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CONVERSION OF THE LNG STORAGE TANKS ATPANIGAGLIA, ITALY, TO DOUBLE

CONTAINMENT STORAGE SYSTEMS

CONVERSION DES RESERVOIRS DE STOCKAGE DE GNL DEPANIGAGLIA (ITALIE) EN SYSTEMES DE STOCKAGE

A DOUBLES RESERVOIRS

Rinaldo RapalliniTechnical Director of LNG Activities

SNAM S.p.A.Pal. INSO 1 - P.zza S. Barbara

20097 San Donato Milanese (MI), Italy

Gregory J. BerthaManager of Engineering

Pitt-Des Moines, Inc.3400 Grand Ave.

Pittsburgh, Pennsylvania 15225, U.S.A.

ABSTRACT

The two 50,000 m3 LNG storage tanks at Panigaglia, Italy, have provided successfulservice since their original construction in 1970. The owner of the LNG facility hasdecided to extend the service life of these tanks. This decision has required a thoroughevaluation of the existing condition of the tanks, as well as updating them to comply withcurrent day safety standards.

The original single containment tanks were converted to double containment storagesystems. A prestressed concrete wall was integrated with the existing steel outer tank toprovide secondary LNG containment along with protection from external loadings.Internal tank pumps were added, and other mechanical and instrumentation systems wereupgraded. The unique configuration of the original tanks posed many challenges to thedesign and construction of the conversion.

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RESUME

Les deux réservoirs de stockage de GNL d'une capacité de 50 000 m3 chacun, situés àPanigaglia (Italie), ont été extrêmement utiles depuis leur construction en 1970. Lepropriétaire de ces installations a décidé de prolonger la durée de service des réservoirs.Cette décision a nécessité à une évaluation serrée de l'état des réservoirs et une remise àniveau de leur conformité aux normes de sécurité actuelles.

Les réservoirs simples d'origine ont été convertis en systèmes de stockage à doublesréservoirs. Une paroi en béton précontraint a été intégrée au réservoir extérieur en acierexistant pour créer un réservoir supplémentaire de GNL ainsi qu'une protection lors deschargements externes. Des pompes réservoirs internes ont été ajoutées et d'autressystèmes mécaniques et d'appareillage ont été rénovés. La configuration unique desréservoirs d'origine a posé un certain nombre de difficultés quant à la conception et laconstruction pendant la conversion.

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CONVERSION OF THE LNG STORAGE TANKS ATPANIGAGLIA, ITALY, TO DOUBLE

CONTAINMENT STORAGE SYSTEMS

PREFACE

Natural gas plays a very important role in Italy since it accounts for more than onequarter of the country’s primary energy demand. The role of natural gas, and particularlyimported natural gas, will escalate in the future due to a number of factors. Italy’s internaloil and coal reserves are insufficient. Natural gas reserves are more encouraging yet thesetoo are unable to follow the continued demand evolution. Additionally, national energypolicy promotes an increased use of natural gas in order to decrease polluting emissions.The annual natural gas demand in Italy currently exceeds 50 billion cubic meters and isprojected to increase to 70 billion cubic meters by the year 2000.

Importation of LNG offers an important opportunity for diversifying supplies inrespect to importation of natural gas by pipeline. The Panigaglia terminal is presently theonly LNG import facility in Italy. Its regasification capacity of 3.5 billion cubic meters peryear is strategically important and meets the national energy diversification objectives.

INTRODUCTION

The import facility at Panigaglia, Italy was placed into service in 1970, and wasdesigned to receive and process LNG as well as other heavier liquefied gases exportedfrom Marsa El Brega, Libya. The import facility includes two 50,000 m3 storage tanks.The facility and tanks have performed well under continuous duty since their initialcommissioning.

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The Original LNG Storage Tanks

In the 1980’s, considering the evolution of the world gas market, SNAM decided torenew the Panigaglia facility. A substantial retrofit of the facility, which eliminated the nolonger necessary cracking process systems for the heavier hydrocarbons and upgraded theLNG vaporization equipment, was completed in 1992. These modifications resulted in aless complex facility with improved reliability and flexibility in order to meet the needs ofthe national network. Many of the original facility components were retained, includingthe ship unloading jetty, LNG pumps, boiloff gas compressors, and the single containmentstorage tanks.

In 1994, SNAM and the Algerian SONATRACH signed a long term agreement for thesupply of LNG to Italy. Considering that the Panigaglia facility has been in successfuloperation over the last three decades, and the need for extending its service life, SNAMdecided to upgrade the storage system to the present day safety standards of the LNGindustry and convert the tanks from single containment to double containment. Aprestressed concrete outer tank wall was added to provide secondary LNG containment,internal tank pumps were installed, and the tank mechanical, instrumentation, and fireprotection systems were upgraded to conform to current industry practices. Thisconversion was done in parallel with the refurbishment of the Algerian liquefactionfacilities in 1995 and 1996. The converted tanks were successfully recommissioned inFebruary of 1997.

SNAM is the owner and operator of the Panigaglia LNG facility. The execution of thestorage tank conversion program was directed by Snamprogetti. Snamprogetti alsoprovided design services to integrate the converted tanks with the remainder of theprocessing facilities. Pitt-Des Moines (PDM) developed the conceptual design of the tankconversion and also performed the detailed design of the double containment system.PDM was also the designer for the original tanks.

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The LNG Storage Tanks at Completion of the Conversion

DESCRIPTION OF THE ORIGINAL LNG STORAGE TANKS

The original single containment storage tanks were of unique construction comparedto modern double wall metal tanks. The inner tank is 9% nickel steel, with a nearly flatsteel roof supported using an internal steel structure on steel columns. The inner tankdimensions are 49.1 m diameter and 26.8 m high. The outer tank is carbon steel, with itsroof supported from the inner roof using a wooden framing structure. The annulusbetween the inner and outer tanks is purged with nitrogen gas. The tank insulation systemconsisted of perlite for the roof, fiberglass blankets in the sidewall annular space, perliteconcrete blocks supporting the inner tank shell, and a composite base insulation systemcomprised of wooden columns with perlite fill. The tanks are supported on elevatedprestressed concrete pile caps. Withdrawal of LNG from the tanks was by gravity flowthrough a series of piping connections in the bottom of the inner tank. A diagram of theoriginal tank configuration can be seen in Figure 1.

The unique configuration of the tanks was influenced by site restrictions and economicconsiderations during the time of the original construction. The flat roof was required dueto a height limitation in the resort community of Panigaglia. The composite wood andperlite base insulation system was chosen as a lower cost alternate to importing cellularglass block insulation. The result of these factors was an intricate wooden support systemwith perlite fill insulation in both the base and roof of the tanks. Although these woodensupport systems complicated the design and construction of the conversion, it was decidedto leave them in place as they had performed well. Protective measures had to be takenduring construction to safely distribute construction loads on the base and roof, and tomaintain the perlite in a dry condition. Due to their considerable weight the new internal

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pump columns could not be supported from either the base or the roof, but rather had tobe suspended from a new concrete platform located above the roof.

Figure 1 - Original Tank Configuration

DESCRIPTION OF MODIFICATIONS REQUIRED FOR THECONVERSION

The principal aspect of the storage tank conversion was the addition of a prestressedconcrete outer tank wall which serves as a secondary container for the LNG. The outertank is in compliance with double containment criteria established by British Standard BS7777.

BS 7777 defines a double containment tank as one designed and constructed such thatboth the inner and outer tanks are capable of independently containing the LNG. Theouter tank is intended to contain LNG leakage from the inner tank, but it is not intendedto contain any vapor resulting from the leakage. The outer tank wall is to be located closeto the inner tank, at a distance not to exceed 6 m, to minimize the pool area of LNG in theouter tank.

The concrete wall was constructed immediately against the original steel outer tankusing the steel tank as an inner form during pouring of the concrete. The steel tank alsoserves as the vapor barrier for the concrete wall to prevent the ingress of water vapor fromthe atmosphere and the escape of nitrogen gas from the annular space. The concrete wallis supported on new concrete piles placed around the original pile foundation. Another

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major feature of the conversion included the replacement of the bottom LNG withdrawalpiping connections with internal tank pumps. A roof top concrete platform, which issupported from the outer concrete wall, was added to service the pumps and associatedvalves and instrumentation. The fiberglass sidewall insulation was replaced with aconventional perlite system incorporating fiberglass compaction control blankets.Extensive upgrades of the tank piping, electrical, instrumentation, and fire protectionsystems were also incorporated. Refer to Figure 2 for a diagram of the converted tankconfiguration.

Figure 2 - Converted Tank Configuration

DESIGN CONSIDERATIONS FOR THE CONVERSION

Codes and Standards

Numerous design and construction standards were utilized for the conversion.

The original tanks were designed and constructed in accordance with the AmericanPetroleum Institute standard API 620, third edition. Appendix Q was added shortly afterthe original tanks were designed. The eighth edition of API 620 including Appendix Q wasused for the design of inner tank components for the conversion.

The British Standards Institution standard BS 7777 was used to define therequirements for the double containment storage system. BS 7777, and its referenced

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standards BS 8110 and BS 8007, were applied for the design of the concrete outer tankwall and design verification of the original concrete pile cap.

The tanks were analyzed for seismic loadings in accordance with the more rigorouscurrent day norms. A site specific response spectrum analysis was performed utilizing theguidelines of the National Fire Protection Association standard NFPA 59A. The Italianseismic code, Decreto Ministro Dei Lavori Pubblici 24, was also followed for the design.

The applicable Italian codes were utilized for wind loadings, miscellaneous concreteand steel structures, and electrical design. Snamprogetti standards were applied for pipingsystems, piping insulation, valves, and instrumentation.

Loading Conditions

Efforts were made in the design of the conversion to minimize any new loadings beingapplied to the original structures which were not intended to be modified, including thefoundation, inner tank shell, tank base insulation system, and tank roof system. Theseoriginal structures were evaluated for any new loading conditions in accordance with thecurrent applicable design standards.

Loads for the design of the double containment storage system were developed per theguidelines of BS 7777. The one exception to BS 7777 requirements was the water levelfor the rehydrotest of the tanks. BS 7777 requires a full height water level, but this wasnot practical for the original inner tank and foundation which were designed for a partialheight water level as permitted by API 620. Therefore, the conversion design was basedon the guidelines for testing of API 620.

The prestressed concrete outer tank wall and its liner system, and the original pile capfoundation, were designed for the pressure and thermal loadings resulting from a gradualrelease of LNG from the inner tank. Full spill and small spill cases were considered.

The original tanks were designed for earthquake forces using an equivalent staticapproach which complied with contemporary codes of practice. Seismic design practiceshave evolved over the years to require the use of more sophisticated analytical techniques.Current day norms require a dynamic response spectra analysis of the composite tankstructure, incorporating soil-structure interaction, and utilizing site specific geotechnicaland seismic response data. An analysis of this type was performed for the conversiondesign. All components of the tank, both original and new, were evaluated for theearthquake loadings developed from the dynamic analysis.

A detailed geotechnical and seismic investigation was conducted. The combined soiland pile stiffness was defined by the investigation and utilized in the seismic analysis. Thedesign response spectra were specified, with a base horizontal ground acceleration of0.057g for the OBE or Operating Basis Earthquake (475 year return period) and 0.145gfor the SSE or Safe Shutdown Earthquake (10,000 year return period).

External blast and impact loading conditions were considered for the design of theconcrete outer tank wall. The blast load was defined by a pressure vs. time relationshipresulting from a vapor cloud explosion in the process area of the facility. Two impact

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loadings were defined. The first was in accordance with the recommendations of BS 7777and consisted of a small nondeformable object such as a valve. The second resulted fromthe presence of a military air field located near the LNG facility and was based on a lightaircraft considered to be a deformable object.

Several fire loadings were evaluated for the conversion design. These consisted of anadjacent tank fire, a pool fire occurring at the LNG spill impoundments located adjacent toeach tank, and an in-tank fire within the concrete outer tank. Heat flux level profiles weredeveloped for the exterior surfaces of the concrete outer tank wall and steel roof for theadjacent tank and spill impoundment fires. A water deluge system was determined to berequired to cool the surfaces of the concrete wall and steel roof for these conditions. Theback radiation heat flux level profile was developed for the exterior surface of the concretewall for the in-tank fire. The concrete wall was designed to sustain the in-tank fire loadingwhile subjected to LNG pressure and thermal loadings on its interior surface.

Other Design Considerations

There were several other factors which influenced the conversion design.

One feature of the original tank construction which permitted the conversion to befeasible was the fact that the original pile cap of the foundation system was prestressed.The pile cap is elevated above grade with an air space of 2.4 m below its underside. Theoriginal design provided prestressing to allow capability to support the tank in the eventthat LNG was released into the earthen impoundment that surrounds the tank andsubmerged the pile cap. The prestressing level was evaluated and was found to besufficient to allow the pile cap to serve as the base component of the outer tank for thedouble containment storage system. The condition of the concrete and prestressing systemwas investigated by observation and sampling.

Another critical feature of the conversion design was the means of supporting thesubstantial mass of the outer concrete wall. Since the existing pile system was not capableof supporting either the dead load of the wall nor the increase in lateral earthquake forcesresulting from the wall’s mass, the wall would have to be supported on a new system ofpiles placed outboard of the original foundation. A stiff structural connection between thenew and original piles could not be utilized as this would transfer too much of the concretewall loads to the original piles. Therefore, it was not feasible to simply key the wall intothe pile cap which would have provided the rigid corner connection which is typical ofdouble containment storage systems. A gap was provided between the base of the wall andthe pile cap to permit differential settlements and lateral earthquake displacements tooccur without allowing the wall to contact the pile cap. It was necessary to develop aflexible corner connection to span this gap that was dependable and would ensure theintegrity of the outer tank for LNG containment.

The solution to the flexible corner connection was a 9% nickel steel membrane linerthat covers the lower portion of the concrete wall. The configuration of this cornerconnection is very similar to the corner protection system used in many current day doubleand full containment tanks. The top of the connection is rigidly embedded into the topsurface of the original pile cap. This membrane will contain LNG yet is sufficiently flexible

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to accommodate differential thermal and settlement movements between the original tankstructure and the new concrete wall.

DESCRIPTION OF THE DOUBLE CONTAINMENT SYSTEMCOMPONENTS

Inner Tank

Modifications to the original inner tanks were minimized in the conversion program.The tanks had performed very successfully and major changes were neither warranted nordesired. Nevertheless, some modifications had to be performed to the inner tanks toevaluate their condition, to permit construction access, and to upgrade the tankmechanical systems. All piping connections to the tank sidewall and bottom were removedand bracing was attached to the sidewall stiffeners for guiding the new internal tank pumpcolumns.

In order to establish their ability to continue to perform successfully, samples of theinner tank shells were removed from both tanks for metallurgical evaluation. Samples weretaken of the 9% nickel steel plate materials and welds from both the 6.4 mm thick uppershell courses and the 14.5 mm thick lower shell course. A series of tests were performedon these materials. Chemical analysis, tensile testing, and fracture toughness testing werecarried out on the plate, weld, and heat affected zone samples. Fracture toughness testingutilized Charpy V-notch as well as crack tip opening displacement methods. In addition,wide plate tests were conducted at -170 C to characterize the resistance to fractureinitiation. It was concluded from these tests that provided a thorough inspection wasperformed to verify that the tanks are free from defects in accordance with current daycode requirements then the tanks could be considered fit for service for the plannedextended life.

A comprehensive nondestructive testing program was conducted on the inner tanks.Radiographic, ultrasonic, and liquid penetrant inspection methods were employed perpresent day industry practices. No detrimental defects were detected by this inspection.

The original inner tank shells were evaluated for the new loading conditions includingthe hydrodynamic earthquake forces and pump column attachments. Additionally, theoriginal inner tank anchor bolts were verified for the earthquake forces. The freeboardavailable for LNG slosh resulting from an earthquake was also checked and proved to beadequate. A finite element fatigue analysis was performed on the inner tank annular plateand its attachment weld to the shell to ensure sufficient cycle life for the future service ofthe tanks.

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Prestressed Concrete Outer Tank Wall

The prestressed concrete outer tank wall is 600 mm thick, 28.5 m high, and has aninside diameter of 52.1 m. The base of the wall is tapered outward to a thickness of 1900mm to accommodate the supporting piles. The wall is prestressed using interior post-tensioning tendons enclosed in grouted ducts. The horizontal tendons are anchored at fourbuttresses spaced equally around the circumference of the tank. The vertical tendons areanchored at the top and bottom of the wall.. The outside surface of the wall is sealed andpainted to provide weather resistance

Foundation System

The original tank is supported on a 610 mm thick prestressed concrete pile cap and204 reinforced concrete piles of 1 m diameter. The new concrete outer tank wall issupported on 44 similar piles. The piles are supported in rock at depths ranging from 10to 40 m. The new piles were not driven to avoid damage to the original tank structure butwere placed by drilling and filling with concrete. The piles are encased in carbon steeljacketing to provide corrosion resistance from the marine environment.

The Foundation and Outer Tank Wall During Construction

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Outer Tank Liner System

The liner which serves as the vapor barrier for the concrete outer tank wall is theoriginal carbon steel outer tank shell. The steel shell was used as an inner form whenpouring the concrete wall. Carbon steel studs were welded to the outer surface of the shellto integrate the liner with the wall. The studs were designed to prevent buckling of theliner under compressive loadings caused by post-tensioning and shrinkage of the concretewall. The original carbon steel outer tank bottom serves as the outer tank base liner. Nomodifications were required for the base liner. The wall and base liners are integrated withthe 9% nickel steel corner connection to complete the vapor barrier.

Tank Insulation

The only substantial change to the tanks’ insulation system was the replacement of thesidewall fiberglass insulation with a perlite system. This system utilized a resilientfiberglass compaction control blanket which was attached to the outer surface of the innertank shell. The compaction control blanket was sized to minimize eternal perlite pressureloads acting on the inner tank shell.

The construction of modifications to the tank piping and instrumentation systemsnecessitated removal and subsequent replacement of the perlite for the tank roof. Theoriginal perlite insulation in the tank base was left in place.

Tank Piping and Instrumentation Systems

The tank piping systems were modified extensively. All original piping connectionswhich ran through the inner tank sidewall and bottom were removed and replaced withconnections through the roof. Process piping which was previously routed from the innertank roof through the sidewall annular space was relocated and supported on the outsideof the tank.

The Internal Tank Pumps and the Roof Service Platform

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The major process piping connections to the tank were replaced. Two new LNG filllines were added including a 24” bottom fill line with mixing nozzle, and a 14” top fill line.A new 24” vapor withdrawal line was installed, a vapor makeup line was added, and thecooldown line was relocated.

The most significant piping modification involved the removal of the four 16” bottomLNG withdrawal connections and replacing them with an internal tank pumping system.Three pump columns were installed, two of 30” diameter and one of 24” diameter. Thepump columns are suspended from the concrete roof service platform located above theouter tank roof and are laterally braced to the inner tank shell. A vibration analysis wasperformed to ensure that the natural frequency of the columns did not coincide with theharmonic operating speed of the pumps.

The tank pressure and vacuum vent sizing was evaluated for the new operatingconditions of the tank. The pressure and vacuum relief system had to be completelyreplaced. A reserve capacity relief valve was also installed in the roof of the outer tank torelieve vapors generated from a release of LNG into the outer tank.

Construction of the Tank Piping Systems

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Modifications were also necessary for the tank purge piping systems. The annularspaces of the tanks are maintained under a continuous nitrogen purge which is kept at apositive pressure utilizing a weighted piston type breather tank. In the original tanks theannular space communicated freely all around the tank, thus a single connection from thetank roof to the breather tank was all that was necessary. The annular space was dividedby the addition of the 9% nickel steel corner liner connection to the sidewall of the innertank. Additional purge piping was installed to connect the isolated base insulation space tothe breather tank.

Upgrades to the tank instrumentation systems were also extensive. Two new servotype level gages with stilling wells were installed in each tank. Stilling wells were addedfor the existing level/temperature/density gages. A new system of temperature monitoringthermocouples was provided for cooldown purposes. Thermocouples were also installedin the base of the annular space for leak detection. Pressure monitoring instrumentationwas also upgraded.

Tank Roof Service Platform

Another significant modification was the addition of a service platform on the tankroof. The platform has an area of approximately 130 m2 and encompasses the tops of thethree pump columns, the LNG discharge piping, and the other major process pipingconnections to the tank. Structural framework of the platform is concrete constructionwhich is cantilevered from the top of the concrete outer tank wall. The working surface ofthe platform is galvanized steel grating. Access to the platform is by means of a spiralstairway attached to the outside surface of the outer tank wall. Lighting was installed onthe platform and stairway.

A system of stainless steel spill protection plates were installed below the platformgrating to protect the carbon steel outer tank roof from spilled LNG. The spill would becollected in a stainless steel trough located at the outer edge of the platform and fromthere it would be directed to grade via a downcomer pipe.

A rotating jib crane was provided on the platform to facilitate removal of the pumpsfrom their columns and lowering them to grade for maintenance. The mast of the crane ishinged to allow the crane to be lowered and stored on the platform thus reducing itsnormal projected height above the tank roof.

Fire Protection Systems

A firewater deluge cooling system was installed on each tank to provide protectionfrom adjacent tank and spill impoundment fires. The original firewater piping systemswere replaced with a completely new system. The roof platform and piping are alsoprotected by the deluge system.

A hazard detection instrumentation system was provided on the roof platform andaround the roof mounted equipment. This system included combustible gas detectors,flame detectors at the discharge of the pressure relief vents, and low temperature detectorsin the spill protection system.

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CONSTRUCTION PROGRAM FOR THE CONVERSION

The tanks were decommissioned during the months of April and May of 1995.Decommissioning included the removal of LNG, isolation of the piping systems, warmupand purging. The tanks were entered and a thorough visual inspection was performed.Construction of the conversion program began in July 1995.

The construction sequence began with the removal of the fiberglass insulation from thesidewall and the perlite insulation from the roof. The insulation removal afforded accessfor the inspection of the inner tank, which included a thorough visual examination andnondestructive testing program. The steel outer tank sidewalls were prepared for concretewall construction by applying the steel studs and installing embedments. Pile installationand load testing paralleled the steel sidewall preparation. The concrete outer tank wall wasconstructed and post-tensioned followed by installation of the roof platform and internaltank pump columns. Mechanical and electrical works both inside and outside the tankwere then completed. Installation of perlite insulation in the tank sidewall and roof was thefinal construction operation. The tanks were purged, cooled down, and recommissioned inFebruary 1997.

The tank conversion involved numerous specialized contractors. The strict scheduleoften required that the contractors work side by side with each other to perform parallelactivities, which mandated a very highly coordinated effort. Snamprogetti was responsiblefor the construction management for the project. In addition to performing the design forthe conversion, PDM provided technical support during the construction of the civil andmechanical works to advise the contractors on the proper execution of the design.

The Tank Roof Service Platform During Construction

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CONCLUSION

The conversion of the Panigaglia LNG storage tanks was a significant engineering andconstruction undertaking. In many respects this effort was much more challenging thannew tank construction. Innovative design concepts and configurations were required to bedeveloped for certain areas of the tanks. Design validation of the original inner tank andpile foundation for the new LNG and earthquake loadings required extensive evaluation ofthe condition of these structures and the application of state of the art analyticaltechniques. Conventional construction methods and sequences were adjusted to suit theunique nature of the project.

All aspects of the conversion were thoroughly considered in the design. Present daysafety standards were rigorously applied. The requirements of BS 7777 for doublecontainment storage systems were fully implemented in the design and construction. Acomprehensive assessment of the original 9% nickel steel inner tank structures wasperformed to assure the integrity of the primary LNG containment. All necessary measureswere taken to have the tanks comply with current engineering practices and to ensure thatthey will perform successfully for many years to come.

In conclusion, the conversion program was successful from both an execution andperformance perspective. The project was completed within a 19 month constructionperiod. The LNG facility was put back into operation in February 1997 and has receivedone ship per week on average since that time. The facility is sending natural gas into thepipeline network at a rate of 6 million cubic meters per day and is proving to be veryflexible and reliable.