OA11995A

OA11995A - Improved system for processing, storing, and transporting liquefied natural gas.

Info

Publication number: OA11995A
Application number: OA1200200021A
Authority: OA
Inventors: Robert M Woodall; Ronald R Bowen; Douglas P Fairchild
Original assignee: Exxonmobil Upstream Res Co
Priority date: 2002-01-22
Filing date: 2002-01-22
Publication date: 2006-04-18

Description

1 119 9 5

IMPROVED SYSTEM FOR PROCESSING, STORING, ANDTRANSPORTING LIQUEFIED NATURAL GAS

This is an application for a certificate of addition to O API patent 11239,hereinafter referred to as the "main patent".

5 FIELD OF THE INVENTION

Both the invention of the main patent and the présent invention relate to animproved System for processing, storing, and transporting liquefied naturalgas (LNG), and more particularly, to a novel System for processing, storing,and transporting LNG at substantially increased pressures and températures io over conventional LNG Systems. The relationship between the invention ofthe main patent and the présent invention is described below.

BACKGROUND OF THE INVENTION

Various tenus are defined in the following spécification. For convenience, aGlossary of tenus is provided herein, immediately preceding the daims. 15 Many sources of natural gas are located in remote area s, gréai distanc.»; fiom any commercial markets for the gas. Sometimes a pipeline is available fortransporting produced natural gas to a commercial market When pipelinetransportation to a commercial market is not feasible, produced natural gas is oftenprocessed into LNG for transport to market The LNG is typically transported via 20 specially built tankers, and then stored and revaporized at an import terminal near the market. The equipment used to liquefy, transport, store, and revaporize natural gas isgenerally quite expensive; and a typical conventional LNG project can cost fiom $5billion to $10 billion, including fîeld development costs. A typical “grass roots” LNGproject requires a minimum natural gas resource of about 280 Gm3 (10 TCF (trillion 25 cubic feet)) and the LNG customers are generally large utilities. Often, natural gasresources discovered in remote areas are smaller than 280 Gm3 (10 TCF). Even fornatural gas resource bases that meet the 280 Gm3 (10 TCF) minimum, very long-termcommitments of 20 years or more fiom ail involved, i.e., the LNG supplier, the LNGshipper, and the large utility LNG customer, are required to economically process, 119 9 5 2 store, and transport the natural gas as LNG. Where potential LNG customers hâve analternative source of gas, such as pipeline gas, the conventional LNG chain of deliveryis often not economically compétitive. FIG. 1 illustrâtes schematically a conventional LNG plant that produces LNG5 at températures of about -162°C (-260°F) and at atmospheric pressure. A typical natural gas stream enters a conventional LNG plant at pressures from about 4830 kPa(700 psia) to about 7600 kPa (1100 psia) and températures from about 21°C (70°F) toabout 38°C (100°F). Up to about 350,000 réfrigération horsepower are needed toreduce the température of the natural gas to the very low outlet température of about 10 -162°C (-260°F) in a conventional two-train LNG plant Water, carbon dioxide, sulfur-containing compounds,“such as hydrogen sulfîde, other acid gases, n-pentaneand heavier hydrocarbons, including benzene, must be substantially removed from the"natural gas during conventional LNG Processing, down to parts-per-million (ppm) ·levels, or these compounds will freeze, causing plugging problems in the process 15 equipment In a conventional LNG plant, gas treaîing equipment is required to remove the carbon dioxide and acid gases. The gas treaîing equipment typically usesa Chemical and/or physical solvent regenerative process and requires a signi-finantcapital investment Also, the operating expenses are high in relation to thosé for otherequipment in the plant Dry bed dehydrators, such as molecular sieves, are required to 20 remove the water vapor. The scrub column and fractionation equipment are used toremove the hydrocarbons that tend to cause plugging problems. Mercury is alsoremoved in a conventional LNG plant since it can cause failures in equipmentconstracted of aluminum. In addition, a large portion of the nitrogen that may beprésent in natural gas is removed after processing since nitrogen wûl not remain in the 25 liquid phase during transport of conventional LNG and having nitrogen vapors inLNG containers at die point of delivery is undesirable.

Containers, piping, and other equipment used in a conventional LNG plant aretypically constracted, at least in part, from aluminum or nickel-containing Steel (e.g., 9 wt% nickel), to provide the necessary fracture toughness at the extremely cold 30 processing températures. Expensive materials with gnod fracture toughness at Inw 119 9 5 températures, including aluminum and commercial nickel-containing Steel (e.g., 9wt% nickel), are typically used to contain the LNG in the LNG ships and at the importterminais, in addition to their use in the conventional plant.

Nickel-containing steels conventionally used for cryogénie températurestructural applications, e.g., steels with nickel contents of greater than about 3 wt%,hâve low DBTTs (a measure of toughness, as defined herein), but also hâve relativelylow tensile strengths. Typically, commercially available 3.5 wt% Ni, 5.5 wt% Ni, and9 wt% Ni steels hâve DBTTs of about -10Q°C (-150°F), -155°C (-250°F), and -175°C(-280°F), respectively, and tensile strengths of up to about 485 MPa (70 ksi), 620 MPa(90 ksi), and 830 MPa (120 ksi), respectively. In order to achieve these combinationsof strength and toughness, these steels generally undergo costly processing, e.g.,double annealing treatment. In the case of cryogénie température applications,industry'currently uses these commercial nickel-containing steels because of theirgood toughness at low températures, but must design around their relatively lowtensile strengths. The designs generally require excessive Steel thicknesses forload-bearing, cryogénie température applications. Thus, use of these nickel-containing steels in load-bearing, cryogénie température applications tends tobe expensive due to the high cost of the Steel combined with the Steel thicknessesrequired. A typieal conventional LNG ship utilizes large spherical containers, known asMoss spheres, to store the LNG during transport These ships currently cost morethan about $230 million each. A typieal conventional project to produce LNG in theMiddle East and transport it to the Far East might require 7 to 8 of these ships for atotal cost of about $1.6 billion to $2.0 billion.

As can be determined from the above discussion, the need exists for a moreeconomical System for processing, storing, and transporting LNG to commercialmarkets to allow remote natural gas resources to compete more efîectively withalternative energy supplies. Furthermore, a System is needed to commercializesmaller remote natural gas resources that would otherwise be uneconomical to 4 119 9 5 deveîop. In addition, a more economical gasification and delivery System is neededso that LNG can be made economically attractive to smaller consumers.

Consequently, the primary objects of the invention of the mainpatent and of the présent invention are to provide a more economical 5 System for processing, storing, and transporting LNG from remote sources to commercial markets and to substantially reduce the threshold size of boththe reserve and the market required to make an LNG project economically feasible.One way to accomplish these objects would be to process the LNG at higher pressuresand températures than is done in a conventional LNG plant, i.e., at pressures higher 10 than atmospheric pressure and températures higher than -162°C (-260°F). While the general concept of processing, storing, and transporting LNG at increased pressuresand températures has been discussed in indusuy publications, these publicationsgenerally discuss constructing transportation containers from nickel-containing Steel(e.g., 9 wt% nickel) or aluminum, both of which may meet design requirements but

15 are very expensive materials. For example, at pp. 162-164 of his book NATURAL GAS B Y SEA The Development of a New Technology, published by Witherby & Co.Ltd., first édition 1979, second édition 1993, Roger Ffooks discusses the conversionof the Liberty ship Sigalpha to carry either MLG (medium condition liquefied gas) at1380 kPa (200 psig) and -115°C (-175°F), or CNG (compressed natural gas) 20 processed at 7935 kPa (1150 psig) and -60°C (-75°F). Mr. Ffooks indicates that although technically proven, neither of the two concepts found ‘buyers’ -largely dueto the high cost of storage. According to a paper on the subject referenced by Mr.Ffooks, for CNG service, i.e., at -60°C (-75°F), the design target was a low alloy,weldable, quenched and tempered Steel with good strength (760 MPa (110 ksi)) and 25 good fracture toughness at operating conditions. (See “A new process for the transportation of natural gas” by R. J. Broeker, International LNG Conférence,Chicago, 1968.) This paper also indicates that an aluminum alloy was the lowest costalloy for MLG service, i.e., at the much lower température of-115°C (-175°F). Also,Mr. Ffooks discusses, at p. 164, the Océan Phoenix Transport design, workmg at a 30 much lower pressure of about 414 kPa (60 psig), with tanks that could be constructed of 9 percent nickel Steel or aluminum alloy; and indicates that, again, the concept did 11995' not appear to offer sufiicient technical or financial advantages to becomecommercialized. See also: (i) U.S. Patent 3,298,805, which discusses the use of a 9%nickel content Steel or a high strength aluminum alloy for making containers for thetransport of a compressed naturel gas; and (ii) U.S. Patent 4,182,254, which discusses 5 tanks of 9% nickel or similar Steel for the transport of LNG at températures from -100°C (-148°F) to -140°C (-220°F) and pressures of 4 to 10 atmosphères (i.e., of 407kPa (59 psia) to 1014 kPa (147 psia)); (iii) U.S. Patent 3,232,725, which discussestransportation of a naturel gas in a dense phase single-fluid State at a température aslow as -62°C (-80°F), or in sonie cases -68°C (-90°F), and at pressures at least 345 kPa 10 (50 psi) above the boiling point pressure of the gas at operating températures, using

containers constructed from materials such as 1 to 2 percent nickel Steel which hasbeen quenched and tempered to secure an ultimate tensile strength approaching120,000 psi; and (iv) “Marine Transportation of LNG-at Intermediate Température”,CME March 1979, by C. P. Bennett, which discusses a case study of transport of LNG 15 at a pressure of 3.1 MPa (450 psi) and a température of-100°C (-140°F) using a storage tank constructed from a 9 % Ni Steel or a 3 1/2 % Ni quenched and temperedSteel and having 9 1/2 inch thick walls.

Although these concepts are discussed in industry publications, to ourknowledge, LNG is not currently commercially processed, stored, and transported at 20 pressures substantially higher than atmospheric pressure andtemperatures substantially higher than -162°C (-260°F). This is likely due to the fact that aneconomical System for processing, storing, and transporting the LNG at suchpressures and températures has not heretofore been conceived.

Therefore, a particular object of the invention of the main patent and 25 of the présent invention is tô provide an improved, economical System forProcessing, storing, and transporting LNG at substantially increasedpressures and températures over conventional LNG Systems. 6 11995

S3JMMARY.QF ΤΉΈ; INVENTION

The main patent covers a container for storing pressurized LNG; a marinevessel for transporting pressurized LNG; a method of, and a System for,

Processing natural gas; a method of, and a System for, transporting 5 pressurized LNG; a method and a System for processing and transporting natural gas; and pressurized LNG derived ffom a process according to theinvention. This application incorporâtes the entire disclosure of the mainpatent, and the description of "the présent invention" herein is a descriptionof the invention of both the main patent and the certificate of addition. 10 Consistent with the above-stated objects of the présent invention, a container is provided for storing pressurized liquefied natural gas (PLNG) at a pressure in thebroad range of about 1035 kPa (150 psia) to 7590 kPa (1100 psia) and at atempérature in the broad range of about -123°C (-190°F) to about -62°C (-80°F), saidcontainer being constructed from materials comprising an ultra-high strength, low 15 alloy Steel containing less than 9 wt% nickel and having adéquate strength and fracture toughness to contain said pressurized liquefied natural gas. The Steel has anultra-high strength, e.g., tensile strength (as defined herein) greater than 830 MPa (120ksi), and a DBTT (as defined herein) lower than about -73°C (-100°F). In order tominimize cost, the Steel preferably contains less than about 7 wt% nickel, and more 20 preferably less than about 5 wt% nickel. Additionally, a System for processing and transporting PLNG is provided. The System of the présent invention produces PLNGat pressures in the broad range of about 1035 kPa (150 psia) to about 7590 kPa (1100psia) and at températures in the broad range of about -123°C (-190°F) to about -62°C(-80°F), and uses the containers of this invention for storing and transporting the 25 PLNG.

The présent invention provides a System for processing natural gas to producePLNG, for storing PLNG, and for transporting PLNG to a user site. The System of theprésent invention includes: (i) a processing plant for converting the natural gas toPLNG with a pressure of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) 30 and a température of about -123°C (-190°F) to about -62°C (-80°F), wherein the 119 9 5

Processing plant consists essentially of - (a) réception facilities for receiving anatural gas stream and removing liquid hydrocarbons from the natural gas; (b)déhydration facilities for removing sufficient water vapor from the natural gas toprevent freezing of the natural gas at the PLNG températures andpressures; and (c)liquéfaction facilities for converting the natural gas to PLNG; - (ii) storagecontainers constructed from materials comprising an ultra-high strength, low alloySteel containing less than 9 wt% nickel and having a tensile strength greater than 830MPa (120 ksi) and a DBTT lower than about -73°C (-100°F); (iii) export terminal (a)including storage containers for storing the PLNG and facilities for transferring thePLNG into transportation storage containers on-board a transportation vessel or,optionally, (b) consisting essentially of facilities for transferring the PLNG intotransportation storage containers on-board a transportation vessel; (iv) transportationvessels, including transportation storage containers, for transporting the PLNG to animport terminal, and, optionally, including on-board vaporizing equipment to couvertthe PLNG to a gas; and (v) import terminal (a) including import storage containers(wherein the import storage containers are land-based, or floating-vessel-based, oroffshore-fixed-structure-based), facilities for transferring the PLNG from thetransportation storage containers to the import storage containers, and facilities forvaporizing the PLNG for delivery to pipelines or usera’ facilities, or, optionally, (b)consisting essentially of import facilities (wherein the import facilities are land-based,or floating-vessel-based, or offshore-fixed-structure-based), including vaporizingequipment, for receiving the PLNG from the transportation storage containers and forconverting the PLNG to a gas and delivering the gas to pipelines or usera’ facilities,or, optionally (c) consisting essentially of facilities for transferring the gas convertedfrom PLNG by on-board vaporizing equipment to pipelines or usera’ facilities at thedock or via offshore mooring connections, such as a single anchor leg mooring(SALM).

This application for a Certificate of Addition to the main patent protects atempérature range which is broadly disclosed in the main patent and whichfalls wholly within the température range mentioned in the daims of themain patent, but which is not specifically claimed in the main patent. 119 9 5

According to one of the aspects of the invention specifically covered by theprésent application, there is provided a method for processing andtransporting natural gas, said method comprising the steps of: (a) processing said natural gas by converting it into pressurized liquefiednatural gas having a pressure of about 1725 kPa (250 psia) to about 7590Kpa (1100 psia) and a température of about -100°C (-148°F) to about -62°C (-80°F); (b) placing said pressurized liquefied natural gas into a plurality ofstorage containers at a first location, wherein said plurality of storagecontainers hâve adéquate strength and toughness to contain said pressurizedliquefied natural gas at said pressure and température conditions; and (c) transporting said plurality of storage containers containing saidpressurized liquefied natural gas away from said first location toward asecond location.

According to another of the aspects of the invention specifically covered bythe présent application, there is provided a System for processing andtransporting natural gas comprising: (a) a processing plant for converting said natural gas to pressurizedliquefied natural gas having a pressure of about 1725 kPa (250 psia) to about7590 kPa (1100 psia) and a température of about -100°C (-148°F) toabout -62°C (-80°F); (b) a plurality of storage containers for receiving said pressurizedliquefied natural gas, wherein said plurality of storage containers hâveadéquate strength and toughness to contain said pressurized liquefied naturalgas at said pressure and température conditions; and (c) at least one marine vessel adapted for holding and transporting saidplurality of storage containers containing said pressurized liquefied naturalgas.

According to a further aspect of the invention specifically covered by the présent application, there is provided pressurized liquefied natural gas derived from a process for liquefying natural gas to produce said pressurized 11995 9 liquefied natural gas as an outlet product at a pressure of about 1725 kPa(250 psia) to about 7590 kPa (1100 psia) and a température of about -100°C(-148°F) to about -62°C (-80°F) and containing at least one componentselected from the group consisting of (i) carbon dioxide, (ii) n-pentane plus, 5 and (iii) benzene, in an amount that would ffeeze in liquefied natural gas at a pressure of about atmospheric pressure and a température of about -162°C(-260°F).

Also covered by the présent application is pressurized liquefied natural gasderived from a process for liquefying natural gas to produce said pressurized 10 liquefied natural gas as an outlet product at a pressure of about 1725 kPa (250 psia) to about 7590 kPa (1100 psia) and a température of about -100°C(-148°F) to about -62°C (-80°F), wherein said process consiste essentially ofthe steps of (i) receiving said natural gas in réception facilities; (ii)removing sufficient water vapor from said natural gas to prevent freezing of 15 said natural gas during said process; and (iii) liquefying said natural gas to produce said outlet product.

In particular, the aspects of the invention covered by the présent applicationdefine the lower limit of the température range of the LNG, as -100°C (-148°F).

20 DESCRIPTION OF THE DRAWINGS

The advantages of the présent invention will be better understood by referring tothe following detailed description and the attached drawings in which: FIG. 1 (PRIOR ART) schematically illustrâtes an exemplary plant forProcessing conventional LNG; FIG. 2 schematically illustrâtes an exemplary plant for processing PLNGaccording to the présent invention; 25 119 9 5 10 FIG. 3 A illustrâtes an end view of an exemplary ship for transporting PLNGaccording to the présent invention; FIG. 3B illustrâtes a side view of an exemplary ship for transporting PLNGaccording to the présent invention;

5 FIG. 3C illustrâtes a top view of an exemplary ship for transporting PLNG according to the présent invention; FIG. 4A illustrâtes an end view of an exemplary ship for transporting PLNGaccording to the présent invention and having an on-board PLNG vaporizer; FIG. 4B illustrâtes a side view of an exemplary ship for transporting PLNG10 according to the présent invention and having an on-board PLNG vaporizer; FIG. 4C illustrâtes a top view of an exemplary ship for transporting PLNG —according to the présent invention and having an on-board PLNG vaporizer; FIG. 5A illustrâtes a plot of critical flaw depth, for a given flaw length, as afonction of CTOD fracture toughness and of residual stress; and 15 FIG. 5B illustrâtes the geometry (length and depth) of a flaw.

While the invention will be described in connection with its preferredembodiments, it will be understood that the invention is not limited thereto. On thecontrary, the invention is intended to cover ail alternatives, modifications, andéquivalents which may be included within the spirit and scope of the invention, as 20 defined by the appended daims.

DETAILED DESCRIPTION OF THE INVENTION PLNG Storage Containers 25

The key to achieving the PLNG plant and transportation vessels of the présentinvention are the storage containers for storing and transporting the PLNG, which isproduced at a pressure in the broad range of about 1035 kPa (150 psia) to about 7590kPa (1100 psia) and at a température in the broad range of about -123°C (-190°F) to 30 about -62°C (-80°F). Storage containers for the PLNG are constructed from matériels 11 119 9 5 comprising an ultra-high strength, low alloy Steel that has both adéquate strength andfracture toughness for the operating conditions of the PLNG System of the présentinvention, including pressures and températures. The Steel has a tensile strengthgreater than 830 MPa (120 ksi), preferably greater than about 860 MPa (125 ksi), and 5 more preferably greater than about 900 MPa (130 ksi). In some applications, a Steelhaving a tensile strength of greater than about 930 MPa (135 ksi), or greater thanabout 965 MPa (140 ksi), or greater than about 1000 MPa (145 ksi), is préférable.

The Steel also preferably has a DBTT lower than about -73°C (-100°F). Additionally,a container is provided for storing pressurized liquefied natural gas at a pressure of 10 about 1725 kPa (250 psia) to about 4830 kPa (700 psia) and at a température of about-112°C (-170°F) to about -79°C (-110°F), wherein said container (i) is constructedfrom materials comprising an ultra-high strength, low alloy Steel containing less than9 wt% nickel and (ii) has adéquate strength and fracture toughness to contain saidpressurized liquefied natural gas. 15 The ultra-high strength, low alloy steels used for constructing containers according to this inventions preferably contain low quantifies of expensive alloys,such as nickel. Preferably the nickel content is less than 9 wt%, more preferably lessthan about 7 wt%, and even more preferably less than about 5 wt%. More preferably,such steels contain the minimum amount of nickel necessary to pro vide the required 20 fracture toughness. Preferably such ultra-high strength, low alloy steels contain lessthan about 3 wt% nickel, more preferably less than about 2 wt% nickel, and evenmore preferably less than about 1 wt% nickel.

Preferably, such steels are weldable. These ultra-high strength, low alloysteels facilitate the construction of containers to transport the PLNG at a substantially 25 lower cost per pound for the Steel than would be possible with currently available alternatives of aluminum or commercial nickel-containing steels (e.g., 9 wt% nickel).Preferably the Steel used for constructing storage containers of this invention is nottempered. However, a tempered Steel having the necessary strength and fracturetoughness may be used for constructing storage containers of this invention. 12 119951

As will be familiar to those skilled in the art, the Charpy V-notch (CVN) testcan be used for the purpose of fracture toughness assessment and fracture contro'l inthe design of storage containers for transporting pressurized, cryogénie températurefluids, such as PLNG, particularly through use of the ductile-to-brittle transitiontempérature (DBTT). The DBTT delineates two fracture régimes in structural steels.At températures below the DBTT, failure in the Charpy V-notch test tends to occur bylow energy cleavage (brittle) fracture, while at températures above the DBTT, failuretends to occur by high energy ductile fracture. Storage and transportation containersthat are constructed from welded steels for the aforementioned cryogénie températureapplications and for other load-bearing, cryogénie température service must hâveDBTTs, as deteimined by the Charpy V-notch test, well below the service températureof the structure in order to avoid brittle failure. Depending on the design, the serviceconditions, and/or the requirements of the applicable classification society, therequired DBTT température shift may be from 5°C to 30°C (9°F to 54°F) below theservice température.

As will be familiar to those skilled in the art, the operating conditions takeninto considération in the design of storage containers constructed from a welded Steelfor transporting pressurized, cryogénie fluids, include among other things, theoperating pressure and température, as well as additional stresses that are likely to beimposed on the Steel and the weldments (see Glossary). Standard fracture mechanicsmeasurements, such as (i) critical stress intensity factor (K,c)> which is a measurementof plane-strain fracture toughness, and (ii) crack tip opening displacement (CTOD),which can be used to measure elastic-plastic fracture toughness, both of which arefamiliar to those skilled in the art, may be used to détermine the fracture toughness ofthe Steel and the weldments. Industry codes generally acceptable for Steel structuredesign, for example, as presented in the BSI publication “Guidance on methods forassessing the acceptability of flaws in fusion welded structures”, often referred to as“PD 6493 :1991”, may be used to détermine the maximum allowable flaw sizes forthe containers based on the fracture toughness of the Steel and weldment (includingHAZ) and the imposed stresses on the container. A person skilled in the art can 119 9 5 13 develop a fracture control program to mitigate fracture initiation through (i)appropriate container design to minimize imposed stresses, (ii) appropriatemanufacturing quality control to minimize defects, (iii) appropriate control of lifecycle loads and pressures applied to the container, and (iv) an appropriate inspectionprogram to reliably detect flaws and defects in the container. A preferred designphilosophy for the System of the présent invention is “leak before failure”, as isfamiliar to those skilled in the art. These considérations are generally referred toherein as “known principles of fracture mechanics.”

The following is a non-limiting example of application of these knownprinciples of fracture mechanics in a procedure for calculating critical flaw depth for agiven flaw length for use in a fracture control plan to prevent fracture initiation in apressure vessel, such as a storage container according to this invention. FIG. 5B illustrâtes a flaw of flaw length 315 and flaw depth 310. PD6493 isused to calculate values for the critical flaw size plot 300 shown in FIG. 5A based onthe following design conditions:

Vessel Diameter: „Vessel Wall Thickness:Design Pressure:Allowable Hoop Stress: 4.57 m (15 ft) 25.4 nam (1.00 in.)3445 kPa (500 psi)333 MPa (48.3 ksi).

For the purpose of this example, a surface flaw length of 100 mm (4 inches),e.g., an axial flaw located in a seam weld, is assumed. Referring now to FIG. 5A, plot300 shows the value for critical flaw depth as a fonction of CTOD fracture toughnessand of residual stress, for residual stress levels of 15,50 and 100 percent of yieldstress. Residual stresses can be generated due to fabrication and welding; andPD6493 recommends the use of a residual stress value of 100 percent of yield stress inwelds (including the weld HAZ) unless the welds are stress relieved using techniquessuch as post weld heat treatment (PWHT) or mechanical stress relief. 11995 14

Based on the CTOD fracture toughness of the pressure vessel Steel at theminimum service température, the vessel fabrication can be adjusted to reduce theresidual stresses and an inspection program can be implemented (for both initialinspection and in-service inspection) to detect and measure flaws for comparison 5 against critical flaw size. In this example, if the Steel has a CTOD toughness of 0.025mm at the minimum service température (as measured using laboratoiy specimens)and the residual stresses are reduced to 15 percent of the Steel yield strength, then thevalue for critical flaw depth is approximately 4 mm (see point 320 on FIG. 5A).Following similar calculation procedures, as are well known to those skilled in the art, 10 critical flaw depths can be determined for various flaw lengths as well as various flawgeometries. Using this information, a quality control program and inspection program(techniques, détectable flaw dimensions, frequency) can be developed to ensure thatflaws are detected and remedied prior to reaching the critical flaw depth or prior to theapplication of the design loads. Based on published empirical corrélations between 15 CVN, KIC and CTOD fracture toughness, the 0.025 mm CTOD toughness generallycorrelates to a CVN value of about 37 J. This example is not intended to limit thisinvention in any way.

The storage containers are preferably constructed from discrète plates of theultra-high strength, low alloy Steel. The joints, including welded joints, of the storage 20 containers preferably hâve about the same strength and fracture toughness as theultra-high strength, low alloy stèel plates. In some cases, an undermatching of thestrength on the order of about 5% to about 10% may be justified for locations of lowerstress within the container. Joints with the preferred properties can be made by anyjoining technique capable of producing the required balance of strength and low 25 température toughness. Exemplary joining techniques are described in the Examplessection hereof. Particularly preferred joining techniques include gas métal arcwelding (GMAW) and tungsten inert gas (TIG) welding. For certain operatingconditions (as described in the Example section hereof) submerged arc welding(S AW), électron beam welding (EBW), and laser beam welding (LBW) may be used. 11995 ’ 15 PLNG Plant

The storage containers described above make feasibie the PLNG processingmethod of the présent invention, which produces PLNG at a pressure in the broad 5 range of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a température in the range of about -123°C (-190°F) to about -62°C (-80°F). Preferably,PLNG is produced and transported at a pressure in the range of about 1725 kPa (250psia) to àbout 7590 kPa (1100 psia) and at a température in the range of about -112°C(-170°F) to about -62°C (-80°F). More preferably, PLNG is produced and transported 10 at a pressure in the range of about 2415 kPa (350 psia) to about 4830 kPa (700 psia)and at a température in the range of about -101°C (-150°F) to about -79°C (-110°F).Even more preferably, the lower ends of the pressure and température ranges forPLNG are about 2760 kPa (400 psia) and about -96°C (-140°F). Within the preferredranges, idéal température and pressure combinations are dépendent upon the

15 composition of the natural gas being liquefied and upon économie considérations. A person skilled in the art may détermine the effect of compositional parameters byreferring to standard industry publications and/or performing equilibrium bubble pointcalculations. Additionally, a person skilled in the art may détermine and analyze theimpact of the different économie considérations by referring to standard industry 20 publications. For example, one économie considération is that as the température ofthe PLNG gets colder, réfrigération horsepower requirements increase; however,colder températures at the increased pressures for PLNG also increase the density ofthe PLNG and, thereby, reduce the volume that must be transported. As thetempérature of the PLNG gets warmer, and the pressure increases, more Steel is 25 required in storage and transport containers, but the réfrigération costs decrease andthe plant effîciency increases.

The followmg description focuses primarily on the economically advantageousdifférences of a System of the présent invention as compared to a conventional systemfor processing LNG. FIG. 2 illustrâtes schematically an exemplaiy plant for 30 processing PLNG according to the présent invention. For comparison purposes, FIG. 119 9 5 16 1 illustrâtes schematically an exemplary plant for processing conventional LNG. Asshown in FIG. 1, an exemplary plant for processing conventional LNG includes feedgas réception equipment 62, gas treating equipment 52, dehydration/mercury removalequipment 56, réfrigération equipment 63, feed scrub equipment 64, fractionationequipment 65, liquéfaction equipment 66, and nitrogen rejection equipment 54.

While standard natural gas liquéfaction equipment can be satisfactorily utilized in aprocessing plant for the présent invention, several steps required in a conventionalLNG plant can be eliminated, and energy needed for cooling the natural gas is greatlyreduced. Thus, in the PLNG process, natural gas that would be consumed forproviding energy in the conventional LNG process can be converted to marketablePLNG. Referring to FIG. 2, the PLNG processing steps preferably include (i) feedgas réception facilities 10 for removing liquid hydrocarbons, (ii) déhydration facilities12, and (iii) liquéfaction facilities 14. An expander plant 16 and fractionation train 18can be used to produce make-up réfrigérants for use in the liquéfaction facilities 14.Altematively either part or ail of the réfrigérants needed for the liquéfaction 14 can bepurchased and/or supplied from some other source. Well-known réfrigérationprocesses can be used to achieve the desired low température of the PLNG. Suchprocesses may include, for example, a single réfrigérant, a multicomponentréfrigérant, a cascade réfrigération cycle or combinations of these cycles.

Additionally, expansion turbines may be used in the réfrigération process. Comparedto a conventional LNG plant, the very large réduction of necessary réfrigérationhorsepower in a PLNG plant according to the présent invention results in a largeréduction in capital costs, proportionately lower operating expenses, and increasedefficiency and reliability, thus greatly enhancing the économies of producing liquefiednatural gas. A plant for producing PLNG according to the présent invention compares to aconventional LNG process as follows. Refening to FIG. 1 and FIG. 2, since theliquéfaction températures in the PLNG plant 8 (FIG. 2) are higher than in aconventional LNG plant 50 (FIG. 1) (that produces conventional LNG at about-162°C (-260°F) and atmospheric pressure), the gas treating equipment 52 (FIG. 1) for 119 9 5 ’i 17 removàl of freezable components such as carbon dioxide, n-pentane plus, andbenzene, which are required in the conventional LNG plant 50, are generally notrequired in the PLNG plant 8 because these naturally occurring components will notnormally freeze and cause plugging problems in the PLNG plant equipment due to the 5 warmer operating températures. If unusually high amounts of carbon dioxide, sulfur-containing compounds, n-pentane plus, or benzene are présent in a natural gasbeing processed by a PLNG plant 8, some gas treating equipment for removal thereofmay be added as needed. Additionally, nitrogen must be removed in the conventionalLNG plant 50 (in the nitrogen rejection facility 54) because nitrogen will not rem a in 10 in the liquid phase during transport of conventional LNG, which is at atmosphericpressure. Moderate amounts of nitrogen in the inlet gas need not be removed in thePLNG plant 8 because nitrogen will remain in the liquid phase with the liquefiedhydrocarbons at the operating pressures and températures of the PLNG process.Additionally, mercury is removed in a conventional LNG plant 50 (in mercury 15 removal equipment 56). Since the PLNG plant 8 opérâtes at much warmer températures than a conventional LNG plant 50 and, therefore, aluminum materialsneed not be used in the containers, piping, and other equipment of the PLNG plant 8,mercury removal equipment will generally not be required in the PLNG plant 8. Theability to omit the equipment required for gas treating, nitrogen rejection, and mercury 20 removal when the composition of the natural gas allows, provides significant technicaland économie advantage.

At the preferred operating pressures and températures of the présent invention,about 3½ wt% nickel Steel can be used in the coldest operating areas of the PLNGplant 8 for the process piping and facilities, whereas the more expensive 9 wt% nickel 25 Steel or aluminum is generally required for the same equipment in a conventional LNG plant 50. This provides another significant cost réduction for the PLNG plant 8compared to the conventional LNG plant. Preferably, high strength, low alloy steelswith adéquate strength and fracture toughness at the operating conditions of the PLNGplant 8, are used to construct the piping and associated components (e.g., franges, 119 9 5 18 valves, and fittings), pressure vessels, and other equipment of the PLNG plant 8 inorder to provide further économie advantage over a conventional LNG plant.

Referring again to FIG. 1, LNG produced in a conventional LNG plant 50 isstored in one or more storage containers 51 at a nearby export terminal. Referring 5 now to FIG. 2, PLNG produced in a PLNG plant 8 may be stored in one or more storage containers 9, constructed of an ultra-high strength, low alloy Steel according tothis invention, at a nearby export terminal. In another embodiment of this invention,PLNG produced in a PLNG plant 8 may be transferred to one or more transportationstorage containers 9, constructed of an ultra-high strength, low alloy Steel according to 10 this invention, on a PLNG transportation vessel, as further described below. A PLNG plant, according to this invention, can be used as a peak shaving plant to allow storage of natural gas as PLNG. For example, a conventional LNGimport terminal receives LNG by ship, stores the LNG, and vaporizes the LNG fordelivery to a gas distribution grid. Stored LNG generates vapors (“boiloff”) as it 15 warms. Usually, the boiloff is withdrawn front the LNG storage container and delivered to the gas distribution grid with the vaporized LNG. During periods of lowgas demand, the boiloff can exceed the volume of vapors required for delivery to thegrid. In such instances, the boiloff is generally reliquefied and stored as LNG untilneeded during periods of high demand. Using the présent invention, the boiloff cari 20 be reliquefied to PLNG and stored until needed during periods of high demand. In another example, a company that provides gas to consumers for home or businessheating typically obtains extra natural gas for distribution to consumers during periodsof peak demand by vaporizing LNG. Using the présent invention, the company canobtain extra natural gas for distribution to consumers during periods of peak demand 25 by vaporizing PLNG. Use of PLNG in peak shaving plants, rather than LNG, can be more economical. 119 9 5 19 PLNG Transportation Vessels

The PLNG transportation vessels of the présent invention contain storagecontainers constructed from ultra-high strength, low alloy steels as described above.The PLNG transportation vessels are preferably marine vessels, e.g., ships, which arepropelled across a body of water from a PLNG export terminal to a PLNG importterminal. The PLNG product has a density that is less than the density ofconventional LNG. Typically, the PLNG product density is about 75% (or less) of thedensity of conventional LNG. Thus, a fleet of ships with a total volume carryingcapacity of about 125% or more than that of a fleet for a conventional project totransport conventional LNG is desired for the System of thepresent invention to carrythe increased production from a more efficient plant as well as the increased volumedue to lower density. FIGs 3A, 3B, and 3C illustrate an exemplary high capacity shipdesigned to cany PLNG. This exemplary PLNG ship 30 holds forty-eight storagecontainers 32 cylindrical in shape with hemispherical or ellipsoidal heads. Thecontainers can also be spherical in shape. The number and dimensions of thecontainers are dépendent upon actual tensile strength of the ultra-high strength, lowalloy Steel, the wall thickness of the containers, and the design pressure, as is familiarto those skilled in the art. PLNG ships are estimated to cost less than conventional LNG ships and hâvesignificantly more carrying capacity than the largest ships currently carryingconventional LNG.

In a preferred embodiment of the présent invention, the containers hold PLNGat températures from about -101°C (-150°F) to about -79°C (-110°F), and this requiressome form of insulation. Currently commercially available industrial insulatingmaterials with good low température insulating properties can be used.

The PLNG ship design offers flexibility in alternatives to meet customers’needs and minimize costs, as more fully described below in the discussion of importterminais. The ship can be designed for a spécifie capacity by adding or deletingPLNG containers. It can be designed to load/unload PLNG in a short period of time 1 1 9 9 5 ^ 20 (typically 12 hours) or load/unload at slower rates up to plant production rates. If thecustomer desires to reduce its import costs to a minimum, the PLNG ship can bedesigned to include on-board vaporization equipment to deliver gas directly to thecustomer, as illustrated by FIGs 4A, 4B, and 4C. The exemplary PLNG ship 40 holdsforty-four storage containers 42 and on-board vaporization equipment 44.

The PLNG ship offers a number of advantages over a conventional LNG ship.Such advantages include substantially larger carrying capacity, a Iower cost, theability to more easily tailor canying capacity to meet customers’ needs, the ability todeliver PLNG in liquid form or to vaporize PLNG on-board to a gas for delivery,Iower pumping costs since PLNG is at a higher pressure (about 2415 kPa (350 psia) toabout 4830 kPa (700 psia) at the preferred conditions) as compared to atmosphericpressure (about 100 kPa (14.7 psia)) for conventional LNG, and shorter constructiontime since the storage containers and associated piping can be pre-fabricated and liftedinto place, thus minimizing labor required on-board the ship. PLNG Export and Import Terminais

The PLNG export terminal can include a dock, storage tanks, and shippingpumps. The PLNG import terminal can include a dock, storage tanks, shippingpumps, and vaporizing equipment. PLNG storage containers at the export terminaland import terminal are preferably constructed from ultra-high strength, low alloysteels that hâve adéquate strength and fracture toughness for the operating conditionsof the PLNG System of the présent invention, including pressures and températures.

Altematively, storage tanks can be eliminated at the PLNG export terminaland/or PLNG import terminal. In a PLNG System with no storage tanks at the exportterminal, produced PLNG is transferred directly from the PLNG plant totransportation storage vessels on-board a PING transportation vessel. In a PLNGSystem with no storage tanks at the import terminal, the import terminal consisteessentially of vaporization equipment or, altematively, each transportation vessel inthe PLNG fleet has standard vaporization equipment on-board to directly couvert the 119 9 5 21 PLNG to pipeline quality gas. For the case where neither the PLNG export terminalnor the PLNG import terminal hâve storage containers, for example, two PLNGtransportation vessels are added to the fleet of PLNG transportation vessels over thenumber that would typically be required to transport and deliver the PLNG to marketusing export and import terminais. Thus, while the other PLNG transportation vesselsare in transit, one of the additional PLNG transportation vessels is moored at theexport terminal, either being filled with or storing the PLNG, and the other additionalPLNG transportation vessels is moored at the import terminal delivering PLNGdirectly to market. In the case of vaporizers on the transportation vessels, suchmooring can be offshore, such as a single anchor leg mooring (S ALM). Thesealternatives are economically advantageous over conventional LNG Systems and cansubstantially reduce the cost of export and import terminais.

EXAMPLES

Example PLNG Storage Containers

As discussed above, containers for storing and transporting PLNG accordingto the présent invention are preferably constructed from ultra-high strength, low alloySteel plates containing less than 9 wt% nickel and having a tensile strength greaterthan 830 MPa (120 ksi). Any such ultra-high strength, low alloy Steel havingadéquate toughness for containing PLNG at operating conditions, according to theknown principles of fracture mechanics as explained above, may be used forconstructing the containers for storing and transporting PLNG of this invention.Preferably, such Steel has a DBTT lower than about -73°C (-100°F).

Recent advances in Steel making technology hâve made possible themanufacture of new, ultra-high strength, low alloy steels with excellent cryogénietempérature toug'hness. For example, three U.S. patents issued to Koo et al.,5,531,842,5,545,269, and 5,545,270, describe new steels and methods for processingthese steels to produce Steel plates with tensile strengths of about 830 MPa (120 ksi), 11995 22 965 MPa (140 ksi), and higher. The steeis and processing methods described thereinhâve been improved and modifîed to provide combined Steel chemistries andProcessing for manufacturing ultra-high strength, low alloy steeis with excellentcryogénie température toughness in both the base Steel and in the heat affected zone 5 (HAZ) when welded. These ultra-high strength, low alloy steeis also hâve improvedtoughness over standard commercially available ultra-high strength, low alloy steeis.The improved steeis are described in a co-pending U.S. provisional patent applicationentitled’ “ULTRA-HIGH STRENGTH STEELS WITH EXCELLENT CRYOGENICTEMPERATURE TOUGHNESS”, which has a priority date of 19 December 1997 10 and is identified by the United States Patent and Trademark Office (“USPTO”) as

Application Number 60/068194; in a co-pending U.S. provisional patent applicationentitled “ULTRA-HIGH STRENGTH AUS AGED STEELS WITH EXCELLENTCRYOGENIC TEMPERATURE TOUGHNESS”, which has a priority date of 19December 1997 and is identified by the USPTO as Application Number 60/068252; 15 and in a co-pending U.S. provisional patent application entitled “ULTRA-HIGHSTRENGTH DUAL PHASE STEELS WITH EXCELLENT CRYOGENICTEMPERATURE TOUGHNESS”, which has a priority date of 19 December 1997and is identified by the USPTO as Application Number 60/068816. (collectively, the“Steel Patent Applications”). 20 The new steeis described in the Steel Patent Applications, and further described in the examples below, are especially suitable for constructing thecontainers for storing and transporting PLNG of this invention in that the steeis hâvethe following characteristics, preferably for Steel plate thicknesses of about 2.5 cm (1inch) and greater: (i) DBTT lower than about -73°C (-100°F), preferably lower than 25 about -107°C (-160°F), in the base Steel and in the weld HAZ; (ii) tensile strengthgreater than 830 MPa (120 ksi), preferably greater than about 860 MPa (125 ksi), andmore preferably greater than about 900 MPa (130 ksi); (iii) superior weldability; (iv)substantially uniform through-thickness microstructure and properties; and (v)improved toughness over standard, commercially available, ultra-high strength, low 30 aUoy steeis. Even more preferably, these steeis hâve a tensile strength of greater than 23 119 9 5 about 930 MPa (135 ksi), or greater than about 965 MPa (140 ksi), or greater thanabout 1000 MPa ( 145 ksi).

First Steel Example 5

As discussed above, a copending U.S. provisional patent application, having apriority date of 19 December 1997, entitled “Ultra-High Strength Steels WithExcellent Cryogénie Température Toughness”, and identiiîed by the USPTO as , Application No. 60/068194, provides a description of steels suitable for use in the10 présent invention. A method is provided for preparing an ultra-high strength Steel plate having a microstructure comprising predominantly tempered fîne-grained lathmartensite, tempered fîne-grained lower bainite, or mixtures thereof, wherein themethod comprises the steps of (a) heating a Steel slab to a reheating températuresuffîciently high to (i) substantially homogenize the Steel slab, (ii) dissolve 15 substantially ail carbides and caibonitrides of niobium and vanadium in the Steel slab,and (iii) establish fine initial austenite grains in the Steel slab; (b) reducing the Steelslab to form Steel plate in one or more hot rolling passes in a first température range inwhich austenite recrystaliizes; (c) further reducing the Steel plate in one or more hotrolling passes in a second température range below about the Tnr température and 20 above about the Ar3 transformation température; (d) quenching the Steel plate at a cooling rate of about 10°C per second to about 40°C per second (18°F/sec - 72°F/sec)to a Quench Stop Température below about the Mj transformation température plus 200°C (360°F); (e) stopping the quenching; and (f) tempering the Steel plate at atempering température from about 400°C (752°F) up to about the Ac, transformation 25 température, preferably up to, but not including, the Ac, transformation température,for a period of tune sufficient to eause précipitation of hardening particies, i.e., one ormore of ε-copper, Mo2C, or the carbides and carbonitrides of niobium and vanadium.The period of time sufficient to cause précipitation of hardening particies dépendsprimarily on the thickness of the Steel plate, the chemistry of the Steel plate, and the 30 tempering température, and can be determined by one skilled in the art (See Glossary 24 11995 for définitions of predominantly, ofhardening particles, of Tnr température, of Ar3,Ms, and Ac, transformation températures, and of Mo2C.)

To ensure ambient and cryogénie température toughness, steels according tothis first Steel example preferably hâve a microstructure comprised of predominantly 5 tempered fine-grained lower bainite, tempered fine-grained lath martensite, ormixtures thereof. It is préférable to substantially minimize the formation ofembrittling constituées such as upper bainite, twinned martensite and MA. As usedin this first Steel example, and in the daims, “predominantly” means at least about 50volume percent. More preferably, the microstructure comprises at least about 60 10 volume percent to about 80_volume percent tempered fine-grained lower bainite, tempered fine-grained lath martensite, or mixtures thereof. Even more preferably, themicrostructure comprises at least about 90 volume percent tempered fine-grained lowerbainite, tempered fine-grained lath martensite, or mixtures thereof. Most preferably, themicrostructure comprises substantially 100% tempered fine-grained lath martensite. 15 A Steel slab processed according to this first Steel example is manufactured in a customary fashion and, in one embodiment, comprises iron and the followingalloying éléments, preferably in the weight ranges xndicated in the following Table I: 119 9 5 25 10

Table I AHoying Elément

Range (wt%) carbon (C) 0.04 - 0.12, more preferably 0.04 - 0.07 manganèse (Mn) 0.5 - 2.5, more preferably 1.0 - 1.8 nickel (Ni) 1.0 - 3.0, more preferably 1.5 - 2.5 copper (Cu) 0.1 - 1.5, more preferably 0.5 - 1.0 molybdenum (Mo) 0.1 - 0.8, more preferably 0.2 - 0.5 niobium (Nb) 0.02 - 0.1, more preferably 0.03 - 0.05 titanium (Ti) 0.008 - 0.03, more preferably 0.01 - 0.02 aluminum (Al) 0.001 - 0.05, more preferably 0.005 - 0.03 nitrogen (N) 0.002 - 0.005, more preferably 0.002 - 0.003 15 V anadium (V) is sometimes added to the Steel, preferably up to about 0.10 wt%, and more preferably about 0.02 wt% to about 0.05 wt%.

Chromium (Cr) is sometimes added to the Steel, preferably up to about 1.0wt%, and more preferably about 0.2 wt% to about 0.6 wt%.

Silicon (Si) is sometimes added to the Steel, preferably up to about 0.5 wt%, 20 more preferably about 0.01 wt% to about 0.5 wt%, and even more preferably about 0.05 wt% to about 0.1 wt%.

Boron (B) is sometimes added to the Steel, preferably up to about 0.0020 wt%,and more preferably about 0.0006 wt% to about 0.0010 wt%.

The Steel preferably contains at least about 1 wt% nickel. Nickel content of25 the Steel can be increased above about 3 wt% if desired to enhance performance after welding. Each 1 wt% addition of nickel is expected to lower the DBTT of the Steel byabout 10°C (18°F). Nickel content is preferably less than 9 wt%, more preferably lessthan about 6 wt%. Nickel content is preferably minimized in order to minimize costof the Steel. If nickel content is increased above about 3 wt%, manganèse content can 119 9 5 26 be decreased below about 0.5 wt% down to 0.0 wt%. Therefore, in a broad sense, upto about 2.5 wt% manganèse is preferred.

Additionally, residuals are preferably substantially minimized in the Steel.Phosphorous (P) content is preferably less than about 0.01 wt%. Sulfur (S) content ispreferably less than about 0.004 wt%. Oxygen (O) content is preferably less thanabout 0.002 wt%.

In somewhat greater detail, a Steel according to this first Steel example isprepared by forming a slab of the desired composition as described herein; heating theslab to a température of fforn about 955°C to about 1065°C (1750°F - 1950°F); hotrolling the slab to fonn Steel plate in one or more passes providing about 30 percent toabout 70 percent réduction in a first température range in which austeniterecrystallxzes, i.e., above about the Tnr température, and further hot rolling the Steelplate in one or more passes providing about 40 percent to about S0 percent réductionin a second température range below about the Tnr température and abové about the

Ar3 transformation température. The hot rolled Steel plate is then quenched at acooling rate of about 10°C per second to about 40°C per second (18°F/sec - 72°F/sec)to a suitable QST (as defined in the Glossary) below about the Ms transformation température plus 200°G (360°F), at which time the quenching is terminated. In oneembodiment of this first Steel example, the Steel plate is then air cooled to ambienttempérature. This processing is used to produce a microstructure preferablycomprising predominantly fine-grained lath martensite, fîne-grained lower bainite, ormixtures thereof, or, more preferably comprising substantially 100% fine-grained lathmartensite.

The thus direct quenched martensite in steels according to this first Steelex ample has high strength but its toughness can be improved by tempering atasuitable température from above about 400°C (752°F) up to about the Ac,transformation température. Tempering of Steel within this température range alsoleads to réduction of the quenching stresses which in tum leads to enhancedtoughness. While tempering can enhance the toughness of the Steel, it notmally leads 119 9 5 27 to substantial loss of strength. In the présent invention, the usual strength loss fromtempering is offset by inducing precipitate dispersion hardening. Dispersionhardening from fine copper précipitâtes and mixed carbides and/or carbonitrides areutilized to optimize strength and toughness during the tempering of the martensitic 5 structure. The unique chemistry of the steels of this first steei example allows fortempering within the broad range of about 400°C to about 650°C (750°F - 1200°F)without any significant loss of the as-quenched strength. The Steel plate is preferablytempered at a tempering température from above about 400°C (752°F) to below theAc, transformation température for a period of time sufficient to cause précipitation of 10 hardening particles (as defined herein). This processing facilitâtes transformation of the microstructure of the Steel plate to predominantly tempered fine-grained lath-martensite, tempered fine-grained lower bainite, or mixtures thereof. Again, theperiod of time sufficient to cause précipitation of hardening particles dépendsprimarily on the thickness of the Steel plate, the chemistry of the Steel plate, and the 15 tempering température, and can be determined by one skilled in the art.

Second Steel Exampls 20 As discussed above, a copending U.S. provisional patent application, having a priority date of 19 December 1997, entitled “Ultra-High Strength Ausaged SteelsWith Excellent Cryogénie Température Toughness”, and identifiedby the USPTO asApplication No. 60/068252, provides a description of other steels suitable for use inthe présent invention. A method is provided for preparing an ultra-high strength Steel 25 plate having a micro-laminate microstructure comprising about 2 vol% to about 10vol% austenite film layers and about 90 vol% to about 98 vol% laths ofpredominantly fine-grained martensite and fine-grained lower bainite, said methodcomprising the steps of: (a) heating a Steel slab to a reheating température .sufficiently high to (i) substantially homogenize the Steel slab, (ii) dissolve 30 substantially ail carbides and carbonitrides of niobium and vanadium in the Steel slab, 1199 5 28 and (iii) establish fine initial austenite grains in the Steel slab; (b) reducing the Steelslab to form Steel plate in one or more hot rolling passes in a first température range inwhich austenite recrystallizes; (c) further reducing the Steel plate in one or more hotrolling passes in a second température range below about the Tnr température and 5 above about the Ar3 transformation température; (d) quenching the Steel plate at acooling rate of about 10°C per second to about 40°C per second (18°F/sec - 72°F/sec)to a Quench Stop Température (QST) below about the Ms transformation température plus 100°C (180°F) and above about the Ms transformation température; and (e) stopping said quenching. In one embodiment, the method of this second Steel 10 example further comprises the step ôf allowing the Steel plate to air cool to ambient température from the QST. In another embodiment, the method of this second Steelexample further comprises the step of holding the Steel plate substantiallyisothermally at the QST for up to about 5 minutes prior to allowing the steel plate toair cool to ambient température. In yet another embodiment, the method of this 15 second Steel example further comprises the step of slow-cooling the Steel plate from the QST at a rate lower than about 1.0°C per second (1.8°F/sec) for up to about 5minutes prior to allowing the Steel plate to air cool to ambient température. In yetanother embodiment, the method of this invention further comprises the step of slow-cooling the steel plate from the QST at a rate lower than about 1.0°C per second 20 (1.8°F/sec) for up to about 5 minutes prior to allowing the steel plate to air cool to ambient température. This processing facilitâtes transformation of the microstructureof the steel plate to about 2 vol% to about 10 vol% of austenite film layers and about90 vol% to about 98 vol% laths of predominantly fine-gramed martensite and fine-grained lower bainite. (See Glossary for définitions of Tnr température, and of Ar3 and 25 Ms transformation températures.)

To ensure ambient and cryogénie température toughness, the laths in themicro-laminate microstructure preferably comprise predominantly lower bainite ormartensite. It is préférable to substantially minimize the formation of embrittlingconstituents such as upper bainite, twinned martensite and MA. As used in this 29 119 9 5 second Steel example, and in the daims, “predominantly” means at least about 50volume percent. The remainder of the microstructure can comprise additionalfine-grained lower bainite, additional fme-grained lath martensite, or ferrite. Morepreferably, the microstructure comprises at least about 60 volume percent to about 80 5 volume percent lower bainite or lath martensite. Even more preferably, themicrostructure comprises at least about 90 volume percent lower bainite or lathmartensite. A Steel slab processed according to this second Steel example is manufacturedin a customary fashion and, in one embodiment, comprises iron and the following 10 alloying éléments, preferably in the weight ranges indicated in the following Table Π:

Table II

Alloying Elément 15 carbon (C)manganèse (Mn)nickel (Ni)copper(Cu) 20 molybdenum (Mo) niobium (Nb)titanium (Ti)aluminum (Al)nitrogen (N) 25

Range (wt%) 0.04 - 0.12, more preferably 0.04 - 0.070.5 - 2.5, more preferably 1.0-1.81.0 - 3.0, more preferably 1.5 - 2.50.1 - 1.0, more preferably 0.2 - 0.50.1 - 0.8, more preferably 0.2 - 0.40.02 - 0.1, more preferably 0.02 - 0.050.008 - 0.03, more preferably 0.01 - 0.020.001 - 0.05, more preferably 0.005 - 0.030.002 - 0.005, more preferably 0.002 - 0.003

Chromium (Cr) is sometimes added to the Steel, preferably up to about 1.0 wt%, and more preferably about 0.2 wt% to about 0.6 wt%.

Silicon (Si) is sometimes added to the Steel, preferably up to about 0.5 wt%, more preferably about 0.01 wt% to about 0.5 wt%, and even more preferably about 30 0.05 wt% to about 0.1 wt%. 119 9 5 30

Boron (B) is sometimes added to the Steel, preferably up to about 0.0020 wt%, and more preferably about 0.0006 wt% to about 0.0010 wt%.

The Steel preferably contains at least about 1 wt% nickel. Nickel content of the Steel can be increased above about 3 wt% if desired to enhance performance after 5 welding. Each 1 wt% addition of nickel is expected to lower the DBTT of the Steel byabout 10°C (1S°F). Nickel content is preferably less than 9 wt%, more preferably lessthan about 6 wt%. Nickel content is preferably minimized in order to minimize costof the Steel. If nickel content is increased above about 3 wt%, manganèse content canbe decreased below about 0.5 wt% down to 0.0 wt%. Therefore, in a broad sense, up 1 CT to about 2.5 wt% manganèse is preferred.

Additionally, residuals are preferably substantially minimized in the Steel.

Phosphorous (P) content is preferably less than about 0.01 wt%. Sulfur (S) content ispreferably less than about 0.004 wt%. Oxygen (O) content is preferably less thanabout 0.002 wt%. 15 In somewhat greater detail, a Steel according to this second Steel example is prepared by forming a slab of the desired composition as described herein; heating theslab to a température of from about 955°C to about 1065°C (1750°F - 1950°F); hotrolling the slab to form Steel plate in one or more passes providing about 30 percent toabout 70 percent réduction in a fîrst température range in which austenite 20 recrystallizes, i.e., above about the Tnr température, and further hot rolling the Steelplate in one or more passes providing about 40 percent to about 80 percent réductionin a second température range below about the Tnr température and above about the

Ar3 transformation température. The hot rolled Steel plate is then quenched at acooling rate of about 10°C per second to about 40°C per second (18eF/sec - 72°F/sec) 25 to a suitable QST below about the Ms transformation température plus 100°C (Î80°F) .and above about the Ms transformation température, at which time the quenching is terminated. In one embodiment of this second Steel example, after quenching is terminated the Steel plate is allowed to air cool to ambient température from the QST.

In another embodiment of this second steel example, after quenching is terminated the 119 9 5 31

Steel plate is held substantially isothermally at the QST for a period of time,preferably up to about 5 minutes, and then air cooled to ambient température. In yetanother embodiment, the Steel plate is slow-cooled at a rate slower than that of aircooling, i.e., at a rate lower than about 1°C per second (1.8°F/sec), preferably for up 5 to about 5 minutes. In yet another embodiment, the Steel plate is slow-cooled fromthe QST at a rate slower than that of air cooling, i.e., at a rate lower than about 1°Cper second (1.8°F/sec), preferably for up to about 5 minutes. In at least oneembodiment of this second Steel example, the Ms transformation température is about 350°C (662eF) and, therefore, the Ms transformation température plus 100°C (180°F) 10 is about 450°C (842°F).

The Steel plate may be held substantially isothermally at the QST by anysuitable means, as are known to those skilled in the art, such as by placing a thermalblanket over the Steel plate. The Steel plate may be slow-cooled after quenching isterminated by any suitable means, as are known to those skilled in the art, such as by 15 placing an insulating blanket over the Steel plate.

Third Steel Example

As discussed above, a copending U.S. provisional patent application, having a20 priority date of 19 December 1997, entitled “Ültra-High Strength Dual Phase Steels

With Excellent Cryogénie Température Toughness”, and identified by the USPTO asApplication No. 60/068816, provides a description of other steels suitable for use inthe présent invention. A method is provided for preparing an ultra-high strength, dualphase Steel plate having a microstructure comprising about 10 vol% to about 40 vol% 25 of a first phase of substantially 100 vol% (i.e., substantially pure or "essentially pure") ferrite and about 60 vol% to about 90 vol% of a second phase of predominantly fîne-grained lath martensite, fîne-grained lower bainite, or mixtures thereof, wherein themethod comprises the steps of (a) heating a Steel slab to a reheating températuresufficiently high to (i) substantially homogenize the Steel slab, (ii) dissolve 30 substantially ail carbides and carbonitrides of niobium and vanadium in the Steel slab, 119 9 5 32 and (iii) establish fine initial austenite grains in the Steel slab; (b) reducing the Steelslab to form Steel plate in one or more hot rolling passes in a first température range inwhich austenite recrystallizes; (c) further reducing the Steel plate in one or more hotrolling passes in a second température range below about the Tnr température and 5 above about the Ar3 transformation température; (d) further reducing said Steel plate inone or more hot rolling passes in a third température range below about the Ar3transformation température and above about the Ar, transformation température (Le.,the intercritical température range); (e) quenching said Steel plate at a cooling rate ofabout 10°C per second to about 40°C per second (18°F/sec - 72°F/sec) to a Quench 10 Stop Température (QST) preferably below about the Ms transformation températureplus 200°C (360°F); and (f) stopping said quenching. In another embodiment of thisthird Steel example, the QST is preferably below about the Ms transformation température plus 100°C (180°F), and is more preferably below about 350°C (662°F).In one embodiment of this third Steel example, the Steel plate is allowed to air cool to 15 ambient température after step (f). This processing facilitâtes transformation of themicrostructure of the Steel plate to about 10 vol% to about 40 vol% of a first phase offerrite and about 60 vol% to about 90 vol% of a second phase of predominantlyfine-grained lath martensite, fine-grained lower bainite, or mixtures thereof. (SeeGlossary for définitions of Tnr température, and of Ar3 and Ar, transformation 20 températures.)

To ensure ambient and cryogénie température toughness, the microstructure ofthe second phase in steels of this third Steel example comprises predominantlyfine-grained lower bainite, fine-grained lath martensite, or mixtures thereof. It ispréférable to substantially minimize the formation of embrittling constituents such as 25 upper bainite, twinned martensite and MA in the second phase. As used in this thirdSteel example, and in the daims, “predominantly” means at least about 50 volumepercent The remainder of the second phase microstructure can comprise additionalfine-grained lower bainite, additional fine-grained lath martensite, or ferrite. Morepreferably, the microstructure of the second phase comprises at least about 60 volume 1199 5 33 percent to about 80 volume percent fine-grained lower bainite, fine-grained lathmartensite, or mixtures thereof. Even more preferably, the microstructure of the secondphase comprises at least about 90 volume percent fine-grained lower bainite,fine-grained lath martensite, or mixtures thereof. 5 A Steel slab processed according to this third Steel example is manufactured in a customary fashion and, in one embodiment, comprises iron and the followingalloying éléments, preferably in the weight ranges indicated in the following Table III:

Table III

Range (wt%) 15 10 ~ Alloying Elément carbon (C) 0.04 - 0.12, more preferably 0.04 - 0.07 manganèse (Mn) 0.5 - 2.5, more preferably 1.0-1.8 nickel (Ni) 1.0 - 3.0, more preferably 1.5 - 2.5 niobium (Nb) 0.02 - 0.1, more preferably 0.02 - 0.05 titanium (Ti) 0.008 - 0.03, more preferably 0.01 - 0.02 aluminum (Al) 0.001 - 0.05, more preferably 0.005 - 0.03 nitrogen (N) 0.002 - 0.005, more preferably 0.002 - 0.003

Molÿbdenum (Mo) is sometimes added to the Steel, preferably up to about 0.8wt%, and more preferably about 0.1 wt% to about 0.3 wt%. 25 Silicon (Si) is sometimes added to the Steel, preferably up to about 0.5 wt%, more preferably about 0.01 wt% to about 0.5 wt%, and even more preferably about0.05 wt% to about 0.1 wt%.

Copper (Cu), preferably in the range of about 0.1 wt% to about 1.0 wt%, morepreferably in the range of about 0.2 wt% to about 0.4 wt%, is sometimes added to the 30 Steel. 20 119 9 5 34

The Steel preferably contains at least about 1 wt% nickel. Nickel content ofthe Steel can be increased above about 3 wt% if desired to enhance performance afterwelding. Each 1 wt% addition of nickel is expected to lower the DBTT of the Steel byabout 10°C (18°F). Nickel content is preferably less than 9 wt%, more preferably lessthan about 6 wt%. Nickel content is preferably minimized in order to minimize costof the Steel. If nickel content is increased above about 3 wt%, manganèse content canbe decreased below about 0.5 wt% down to 0.0 wt%. Therefore, in a broad sense, upto about 2.5 wt% manganèse is prefeiTed.

In somewhat greater detail, a Steel according to this third Steel example isprepared by forming a slab of the desired composition as described herein; heating theslab to atemperature of ffom about 955°C to about 1065°C (1750°F - 1950°F); hotrolling the slab to foim Steel plate in one or more passes providing about 30 percent toabout 70 percent réduction in a fîrst température range in which austeniterecrystallizes, i.e., above about the Tnr température, further hot rolling the Steel platein one or more passes providing about 40 percent to about 80 percent réduction in asecond température range below about the Tnr température and above about the Ar3 transformation température, and finish rolling the Steel plate in one or more passes toprovide about 15 percent to about 50 percent réduction in the intercritical températurerange below about the Ar3 transformation température and above about the Ar,transformation température. The hot rolled Steel plate is then quenched at a coolingrate of about 10eC per second to about 40°C per second (18°F/sec - 72°F/sec) to asuitable Quench Stop Température (QST) preferably below about the Mstransformation température plus 200°C (360°F), at which time the quenching is 35 119 9 5 terminated. In another embodiment of this invention, the QST is preferably belowabout the Ms transformation température plus 100°C (180°F), and is more preferably below about 350°C (662°F). In one embodiment of this third Steel example, the Steelplate is allowed to air cool to ambient température after quenching is terminated. 5 In the three example steels above, since Ni is an expensive alloying element, the Ni content of the Steel is preferably less than about 3.0 wt%, more preferably lessthan about 2.5 wt%, more preferably less than about 2.0 wt%, and even morepreferably less than about 1.8 wt%, to substantially minimize cost of the Steel.

Other suitable steels for use in connection with the présent invention are 10 described in other publications that describe ultra-high strength, low alloy steelscontaining less than about 1 wt% nickel, having tensile strengths greater than 830MPa (120 ksi), and having excellent low-temperature toughness. For example, suchsteels are described in a European Patent Application published February 5,1997, andhaving International application number: PCT/JP96/00157, and International 15 publication number WO 96/23909 (08.08.1996 Gazette 1996/36) (such steels preferably having a copper content of 0.1 wt% to l .2 wt%), and in a pending U.S.provisional patent application with a priority date of 28 July 1997, entitled “Ultra-High Strength, Weldable Steels with Excellent Ultra-Low Température Toughness”,and identified by the USPTO as Application No. 60/053915. 20 For any of the above-referenced steels, as is understood by those skilled in the art, as used herein “percent réduction in thickness” refers to percent réduction in thethickness of the Steel slab or plate prior to the réduction referenced. For purposes ofexplanation only, without thereby limiting this invention, a Steel slab of about 25.4 cm(10 inches) thickness may be reduced about 50% (a 50 percent réduction), in a first 25 température range, to a thickness of about 12.7 cm (5 inches) then reduced about 80%(an 80 percent réduction), in a second température range, to a thickness of about 2.5 cm(1 inch). Again, for purposes of explanation only, without thereby limiting thisinvention, a Steel slab of about 25.4 cm (10 inches) may be reduced about 30% (a 30percent réduction), in a first température range, to a thickness of about 17.8 cm (7 30 inches) then reduced about 80% (an 80 percent réduction), in a second température 1199 5 36 range, to a thickness of about 3.6 cm (1.4 inch), and then reduced about 30% (a 30percent réduction), in a third température range, to a thickness of about 2.5 cm (1 inch).As used herein, "slab" means a piece of Steel having any dimensions.

For any of the above-referenced steels, as is understood by those skilled in the 5 art, the Steel slab is preferably reheated by a suitable means for raising the températureof substantially the entire slab, preferably the entire slab, to the desired reheatingtempérature, e.g., by placing the slab in a fumace for a period of time. The spécifiereheating température that should be used for any of the above-referenced Steelcompositions may be readily deteimined by a person skilled in the art, either by 10 experiment or by calculation using suitable models. Additionally, the fùmace température and reheating time necessary to raise the température of substantially theentire slab, preferably the entire slab, to the desired reheating température may bereadily determined by a person skilled in the art by référencé to standard industrypublications. 15 For any of the above-referenced steels, as is understood by those skilled in the art, the température that defines the boundary between the recrystallization range andnon-recrystallization range, the Tnr température, dépends on the chemistry of the Steel,and more particularly, on the reheating température before rolling, the carbonconcentration, the niobium concentration and the amount of réduction given in the 20 rolling passes. Persons skilled in the art may détermine this température for each Steelcomposition either by experiment or by model calculation. Likewise, the Ac„ Ar„ Ar3,and Ms transformation températures referenced herein may be determined by persons skilled in the art for each Steel composition either by experiment or by modelcalculation. 25 For any of the above-referenced steels, as is understood by those skilled in the art, except for the reheating température, which applies to substantially the entire slab,subséquent températures referenced in describing the Processing methods of thisinvention are températures measured at the surface of the Steel. The surfacetempérature of Steel can be measured by use of an optical pyrometer, for example, or 30 by any otherdevice suitable for measuring the surface température of Steel. The 37 1 19 9 5 cooling rates referred to herein are those at the eenter, or substantially at the center, ofthe plate thickness; and the Quench Stop Température (QST) is the highest, orsubstantially the highest, température reached at the surface of the plate, afterquenching is stopped, because of heat transmitted from the mid-thickness of the plate. 5 For example, during processing of experimental heats of a Steel composition according to the examples provided herein, a thermocouple is placed at the center, orsubstantially at the center, of the Steel plate thickness for center températuremeasurement, whiîe the surface température is measured by use of an opticalpyrometer. A corrélation between center température and surface température is 10 developed for use during subséquent processing of the same, or substantially thesame, Steel composition, such that center température may be determined via directmeasurement of surface température. Also, the required température and flow rate ofthe quenching fluid to accomplish the desired accelerated cooling rate may bedetermined by one skilled in the art by reference to standard industry publications. 15 A person of skill in the art has the requisite knowledge and skill to use the information provided herein to produce ultra-high strength, low alloy Steel plateshaving suitable ultra-high strength and toughness for use in constructing thecontainers for storing and transporting PLNG of the présent invention. Other suitablesteels may exist or be developed hereafter. AU such steels are within the scope of the 20 présent invention. A person of skill in the art has the requisite knowledge and skill to use theinformation provided herein to produce ultra-high strength, low alloy Steel plateshaving modified thicknesses, compared to the thicknesses of the Steel plates producedaccording to the examples provided herein, while still producing Steel plates having 25 suitable high strength and suitable cryogénie température toughness for use in theSystem of the présent invention. For example, one skiUed in the art may use theinformation provided herein to produce a Steel plate with a thickness of about 2.54 cm(1 inch) and suitable high strength and suitable cryogénie température toughness foruse in constructing a storage container of the présent invention. Other suitable steels 119 9 5 38 may exist or be developed hereafter. Ail such steels are within the scope of theprésent invention.

Containers constructed from any suitable high strength, low alloy Steel asdescribed herein, such as any of the steels described in this Example, are sizedaccording to the needs of the PLNG project in winch the containers will be utilized. A person skilled in the art may use standard engineering practices and referencesavailable in the industry to détermine the necessary dimensions, wall thickness, etc.for the containers.

When a dual phase Steel is used in the construction of containers according tothis invention, the dual phase Steel is preferably processed in such a manner that thetime period during which the Steel is maintained in the intercritical température rangefor the purpose of creating the dual phase structure occurs before the acceleratedcooling or quenching step. Preferably the processing is such that the dual phasestructure is foimed during cooling of the Steel between the Ar3 transformationtempérature to about the Ar, transformation température. An additional préférence forsteels used in the construction of containers according to this invention is that the Steelhas a tensile strength greater than 830 MPa (120 ksi) and a DBTT lower than about-73°C (-100°F) upon completion of the accelerated cooling or quenching step, i.e.,without any additional processing that requires reheating of the Steel such astempering. More preferably the tensile strength of the Steel upon completion of thequenching or cooling step is greater than about 860 MPa (125 ksi), and morepreferably greater than about 900 MPa (130 ksi). In some applications, a Steel havinga tensile strength of greater than about 930 MPa (135 ksi), or greater than about 965MPa (140 ksi), or greater than about 1000 MPa (145 ksi), upon completion of thequenching or cooling step is préférable.

For containers that require bending of the Steel, e.g., into a cylindrical shape,the Steel is preferably bent into the desired shape at ambient température in order toavoid detrimentally affecting the excellent cryogénie température toughness of theSteel. If the Steel must be heated to achieve the desired shape after bending, the Steel 39 is preferably heated to a température no higher than about 60Q°C (1112°F) in order topreserve the bénéficiai effects of the Steel microstructure as described above.

The desired variables for a PLNG container, e.g., size, geometry, materialthickness, etc., are dépendent upon operating conditions such as internai pressure, 5 operating température, etc., as will be familiar to those skilled in the art. For the mostdemanding low température designs, the DBTT of the Steel and the welds is veryimportant. For designs with somewhat higher operating températures, toughness isstill an important issue, but the DBTT requirements will likely be less demanding.

For example, when the operating température increases, the required DBTT will also 10 increase.

In order to construct^pntainers for use in the présent invention, a suitablemethod of joining the Steel plates is employed. Any joining method that will providejoints with adéquate strength and fracture toughness for the présent invention, asdiscussed above, is considered to be suitable. Preferably a welding method suitable 15 for providing adéquate strength and fracture toughness to contain said pressurized liquefied natural gas is used to construct containers of the présent invention. Such awelding method preferably includes a suitable consumable wire, a suitableconsumable gas, a suitable welding process, and a suitable welding procedure. Forexample, both gas métal arc welding (GMAW) and tungsten inert gas (TIG) welding, 20 which are both well known in the Steel fabrication industry, can be used to join theSteel plates, provided that a suitable consumable wire-gas combination is used.

In a first example welding method, the gas métal arc welding (GMAW)process is used to produce a weld métal chemistry comprising iron and about 0.07wt% carbon, about 2.05 wt% manganèse, about 0.32 wt% Silicon, about 2.20 wt% 25 nickel, about 0.45 wt% chromium, about 0.56 wt% molybdenum, less than about 110ppm phosphorous, and less than about 50 ppm sulfur. The weld is made on a Steel,such as any of the above-described steels, using an argon-based shielding gas withless than about 1 wt% oxygen. The welding heat input is in the range of about 0.3kï/mm to about 1.5 kJ/mm (7.6 kJ/inch to 38 kJ/inch). Welding by this method 30 provides a weldment having a tensile strength greater than about 900 MPa (130 ksi), 11995 40 preferably greater than about 930 MP a (135 ksi), more preferably greater than about965 MPa (140 ksi), and even more preferably at least about 1000 MPa (145 ksi).Further, welding by this method provides a weld métal with a DBTT below about-73°C (-100°F), preferably below about -96°C (-140°F), more preferably below about-106°C (-160°F), and even more preferably below about -115°C (-175°F).

In another example welding method, the GMAW process is used to produce aweld métal chemistry comprising iron and about 0.10 wt% carbon (preferably Iessthan about 0.10 wt% carbon, more preferably from about 0.07 to about 0.08 wt%carbon), about 1.60 wt% manganèse, about 0.25 wt% Silicon, about 1.87 wt% nickel,about 0.87 wt% chromium, about 0.51 wt% molybdenum, less than about 75 ppmphosphorous, and less than about 100 ppm sulfur. The welding hëàt input is in therange of about 0.3 kJ/mm to about 1.5 kJ/mm (7.6 kJ/inch to 38 kJ/inch) and a preheatof about 100°C (212°F) is used. The weld is made on a Steel, such as any of theabove-described steels, using an argon-based shielding gas with. less than about 1 wt%oxygen. Welding by this method provides a weldment having a tensile strengthgreater than about 900 MPa (130 ksi), preferably greater than about 930 MPa (135ksi), more preferably greater than about 965 MPa (140 ksi), and even more preferablyat least about 1000 MPa (145 ksi). Further, welding by this method provides a weldmétal with a DBTT below about -73°C (-100°F), preferably below about -96°C(-140°F), more preferably below about -106°C (-160°F), and even more preferablybelow about -115°C (-175°F).

In another example welding method, the tungsten inert gas welding (TIG)process is used to produce a weld métal chemistry containing iron and about 0.07 wt%carbon (preferably less than about 0.07 wt% carbon), about 1.80 wt% manganèse,about 0.20 wt% Silicon, about 4.00 wt% nickel, about 0.5 wt% chromium, about 0.40wt% molybdenum, about 0.02 wt% copper, about 0.02 wt% aluminum, about 0.010wt% titanium, about 0.015 wt% zirconium (Zr), less than about 50 ppm phosphorous,and less than about 30 ppm sulfur. The welding heat input is in the range of about 0.3kJ/mm to about 1.5 kJ/mm (7.6 kJ/inch to 38 kJ/inch) and apreheat of about 100°C(212°F) is used. The weld is made on a Steel, such as any of the above-described 11995 Ί . ' 41 steels, using an argon-based shielding gas with less than about 1 wt% oxygen.

Welding by this method provides a weldment having a tensile strength greater thanabout 900 MPa (130 ksi), preferably greater than about 930 MPa (135 ksi), morepreferably greater than about 965 MPa (140 ksi), and even more preferably at least 5 about 1000 MPa (145 ksi). Further, welding by this method provides a weld métalwith a DBTT below about -73°C (-100°F), preferably below about -96°C (-140°F),more preferably below about -106°C (-160°F), and even more preferably below about-115°C (-175°F).

Similar weld métal chemistries to those mentioned in the examples can be10 made using either the GMAW or the TIG welding processes. However, the TIG welds are anticipated to_have lower impurity content and a more highiy refinedmicrostructure than the GMAW welds, and thus improved low température toughness.

In one embodiment of this invention, submerged arc welding (SAW) is used asa joining technique. A detailed discussion about SAW can be found in Chapter 6 of 15 the Welding Handbook, Volume 2, Welding Processes, 8th ed., American WeldingSociety, pages 191-232 (1995).

Submerged arc welding (SAW) is a welding technique that is often used for itsadvantage of high métal déposition rate. It can be more economical for certainapplications because more welding material can be applied per unit time than for other 20 welding techniques. One potential drawback for SAW, when used tojoin ferritic steels for low température applications, is insufScient or variable toughness. The lowtoughness can be caused by factors such as large grain size and/or higher than desiredinclusion content The large grain size is created by the high heat input aspect ofSAW which is also the feature that enables high déposition rate. One other potential

25 problem with SAW when applied to heat sensitive, high strength Steel, is HAZ softening. The high heat input feature of SAW causes more extensive softening in theHAZ as compared to gas métal arc welding (GMAW) or tungsten inert gas (TIG)welding.

For some PLNG container designs, the SAW technique may be suitable. The 30 decision to use SAW will be based primarily on a balance of économies (weld 119 9 5 42 déposition rate) versus achieving adéquate mechanical properties. It is possible totailor a spécifie SAW welding procedure to a particular PLNG container design. Forexample, if it is desired to limit HAZ softening and reduce weld métal grain size, aSAW procedure can be developed that utilizes an intermediate heat input. Instead ofenabling the very high déposition rates at heat inputs above about 4 kJ/mm (100kJ/in), heat inputs in the range of about 2 kJ/mm to about 4 kJ/mm (50 kJ/in to 100kJ/in) might be used. At values lower than this intermediate range, SAW would likelybecome less désirable than GMAW or TIG welding. SAW can also be used with an austenitic weld métal. Weld toughness issomewhat easier to achieve due to the high ductility of the face centered cubicaustenite. One drawback of an austenitic weld consumable is the expense which ishigher than for most feiritic consumables. The austenitic material contains significantamounts of expensive alloys like Cr and Ni. However, it may be possible for aparticular PLNG container design, to offset the austenitic consumable expense withthe higher déposition rate allowed by SAW.

In another embodiment of this invention, électron beam welding (EBW) isused as a joining technique. A detaiîed discussion about EBW can be found inChapter 21 of the Welding Handbook, Volume 2, Welding Processes, 8th ed.,American Welding Society, pages 672-713 (1995). Several inhérent features of EBWare particularly suited for use in service conditions requiring both high strength andlow température toughness. A problem relating to the welding of most high strength steels, i.e., steels havingyield strengths greater than about 550 MPa (80 ksi), is the softening of the métal in theheat-affected zone (HAZ) resulting from many conventional welding processes, such asshielded métal arc welding (SMAW), submerged aie welding (SAW), or any of the gasshielded processes, suçh as gas métal arc welding (GMAW). The HAZ may undergolocal phase transformation or annealing during welding-induced thermal cycles, îeadingto a significant, i.e., up to about 15 perçoit or more, softening of the HAZ as comparedto the base métal prior to exposure to the heat of welding. While ultra-high strengthsteels hâve been produced withyield strengths of830 MPa (120 ksi) or higher, many of 1 1 9 9 5’ 43 these steels fail to meet the weldability requirements necessary for extreme lowtempérature service, such as those required for piping and pressure vessels for use in theprocesses disclosed and claimed herein. Such materials typically hâve a relatively highPcm (a well-known industry term used to express weldability), generally greater than 5 about 0.30, and sometimes above 0.35. EBW mitigates some of the problème resulting from conventional welding techniques, such as SMAW and SAW. Total heat input is significantly less than arcwelding processes. This réduction in heat input reduces the alteration of manyproperties of the Steel plates during the joining process. In many cases, EBW produces 10 a welded joint that is stronger and/or more résistant to brittle fracture in cold températureservices than similar joints produced by arc welding. EBW, when compared to arc welding the same joint, results in a réduction ofresidual stresses, HAZ width, and mechanical distortion of the joint, along with apotential improvement in HAZ toughness. The high power density of EBW also 15 facilitâtes single pass welding, thus also minimizing the time the base métal of the Steelplates are exposed to elevated températures during the joining process. These featuresof EBW are important in minimizing the deleterious effects of welding on heat sensitivealloys.

Furthermore, EBW Systems using reduced pressure or high vacuum welding 20 conditions resuit in a high purity environment reducing weld pool contamination. The réduction of impurities in the électron beam welded joint results in enhanced weld métaltoughness produced by reducing the amount of interstitial éléments and inclusions. EBW is also extremely flexible in that a great number of process controlvariables may be independently controlled (e.g. vacuum level, working distance, 25 accelerating voltage, beam current, travel speed, beam spot size, beam deflection, etc.).

Assuming proper joint fit-up, no Aller métal is required for EBW thus resulting in awelded joint of homogeneous metallurgy. However, shims of Aller métal can be used tointentionally alter the metallurgy of the EBW joint and enhance mechanical properties.Strategie combinations of beam parameters and use/omission of shims permits tailoring 30 of the weld métal microstructure to produce the desired combination of strength and 119 9 5 44 toughness.

The overall combination of excellent mechanical properties and low residualstresses also permits élimination of post weld heat treatment in many cases, even whenthe thickness of the plates joined is one to two inches or more. 5 EBW may be conducted at high vacuum (HV), medium vacuum (MV), or no vacuum (NV). HV-EBW Systems produce welds with a minimum of impurities.However, high vacuum conditions can cause loss of critical volatile éléments (e.g.chromium and manganèse) when the métal is in the molten State. Depending on thecomposition of the Steel to be welded, the loss of a portion of certain éléments may 10 impact the mechanical performance of the weld. Furthermore, these Systems tend tobe large and unwieldy, and difficult to use. NV-EBW Systems are mechanically lesscomplicated, more compact, and generally easier to use. However, NV-EBWProcessing is more limited in its application in that the beam tends to diffuse, scatter,and become less focused and less effective when exposed to air. This tends to limit 15 the thickness of plates that may be welded in a single pass. NV-EBW is also moresusceptible to weld impurities which may resuit in welds with lower strength andtoughness than higher vacuum EBW. Therefore, MV-EBW is the preferred option forconstruction of the containers of the claimed invention. MV-EBW offers the bestbalance of performance and weld quality. 20 In another embodiment of this invention, laser beam welding (LBW) is used as a joining technique. A detailed discussion about LBW can be found in Chapter 22 ofthe Welding Handbook, Volume 2, Welding Processes, 8th ed., American WeldingSociety, pages 714-738 (1995). LBW offers many of the same advantages as EBW,but is more limited in application in that currently available EBW is capable of single 25 pass welds on a wider range of plate thickness. A person of skill in the art has the requisite knowledge and skill to use the information provided herein to weld ultra-high strength, low alloy Steel plates toproduce joints having suitable high strength and fracture toughness for use inconstructing the containers and other components of the présent invention. Other 119 9 5 45 suitable joining or welding methods may exist or be developed hereafter. Ali suchjoining or welding methods are within the scope of the présent invention.

While the foregoing invention has been described in ternis of one or morepreferred embodiments, it should be understood that other modifications may be made 5 without departing from the scope of the invention, which is set forth in the followingdaims. 46 119 9 5 the température at which austenite begins to form during heating;

Glossarv of terms:

Ac, transformation température:

Ac3 transformation température:

Ar, transformation température:

Ar3 transformation température: cryogénie température: CTOD: CVN: the température at which transformation of ferriteto austenite is completed during heating; the température at which transformation ofaustenite to ferrite or to ferrite plus cementite iscompleted during cooling; the température at which austenite begins totransform to ferrite during cooling; any température lower than about -40°C (-40°F); crack tip opening displacement;'

Charpy V-notch; DBTT (Ductile-to-Brittle

Transition Température): delineates the two fracture régimes in structural steels; at températures below the DBTT, failuretends to occur by low energy cleavage (brittle)fracture, while at températures above the DBTT,failure tends to occur by high energy ductilefracture; EBW: électron beam welding; substantially 100 vol%; essentially pure: 119 9 5 47

Gm3: GMAW: 5 hardening particles HAZ: intercritical température range: 10 Κ,ό 15 kJ: kPa: ksi: 20 LBW: low alloy steel: 25 MA: maximum allowable flaw size: billion cubic meters; gas métal arc welding; one or more of ε-copper, Mo2C, or the carbidesand carbonitrides of niobium and vanadium; heat affected zone; from about the Ac,· transformation températureto about the Ac3 transformation température onheating, and from about the Ar3 transformationtempérature to about the Ar, transformationtempérature on cooling; critical stress intensity factor; kilojoule; thousands of Pascals; thousands of pounds per square inch; laser beam welding; a steel containing iron and less than about 10 wt%total alloy additives; martensite-austenite; critical flaw length and depth; 119 9 5 48

Mo2C: a form of molybdenum Carbide; MPa: millions of Pascals; Ms transformation température: the température at which transformation of austenite to martensite starts during cooling; Pcm: · a well-known industry term used to express weldability; also, Pcm=(wt% C + wt% Si/30 + (wt% Mn + wt% Cu + wt% Cr)/20 + wt% Ni/60+ wt% Mo/15 + wt% V/10 + 5(wt% B)); PLNG: pressurized Iiquefied natural gas; ppm: parts-per-million; predominantly: at least about 50 volume percent; psia: pounds per square inch absolute; quenching: as used in describing the présent invention, accelerated cooling by any means whereby a fluid selected for its tendency to increase the cooling rate of the Steel is utilized, as opposed to air cooling; quenching (cooling) rate: cooling rate at the center, or substantially at thecenter, of the plate thickness; 49

Quench Stop Température: 5 QST: SAW: 10 SALM: slab: TCF: 15 tensile strength: TIG welding: 20

Tnr température: USPTO: 119 9 5 the highest, or substantially the highest, température reached at the surface of the plate, after quenching is stopped, because of heat transmitted from the mid-thickness of the plate;

Quench Stop Température; submerged arc welding; single anchor leg mooring; a piece of Steel having any dimensions; trillion cubic feet; in tensile testing, the ratio of maximum load tooriginal cross-sectional area; tungsten inert gas welding; the température beiow which austenite does notrecrystallize;

United States Patent and Trademark Office; and 119 9 5 50 weldment: a welded joint, including: (i) the weld métal, (ii) the heat-affected zone (HAZ), and (iii) thebasemétal in the “near vicinity” of the HAZ. Theportion of the base métal that is considered 5 within the “near vicinity” of the HAZ, and therefore, a part of the weldment, variesdepending on factors known to those skilled inthe art, for example, without limitation, thewidth of the weldment, the size of the item that 10 was welded, the number of weldments required to fabricate the item, and the distance betweenweldments.

Claims

11995 51 CLAIMS

1. A method for processing and transporting natural gas, said method comprisingthe steps of: 5 (a) processing said natural gas by converting it into pressurized liquefiednatural gas having a pressure of about 1725 kPa (250 psia) to about 7590 Kpa(1100 psia) and a température of about -100°C (-148°F) to about-62°C(-80°F); 10 (b) placing said pressurized liquefied natural gas into a plurality of storagecontainers at a first location, wherein said plurality of storage containers hâveadéquate strength and toughness to contain said pressurized liquefied natural gasat said pressure and température conditions; and 15 (c) transporting said plurality of storage containers containing saidpressurized liquefied natural gas away from said first location toward a secondlocation. 20 '

2. A System for processing and transporting natural gas comprising: (a) a processing plant for converting said natural gas to pressurizedliquefied natural gas having a pressure of about 1725 kPa (250 psia) to about7590 kPa (1100 psia) and a température of about-100°C (-148°F) to about 25 -62°C (-80°F); (b) a plurality of storage containers for receiving said pressurized liquefiednatural gas, wherein said plurality of storage containers hâve adéquate strengthand toughness to contain said pressurized liquefied natural gas at said pressure 30 and température conditions; and (c) at least one marine vessel adapted for holding and transporting saidplurality of storage containers containing said pressurized liquefied natural gas. 119 9 5 52

3. Pressurized liquefied natural gas derived from a process for liquefying naturalgas to produce said pressurized liquefied natural gas as an outlet product at apressure of about 1725 kPa (250 psia) to about 7590 kPa (1100 psia) and a 5 température of about -100°C (-148°F) to about -62°C (-80°F) and containing at least one component selected from the group consisting of (i) carbon dioxide,(ii) n-pentane plus, and (iii) benzene, in an amount that would freeze inliquefied natural gas at a pressure of about atmospheric pressure and atempérature of about -162°C (-260°F). 10

4. Pressurized liquefied natural gas derived from a process for liquefying naturalgas to produce said pressurized liquefied natural gas as an outlet product at apressure of about 1725 kPa (250 psia) to about 7590 kPa (1100 psia) and atempérature of about -100°C (-148°F) to about -62°C (~80°F), wherein said 15 process consiste essentially of the steps of (i) receiving said natural gas in réception facilities; (ii) removing sufficient water vapor from said natural gasto prevent freezing of said natural gas during said process; and (iii) liquefyingsaid natural gas to produce said outlet product.