Method of gas transportation in permafrost zone

Transcription

1 Method of gas transportation in permafrost zone V.P. Antonov-Druzhinin Region, Russia, CIS Abstract The report deals with the authors method of gas transportation in permafrost zone with ecologically safe of natural geosystem in permafrost areas. These problems are addressed through optimization of processes of mass and energy exchange between pipeline and surrounding medium under maximum reduction of investment costs and simplification of gas treatment before transportation. 1 Introduction The main task of our investigations is to ensure accident-free operation of oil and gas transport units, safety of people. A chilled gas pipelining experiment has been performed on a test site of an operating gas pipeline. In the course of the experiment we have received data on the freezing peculiarities of different soils in the gas pipeline base. The freezing rate was found to be 0.5 to 1.6 mm/hour and that of the pipeline displacement mm. Computer simulation of the temperature fields of the soil around the gas pipeline being chilled has been carried out. The simulation results are in good agreement with field test experiments. Classification of permafrost zone geosystems (landscapes) reflecting ecological hazard of natural gas pipelining has been developed. It is known that the formation of a soil zone exposed to all the year round thermal effect with the "ice-water" phase transition in soil is a

2 304 Marine, Offshore and Ice Technology process the most typical of the northern gas pipelines in a permafrost zone. This process manifests itself in: 1) the formation of thaw areas beneath the pipelines when gas is transported at a temperature above zero (t OO) - "warm" gas; 2) the formation of freezing areas during the transportation of gas chilled to a subzero temperature (t C<0) - "cold" gas; 3) the formation of freezing areas within thaw areas with the change-over from "warm" to "cold" gas transportation. 2 Field Testing Typical of a permafrost zone of Western Siberia are the following natural landscape stow type: pretundra forests (light forests) growing on unfrozen sands (natural type 1) and on permafrost clay loams and sandy loams with mean annul soil temperatures (t%y) of about 0 to 1.0 C; tundras on permafrost sandy loams, clay loams and clays with t^ of-3.0 to -3.5 C (natural type II); peat bogs with t^ of-3.0 to -4.0<>C and -4.0 to -5.0 C (natural type III: type III-l, free from vein ice; type III-2, containing vein ice); high grass bogs with t^ of-2.0 to +1.0<>C (natural type IV). The main natural peculiarities of the gas pipeline operation in southern forest-tundra were represented in one of our experiments which was carried out on a test site of an operating gas pipeline route. The length of this site is about 2.5 m. According to the program of this experiment the transported gas temperature was changing from positive to negative values. This enabled to observe different types of interaction between the gas pipeline and its soil base. It was found that during the transportation of gas at low temperatures (t <0) ("cold" gas) the rate of anthropogenic freezing was as follows. -in coarse and medium coarse sand mm/h (for natural type I) -in sand-loam soils mm/h (for natural type II) -in peaty-mineral soil massif mm/h (for natural type III) -in water-logged soils and bogs mm/h (for natural type IV). 3 Computer Simulation Following the full-scale field experiment the computer and analog simulation of the temperature field patterns in the soils surrounding the gas pipeline was carried out. The thermophysical characteristics of the soils used in computations are presented in Table I

3 Marine, Offshore and Ice Technology 305 itype Hated peat,2.5 : sand peat, 1 IT.! 200 sandy loam i As an example, we compare the temperature field pattern in the soil surrounding the gas pipeline only for nature! type III-2 (calculated type 2). It is characterised by most complex lithologic structure texture of the gas pipeline soil base ( a peat layer containing ice veins occurs on sands with inclusion of small layers of clay loams and sandy loams ). The most complex are also the patterns of the rock zone exposed to all the year round thermal effect with "icewater" phase transition in soil and its temperature field. Other natural types are characterized by a similar lithologic texture of a soil base and therefore by a simpler temperature field patterns are in good agreement with measurement data. The formation and dynamics of an unfrozen ground "bulb" beneath the gas pipeline with change-over from "warm" to "cold" gas transportation is a very danderous phenomenon which may result in the mass and energy redistribution in the gas transporting geotechnical system. The latter may be associated with unforeseen overtension and breakage of pipes followed by unfavorable ecological after effects. As a result of our investgations we have developed classification of permafrost zone geosystems according to the ecological hazard to the construction and operation of gas pipelines within their boundaries. The most favorable conditions for gas pipelining (for both "warm" and "cold" gas) in the permafrost zone arise in the natural geosystems (landscapes) corresponding to type I The most unfavorable ecological after-effects may be expected in geosystems corresponding to natural type III-2. Special

4 306 Marine, Offshore and Ice Technology sufficiently dangerous conditions for the transportations of both "warm" and "cold" gas arise on the watercourse within the bogs but this problem exceeds the framework of this report because of its limited volume. To this effect we have developed a number of mathematical methods including an express-method for determination of the soil thawing and freezing depth within the zone of the direct thermal effect of the gas pipeline. This method is very convenient for use because it provides reliable information concerning freeze-thaw phenomena of the soil surrounding gas pipeline, it is very simple and obvious. 4 Large-Scale Experimental Research of Pipelines in Permafrost Zone Field test and large-scale experimental research of pipelines in permafrost zone under conditions of planes (similar to those of the permafrost zone of Western Siberia) must take into consideration at least five types of gas pipelining natural conditions. In our approach to field tests of pipelines the object of investigation is not only the gas pipeline as a technical object but the gas pipeline and its natural environment as an integral gas transpiration system. Such an approach is called engineering-geological monitoring. Under this term we understand observation, data accumulation, prediction of the geotechnical system development as a natural anthropogenic object. The main aim of engineeringgeological monitoring is optimization, provision of reliability and ecological safety of the geotechnical system. At present the service of engineering-geological monitoring is carrying out observation on km sites of operating gas pipelines. These are main sites of gas transporting systems of Western Siberia, which are operating under permafrost conditions. The observation system was designed with account of methodical backgrounds mentioned in the first section of the author's paper. In the process of observation we determined: 1) the thermal field in the soil surrounding gas pipeline; 2) the dynamics of direct thermal effect with "ice-water" phase transition in soil; 3) the ground water level beneath the gas pipeline; 4) displacement of gas pipeline and other technical characteristics.

5 Marine, Offshore and Ice Technology Optimization of Gas Transporting Geotechnical System in Permafrost Zone The brief summary may be presented as follows: 1. The "warm gas" pipelining is unreasonable because it is connected with the permafrost thawing in the pipeline base and the activization of cryogenic processes dangerous for gas pipelines - thermokarst, thermal erosion, ground subsidence etc. 2. The "cold gas" pipelining is dangerous because of the activization of frost heaving and fracturing of soils and the formation of frozen earth materials beneath the pipelines. The temperature condition of the transported gas developed by us permits to prevent dangerous cryogenic processes in these zones. Thereat seasonal gas pipeline displacements become comparable with the extent of seasonally heaving of clay loam in a seasonally nature As a result, the gas pipeline remains at the elevation mark (Fig. la). In case of violation of this condition the forces of cryogenic heaving may affect the gas pipeline. This results in the deviation from a design position (Fig Ib) and possible dangerous displacements (Fig. Ic). The data presented are obtained during field nesting of pipelines in the process of engineering-geological monitoring of the gas transporting systems in the North of Western Siberia. The pipe displacement illustrates in Fig. la is determined on the gas pipeline fixed in soil with anchor devices and transporting gas under temperature condition developed by us. We have presented the methodical backgrounds and the results of large scale experimental research of pipelines in the permafrost zone being carried out by the department of nothern geotechnical systems of the trust of Engineeringgeological Monitoring and Research. It is worth noting that the development of temperature condition for gas pipelining, which permits to control cryogenic processes, is base on a special permafrost prediction Using the computational algorithm we have simulated a problem of finding the monthly average gas temperature which ensures a 0.5 m thaw radius around the pipe all year round. The simulation results are presented in Fig Method of Gas transportation in permafrost zone. The approach presented was patented in the USSR and in Russia. The invention is devoted to increase of pipeline reliability and

6 308 Marine, Offshore and Ice Technology ecological safety when transporting the natural gas through a pipeline operating in permafrost zone. The objective is met through the controlled gas chilling after compriming. Gas temperature is kept equal to the temperature of water lying over permafrost, specifically ranging from 1 C to 5 C. The temperature regime proposed (1-5 C) allows to keep thaw radius around the pipe which separate a pipe body from permafrost ground, preventing a pipeline from dangerous freezing processes. In winter it is possible that thaw radius decreases gradually as a result of freezing down to the close contact of frozen ground with pipeline body, allowing to the cryogenic heaving forces to affect the pipeline. When the pipe body touches frozen ground, a brief controlled gas temperature increase to 6-10 C is conducted, this regime is kept until the thaw radius regains to its original value. The transported gas temperature range from 1 C to 5 C is common for the temperatures of water over permafrost, i.e. the medium which presents over pipeline in seasonal thaw layer at any average annual temperature of permafrost ground. By this means the proposed temperature regime makes it possible for the pipeline's and its natural environment's temperatures not to come into conflict. In the case of long-term transportation of gas with high temperature (10-20 C and more) the change-over to transportation of gas with temperature 1-5 C in terms of proposed approach allows to conduct a partial regeneration of permafrost grounds at pipeline foundation with the help of natural cold of permafrost surrounding thaw aureole and the decrease of heat from pipe to ground. Practical realization of approach is following. First, the gas is prepared for the transportation on a setup for Combined Gas Preparation (SCGP) or on a Compressor Station (CS). Then gas is chilled on the Aggregate of Air Cooling (AAC) or on the Gas Chilling Station (GCS) In this process the output gas temperature of SCGP or CS after AAC or GCS must have such a value that the temperature of pipe wall (for the heat isolated pipeline it is equal to transported gas temperature) was ranging from 1 to 5 C. The temperature of ground foundation of gas pipeline is controlled on the principle of correlation of sensor parameters (resistance, potential or frequency) and temperature. When the temperature of transported gas is ranging from 1 to 5 C (proposed temperature interval), the minimal level of heat pollution of environment and maximal stability of disperse system are observed.

7 Marine, Offshore and Ice Technology 309 When transported gas temperature is lower (e.g. in the range from 0 to PC), the stability of disperse system and the heat pollution of geological environment parameters practically correspond to those for gas transportation in temperature range from 1 to 5<>C. But in this case the technical complexity of such a temperature (0-PC) provision increases, and the opportunity of dangerous freezing processes activation on the deviation of transported gas temperature to the nagative values arises as well. Temperature increase over 5 C drastically changes the level of heat pollution of geological environment and decreases system stability. Scheme of geodesic mark installation 3 2 \ 50 la. Seasonally-cyclic displacement!? 1.2 o -50 _ Ib.The displacement resulting from the effect of criogenic heaving vm ix xxi xii i ii in ivvviviiviiiixxxixn la.dangerous displacements i i i ri i i Bi ix x xi xii i ii in' iv v vi' vii vin Ix x i xii 19&8 19&9 Figure 1. Seasonal displacement of the gas pipeline

8 310 Marine, Offshore and Ice Technology t c month gas temperature the temperature of soil surface Figure 2. Average monthly gas temperature provide the minimum thaw area (0,5m) The first year of operating, The initial Q soil temperature -7 C