SAFETY PARAMETERS AND RISK ANALYSIS AT OPERATION OF OAO GAZPROM GAS TRANSPORT SYSTEM

Size: px

Start display at page:

Download "SAFETY PARAMETERS AND RISK ANALYSIS AT OPERATION OF OAO GAZPROM GAS TRANSPORT SYSTEM"

Aleesha Walton
5 years ago
Views:

1 SAFETY PARAMETERS AND RISK ANALYSIS AT OPERATION OF OAO GAZPROM GAS TRANSPORT SYSTEM V.S. Safonov, VNIIGAZ G.E. Odisharia, VNIIGAZ V.I. Rezunenko, OAO GAZPROM For today, Gazprom faces an acute challenge to achieve steady development under condition of substantial capital consumption As of 00, GAZPROM operates over 5,000 kilometers of trunk gas pipelines and lateral gas pipelines and more than 50 compressor stations (CS), as well as 5,00 kilometers of product pipelines, including unstable condensate (broad fraction of light hydrocarbons) pipelines. GAZPROM succeeded in slightly lowering the accident rate on its gas pipelines in the last few years, in many respects through ever extensive use of in-pipe diagnostics. Specific accident rate data (accidents per 000 kilometers a year) for the period from 99 through 000 are given in Table. It is also known that Russia's unified gas-supply system (UGSS) is being aging. By now 34% of gas pipelines (out of 5,600 kilometers) have turned 0 years of service life, some 5% - passed 30 years (conditionally normative service life) and 3,5% - exceeded 40 years service life. Marked increase in a number of accidents caused by stress corrosion cracking (SCC or stress corrosion) is of particular concern. Years Reduced accident rate, accidents/000 0,60 0,8 0,4 0,97 0,08 0,39 0,63 0,43 0,8 0, km Number of corrosion-caused accidents Including due to stress-corrosion Table. Gas pipeline accident rate Proceeding from the statistical gas pipeline accidents information related to Russia's northern regions it was found that the section l = l l rupture probability within a section of length L between compressor stations can be expressed as l l λсрkkl р D -α -α L L - P l-l= [e e ] -α -e where λ ср. is a mean accident rate value throughout the entire UGSS, K p and K D are coefficients to take into account the effect, which regional and engineering factors exert on the accident rupture rate, α is accident irregularity coefficient along the leg length (ά.7) Accident ruptures of gas main pipelines, lateral gas pipelines and CS gas pipelines are related to physical effects of two types: internal - non-stationary gas-dynamic processes occurring in pipelines themselves and determining the atmospheric natural gas emission dynamics; external - determining the effect of high-pressure pipeline section rupture on the environment. External effects being followed with: - formation of compression waves due to natural gas expansion to atmosphere when pressurized gas being released from the ruptured pipeline, also, compression waves to form in the event of gas "plume" (cloud) ignition, that is owing to its combustion products expansion. - fragmentation and separation of fragments of the ruptured pipeline section; - potential gas ignition and environmental fire thermal action

2 With the aim of examining the above processes VNIIGAZ has developed and verified a special system of mathematical models and numerical programs. Here, human beings, process equipment, buildings and nature components were considered as recipients of detrimental effect. The probability of detrimental effect was calculated proceeding from the assumption of normal distribution law (Gaussian integral) using probit-functions (Pr = a + b ln D, where D is a dose of detrimental effect). The calculation results of thermal detrimental effect are presented below in Fig..,0 0,8 Probability of fatality 0,6 0, , 0, Human position with respect to rupture "center", m. Figure. Probability of fatalities in case of gas pipeline ruptures followed by gas ignition if people to leave the risk area at a speed of,5 m/sec. ( -D c (conditional) = 700 mm; P = 5,5 MPa; - D c =,000 mm, P = 5,5 MPa; 3 - D c = 00 mm; P = 5,5 MPa; 4 -D c -= 400 mm; P = 7,5 MPa). A peculiar feature of a gas pipeline as a hazardous source is that the recipient being positioned normal to the pipeline's axis at some distance could be injured if "accident occurs at any point of some effect section" ( x ; x ) (see Fig. where R 00, R are radiuses of absolute that is 00% effect zone and safety zone that is % effect).

3 z y R R 00 R 00 R R -x -x 00 x 00 x x МГ x x 00 x Figure.. Calculation diagram of probable human injury in case of trunk gas pipeline accident. Figure. 3 shows influence of the trunk gas pipelines characteristics (complex P D, where P is working pressure, D is diameter) on the safety zone boundaries. Safety zone values used for gas pipeline with D c = 400 mm and P = 7,5 MPa are taken as a unity [].,0 0, ,8 0,7 P D /P400 D 400 0,6 0,5 0,4 0,3 0, 0, 0, Distance Расстояние from от the "центра" center разрыва, of disruption м Figure. 3. Relationship between the % human effect zone boundaries and the trunk pipeline characteristics; baric effect ( - 5) and thermal effect (6): -lung affection, - chaotic shock wave movement, 3-ear-drum rupture, 4-injury of people in partially destroyed building, 5 injury of people in completely destroyed building

4 As of 00, Gazprom operated some 700 compressor workshops (CW) (comprising more than 4,000 gas pumping units (GPU) with installed capacity of 4.6 MW). Among them GPU being operated (years): up to 5 years - 60 units; 6 to 0 years - 94 units; to 5 years - 38 units; 6 to 0 years - 68 units; to 5 units - 95 units; 6 to 30 years - 9 units and over 30 years - 8 units. Thus, for today over 43% of all workshops have exceeded operational period of 5 years, which is considered as threshold from viewpoint of maintaining normative % probability of failure. As for types of equipment, breakdown of accident failures for this period (as % to the total number) was as follows: pipeline metal, including welds - 46 failures, tee joints - 3 failures, valves - 0 failures, dust catchers - 9 failures, non-return valves - 6 failures, air cooling units - 3 failures and others - 3 failures. Note that as for gas-pressure vessels, 4% of total failures account for vessel body defects and 57% - branch pipe defects. And 83% of defects were of fabrication origin and only 9% were related to operation. It follows from the analysis of statistical data on compressor stations failures and accidents for the last 0 years that the main causes and factors to have contributed to failures and accidents are (%): increased pipeline vibration and subsidence of rock under pipelines and pipeline supports - 49 failures; equipment manufacture defects (primarily shaping parts and fixtures) -3 failures; installation errors - 7 failures and corrosion and wearing - failures. By seriousness of CW failures these can be conditionally classified into five categories: 5 - accidents with heavy material losses and emergency CW shutdown; 4 - CW emergency shutdown after defect being detected; 3 - unscheduled SW shutdown to remove defects; - GPU shutdown; - defect to be eliminated during scheduled CW or GPU outage. Failure breakdown for the period being considered by categories (%): 5-3; 4-4; 3-6; - 3; , 4 and 3 seriousness failures categories accounted for 53% of the overall failures So, as of 000, the expected conservative approximation rate of large-scale rupture accidents followed by fires and gas explosions (category 5) can be assumed to be equal to,7.0- per compressor workshop a year. In the last few years GAZPROM has been carrying out a complex of works aimed at considerable increase in volume of CS diagnostic assistance and its quality, which improves their reliability and safety. As noted above, as of today there are about 700 compressor workshops operating in the industry. Approximately 70 of these CWs are provided with permanent buildings to house GPUs in groups. The buildings include GPU halls, a compressor room, oil supply system, a battery room, control system and other services. Gas escape and its explosion inside GPU hall and compressor room is one of the causes, which might lead to accident CW building destruction. A proper mathematical model has been developed to analyse these processes. As an example, let us consider a gas explosion to occur inside GPU hall designed for 6 6mx5mx5,5m GTK-0-4 units (volume being 9,300 cu. m). The building is equipped with 38 window openings measuring,7mx3,4m (3 mm single glass). Figure. 4 shows the results of calculated overpressure dynamics, if methane-air mixture deflagration burning is occurring inside the completely gassy GPU hall, for accident scenarios: - small gas escape and low turbulization throughout the hall; - violent jet gas escape and intense air mixture turbulization. The first pressure peak corresponds to window glass opening with respect to time when combustion products start going to atmosphere. It has been established that in the example being considered the window glass area does not provide the building structure integrity in case of internal deflagration burning of methane-air mixture.

5 Overpressure, kpa.. Time, ms Избыточное давление, кпа Время, мс Time, milli sec. Figure 4. Overpressure dynamics in case of gas explosion inside GPU hall of the compressor workshop. Destruction (damage) of CW buildings and also individual GPU covers is likely to occur both in case of internal explosions and in case of the GPU piping failure (compressed gas energy) []. As the analysis demonstrated, accident ruptures of pipelines and GPUs followed with gas ignition at the compressor station (CS) "upstream side" are the most hazardous to personnel. As an example, Figure. 5 shows distribution of working places as regards risk levels influencing the personnel of one of the gas-transport enterprise "Volgotransgaz's CS, which is operating a system consisting of 6 trunk gas pipelines (D = 400 mm, P = 7,5 MPa). Количество Number рабочих of risk-affected мест, подверженных working places риску Уровень Risk level, риска, /year. /год Figure 5. Distribution of working places as regards risk levels. As mentioned above, GAZPROM's network of pipeline includes product pipelines 5,00 kilometres long, namely, pipelines to carry unstable condensate (broad fraction of light hydrocarbons). The specific character of risk analysis as applied to such pipelines is illustrated by an example of a

6 050 km twin product pipeline "Surgut-Tsentr" being designed (D c = 400 mm, P = 6,4 MPa) intended for pumping 6,4 tons of unstable condensate annually. The analysis of statistical information on the systems of liquid pipelines of Western Europe, USA, Canada and Russia has shown that the frequency of seal failures on middle and large diameter pipelines leading to ShFLU leakage in a range of 0-00 kg/s accounts for 0. per 000 km per year. For separate sections located near the areas of high industrial activity, high density of population and complicated engineering and geological characteristics the value 0. (thou. km-year) - was increased based on the above influence factors. By its component composition, ShFLU is a uniform mixture of light hydrocarbons, more than 90% of which can not exist thermodynamically as a liquid under atmospheric conditions. Therefore, emergency leakage of ShFLU from a damaged pipeline will be accompanied by its intensive and practically complete evaporation with the formation of explosive HC clouds. These clouds are able to move in the surface layer of the atmosphere under the influence of wind to long distances, Fig. 6 []. Radius of liquid mirror, m; Радиус зеркала жидкости, м G G=50 = Kg/s кг/с τ= τ=4,4 4,4 min мин Время, Time, min мин Интенсивность испарения Intensity of evaporation from the с поверхности, кг/с surface, kg/s; Figure 6. The dynamics of ShFLU spreading and evaporation on ground surface (loam, summer) Non-stationary hydrodynamic processes taking place in pipeline having considerable leak failures differ from the other wave processes and thus can be easily distinguished by operators at compressor stations with the help of ordinary flow and pressure meters. However, due to a relatively low velocity of wave process distribution in ShFLU (no more than 800 m/s) and considerable distances between compressor stations (up to 50 km), the time for identifying an emergency process by operators at CS and taking an adequate solution on closing gate valves ( T*) may account for 0 30 min, Fig.7

7 Time of response formation ( P = atm) per CS, min Leakage from cracks Kg /s rapture on full profile A distance from CS to rapture point, km. Figure 7. Leakage intensity vs. KS response During the above given time intervals several tons of ShFLU can be released as a gas-liquid mixture, i.e. at a typical velocity of ShFLU vapor distribution in the atmosphere (-5 m/s) the dangerous zone mar reach over km. A general consequence of risk factor calculation includes the stages as follows: provision of a general map of the territory and specific features of its infrastructure; identification of possible sources of ignition of HC cloud; finding of inhabitants distribution by their remoteness from the pipeline; calculation of the probability of reaching by a HC cloud of different points of the adjacent territory taking into account the all possible variants of ShFLU emergency leakage and for the whole year spectrum of a change in meteorological parameters for the area concerned; construction of the potential risk field considering a real distribution of ignition sources and inhabitants; calculation of inhabitant distribution in a settlement by risk levels (probability of their death in a HC cloud burning in deflagration regime). Specific technical decisions on providing the necessary safety of the pipeline have been developed for all settlements (more than 400 with a total population of 00 thousand people) near which the pipeline runs and which have an increased risk indexes. REFERENCES. A.D. Sedych, V.S. Safonov, G.E. Odisharia, A.A. Shvyriaev. The Use of Risk Analysis Method During the Declaration of Industrial Safety of Gazprom s Enterprises. Reliability and Certification of Oil-and-Gas Equipment, 00, 4.. A.D. Sedych, A.I. Gritsenko, V.S. Safonov, G.E. Odisharia, S.V. Ovcharov, A.A. Shvyriaev. Risk Analysis of Gas Industry Enerprises. New high technologies in gas, power and communication sectors. Vol. 0, Book, pp. 8-4.

Abstract. 1 Introduction

Abstract. 1 Introduction Risk assessment study of the mutual interactive influence of working procedures on terminals handling dangerous goods in port of Koper (Slovenia) L. Battelino Water Management Institute, Maritime Engineering