Impact of
Permafrost
Permafrost and Pipelines
Permafrost and other Facilities
This section reviews several of the key issues raised by the presence of permafrost when a pipeline or other structure is built in boreal environments. Both the cause of the issue and potential methods to minimize its impact are discussed.
Permafrost and Pipelines
Permafrost can have a variety of impacts on pipeline construction. This is because any interaction between the permafrost and the pipeline can change the equilibrium between the permafrost and its environment.
For example, heat may be transfered from the pipeline into the permafrost causing the permafrost to thaw. If the permafrost is ice-rich, it can shift and subside. The resulting stress can bend, wrinkle, or potentially rupture the pipeline. In order to minimize the risk of this occuring, several steps can be taken. The first of these is to chill the natural gas to 0°C. This minimizes the thermal gradient between the pipeline and the surrounding permafrost and thus reduces the transfer of heat. Secondly, the pipeline trench may be backfilled with coarse gravel. Because of the large amount of air that would be trapped in the spaces between the gravel, the gravel acts to insulate the permafrost from the effects of the pipeline. Finally, the pipeline can be anchored in place using large - up to ten metric tonne - concrete weights or an overlaying earthwork berm (Williams, 1986). Due to their immense size and weight, these achors hold the pipeline in place even if the surrounding ground shifts somewhat.
A second important consideration is drainage. In areas characterized by ice-rich permafrost, if the permafrost thaws, pockets of moisture may become trapped against the pipe (Harris, 1986).This water could then facilitate corrosion and pitting of the pipe. Again, the end result is a loss in pipeline integrity, i.e. leaks. To reduce this danger, several steps can be taken. For example, the exterior of the pipe is commonly coated with a hydrophobic epoxy resin. This coating prevents the water from coming into direct contact with the pipe (Kennedy, 1984). Additionally, the trench may be filled with material, such as gravel, that will facilitate the drainage of water way from the pipe.
engineers to locate
bending, corrosion, or other damage to the pipeline which could have a
detrimental impact (Natural Gas Supply Association). This damage can then be repaired promptly.
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Permafrost and other Facilities
Beyond stringing and maintaining the pipeline, a variety of other facilities are often needed when building a natural gas pipeline system. These include buildings such as the Gathering and Compression Stations, roadways, and airstrips to facilitate the movement of both people and equipment into and out of the area (Mackenzie Gas Project). Permafrost is a consideration when building these structures as well.
One of the chief issues when erecting a building in boreal regions is how will the building impact the permafrost. If the foundation does not properly insulate the underlaying ground from the building, the permafrost could thaw and shift
Frost heave can also be an issue when building pave roadways or airstrips. And, as with buildings, insulating the road from the permafrost beneath it is the most common remediation technique (Brown, 1970). This can be accomplished several different ways. For example, a layer of foam isulation may be sandwiched between the paved surface and the underlying substrate. But, because this can be quite costly, it is common to again use a layer of coarse gravel. The road is then paved over this layer of gravel that is at least one meter thick.
If the road is unpaved it is most economical simply to grade the road frequently in order to ensure it is level and passable.
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Permafrost and Pipelines
Permafrost and other Facilities
This section reviews several of the key issues raised by the presence of permafrost when a pipeline or other structure is built in boreal environments. Both the cause of the issue and potential methods to minimize its impact are discussed.
Permafrost and Pipelines
Permafrost can have a variety of impacts on pipeline construction. This is because any interaction between the permafrost and the pipeline can change the equilibrium between the permafrost and its environment.
For example, heat may be transfered from the pipeline into the permafrost causing the permafrost to thaw. If the permafrost is ice-rich, it can shift and subside. The resulting stress can bend, wrinkle, or potentially rupture the pipeline. In order to minimize the risk of this occuring, several steps can be taken. The first of these is to chill the natural gas to 0°C. This minimizes the thermal gradient between the pipeline and the surrounding permafrost and thus reduces the transfer of heat. Secondly, the pipeline trench may be backfilled with coarse gravel. Because of the large amount of air that would be trapped in the spaces between the gravel, the gravel acts to insulate the permafrost from the effects of the pipeline. Finally, the pipeline can be anchored in place using large - up to ten metric tonne - concrete weights or an overlaying earthwork berm (Williams, 1986). Due to their immense size and weight, these achors hold the pipeline in place even if the surrounding ground shifts somewhat.
A second important consideration is drainage. In areas characterized by ice-rich permafrost, if the permafrost thaws, pockets of moisture may become trapped against the pipe (Harris, 1986).This water could then facilitate corrosion and pitting of the pipe. Again, the end result is a loss in pipeline integrity, i.e. leaks. To reduce this danger, several steps can be taken. For example, the exterior of the pipe is commonly coated with a hydrophobic epoxy resin. This coating prevents the water from coming into direct contact with the pipe (Kennedy, 1984). Additionally, the trench may be filled with material, such as gravel, that will facilitate the drainage of water way from the pipe.
 |
In addition to the steps that can be taken
at the time of construction, careful maintenance of the pipeline is
important to minimizing the impact of the pipe on the permafrost. This
regime includes aerial and ground-based inspections of the pipeline and
the periodic inspection of the pipeline using Pipeline
Inspection Tools known as PIGs. PIGs are robotic devices that
travel
through the pipeline and, using a combination of ultrasonic and
magnetic |
| A Pipeline Inspection Tool or
PIG. Photo courtesy of the Natural Gas Supply Association. |
sensors, detects pipeline orientatation,
wall thickness, and other characteristics. This information can then be
used by |
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Permafrost and other Facilities
Beyond stringing and maintaining the pipeline, a variety of other facilities are often needed when building a natural gas pipeline system. These include buildings such as the Gathering and Compression Stations, roadways, and airstrips to facilitate the movement of both people and equipment into and out of the area (Mackenzie Gas Project). Permafrost is a consideration when building these structures as well.
One of the chief issues when erecting a building in boreal regions is how will the building impact the permafrost. If the foundation does not properly insulate the underlaying ground from the building, the permafrost could thaw and shift
| unevenly. In the past, this was most
commonly dealt with by placing the building atop wooden pilings. While
this method is still used, it is more common, especially for largerer
buildings, to use a heavy concrete pad which typically exceeds 30
cm in thickness. This thick steel-reinforced concrete minimizes the
flow of heat from within the building into the ground where it could
degrade the permafrost. If necessary, this pad |
 |
| might overlay a gravel pad that can exceed
one meter in thickness. The gravel, as it does when used during
pipeline construction, prevent the flow of heat to the permafrost.
However, because gravel can occasionally shift, it needs to be replaced
periodically (Brown, 1970). |
This building in Dawson City, YK
shows clears signs of settling due to the uneven thawing of the
underlying Permafrost. Photo courtesy of Joan and Uell Hauser. |
Frost heave can also be an issue when building pave roadways or airstrips. And, as with buildings, insulating the road from the permafrost beneath it is the most common remediation technique (Brown, 1970). This can be accomplished several different ways. For example, a layer of foam isulation may be sandwiched between the paved surface and the underlying substrate. But, because this can be quite costly, it is common to again use a layer of coarse gravel. The road is then paved over this layer of gravel that is at least one meter thick.
If the road is unpaved it is most economical simply to grade the road frequently in order to ensure it is level and passable.
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