Nat Hazards DOI 10.1007/s11069-016-2308-y ORIGINAL PAPER
Geohazards and thermal regime analysis of oil pipeline along the Qinghai–Tibet Plateau Engineering Corridor Wenbing Yu1 • Fenglei Han1,2 • Weibo Liu1,2 Stuart A. Harris3
•
Received: 22 December 2015 / Accepted: 18 March 2016 Ó Springer Science+Business Media Dordrecht 2016
Abstract This paper investigates the influence of geohazards on the existing oil pipeline and the potential interaction between the proposed new oil pipeline and preexisting transportation structures along the Qinghai–Tibet Plateau Engineering Corridor. The current Golmud–Lhasa oil pipeline has been seriously affected by retrogressive thaw slumps caused by surface water being channeled through culverts causing serious erosion problems. Climate data show that the air temperature increased at a rate of 0.0281 °C/a for the past 60 years along the corridor. To design the new pipeline, the effects of revegetation, climate warming and pipe insulation on permafrost have been simulated using numerical modeling. A warm oil pipeline would potentially lead to significant thawing of the permafrost foundation. When climate warming is not considered, insulation of the buried pipe could keep the permafrost stable. Revegetation and the use of utilidors could counteract the influence of heat input from the oil pipe, and even a 1.1 °C/50a climate-warming rate. However, for the 2.6 °C/50a climate-warming-rate scenario, they are inadequate to keep the permafrost stable. Vegetation cover is important to reduce the effect of climate warming on both the natural and the human-impacted permafrost. Revegetation after construction is important to protect the permafrost environment as well as the oil pipeline itself. Keywords Permafrost Climate warming Oil pipeline Hazards Anthropogenic impacts Revegetation
& Fenglei Han
[email protected] 1
State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, Gansu, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Department of Geography, The University of Calgary, 2500 University Dr. N. W., Calgary, AB T2N 1N4, Canada
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1 Introduction Climate warming and anthropogenic impacts modify the heat balance of the ground, which can lead to extensive degradation of permafrost. Many studies have focused on this issue (Washburn 1980; Romanovsky et al. 2010; Yang et al. 2010; Olsen et al. 2011; Johansson et al. 2011; Kokelj and Jorgenson 2013; Shiklomanov and Nelson 2013). Degradation of permafrost can cause surface settlement, and structures are then subject to thaw-induced damage. The Qinghai–Tibet Plateau Engineering Corridor (QTPEC) is located on the Qinghai– Tibet Plateau between Golmud in the north and Lhasa. The plateau has an unusually large rate of climate warming as well as intensive disturbance of natural conditions by anthropogenic development. There are several key engineering projects in this crowded corridor, such as the Qinghai–Tibet Railway, the old Qinghai–Tibet Highway, the new Expressway, the old Golmud–Lhasa oil pipeline. This paper focuses on the hazards of the Golmud–Lhasa oil pipeline and its interaction with permafrost. Many problems have been encountered during the period of operation (He and Jin 2010). By now, many studies have been carried out on the performance of the old oil pipeline built on permafrost, including field observations and numerical analysis (Rowley et al. 1973; George 1995; Hastaoglu and Hakin 1996; Leonid and Eli 1999; Li et al. 2010; Wen et al. 2010; Zhang et al. 2010). Most of the past research has been focused on the stability of the oil pipelines located in areas at high latitude (e.g., Canada and Alaska). There are few reports related to the oil pipelines at low latitudes such as on the Qinghai–Tibet Plateau where the ecological system is very fragile and the short-wave solar radiation and night-time long-wave reradiation are significantly greater. This paper examines the existing and potential hazards of the Golmud–Lhasa oil pipeline caused by natural and human impacts. Numerical simulations are used to investigate the effect of the new oil pipeline on the permafrost environment under a variety of conditions to provide information for its proper design and construction along the QTPEC.
2 The Golmud–Lhasa oil pipeline project and climatic conditions The Golmud–Lhasa oil pipeline was constructed during 1973–1977 (Fig. 1). It is 1076 km long, and about 965 km of it is located at an altitude above 4000 m. The highest elevation along the pipeline route is about 5231 m at the Tanggula Pass. Five hundred and fifty kilometers of the pipeline overlies continuous permafrost, and 82 km is built on island permafrost (He and Jin, 2010). The permafrost table is at 0.8–4.8 m depth, although the anthropogenically disturbed permafrost table is much deeper at 2.8–10.0 m under the asphalt pavement (Jin et al. 2008). Using the mean annual ground temperatures (MAGTs), the permafrost on the QTPEC is divided into four types: the cold permafrost (MAGTs B -2.0 °C), the mid-warm permafrost (-2.0 °C \ MAGTs \ -1.0 °C), the warm permafrost (-1.0 °C B MAGTs B -0.5 °C), and the warmer permafrost (MAGTs [ -0.5 °C) (Wu and Liu 2005). Warm permafrost and warmer permafrost account for about 275 km of the route, 40 % of which is ice-rich permafrost. This is readily affected by climate change and anthropogenic factors and then becomes unstable (Wu et al. 2003). Xidatan is at the north boundary of permafrost, and Anduo lies at the south boundary. Talik exists along the river flood plains in the permafrost area.
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Fig. 1 Projects and permafrost distribution along the QTPEC [revised from Jin et al. (2008), He and Jin (2010)]
The 739-km old Golmud–Lhasa oil pipeline is scheduled to be decommissioned due to corrosion of the pipes, and a new, larger diameter, 1076-km oil pipeline from Golmud to Lhasa is being planned. It will start at the Xueshui River (30 km south of Golmud City)
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Monthly precipitation (mm)
90
15
80
Monthly precipitation (mm)
70
Mean monthly air temperature (°C)
10 5
60 50
0
40
-5
30
-10
20 -15
10 0
Jan-59 Feb-59 Mar-59 Apr-59 May-59 Jun-59
Jul-59 Aug-59 Sep-59 Oct-59 Nov-59 Dec-59
-20
Mean monthly air temperature (°C)
and end at the Lhasa distribution station (18 km west of Lhasa city). The new oil pipeline will closely parallel to the old one. The pipe size will increase to 300 mm in diameter, and the temperature of the oil passing through it will be about 15 °C. In the case of the old Golmud–Lhasa oil pipeline, the temperature of the oil varies from -5 to 9 °C and the pipe diameter is only 159 mm. The new pipeline is designed to be buried, and the vegetation will be removed during construction. The climate in the corridor is dry, cold, and windy, with strong solar radiation and large temperature difference between day and night. The recorded warmest air temperature since 1951 is 24.7 °C (June 29, 1988), and the warmest ground temperature is 65 °C (July 14, 1994). The recorded coldest air temperature since 1951 is -45.2 °C (Jan 6, 1986), and the coldest ground temperature is -51.5 °C (Jan 6, 1986). The mean snowfall accumulation is small, and 83.5 % of the total precipitation occurs in summer (Fig. 2). In general, the annual precipitation in the QTPEC is low, although it has reached high values in some years, e.g., it reached 1018 and 908 mm in 1967 and 2010, respectively, at Golmud meteorological station (elevation 2807 m). The average precipitation along the corridor is 399 mm (from 1955 to 2013), but in the permafrost area of the corridor, the mean precipitation is only 342 mm for the same time period. The recorded minimum precipitation was only 136.3 mm in 1984 at Wudaoliang meteorology station (elevation 4612 m). The actual precipitation in the corridor is influenced by the southeastern Asian monsoons and local factors, especially topography. Though the mean precipitation is small, it is concentrated in the 3 months in summer, and causes flash flooding and significant surface erosion of the natural ground and in the areas of construction. In the region along the QTPEC, the mean annual air temperature (MAAT) started increasing between 1967 and 1980 (Fig. 3). The data in Fig. 3 are from the Share Center of Chinese Meteorology Data. All the data from the seven stations show the same warming trend. The average rate of warming in the permafrost region of the corridor is 0.281 °C/10a, but it rises to 0.447 °C/10a in the seasonally frozen area of the corridor.
Year Fig. 2 Typical precipitation and air temperature regime during 1959, Tuotuohe meteorology station, E34°130 , N92°260 , elevation 4533 m
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Mean annual air temperature (oC)
Nat Hazards 9 Lhasa
7 5
Golmud
3
Dangxiong
1
Naqu
-1 Anduo,southern edge of permafrost zone
-3 -5
TuotuoHe,permafrost zone
-7
Wudao Liang,permafrost zone
-9 1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
Year Fig. 3 Thirty-year moving average of the mean air temperatures along the QTPEC. The seven meteorology stations shown in Fig. 3 are from the south (Lhasa, elevation 3649 m) to the north (Golmud, elevation 2807 m) along the corridor
3 Hazards of the Golmud–Lhasa oil pipeline The main factors that have affected the stability of the old buried Golmud–Lhasa oil pipeline include degradation of permafrost, concentrated rainfall in summer, and human activities. Since the operation of the Golmud–Lhasa oil pipeline began in 1977, there have been more than 30 product leaks and at least four pipeline ruptures. Normally, these pipelines are emplaced separately from other transportation facilities, but along the Qinghai–Tibet Transportation Corridor, there has been a succession of individual projects built at different times in a fairly narrow belt. While this minimizes damage to the environment, the effects of a new construction on the existing infrastructure have been omitted from full consideration in the design stage, and this has shortened the life of the original oil pipeline as explained below.
3.1 Thermokarst hazards affecting the oil pipeline The old oil pipeline was the first structure to be built along the QTPEC. This meant that it had a free choice of its route, and another construction had to be built alongside it. In places where the available width is limited, this creates problems. According to the available incomplete statistics, there have been more than ten cases of thermokarst-related oil pipeline problem in the permafrost zone of the QTPEC (Fig. 4). Figure 4a shows a case of thermokarst involving a slope hazard. The oil pipeline had been relocated in 2001–2004 because a retrogressive thaw slump destroyed the old pipeline that used to be located about 30 m downhill from the reconstructed one. The retrogressive thaw slump was caused by the excavation during the construction of the Qinghai–Tibet Highway in the 1950s. Because of continued human activity along the Highway, this slump has been expanding fast for the past 10 years. The upper edge of the retrogressive thaw slump slope has crossed the reconstructed oil pipeline and is expanding on to the higher parts of the slope that is underlain by continuous permafrost. The bulk of the limited precipitation falls as sudden downpours of rain in summer. Overland runoff is ponded against the highway embankment and passes through culverts as a concentrated flow. This causes gully erosion below the
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Qinghai Tibet Railway
The reconstructed oil pipeline
Oil pipeline
Buried optical cable
Retrogressive thaw slump
Thermokarst lake
(a)
(c) Buried oil pipeline
Precipitation
Top
Evaporation
Extending direction Buried pipeline
Lake bed
Qi ng ha i-T ibe tH
(b)
Meltwater
Foot igh
wa y
Permafrost table
Permafrost degradation direction
(d)
Fig. 4 Thermokarst hazards of the old oil pipeline along the QTPEC. Figure 4a shows a case of a retrogressive thaw slump exposing the oil pipeline at Fenghuo Mountain area. Figure 4b illustrates the progress of thermokarst slumping of the hill slope. The first stage in the development of the thermokarst slump was the cutting of the foot of the slope in 1950s, as indicated by the dotted arrow. The second stage occurred as a result of the construction of the QTR embankment affecting the reconstructed oil pipeline in 2001–2004. The solid arrows indicate the second direction of slumping. Thus, human activity is the primary cause of the renewed thermokarst erosion, with the climate warming and concentrated precipitation speeding up the thawing and erosion of the permafrost. Figure 4c shows a thermokarst lake threatening to erode the ground over the oil pipeline. Figure 4d shows the typical thermal regime of an unpenetrated thermokarst lake on QTPEC and its effect on the nearby permafrost
culvert, while the ponded water tends to develop thermokarst depressions. When the road is built on a side slope, retrogressive thaw slumps can develop starting at the base of the embankment in the area affected by the thermokarst. The exposed ground ice is easy to thaw, and the heavy rainfalls in summer accelerate the erosion of the foot of the slope. In the case shown in Fig. 4a, the excavations during the construction of the reconstructed oil pipeline resulted in another retrogressive thaw slump developing at the upper disturbed part of the slope. The excavated trench for the buried oil pipe was not compacted, and there is no vegetation on the top, which has resulted in the development of a seepage and warmer zone (Fig. 4b). Thermokarst lake hazards are another threat to the oil pipeline along QTPEC. They are widespread in the permafrost areas of the flatter land between the mountain ranges (Wu et al. 2014; Niu et al. 2011; Huang et al. 2013). Gale force winds occur on an average about 149–283 days each year, causing wave erosion of the banks of lakes and speeding up the increase in their size (Fig. 4d). The warming climate aids this process by
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increasing the temperature of the underlying permafrost and increasing the thickness of the active layer. These thermokarst lakes can cause profound ecosystem changes and can lead to serious damage or even collapse of human infrastructures (Shiklomanov and Nelson 2013). Figure 4c shows an example of damage done to the Golmud–Lhasa oil pipeline by expansion of a thermokarst lake. The oil pipeline is currently located close enough to the lake to be submerged in summer. The destruction of the vegetation cover at the north side of the lake is serious and is mainly caused by anthropogenic impacts (construction of the oil pipeline and the QTR). The collapse of the lakeshore has been increasing, so as to threaten the oil pipeline. Proper engineering measures should be adopted to prevent the lakeshore from extending, such as the restoration of the vegetation and lakeshore reinforcement with special active cooling techniques such as thermosiphons or rocks. As for the new oil pipeline, it is better to avoid the thermokarst lakes altogether as far as possible.
3.2 Pipeline hazards caused by multiple factors The subsequent construction of the road and QTR has had a significant influence on the Golmud–Lhasa oil pipeline. The QTR was built in 2001, and most of the old pipeline is located downhill of the railway embankment. The precipitation from the heavy rainfall can only move downslope through culverts placed periodically under the embankment. This concentrated drainage from the culverts concentrates it in specific locations which often cause erosion and gullying of the ground surface. These gullies wash the soil away from around the buried oil pipeline, leaving it exposed and unsupported. According to field investigations and incomplete statistics, there are more than 50 sites where this erosion hazard has taken place. Figure 5a presents an example of scour hazard of the oil pipeline. In order to avoid a problem with icings, the oil pipeline was located at a higher elevation on a hill slope. After the QTR was built, the pipeline was washed out at the both locations of water discharge through culverts beneath the QTR embankment. A gabion was used to protect the exposed oil pipe, but this measure has shortcomings in permafrost regions,
(a)
Culvert
(c)
Bridge Railway Site 1 Oil pipeline
The bended uplied oil pipeline Gabion (b)
Aufeis Highway Site 2
(d)
Fig. 5 Erosion and bend-uplift hazards of the existing oil pipeline along the QTPEC. The pipeline was washed out at the two sites in Fig. 5a. Figure 5b shows the gabion installed to try to support the pipeline and decrease erosion. In Fig. 5c, an uplifted section of pipe occurs at site 1, located at N35°400 20.7900 , E94°020 59.3800 at an elevation of 4681 m. The photo was taken on June 6, 2012. Site 2 shows a similar uplifted section located at N34°370 02.7800 , E92°470 54.5700 . The elevation is 4718 m
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especially in the ice-rich strata. Measures that are more effective need to be devised (Fig. 5b). There is also an upheaval (bend-uplift) deformation hazard affecting the old Golmud– Lhasa oil pipeline. Most of this kind of hazard occurs at the shoulders of the river bank or in the gullies. It is characterized by bend deformation so that the pipeline rises above the ground surface (Fig. 5c, d). It is the result of the combined action of multiple factors, such as the differences in frost heave, thaw consolidation, surface subsidence, and surface erosion. There are other cryogenic hazards for the oil pipeline in the QTPEC, such as seasonal ice mounds, icings, and thermokarst-induced landslides. Although the number of these hazards is small, it costs a lot to control them. For the new, enlarged Golmud–Lhasa warm oil pipeline, the route selection is very important. Avoiding the disaster sites is the best way. However, in many cases, there is no more space with suitable geology and hydrology for the new infrastructures within the corridor. Therefore, unless the corridor is widened or a different route is used, the new oil pipeline will have to be built alongside the old one and anti-hazard measures will have to be considered during the design stage.
4 Influence of the warm oil pipeline on permafrost under different conditions The permafrost degradation will affect the oil pipeline. At the same time, the oil pipeline will affect the permafrost too. In order to evaluate the influence of warm oil pipe on permafrost, the numerical method was used. Vegetation restoration, climate warming, and pipe insulation are also considered in the simulation.
4.1 Theoretical model The problem of the influence of a warm oil pipe on permafrost is an unsteady heat transfer process. Phase change between ice and water occurs during the process. The mathematical model of it can be simplified as a two-dimensional heat transfer model. The differential equation is as follows: o oT o oT oT þ ¼ qC k k ð1Þ ox ox oy oy ot where C is equivalent thermal capacity (J/kg K) of soil, k is thermal conductivity of soil (W/m K), q is natural density of soil (kg/m3). Supposing the phase change happens at the temperature range Tm ± DT, the equivalent heat capacity and heat conductivity coefficient can be obtained by using the apparent heat capacity method to make up the equivalent heat capacity (Bonacina and Comini 1973). 8 T\ðTm DT Þ > < Cf L Cf þ Cu ð2Þ Ce ¼ þ ðTm DT Þ T ðTm þ DT Þ > 2 : 2DT Cu T [ ðTm þ DT Þ
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8 T\ðTm DT Þ k > < f ku kf ke ¼ kf þ ½T ðTm DT Þ ðTm DT Þ T ðTm þ DT Þ > 2DT : ku T [ ðTm þ DT Þ
ð3Þ
The problem discussed in this paper is strongly nonlinear, and it is hard to determine its analytic solutions. Only numerical methods can solve this problem. The finite element equation is obtained through the Galerkin method (Lai and Zhang 2003). oT ½M þ ½K fT g ¼ fF g ð4Þ ot The Crank–Nicolson method is used to discrete Eq. 4. The numerical solution of the temperature problem can be obtained by solving the equations by the time growth method (Crank and Nicolson 1947). Based on the theoretical model, the software ANSYS was adopted to simulate the thermal regime of the permafrost affected by the warm oil pipeline.
4.2 Geometric model According to the Golmud-Lhasa oil pipeline reconstruction project and the typical strata of QTPEC, the calculated geometric model is presented (Fig. 10). The buried depth of the pipe is 1.6 m (from the bottom of the pipe to the ground surface). The simulated time span was 50 years.
4.3 Boundary conditions and physical parameters In the past, the vegetation was removed and not replaced after the construction along the QTP, resulting in significant permafrost degradation. Accordingly, the two different effects of the bare ground and vegetated ground on permafrost temperatures are compared in this paper. Two climate-warming trends of 1.1 and 2.6 °C/50a are applied in the numerical simulation (Li and Cheng 1999; Qin 2002). In order to investigate the best method to mitigate the influence of the warm oil pipeline, different pipe insulation thicknesses are considered. There are eighteen operational cases simulated in total (Table 1). In Table 1, ‘‘Bare’’ means that the vegetation is not replaced after it is removed during construction. ‘‘Covered’’ means that the ground is revegetated after it is removed during construction. The boundary conditions are determined according to the annual mean air temperature and the boundary layer theory. The key element of this theory is that the ground surface
Table 1 Operation cases of the numerical simulation Temperature boundary
T1
T1a
T1b
T2
T2a
T2b
Ground condition
Bare
Bare
Bare
Covered
Covered
Covered
Climate-warming rate/°C/50a
0
1.1
2.6
0
1.1
2.6
Insulation thickness/cm
0
3
5
0
3
5
0
3
5
0
3
5
0
3
5
0
3
5
In Table 1, ‘‘Bare’’ means that the vegetation is not replaced after construction. ‘‘Covered ’’ means that the ground is revegetated after construction
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temperature is higher than air temperature, which is dependent on the type of ground surface. The ground surface-warming regime is different in different areas. Wu et al. (1988) described the statistical relations between the air temperature and the ground surface temperature of different surface types along the Qinghai–Tibet Plateau Engineering Corridor (QTPEC). By this method, the ground boundary conditions can be calculated. Generally, the air temperature is easy to obtain. So far, this reference is almost the only evidence that the temperature boundary is determined when researchers do the numerical simulation on infrastructures along the QTPEC. So, based on the mean annual air temperature of -5.2 °C at the Beilu River area and the reference (Wu et al. 1988), the six temperature boundary functions are obtained. For the bare ground surface cases, the temperature boundary functions are as follows: 2p p ð5Þ th þ T1 ¼ 1:2 þ 12 sin 8760 2 1:1th 2p p T1a ¼ 1:2 þ ð6Þ th þ þ 12 sin 8760 2 50 365 24 2:6th 2p p T1b ¼ 1:2 þ th þ ð7Þ þ 12 sin 8760 2 50 365 24 For the revegetated surface cases, the temperature boundary functions are as follows: 2p p ð8Þ T2 ¼ 2:7 þ 12 sin th þ 8760 2 1:1th 2p p T2a ¼ 2:7 þ th þ ð9Þ þ 12 sin 8760 2 50 365 24 2:6th 2p p T2b ¼ 2:7 þ th þ ð10Þ þ 12 sin 8760 2 50 365 24 In Fig. 6, edge AB is the heat flux boundary. According to the test data in this area, the ground temperature gradient at 30 m depth is 0.38 °C/m. Edges AD and BC are adiabatic boundaries. The temperature of warm oil in the pipe is kept at 15 °C. The parameters used in the numerical simulation are shown in Table 2. The parameters are determined according to the typical strata and permafrost type along the Golmud– Lhasa oil pipeline. This model is calibrated by the measurements made in the undisturbed permafrost table in the Beilu River area, which is at 2.3–3.0 m depth. The calculated undisturbed permafrost is about 2.81 m, which shows that the model is reasonable and can be used to predict the changes in permafrost, affected by climate warming, different ground surface type, etc.
4.4 Results and analysis 4.4.1 Influences of the temperature boundary on the permafrost when the oil pipeline is not insulated The permafrost table (the maximum thawing depth in 1 year) is one of the most important indicators of the state of the permafrost environment. The ‘‘artificial permafrost table’’ and
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C
D Sandy soil
Pipeline Silty clay
Weathering mud rock
Unit:m B
A
Table 2 Thermal parameters
Physical parameters
kg m-3
J/kg K
q
Cf
J/m h K Cu
kf
ku 6854
Sandy soil
1710
906
1170
8921
Silty clay
1280
1275
1730
6022
3931
Weathering mud rock
1800
1222
1608
7632
5112
500
1800
1800
126
126
Insulation
‘‘natural permafrost table’’ are used in the following analysis. The term ‘‘artificial permafrost table’’ is used for the permafrost table that is affected by human activity and climate. The term ‘‘natural permafrost table’’ is used for the permafrost table that is only affected by climate. The permafrost table under different conditions in different years is presented in Fig. 7. The curves in all figures represent the permafrost table. Figure 7a shows that the artificial permafrost table of case T1 under the oil pipeline is about 7.49 m in the 50th year. It is about 5.16 m for case T2. The difference is 2.33 m. The natural permafrost table under the natural ground (away from the oil pipe) had no obvious change in 50 years. Figure 7b shows that, if the temperature rises 1.1 °C in the next 50 years, the artificial permafrost table in the case, T1a will reach to 9.62 m after 50 years of operation. It is about 6.07 m of case T2a. The difference is 3.55 m. The natural permafrost table has a slight increase in depth. Figure 7c indicates that if the temperature rises 2.6 °C in the next 50 years, the artificial permafrost table of case T1b will reach 13.72 m in the 50th year. It is about 7.92 m case T2b. The difference is 5.78 m. The natural permafrost table in the two cases shows a significant increase. The natural permafrost table of case T1b is much larger than that in the T2b case. Figure 7 shows that the warm oil pipe has a significant influence on the permafrost environment. A non-insulated warm oil pipe could cause a very large thaw zone. However, a good vegetation cover is very favorable for permafrost protection in all cases.
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a
T1
T2
0 T2a
2
2
4
4
Depth (m)
Depth (m)
b
0
6 8 initial 1 year 5 years 10 years 20 years 30 years 50 years
10 12 14
6 8 initial 1 year 5 years 10 years 20 years 30 years 50 years
10 12 14
16
T1a
16
0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
Distance (m)
c
8
10
12
14
16
18
20
Distance (m)
0 T2b
T 1b
2
Depth (m)
4 6 8 initial 1 year 5 years 10 years 20 years 30 years 50 years
10 12 14 16
0
2
4
6
8
10
12
14
16
18
20
Distance (m)
Fig. 7 Shape of the permafrost table for the non-insulated cases under different boundary conditions. The legends in denote the operational time
0 -2
Depth (m)
-4 -6 -8 -10 -12 -14
T1
T1a
T1b
T2
T2a
T2b
-16 0
10
20
30
40
50
Time (a) Fig. 8 Shape of the permafrost table for the different cases when the oil pipeline is not insulated
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Figure 8 shows the changing process of the artificial permafrost table. Figure 8 shows that the melting rate of permafrost under the oil pipeline gradually decreases and then reaches equilibrium when climate warming is not considered. When continuous climate warming is considered, the permafrost keeps melting during the whole simulation time span. For the vegetation cover and the no climate-warming case T2, the time needed for the permafrost table to reach equilibrium is about 8 years. However, it is about 28 years for the bare and non-climate-warming case T1.
4.4.2 Influences of the temperature boundary on permafrost when the oil pipeline is insulated In order to investigate the thermal regime of the permafrost after the warm oil pipeline is insulated, 3-cm-thick and 5-cm-thick insulation materials are simulated under different temperature boundaries (Table 1). Figures 9, 10 and 11 show the results. T1–3 is used to represent the case of bare ground and 3-cm-thick insulation material, and so on. The permafrost table after the first year is 4.74, 3.23, and 3.15 m for cases T1, T1–3, and T1–5, respectively. In the 50th year, it is 7.49, 3.29, and 3.15 m, respectively. The difference between case T1–3 and case T1–5 is about 14 cm. For both insulated cases, the permafrost table shows almost no changes as time goes by (Fig. 9). The depth of the permafrost table after the first year is 4.14, 2.87, and 2.72 m for cases T2, T2–3, and T2–5, respectively. In the 50th year, it is 5.16, 2.87, and 2.72 m, respectively. The difference between case T2–3 and case T2–5 is about 15 cm. For both the insulated cases, the permafrost table hardly changes over the 50 years (Fig. 10). The permafrost table in the first year is at depths of 4.74, 3.23, and 3.09 m for cases T1a, T1a–3, and T1a–5, respectively. In the 50th year, it is 9.62, 3.83, and 3.35 m, respectively. The difference between case T1a–3 and case T1a–5 is about 48 cm (Fig. 11). The permafrost table for the other scenarios is not illustrated, but the results are shown in Table 3. Table 3 shows that the permafrost table of the 50th year is 13.7, 6.92, and 5.16 m for cases T1b, T1b–3, and T1b–5, respectively. The difference between case T1b–3 and case T1b–5 is about 76 cm. It is 6.07, 2.92, and 2.72 m for cases T2a, T2a–3, and T2a–5, respectively. The difference between case T2a–3 and case T2a–5 is about 20 cm. It is 7.92, 3.69, and 3.35 m for cases T2b, T2b–3, and T2b–5, respectively. The difference between case T2b–3 and case T2b–5 is about 34 cm.
a
b
0 T1-3
T1
0
4
4
6
6
8 10 12
8
initial 1 year 50 years
14
16
T1
10 12
initial 1 year 50 years
14
T1-5
2
Depth (m)
Depth (m)
2
16 0
2
4
6
8
10 12 14 16 18
Distance (m)
20
0
2
4
6
8
10 12 14 16 18
20
Distance (m)
Fig. 9 Shape of the permafrost table for the insulated cases under boundary condition T1
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a
b
0 T2-3
2
T2
T2-5
2
4
T2
4
Depth (m)
Depth (m)
0
6 8 10 12
8 10 12
initial 1 year 50 years
14
6
initial 1 year 50 years
14
16
16
0
2
4
6
8
10 12 14 16 18
20
0
2
4
6
Distance (m)
8
10 12 14 16 18
20
Distance (m)
Fig. 10 Shape of the permafrost table for the insulated cases under boundary condition T2
a
b
0 T 1a-3
2
T 1a
T1a
4
Depth (m)
Depth (m)
T 1a-5
2
4 6 8
initial 1 year 5 years 10 years 20 years 30 years 50 years
10 12 14 16
0
0
2
4
6
6 8
initial 1 year 5 years 10 years 20 years 30 years 50 years
10 12 14
8
10 12 14 16 18
Distance (m)
20
16
0
2
4
6
8
10 12 14 16 18
20
Distance (m)
Fig. 11 Shape of the permafrost table for the insulated cases under boundary condition T1a
From Table 3, it can be seen that vegetation cover and pipe insulation can weaken to some extent or eliminate the effects of a smaller climate-warming trend on the permafrost foundation.
5 Discussion and conclusions Based on field investigations, this paper introduces the negative effect of climate, human activity, and permafrost degradation on oil pipeline construction and stability, and studies the effect of the oil pipeline, vegetation, and climate warming on the permafrost environment by the numerical method. The simulation does not consider the effect of slope or aspect, and assumes no interference by surface or underground movement of substantial quantities of water. But this does not affect the results which can provide a guide for the oil pipeline engineer and the environmental protection department. Analysis of the climate, hazards of oil pipeline, and numerical simulations yields the following conclusions:
123
5.96
6.50
6.95
7.23
7.49
Fifth year
tenth year
20th year
30th year
50th year
3.29
3.29
3.29
3.29
3.29
3.23
2.81
3
3.15
3.15
3.15
3.15
3.15
3.15
2.81
5
The unit of insulation thickness is in cm
2.81
4.74
First year
0
Insulation
Original
T1
Boundary
9.62
7.75
7.12
6.53
5.96
4.74
2.81
0
T1a
3.83
3.57
3.46
3.38
3.35
3.23
2.81
3
3.35
3.26
3.18
3.15
3.12
3.09
2.81
5
13.7
9.14
7.69
6.64
5.96
4.74
2.81
0
T1b
Table 3 Depth of the permafrost table under the oil pipeline (unit: m)
6.92
4.45
3.89
3.52
3.40
3.23
2.81
3
5.16
3.77
3.46
3.23
3.21
3.09
2.81
5
5.16
5.16
5.13
5.11
4.94
4.14
2.30
0
T2
2.87
2.87
2.87
2.87
2.87
2.87
2.30
3
2.72
2.72
2.72
2.72
2.72
2.72
2.30
5
6.07
5.65
5.39
5.16
4.91
4.14
2.30
0
T2a
2.92
2.92
2.92
2.92
2.92
2.87
2.30
3
2.72
2.72
2.72
2.72
2.72
2.72
2.30
5
7.92
6.27
5.73
5.22
4.88
4.09
2.30
0
T2b
3.69
3.21
3.04
2.95
2.92
2.87
2.30
3
3.35
3.04
2.92
2.81
2.75
2.72
2.30
5
Nat Hazards
123
Nat Hazards
(1)
(2)
(3)
(4)
The Qinghai–Tibet Plateau Corridor has been warming at a rate of 0.0287 °C/a for the past 60 years in the permafrost area of QTPEC. Human activities and climate warming have been causing a general deterioration of the permafrost environment. This permafrost degradation leads to instability of infrastructures. The human-induced damage to the natural vegetation, climate warming, concentrated precipitation, complex freeze–thawing processes in the active layer, thermokarst terrain, and other engineering projects along the QTPEC (highway, railway) have caused several kinds of hazards for the Golmud–Lhasa oil pipeline. A warm oil pipeline would lead to significant thawing of the underlying permafrost foundation. When climate warming is not considered, proper pipe insulation could keep the permafrost and pipe stable. Revegetation and pipe insulation could balance out the effect on the warm oil pipe together with a 1.1 °C/50a climate-warming trend on the underlying permafrost. Unfortunately, in the case of 2.6 °C/50a climate warming, revegetation and pipe insulation cannot maintain the stability of the permafrost foundation. This will need additional active cooling techniques, and new design criteria need to be considered. Vegetation cover is important to reduce the effect of climate warming on both the natural and human-impacted permafrost. Therefore, it is suggested to recover the vegetation after the construction to protect the permafrost environment and the oil pipeline itself.
Acknowledgments This study is supported by the National Natural Science Fund (41571070, 41461016) and the Fund of the National Key Basic Research and Development Program (2012CB026102).
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