Maintainability Analysis of Remotely Operated LNG Marine Loading Arms Based on UNE 151001 Standard

Abstract

The operation of liquefied natural gas (LNG) marine loading arms plays a pivotal role in the efficient transfer of LNG from maritime vessels to downstream facilities, underpinning the global LNG supply chain. Despite their criticality, these systems frequently encounter operational challenges, notably slow coupling speeds and increased downtimes driven by maintenance demands. Addressing these challenges, Physical Asset Management principles advocate for maximizing process availability by minimizing both planned and unplanned outages. Recognizing maintainability as a key equipment attribute, this document proposes a procedure that extends the use of the UNE 151001 standard to evaluate the maintainability of physical assets. This proposal incorporates into traditional RCM a step for the selection of maintenance levels proposed in the standard, as well as the use of the AHP technique for selecting the weights used during the analysis process. Finally, an aggregated maintainability indicator is presented, which will allow for better evaluation, comparison, and monitoring of this characteristic in one or more industrial assets. To demonstrate its feasibility and utility, the proposed procedure is applied to a set of LNG marine unloading arms. This study identifies pivotal areas for improvement and devises strategic action plans aimed at enhancing asset’s maintainability. The outcomes of this analysis not only provide a roadmap for augmenting operational efficiency but also furnish empirical justification for the requisite investments in maintainability enhancements, thereby contributing to the resilience and sustainability of LNG logistics infrastructure.

1. Introduction

Maritime transport is the main mode of international trade (up to 80% of goods); thus, marine structures are critical because of their specific influence on trade [1]. Discharge arms are remotely operated equipment that allow the transfer of LNG from a ship to the terminal. The types of discharge arms used in the LNG industry are liquid or vapor (vapor returning to the ship during high-flow discharge). Due to the high impact of a failure if one of these pieces of equipment operates outside the operational parameters, they are constantly monitored through pressure, temperature, and position instrumentation [2]. However, the aspect that most affects the availability of the equipment is the difficulty in performing maintenance, as it requires access to high points, in adverse weather conditions, and with a level of corrosive aggressiveness that requires inspection frequencies higher than those of other LNG terminals [3]. For this reason, it is necessary to analyze and improve the maintainability of such equipment continuously and repeatedly. This work proposes a framework based on the UNE 151001 standard to conduct these analyses. The proposed framework integrates or connects the application of this standard within the context of RCM to link the analysis of asset topology, criticality analysis, and the proposal of maintenance strategies with the maintainability analysis considering the various levels or categories of maintenance interventions suggested by the UNE 151001 standard. The purpose is to reduce the efforts and difficulties in executing such interventions. Additionally, the use of a multicriteria decision-making support technique is proposed for defining the weights used in evaluating the various maintainability attributes during the application of the standard.

This work consists of a section presenting a theoretical background, followed by a brief literature review in Section 3. Subsequently, the proposed framework is described in Section 4. To illustrate and validate our proposal, a case study applied to LNG unloading arms is presented in Section 5. Finally, Section 6 discusses the results and presents the conclusions of the work.

2. Theoretical Background

According to the UNE-EN 13306 standard [4], availability is defined as “the ability of an element to be in a state in which it can perform its function, when and how required, under given conditions, assuming that the necessary external resources are available”. Availability is estimated using the following Equation (1):

$$A=\frac{TBF}{TBF+TTR}$$

(1)

where the variables represent the following:

A: availability;

TTR: time to repair;

TBF: time between failures.

From the above, one can see that availability is directly affected by the failure rate and the associated repair times. By evaluating the failure rate and the consequences of such failures, the intrinsic risk level in the production process can be determined. The failure rate refers to the number of failures over a specific period of time, and the consequence can be evaluated from different approaches, such as the following:

• Operational flexibility;
• Quality of service or product;
• Operational safety;
• Personnel safety;
• Environment;
• Failure costs;
• Operating costs;
• Maintenance costs;
• Downtime.

These factors collectively influence the reliability and efficiency of the physical asset and, by extension, impact the availability of the entire system. An understanding and assessment of these components are critical for maintaining functionality and safety, while also dealing the financial implications associated with downtime and repairs.

Conventional design methods of predictive health management (PHM) have become unfeasible and fail to meet the demands of systems with intricate structural and functional complexities. Within the scope of PHM frameworks, the designs focusing on testability and maintainability encounter significant challenges [5]. The maintainability of industrial equipment is defined as the capability of this equipment to be maintained or repaired in a timely, safe, and effective manner, ensuring its continued availability at the lowest possible cost and effort. This encompasses aspects such as the ease with which failures can be diagnosed and corrected, the simplicity of carrying out preventive and corrective maintenance tasks, and the speed with which the equipment can be returned to operational status after a shutdown or failure.

Maintainability is a critical aspect in the design and operation of industrial equipment, as it directly impacts operational efficiency, maintenance costs, and workplace safety. On the other hand, improving maintainability generally requires re-engineering, additional investments, and, in some cases, changes to the operational context. One tool that allows for the improvement of consequences is Maintainability Analysis.

A well-known methodology for the generation of maintainability indicators and the subsequent proposal of improvements to enhance the maintainability of equipment is the UNE 151001 standard [6]. Such a standard offers a procedure for the analysis of maintainability through a set of structured phases and assessments. From this analysis, recommendations can be made to alter the design of equipment, as well as the usual maintenance procedures. Among the most outstanding characteristics of the standard are its extensiveness, which can make it complex to use, and its qualitative nature, which can generate biases and make it more difficult to trace its evaluations. In this paper, we propose a method to integrate this standard within an RCM context to improve the traceability of the assessments carried out during its application. Similarly, we incorporate the AHP technique for defining the weights during the level-specific maintainability analysis. Finally, in order to address the lack of a single maintainability indicator, we propose an aggregated indicator, which is constructed from the aforementioned AHP-based procedure. The following section provides a brief review of the literature regarding maintainability analysis techniques.

3. Brief Literature Review

The MTTR (mean time to repair) index is the most widely used quantitative metric for maintainability. It is calculated using statistical data on the duration of repairs. Despite its widespread use, studies such as that of [7] suggest that maintainability metrics alone are not sufficient to accurately estimate maintenance efforts. They propose complementing them with other factors such as complexity and design quality. The method proposed by Wu Zhenya and Hao Jianping [8] to evaluate the maintainability of complex systems is based on grouping maintenance time distributions and applying a demonstration index for each group. Lu, Liu, Dong, and Liang [9] present a technique that uses colored stochastic Petri nets (SCPN) to simulate the maintenance process and calculate performance indicators, such as MTBF and MTTR. Their methodology also considers determining optimal maintenance strategies based on simulation results. Goulden Eldon [10] introduces a proactive analytical method that employs failure mode and effects analysis to improve system reliability and efficiency. Similarly, Elevli, Uzgoren, and Taksuk [11] conduct a study on the maintainability of mechanical systems in electric cable shovels, using failure mode and effects analysis to propose improvements that increase their reliability. Luo, Ge, Zhang, and Yang [12] develop a three-step process to evaluate maintainability from the design phase, based on design attributes that can influence maintainability. This approach provides valuable information to improve maintainability from the early stages of development. Finally, Nabizadeh et al. [13] study the reliability and maintainability of material handling machinery in a mine, concluding that factors such as equipment age and maintenance practices are crucial to its performance.

A significant limitation in the aforementioned analyses is the absence of historical data for analysis when dealing with new equipment, which prevents obtaining conclusive statistical analyses. The literature suggests a trend towards the proactive evaluation and design of maintainability, using advanced technologies to anticipate and resolve problems. These technologies allow for the simulation of maintenance scenarios and prediction of the impacts of design changes on equipment maintainability [14]. Augmented reality methods and digital twins are examples of how technologies can enhance the assessment and design of maintainability. In the studies by Khalek et al. [15] and Guo et al. [16], these technologies enable early identification of potential problems and help to optimize design for maintainability. Artificial intelligence (AI) and machine learning (ML) are increasingly being used to analyze data from Internet of Things (IoT) devices and connected systems to predict and prevent maintenance issues. These technological advances allow companies to anticipate failures and optimize maintenance management, including maintainability. The management of uncertainty in maintainability analyses is another field of growing interest. Fuzzy logic and other methods that consider uncertainty and vagueness are becoming more significant, as illustrated in the fuzzy quality model of Yilmaz and Buzluca [17] for microservice architectures.

A trend that is noticeable in the analysis of the literature on maintainability studies is the aspect of human factors. The work reported by [18] addresses the influence of human and organizational factors on the reliability and maintainability of CNC turning centers, highlighting how human intervention affects the efficiency of maintenance processes. These studies illustrate a shift toward the incorporation of human and ergonomic considerations into design for maintainability, recognizing the importance of human–machine interfaces and ergonomics in maintenance efficiency. A study on helicopters [19] shows how the integration of human factors into design can result in more efficient and less error-prone maintenance.

The assessment of maintainability is becoming a more holistic practice that utilizes a blend of quantitative and qualitative analyses, greatly benefiting from advanced simulation and visualization [20]. As technologies continue to evolve, we can expect the strategies for evaluation and design for maintainability to become even more sophisticated and effective, allowing companies to not only anticipate maintenance problems but also improve operational efficiency and the long-term reliability of equipment [21].

In summary, the literature on design for maintainability, although not extensive, is rich in innovative and practical proposals that move towards the integration of AI and ML for proactive and sustainable evaluation and design. The use of digital tools is becoming a standard to optimize maintainability from the earliest design phases, with a strong focus on simulation and augmented reality to identify potential problems and optimize maintainability. Digital twin technology [22] and virtual reality are also gaining ground as effective methods for testing and improving maintainability in safe and controlled environments.

4. Methodology

This proposal involves defining a procedure that integrates RCM with maintainability analysis using the UNE 151001 standard as a basis. The proposed procedure begins with selecting the asset or system to be analyzed. Subsequently, with the support of a model we have called Maintainability Centered Analysis, the maintenance levels on which the maintainability analysis will be performed are identified. Once the levels are selected, the procedure governed by UNE 151001 is applied. At this point, we propose an alteration to the procedure: the use of the AHP technique to determine the weights for each analyzed level and the attributes, concluding with the definition of a general maintainability indicator for the equipment.

The main information inputs for executing the proposed procedure through the framework are as follows:

• ISO 14224 standard, through which the taxonomy will decompose the asset or system into subsystems and/or components [23];
• A methodology to determine the criticalities in the assets’ taxonomy;
• The RCM FMECA, which will identify the critical failure modes and maintenance strategies (type and frequency of interventions) [24];
• The UNE 151001 standard as the basis for the maintainability analysis procedure;
• The AHP technique as an alternative method to find the weight values used by the mentioned standard.

Once the critical failures and the associated maintenance strategies for each have been identified through RCM, the selection of the maintenance action level specified in the UNE 151001 standard, required for each of these critical failures, will proceed. A decision diagram, as illustrated in Figure 1, is proposed as a means of facilitating this process. This diagram, developed based on the RCM decision diagram, allows for the identification of the maintenance levels of the asset under analysis and its critical failure modes, which will be used to continue with the maintainability analysis.

Additionally, we propose expanding the RCM Decision Sheet by adding a new section, labeled MCA (Maintenance-Centered Analysis). This new section enables the analysis of how different failure modes are connected to maintenance strategies and the maintenance levels to which these strategies belong, in accordance with the UNE 151001 standard. By incorporating these elements, the new set of columns offers a comprehensive framework to evaluate and optimize the maintenance processes, facilitating improved identification and alignment between the maintainability analysis and the previously executed RCM.

Once the decision diagram is applied, the results are recorded on a sheet, which we have called the MCA Decision Sheet (Figure 2). This sheet is proposed based on the decision sheet suggested by RCM and the conclusions obtained from the MCA Decision Diagram (Figure 1).

Subsequently, once the appropriate levels have been selected, the procedure defined by the UNE 151001 standard is applied. This entails considering the identified levels and assigning values to the maintainability attributes in accordance with the procedure outlined in the following section.

4.1. Maintainability Assessment Based on UNE151001

Maintainability is a fundamental characteristic of industrial devices, and its assessment is based on the capability of these devices to be maintained under specific conditions of use. This evaluation requires a detailed description of the operating conditions of the device, which include not only the way in which it is used but also its layout in the plant and the surrounding environmental conditions.

To obtain objective measurements of maintainability indicators, it is important that the personnel responsible for the evaluation are properly trained and qualified and that there is a regulatory framework. This ensures that the results are accurate and reliable.

It is within this framework that the maintainability evaluation will be carried out following the guidelines and directives established in the standard UNE 151001:2011 as the main reference. This standard provides a robust and standardized framework for measuring and evaluating maintainability in industrial devices. By following this standard as a reference, it is ensured that the evaluation process is conducted consistently and reliably, facilitating the comparison and analysis of maintainability across different devices and situations.

It is important to emphasize that the use of a reference standard such as UNE 151001:2011 provides coherence and uniformity in the assessment of maintainability, which contributes to obtaining precise and meaningful results. Additionally, it facilitates communication and understanding of the results among different stakeholders, which is important for making informed decisions in asset management and maintenance.

The application of the UNE 151001 standard enables the evaluation of maintainability indicators under a range of conditions of use and at different stages of the device’s lifecycle. During the device’s preparation phase, the evaluation of these indicators can assist in improving its design to facilitate future maintenance tasks. Subsequently, during the operational phase, the evaluation is conducted on a periodic basis to ascertain the ease of maintenance under specific operational conditions, which may change over time. Through the application of the UNE 151001 standard, maintainability indicators can be evaluated under different conditions of use and at different stages of the device’s lifecycle. During the device’s preparation phase, the evaluation of these indicators can help to improve its design to facilitate future maintenance tasks. Subsequently, during the operation phase, the evaluation is carried out periodically to measure the ease of maintenance under specific operating conditions, which may change over time.

When evaluating maintainability indicators during the operation phase, the inherent characteristics of the device itself are being assessed, not the organization responsible for its maintenance. For example, when the training of maintenance personnel is mentioned as an attribute of maintainability, it refers to the capacity and qualification of the personnel in charge of carrying out the maintenance actions according to predefined levels. The current level of training of the maintenance staff at that moment is not being evaluated. The determination of maintainability indicators offers various advantages in decision-making and the management of industrial devices. These indicators are useful for the following:

• Compare Systems: By evaluating the maintainability of various systems or devices, informed decisions can be made when selecting the most appropriate one based on its ease of maintenance;
• Improve Design: Maintainability indicators can guide the design of devices, ensuring that they are aligned with maintenance requirements and facilitating future maintenance operations;
• Optimize Maintenance: For devices in operation, the periodic evaluation of indicators allows identifying areas for improvement in terms of maintainability. This can result in the implementation of more efficient maintenance practices;
• Adapt to Changes: In the face of potential changes in the location or industrial environment of a device, the evaluation of maintainability indicators helps to foresee and address potential maintenance-related issues.

The evaluation of maintainability indicators is applicable to any type of industrial device. It is also applicable in the preparation and operation phases of the device. Therefore, for the maintainability measurement of a piece of equipment to be comparable and useful, it is important to detail the moment, in the life cycle of the device, at which the maintainability indicator is evaluated and, if applicable, the conditions of use and the environment of the device at that time.

The standard provides a characterization of two types of attributes or characteristics of the equipment:

General Attributes: These attributes are of a general nature and affect any level of the device’s maintenance. They include characteristics that are relevant at all times and in any phase of the device’s life cycle. Examples of general attributes could be the ease of access to components, the availability of detailed technical documentation, the training required for maintenance personnel, among others. These attributes provide a general assessment of the device’s maintainability;

Specific Attributes: Specific attributes are those that depend on the specific level of maintenance to be performed on the device. These attributes vary according to the type of intervention or level of maintenance considered. For example, the ease of disassembling a particular component might be a specific attribute for preventive maintenance, while the availability of spare parts might be a specific attribute for corrective maintenance. These attributes are directly related to the maintenance actions that will be carried out at a specific level of intervention.

The evaluation of the specific attributes of a device’s maintainability is based on the classification of maintenance actions into five distinct levels. Each level of maintenance corresponds to a specific set of activities and tasks related to the maintenance of the device. The standard proposes the use of a grading scale consisting of five values, ranging from 0 to 4 points, to assess each of these maintenance levels.

This five-value scale allows assigning a score to specific maintainability attributes based on the maintenance level they are related to (0 to 4). Higher values on the scale indicate better performance or greater ease in fulfilling that attribute at that specific level of maintenance, while lower values indicate the opposite. The use of this grading scale facilitates obtaining a quantitative and objective evaluation of the device’s maintainability at each of the considered maintenance levels. This provides a detailed view of how maintainability behaves in different situations and levels of intervention.

4.1.1. Maintenance Levels

According to the complexity of the tasks and the number of resources necessary for their execution, they will be classified into different levels of maintenance. This classification ranges from Level 1 to Level 5. This allows us to distinguish between more- or less-invasive maintenance activities, and those that are demanding from an economic point of view or in terms of availability.

• Level 1, Simple maintenance actions in operation: preventive or corrective activities carried out by the operator without generating equipment downtime;
• Level 2, Maintenance actions with component exchange: preventive or corrective activities carried out by maintenance staff where components or parts are exchanged, generating downtime;
• Level 3, Identification and diagnosis of failures: actions where the operator or maintainer, after taking the equipment out of service, identifies and locates the cause of the failures;
• Level 4, Overhauls: a set of inspections and preventive or corrective maintenance activities that require the complete or partial disassembly of the device. The objective of an overhaul is to ensure the safe and reliable operation of the equipment. Its frequency can depend on hours of operation or operating cycles;
• Level 5, Renewal: proactive maintenance operations, which can include the implementation of improvements, upgrades, or major reconstruction. In some cases, these types of overhauls can positively influence the Asset Life Cycle.

Table 1. Maintenance levels according to UNE 151001:2011.

Level	Description	Personnel Needed	Resources	Unavailability Estimated Time
1	Simple maintenance actions without device shutdown	Plant operators and/or maintenance personnel	Common tools defined in the instructions for use and maintenance	None
2	Maintenance actions involving functional component exchange	Plant operators and/or maintenance personnel	Common tools defined in the instructions for use and maintenance and regular spare parts	Minutes
3	Fault identification and diagnostics	Plant maintenance specialized personnel and/or specialized companies	Tools and measuring devices, testing equipment, etc.	Hours
4	Inspections	Team composed of specialized technicians. In a specialized maintenance workshop, on-site or off-site	Specialized tools in general, testing devices, control equipment, etc.	Days
5	Renovation, reconstruction, and/or major repairs	Full and versatile technical team. In the central workshop of the manufacturer	Equipment close to the manufacturing of the element under study, owned by the manufacturer	Weeks

shows a scheme representing the different classifications of maintenance levels where the personnel involved in the equipment, the means required, and the execution time and, therefore, the downtime of each maintenance level are specified.

4.1.2. Maintainability Attributes

The evaluation of a device’s maintainability indicators involves considering various attributes that can be classified into three main groups: attributes related to design, attributes related to personnel requirements and working conditions, and attributes related to the need for logistical support. It is important to emphasize that these attributes are related to the inherent capabilities of the device being evaluated and are not related to the skills or capabilities of the maintenance organization operating the device.

Design-related attributes: These attributes refer to the characteristics and configuration of the device that facilitate or hinder maintenance operations. They include aspects such as the accessibility to components, the modularity of the design, and the availability of technical information;

• Attributes related to personnel requirements and working conditions: These attributes consider the training and qualification needs of maintenance personnel, as well as the working conditions necessary to carry out maintenance tasks efficiently and safely;
• Attributes related to the need for logistical support: These attributes focus on the availability of spare parts, tools, and the necessary technical documentation to perform maintenance activities. They also cover the ease of transporting spare parts and waste management;
• Attributes related to design: Within the category of design-related attributes, eight key aspects are considered that influence the maintainability of the device:
  • Accessibility: This attribute assesses whether there is provision for adequate access to components that will require maintenance, such as doors, sliding racks, etc.;
  • Assembly/Disassembly: Refers to the ease with which components can be removed and replaced in the different subsystems of the device. This ease is related to the presence of joints, welds, and the size and weight of the components;
  • Standardization: Evaluates the compatibility of spare parts with other similar materials when replacing a component of the device. This depends on the choice of standards during design, such as the selection of bearings, seals, and dimensional and functional tolerances;
  • Simplicity: Considers whether there is a reduced number of unnecessary elements and assemblies in the device;
  • Identification: This attribute assesses whether the device provides clear identification and prioritized labeling of the elements that require maintenance, as well as the inspection and testing points. This can include color coding, labeling, and the use of symbols that direct maintenance personnel quickly to the areas that need attention, which can significantly reduce the time needed for troubleshooting and repairs;
  • Surveyance: Refers to the inclusion of critical parameter indicators and alarms that can predict failures in the device. This aspect of design involves integrating sensors and diagnostic systems that alert operators before a failure occurs, enabling preventative maintenance and reducing unplanned downtime;
  • Modularization: Evaluates whether the device’s design is divided into separate parts or assembly units so that, in the event of a failure, it is not necessary to dismantle the entire equipment but only the affected part. Modular design can make maintenance tasks more manageable and less time-consuming as it allows for the replacement or repair of individual modules instead of more complex repairs that involve the entire system;
  • Tribology: Considers the device design and the appropriate selection of material quality, taking into account aspects such as lubrication, friction, and wear. This is fundamental for reducing maintenance requirements and extending the lifespan of both the device and its components. Proper tribological design can prevent premature wear and failure, ensuring that components last for their intended service life and beyond;

• Attributes related to personnel and working conditions:

Within the category of attributes related to maintenance personnel and working conditions, three aspects are considered:

• Ergonomics: This evaluates the necessary space requirements to ensure adequate working conditions during maintenance activities. This attribute also takes into account the location and space requirements for handling materials when interventions are made on the physical system;
• Training: Refers to the training and skills necessary for maintenance personnel to carry out the required type of work effectively and safely;
• Site: Assesses the requirements related to the environmental and surrounding conditions in which maintenance activities will be conducted. This includes aspects such as temperature, lighting, and other factors that ensure maintenance is carried out in safe and suitable conditions;

• Attributes related to logistic support:

Within the category of attributes related to the need for logistical support, six key aspects are considered:

• Spare Parts: This evaluates the requirements related to the storage and handling of spare parts necessary for the device. This includes aspects such as spare parts inventory, their availability, and the efficiency of their management;
• Tools and Equipment: Refers to the requirements related to the tools and equipment necessary to carry out maintenance activities. This implies evaluating their functionality, ergonomics, and ease of acquisition;
• Documentation: Assesses the availability of relevant documentation, such as maintenance manuals, procedural guides, and any necessary information required to carry out maintenance actions effectively;
• Relationship with the Manufacturer: Considers the coordination and communication between the maintenance personnel at the plant and the device manufacturer. This can include aspects such as a common language, similarity of management systems, geographical distance, and other factors that may affect maintenance efficiency;
• Personnel Organization: Evaluates the amount of personnel required to carry out maintenance operations and the possibility of dividing tasks into parallel activities to improve efficiency;
• Interdepartmental Coordination: Considers the need for coordination with other departments, individuals, or the acquisition of special permits that may be required to carry out maintenance activities effectively.

4.2. Attributes of Maintainability and Maintenance Levels

The relationship between maintainability attributes and maintenance levels can be better understood by dividing these attributes into two main groups: general attributes and specific attributes for each maintenance level, aggregating a set of six attribute sets.

General attributes may include features such as the overall design simplicity or standardization of components, which affect maintenance at every level. In contrast, specific attributes could be particular to preventive, corrective, or overhaul maintenance activities, such as the ease of disassembling a specific part during routine checks or the need for specialized tools during major overhauls. Therefore, they must be evaluated in a detailed and adapted manner for each corresponding level of maintenance. Understanding and assessing these attributes accordingly ensures that maintenance efforts are appropriate and effective for the specific demands of each maintenance level, ultimately contributing to better equipment reliability, reduced downtime, and more efficient use of resources.

4.2.1. General Attributes

The following eight general attributes should be considered in the evaluation of a device’s maintainability, as they influence all levels of maintenance. Each attribute is presented along with its nature in parentheses:

• Simplicity (G1) (Design);
• Identification (G2) (Design);
• Modularization (G3) (Design);
• Tribology (G4) (Design);
• Ergonomics (G5) (Maintenance Staff);
• Standardization (G6) (Design);
• Surveillance (G7) (Design);
• Relationship with the manufacturer (G8) (Logistical Support).

4.2.2. Specific Attributes

These attributes are applicable at the location where the device maintenance is carried out, which may differ from its usual location, especially in the last two levels of maintenance. The following nine maintainability attributes should be considered for each maintenance level, and the nature of each attribute is indicated in parentheses:

• Accessibility (V1) (Design);
• Assembly/Disassembly (V2) (Design);
• Training (V3) (Maintenance Personnel);
• Personnel organization (V4) (Logistical Support);
• Environment (V5) (Maintenance Personnel);
• Spare parts (V6) (Logistical Support);
• Tools and equipment (V7) (Logistical Support);
• Coordination (V8) (Logistical Support);
• Documentation (V9) (Logistical Support).

4.2.3. Evaluation of Indicators

The evaluation of six maintainability indicators is required: a general maintainability indicator that is obtained from the evaluation of the general attributes of the equipment plus five specific maintainability indicators, one for each level of maintenance (see Table 1), which will be named “maintainability indicator for level i of maintenance”. Each maintainability indicator is calculated by weighting the various maintainability attributes and using a specific algorithm for its calculation. The weights assigned to each attribute reflect their relative importance in the particular conditions of the indicator’s evaluation and represent the proportional contribution of each attribute to the maintainability indicator. These evaluation conditions, as well as the weights assigned to each attribute, must be supported and justified in the corresponding report.

The weights assigned to each attribute must be in the range of 0 to 1, ensuring that the sum of all the weights used to calculate a specific maintainability indicator for a given element is equal to unity. That is, the equation must comply with the following condition:

$$p_{Ai}=\left\{0,1\right\}$$

(2)

$${\displaystyle \sum }_{i=1}^n{p}_{Ai}=1$$

(3)

where the variables represent the following:

$p_{Ai}$ = Relative weight of attribute A_i in the computation of the indicator, which can be general attributes ($G_i)$ or a specific indicator of a maintenance level ($V_i)$;

i = attribute indicator;

n = number of attributes.

4.3. Level Maintainability Indicator

The calculation of the maintenance indicator is accomplished using the following equation:

$$IM{G}_j={\displaystyle \sum }_{i=1}^9{V}_{ij}{p}_{vij};con j=1\dots 5$$

(4)

where the variables represent the following:

j = Number indicating each of the five maintenance levels;

i = Number indicating each of the eight specific maintenance attributes;

V_ij = Value, ranging from 0 to 4, of each of the nine specific maintainability attributes for each of the five maintenance levels;

p_vij = Value, between 0 and 1, representing the weight of each of the nine specific maintainability attributes for each of the five maintenance levels.

To obtain the weights of each attribute and better capture the opinions of the specialists participating in the maintainability evaluation process, the use of the AHP method is proposed here. The following section describes the main elements and phases of the AHP method as is suggested to be applied in this paper.

Weight Calculation

Each aspect of maintenance and each characteristic of the asset contributes to or impacts its maintainability differently. Therefore, a critical step to obtain a rational evaluation result is to quantify the weight of each factor defined in the standard. To determine the weight of these factors, we propose a variation to the method suggested by the UNE 151001 standard. This variation involves using the Analytic Hierarchy Process (AHP).

The AHP uses a ranking scale of judgments to evaluate the factors. A nine-level numerical scale is adopted for pairwise comparisons, allowing for the effective distinction of factors that cause different perceptions. The fundamental scales of the AHP are described in Saaty (1987). The AHP involves constructing a hierarchical structure, creating a judgment matrix, and performing weight calculations and consistency checks. The hierarchical structure helps analyze the decision problem structurally.

To integrate this method into the application of the UNE 151001 standard, we propose the disaggregated calculation of two maintainability indicators. The first indicator corresponds to the Intrinsic Maintainability of the Asset (IMG). This indicator will be obtained as proposed by the standard. The second indicator is an Aggregated Maintainability Indicator ($\overline{\mathrm{IMN}}$), which considers the evaluations of the maintainability attributes of up to 5 levels suggested by the standard and their respective IMN_i indicators (Table 1). Figure 3 illustrates the hierarchical structure used to define the weights of the different attributes and indices in the maintainability evaluation by level of a given physical asset.

The first level represents the goal, which is to obtain the aggregated indicator that combines the maintainability indicators IMNi (one for each analyzed maintenance level) from the standard. The second level of the hierarchy includes the maintenance levels (Table 1) proposed by the standard and determined in the MCA analysis. The third level lists the 9 attributes related to each of the maintenance levels defined by the standard (Vi).

The application of this technique to obtain the Aggregated Maintainability Indicator ($\overline{\mathrm{IMN}})$ is illustrated through the following example.

Step 1: Definition of the Hierarchy (Figure 3). First Level: Calculation of the Aggregated Maintainability Indicator ($\overline{\mathrm{IMN}}$). Second Level: This is where the Maintenance Levels selected by the MCA are located. Third Level: This consists of the 9 Attributes (Vi) in each level described in the second level of the hierarchy;

Step 2: Construction of the Comparison Matrix Comparison of the Maintenance Levels (Second Level of the hierarchy) in terms of their contribution to the ($\overline{IMN}$). Suppose the MCA indicated the need to perform maintainability analysis at the 5 levels of the standard, and suppose that the specialists provided the following judgments (on a Saaty scale of 1 to 9) for the Comparison Matrix for the maintenance levels (

Table 2. Comparison matrix (level of maintenance).

	Level 1	Level 2	Level 3	Level 4	Level 5
Level 1	1	1/3	1/5	1/7	1/9
Level 2	3	1	1/3	1/5	1/7
Level 3	5	3	1	1/3	1/5
Level 4	7	5	3	1	1/3
Level 5	9	7	5	3	1

);

Step 3: Construction of the Comparison Matrix for the Maintainability Attributes.

Table 3. Comparison matrix (maintainability attributes).

	V1	V2	V3	V4	V5	V6	V7	V8	V9
V1	1	3	5	7	9	1	3	5	7
V2	1/3	1	3	5	7	1/3	1	3	5
V3	1/5	1/3	1	3	5	1/5	1/3	1	3
V4	1/7	1/5	1/3	1	3	1/7	1/5	1/3	1
V5	1/9	1/7	1/5	1/3	1	1/9	1/7	1/5	1/3
V6	1	3	5	7	9	1	3	5	7
V7	1/3	1	3	5	7	1/3	1	3	5
V8	1/5	1/3	1	3	5	1/5	1/3	1	3
V9	1/7	1/5	1/3	1	3	1/7	1/5	1/3	1

shows the Comparison Matrix for the Attributes within each Maintenance Level. (For illustration purposes, only the matrix for one maintenance level will be shown);

Step 4: Calculation of Eigenvectors and Eigenvalues:

The AHP methodology includes a Consistency Verification through the calculation of a Consistency Index (see for more details: [25]).

Once the consistencies of the comparison matrices have been verified, it is possible to proceed with obtaining the relative importance among the maintenance levels. The normalized eigenvector, eigenvalue, and RC index must be calculated according to Saaty’s methodology. Thus, the normalized eigenvector is as follows:

V = (0.0329, 0.0636, 0.1296, 0.2638, 0.5100)

Based on this eigenvector, and sorting from highest to lowest, the order of relevance among the 5 maintenance levels can be obtained. In this case, the order from the highest weighting level to the lowest is as follows: levels 5, 4, 3, 2, and 1.

For each level, the 9 attributes (Vi) are evaluated, and the corresponding eigenvectors are obtained. For example, in Level 5, the eigenvector is as follows:

V = (0.1245, 0.1182, 0.1105, 0.1009, 0.0915, 0.1245, 0.1182, 0.1105, 0.1012)

These values indicate the order of priority or weights of the 9 attributes when analyzing maintainability at this specific level. These are the following, ordered from highest to lowest: attributes 1, 6, 7, 2, 3, 8, 4, and 5. In order to ascertain the weights of each attribute in relation to each maintenance level, it is necessary to obtain all the eigenvectors for each level. Once all the eigenvectors have been obtained, the calculation of the Aggregated Maintainability Indicator ($\overline{IMN}$) can proceed. This indicator can be obtained by multiplying the weights matrix by the transposed eigenvector of the maintenance levels. At the end of the detailed procedure, two maintainability indicators are obtained: the Intrinsic Maintainability Indicator (IMG) and the Aggregated Maintainability Indicator ($\overline{IMN}$).

In the following section, a case study is presented, using the proposed methodology, to assess the maintainability of remote-controlled LNG unloading arms.

5. Case Study

Liquefied natural gas (LNG) is an efficient solution for transporting Natural Gas (NG) over long distances by sea or land when constructing a pipeline is not technically or economically feasible. Liquefied natural gas is natural gas that, when supercooled to cryogenic temperatures, reduces in volume by six hundred times, allowing for its transportation in insulated containers to be later stored and regasified. The transfer from storage and transfer vessels is carried out using equipment designed for cryogenic service, such as rigid pipelines, flexible pipelines, and unloading arms. Currently, the coupling process of a set of LNG marine loading arms is carried out thanks to remote operation. This coupling process involves the LNG marine loading arm (along with its control system), the remote operator, and the LNG vessel system. The unloading arms are articulated pipelines that move with the help of hydraulic cylinders and counterweights. They also have an emergency release system called PERC (Powered Emergency Release Coupling). This system allows the safe physical disconnection between the unloading arms and the vessel’s manifold.

The case study presented corresponds to unloading arms of a regasification plant located in Chile; these arms function to extract LNG from the vessel and transfer it to the storage tanks (Figure 4). A discharging process lasts 24 h, of which approximately 12 h correspond to the actual fluid transfer time. On average, a vessel discharges about 120,000 m³ of LNG at an approximate discharge flow of 12,000 m³/h. To achieve this discharge flow, the terminal has three LNG unloading arms. The terminal analyzed in this paper considered, during its engineering stage, a configuration of N + 1 liquid arms, in addition to a backup vapor arm through a hybrid arm that can function as liquid or vapor.

The unloading arms are considered to be of critical importance to the overall operation. In accordance with the prevailing norms within the domain of asset-intensive organizations, the reliability areas are engaged in efforts to enhance the performance of these arms, as evidenced by improvements in reliability, availability, and maintainability indicators.

5.1. Maintainability Indicators in Unloading Arms

To begin the maintainability evaluation process, the first source of information is the RCM analysis, from which the MCA analysis is performed, and the procedure defined by the MCA decision diagram and the MCA Decision Sheet is applied. Following this analysis, three maintenance levels were identified. These are described in the following section.

5.1.1. Maintenance Levels for Unloading Arms

Table 4. Levels of maintenance.

Level	Descripction	Personnel Needs	Resources	Unavailability Estimaded Time
1	Monthly Mechanical Maintenance: Operational Testing Inspection	Operations Mechanical Maintenance Mechanical Contractor Support	Gas detector and Snoop for leak detection. Inspection checklist.	None
1	Predictive Quarterly Maintenance: Tribology	Predictive Maintenance	Oil sampling kit.	None
3	Quarterly Electrical Maintenance: Insulation Measurement	Electrical Maintenance Electrical Contractor Support	Sensor activation paddles.	Hours
4	Quarterly Mechanical Maintenance: Testing Inspection Lubrication	Operations Mechanical Maintenance Mechanical Contractor Support Scaffolding Contractor Support	Gas detector and Snoop for leak detection. Scaffolding. Greasing equipment. Inspection checklist.	Days
4	Biannual Mechanical Maintenance: Testing Inspection Minor Lubrication	Operations Mechanical Maintenance Mechanical Contractor Support Scaffolding Contractor Support	Gas detector and Snoop for leak detection. Scaffolding. Greasing equipment. Inspection checklist.	Days
4	Annual Mechanical Maintenance: Testing Inspection Major Lubrication PERC Internal Inspection Torque Verification Cable Measurement	Operations Mechanical Maintenance Mechanical Contractor Support Scaffolding Contractor Support	Gas detector and Snoop for leak detection. Scaffolding. Greasing equipment. Inspection checklist.	Days

presents a summary that exposes the different maintenance plans, including the required personnel resources and involved specialties, the required downtime, the frequency of execution of each activity, the means or resources required, and finally the origin of the maintenance strategy, but incorporating the level of maintenance analysis proposed by the UNE 151001 standard.

When analyzing the maintenance activities carried out by the mechanical area with quarterly and semi-annual frequencies, they differ only in the scope of lubrication. In the case of annual maintenance, cable tension measurements and joint torque verification are added. For these three maintenance frequencies, it is necessary to assemble work platforms to access inspection and lubrication points.

In the case of monthly maintenance by the mechanical area and quarterly maintenance by the electrical and predictive areas, they can be performed at ground level, considering only that there are no interferences from other activities on the same arm at the same time.

For monthly mechanical maintenance, it is performed at ground level with general checks that can be coordinated with the operator during a window between ships or even executed during the unloading process. Therefore, this maintenance frequency does not generate major impacts and can be executed without affecting operations. Something similar happens with predictive maintenance, which consists of oil sampling, but in this particular case, this sampling does not generate unavailability since the equipment has been provided with a sampling system that allows the inspection to be carried out without operational impact.

In summary, the quarterly, semi-annual, and annual maintenance actions are invasive as they generate equipment downtime (ranging from 1 to 3 days). However, these maintenance actions, if having proper access, could be carried out in the windows between ships without affecting operations, and due to the need to erect scaffolding at these points, maintenance times increase considerably. This represents a significant challenge for the planning area to be able to execute these activities since other factors are added:

• The annual vessel schedule ranges from 30 to 52 per year;
• Restrictions on work at height with winds over 16 knots;
• Average historical dock availability of 73.88% due to weather conditions (affects ship scheduling and consequently maintenance scheduling);
• Impossibility to keep scaffolds on an arm during a discharge.

5.1.2. Calculation of the General Maintainability Indicator

Once the different attributes of the equipment under evaluation have been analyzed, it is necessary to calculate the General Maintenance Indicator (IMG), for which the importance of each of the general attributes must be defined. These general attributes, as mentioned previously, do not depend on the maintenance level.

To determine the importance, the following equivalence was defined, where PGi will be the “Level of importance for maintenance” and will take a value from 0 to 4 (

Table 5. Importance Level/PGi Equivalence.

Importance Level	PGi
Very High	4
High	3
Medium	2
Low	1
Very Low	0

and

Table 6. Weights of each attribute.

Attribute ID	Attribute	Importance	PGi (0–4)	PGi (0–1)
G1	Simplicity	High	3	0.12
G2	Identification	High	3	0.12
G3	Modularization	High	3	0.12
G4	Tribology	Very High	4	0.16
G5	Ergonomics	Very High	4	0.16
G6	Standardization	High	3	0.12
G7	Surveillance	High	3	0.12
G8	Relationship with OEM	Medium	2	0.08

). Additionally, the weight must be considered, which is obtained from Equation (2).

In the following paragraphs, the PGi for each attribute of the IMG is discussed; additionally, a brief justification for the importance or weight assigned to each point is provided:

Simplicity (G1): High importance because, with five discharge arms and a series of maintenance activities to be executed, simplicity is relevant to seek efficiencies in execution;

Identification (G2): High importance because it aids in efficiency in intervention times by minimizing the time required for component or item identification;

Modularization (G3): High importance considering that, in the case of a major intervention in a specific component, such as a specific rotary joint, the possibility of disassembling only that rotary joint significantly reduces repair times;

Tribology (G4) and Ergonomics (G5): Both attributes were rated as very-high importance as they have a considerable impact on the maintenance times currently recorded;

Standardization (G6): This attribute was rated as high because having interchangeable components provides greater flexibility in the event of a failure requiring component replacement;

Surveillance (G7): High importance because adequate monitoring of equipment operation provides information and even actions to protect the equipment against potential catastrophic failures;

Relationship with the manufacturer (G8): Regarding the manufacturer, this point was rated as medium importance because, once the equipment is in operation, it is not susceptible to vendor assistance except after major events such as an earthquake.

In

Table 7. Calculation of the General Maintenance Indicator (IMG).

Attribute ID	Attribute	Scale		Attribute Assessment	Weight
				Gi (0–4)	Pgi (0–1)
G1	Simplicity	0	High number of components with redundancy of elements, very easily visible.	3	0.12
		4	Optimized number of components, reduced and without any redundancy.
G2	Identification	0	No identification.	2	0.12
		4	Complete identification. Everything is visible when facing the device.
G3	Modularization	0	Changing all units is very complicated, always requiring moving another unit.	2	0.12
		4	Excellent modularization.
G4	Tribology	0	From 0% to 10% of properly selected elements.	1	0.16
		4	From 80% to 100% of properly selected elements.
G5	Ergonomy	0	Maintenance tasks are very difficult to carry out. Weight, size, and shape of the elements to be manipulated are very uncomfortable, causing great fatigue to the operator. Inadequate space provided.	1	0.16
		4	Excellent ergonomics of the device, almost all maintenance work is performed very comfortably and quickly.
G6	Standardization	0	Very poor standardization. High difficulty in finding spare parts in the market. High need for spare parts storage.	3	0.12
		4	Good standardization. Great ease in finding spare parts in the market, at competitive prices. No need to store spare parts.
G7	Surveyance	0	There are no indicators of any equipment condition, and, therefore, it is impossible to make any diagnosis about its operating state.	2	0.12
		4	There are necessary indicators to know the state of the device, and they are easily monitorable on the device.
G8	Relationship with OEM	0	Very poor coordination. Distance between maintainer and manufacturer, another language is used, different communication means are necessary, etc.	2	0.08
		4	Good coordination. Proximity between maintainer and manufacturer, the same language is spoken, there are no communication difficulties, standard communication means are used.
				IMG = 1.92	1.92

, a consolidated sheet is presented showing the results of the evaluation of each attribute and the value of the General Maintenance Indicator (IMG).

Computation of the level maintenance indicator

Once the General Maintenance Indicator (IMG) has been obtained, the calculation of the Level Maintenance Indicators (IMN) must be carried out for each level and for their specific attributes. As mentioned in Section 4.2, to determine the importance or weights, the aforementioned AHP procedure was followed. In addition, each attribute is evaluated using the scale shown in

Table 8. Importance level/ P_Vi equivalence.

Importance Level	PVi
Very High	4
High	3
Medium	2
Low	1
Very Low	0

. The values, with PVi and pvi corresponding to levels 4.3 and 1, are shown in

Table 9. Maintenance Level 4 indicator.

Attribute ID	Attribute	Scale		Attribute Assessment	Weight
				Vi (0–4)	Pvi (0–1)
V1	Accessibility	0	Very difficult access, requires moving things, displacements, etc.	0	0.160
		4	Very good access.
V2	Assembly/disassembly	0	Many difficulties: Many tools and equipment are required. Weight, size, and volume of materials are too significant.	0	0.120
		4	Very easy to assemble and disassemble.
V3	Training	0	Requirements too high for the level of maintenance to be performed.	4	0.120
		4	Training requirements are appropriate for the level of maintenance.
V4	Organization	0	Many people are needed to perform maintenance tasks, more than four or five people.	3	0.120
		4	Only one person is needed, who has the freedom to perform maintenance activities when deemed appropriate.
V5	Site/Location	0	Environment where maintenance tasks are performed is very dangerous.	1	0.120
		4	Environment is very safe. There are protections, no need to approach dangerous components, correct isolations, and signaled interlocks, etc.
V6	Spare Parts	0	Storage of many spare parts is necessary, which are also difficult to handle.	4	0.080
		4	Limited spare parts storage that is also easily manageable.
V7	Tools	0	Many tools and/or equipment are required, which are not readily available.	4	0.080
		4	Few tools and/or equipment are required, and these are standard.
V8	Coordination	0	Great difficulty in performing maintenance tasks. Coordination between many departments, people, permit requests, etc., is necessary.	2	0.120
		4	Maintenance tasks are easily scheduled and performed with few coordination needs with other departments.
V9	Documentation	0	There is no adequate documentation to perform maintenance tasks and solve problems.	3	0.080
		4	There is very good and complete documentation.
				IMN4	2.08

,

Table 10. Maintenance Level 3 indicator.

Attribute ID	Attribute	Scale		Attribute Assessment	Weight
				Vi (0–4)	Pvi (0–1)
V1	Accessibility	0	Very difficult access, requires moving things, displacements, etc.	3	0.160
		4	Very good access.
V2	Assembly/disassembly	0	Many difficulties: Many tools and equipment are required. Weight, size, and volume of materials are too significant.	2	0.120
		4	Very easy to assemble and disassemble.
V3	Training	0	Requirements too high for the level of maintenance to be performed.	4	0.120
		4	Training requirements are appropriate for the level of maintenance.
V4	Organization	0	Many people are needed to perform maintenance tasks, more than four or five people.	3	0.120
		4	Only one person is needed, who has the freedom to perform maintenance activities when deemed appropriate.
V5	Site/Location	0	Environment where maintenance tasks are performed is very dangerous.	3	0.120
		4	Environment is very safe. There are protections, no need to approach dangerous components, correct isolations, and signaled interlocks, etc.
V6	Spare Parts	0	Storage of many spare parts is necessary, which are also difficult to handle.	4	0.080
		4	Limited spare parts storage that is also easily manageable.
V7	Tools	0	Many tools and/or equipment are required, which are not readily available.	4	0.080
		4	Few tools and/or equipment are required, and these are standard.
V8	Coordination	0	Great difficulty in performing maintenance tasks. Coordination between many departments, people, permit requests, etc., is necessary.	4	0.120
		4	Maintenance tasks are easily scheduled and performed with few coordination needs with other departments.
V9	Documentation	0	There is no adequate documentation to perform maintenance tasks and solve problems.	3	0.080
		4	There is very good and complete documentation.
				IMN3	3.28

and

Table 11. Maintenance Level 1 indicator.

Attribute ID	Attribute	Scale		Attribute Assessment	Weight
				Vi (0–4)	Pvi (0–1)
V1	Accessibility	0	Very difficult access, requires moving things, displacements, etc.	4	0.160
		4	Very good access.
V2	Assembly/disassembly	0	Many difficulties: Many tools and equipment are required. Weight, size, and volume of materials are too significant.	4	0.120
		4	Very easy to assemble and disassemble.
V3	Training	0	Requirements too high for the level of maintenance to be performed.	4	0.120
		4	Training requirements are appropriate for the level of maintenance.
V4	Organization	0	Many people are needed to perform maintenance tasks, more than four or five people.	3	0.120
		4	Only one person is needed, who has the freedom to perform maintenance activities when deemed appropriate.
V5	Site/Location	0	Environment where maintenance tasks are performed is very dangerous.	3	0.120
		4	Environment is very safe. There are protections, no need to approach dangerous components, correct isolations, and signaled interlocks, etc.
V6	Spare Parts	0	Storage of many spare parts is necessary, which are also difficult to handle.	4	0.080
		4	Limited spare parts storage that is also easily manageable.
V7	Tools	0	Many tools and/or equipment are required, which are not readily available.	4	0.080
		4	Few tools and/or equipment are required, and these are standard.
V8	Coordination	0	Great difficulty in performing maintenance tasks. Coordination between many departments, people, permit requests, etc., is necessary.	4	0.120
		4	Maintenance tasks are easily scheduled and performed with few coordination needs with other departments.
V9	Documentation	0	There is no adequate documentation to perform maintenance tasks and solve problems.	3	0.080
		4	There is very good and complete documentation.
				IMN1	3.68

, respectively.

5.2. Computation of the Aggregated IMN ($\overline{IMN}$)

Once the maintainability indicators per level (IMNi) have been calculated, the Aggregate Maintainability Index of the levels ($\overline{\mathrm{IMN}}$) can be calculated. As previously stated, the values of the eigenvector of the weights obtained with the comparison matrix at the second level of the AHP hierarchy are used as weights. This is accomplished by considering the three maintenance levels selected through the MCA procedure. These values are then used to calculate the weighted average of the obtained IMNs. In this instance, the values of the eigenvector of the weights correspond to the following:

V = (0.156, 0.185, 0659).

It is evident that the levels of maintenance that result in more stoppages and require more resources were the ones that were most highlighted as critical by the specialists. Finally, the value of the Aggregate Maintainability Index can be calculated as follows:

$$\overline{\mathrm{IMN}}=0.156·3.68+0.185·3.28+0.659·2.08=2.55$$

The subsequent section will present the findings of the analysis of the maintainability of the unloading arms.

5.3. Analysis of the Results of IMG

For the General Maintenance Indicator (IMG), the major gaps lie in Tribology (G4) and Ergonomics (G5). Primarily, this is due to the former requiring greasing activities at each bearing, given the low ergonomics for performing the task and the complexity of accessing the greasing points.

Regarding Surveillance (G7), the gaps are associated with the equipment having operational condition monitoring to protect the equipment, terminal, and vessel against deviations from their operating windows. An area for improvement is implementing a condition monitoring system for the equipment that can determine the health condition of the equipment in addition to its operational condition.

For the Relationship with the Manufacturer (G8), while there are gaps, they are not significant except in cases requiring major equipment interventions, such as a major earthquake that would necessitate manufacturer assistance to support potential repairs.

The rest of the attributes have a rating of three, which can be interpreted as “good”. Therefore, they will not be considered for an improvement action plan in this analysis.

The following radar chart (Figure 5) presents the results of the General Maintenance Indicator (IMG) for each of the evaluated General Attributes.

5.3.1. Results of Maintenance Level 4 Maintenance Indicator Evaluation

In this particular case, the evaluated maintenance corresponds to quarterly, semi-annual, and annual maintenance tasks in the mechanical area. These tasks involve greasing the arm bearings and require scaffold platforms to access various points.

For the Maintenance Level 4 Maintenance Indicator, the major gaps are in Accessibility (V1), which was rated as zero primarily due to significant difficulties in accessing greasing points. This includes points defined by the manufacturer as well as additional points defined by GNLQ to ensure proper functioning and equipment lifespan.

Regarding Environment (V5) and Coordination (V8), the ratings were one. This is explained by the environmental conditions requiring work at heights, with the risk of falling into the sea, in adverse weather conditions of wind, rain, and low temperatures. The rating for Coordination reflects the necessary coordination for executing these tasks, involving operational blockades, operator support, scaffold platform assembly, and coordination of the various teams required for maintenance execution.

For Training (V3), Organization (V4), Spare Parts (V6), Tools (V7), and Documentation (V9), no significant gaps were identified for improvement in this initial analysis. The radar chart below presents the various ratings of the Maintenance Level 4 Maintenance Indicator (Figure 6).

5.3.2. Results of Maintenance Level 3 Maintenance Indicator Evaluation

In this level of evaluation, the quarterly electrical maintenance was rated, which is executed without the need for scaffold platforms assembly. However, it may lead to corrective actions if deviations from the measurements occur.

In this particular case, the only identified gap is in Assembly/Disassembly (V2), which is related to the replacement of bolts for the dielectric joint of the PERCs. This assembly/disassembly involves replacing bolts when the measurements indicate a resistance lower than 1000 ohms as indicated in the equipment manual. The radar chart below (Figure 7) presents the various ratings of the Maintenance Level 3 Maintenance Indicator.

5.3.3. Results of Maintenance Level 4 Maintenance Indicator Evaluation

In this level of evaluation, the monthly mechanical maintenance was rated, which is carried out without the need for scaffold platforms assembly and can even be performed during normal equipment operation.

In this maintenance level, no gaps to be improved were identified as all attributes received a rating of three or higher. The radar chart below (Figure 8) presents the various ratings of the Maintenance Level 1 Maintenance Indicator.

5.4. Sythesis

From the results obtained from the evaluation of the different attributes, the attributes of Tribology (G4) and Ergonomics (G5) can be highlighted as having the greatest impact on equipment maintainability. This is mainly since the equipment was not designed for recurrent maintenance activities at those difficult-to-access points for inspection and lubrication.

The need to carry out these lubrication and inspection activities stems from the environmental conditions, contamination, and corrosiveness present in the operational context of the equipment, which is far from the conditions considered by the manufacturer in the design. These adverse results provide scientific backing to justify the necessary CAPEX investments to improve this situation and increase equipment availability.

The suggested investments to improve equipment maintainability are related to material improvement, implementation of automatic lubrication systems, and implementation of inspections using drones.

5.5. Reflection

The quest for greater process availability often revolves around the analysis of an element’s ability to perform a required function under specific conditions for a determined period and its capability, under specific usage conditions, to be preserved or restored to a state where it can perform the required function when maintenance is conducted following established conditions, procedures, and utilizing appropriate resources, namely reliability and maintainability, respectively. In the former case, there are a plethora of methodologies for evaluation, improvement, and reduction in its impact on the business, among other things. In contrast, the available methodologies for maintainability are limited and relatively unknown in the industry. In particular, this work was based on the recommendations of the UNE 151001 standard to determine maintainability indicators. Moreover, the use of these methodologies helps understand the importance and impact of including reliability areas in engineering processes from the stage of investment study through design, manufacturing, commissioning, start-up, and finally throughout the entire lifecycle until decommissioning. The maintainability deficiencies identified in this text could have been circumvented during the design phase had the relevant aspects of equipment maintainability been considered. Consequently, the “Design for Maintainability” methodology can assist in addressing future maintainability issues from an engineering perspective, which will consequently have a positive effect on availability.

6. Conclusions

The main objective of this document has been to adapt and apply a method for evaluating Maintainability, based on the UNE 151001 standard, grounded on a set of indicators that allow quantifying the maintainability of liquefied natural gas (LNG) discharge arms. This evaluation aims to facilitate the comparison of LNG discharge arms, promote improvement in design regarding maintainability requirements, or simply optimize maintenance procedures for these arms.

To adapt the method to LNG discharge arms, specific maintainability attributes for this type of industrial devices were initially defined and classified, following the guidelines of the UNE 151001 standard. These attributes were categorized considering both general and specific scopes in relation to maintenance levels. The maintenance levels for LNG discharge arms were established based on the complexity of maintenance tasks. After determining the maintenance level of the arm, each attribute was evaluated, conducting a weighted-average calculation for each maintenance level.

The applied method allows for the consideration of a range of factors that are pertinent to each attribute, with the aim of aligning with the operational and structural characteristics of LNG discharge arms. Consequently, six measures were derived: one for the general indicator and five for each maintainability indicator at different maintenance levels. The results and their graphical representation provide a clear and comprehensive insight into the maintenance performance at different levels. The applied method contributes to the enhancement of the maintainability of LNG discharge arms throughout their life cycle stages. The application of this method in a case study demonstrated the significance of providing adequate training for those responsible for conducting the evaluation, emphasizing simplicity and its adaptability to any device. However, it necessitates a specific interpretation and adaptation of attributes and maintenance levels that align with the peculiarities of each LNG discharge arm. Future research in this field, guided by the UNE 151001 standard, may extend to the practical application of this method to other industrial devices.