**1. Introduction**

The most environmentally friendly form of hydrocarbon combustion is natural gas. It is available in large quantities, has a wide range of applications, helps fulfill the rising need for energy worldwide, and can work in tandem with other forms of renewable energy. In addition, natural gas is an essential resource for such industries as heating and power generation, manufacturing, and transportation, not only in Europe but all around the world.

Even though the combustion of natural gas releases greenhouse gases, it produces a far lower amount of carbon dioxide (CO2) and other air pollutants than the vast majority of the fuels it is gradually replacing, particularly coal. The use of natural gas has increased dramatically over the last decade, making up over a third of the increase in total energy demand. This is higher than any other fossil fuel.

Natural gas is currently responsible for around a quarter of the world's electrical generation. It is anticipated that it will play a significant part in easing the transition to energy systems that produce zero net emissions over the medium range; however, its utility over the longer term remains unknown in a future where renewable energy sources predominate. Recently, natural gas pipelines have become vital for the functioning of every country, since natural gas that is transported via pipelines contributes to the economic expansion of cities and industries. It is anticipated that natural gas pipelines will continue to be significant for the global economy because they have the ability to transport hydrogen that is created from natural gas or electrolysis and has the potential to be a game changer in the transition to a cleaner source of energy [1]. Additionally, after goods derived

**Citation:** Koulinas, G.K.; Demesouka, O.E.; Bougelis, G.G.; Koulouriotis, D.E. Risk Prioritization in a Natural Gas Compressor Station Construction Project Using the Analytical Hierarchy Process. *Sustainability* **2022**, *14*, 13172. https://doi.org/10.3390/ su142013172

Academic Editors: Albert P. C. Chan, Srinath Perera, Xiaohua Jin, Dilanthi Amaratunga, Makarand Hastak, Patrizia Lombardi, Sepani Senaratne and Anil Sawhney

Received: 5 September 2022 Accepted: 9 October 2022 Published: 14 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

from petroleum, natural gas is the principal energy resource in the Euro area while it is considered as the most significant energy source for the manufacturing industry.

During the last few decades, many studies contributed to the relative scientific field of risk management in natural gas infrastructures. Simonoff et al. [2] developed risk measures and scenarios to better understand how consequences of pipeline failures are linked to causes and other incident characteristics, and [3] proposed a model for quantitative risk assessment on metering stations and metering-regulation stations for natural gas with natural ventilation. In addition, ref. [4] described an application of a methodology for quantitative risk assessment that considered failure frequencies found in a public database, and consequences were computed as a function of pipe diameter and operating pressure for each network's section. Also, ref. [5] performed job-hazard assessment to predict hazards while executing nonroutine tasks in gas transmission stations, while [6] developed a model for accident classification in the natural gas sector according to possible fatalities, using rough set theory and decision rules. Recently, ref. [7] assessed the safety state of oil and gas activities and identified risk factors that cause hazards to people and to the environment using formal risk assessment and Bayesian networks.

Additionally, some studies used multicriteria decision analysis methods for oil and gas industry applications, such as [8], who presented a new methodology for identifying and assessing risks simultaneously by applying a multiattribute group decision-making technique. In the study of [9], the researchers proposed an approach for pipeline route selection based on SWOT analysis and the Delphi method for determining decision-makers' beliefs, and then the PROMETHEE model was used to integrate these beliefs with subjective judgments and identify the suitable pipeline route. Paradopoulou and Antoniou [10] performed REGIME multicriteria decision analysis to prioritize alternative LNG terminal locations on the island of Cyprus in the Mediterranean Sea, while Strantzali et al. [11] proposed a decision-support tool that embodies multicriteria analysis, using the PROMETHEE II method, for the evaluation of potential LNG export terminals in Greece. A comprehensive literature review and a framework for classification of decision-support methods used for technical, economic, social and environmental assessments within different energy sectors including upstream oil and gas, refining and distribution can be found in the study of [12].

In the recent studies of Marhavilas et al. [13–16], a combination of both the typical and fuzzy AHP and HAZOP method used for risk assessments in the sour crude oil industry.

The applications of AHP and fuzzy AHP in the health and safety research field include a wide range. In their study, [17] used AHP for measuring health and safety awareness in selecting a maintenance strategy within the Norwegian oil and gas industry, while [18] used a fuzzy extension of AHP with trapezoidal fuzzy numbers for safety evaluations in hot and humid workplaces. Additionally, [19] presented a framework for safety risk assessments in construction projects that was based on the cost of a safety model and the analytic hierarchy process, and [20] developed a methodology for safety device selection that used AHP and mechanical hazard classification. Podgórski [21] used typical AHP for evaluating how workplace safety and health management systems are working. In addition, [22] applied nonlinear fuzzy analytic hierarchy process and logarithmic fuzzy preference programming for performing safety evaluations within coal mines in China, and Xie et al. [23] developed a technique for evaluating the environmental quality of two commercial buildings. Janackovic et al. [24] ranked and selected occupational safety indicators using fuzzy AHP, and Kasap and Subasi [25] employed fuzzy AHP to quantify occupational risk in open pit mining. Additionally, Carpitella et al. [26] optimized system maintenance by combining reliability analysis with multicriteria techniques like fuzzy TOPSIS and AHP. Recently, [27] applied a combination of the Pythagorean fuzzy AHP and VIKOR method for health and safety risk assessment in dangerous workplaces, while Koulinas et al., [28] and Marhavilas et al., [29] used fuzzy AHP and real data to perform risk assessments in construction projects.

The literature review above is summarized in Table 1.


**Table 1.** Summary of the relative literature.

A great survey on risk analysis and assessment methodologies in the workplace can be found in the study of [30]. In addition, [31] provided a systematic literature review on the use of risk-acceptance criteria in occupational health and safety risk assessment.

The present approach intends to serve as a practical tool for knowledge and expertise transfer. The remaining five sections of the paper are: describing the analytical hierarchy process, presenting the compressor station, explaining the suggested framework, describing the application, and discussing the findings.

### **2. The Concept of the Analytical Hierarchy Process**

The analytical hierarchy process (AHP) proposed by Saaty [32] is a well-known approach for evaluating many criteria in which the factors at hand are arranged in a hierarchical manner. It is founded not just on mathematics but also on human psychology, fusing together rational thought with emotional inclination. The ability to incorporate qualitative and quantitative criteria during the evaluation is one of the benefits of using this method. Another benefit is the ability to use the experience, knowledge, and intuition of the person making the decision when determining the weights of the elements. On the other hand, the subjective character of the modeling process is the fundamental flaw of this approach and, more generally, of similar multicriteria methods. This implies that the methodology cannot ensure that the judgments will be absolutely accurate.

The AHP approach allows for the multicriteria problem to be organized into a hierarchical structure. Following this, the local and global priorities for the problem's criteria and subcriteria may be defined using pairwise comparisons and weightings. During the process of conducting the pairwise comparisons, the AHP takes the judgments of the decision-maker regarding how important one criterion is in comparison to another as its input. As an output, the AHP generates a ranking according to the importance of each criterion and/or subcriterion of the analysis. A standard scale (Table 2) is used in order to convert the qualitative estimates of importance that the decision-maker has into numerical values.

**Table 2.** The basic scale of the AHP method [33].


The fact that the approach examines the input judgments of the decision-maker for any possible instances of inconsistency is a feature that is highly significant to the method. The latter leads in an improvement in overall quality. In this particular study, we employ standard AHP in order to rate the risks identified for every task of the project.

#### **3. Description of a Natural Gas Compressor Station**

A compressor station is an essential component of a natural gas pipeline network, which transports natural gas from specific producing sites to the end customers. In this paper, the case of the Kipi Compressor Station of the Trans Adriatic Pipeline (TAP) is studied [34]. Distance, friction, and elevation variances inhibit the flow of natural gas via a pipeline and lower pressure. The compressor stations are ideally located throughout the collection and transportation pipeline network to assist maintain gas flow rate to the clients. Because the gas has a tendency to slow down as it passes through the pipeline network, engineers build compressor stations along the pipeline to maintain the gas flowing toward its destination.

During times of low demand, compressor stations are also able to deliver natural gas to storage sites in the surrounding region. In addition, the passage of the gas through the pipeline results in the formation of water droplets and various types of hydrocarbons inside the gas itself. Scrubbers, strainers, and filters are used in compressor stations to remove dirt and other contaminants from the flow of gas, in addition to separating the aforementioned objects.

#### *3.1. The Natural Gas Compression Process*

As described in Figure 1, and in [34], initially, the gas enters the station through the yard piping, which is the term given to the network of pipes that link the main gas pipeline to the compressor station.

The gas is routed through a number of filters and scrubbers in the yard by means of pipes, which eliminates any liquid or solid pollutants that may be present in the gas stream. After that, it goes back into the pipe at the compression station yard and enters a compressor unit. The compressor works to repressurize the gas so that it will flow steadily through the primary natural gas pipeline network. However, the process of increasing the pressure of the gas results in the generation of heat, which needs to be controlled. As a solution to this issue, the compressor station is equipped with a cooling system that is meant to remove the additional heat. This is often accomplished by employing a number of fans to assist in chilling the pipes as they reflect the heat away. Because of this cooling

process, which also involves shifts in pressure and temperature, part of the liquid that was present in the gas condenses and separates itself from the primary flow of gas.

**Figure 1.** The flowchart of the natural gas compression process.

Other operations, such as the addition of mercaptan, the smell of which is sulfurous and indicates the existence of natural gas, may be a part of the process at the gas compressor station once the pressure of the gas has been reestablished.

A comprehensive system monitoring, gas pressure monitoring, and safety control apparatus are some of the other components that are often present in a gas compressor station. In the event that there is a disruption in the power supply, backup generators are an important component that plays a role in helping to maintain the natural gas pipeline running continuously and evenly.

Given that the compressor station is designed to filter, meter and compress natural gas for further transportation through the pipeline network, it mainly consists of the following:


The gas is brought in by a scraper reception facility, which serves as the operational interface between the pipeline and the station. The station is where the gas is compressed. The natural gas that is being transported via the gas transmission pipeline is brought into the compressor station after it has gone through the scraper reception facilities. Before the gas can enter the metering and compression units, it must first be passed through an intake separator, which removes any solid particles and free water that may be present in the gas stream.

Two distinct steps of separation will make the separation process simpler. The droplets in the gas stream are subjected to gravity and/or centrifugal forces during the initial step of the separation process. After this initial stage, there is a second stage that is comprised of cartridges that use coalescing effects in order to produce liquid droplets that are of a larger size. Last but not least, the gas stream progresses via a demisting and vane step, which gets rid of the bigger droplets. The contaminants and liquids that have been separated will be collected in a sump located below the separator in the form of a horizontal pipe.

For reasons of custody, it is necessary to measure the volume of natural gas that is delivered to the gas transmission system located farther downstream. This will be accomplished by the utilization of ultrasonic flow meters (USM). In addition to this, the measured amount of gas flow will be utilized in the process of controlling the performance of the compressor.

A gas analyzing unit will perform an examination for the purpose of custody to determine the quality of the natural gas that is being transported from the upstream gas transmission system (GAU). The measurement will be carried out mechanically, either constantly or discontinuously, depending on how the relevant network code specifies it should be done.

The process gas chromatograph (PGC) is the primary component of the gas analyzing unit (GAU) system, which is designed to analyze at least the following parameters:


Because this is the primary gas entry point to the pipeline, the gas will be analyzed in more depth than it will be at the intermediate stations, which will merely monitor the concentration of C1 to C6 hydrocarbons and the hydrocarbon dew point. The numbers needed for the flow calculation, such as density and compressibility factor, are computed based on the results of measuring the composition of the gas. In addition, this composition provides the information necessary to construct indices such as the Net and Gross calorific value, as well as the Wobbe index. Additional quality-control methods are used for the purpose of monitoring the gas when it is introduced into the pipeline system.

The gas will enter the gas compression units once it has completed its journey via the gas metering unit. Depending on the capacity of the station, gas turbine-driven turbo compressors are anticipated to be utilized for the purpose of compressing the gas. The compressors are set up in a parallel configuration. Each compressor unit is built with unit shutoff valves, which may be used to separate the compressor unit on either the suction or discharge side. When a gas first enters the suction of a compressor, it is sent via a suction strainer on its way to the suction line. This serves as a protective measure against the formation of bigger deposits in the suction line. A flow meter is utilized on the gas supply before it is allowed to enter the compressor proper. After that, the gas will be compressed by a turbo compressor that has three different rotors, or impellers. A gas turbine will serve as the source of propulsion for the turbo compressor.

After exiting the compressor at the specified pressure, the gas then travels to the discharge header, where it is directed through the discharge check valve and the unit shutoff valve en route.

In the event of low flow, turbo compressors are prone to surging, which has the potential to cause the machine's destruction. A short recycling with a hot bypass valve (HBV) and a longer cooled recycle with an antisurge control valve (ASV) are both designed and put into the system so as to prevent surge operation from occurring. Controlling the machine at low flows is the antisurge valve (ASV), which prevents the machine from running too closely to the surge area. In the event that the antisurge valve does not respond quickly enough to rapid transients in the process, the hot bypass valve (HBV) will open entirely, which will cause the machine to trip. At each machine, the necessary process parameters are monitored. These include flow, suction and discharge pressure, and temperature. It is important to keep in mind that the antisurge cycle, namely, the cooler, is intended to be used with a single compressor unit. On the other hand, in the event

that the units have to be run with a low flow, the station recycling valve will be used to accomplish this.

The same lines that are utilized for recycling will also be used for the purposes of starting up. The presence of these separate lines makes it possible for the compressor to begin functioning, even when other equipment is already in use. This starting line will be sent to the beginning of the startup header. The gas that will be used in the gas turbine will come from the suction header of the compressor. Nevertheless, in conformity with EN 12583, this line also features a separate shutoff valve that may be utilized if necessary. The fuel gas is being supplied by the fuel gas unit, and it will then be sent further to the gas turbine through a direct channel.

Another gas line is run all the way to the compressor seal gas panel from the side of the compressor that discharges the gas. This is necessary because the compressor needs a steady gas flow to the dry gas seals in order to function properly. Additionally, this gas flow is necessary even while the devices are in their pressurized stop position. As a result, it will be obtained from a position that is not directly associated with the shutoff valves that control the compressor unit. The gas will be extracted from the discharge side because the pressure has to be slightly greater than the suction or settle-out pressure. Purging of the tandem dry gas seal will be accomplished with the usage of the gas (primary and secondary seals). Air will be used to clean the tertiary compressor seal once it has been purged. Because there is always some quantity of seal gas that enters the process lines via the machine, the lines need to be depressurized during prolonged standstills (for example, to the suction line) in order to guarantee that there is adequate driving force for the flow of seal gas. It is impossible to prevent some of the seal gas from escaping through the vent lines of the dry gas seals, hence this is an inevitable aspect of the sealing system. In order to prevent the release of greenhouse gases, any air that escapes via the primary vent line (the connection between the primary and secondary seals) will be burnt in the boiler unit.

The gas will then be sent to a gas chiller when it has completed its journey through the compression unit. This cooler is necessary because a maximum temperature of 50 degrees Celsius must be maintained for the gas that is directed toward the pipeline. The cooler, also known as the transportation cooler, has a total of five compartments. Out of these five bays, four bays are required for duty, while the remaining bay serves as a standby bay. Each bay has the necessary number of one-pass heat exchanger bundles as well as two fans. A temperature measurement device located in the discharge header of the cooler is used, in conjunction with variable speed drives for the fans' motors, to maintain a consistent temperature at the cooler's output. In the event that the output temperature cannot be attained for whatever reason, the flow originating from the compressors will be lowered in the appropriate proportion. In the event that this preventive precaution is not enough, the compressors will be turned off. After that, the gas is transferred to the pipeline system using devices known as scraper launchers. Due to the fact that the design of the station is somewhat elevated above the design of the pipeline, it is anticipated that there will be a pressure shutoff valve at the station outlet, which will also serve as the station's shutoff valve. A startup cooler will also be provided, in addition to the transit cooler that was already mentioned. After being connected to the compressor's startup header, the cooler is then routed back to the suction header of the compressor. This refrigerator is constantly operational, and its entire capacity may be accessed at any time. A temperature measurement device located in the discharge header of the cooler is used, in conjunction with variable speed drives for the fans' motors, to maintain a consistent temperature at the cooler's output. The second reason for having this cooler is so that it can offer cooling capacity in the event that a compressor is working inside the antisurge area. In this scenario, gas is redirected from the compressor discharge line via the cooler and back to the compressor suction.

A scraper launcher facility serves as the operational interface between the pipeline and the compressor station. This is where the gas is launched once it has been compressed at the station.

Regarding the utility systems, these are described in the following sections.
