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COST EFFICIENT LNG STORAGE TANK CONSTRUCTIONBY HIGH PRODUCTIVITY WELDING

CONSTRUCTION ECONOMIQUE DE RESERVOIRS DESTOCKAGE DU GAZ NATUREL LIQUEFIE PAR

SOUDAGE A HAUT RENDEMENT

Jörgen StrömbergSusan Sun-Hi Pak

ESAB AB, P O Box 8004SE-402 77 Gothenburg, Sweden

ABSTRACT

The use of Natural Gas as a source of energy is growing in many parts of the world.New reserves are discovered almost every year and new projects are in progress to exploitthe gas. Natural Gas as for producing energy is environmental friendly due to its cleanburning. Many of the gas reserves are however, generally located far from the mainconsumption areas. This leads to a need for facilities to transport gas and construction oflarge storage tanks in receiving and export terminals.

Liquefaction of the gas reduces the volume by a factor of more than 600, whichsimplifies the storage and transportation. For example Liquefied Natural Gas (LNG)Methane will liquefy at -163°C and is therefore stored or transported around -170°C. Atthis low temperature only stainless steel, aluminium or quenched and tempered 9% nickelsteels have the fracture toughness and crack arrest properties required for safeconstruction of tanks and vessels. For large and land based storage tanks (50-160.000 m3 )9% nickel steel is used because of its high strength. These tanks are at atmosphericpressure and refrigerated. The development heads also for larger wall thickness in order tomake it possible to increase the size and thus the storage capacity for both pressurevessels and storage tanks in the future. This create needs for even more productivewelding methods to assembly and construct enlarged tanks.

This paper will present recent developments in welding technology for the fabricationof large welded tanks in 9% nickel steels. Strength and quality of welded joints are keyelements for an economic design of the tank and safe operating. The properties andweldability of the 9% nickel steels and welding consumables will be discussed.Continuing with a survey of the special consumables that have been developed to meetthe high tensile strength properties (>690 MPa) required for these applications but at thesame time give high impact toughness at low temperature as -196°C. Example ofproductive welding procedures for girth and vertical welding in the fabrication of a LNGstorage tank will be given.

Several large terminals have been built the last 10-15 years in Europe, Middle Eastand Far East. Some large projects have recently been completed in Puerto Rico, Trinidadand South Korea among others. References will be given of the technology used in someof these projects.

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Figure 1. LNG Storage concepts for 2000 & beyond (on courtesy of CBI)

LNG storage tank construction

Most tanks being built today are of the double wall, double bottom design with an outerprestressed concrete tank or an outer concrete wall. The inner self-supporting “open top”tank is made of 9% nickel steel thermally insulated and covered with a suspendedaluminium roof. The full containment concrete tank is lined with an inner shell of carbonsteel to take up the liquid and provide a vapour-tightness barrier of the concrete containerin case of a leakage. The lower part of this outer shell and the bottom part between theshells may also be in 9% nickel steel as a safety.

The outer concrete tank is also a protection against external impact. The doublecontainment tank has the inner double wall tank and an outer concrete wall lined with 9%nickel steel designed to be able to contain the liquid but not the vapour. The inner tank isreinforced with several ring stiffeners. There is no pressure except the hydrostaticpressure from the liquid height and the wall thickness of the tank needs to be largest at thebottom and can successively be smaller. The bottom shell course may be 25-30 mm andthe top shell course may be 10 mm depending upon the height and design.

All pipes for the loading or unloading the tank is through the roof and there are no otheropenings for access into the tank once the tank is completed. First the foundation ofreinforced concrete is cast with embedded anchor straps for holding down the tank. Incase of very seismic location dampers underneath will be used to take up earthmovements. The normal procedure is then to build the outer concrete tank first and thenbuild the inner 9% nickel steel tank under the cover of the outer tank. The bottom of 9%nickel steel plate is laid out and welded together. The shell is constructed as a series ofconcentric rings starting with the bottom and finishing with the top.

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Figure 2. LNG tank built in Greece 1996; Noell-Whessoe Ltd UK

Above ground storage tanks being built today are normally between 65.000 to100.000 m3. The height determines the hydrostatic pressure from the liquid and theallowable design stress thus limits the height. Tanks up to 200.000 m3 may be designedwith a maximum of 30 mm shell thickness which is currently being considered as areasonable limit for the 9% nickel steel.

The large LNG ocean-going gas tankers for sea transportation are constructed withspherical aluminium tanks to reduce weight but some of the earliest ships were made withspherical 9% nickel steel tanks.

Materials for LNG tanks

The base material for the tank containing the liquid gas at below -170°C must remainductile and crack resistant with the highest level of safety. The material must also permitwelding without any risk of defects inducing brittle fracture. Stainless steels, aluminiumand 9% nickel steels can be used as they do not have a ductile/brittle transitiontemperature. However, in practice aluminium and stainless steel has become uneconomicfor large land-based tanks but aluminium alloys are used for the large spherical tanks ingas tankers because of the lower weight.

9% nickel steel provides an attractive combination of properties at a moderate price. Ahigh corrosion resistance is not required for LNG tanks. The high strength of the steelenables a reduction of the wall thickness. The chemical composition and mechanicalproperties for some cryogenic materials are given in table 1.

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Table 1. Typical properties for some materials used in cryogenic applications

Chemical compositionand mechanicalproperties

ASTM 304 LEN 10088-1/

1.4305

ASTM A 553-1EN 10028-4/

X 8 Ni 9

~ASTM 645EN 10028-4/

12 Ni 19

AA 5083Aluminium -alloy Al bal.

C max % 0.03 0.13 0.12Mg 4.5%Mn 0.8%

Mn max % 2.0 0.9 0.8Cr % 16.5Ni % 9.5 9 5Rp 0.2 MPa min 190 585 390 145Rm MPa 490 690 530-710 290Charpy V °C > 60-196 > 70-196 > 34-120 *)

*) No decrease in toughness with temperature

Steels alloyed with nickel are used in many cryogenic applications since nickel improvesthe quenchability and improves the notch toughness at low temperatures. Steels with3.5% nickel, 5% nickel and 9% nickel are used at temperatures below -50°C. Attemperatures below -104°C down to -196°C mainly the 9% nickel steels are to be used.The 9% nickel steel was developed in the early 40´s and has since then become the steelfor LNG tanks. For lower temperatures (liquid hydrogen -252.8°C) stainless steels areused for vessels.

The excellent low temperature notch impact properties of 9% nickel steels arise from thefine grained structure of tough nickel-ferrite free from embrittling carbide networks. Theoptimum microstructure and mechanical properties are obtained by a carefully controlledheat-treatment in the production of the steel.

Double-normalising or quenching followed by tempering will produce a structurecontaining nickel rich ferrite and stable high carbon austenite giving good strength andlow temperature properties. Water quenching or air cooling from temperatures in theregion of 800°C gives a structure containing low carbon martensite and bainite.Subsequent tempering around 570°C produces strong, tough high nickel ferrite andcarbides and reforms some austenite in which the carbides dissolve. To improvetoughness the steel has also a high degree of cleanliness with low amounts of impuritiesas P and S and low oxygen content.

One important toughness criteria is the behaviour in the presence of a sharp crack. Theyielding at the root of a preformed sharp crack under stress is measured as CTOD valueand a minimum of 0.3 mm at -170°C is normally specified. In a brittle material little or noplastic deformation takes place before rapid cracking occurs.

The weldability of 9% nickel steel is excellent and the steel is not suceptible to crackingand shows little or no detoriation of the properties by the heat inputs normally used duringthe welding with procedures as described later. Heat input should not exceed 3 kJ/mmand interpass temperature must be limited to max 100-150°C.

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Some of the properties of plate material and weld metal after welding with SAW with aheat input of 1.5 kJ/mm are given in table 2. A small increase in hardness is obtained inthe HAZ due to changes in the structure by the thermal cycle.

Table 2. Typical properties of welds and plate material

HardnessHv 10

CTODmm -170°C

Charpy VJ -196°C

Plate 237-240 0.40-0.80 200Weld metal 238-245 0.50-0.56 100Heat affected zone 259-330 0.40-0.46 125

There is no need for preheating when welding and no post heat-treatment is required.Practical problems in welding may arise from magnetic forces as 9% nickel is easilymagnetized in the production and in handling of the plates. The plates are delivereddemagnetized from the steel work with a maximum 50 gauss residual magnetism.

Welding of 9% nickel steel tanks

One of the most important steps in the fabrication of a storage tank is welding. All thedifferent steps must be carefully considered such as joint preparation of the base material,alignment of the plates, positions involved, welding procedures and quality control duringand after the welding.

9% nickel steel has very good weldability and Shielded Metal Arc Welding (SMAW) andSubmerged Arc Welding (SAW) are used extensively in site fabrication. Metal Inert Gaswelding has been used occasionally mainly for components in the workshop. Weldingwith Fluxed Cored Wire (FCW) is being developed but does not yet appear to be readyfor these applications.

SMAW offers maximum flexibility with the minimum of equipment. SMAW is usedmanually for vertical welding the shell plates together as well as for the positionalwelding of ring stiffeners joints.

Unless the constructions are heavily restrained there is no need for preheating or post heattreatment when welding 9% nickel steels. Stress relieving is also not required. Interpasstemperature should be kept below 150°C. The peak hardness in HAZ will reach 250-320HV 10 at normal heat inputs between 1-3 kJ/mm. Heat input must be limited because ofthe risk of embrittlement and softening of the heat-treated base material.

SAW is essentially an automatic high productivity welding method and is suited for allthe long circumferential welds for joining the shell courses together. SAW is alsofrequently used for joining of the bottom plates.

Welding consumables

During the years many different types of weld metals have been used for welding 9%nickel steel LNG tanks. Matching 9% nickel weld metal cannot be used, as a heattreatment would be required, as for the steel, to obtain the desired properties. The strength

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of the weld metal should ideally be approaching the high strength of the steel and it mustas well have high ductility at the liquefied gas temperature -170°C.

In the early days austenitic tungsten alloyed stainless steels weld metals were used.Typical tensile strength was about 650 MPa. The austenitic weld metals have about 40%higher thermal expansion coefficient than the 9% nickel steel which will cause stressesduring cooling and heating. Later on mainly nickel based weld metals have been usedwhich have increased low temperature ductility and also a thermal expansion very closeto the steel. The ductility of the weld metal at cryogenic temperatures is also much higherfor the nickel based alloys than for the austenitic weld metals. Most emphasise has beenon the ductility and crack arresting properties of the weld metal to eliminate the risk of acatastrophic brittle fracture of a tank. Obviously the welded joints are of crucialimportance for the safety and consequently meticulous testing of the welded joints are offundamental importance.

Two types of weld metal have been developed to meet the very high requirements by acarefully balanced chemical composition. The principal compositions are nowstandardised in AWS (A5.11-90, A5.14-89), although commercially availableconsumables may give quite different properties and welding characteristics. Basicelectrode coatings and basic submerged arc welding fluxes are considered mandatory togive a clean deposit with low amount of microslag. The nickel based weld metal canabsorb hydrogen and is not sensitive for cracking.

Testing by crack tip opening displacement (CTOD) is required to determine brittlefracture initiation and arresting properties at service temperature. Covered electrodes aresupplied in vacuum packed boxes to prevent any moisture pick-up during handling andstoring. The electrodes are designed to give easy slag removal, arc striking and to givesafe reliable result and minimum repair.

Table 3: Typical weld metal composition and mechanical properties

Typical weld metalcomposition and

mechanical properties

OK 92.45AWS

ENiCrMo-3

OK 92.55AWS

ENiCrMo-6

OK Autrod 19.82/OK Flux 10.16

AWS ERNiCrMo-3

C %< 0.03 < 0.08 0.01

Mn % 0.4 3 0.4Cr % 21 13 20Ni % 64 70 60Mo % 9.5 6.5 9W % 1.5Nb % 3.3 1.3 3

Rp0.2 MPa 480 450 450Rm MPa 780 710 700

Charpy-V 50 J -196 °C 85 J -196 °C 80 J -196 °CLat. Exp. mm 0.9 1.0-1.5 1.0-1.5

Current DC AC/DC DC

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Design criteria

The predominant standards for LNG storage tanks are API 620 section Q, ASME/AWSand BS7777. The allowable design stress is a certain fraction of either the yield strengthor the tensile strength. Normally inspection authorities and specifications require Rp0.2>430 MPa and Rm >690 MPa for the weld metal allowing full use of the allowablestrength for the 9% nickel steel. Project specifications often require minimum impact>50-70 J and lateral expansion >0.38 mm at -1960C. CTOD value may be specifiedminimum 0.3 mm.

Manual welding

OK 92.45 (ENiCrMo-3) and OK 92.55 (ENiCrMo-6) have typical compositions andmechanical properties as in table 3. The tensile strength of OK 92.45 is much higher thanthe tensile strength of OK 92.55) and consequently the ductility is lower.

OK 92.55 is designed principallyfor AC welding as this is oftenpreferred for welding 9% nickelsteels to minimise effects of themagnetic arc forces as the steel iseasily magnetised during handlingand welding. The productivity isalso higher with OK 92.55 becauseof high metal content in the coating.OK 92.45 is recommended whenthe highest tensile strength isrequired and offer some operationaladvantage in certain joints.

A typical welding procedure withOK 92.55 in vertical welding ofbutt joints in 25 mm thick 9%nickel steels is shown in figure 3.The normal procedure qualificationsinclude transverse tensile tests, bendtests and impact tests of every typeof joint, position and /or thicknessof plate.

Figure 3. Welding procedure SMAW verticalwelding of shell plates

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Mechanized welding

Submerged Arc Welding is used mainly for the circumferential welds of the shells and forjoining of the bottom plates. It is performed with the welding machine hanging on aplatform on the rim of the shell plates. The welding flux is supported by a rubber belt.The combination of a basic agglomerated flux OK Flux 10.16 and a nickel based solidwire OK Autrod 19.82 (ERNiCrMo-3) will result in a weld metal of similar properties asthe weld metal from SMAW. With increased plate thickness the productivity of thewelding operation becomes even more important.

The volume of weld metal is determined by the joint preparation. For the weld metalquality, such as crack resistance, the dilution of the base metal must be limited. The sizeof the weld pool must be restricted to avoid overflow and therefore many runs have to bewelded. Heat input must be kept low to avoid softening or embrittlement of the basemetal. Welding is normally carried out with either 1.2 mm or 2.4 mm wire. With a goodpreparation and proper parameters SAW can be used without welding the root runs withSMAW. One typical welding procedure with 2.4 mm wire is shown in figure 4a. Recentdevelopment of SAW with two wires 1.6 mm (“Twin-arc“) has greatly increased thedeposition rate and consequently reduced the welding time and cost.

Figure 4a. Welding procedureSAW circumferential welding ofshell single wire φ 2.4 mm

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Figure 4b. Welding procedureSAW circumferential welding ofshell “Twin-arc” 2 x φ 1.6 mmwire

Quality control

The consequenses in case of any failure in the welded joints imply a rigorous qualitycontrol of every step in the fabrication. Every production batch of the consumables istested in the presence of inspection authorities to conform to the specifications regardingchemical composition and mechanical properties of the weld metal. In theelectrode/wire/flux roduction also the individual steps in the production are monitored inthe highest level of Quality Assurance. The welding procedures are also qualified for eachtype of joint and thickness. The welded joints during the fabrication are controlled bynon-destructive tests (visual examination, x-ray and ultrasonics) and all procedures arerecorded identifying all relevant parameters.

Review of recent LNG tank projects

Table 4 lists several LNG tank projects where the described welding technology andconsumables have been used in the fabrication. Included are also some projects forethylene tanks using 5% nickel steel (-104°C) but welding with the same consumables forconvenience and added safety. Two spherical 5% nickel steel tanks for ethane (-88.4°C)with 33 mm wall thickness were built in France conforming to the French standardCODAP. The original idea of using 3.5% nickel steel was discarded as 5% steel offersuperior weldability with no need for preheating or postheating and the overall cost couldthus be reduced.

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The largest above ground LNG tank project until now is two double containment tanks of160.000 m3 (EcoElectrica Puerto Rico). The 9% nickel steel tanks are 80 m in diameterand the liquid heigth is 31.5 m. Initially the gas from the first completed tank will befueling a power plant of 500 MW.

Large ocean-going tankers today mostly have spherical tanks of aluminium, a designdeveloped by Moss Verft in Norway, but in the beginning also 9% nickel steel tanks wereused.

Recently a gas tanker (”Mystic Lady”) built in the early 70´s with 9% nickel steelspherical 87.000 m3 tank capacity was repaired and rebuilt by welding with OK 92.55 andnow beginning a 17 year charter contract.

Due to increased attention to safety and the enviromental issues, two single containmentLNG tanks built in the early days in La Spezia, Italy were rewamped 1995 by adding aconcrete containment lined with 9% nickel steel.

Figure 5. Mechanical welding with welding machine hanging on a platform

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Reference list LNG 9% nickel steel projectsAll projects welded with OK 92.55 covered electrodes. Most of them also using SAW withOK Autrod 19.82 + OK Flux 10.16. Most of the stainless piping welded with OK 61.35 or OK 63.35.

Year Company State/City Country Project1986 ATB Brescia Italy LNG project1988 PDM/Tasmi Cartagena Spain 1 LNG tank 55.000 m3

1988 LGA/T-Larsen 5% Ni Hazira India 2 ETH-tanks 17.500 m3

1990 CBI/Enagas Huelva Spain 1 LNG tank 110.000 m3

1990-1991 PDM/Technigaz Marmara Turkey 3 LNG tanks 85.000 m3

1992 CBI/Petrokemya Saudi 1 LNG tank 55.000 m3

1992 CBI Delaware USA LNG project1992 CBI California USA LNG project1992 LGA/T-Larsen India LNG project

1994-1995 PDM/Technigaz/Chioyda Qatar 3 LNG tanks 85.000 m3

1994 PDM Argentina LNG project1994 CBI Australia Propane tanks

1995 Motherwell Bridge India LNG tank1995 PDM/Snamprogetti La Spezia Italy 2 LNG tanks revamping1996 CBI Kuwait LNG tank

1996 Noell-Whessoe Depa Greece 2 LNG tanks 65.000 m3

1996 PDM/Technigaz/Chioyda Qatar 1 LNG tank 85.000 m3

1996 Noell-LGA (5% Ni) Korea 1 ETH-tank 27.000 m3

1997 Technigaz/Sofregaz Ping Hu Shanghai LNG tank 20.000 m3

1997 Noell-Whessoe Point Fortin Trinidad 2 LNG tank 102.000 m3

1997 CBI/Pertamina Bontang Indonesia LNG tank

1998 Daelim/TKK Inchon Korea 4 LNG tanks 100.000 m3

1998 PDM/Enron Penuelas Puerto Rico 2 LNG tanks 160.000 m3

1998 Schelde Sicon/Whessoe Kårstö Norway 1 ETH tank 25.000 m3

1999 Tissot (5% Ni) Marseilles France 2 Spher, ETH tanks 1.200 m3

2000 CBI K Sembawang Indonesia 1 LNG Ship repair 87.000 m3

2000 Egegaz/CB (OK 92.45) Izmir Turkey 2 LNG tanks 140.000 m3

Summary

A review has been given of current technology for welding of above ground LNGstorage tanks. Many projects are being in progress all over the world and with theneed for energy a large number of 9% nickel steel tanks will be built in the future.Larger volume tanks are desirable for the economy of the projects and thedevelopment also will lead to larger wall thicknesses. Therefore both 9% nickel steeland corresponding welding consumables and welding methods will be developed. Acomprehensive project has been launched by the Belgian Welding Institute togetherwith 9% steel manufacturers and welding companies to investigate steels and weldingtechnology for thicknesses between 40-100 mm.

Even more importance will be given on welding productivity for future larger tanks. It isestimated that new technology as flux cored wire welding will be further developed andmore utilised in the future. The design of the LNG tanks will likely remain fundamentallythe same but floating liquefaction plants and storage terminals are being projected.

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Acknowledgements

Mrs Solveig Rigdal ESAB AB, SwedenMr Lennart Johannesson ESAB AB, SwedenMr Jean-Claude Finet Industeel Usinor, Belgium