1. Introduction
Since 2010, the International Convention for the Prevention of Pollution from Ships (MARPOL) Protocol Annex VI has been enforced to minimize sulfur oxide (SO
X), nitrogen oxide (NO
X), and particulate matter emissions [
1]. Emission control areas (ECA) have been established in several regions that prohibit ships from producing excess pollutants. According to Annex VI Regulations 13 and 14, ships must reduce SO
X and NO
X emissions to 0.10% m/m and 3.4 g/kWh, respectively, inside the ECA [
1]. To conform to these regulations, liquefied natural gas (LNG) can be utilized as an alternative fuel for ships. As a result, there is significant demand for LNG-fueled ships—116 vessels have been in service since 2018, with 120 more under construction or with confirmed orders [
2]. Consequently, it is predicted that LNG consumption will increase from 1.27 to 3.01 million tonnes per annum for the shipping industry alone [
3]. This would require a new LNG bunkering facility in each port. There are several LNG bunkering methods, including truck-to-ship (TTS), pipeline-to-ship (PTS), and ship-to-ship (STS). These methods are applicable depending on operating conditions, port topography, and the allowable vessel size in the port [
4,
5]. TTS is unlikely to be suitable to meet the significant demands for LNG bunkering in the future. However, its benefits include high accessibility, easy operation, and low initial investment, and it remains a popular method [
6]. PTS and STS methods are more feasible for increasing LNG storage capacity. In addition to its suitability for addressing the LNG capacity issue, STS can overcome topographic disadvantages and is suitable for ships with short port turnaround times [
7]. Since STS requires a bunkering vessel unit, past research has been concerned with the design of the LNG storage tank. The common shape of the LNG storage tank is the IMO Type-C independent tank, which can sustain the pressure increases up to the maximum design pressure without affecting the gas-fuel supply and it does not need a boil-off gas treatment [
8]. However, the volume efficiency is lower than other types of non-pressurized LNG storage tanks [
8]. Past research presented a study to optimize the LNG storage tank shape by adopting the Lattice pressure vessel which can increase the volume efficiency [
9]. Another improvement in the LNG bunkering research is the study of the characteristics and control of the heading angle of the floating LNG bunkering terminal. The optimum heading angle along ships during the bunkering process can improve the loading and off-loading performance. It can reduce the relative motions of the moored vessels [
10].
In regards to the research on risk modeling, the SAFEDOR risk model identifies generic types of accident affecting LNG carriers, which are collision, grounding, contact, fire/explosion, and accidents during loading/unloading [
11]. These accident types contribute most to risk according to their probability and severity based on data from the 1964 to 2005 accident database. These categories of accident can happen on all types of ship. The possibility of accident escalation is significant for LNG carriers due to the volatility of the cargo [
11]. For example, a collision accident on an LNG carrier that causes damage to the cargo hold or LNG line could lead to LNG leakage.
Storage/distribution facilities and loading/unloading activities are associated with frequent accidental gas releases which are related to the LNG bunkering of ships, according to the electronic major accident report system (eMARS) database [
12]. Unignited LNG release creates significant risks, such as asphyxiation, cold vapor inhalation, and cryogenic burns to personnel [
6,
13]. When the released gas accumulates and is ignited, it can lead to deflagration or detonation, which could endanger objects surrounding the LNG bunkering ship [
6,
14].
Computational fluid dynamics (CFD) simulation can assist in the evaluation of safety and loss prevention in the design of LNG bunkering ships. CFD is a reliable method that can simulate complex gas dispersion, jet or pool fires, and vapor cloud explosion (VCE). Several studies have been undertaken utilizing these kinds of CFD models for ships or offshore structures [
15,
16,
17,
18,
19,
20]. Tools such as Kameleon Fire Ex (KFX) incorporate both gas boiling and spreading problems, a turbulence model, interactions with obstructions, and heat transfer problems to simulate unignited gas release [
21,
22,
23].
CFD simulation can also be useful for predicting the boundary of hazardous or critical zones. This zone is designated using a three-dimensional geometry to represent the occurrence of flammable gas in the event of an accidental release [
7,
18,
24]. Several studies have used the critical zone as a primary outcome that is determined by measuring the concentration of released gas above its lower flammability limit (LFL) [
6,
25,
26,
27,
28,
29]. The purpose of the critical zone is to eliminate ignition sources around the bunkering installation, and to restrict access to it so that only essential personnel and activities are allowed in this zone [
7,
29,
30]. This can help to prevent personnel injuries or fatalities, as well as the possibility of VCE, by minimizing ignition sources in the zone.
Further types of risk analysis, such as advanced cryogenic risk analysis (ACRA), is suggested for cases of temperature reduction due to cryogenic release in any offshore or onshore unit [
31]. This method includes assessment of the presence of structures, equipment, barriers, and wind conditions for incorporation into a CFD simulation. It also involves heat transfer calculation to estimate the ductile to brittle transition temperature (DTBTT) of the exposed structure [
13,
31]. Such embrittlement can weaken the structural steel and escalate to a structural collapse, particularly if an accidental load strikes the ship [
13,
32]. Therefore, the temperature profile obtained in a CFD simulation can be utilized for advanced cryogenic spill protection optimization (ACSPO) for a load. The ACSPO approach involves thermal–structural analysis that is simulated using the finite element (FE) method. It is useful to estimate the load capacity of a structure in the event of cryogenic exposure [
31].
Table 1 presents past studies that have involved temperature reduction analyses of structures.
The purpose of the present study was to develop LNG release scenarios that take into account leak and environmental parameters, as well as to provide an analysis of temperature reduction in the structure of LNG bunkering ships in the event of an accidental LNG release, and to determine critical zones based on the flammability of released gas.
Figure 1 presents a framework for the study that highlights two important steps in the temperature reduction analysis.
I. Accidental Release Scenario: Details of the bunkering system were provided for an LNG bunkering ship. Damaged parts of the ship geometry owing to a collision, and the ship geometry comprising both intact and damaged elements, were represented. The CFD analysis considered variables such as leak size, mass flow rate, reservoir pressure, leak position, and leak duration. Wind speed, wind direction, and ambient temperature were also considered. The thermal properties of LNG and steel, such as density, thermal conductivity, and specific heat, were considered in the KFX material settings. A total of 72 scenarios were then selected for CFD analysis.
II. CFD Simulation: All LNG release scenarios were simulated in KFX. To obtain a suitable grid number and time interval for the LNG release model, a grid convergence test (GCT) and iteration convergence test (ICT) were conducted. The result of gas cloud volume analysis was then used to investigate gas accumulation and dispersion. At the LNG flammability limits, which represent the gas contour, the critical zone was also determined. Finally, to predict damaged components as a result of cryogenic effect, the temperature profile in the LNG bunkering structure was obtained.
6. Conclusions
This paper offers a consequence analysis for an LNG bunkering ship in the event of an accidental LNG release, considering both intact and damaged conditions. A previous ship collision study was utilized to define the wrecked part for the damaged ship. The procedure for determining the LNG leakage scenario is presented, and the leakage and environment parameters are considered. Data from the previous collision study on depth penetration were utilized to build the ship’s geometry. Several variables, such as leak diameter, wind speed, wind direction, and depth penetration, were defined by considering their frequencies, to obtain a credible leakage scenario. A total of 72 scenarios were generated in the study.
Grid and iteration convergence tests were conducted to obtain adequate numbers of grids and iterations used for the main CFD analysis. A KFX validation is presented, using a UK HSL liquid hydrogen release experiment, which was modeled in KFX. The validation produced a satisfactory result in which the discrepancy for the solid temperature between the KFX and UK HSL test did not reach 15% of the COV. With respect to the results of the CFD analysis, key points are summarized below:
The gas dispersion characteristics are inferred from the gas cloud volume and its shape. The wind and obstructions exert the main influence on the formation of the gas cloud. The CFD result shows that the leakage in the pipe involves a large accumulation of gas due to its position near an obstacle that causes the released gas to be re-entrained into the release path. For leakage in the valve, the gas cloud can be easily dissipated and mixed with the air since there is no significant obstacle to disturb its release path.
The steel temperature reduces significantly in the stern trunk wall as a result of leakages from the pipe. The cold gas exposes this section due to the leak point adjacent to the stern trunk wall. For leakages from the valve, the cold gas was already expanded when it reached the stern trunk wall. Thus, the temperature reduction in this case was minor. Overall, the cold gas did not reach the broken part of the damaged ship, which was inside the cargo hold. As a result, there was no major difference in the cooling effect between the intact and damaged ships.
A profile of steel temperature was retrieved from KFX to ANSYS/LS-DYNA. The temperature reduction was significant for the leakages from the pipe, and was typically below 200 K for a 50 mm leak diameter. Since the cold gas was built adjacent to the leak point, it had no noticeable impact on the ship’s structure for 3 and 10 mm leak diameters.
This study is limited in only assessing simulation of gas dispersion to obtain the steel temperature profile. Pipe leakages with 50 mm leak diameter represent interesting cases for future structural strength analysis using FE since the cryogenic flow in these cases severely exposed the structure of the stern trunk wall. This part must be checked for indications of cracks or embrittlement due to the cooling effect, using finite element analysis.