The Application of Risk Based-Design (RBD) Methodology For LNG Carrier Under Varied Sea State

. The energy transition issue is being pursued following the outcomes of the 22nd Summit by the G-20 group. The use of Liquified Natural Gas (LNG) fuel has emerged as one of the solutions in addressing the energy transition challenge. In line with the issuance of the Ministry of Energy and Mineral Resources Decision in January 2022, the Indonesian Government will increase the utilization of natural gas in the form of LNG for the electricity generation sector. LNG fuel is considered to possess numerous advantages and is more environmentally friendly. The distribution of LNG fuel is often carried out by tanker ships equipped with cryogenic tanks called LNG carrier. However, challenges arise when ships operate in waters which has high sea state condition, where the vessel faces the risk of accidents such as sloshing, increased motion response, stability disruptions, and other. Therefore, this paper provide a comprehensive risk analysis to minimize the existing accident risks caused by varied sea state condition using Risk-Based Design (RBD). RBD is a structured and systematic method to ensure safety levels and cost-effectiveness and utilizing risk analysis.


Introduction
Use At the close of the year 2022, the G-20 group convened the 22nd High-Level Conference (HLC) held in Bali, Indonesia.The G-20 comprises 19 countries and one European Union institution, with Indonesia being one of them and having the privilege of hosting and presiding over the 22nd G-20.The three main issues that Indonesia, in its capacity as the President, is advocating for in the 22nd G-20 High-Level Conference are sustainable energy transition, digital transformation, and global health architecture [5].Energy transition is the process of shifting from the use of environmentally unfriendly fossilbased energy sources to clean and eco-friendly energy sources.The energy transition is continuously promoted, including through the utilization of Indonesia's natural gas potential, specifically Liquified Natural Gas (LNG), which is claimed to offer numerous advantages in both the power generation sectors and transportation sectors [1].
The provision of LNG as a fuel for power generation is predominantly carried out by largesized vessels commonly known as LNG carriers.On these vessels, LNG storage tanks are typically installed either above or within the ship.However, challenges emerge in supplying LNG to remote and small archipelago regions, typically characterized by shallow water depths.The use of large-sized LNG carriers can potentially grounding due to the difference in draft between the vessel and water depth.The small LNG carrier scheme is one of the solutions to address this issue [9].Another challenge is that LNG carriers are classified as Type 1G vessels, which means they are gas carriers that must remain intact even when faced with severe damage, and their cargo tanks must be located far from the ship's hull.This challenge is compounded by the maritime conditions in the Eastern Indonesian waters, which are prone to high waves where the vessel faces the risk of accidents such as sloshing, increased motion response, stability disruptions, and other [2] [4].One method that can be employed to address these issues is the Risk-Based Design (RBD) approach.Risk-Based Design is a design process that supports risk assessment or design based on the results of risk assessment.RBD is a structured and systematic method with the aim of ensuring safety performance and cost-effectiveness through the use of risk analysis and cost-benefit assessment.

Small LNG Carrier
The small LNG carriers generally with volumes up to 10,000 m3 have a configuration of 2 cargo tanks, and for sizes larger than 10,000 m3 up to 20,000 m3, they typically have 3 cargo tanks.Meanwhile, a configuration of 4 tanks is found in ships with volumes ranging around 30,000 m 3 .[7].These configurations are shown in the Figure 1 below.The LNG carrier volume taking into consideration is small LNG carrier with the cargo capacity below 1500 m 3 .

Indonesia Sea State
The determination of sea states in Indonesian waters is conducted to understand the distribution of areas prone to high waves.Sea state data is obtained from BMKG based on the distribution of the highest wave data [4].The wave conditions are classified as extreme, with an annual wave distribution having a significant wave height reaching up to 3 meters.When compared to sea state data distribution as indicated in Table 1 below, the waters in the eastern regions of Indonesia have a sea state range from sea state 0 to sea state 7.

Risk Based-Design
Risk-Based Design (RBD) for ships is a relatively new field of knowledge and engineering that has been developed in relation to ship design, construction, operation, and regulation.RBD, or Risk-Based Design, is a design process that supports risk assessment or design based on the results of risk assessment.RBD is a structured and systematic method with the objective of ensuring safety performance and cost-effectiveness through the use of risk analysis [6].

Functional and Failure Identification
The process of functional identification is carried out with the aim of detailing the functions of main component of Small LNG carrier.This process begins at the system level, then breaks down to the subsystem level, and finally reaches the minimum level that needs to be achieved.This step allows for a deep understanding of the roles and contributions of each component of Small LNG Carrier.By detailing these functions, a comprehensive understanding of the relationships between parts can be built, facilitating the identification of potential failure modes, and aiding in the planning of effective maintenance strategies [8].

FMEA (Failure Mode and Effect Analysis)
The Failure Mode and Effect Analysis (FMEA) analysis begins by defining the failure modes of a component.Subsequently, the impact of each potential failure mode is identified.Recommendations for corrective actions are also proposed to address or prevent these failures.During the analysis, the Residual Risk Priority Number (RPN) is calculated to provide a better understanding of the risk level and the necessary priority of improvements for that component.The Risk Priority Number is determined by three factors: severity, occurrence, and detection levels [3].These factors are described in Table 2, Table 3 and Table  4. It's will help assess the seriousness of the potential failure, the likelihood of its occurrence, and the ability to detect it before it causes significant consequences.The calculated RPN serves as a quantitative measure to prioritize and guide the implementation of corrective actions, emphasizing components with higher risk level.

Result and Discussion
The result of risk based design is to provide design requirement from identified failure effect for Small LNG Carrier components on each sea state condition.The design requirement will determined based on the value of risk priority number (RPN) for each failure effect as simulated in the Table 5 below.

Risk Identification
The failure mode will be achieved after the risk identification is determined.The risk identification is regrading to Small LNG Carrier when operated under varied sea state conditions.These risks will lead to several failure conditions of ship depend on the main component.The selected main component are the LNG tank and hull ship, its are because these two components have significant role when Small LNG Carrier is being operated.The result of risk identification is indicated in Table 6

Failure Mechanics
Every identified hazard is analysed to understand the mechanisms through which failure may occur when each main component is at risk.The failure mechanisms typically consist of failure modes, the effects of failure, and how failure can be detected.Failure mechanisms are identified based on each hazard risk that occurs in each main component.The results are presented in Table 7 below.

Failure Mitigations
Failure mitigation is a method used to reduce the potential for failure when an LNG ship faces existing hazard risks.Failure mitigation will be used as a reference in small LNG ship design activities.The results are presented in Table 8

RPN Value
Risk Priority Number (RPN) is utilized to determine the criticality of ship components concerning the occurrence of failures.The RPN value is the result of multiplication from severity x occurrence x detection of each failure effect.The results of the RPN assessment for ship components under each sea state in Indonesian waters are presented in Table 9 and Table 10 below.The result of RPN value show in line with the increasing of sea state conditions.The highest Total average RPN value occurs in the sea state 7 with 320.Among of all failure effect, Sink and Ship bottom fracture had the highest value of RPN with 560.These failure effect caused by the increasing of wave height that will lead to risk of the increasing of stress and bending moment in the midship area.

The Relationship Between Risk and Ship Design
The relationship between risk and ship design is a crucial aspect of naval engineering and management.Ship design involves creating vessels that meet specific operational requirements while minimizing various risks associated with their construction, operation, and lifecycle.In the other words, in the way of increasing of sea state condition the Small  The ship design requirements are divided into 3 group based on the result of total average RPN Value.The 1 st group is consisting of sea state 0 into 3 where the total average RPN is under100.In this group, there is no design requirement for the Small LNG Carrier.Except in sea state 3 and 4, the surface area effect must be considered to minimize the failure effect.
The 2 nd group is consisting of sea state 4 and 5 where the total average RPN is between 100 and 200.There are many design and operational requirements shall be considered to minimize the failure effect.In the 3 rd group is consist of sea state 6 and 7 where the total average RPN is above 200.In this group, the material selection is considered as special design requirement to minimize the failure effect.It is because if the vessel face higher wave so the material to construct also has higher strength limit.But if these design requirements is not able for small LNG carrier to operate in sea state 6 and 7, re-routing based on weather forecasts is the best way to avoid the risk of failure effect.

Conclusion
The small LNG carrier has several design requirements to operate under varied sea stated conditions.The occurrence of failure getting higher in the way of the sea state condition.The highest RPN are failure on sink and bottom fracture where it is the priority risk shall be considered.The failure that affects the whole system of ship are sink, bottom fracture, bow

Table 2 .
Severity Level

Table 3 .
Occurrence Level

Table 4 .
Detection Level

Table 5 .
The Simulation Between Risk and Ship Design

Table 7 .
Risk Identification

Table 9 .
RPN Value Sea State 0 to 4

Table 10 .
RPN Value Sea State 5 to 7 LNG Carrier will have more specific requirement to prevent the failure effect of each sea state condition.This relationship tabulated in Table11below.

Table 11 .
The Relation Ship Between Risk and Ship Design , 02004 (2024) BIO Web of Conferences https://doi.org/10.1051/bioconf/2024890200489 SRCM 2023 fracture and ship turn over.These cases have high occurrence in the sea state 6 & sea state 7. Hence, the Small LNG carrier design is very hazardous to operate under sea state 6 and 7.