African Fusion November 2022

Welding consumables for LNG vessels

Welding solutions for the fabrication of LNG storage tanks and carrier vessels

Pierre Gérard, LNG expert and global Vice President for key accounts at voestalpine Böhler Welding, introduces some of the innovative welding solutions and consumables de veloped for modern LNG storage tanks and carrier vessels.

E nergy has become a key topic for many countries considering the en ergy 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 emis sions 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 emis sion for marine applications have driven the shipbuilding industry to increasingly consider LNG as a cleaner fuel. The technology for LNGwas 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 countrieswanting to extend their energy source portfolio. Sizes of LNG tanks and tankers have increased in order to reduce transportation and storage costs, with cur rent typical gas storage tanks able to con tain more than 200 000 m 3 of liquefied gas. voestalpine Böhler Welding has been a reliable partner for high- and nickel-alloy consumables designed for the LNG industry formore than 20 years, havingworkedwith 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 ap plications 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. Mechani cal properties are achieved thanks to a double heat treatment of the steel during production. The first steel of choice for LNG con tainment 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 ASTMA645 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 cryo genic 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 minimumrequirementsof theAPI 620code, engineers tend to apply safety factors that often require a minimum value of 55 J or even 70 J at these extreme temperatures. The selection of appropriate welding con sumable is primarily done based on the requirements of themechanical 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 pro cedure against the design standards. Welding consumables for nickel steel welding

IGC/IGF international standards for liquefied gas carriers cover the LNG carrier vessels as well as vessels using LNG as fuel.

Current typical LNG gas storage tanks are able to contain more than 200 000 m 3 of liquefied gas.

26

November 2022

AFRICAN FUSION