عنوان مقاله [English]
Suitable sites for waste disposal must leave the least environmental effects while being executable in various aspects. Combination of AHP and GIS is a popular approach used for selecting suitable waste disposal sites, since AHP classifies and prioritizes selected sites based on different types of information layers and GIS provides an effective way for data management and display. Various studies have been recently conducted to select suitable sites for waste disposal using GIS and AHP. Rahimi et al. selected a sustainable site for urban solid waste disposal in Mahallat, Iran, using AHP and GIS.
Fourteen environmental, economic and social parameters affecting sustainability of landfills were examined in this study and a site was selected in vicinity of this city as the most suitable landfill for solid waste disposal. Improper disposal of waste produced in oil-based drilling of oil and gas wells not only increases costs, but also cause the aforementioned problems. Thus to prevent these problems, it is necessary to select appropriate landfills for this kind of waste.
Although, Iranian Offshore Oil Company (IOOC) is responsible for most of oil and gas extraction from Iranian fields in the Persian Gulf, no specific solution has been provided for selection of suitable locations for drilling waste produced in these areas. The present study seeks to select suitable sites for disposal of drilling waste produced in east of the Persian Gulf Iranian oil and gas fields in Lavan Island using AHP method and GIS software.
Material and methods
Oil and gas fields in Qeshm, Kish, Siri and Lavan operational areas are located in the eastern part of the Persian Gulf. Lavan is one of the islands of Hormozgan province in the Persian Gulf. It is about 2.5 kilometers long and 4.8 kilometers wide.
Oil Based Drilling Fluid used in Iranian Oil and Gas Fields in the Persian Gulf
Due to the type and depth of formation (layers of shale and deep reservoirs), oil based drilling fluids are generally used in Iranian oil and gas fields in the eastern Persian Gulf (Qeshm, Kish, Siri and Lavan).
The main component of oil based drilling fluid is petroleum hydrocarbons, especially those with high flashpoint. Usually diesel fuel is used as the main component, which may be added up to 90% to the fluid used in drilling operations.
Drilling waste produced in Iranian oil and gas fields in the Persian Gulf
Based on the latest statistical information provided by IOOC, annually drilling waste is generated by this company in the Persian Gulf, which has declined in recent years due to a reduction in excavation activities. The volume fractions of humidity and oil in the drilling waste are 65% and 30%, which according to the standards of Iranian DOE and HSE unit of IOOC must be reduced to 15% and 1%, respectively after the recycling process.
Analytic hierarchy process
The analytic hierarchy process (AHP) is a logical framework that divides complex decisions into hierarchical structures and thus, simplifies their understanding and analysis. This process can be used when decision-making faces some alternatives.
Site selection in land-related sciences is an operation through which an expert presents needs, objectives, and information related to the current situation to find the best choice among available alternatives for the concerned land use. The main objective of site selection is to ensure that considering all limitations and available facilities, human activities in the selected site is consistent with the surrounding environment. Nowadays, GIS is used to reach a more scientific and realistic site selection. GIS is a coherent system of hardware, software, data, which allows the storage, analysis, transfer, and recovery of input data and makes it possible to publish the output data as maps, tables, and models of geographical zones.
The present study is applied in terms of its objectives and descriptive-analytical in terms of its methodology. The criteria and sub-criteria (layers) involved in site selection for drilling waste disposal in Lavan Island were chosen based on the specifications of the region, recommendations of experts, and related literature. Base data were collected from various sources such as IOOC, Iranian department of environment, and geological survey and mineral exploration of Iran. Accordingly, 15 information layers (sub-criterion) affecting waste disposal site selection in Lavan Island were introduced and classified into three indices.
These information layers include industrial building, slope, elevation, gas lines, oil lines, oil storages, roads, population centres, industrial regions, land use, airport, fault line, vegetation, river, and geology which have been classified as technical-economic, social-cultural, and environmental indices (criteria).
Figure 1 depicts the hierarchical tree of site selection for disposal of drilling waste produced in eastern Persian Gulf Iranian oil and gas fields in Lavan Island.
Figure 1. The hierarchical tree of site selection for oil based drilling waste in Lavan Island
Results and discussion
Properties of each layer (layer values) were weighted in GIS environment based on al-saati method and experts’ opinions. Classification, weighting and normalization of effective layers used for selecting appropriate sites for oil drilling waste disposal in Lavan Island were performed and results were used to prepare a weighed map for each layer. These maps were combined in the final step to obtain the proposed map for waste disposal site. Figures 2 shows the weights assigned to information layers prepared for waste disposal in lavan Island.
Figure 2. Weights assigned to information layers prepared for oil based waste disposal in lavan Island
After the internal weighing of each layer, the AHP model was used to prepare the final map for the optimal site. Weighing each of these 15 layers is one of the most important stages of this model in which significance of each layer is expressed compared to other layers. The ultimate normalized weight of each layer was calculated by an AHP matrix with an inconsistency rate lower than 1.0. Chart 1 shows the ultimate normalized weight of each layer which will be used in overlapping operations to find appropriate sites for oil based drilling waste disposal in Lavan Island.
Chart 1. Importance of weights assigned to layers in the selection of oil based waste disposal site in Lavan
Results indicate that distance from population centers, distance from roads, distance from rivers and distance from airport are the most important parameters used to select appropriate sites for oil based waste disposal in Lavan Island. Results confirm the sensitivity of environmental and socio-cultural criteria for oil based drilling waste disposal in Lavan Island.
Then, information layers were integrated using weighted overlay method in AHP to obtain the final map of the appropriate region for waste disposal. In this stage, the layers were overlapped based on their level of effectiveness in GIS environment and the final site selection map was prepared for waste disposal in Lavan Island (see Figure 3). The appropriate sites for waste disposal were classified into 5 classes (from “very good” to “very poor”) and depicted in this map.
Figure 3. Classification of selected sites for oil based waste disposal in Lavan Island
Spatial analysis of final maps shows that some regions in the center of Lavan Island (sites number 1, 2, 3, 4 and 5)
are appropriate for drilling waste disposal due to their distance from population centers, roads, rivers, and the airport. These barren lands are the farthest sites from urban centers, roads, rivers, and the airport. Therefore, construction of waste disposal sites in these regions of Lavan Island is suggested in the final map to decision-makers. Figure 4 shows the prioritized waste disposal sites in Lavan Island.
Figure 4. Prioritization of oil based waste disposal sites in Lavan Island
The present study was performed due to the lack of similar studies on waste disposal site selection in this region. GIS and AHP were used to select suitable sites for the disposal of drilling waste in Lavan Island. This drilling waste is produced in the Iranian oil and gas fields in the eastern parts of the Persian Gulf. Effective factors were weighted in different layers of GIS environment and weighted maps were prepared. Priorities were selected using the AHP, and site selection for drilling waste disposal was performed in GIS. Distance from rivers was recognized as the top priority parameter in environmental criteria due to the importance of environmental standards and avoiding surface water pollution. Moreover, distance from population centers, roads, and the airport were selected as top priorities in social-cultural sub-criteria due to the importance of the Island residents’ health and beauty of the landscape. Information layers were thus produced and combined using weighted overlay method in AHP to reach the final maps of suitable locations for oil based waste disposal in GIS. In accordance with effective criteria in the waste disposal site selection, suggested sites were classified into five classes ranging from “very good” to “very poor”. Accordingly, some sites located in the central part of Lavan Island were selected as appropriate sites for the disposal of drilling waste due to their distance from urban and population centers, roads, rivers, the airport, and so forth