrix below.

110-5

110-4

110-3

110-2

$10M$1K$100K$10K

5

4

3

2

1

A B C D E

CONSEQUENCE

medium risklow risk

high risk

rapid shell

ailure

rapid

bottom ailure

small

bottom leak

small shell

leak

medium-high

risk

Total LoF andprobability

weighted CoF

Figure 9-1: A Risk Matrix Showing the Results of the Example Case (including consequenceaversion)

The shading of the various levels of risk determines how the scenarios are rated. Manycompanies may choose to leave the matrix un-shaded. Based on the example above, the

highest risk scenarios are the small tank bottom leaks. This example demonstrates theconsequence-aversion nature of the shading in the above matrix. Note that any scenariowith a very high consequence (Level E) is rated as a medium-high risk or higher,regardless of the likelihood.


52/61




9.3 Risk Calculations

The approach described under Section 9.2 considers the likelihood of specific failurescenarios and their related costs. This does not necessarily capture the situation that onespecific tank might impose a much larger risk than another, for example due to difference

in Tank type, contained fluid, etc. For instance, a Gasoline Tank most likely imposes amuch larger overall risk than an Asphalt tank. The objective of this module is to considerand quantify the combined risk for the whole tank, in order to differentiate the Highrisk tanks.

Calculation of the total Tank Risk value is based on the probability weightedconsequence (CoF) for the tank, including all relevant scenarios, as opposed to adding upthe risk related to each scenario. This will fix the Tank in one consequence category (Athrough E), instead of moving across the entire consequence spectrum of the Risk Matrix.

The Risk Calculation steps are illustrated on the Flow Chart in Figure 9-2 below.

Figure 9-2 Risk Calculation, and Displayed Factors for Risk, LoF and CoF in the API RBI Software

Risk

Calculation

LoF = A + BCalculated Input data:

A LoFLeak

B LoFRupture

C Env.CoFLeak

D Env.CoFRupture

E Fin.CoFLeak

F Fin.CoFRupture

Display as LoF

CoF = 0.9972 (C + E) + 0.0028 (D + F)

IncludeEnvironmentalConsequence

?

No

Yes

Display CoF

CoF = (0.9972 E) + (0.0028 F)

Display CoFCategory

(based on matrix)

Financial Risk

= A E + B F

Environmental Risk

= A C + B D

Environmental Risk

= 0

IncludeEnvironmental

Consequence

?

No

Yes

Risk ($/yr) = Financial Risk

+ Environmental Risk Display as Risk ($/yr)


53/61


54/61


55/61




Future Case from the risk of the Base Case. In mathematical terms, the cost-benefit ratiois calculated as follows:

ScoreSystemModifiedScoreSystemCurrent

onModificatiofCostRatioBC

=

Equation 9-1

The cost-benefit ratios of a number of future cases can then be compared. In theory, thecase with the lowest cost benefit ratio would be the desired option. Any ratios less than1.0 would be considered cost-effective.


56/61




10. INSPECTION PLANNING

10.1 Objective

The objective is to determine the most appropriate time and level of inspection to beperformed while maintaining the risk associated with the tank operation at an acceptable

level. The inspection planning process should be combined with other risk mitigationefforts such as installing a Liner or CP system. This inspection planning process isiterated until an acceptable level of risk or situation is achieved.

10.2 Inspection Planning Criteria

Inspection can simply be planned by setting a due date for the next inspection, assume thetype of inspection to be done and calculate the predicted risk at this point in time.

Alternatively, the inspection planning can be based on the Likelihood modification factor(MFar/t) in Table 3-8. The MFar/t factor is derived from equipment age, corrosion rateoriginal thickness and inspection efforts, and does determine the progression of risk for

the Tank over time. Thus, the MFar/t factor is the most appropriate risk variable andreference for planning future inspection.

For this module, it is suggested to use a set of targets for the MFar/tfactor that relate to therisk matrix in Figure 9-1. A set of suggested inspection MFar/ttargets is shown on Figure10-1 below. These targets should be evaluated by the user and set as found mostappropriate.

10.3 Manual Inspection Planning

Inspection planning can be done manually by taking additional inspections into accountand varying the time for inspection in the modification factors and LoF calculations.

This is repeated until a target MFar/tfactor is achieved, or the best achievable MFar/tfactoris obtained.

10.4 Automated Inspection Planning

The API RBI Software contains an Automated Inspection Planning (AIP) module. Thisworks on a Global (Batch several Tanks) level as well as on a Detailed (Equipment one Tank) level, i.e. the user is able to set/alter acceptance criteria on both the Global andDetailed levels.

Two methods of Inspection Planing are available:

10.4.1 Inspection (Equipment Level only):

Input:

Plan Ending Date

Number of Inspections

Inspection Effectiveness


57/61




Optimize Inspection Type with regard to Non Intrusive, Intrusive, or Both not affecting the software calculations, but appears as an instruction to theinspector.

Output:

Risk reduction based on entered inspection

10.4.2 Target (Batch and Equipment Level):

Input:

Generic Plan Ending Date for all Tanks

Minimum years between inspections

Target Values for MFar/t

Optimize Inspection Type with regards to Non Intrusive, Intrusive, or Both not affecting the software calculations, but appears as an instruction on theInspection Planning Form.

Output:

Proposed Inspection (required to meet target or as close as achievable)

Risk reduction based on calculated/proposed inspection

10.4.3 Automated Inspection Planning In the API RBI SoftwareThe suggested target factors relate only to the MFar/t. The target factors can be edited bythe user. The ranges, however, are fixed for both the MFar/tand CoF.

Target Factors: The user sets acceptable MFar/t factors in a 5 x 5 matrix. The matrixconsists of Likelihood categories (quantified in MFar/t ranges) on the vertical axis andConsequence categories (quantified in CoF $ ranges) on the horizontal axis. The fixedranges for the Likelihood and Consequence categories are:

Likelihood

CategoryMFar/tRange

Consequence

CategoryCoF Range ($)

1 < 2 A < 10,000

2 < 20 B < 100,000

3 < 100 C < 1,000,000

4 < 1000 D < 10,000,000

5 > 1000 E > 10,000,000

Target values are set for all tanks in the one and same matrix. The AIP risk matrix withtarget values is shown in Figure 10-1 below.


58/61




Note: For a given consequence category only the Modifying Factor is considered forAutomated Inspection Planning (AIP), i.e. by added inspection and changing theLikelihood, the equipment risk position will move vertically in the matrix.

In principle, the user sets a maximum value for the Modifying Factor in each Risk

Category field. Given the Risk Category field for each tank, the program determines ifinspection is required, and if so, which inspection is required to reduce the futureModifying Factor to the corresponding target value. In case the target value for the futureModifying Factor can not be met, the program returns the minimum inspection that givesthe lowest achievable future Modifying Factor. The program derives the minimumeffectiveness and amount of inspection required to reduce the future Modifying Factor tothe target level. The target factor shown in Figure 10-1 are the same as the default valuesin the API RBI Software (November 2001).

Figure 10-1: AIP Risk Matrix for Tank Bottoms

The AIP works on both the Equipment and the Batch level. The software provides a setof default Modifying Factor settings with the option for the user to overwrite these in theProgram Settings Screen (Batch Level) or on the Inspection Planning Screen (Equipment

Level). The Default Inspection Targets are presented in a 5 x 5 matrix, and with a resetbutton to allow the user to return to the Default settings.

The target values are used for all damage mechanisms (presently thinning is the onlydamage mechanism considered for tanks). The target value matrix is defined in theProgram Settings (similar to Custom Inspection Cost).

5

4

3

2

1

A B C D E

CONSEQUENCE

330

LIKELIHOOD

330 250

50 25

100100

2

20

25

2

10

10

2

100

50

20

2

150

20

100

150

100

20

2

$10K $100K $1M $10M

1000

100

20

2


59/61


60/61




11. REFERENCES

American Petroleum Institute, A Survey of API Members Aboveground Storage TankFacilities, Washington, 1994.

American Petroleum Institute,API Publication Number 340:

Liquid Release Preventionand Detection Measures for Aboveground Storage Facilities.Washington, 1997.

American Petroleum Institute,API Standard 650: Welded Steel Tanks for Oil Storage, 9th

Edition. Washington, 1993.

American Petroleum Institute, API Recommended Practice 651: Cathodic Protection ofAboveground Petroleum Storage Tanks, 2ndEdition. Washington, 1997.

American Petroleum Institute, API Recommended Practice 652: Lining of AbovegroundPetroleum Storage Tank Bottoms, 2ndEdition. Washington, 1997.

American Petroleum Institute, API Standard 653: Tank Inspection, Repair, Alteration,and Reconstruction, 2ndEdition, Addendum 1 (1996), Addendum 2 (1997). Washington,1995.

American Society for Testing and Materials,ASTM Standard G 57 95a: Standard TestMethod for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode

Method, West Conshohocken, PA, 1995.

ASM International, ASM Handbook: Vol. 13 -- Corrosion, Formerly Ninth Edition Metals Handbook. United States, 1987, 909.

Baldock, P.J., Accidental Releases of Ammonia: An Analysis of Reported Incidents.Loss Prevention, 13(1985) 35-42.

Bello, G.C. and V. Colombari, The human factors in risk analyses of process plants: thecontrol room operator model TESEO. Reliability Engineering, 1 (1980) 3.

Bonaparte, R., Giroud, J.P. & B.A. Gross, Rates of Leakage Through Landfill Liners,Geosynthetics 89 Conference, San Diego, CA.

Christensen, R.A. and R.F. Eilbert, Aboveground Storage Tank Survey, Final Report.Entropy Ltd., Lincoln, MA, 1989.

COVO,Risk Assessment of Six Potentially Hazardous Industrial Objects in the RijnmondArea A Pilot Study, A report to the Rijnmond Public Authority. D. Reidel PublishingCompany, Dordrecht, 1982, 2-349.

Giroud, J.P. & Bonaparte, R., "Leakage through Liners Constructed withGeomembranes-Part II. Composite Liners, Geotextiles and Geomembranes, 8 (1989) 71-111.


61/61



Industrial Advisory Committee on Fracture Avoidance, Learning from experience Industry reviews fracture avoidance practices for large tanks. Metal Construction, 12(1987) 699-704, 1(1988) 16-19.

Kletz, Trevor,An Engineers View of Human Error, 2ndEdition. Institution of Chemical

Engineers, Rugby, 1991, 96-98.

Lees, Frank P., Loss Prevention in the Process Industries, vols 1-3, 2nd Edition.Butterworths, London, 1996, A8/5, A14/11,12.

Mesloh, RaymondE., Marschall, Charles W., Buchheit, Richard D., and John R. Kiefner,Battelle determines cause of Ashland tank failure. Oil & Gas Journal, September 26,1988, 49-54.

Savageau, David and Geoffrey Loftus, Places Rated Almanac, 5thEdition. Macmillan,New York, 1997, 182-250.

Strack, Otto D.L., Groundwater Mechanics, Prentice Hall, Englewood Cliffs, 1989.

U.S. Nuclear Regulatory Commission,Reactor Safety Study -- An Assessment of AccidentRisks in U.S. Commercial Nuclear Power Plants, Appendix III: Failure Data. NUREG-75/014, WASH-1400, NTIS, Washington, 1975, III-81-A.

581_O - Aboveground Storage Tanks

Documents

Transcript of 581_O - Aboveground Storage Tanks