Pierre Gérard, LNG expert and global Vice President for key accounts at voestalpine Böhler Welding, introduces some of the innovative welding solutions and consumables developed for modern LNG storage tanks and carrier vessels.
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Energy has become a key topic for many countries considering the energy transition and the necessity for low carbon emission energies, with liquid natural gas (LNG) being a key player in the energy market. Not only for carbon emissions but also in terms of energy security, LNG brings accessible reserves to markets where gas was not previously available. In addition, new regulations on sulphur emission for marine applications have driven the shipbuilding industry to increasingly consider LNG as a cleaner fuel.
The technology for LNG was established in the 60s, with the first LNG commercial shipments done from Algeria to France and the UK, and from Louisiana to Europe. Liquefaction, transport and gasification operations have been optimised since then and several international engineering and contractor companies are reliable partners to countries wanting to extend their energy source portfolio. Sizes of LNG tanks and tankers have increased in order to reduce transportation and storage costs, with current typical gas storage tanks able to contain more than 200 000 m3 of liquefied gas.
voestalpine Böhler Welding has been a reliable partner for high- and nickel-alloy consumables designed for the LNG industry for more than 20 years, having worked with most of the specialised EPC companies to deliver high quality welding consumables to all continents: Europe, the America’s, Asia, Oceania and Africa.
Different kinds of nickel (Ni) steels are available and applicable for storage applications requiring extremely low design temperatures. These are the materials of choice for cryogenic applications when liquid gas has to be stored or transported.
Between the 5% and the 9% Ni is the A645 Grade B, with nickel content slightly above the Grade A but having mechanical properties like those of 9% Ni. Lowering the nickel content has a direct impact on the material cost, which is the reason for the attractiveness of that grade. Mechanical properties are achieved thanks to a double heat treatment of the steel during production.
The first steel of choice for LNG containment tanks is the 9% Ni, which shows excellent fracture toughness at cryogenic temperatures. Typically, ASTM A553 Type I steel is selected.
But with current pressure on nickel costs, several engineering companies have been considering ASTM A645 Grade B, which contains a lower percentage of nickel – in the range of 6% – making this steel about 10% cheaper than the standard A553 while retaining the same mechanical properties.
Two of the main design codes for cryogenic tanks are the API 620 and EN 14620. For design temperatures below -165 °C, material has to be tested at -196 °C and must meet the minimum requirements of Table 3.
Typical Charpy impact tests are done on transverse specimens for which API requires a minimum of 27 J at -196 °C for a 10×10 mm specimen. In addition to these minimum requirements of the API 620 code, engineers tend to apply safety factors that often require a minimum value of 55 J or even 70 J at these extreme temperatures.
Welding consumables for nickel steel welding
The selection of appropriate welding consumable is primarily done based on the requirements of the mechanical properties of the weld metal. Also, though, welding operations will have an influence on the mechanical properties of the heat affected zone of the base material as well as on the diluted weld metal. Due to this influence, the application standards will require a Welding Procedure Qualification Record (WPQR) to prove conformance of the procedure against the design standards.
Nickel base alloys are usually chosen for welding operations.
Three welding processes are usually considered for the welding of LNG containment tanks. The submerged arc process is used to weld the horizontal joints in the 2G position, while SMAW stick electrodes or flux cored wires are used for vertical welding.
The choice between flux cored wire or stick electrodes is based mostly on the welding capabilities of the welders and the automation experience of the contractor. When welding with flux-cored wires, the vertical up joints can be welded using tractors, which enables drastically increased welding productivity.
voestalpine Böhler Welding supports contractors with the welding procedures qualifications of the above consumables as well as with welding equipment and the mechanisation requirements. Table 6 outlines the impact properties that can be achieved for design temperature of -196 °C
Liquid natural gas tankers
LNG tankers are used to transport liquefied gas from the region of extraction and production to the country of utilisation. Different designs exist, with some of them using nickel steels, as for the storage tanks.
Ultimate tensile strength, yield strength as well as impact toughness at design temperatures are key factors for the steel selection.
IGC/IGF codes are international standards for liquefied gas carriers. They cover the LNG carrier vessels as well as vessels using LNG as fuel. As for the storage tanks, design temperature defines the impact test temperature. According the IGC codes, for LNG carriers having a design temperature of -165 °C, only three base materials can be selected for use: austenitic stainless steels and 9%-5% nickel steels. In addition, the IGC code allows special heat treated 5% nickel steel for design temperature down to -165°C, provided the impact test is carried out at -196°C.
For 9% nickel steels, the same welding consumables as for the onshore tanks can be used. The main difference with the onshore tank welding is that carrier vessels are welded in shipyards, and their LNG tanks are welded in roof covered workshops, which enables the use of welding processes not possible for on-site welding of onshore tanks.
voestalpine Böhler Welding has developed a unique welding consumable for gas metal arc welding (GMAW) of austenitic stainless steel structures, which is purpose designed for the welding of nickel steels with impact testing requirements at -196°C and a minimum ultimate tensile strength at room temperature of 690 MPa. The main advantage of this solution against nickel-based welding consumables is the cost of the product, being a stainless steel.
Both A553 Type I and A645 grade B steels have been successfully welded using this innovative solution and all the mechanical requirements specified in the standards have been met. This innovative solid wire GMAW solution produces typical mechanical properties as shown in Table 6, where tensile test was performed on transversal specimens and the welding consumables wire used had a diameter of 1.2 mm.
voestalpine Böhler Welding has been a partner for the welding of LNG components for more than 20 years and closely follows market demand and technical trends, leading to the development of innovative and cost effective welding consumables and process solutions for LNG storage tanks and tankers.
