Reliability evaluation of marine flowmeters and transmitters under cryogenic hydrogen-storage conditions based on FMEA and QFD

2.1 FMEA Method

FMEA is a reliability assessment procedure. It is designed to systematically predict potential failures that may occur during the design, manufacturing, and operational stages of a system or product. It analyses the causes and consequences of each failure to establish preventive measures in advance. This method divides the functions of each component within a complex system into detailed elements and evaluates the effects of any malfunction on the overall system both qualitatively and quantitatively, thereby enabling a failure-prevention-oriented design approach [12][13].

The overall procedure for the FMEA used in this study is summarized in Figure 1. The analysis begins by defining the functional structure of the target device. As most systems include numerous subsystems and components, the degree of segmentation must be carefully selected to ensure consistency with the analysis objectives. The level of detail must be sufficient for the proposed improvement measures to be applicable in the actual design or maintenance stages.

Figure 1:

Procedure of FMEA

Once the functional breakdown is completed, the potential failure modes for each component are identified, and their resulting effects, such as performance degradation or system shutdown, are examined. The causes of failure include material fatigue, structural damage, vibration, temperature fluctuations, thermal shock, electrical abnormalities, wear, and corrosion; environmental and human factors are also considered [14].

Three evaluation parameters are assessed for each failure mode: severity (S), occurrence (O), and detection (D). The combined product of these parameters represents the risk priority number (RPN), which is used to compare the relative risk significance of individual failure modes within the system. The RPN serves not as an absolute risk value, but as an indicator that prioritizes potential failures within the same system.

This method effectively captures rare but high-impact failures, as well as recurring faults that are not easily detected through routine monitoring. Consequently, it provides practical guidance for enhancing the reliability during system operation by reinforcing weak points at the design stage or adjusting maintenance strategies [15]-[17].

2.2 Functional Analysis of Flowmeter System

The flowmeter is a key device in the fuel-supply system of a ship that precisely measures the flow rate and density of the fluid moving between the storage tank and fuel line [18]. The measured data are transmitted to the control system through a transmitter, directly contributing to fuel control, valve operation, leak detection, and combustion efficiency. Because liquid hydrogen exhibits large density fluctuations with temperature and repeated vaporization and condensation, even minor flow deviations can critically affect system safety and efficiency.

In this study, functional analysis was conducted on a Coriolis-type mass flowmeter commonly used in marine fuel-storage systems. Figure 2 shows the analyzed sensor [19] and Table 1 presents the results. The device measures the mass flow by detecting the Coriolis force generated as the vibrating tube interacts with the flowing fluid, and simultaneously determines the density from the vibration-frequency change [20][21].

Figure 2:

Structure of flowmeter

Table 1:

Functional analysis of flowmeter

The transmitter amplifies, converts, and compensates the sensor signal for transmission to the control system. It includes power, signal processing, and communication circuits that must maintain a stable operation under cryogenic, vibrational, and humid conditions. Noise suppression and insulation reliability are critical for minimizing the output errors from signal interference.

The flowmeter operates as an integrated system in which the sensor and transmitter coordinate the flow measurements, signal processing, and data transmission. Identifying the functional roles of each component and analyzing the potential failure modes form the foundation of FMEA.

2.3 QFD Analysis Method

QFD is a quality-design methodology used in the early stages of system or product development to systematically analyze user requirements and translate them into corresponding technical elements to be addressed during the design process. By quantifying user expectations and operational requirements and linking them to technical specifications, QFD enhances product completeness and reliability [22][23]. This approach is particularly effective for sensor devices operating under complex conditions, where a single malfunction may affect the entire system, as it ensures reliability during the design stage.

In this study, QFD analysis was conducted for marine flowmeters and transmitters used in cryogenic liquid-hydrogen storage systems. Because a flowmeter measures fuel flow in real time, its accuracy and stability are directly related to the operational safety of the vessel. Therefore, the analysis defines key quality attributes by incorporating user and classification society requirements, from which the corresponding technical-test items are derived. The primary requirements include measurement accuracy, long-term stability, cryogenic resistance, moisture resistance, vibration durability, electromagnetic compatibility, maintenance convenience, and overall reliability. These factors are selected by considering both the actual shipboard operating conditions and physical characteristics of liquid-hydrogen systems [24]-[27].

The analysis was executed in a two-stage process to link failure risks to technical-test items systematically. In the first stage (Level 1), the failure modes identified in the FMEA were mapped against the user requirements to determine the critical design components. The importance scores derived from this stage were then carried over to the second stage (Level 2), where they served as weighting factors to prioritize the environmental-test items. The key tests included low-temperature endurance, vibration, humidity, voltage, current tolerance, and lifetime testing. Each test item was prioritized based on its potential likelihood of failure and impact on performance.

The results can be applied in the early design stage of flowmeter development to define the design criteria and testing conditions, while also providing a foundation for enhancing environmental reliability and establishing certification and qualification testing plans.

2.4 Evaluation Framework for FMEA and QFD

In this study, the risk assessment for the FMEA was performed using two parameters: severity (S) and occurrence (O). The detection (D) factor was excluded based on the assumption that the target transmitters were integrated into a continuous monitoring system with self-diagnostic capabilities. Because failures, such as signal irregularities or communication loss, trigger immediate alarms, the probability of a failure remaining undetected was considered negligible. Therefore, to prioritize risks based solely on the magnitude of the impact and frequency, the risk level was defined as the product of severity and occurrence. This evaluation framework was established with reference to the MIL-STD-882D guideline [28], and the risk matrix and acceptance levels are presented in Tables 2 and Table 3.

The assessment was performed by two experts with over ten years of professional experience in marine instrumentation. Their evaluation was based on failure data collected from the LNG-carrier flowmeter-operation records and supplementary technical information provided by the manufacturer.

Table 2 shows the combined assessment values for severity and occurrence. Each parameter is divided into three levels (low, medium, and high), and the intersection of the two parameters determines the total risk level, which ranges from 1 to 9. Higher numerical values indicate a greater combined impact and frequency of occurrence.

Table 2:

Risk-assessment values

Table 3 lists the corresponding risk-acceptance regions. A risk level of 9 belongs to the intolerable region, indicating that the associated failure must be corrected or mitigated before system operation. Levels 3–7 fall within the “as low as reasonably practicable” (ALARP) region, where the risk can be accepted only if further reduction is impractical or economically disproportionate. A risk level of 1 is broadly acceptable, indicating that the associated failure has a negligible impact on safety and performance.

Table 3:

Risk-acceptance levels

The severity levels summarized in Table 4 describe the degree of effect that a failure exerts on system operation. A high severity level (3) refers to failures that may cause critical incidents, such as explosions, leakages, or fires, or lead to a complete shutdown of the fuel-supply system. A medium level (2) may degrade system performance or cause equipment damage. A low level (1) represents minor effects, in which the system remains operational with manageable warnings or replaceable components.

Table 4:

Severity levels

The occurrence levels shown in Table 5 indicate the likelihood of failure. A high occurrence level represents failures that appear frequently during service life or are commonly observed in similar systems. A medium level denotes events that may occur once during operation or have theoretical possibilities based on the design conditions. A low level corresponds to rare or unreported events with a minimal probability in an actual service.

Table 5:

Occurrence levels

This structured evaluation framework allows the identification of intolerable and high-priority failure modes that require immediate design improvement, as well as the classification of acceptable or manageable risks that can be addressed through preventive maintenance and monitoring.

3.1 FMEA Results

Thirty potential failure modes were identified based on the FMEA conducted for the flowmeter applied to liquid-hydrogen storage and supply systems of ships. This evaluation considered the functional structure, operational environment, and stress factors that occur under cryogenic and vibrational conditions. The assessed severity and occurrence ratings for each failure mode were used to calculate the RPN, as summarized in Table 6.

Table 6:

FMEA result of flowmeter

Among the analyzed failure modes, sensor-tube deformation was evaluated as having the highest risk level, with an RPN value of 9. This failure is likely to occur under cryogenic conditions owing to repetitive thermal contraction and vibrational loading, which can cause significant functional losses, such as flow-measurement errors or process interruptions.

In addition, the sensor-tube fracture, driver-coil operation failure, and pickup-coil output instability showed relatively high risk levels (RPN 7). These failures were within the ALARP region, indicating that the risk is conditionally acceptable when appropriate maintenance and inspection measures are implemented. These are primarily associated with material fatigue, insulation degradation, and sensitivity reduction under continuous vibration, all of which may cause inaccurate flow measurements or temporary system shutdowns.

The results indicate that the electrical and mechanical components of the flowmeter are both critical for maintaining measurement stability in cryogenic hydrogen environments. Accordingly, sensor-tube deformation was selected as a representative intolerable failure mode requiring structural and material design improvements. Table 7 lists the design recommendations and RPN-reduction results derived from the proposed preventive measures.

Table 7:

Critical failure mode and corresponding design recommendations

3.2 QFD Result

Table 8 presents the results of the first-stage QFD analysis and Table 9 summarizes the second-stage results. The first-stage evaluation was performed by examining the correlation between the failure modes of each component in the flowmeter and user requirements. The primary evaluation criteria consisted of eight technical factors, as summarized in Table 8, which represent the essential performance and environmental-reliability requirements of the system.

Table 8:

QFD result of flowmeter (Level 1)

Table 9:

QFD result of flowmeter (Level 2)

The analysis revealed that the components of the sensor section, which measure the mass flow through the Coriolis effect generated by fluid vibration, exhibited the highest importance scores. In particular, the sensor tube, driver coil, and pickup coil were identified as critical components that directly affect the measurement accuracy and output stability through their structural integrity and signal sensitivity. These components are prone to stiffness degradation and coil-insulation damage caused by repeated thermal contractions and vibrations under cryogenic conditions, leading to measurement errors and signal instabilities.

Within the transmitter, the Micro controller unit(MCU), RS-485 circuit, and output-signal circuit exhibited relatively high importance scores. These circuits are responsible for signal amplification, conversion, and transmission, and are thus vulnerable to electromagnetic interference and electrical stress, which may result in signal fluctuation or loss of the control function. In contrast, mechanical components such as the cover and housing have a limited direct influence on measurement accuracy, but may experience indirect degradation owing to vibration and moisture ingress during long-term operation.

In the second-stage QFD analysis, the importance scores obtained from the first stage were used as weighting factors to assess the relative importance of the environmental-test items related to each failure mode. The results indicated that the humidity test received the highest total score, followed by the lifetime, low-temperature, and vibration tests. This outcome reflected the operational environment of the marine flowmeter, which was continuously exposed to combined factors, such as humidity, temperature variation, and vibration. These factors can lead to insulation deterioration, signal drift, and cumulative sensor fatigue, demonstrating their significance as essential verification parameters for reliability evaluation.

Overall, the QFD analysis established a priority framework for developing environmental testing and design-validation strategies for flowmeters. In particular, humidity, lifetime, and low-temperature tests were identified as the most critical evaluation parameters for maintaining stable performance in cryogenic liquid-hydrogen environments. These findings complemented the FMEA results by providing practical guidance for improving the high-risk components identified in the earlier analyses.

Author Contributions

Conceptualization, Y. G. Kim; Methodology, Y. G. Kim; Software, J.M. Lee; Formal Analysis, J.M. Lee; Investigation, J.M. Lee; Resources, Y. G. Kim; Data Curation J.M. Lee; Writing-Original Draft Preparation, Y. G. Kim; Writing-Review & Editing, J.M. Lee; Visualization, J.M. Lee; Supervision, Y. G. Kim; Project Administration, Y. G. Kim; Funding Acquisition, Y. G. Kim.