Pipeline & Gas Journal / June 2003 / www.pipelineandgasjournal.com

by J. Randolph Kissell, The TGBPartnership, Alan Geis, ColonialPipeline Company, Atlanta, GA.

n the past, steel roofs on petrole-um storage tanks have oftenbeen a challenge to maintain.Traditionally, more volatile stockssuch as gasoline were stored in

external floating roof tanks, while coneroof tanks were usually built for lessvolatile products such as diesel. In eachcase, the results were mixed. Externalfloating roofs often sink when drains failto operate properly or snow accumulateson the decks. Cone roofs frequently cor-rode more rapidly than the tanks theycover, requiring replacement.

To address these problems, aluminumdomes were first used as fixed roofs forpetroleum storage tanks in the late 1970s.These domes clear-span the tank with abolted, triangulated aluminum frame cladwith aluminum sheet and can be installedwithout emptying the tanks, since theyare bolted together on site without weld -ing. The cladding is 3003-H16 aluminumsheet, usually 0.050-inch thick, and theframes are 6061-T6 aluminum extrusions.

Beginning in 1977, when Conservatekinstalled 14 domes ranging from 24 to 120feet in diameter for Arco in the Northeast,aluminum domes rapidly captured themarket for retrofitted fixed roofs.Aluminum domes became so popular thatthe American Petroleum Institute addedthem in 1992 to API Standard 650, WeldedSteel Tanks for Oil Storage, as Appendix

G. The aluminum roofs were expected toresist corrosion without coatings andremain leak-tight with little maintenance.

In 2002, 25 years after the first alu -minum domes were built on API tanks,the TGB Partnership surveyed two earlyinstallations to determine whether theanticipated benefits have been realizedand which dome features among variousalternatives have proven to be the best.

Colonial PipelinesPioneering Effort

Colonials pipeline system extends fromHouston across the Gulf States, up the east-ern seaboard, and terminates near the NewYork harbor area. More than 2 million bar -rels a day of petroleum products such asgasoline and fuel oil move through thepipeline. In 1989 Arnold Barrett, then ChiefMechanical Engineer for Colonial,explained Colonials situation as follows:

Tankage is the key to efficient and suc-cessful operational scheduling at Colonial...Breakout tanks are used for operationaltransfer of products between pipeline seg -ments operating at different rates and ondifferent scheduling sequences. As a result,these tanks are operated almost constantly.It is not unusual for some breakout tanks tobe filled and emptied two or more timesduring a 5-day product cycle.

The original Colonial system was con -structed in 1963; operations began in1964. During the early 1970s, whenColonials external floating roofs beganshowing serious deterioration from opera-tional wear, an intensive repair programwas established. An effort was made toprolong the useful roof life.

By the late 1970s, it became apparentthat an orderly replacement programwould be needed to prevent crisisreplacements under duress and to preventthe emergency construction problems andoperational disruption caused by unex-pected roof failures. By 1978, the program

had been devised and set in place.Colonial evaluated several roof-replace -

ment options. Costs and relative advantagesof various options were compared. [An alu-minum] geodesic dome roof with [an alu-minum] internal floating deck was adoptedas the companys standard roof design forfuture roof replacements, as well as fororiginal equipment for new tanks.

Colonial had been one of the first com -panies to use aluminum dome roofs, pur-chasing seven aluminum roofs in 1979 fornew tanks at their Pelham, AL facility.Then, as Barrett described, Colonialembarked on a pioneering effort toreplace external floating roofs with alu -minum domes and aluminum internalfloating roofs. As part of this effort,Colonial replaced all external floatingroofs at their Greensboro, NC facility inthe 1980s with aluminum dome roofs.

The 2002 InspectionIn November 2002, TGB inspected the

domes built by Conservatek at ColonialsGreensboro facility. The oldest had beeninstalled on Tank 830, 140 feet in diame -ter by 51 feet high, which had originallybeen built in 1963 by Chicago Bridge &Iron with an external floating roof. In lessthan 20 years, the roof needed replace -

I

Revisiting Aluminum Domes For API StorageTanks

Photo 1: Tank 830 at Colonial PipelineCompanys Greensboro, NC facility.

Kissell Geis

Photo 3: Close-up of batten bars used toclamp the dome cladding to the dome frame.

Photo 2: 200-foot diameter MTBE storagetank at Texas Petrochemicals Pasadena,TX facility.

Lighter Material Shows Its Mettle

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Lighter Material Shows Its Mettle

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ment, and in 1983, Colonial awarded thework to Conservatek.

Today, the aluminum dome roof (Photo1), has outlived its predecessor and showsfew signs of age. The weather seals andcaulk remain resilient and pliable, and thealuminum components and stainless steelfasteners show no signs of corrosion. Whilethe tank shell required repainting in 1989,the aluminum dome has received no coat-ings and little maintenance of any kind.

Other dome roofs constructed byConservatek at Colonials Greensboro facil-ity were in the same condition. Each steeltank the domes covered had requiredrepainting since the domes were erected,while the domes remain as installed withnothing more than a mill finish.

The dome on Tank 830 was attached tothe steel tank in a manner that preventedthe dome from moving relative to the topof the tank. This is referred to as a fixedsupport. For this type of support, the top ofthe steel tank resists the tension induced bygravity loads such as dead load or snow onthe dome and compression caused by winduplift on the roof. Colonial was the firstcompany to use fixed supports, pioneeringthis concept at their Pelham facility. Theother domes at Greensboro were attachedto the steel tank so as to allow the dome tomove radially inward or outward relative tothe center of the tank. This type of supportis called sliding.

For sliding supports, the base ring of thealuminum dome resists the tension inducedby gravity loads on the dome and compres-sion induced by wind uplift. API Standard650 allows both support types, but sincealuminum is more expensive than steel,domes with fixed supports are usuallymore cost-efficient than domes with slidingsupports, especially for dome profiles com-monly used on API tanks and on largetanks. On the other hand, retrofitting thetop of the steel tank to resist the domesradial reactions may necessitate welding,requiring that the tank be out of service.

When loads such as snow or wind act onthe domes, the domes deflect. Although thisdeflection is imperceptible from a distance, itis on the order of several inches in domes ofthe size at Greensboro. Fixed-support domesdeflect significantly less than sliding-supportdomes because the steel ring at the base ofthe dome is considerably stiffer than an alu-minum ring. (Steels modulus of elasticity isnearly three times that of aluminum, while itsdesign strength is about the same as that ofthe aluminum alloy used in the domes). Afteryears of service, however, there was no per-ceptible difference in the condition or per-formance of the sliding-support domes com-pared to the fixed-support dome.

In constructing the domes atGreensboro, Conservatek used a stainlesssteel pin so the dome supports couldrotate relative to the top of the tank. Thisminimizes bending of the tank top fromthe dome, prolonging the life of the tank.

A primary concern with any roof is

weather tightness. The day after the fourthwettest October on record, with 9 inchesof rain over the previous four weeks, nowater was visible on the floating roofsbeneath the domes.

Since the Greensboro terminal is relativelyclose to an international airport and interstatehighway, there were initial concerns aboutpossible glare from the aluminum domes.Additionally, the durability of mill finish alu-minum in tank service had not yet beendemonstrated. While one dome at the sitewas painted, all of the others were furnishedwithout any coatings. The mill finish domeshave weathered to a uniform light gray, andglare and durability are of no concern today.

In summary, the inspection ofColonials Greensboro facility indicatedthat aluminum domes had performed wellfor rigorous service on gasoline tanks. Buthow had they performed for other typesof petrochemical service?

A Large DomeIn 1985, Conservatek erected the largest

aluminum dome ever built for petroleumservice up to that time. The roof coveredan existing 200 foot diameter methyl terti-ary butyl ether (MTBE) external floatingroof storage tank at Texas Petrochemicalsin Pasadena, TX, (Photo 2). MTBE storageis particularly sensitive to contaminationby water since the product is water solu-ble. Water can result in the product notmeeting specifications, so the product issampled every day.

Texas Petrochemicals had not experi-enced problems with external floatingroof deterioration, however, the sealsbetween the external floating roof and thetank shell allowed too much water toenter the tank. The purpose of the alu-minum dome roof was to eliminate watercontamination of the product.

Just off the Houston Ship Channel in aheavily industrial area, the site is exposedto the heat and humidity of the long Texassummers and Gulf Coast hurricanes, a realconcern for an installation that soars near-ly 80 feet above grade. During TGBsNovember 2002 inspection, the ownersaid that it has never performed any main-tenance on the dome nor found any evi-dence of roof leakage.

In fact, when Texas Petrochemicalsexpanded their MTBE storage capacity in1995 by adding two 75-foot diameter, 48-foot tall tanks, they used aluminum domeroofs for those as well. All of the tankshave been in continuous service since theroofs were installed. The dome seals andcaulk were flexible, and there was no evi-dence of corrosion of the aluminum com-ponents or stainless steel fasteners.

The domes aluminum cladding is heldin place by batten bars, (Photo 3), a customaluminum extrusion with grooves to retaina weather seal gasket on either side of thecenterline of the batten. API Standard 650G.2.5.2 requires that gaskets comply with

either ASTM C509, Standard Specificationfor Elastomeric Cellular Preformed Gasketand Sealing Material, or FederalSpecification ZZ-R-765, Rubber, Silicone.On the Colonial domes, silicone manufac-tured to ZZ-R-765 was used for the seals.

All the Texas Petrochemicals dome roofshave neoprene weather seal manufacturedto ASTM C509. Both the neoprene seals atthe Pasadena site and the silicone seals inGreensboro were pliable and resilient inthe 2002 inspection, and there was no per-ceptible difference in the condition or per-formance of the different seal materials.

There is a great variety of performanceamong elastomers depending on the speci-fications to which they are produced, andthe gaskets used on aluminum domes areno exception. Some flange gaskets ofunspecified material on nozzles in theColonial roofs, for example, showed deteri-oration where exposed to the elements.Both ZZ-R-765 and ASTM C509 require thatelastomers meet high and low temperaturetest requirements, however, and so providesome assurance of outdoor suitability, if notcompatibility with product in the tank.

While silicone has suffered some failuresas a dome seal in petroleum service, it hasbeen adequate for freely-vented applicationsof the type used on the tanks in this survey.Neoprene is less costly than silicone andmore resistant to petroleum products. Thissuggests that ASTM C509 neoprene seals arethe better choice for petroleum service,offering better performance at lower cost.

The 200-foot diameter dome atPasadena was built with fixed supports,similar to Colonials Tank 830 previouslydescribed, while the 75-foot diameterdomes were built with sliding supports.The tension induced by gravity loads atthe base of the 200-foot diameter tankwas large enough to require installation ofa steel stiffening ring on the tank topwhen the dome was built. As atGreensboro, though, there was no meas-urable difference in durability or leaktightness of the fixed-support dome com-pared to the sliding-support domes.

ConclusionsFixed-support and sliding-support

domes performed equally well at the sitessurveyed for this article. This suggests,then, that the choice of support can bebased on costs and conditions for the spe-cific case at hand, without concernsregarding performance. For the freelyvented storage tanks at Greensboro andPasadena, silicone and neoprene weatherseals have proven equally durable.

A quarter century after their introduc-tion, aluminum domes have now real-ized the benefits anticipated when theywere first conceived, and proved to bethe most durable fixed roofs for petrole-um tanks. Thousands of domes havebeen built and every major oil companyhas used them. At the various sites andfor the types of service recently inspect-

Pipeline & Gas Journal / June 2003 / www.pipelineandgasjournal.com

ed, the domes have required no coat-ings to last indefinitely in continuouspetroleum storage service.

Authors: Randy Kissell co-foundedthe TGB Partnership, an engineering firmspecializing in aluminum structures, in1993 and has been involved in the design,fabrication, and erection of aluminumstructures since 1978. He co-authoredAluminum Structures - A Guide to TheirSpecifications and Design, published byJohn Wiley and now in its 2nd edition,

and co-holds two patents, including onefor an aluminum bridge deck. He is secre-tary of the Engineering AdvisoryCommittee of the Aluminum Association,responsible for the Specification forAluminum Structures, used throughoutthe U.S. for aluminum structural design.He is also chairman of the ASME B96Committee for Welded Aluminum AlloyStorage Tanks, secretary of the AmericanWelding Societys Subcommittee onAluminum Structures, and a member of

the ASTM Light Metal Alloys committeeand the American Society of CivilEngineers Load Standards Committee.

Alan Geis is a Tank Coordinator forColonial Pipelines system integrity pro-gram and has worked for Colonial for 16years in various tank related assignments.Geis holds a BS (1985) in business man-agement from Oklahoma State Universityand is a member of the API subcommitteeon pressure vessels and tanks, subgroupdesign. P&GJ