PSI - Issue 48

Haris Nubli et al. / Procedia Structural Integrity 48 (2023) 73–80 Nubli et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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contributors to risk, with high probabilities and severe consequences based on accident databases from 1964 to 2005 (Vanem et al., 2008). These generic accidents can occur on all ship types, but the risk of accident escalation is higher for LNG carriers due to cargo volatility. For instance, a collision on an LNG carrier that damages the cargo hold or LNG line may cause LNG leakage. Hence, it is crucial to prioritize risk mitigation measures in the design and operation of LNG carriers and implement effective safety management systems to prevent or minimize the consequences of potential accidents. In order to evaluate the hazards of cryogenic release on offshore or onshore units, Advanced Cryogenic Risk Analysis (ACRA) is suggested as a further risk analysis approach (Lloyd’s Register, 2015) . This method incorporates factors such as structures, equipment, barriers, and wind conditions into Computational Fluid Dynamics (CFD) simulation, as well as heat transfer calculation to estimate the DBTT of the exposed structure (Lloyd’s Register, 2015; Pujol et al., 2016). Figure 1 exhibits the example of CFD- based heat transfer analysis to the ship’s plate under cryogenic flow (Nubli et al., 2022a). The embrittlement of structural steel due to cryogenic exposure can weaken the steel's mechanical properties and potentially cause a structural collapse under an accidental load (Paik et al., 2020). Therefore, the temperature profile obtained from CFD simulation can be utilized as a load for Advanced Cryogenic Spill Protection Optimization (ACSPO). ACSPO considers a thermal-structural analysis simulated by the Nonlinear Finite Element Analysis (NLFEA) method and can be used to estimate the load capacity of a structure in the presence of cryogenic exposure (Lloyd’s Register, 2015) . In conclusion, the hazard of LNG is various which can result in various consequences. Particularly, the cryogenic temperature can weaken the structure. Thus, comp-rehensive structural and consequence analyses must be adopted in the preliminary design of the ships carrying LNG.

(a) (b) Fig. 1. Dispersion and heat transfer case of an accidental LNG release: (a) gas temperature, (b) steel temperature contours (Nubli et al., 2022a). 3. Cryogenic Tensile Test The use of carbon steel in the hull structures of ships that carry liquefied natural gas (LNG) can be problematic due to its susceptibility to the ductile-to-brittle transition temperature (DBTT) phenomenon. This type of steel can be prone to damage when exposed to cryogenic temperatures, such as those experienced by LNG, which can be as low as - 163°C. Hence, the use of higher-strength steel grades such as AH, DH, EH, and FH is recommended for these ships, governed by the IGC code. Recent studies have explored the effects of cryogenic temperatures on the mechanical properties of various steel grades. For example, full-scale collapse testing conducted by Paik et al. (2020) revealed the lower yield strength of AH32 in compression compared to tension under cryogenic temperatures. At sub-zero temperatures, steels exhibit a pronounced phenomenon known as hardening, which manifests as an increase in both yield and ultimate tensile strengths. Extensive research has been conducted to explore this behavior. For instance, Cho et al. (2014) conducted a tensile test on DH36 steel and observed an increase in yield and ultimate tensile strengths by 21.6% and 12.1%, respectively, when the temperature decreased from room temperature to -50oC. Similarly, Park et al. (2015) performed a study involving a tensile test on DH36 steel at temperatures ranging from room temperature to -60 oC. Their findings indicated significant enhancements in yield and ultimate tensile strengths, reaching up to 15.73% and 14.27%, respectively. Notably, both studies assumed quasi-static loading conditions,