ClassNK has issued a General Design Approval (GDA) for Low-pressure LCO2 Cargo Tank developed by Mitsubishi Shipbuilding Co., Ltd. This approval verifies the tank’s compliance with relevant rules at the detailed design level*1. GDA for a liquefied CO₂ cargo tank made of KF460 steel and applying technology that omits PWHT*2 marks the world’s first case.
This GDA includes a tank safety assessment process based on a fracture mechanics technique called Engineering Critical Assessment (ECA)*2 to omit post-weld heat treatment (PWHT)*3, which can be a bottleneck in tank manufacturing. An Approval in Principle (AiP) for Mitsubishi Shipbuilding's ECA evaluation process was issued in September 2024*4.
Based on the detailed design evaluation conducted by the Company and the material property assessment for a new steel conforming to NK standard 'KF460' developed by Nippon Steel Corporation, the structural integrity of the tank under the specified design temperature conditions has been demonstrated through ECA. In light of these assessments, ClassNK conducted a review of the tank in accordance with Part N of its 'Rules and Guidance for the Survey and Construction of Steel Ships'. Upon confirming that the tank meets the prescribed requirements even without PWHT, ClassNK has issued the GDA. This marks the first time ClassNK has issued a GDA for a LCO2 cargo tank.
ClassNK will continually strive to contribute to advanced carbon neutral initiatives through safety assessments and more.
Illustration for an installation of the Low-pressure LCO2 tanks (Courtesy of Mitsubishi Shipbuilding)
1- At an early stage of design, or prior to the determination of a specific vessel for implementation, the design is reviewed based on existing regulations such as international conventions and ship classification rules, and a certificate is issued as proof of compliance with the relevant requirements.
This process helps prevent regulatory revisions or rework in later development stages and contributes to reducing the time required for examination upon class registration. It can also serve as a technical basis for externally demonstrating the soundness of the design.
Depending on the stage of the review, Approval in Principle (AiP) is generally applied to basic designs or earlier stages, while General Design Approval (GDA) is applied to standardized products at the detailed design level.
2- Engineering Critical Assessment (ECA)
An evaluation method based on fracture mechanics to determine whether defects found in welds, etc. will cause phenomena such as collapse during the design life of a structure.
3- Post Weld Heat Treatment (PWHT)
Heat treatment to reduce residual stress in the weld zone by reheating and holding the weld zone to approximately 600°C after welding. Generally, this treatment is performed in the heat treatment furnace, since it is reheated and maintained to a high temperature of about 600°C.
When the products are of large size, furnace size may cause bottlenecks in the manufacturing process.
4- Related Press Release: ClassNK issues Approval in Principle (AiP) for large liquefied CO2 carriers of 50,000 m³ and 23,000 m³ - Working towards ensuring stable supply of liquefied CO2 carriers for support of CCS projects
