Impact of hydro-thermal behaviour around a buried pipeline in cold regions
Introduction
In cold regions, buried pipeline systems, which should be installed and operated in the permafrost/frost-susceptible areas, keep the most economical and efficient ways for the transportation of the natural oil and gas. During the process of frost heaving and thaw settlement of underlying soils along a buried pipeline, the impact of the temperature gradient can cause significant migration of the soil moisture content or growth of the cryogenic suction of soils, leading to temperature and water content redistribution in the surrounding soils. Basically, the impact of hydro-thermal behaviour around the buried pipeline is completely governed by the thermal interaction between the ambient surroundings and pipelines. When the complicated hydro-thermal process is determined by considering the boundary condition variations and pipe-soil thermal interaction, the evolution of the temperature field and moisture field can be evaluated, aiding in assessment of the hydro-thermal phenomenon impact on the stability performance of the surrounding soils.
According to previous investigations, there are three important factors to cause frost heave: (1) soil type, which can be frost-susceptible soil, (2) water supply, which is the material source for frost heave in the surrounding soils, and (3) subfreezing temperature, which can be able to drive the propagation of frost heave (Xu et al., 2001; Andersland and Landanyi, 2004; Lai et al., 2009). If one of these three conditions is not met, frost heave will not occur. Generally, when temperature is below the freezing point of soils, the pore water in the surrounding soils will just freeze in situ, and ice-water phase change will occur. In fact, ice lenses around a buried pipeline are formed when enough water migrates to the freezing front and it causes excessive deformations in the surrounding soils. For example, when the temperature of medium transported by pipelines is below the freezing point of the surrounding soils, temperature difference will be formed and cause a strong heat exchange between the pipeline and surrounding soils. However, frost bulb will form in the surrounding soils, and frost heave will occur, and then trigger the buried pipeline to move upward, resulting in buckling instability of the buried pipeline in cold regions.
Generally, the pore water in the pores of the surrounding soils freezes into ice with the increasing of volume by approximately 9% and leads to frost heaving, which is followed by thawing of pore ice and weakening of underlying soil shear strength, thus leading to thaw settlement (Xu et al., 2001; Andersland and Landanyi, 2004; Lai et al., 2009). That is, unfrozen water in the surrounding soil pores migrates towards the freezing front by temperature gradient, which can cause the occurrence of frost heaving in the ground, as well as the thawing of permafrost beneath the pipe. Excess pore water pressures are generated that weaken the bearing capacity of the surrounding soils and increase buoyancy forces around the pipe. This kind of freezing and thawing action can potentially cause many engineering problems involving fracturing and buckling of pipelines, cracking and settlement of pavement and damage to building foundations (Dallimore, 1985; Rajani, 1992; Nixon and Burgess, 1999; Huang et al., 2004; Xu et al., 2010; He and Jin, 2010; Xu et al., 2018; Li et al., 2019; Wang et al., 2019a).
Engineering practices, such as Norman Wells Oil Pipeline (NWOP) (Nixon and Burgess, 1999; Oswell, 2011), Trans-Alaska (Alyeska) Pipeline System (TAPS) (Huang et al., 2004; Jin and Max, 2005), China-Russia Crude Oil Pipeline (CRCOP) (Xu et al., 2010; Wang et al., 2016; Wang et al., 2019a, Wang et al., 2019b) and Golmud-Lhasa Oil Pipeline (GLOP) (He and Jin, 2010) have shown that frost heaving and thaw settlement caused by temperature differences, which triggers instability and buckling of the pipelines, are dynamic and complicated hydro-thermal interaction processes. The uplift buckling of the Norman Wells pipeline at KP 5.2 caused by the temperature differences and initial pressure, and the pipeline at approximately KP 5.2 is currently experiencing upward displacement along a 25-m short length of pipe (Nixon and Burgess, 1999), as shown in Fig. 1. However, Li et al. (2019) summarized and analysed the influences factors and instability mechanism of a buried pipeline in cold regions. In addition, periglacial geohazards, involving frost heaving and thaw settlement, surface erosion and landslides, lead to buckling deformation and even disruption of the buried pipeline. Crude oil spilled from the buried pipeline triggers environmental pollution and in turn causes economic losses and geotechnical problems. To a large extent, the original hydro-thermal balance around a buried pipeline was disturbed by construction of the pipeline. Most notably, the temperature and moisture redistribution in the surrounding soils was subjected to the impact of the thermal interaction between the ambient surrounding and pipeline. However, the buried pipeline has some degree of damage due to the effects of the freezing and thawing cycles. In addition, this cycle can lead to fracturing and damage of the buried pipeline, and this kind of engineering disaster may threaten safe operations and even shorten the service life cycle of buried pipelines in cold regions. Therefore, frost heave and thaw settlement of the surrounding soils are a key factor in determining the stability and safety of a pipeline. Hence, it is crucial to solve this kind of engineering problem in the soils surrounding buried pipelines in cold regions.
Thus far, many researchers have focused on theory and engineering application investigations regarding buried pipelines, and numerical studies have been conducted to better understand the mechanical behaviour of buried pipelines due to frost heaving and thaw settlement in cold regions (Foriero and Ladanyi, 1995; Lai et al., 1999, Lai et al., 2009; Yu et al., 2014; Li et al., 2018a; Wang et al., 2019a). Hydro-thermal (TH) coupled models are used to predict and estimate the frost heaving and thaw settlement of soils surrounding pipelines (Carlson and Nixon, 1988; Nixon, 1990, Nixon, 1991, Nixon, 1992; Foriero and Ladanyi, 1995; Lai et al., 1999; Yu et al., 2014; Di et al., 2016; Haxaire et al., 2017; Ma et al., 2018). Konrad and Morgenstern (1984) developed a coupled heat transfer model between saturated soils and pipelines by using a segregation potential concept to explore the effects of pipeline heat conduction characteristics. Lai et al. (1999) developed a mathematical model of the temperature, seepage and stress fields based on the theory of heat transfer, seepage flow and frozen soil mechanics to predict and estimate the mechanical behaviour of a tunnel lining. Selvadurai et al., 1999a, Selvadurai et al., 1999b adopted a semi-coupled thermo-hydro-mechanical model to investigate the behaviour of a buried pipeline in a discontinuous frozen heave region. Kim et al. (2008) used a quasi-two-dimensional explicit finite difference model based on a segregation potential concept to predict pipe displacement. Nishimura et al. (2009) improved a fully thermo-hydro-mechanical coupled model based on a unified effective-stress framework that considers freezing and thawing in water saturated soils to simulate the thermal, hydraulic and mechanical fields around a pipeline. Wen et al. (2010) developed a simple thermal elasto-plastic finite element computational model based on empirical frost heaving and thaw settlement coefficients to calculate the stress and deformation behaviours of a pipeline; the researchers proposed that for buried pipelines in cold regions, frost heaving hazards should receive more focus than thaw settlement hazards. Yu et al. (2014) used a heat-moisture coupled model to investigate the influence of a warm oil pipeline on the stability of a permafrost foundation and proposed that the 8-cm-thick insulation layer was sufficient to stabilize the permafrost foundation based on numerical studies. Zhao (2014) developed a thermo-hydro-mechanical coupled model based on a porosity function to simulate frost action during a full freeze-thaw cycle in frost-susceptible soils and adopted this model to simulate heaving and thawing of soils around a pipeline. Bekele et al. (2017) used an isogeometric analysis (IGA) based on numerical models to simulate thermal, moisture and mechanical fields around a pipeline in freezing ground, and Haxaire et al. (2017) developed a constitutive model describing the mechanical behaviour of frozen soils as a function of temperature to simulate the thermal interaction between the pipeline and underlying soils by a geotechnical finite element code applied to a practical thermo-hydro-mechanical boundary value. Wang et al. (2019b) designed and established along the China-Russia Crude Oil Pipeline (CRCOP) in-situ monitoring system used for monitoring ground temperature and water content in the surrounding soils, vertical settlement of pipeline as well as frost bulb and thaw bulb around the pipelines. These studies have been undertaken to either develop new coupled thermo-hydro-mechanical models or to improve already existing models to evaluate and predict heat transfer characteristics between buried pipeline and surrounding soils.
To some extent, these theoretical, practical and numerical investigations could provide some references for the design and maintenance of buried pipelines in cold regions. However, several noticeable characteristics and phenomena exist in the freezing and thawing processes around the buried pipeline involving water migration, ice-water phase transitions, and temperature-related physical parameters, especially the heat-moisture convection process during heat transfer; the thermal insulation layer and temperature boundary variation parameters were simplified or neglected in previous studies. Furthermore, it is vital to understand the thermal conductivity of the thermal insulation layer for the design and construction of buried pipelines in cold regions. Unfortunately, the relationship among the thermal insulation effects, thermal conductivity and thickness as well as temperature boundary conditions were not established in those previous studies. In this paper, we first established a numerical hydro-thermal coupled model considering latent heat caused by ice-water phase transition, heat convection of pore water and thermal interaction between buried pipelines and ambient surroundings to investigate freezing and thawing actions. Then, the relationship among the thermal insulation effects, thermal conductivity and thickness as well as temperature boundary condition is obtained from the results of the coupled model computations and could be used to predict and estimate the effects of these parameters. Subsequently, moisture migration of pore water in the surrounding soils and the thermal interaction between the pipeline and ambient surrounding soils with temperature boundary condition variations in cold regions were simulated for 50 years to evaluate the hydro-thermal behaviour of soils surrounding the buried pipeline. This study could aid in better understanding the complicated dynamic freezing and thawing interaction processes and their effects on buried pipelines and may serve as a reference for the design of a thermal insulation layer for buried pipelines in cold regions.
Section snippets
Mathematical model and equations
Generally, the soils surrounding a buried pipeline can be isotropic and elastic porous medium and described as a multiphase continuous porous medium that are fully frozen, partially frozen or unfrozen. Currently, for the convenience and simplification of research, gaseous phase and dissolved salt are not considered in the unfrozen condition, and the components are assumed to be composed of pore water (liquid phase) and solid grains (solid phase). The partially frozen soil is considered to
Model verification
Lai et al. (2014) performed five groups of one-sided freezing experiments for silty clay columns in an open system. Three samples are 10-cm in height and 10-cm in diameter, and two samples are 20-cm in height and 20-cm in diameter. In these experiments, the top plate (cold end) and bottom plate (warm end) temperature boundaries are controlled by circulating alcohol liquids through the test chamber, and the no-pressure water supplement system is connected with the bottom plate through a plastic
Numerical results and analyses
After the buried pipeline is installed and constructed, the original freeze-thaw state of the surrounding soils will be disturbed and change under continuous coupling effect of crude oil/gas and air temperature. The thermal regime and water filed in the surrounding soils redistribute with variations of heat convection and heat transfer among air, crude oil/gas and surrounding soils. Therefore, the thermal insulation layer of the pipeline, which should be used as an economical and effective
Discussion
In this study, the hydro-thermal coupled model is established, which can improve the thermal variation and moisture distribution simulation results of surrounding soils in cold regions much better than a single heat transfer model. However, some issues remain to be discussed to investigate the hydro-thermal behaviour impact of the surrounding soils along the pipeline. First, based on the aforementioned investigations and results, the thermal insulation layer can effectively protect the pipeline
Conclusions
In this paper, as a common pipeline engineering problem, buried pipelines in cold regions are severely damaged by freezing and thawing actions. To avoid frost damage, a hydro-thermal coupled model for buried pipelines in cold regions is established. Based on a series of simulations and investigations of the hydro-thermal behaviour impact of the surrounding soils on a buried pipeline in cold regions, the relationship among the thermal insulation effect, thermal parameters and thickness, and
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
This research was supported by the National Key Research and Development Program of China (Grant No. 2018YFC0809605, 2018YFC0809600), the National Natural Science Foundation of China (Grant No. 51668055), and the Open Fund of State Key Laboratory of Frozen Soil Engineering of China (Grant No. SKLFSE201708), and the First Division Aral Science and Technology Project of China (Grant No. 2019GJJ02).
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