TANK LEAK DETECTION AS A RISK MANAGEMENT TOOL

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49.0122510.811D ET E C TE 0.07849.0122DEDICATIONTANK LEAK DETECTION AS A RISK MANAGEMENT TOOL

510.811E XECUTIVEEVOLUTIONStorage tanks are an integral part of theinfrastructure of the oil industry. Imaginean oil industry without storage; it wouldbe a very different place. All operationswould have to happen in real time.Refining could only take place whena ship was discharging crude oil andanother ship was receiving therefined products.SUMMARYAt a terminal, ship discharges could onlytake place when sufficient road or railcarswere available to receive the products.Then imagine the low discharge rate andthe time this would take.The industry would operate at a verymuch slower pace and on a very muchsmaller scale without storage facilities,if indeed it could operate at all.In times of global or regional instabilities,storage tanks also provide a means ofsecuring supply to countries. Termedstrategic petroleum reserves, it isestimated that this totals around 4.1billion barrels globally. This may be inthe form of crude oil or refined product.

1530.9741428.086LOCATI O NOPTIMISATIONNAVIGATIONWhile the importance of storage tanks is clear, they do need to be in the right place for them to be of use.Easy access to shipping is vital, hence the majority of storage tank farms are built on the coast at a large port.Furthermore, the terminal must connectseamlessly with the users of the refinedproducts either via pipelines (perhaps toa distribution terminal) or via loadinggantries for internal distribution by roadand rail. Such locations are relativelyunique. The increase in size of oceangoing tankers has further restrictedthe locations suitable for largestorage terminals.This indicates that there is a vital link inthe supply chain with limited potential forgrowth. This growth (i.e. building newtanks) is further limited by environmentaland planning restrictions; society simplydoes not want more tanks to be built.The oil industry therefore has a scenariothat requires that the current stock ofstorage tanks must be used at maximumefficiency at all times. However, this isat odds with another requirement: thatof safety.HEALTH AND SAFETYREQUIREMENTS ANDENVIRONMENTAL/ECONOMICCONSIDERATIONSAll liquids associated with the oilindustry, from crude oil to the plethoraof refined products, are hazardous tohealth and to the environment. Wheneven the smallest of releases into theenvironment is to be avoided, storingthem in large quantities only servesto magnify the issue. Storage tankintegrity is therefore critical from anenvironmental point of view. There arealso economic considerations: the liquidsare valuable, some more so than othersand therefore a leak is a financial loss.If a leak does occur then the cost of anyclean-up can be significant. On top of that,there may be punitive costs applied plusthe impact on the company’s reputation.But what can be done to minimise thepossibility of a leak from a storage tank?MANAGING RISKThis starts with the design and buildof the storage tank. International codesare available, for example API 650,which give guidance on the matter.The following is an extract fromthat standard:1.1.1 This standard covers material,design, fabrication, erection, and testingrequirements for vertical, cylindrical,aboveground, closed- and open-top,welded steel storage tanks in varioussizes and capacities for internal pressuresapproximating atmospheric pressure(internal pressures not exceeding theweight of the roof plates), but a higherinternal pressure is permitted whenadditional requirements are met (see1.1.10). This standard applies only totanks whose entire bottom is uniformlysupported and to tanks in nonrefrigerated service that have amaximum operating temperatureof 90 C (200 F) (see 1.1.17).1.1.2 This standard is designed to providethe petroleum industry with tanks ofadequate safety and reasonable economyfor use in the storage of petroleum,petroleum products, and other liquidproducts commonly handled and storedby the various branches of the industry.This standard does not present orestablish a fixed series of allowable tanksizes; instead, it is intended to permit thepurchaser to select whatever size tankmay best meet his needs. This standardis intended to help purchasers andmanufacturers in ordering, fabricating,and erecting tanks; it is not intended toprohibit purchasers and manufacturersfrom purchasing or fabricating tanks thatmeet specifications other than thosecontained in this standard.While testing forms part of this standard,it refers to testing performed during theconstruction/fabrication process to ensurethat the tank is put into service in a fitstate, i.e. without leaks and structurallysound. There will be more on this later asmany integrity issues relate to tankswhen they are first put into servicefollowing construction or following majorrepair work.Once the tank is in service, what stepscan be taken to give confidence that thetank will not leak? Again, standards areavailable that give guidance as to therecommended checks and tests to beperformed. Typically, codes are API 653and EEMUA 159.The inspections performed can beclassified as follows:Routine in-service inspection The external condition of the tank shallbe monitored by close visual inspectionfrom the ground on a routine basis. The interval of such inspections shallbe consistent with conditions at theparticular site, but shall not exceedone month. This work shall be performed bycompetent personnel, but notnecessarily an authorised inspector (asdefined by the standard).External inspection Visual external inspection to beperformed by an authorised inspectorat least every 5 years or less if theshell plate corrosion ratedictates otherwise.

Ultrasonic thickness inspectionHOW OFTEN? External ultrasonic thickness testing ofthe tank shell used to determine rateof uniform general corrosion while thetank is in service.The frequency of internal inspections isdependent upon knowledge of platecorrosion rates. Where this informationis available, either through previousinternal inspections or is anticipatedbased on tanks in similar service, asimple calculation can be performed toensure a minimum plate thickness is notreached over a known period of time.Not withstanding this, the standardindicates that the internal inspectioninterval should not exceed 20 years. Inspection intervals can varydependent upon whether the corrosionrate is known or not. When it is not,the maximum interval should be 5years. When it is then the interval isdetermined by calculation based oncurrent shell thickness, minimumallowable shell thickness and theknown corrosion rate. If this figure isgreater than 15 years then a maximumof 15 years between inspectionsshould be adopted.Cathodic protection surveys Where exterior tank bottom corrosionis controlled by a cathodic protectionsystem, periodic surveys of the systemshall be conducted in accordance withAPI RP 651. This work shall beperformed by a competent person.Internal inspectionThe purpose of the internal inspectionis as follows: Ensure that the bottom is not severelycorroded and leaking. Gather plate thickness data. Identify and evaluate tank bottomsettlement.When corrosion rates are not knownand similar service experience is notavailable an internal inspection shouldbe performed to determine bottomplate thickness. This should beperformed within 10 years of the tankbeing put into service.In practice, it is this 10-year period thathas been adopted as the target forinternal inspection intervals by manyorganisations. In some countrieslegislation is in place that requires tanksto be taken out of service for an internalinspection and re-calibrated on theirtenth anniversary. In the UK suchlegislation is not in place, but an operatormust have in place a documentedmaintenance plan/policy that sets outwhat actions will be taken, and when,to ensure that the facility (which includesthe storage tanks) is operated in asafe manner and the potential forcontamination of the environment isminimised. This document can be subjectto review by the Environment Agencyand may be included in any licencegranted for the operation of the facility.GOING INSIDEMany operators write into theirmaintenance policy that tanks will besubject to an internal inspection every10 years, thereby meeting requirementsas set out by the API document.But what happens if the tank cannot betaken out of service at this time?The act of tank entry raises a number ofissues: operational, financial, and healthand safety – all giving good reasons fornot entering a tank unless it is absolutelynecessary. For example, entering a tankthat has contained hydrocarbon liquids orindeed other hazardous liquids hassignificant health and safety implications.The tank is an enclosed space and thestored products are hazardous to health.Strict controls are required to minimisepotentially life-threatening risks to thoseentering the tank. Therefore, manyoperators have looked for alternativemeans of ensuring the integrity oftheir tanks, without the need for entryunless absolutely necessary (i.e. if workneeds to be done) and have adoptedwhat is termed as an RBI (risk-basedinspection) approach.Section 6.4.3 of API 653 outlines RBI.Operators use this to justify not enteringtheir tanks. However, this may beconsidered somewhat weak byregulators as it does not provide positiveproof that the tank is tight. In order todemonstrate the conclusions reachedconcerning the integrity of the tank,based on the various factors consideredin their risk-based analysis, operatorsshould ideally include a precision leaktest as a final check. The precision leaktest will document and confirm that theirtank is not leaking, supporting that theiractions have been sufficient.

EXTRACT FROM API 6536.4.3 Alternative InternalInspection IntervalAs an alternative to the procedures in6.4.2, an owner-operator may establishthe internal inspection interval usingrisk-based inspection (RBI) procedures.Combining the assessment of thelikelihood of tank leakage or failure andthe consequence of tank leakage orfailure is the essential element of RBI.A RBI assessment may increase ordecrease the internal inspection intervalsobtained using the procedures of 6.4.2.1.The RBI process may be used toestablish as acceptable the risk of aminimum bottom plate thickness at thenext inspection interval independent ofthe values in Table 6-1. The RBIassessment may also increase ordecrease the 20-year inspection intervaldescribed in 6.4.2.1. The initial RBIassessment shall be reviewed andapproved by an authorised inspectorand an engineer(s), knowledgeable andexperienced in tank design (includingtank foundations) and corrosion.The RBI assessment shall besubsequently reviewed and approved byan authorised inspector and anengineer(s), knowledgeable andexperienced in tank design (includingtank foundations) and corrosion, atintervals not to exceed 10 years, or moreoften if warranted by changes in service.After an effective RBI assessment isconducted, the results can be used toestablish a tank inspection strategy andbetter define the most appropriateinspection methods, appropriatefrequency for internal, external andon-stream inspections, and preventionand mitigation steps to reduce thelikelihood and consequence of a tankleak or failure.Factors that should be considered intank RBI assessments include, but arenot limited to, the following:Likelihood Factors: Original thickness, weld type, and ageof bottom plates. Analysis methods used to determinethe product-side, soil-side and externalcorrosion rates for both shell andbottom and the accuracy of themethods used. Inspection history, including tankfailure data. Soil resistivity. Type and quality of tank pad/cushion. Water drainage from bund area. Distance to environmental receptors. Effectiveness of leak detectionsystems and time to detection. Mobility of the product in theenvironment, including for releasesto soil, product viscosity andsoil permeability. Sensitivity characteristics of theenvironmental receptors tothe product. Cost to remediate potentialcontamination. Type/effectiveness of cathodicprotection system and maintenancehistory. Cost to clean tank and repair. Operating temperatures. Loss of use. Effects on internal corrosion rates dueto product in service. Impact on public safety and health. Internal coating/lining/liner type, ageand condition. Use of steam coils and waterdraw off details. Quality of tank maintenance, includingprevious repairs and alterations. Design codes and standards and thedetails utilised in the tank construction,repair and alteration (including tankbottoms). Materials of construction. Effectiveness of inspection methodsand quality of data. Functional failures, e.g. floating roofseals, roof drain systems, etc. Settlement data. Tank bottom details (single, double,Release Prevention Barrier (RPB,internal reinforced linings, etc.)Consequence Factors: Product type and volume. Mode of failure, e.g. slow leak to theenvironment, tank bottom rupture ortank shell brittle facture. Dike containment capabilities (volumeand leak tightness). Identification of environmentalreceptors such as wetlands, surfacewaters, ground waters, drinking wateraquifers, and bedrock.More qualitative approaches may beapplicable that do not involve all of thefactors listed above. In these cases,conservative assumptions must be usedand conservative results should beexpected. A case study may benecessary to validate the approach.The results of the RBI assessment are tobe used to establish a tank inspectionstrategy that defines the mostappropriate inspection methods,appropriate frequency for internal,external and on-stream inspections, andprevention and mitigation steps toreduce the likelihood and consequenceof tank leakage or failure.By including a precision leak test, thiswould confirm that the procedures put inplace are working and that deferment ofthe internal inspection is justified.The key is that the maintenance planprovides sufficient detail of the approachto be adopted.The advantages to a risk-basedinspection approach are clear. By movingaway from a strict calendar-basedinspection programme this more flexibleapproach allows an operator to maximiseutilisation of the tank assets to meetoperational needs. It also encourages amore focused approach to maintenancespend, targeting those tanks that aremost in need of maintenance rather thanthose that have reached a certain age,but may be completely serviceable.

HOW TO1224.3052040.079L EAK T ESTLOCATIONTRACTIONThe next section considers various options for performing leak tests to verify the RBI findings. Obviously, if thetank is fitted with a permanent leak detection system or has a double bottom with interstitial monitoring thenthese should be capable of providing the verification required. However, many tanks have only a single bottomand do not have any permanent leak monitor systems in place. This section considers what techniques areavailable in such circumstances.Clearly, for the test to provide thisconfirmation it must be suitable forthe application and ideally have had itsperformance verified by an independentthird party.An immediate response is that theautomatic gauging system used for stockdetermination can be used during quietperiods to act as a leak detectionsystem. On the face of it this seems asensible approach, utilising equipmentalready installed on the tank andproviding constant feedback of levelmeasurement to a central control room.They are normally calibrated beforeinstallation and typically thereafter on aregular basis. Indeed, many systems aremarketed as having leak detection builtin to their functionality. However, oncloser examination, does it really providethe level of discrimination required toconfirm that a tank is tight?Current level gauging systems claim abest measurement capability of 0.5mm. If we consider a tank ofdiameter 30m the surface area of thetank is 707m2. A level change of 0.5mmequates to a volume of 354 litres. It isonly once a volume change of thismagnitude or greater has occurred that aleak could be identified with someconfidence. Remember as well thatthroughout the test period the tankcontents and the shell will be subject tothermal gains and losses.As the system measures level directly,changes in liquid temperature will impactdirectly on level, perhaps triggering falseleaks or masking real leaks. Based onthe above, it is suggested that the use ofexisting level gauges does not give theconfidence required to determinewhether a tank is leaking or not, exceptin situations when the leak is very large.An alternative is the use of acousticemissions. This technology uses an arrayof transducers located at intervalsaround the tank that listen for the noisescreated by active corrosion andpotentially for the noise created byleaking liquid. Careful interpretation ofthe received signals against a databaseof known responses enables theoperator to map the complete tank floorand highlight the level of active corrosiontaking place. This is extremely useful asit provides the owner (and specificallythose charged with maintaining thetanks) with details of where corrosionactivity is high and logically where thereis a greater likelihood of a leak pathbeing present. For the test to take placethere must be product in the tank, but itmust be static. As the technologymonitors sound the test area must beacoustically quiet during the test period(this could be a number of hours)otherwise the low-level noise generatedby the active corrosion is swamped byextraneous noise potentially masking anactual occurrence.While not strictly a leak test it doesprovide information that can be usedto verify the assumptions made fromthe RBI. For example, if RBI points to highrates of corrosion and this is supported byacoustic testing then the tank is a primecandidate for early internal inspection. IfRBI suggests little corrosion and this iscorroborated by acoustic testing, then thetank can remain in service withincreased confidence.However, acoustic emission is aqualitative test; monitoring oneparameter (sound) and from that,predicting another parameter (corrosionactivity) in conjunction with a databaseof known responses. A better approachwould be to apply a quantitativeapproach to tank leak determinationthrough precision, mass-based testing.A mass-based technique monitors thepressure head generated by the columnof liquid in the tank over a period of time.The basic equation that is applied by thetechnique is:p ρ*g*hwhere, p is the pressure generated by thecolumn of liquid ρ is the density of the liquid in the tank g is the constant term, accelerationdue to gravity h is the height of the liquid columnChanges in liquid temperature result in achange in density and therefore a changein volume. If the liquid is constrainedwithin a tank then any change in volumewill manifest itself as a change in level,yet there is no physical loss or gain ofliquid by the system. With a leakdetection system based on levelmeasurement, even small temperaturefluctuations are sufficient to mask anyleaks that may be present. By monitoringpressure rather than level, the impact ofchanging liquid temperature is neatlyeliminated. Considering the aboveequation, as temperature changes thedensity of the liquid also changes.

Schematic data plot showing diurnal effects – circled in red. Courtesy of Mass Technology Corporation.Within a fixed container thecorresponding change in volume of theliquid will result in a change in the heightof the column. However, since thechanges in density and liquid height areinversely proportional the net result inpressure change is zero.However, the shell of the tank containingthe liquid is not immune fromtemperature changes. A good example isthe effect of solar radiation on the tank:as the sun rises in the morning the effectis to warm the tank

possibility of a leak from a storage tank? MANAGING RISK This starts with the design and build of the storage tank. International codes are available, for example API 650, which give guidance on the matter. The following is an extract from that standard: 1.1.1 This standard covers material, design, fabrication, erection, and testing

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