Evaluating pollution degree of oil spills on the sea surface

. This article analyses influencing factors of pollution degree of oil spills on the sea surface and establishes corresponding evaluation system, thus combines fuzzy comprehensive evaluation method and analytic hierarchy process to evaluate pollution degree of oil spill accidents in the Bohai Sea. Based on the evaluation system, oil spill accidents in Penglai 19-3 oilfield and Suizhong 36-1 oilfield central platform are classified as serious pollution and light pollution respectively, which is consistent with relevant institutions and scholars, proving the rationality of the evaluation system and parameter selected.


Introduction
In recent years, there have been several oil spill accidents in the Bohai Sea [1][2][3][4][5]. Oil spills into seawater, then drifts and diffuses under the influence of environmental dynamic fields, which causes pollution to the marine [6][7][8][9][10]. There are some differences among pollution degree of oil spill accidents to some extent [11][12][13]. Evaluation on pollution degree affects emergency response, hazard assessment and loss compensation [14]. Therefore, classification of pollution degree of oil spills on the sea surface is of significance to determine the corresponding emergency and respond to oil spill hazards efficiently and scientifically [15]. This paper applies a multi-level fuzzy comprehensive evaluation method to establishing pollution evaluation system of oil spills on the sea surface in section Evaluation System of Oil Spill Pollution. Section Membership of Evaluation Indexes elaborates membership degree of each evaluation index. Analytic hierarchy process is used to determine weight of each index in Section Weights of Evaluation Indexes. Pollution degree of oil spills on the sea surface is evaluated in Section Multi-level Fuzzy Comprehensive Evaluation and pollution degree of oil spill accidents in Penglai 19-3 oilfield and in Suizhong 36-1 oilfield is analysed in Section Case Analysis of Oil Spills on the Sea Surface. speed U 41 , wind direction U 42 , visibility U 43 }, U 5 ={current velocity U 51 , current direction U 52 , water temperature U 53 , wave height U 54 }, U 6 ={risk identification and control capabilities U 61 , emergency resource support capabilities U 62 , emergency processing capabilities U 63 , emergency plan preparation capabilities U 64 , emergency recovery capabilities U 65 }. Pollution degree is divided into five levels and comment set is expressed as V= {V 1 , V 2 , V 3 , V 4 , V 5 } in which V j (j = 1, 2, ..., 5) represents five pollution levels, namely extremely light, light, medium, heavy and serious pollution respectively.

Membership of Evaluation Indexes
Membership of evaluation indexes [31] is quantitative description of comment set on factor set and normally determined by membership function or subset table of membership [32]. Among them, membership of subindicators such as oil spills, marine sensitivity and offshore distance is determined by establishing membership function curve and membership of other indexes is determined by constructing membership subset table.

Membership function of oil spill amount
Combining decree No.15 of the Ministry of Transport of the people's Republic of China on statistical measures for water traffic accidents (2014) with British method of dividing points for oil spill evaluation, a triangular and trapezoidal method is applied to determining membership function of oil spill amount. For example, membership function of oil spill amount t for extremely light pollution V 1 is

Subset table of sub-indexes of oil properties
Membership of oil properties as toxicity, persistence, flammability and viscosity are invested [33]. Toxicity of crude oil depends on its composition, corresponding proportion, physical and chemical properties of each composition [34]. According to hydrocarbon compounds, crude oil can be divided into paraffins, paraffins and naphthenes, naphthenes, naphthenes and aromatic hydrocarbons as well as aromatic hydrocarbons.
Persistence refers to the ability to remain in the water without being degraded [35]. In general, the greater density of oil spills, the stronger persistence and the longer retention time, the more serious damage to the marine. According to API°, crude oil can be divided into light (>34), medium (20~34), heavy (10~20) and extra thick crude oil (<10).
Flammability of oil spills brings potential danger to emergency operation and it depends on flash point of oil spills [36]. The lower the flash point, the more dangerous of oil spills. Accordingly, oil products can be divided into lubricating oil and grease (>125 ℃), diesel and heavy oil (45~125 ℃), kerosene (28~45 ℃), solvent oil and gasoline (<28 ℃).
Viscosity represents fluidity of oil spills. The greater the viscosity, the higher the membership value assigned to a higher pollution level [37]. On account of viscosity, crude oil can be divided into conventional oil (<100 mPa·s), heavy oil (100~10000 mPa·s), extra heavy oil (10000~50000 mPa·s) and super heavy oil (> 50000 mPa·s).

Subset table of indexes of oil spill location
Oil spill areas can be divided into sensitive and nonsensitive areas [38]. The richer the sensitive resources near the spill location, the more serious the pollution will be [39]. Once sensitive area is polluted, oil will cause heavy losses and huge ecological disasters [40].
The closer the spill location to shore, the higher the pollution degree [41]. Function curve of membership of offshore distance is illustrated.
With reference to Environmental Sensitivity Index (ESI) [42][43], coastlines can be divided into bedrock coast, gravel coast, plain coast, mangrove coast and coral reef coast in China and the membership subset is shown.

Subset table of indexes of meteorological factors
Meteorological factors such as wind speed, wind direction and visibility influence pollution degree of oil spills on the sea surface [44].
Wind speed [45] determines wave size to some extent, promotes dispersion of oil spills, expands pollution area and adversely affects the recovery and treatment of oil spills, resulting in increased pollution level. According to wind speed, wind levels can be divided into level 1 (<1.5 m/s), level 2 (1.5~5.4 m/s), level 3 (5.4~10.7 m/s), level 4 (10.7~20.7 m/s) and level 5 (>20.7 m/s).
Wind direction [46] affects drift trajectory and diffusion direction of oil spills, and may cause serious pollution to coastal sensitive areas. According to the angle between wind direction and coastal sensitive area, wind direction is divided into three levels: reverse coastal sensitive area, a certain angle with coastal sensitive area and forward sensitive coastal area.

Sub-indexes of Meteorological Factors
Weight Level Comment Set Wind speed 0.5278

Subset table of hydrological factors
Hydrological factors as current velocity, current direction, water temperature and wave height affect pollution degree of oil spills on the sea surface [48]. Current velocity [49] increases oil diffusion and is not conducive to oil spill recovery, which aggravates oil spill pollution, and it can be divided into level 1 (<0.5 m/s), level 2 (0.5~1 m/s), level 3 (1~1.5 m/s), level 4 (1.5~2 m/s) and level 5 (>2 m/s).
Current direction and wind direction jointly determine drift direction of oil spills. Drifting to coastal sensitive resources may cause more serious harm to coastal ecological environment. According to the angle between current direction and coastal sensitive area, current direction is divided into 3 levels: reverse, at an angle and forward.
Wave disturbance [51] promotes stirring of oil spills and water, accelerates formation of emulsions and some oil break into small liquid clumps or droplets, increasing the difficulty of oil spill recovery. Wave disturbance is measured by wave height which can be divided into level 1 (<0.5 m), level 2 (0.5~1.5 m), level 3 (1.5~3 m), level 4 (3~9 m) and level 5 (>9 m).

Table4. Membership subset of sub-index of hydrological factors
Sub-indexes of Hydrological

Subset table of emergency measures
Emergency measures play an important role in controlling oil spills and reducing pollution [52].

Membership of Evaluation Indexes
AHP is used to determine the weight of each index [54] and weights of oil properties and oil spill position are present respectively. Since the consistency ratio CR [55][56] is less than 0.1 for the weight of each index, according judgment matrix has good consistency, that is, the weight distribution is reasonable.

Multi-level Fuzzy Comprehensive Evaluation
In the fuzzy comprehensive evaluation,  represents a fuzzy matrix synthesis operator in B A R   [57]. This paper selects a weighted average operator [58][59], which indicates all factors are balanced according to the weight and it is applicable to the effect of various factors. Evaluation system for pollution degree of oil spills on the sea surface is a second-level fuzzy evaluation. Taking oil properties as an example to illustrate the process of the first-level fuzzy comprehensive evaluation, the membership matrix R2 can be obtained from the subset table of toxicity, persistence, flammability and viscosity. ( , , , , ) ( , , , ) Therefore, the total evaluation matrix R of the fuzzy comprehensive evaluation system can be obtained as The second-level fuzzy comprehensive evaluation The location is about 70 km from the nearest coast Longkou coast and the shoreline is sandy type. Coastal sensitive resources include nature reserves, marine special protection areas, natural landscapes, historical and cultural heritage areas, fishery waters, coastal tourist areas, port shipping areas and special utilization areas in adjacent marine. State Oceanography Bureau implemented three-dimensional all-weather surveillance and monitoring on the oil spill accident the first time.
However, due to the deliberate delay of the responsible party for the accident, ConocoPhillips did not fully control the spill situation, the progress of oil spill investigation and plugging was slow, oil spills in platform B and C was basically controlled until June 19 and June 21 respectively. At the time of oil spill accident, wind on the sea surface was about 5.7 m/s and there was a certain angle between wind direction and coastal sensitive resources. Current velocity was approximately 0.5~1 m/s and it was directly opposite coastal sensitive resources, wave height was less than 1m. The spilled oil is medium or heavy, mainly isoparaffin or cycloalkane but less n-paraffin, with low alkane content (32.8%~44.6%) and higher colloid content ( [63], this paper considers the emergency measures on the oil spill accident at a good level. Adopting the relevant data by Liu Jie [64], the secondlevel fuzzy comprehensive evaluation method is applied to evaluating the pollution degree of the oil spill and the total evaluation matrix is obtained as B= (0.2043 0.6071 0.2007 0.0476 0.0039). According to the principle of maximum membership, oil spill accident on the central platform of Suizhong 36-1 oilfield is classified as light pollution, which is consistent with the conclusion drawn by Liu Jie [64].

Conclusion
In this paper, multi-level fuzzy comprehensive evaluation method is applied to establishing pollution evaluation system for oil spills on the sea surface and analytic hierarchy process combining qualitative and quantitative analysis is applied to determining weights of each index.
(1) Evaluation system of oil spill pollution on the sea surface is established by multi-level fuzzy comprehensive evaluation method. There are 6 first-level evaluation factor sets and 23 second-level evaluation factor sets. Pollution degree of oil spills is divided into 5 levels, which are respectively extremely light, light, medium, heavy and serious pollution.
(2) Membership degree of sub-indexes such as oil spill amount, marine sensitive resources and offshore distance is determined by establishing membership function, while membership degree of other indexes is determined by constructing membership subset table.
(3) Analytic hierarchy process is applied to determining weight of each sub-index and the fuzzy operator in the multi-level fuzzy comprehensive evaluation is determined by a weighted average model, thus the sub-indexes can be balanced and considered in accordance with the corresponding weight.
(4) Based on the principle of maximum membership, oil spill accidents in Penglai 19-3 oilfield and in the central platform of Suizhong 36-1 oilfield are evaluated as serious pollution and light pollution respectively, which signifies feasibility and rationality of the evaluation system of pollution degree of oil spills on the sea surface.