Synonyms

Conduit; Pipeline network; Tube

Definition

A long system of pipes, typically metal but which can be made of plastics or composite fibers, used to convey natural gas, oil, water, and related liquids across long distances in isolation from the environment, either above or below ground (but the latter is most common), and in which flow is controlled by a series of valves, pumps, and control devices along the network.

Introduction

The global pipeline network is vast, covering millions of kilometers of geology and terrain while transporting essential liquid goods within and between nations and to users around the world. In doing so, pipelines are exposed to more geological conditions than almost any other infrastructure resource, except perhaps roads and railways. In urban environments, pipes and pipelines distribute everything from water and sewage to gas. Pipe segments convey water beneath roadways as culverts and perform a critical part of the storm water drainage systems of a city.

Damage to any part of a pipeline system can have dramatic economic, environmental, and social impacts, threatening the viability for what remains, at least at present, a critical human infrastructure.

Engineering geology and geomorphology help to reduce threats to pipelines at every stage of their life cycle, from prefeasibility and routing, through operations and even deactivation, by delivering a deeper understanding of the ground conditions and processes acting on the network of pipe.

Prefeasibility

At prefeasibility stages, a pipeline or utility company must decide whether or not there is an economic route between source and destination to build a pipe. Considerations are increasingly social and environmental; however, engineering geology helps define and characterize the types of ground that various route options will traverse, and the relative geological challenges along those routes. Geology and geomorphology are interpreted in a general manner to provide route data related to:

  • Topography (where more relief in a shorter distance generally increases the challenge of building a pipeline)

  • Depth and type of bedrock (where harder bedrock close to the surface means expensive excavations)

  • Seismic conditions (does the route cross active faults)

  • Existence of landslides (ground movement presents significant challenges to the construction and operation of pipelines)

  • Existence of soluble bedrock (subsidence and cavities present significant challenges to the construction and operation of pipelines and present unique challenges to the environment should a release occur at any time)

  • Existence of permafrost and thermokarst terrain (where the potential to change existing conditions by melting of ground ice presents significant challenges to the construction and operation of pipelines)

  • The type and nature of rivers (particularly related to the potential depth of the active bed, the lateral and vertical stability, and the height and composition of the banks on either side). Sometimes at the prefeasibility stage it is sufficient to know how many major rivers must be crossed

  • The depth and general properties of soil and rock (particularly where soil may be aggressive and increase corrosive action on the pipeline, or where soil properties combined with topography, may present ground movement hazards – in cold regions this may include frost heave)

  • The amount of water bodies (including muskeg and organic soils) to be crossed that might require special treatment and equipment during construction

The engineering geologist may be asked to consider several route options and to provide an objective comparison between those routes and the challenges they present for environmental impact assessments. Specific types of constraints tend to be grouped to portions of the landscape, and comparisons may be made by creating geological and geomorphological models of the constraints and conditions that may be faced (Figs. 1 and 2). The routes are mapped according to the amount of time in each of the geological and geomorphological archetypes, and in this manner, they can be compared. Geological and geomorphological challenges are typically overlaid with social, biological, and other environmental studies to determine the ultimate path.

Pipes/Pipelines, Fig. 1
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Geomorphological models of terrain in (a) arid wadis and (b) a plains-type river valley

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Pipes/Pipelines, Fig. 2
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Geomorphological models of two types of terrain in northern latitude mountains. Figure modified from Guthrie (2011)

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Feasibility and Design

At the feasibility and design stages, considerably more detail is required to determine the nature of the ground to be crossed and how it might affect the pipeline. Typically, detailed terrain and hazard mapping is conducted to refine the route and provide specific information about the type and nature of constraints to pipeline construction or operations along the way. This information may include:

  • A landslide inventory that includes the type and nature of landslides crossed by the pipeline (or proposed to be crossed by the pipeline)

  • Seismicity of the region including presence of faults, amount of fault displacement, potential for liquefaction, and dynamic ground motion

  • Erosion potential related to the right of way, construction back fill, and subsurface (piping) erosion

  • Uplift displacement (frost heave)

  • Characteristics and extent of ground ice (permafrost) and thermokarst hazards

  • Potential geochemical concerns such acid rock drainage and/or karstification

  • Watercourses and associated hazards (scour, bank erosion and migration)

  • Unique soil structures such as boulders/cobbles in soil mass, presence of competent bedrock, or sensitive soils

  • Hydrotechnical hazards including scour, bank erosion, channel migration, and buoyancy

Detailed maps are normally produced along the route (Fig. 3) and spatial data are correlated to data gathered in subsurface investigations (geotechnical boreholes). In urban areas, a comprehensive or very targeted subsurface investigation is typical.

Pipes/Pipelines, Fig. 3
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Detailed mapping for a pipeline route. Terrain mapping (a) provides fundamental information about the type and nature of ground crossed by a pipeline. Codes are correlated to geotechnical and engineering properties of the soil. Hazard or Constraints mapping (b) provides specific construction advice based on constraints encountered on the route

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Additional geotechnical analyses may be required depending on the results of the surface and subsurface investigations. Such analysis often includes detailed slope models and factor of safety analysis under a variety of conditions. It may also include developing soil-spring interaction models with the pipeline engineers to better predict the performance of pipe under different conditions (grade, wall thickness, pressure, etc.).

Constraints at major watercourse crossings are increasingly being solved using trenchless techniques (such as horizontal directional drilling). Here geotechnical engineering provides critical subsurface information about the conditions along the expected drill path. Data come from geotechnical drill holes and geophysical surveys are combined to give a clear picture of the subsurface conditions. Using these data, a construction crew can select the best boring method, drill bits and path, avoiding hydraulic fracturing, construction delays, cost overruns, and even failure to successfully complete the bore (Fig. 4).

Pipes/Pipelines, Fig. 4
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Subsurface investigation for a trenchless crossing of a river (also avoiding steep slopes on the left bank). Data from geotechnical boreholes (see column in left third of picture) are combined with geophysics to provide subsurface conditions along the drill path

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Finally, traditional geotechnical inputs to foundation design provide support to the construction of facilities built to process and manage gas and liquids moved by pipelines.

Operations and Maintenance

Pipelines have a limited design life; however, like many human structures, they frequently persist and perform beyond the original intended years. They are impacted over their lifespan by a multitude of factors that impact their integrity. Those factors include corrosion, metal loss, physical damage, and, importantly for engineering geology, ground movement.

Ground movement may be a result of frost heave, liquefaction, landslides, river scour (the movement of the active portion of the river bed), chemical weathering, or melting of ground ice. Whatever the reason, ground movement may induce sudden strain on a pipeline and threaten its overall integrity. Given the vast distances covered by pipelines, geotechnical and hydrotechnical hazards are common.

Pipeline failures related to ground movement (in this case, failure is understood to mean an unintended condition where the pipeline no longer isolates its contents from the surrounding environment) occur at a rate between 0.02 and 0.03 failures/1000 km/year in North America and Europe, up to an order of magnitude higher in South American countries and up to an order of magnitude less in Australia (Porter et al. 2016). Not surprisingly, most of the reported failures are related to slopes, and statistics are as much a reflection of geological conditions as they are of practice.

Engineering geologists may be asked to examine moving or potentially unstable ground to determine the actual threat to a pipeline. These investigations can be substantially more detailed and focused than investigations at a routing stage because they relate to a clearly defined problem and location.

Desktop studies include a review of the original terrain mapping or geomorphology, surficial geology, bedrock geology, and where available, the pipeline design documents. Updated aerial photograph, satellite imagery, LiDAR, and InSAR data may be available for interpretation providing a substantially superior understanding of ground conditions than were available when the pipeline was originally put into the ground (Fig. 5). Similarly, engineering geologists will work with integrity engineers to understand the results of in-line inspections that use inertial measurement units to detect bending strains and their locations on the pipeline itself.

Pipes/Pipelines, Fig. 5
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Aerial photograph (a) and LiDAR (b) image of a pipeline right of way that crosses a large old landslide

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For slopes or subsidence, a field program may include geotechnical drill holes, field inspections for and surveys of characteristic features of ground movement such as tension cracks, head scarps, transverse scarps, back tilted blocks or rotated ground, toe bulges, lateral shear zones, horst-and-graben features, isolated ponds, caves or lesser karst features, and disturbed vegetation.

For rivers, a field program may include geotechnical drill holes (on banks), field assessment of river morphologies and processes, assessments of vertical and lateral stability, and assessment of bank erosion hazard.

Probability of failure is often estimated based on the data gathered whereby the engineering geologist determines the likelihood that observed ground movement will induce a strain on the pipe (bending strain, vortex induced vibration, impact force, etc.) and the integrity engineers then identify the amount of strain that the specific pipe can handle (usually as a measure of elastic strain and total strain).

As technology and understanding improves, pipeline operators are increasingly focused on widespread geohazards programs, which aim to find and prioritize threats to existing lines before they become acute. There is, therefore, an operational component of pipeline integrity that is informed by engineering geologists and is similar to the feasibility and design stages described above. The fundamental difference is that, in this case, the pipeline is already fixed in place, and accurate identification of hazards (and possible mitigations) is critical.

Mitigation

The mere existence of a hazard or potential hazards does not necessarily mean that a pipeline will fail. Engineering geologists are therefore often asked to provide input to mitigation options. For very slow, episodic slopes, or slopes where movement is uncertain such as the case with very old landslides detected using LiDAR (Fig. 5), instrumentation or a repeatable monitoring program is often useful.

Monitoring is traditionally completed by surveying known points repeatedly through time or by installing slope inclinometers, slope accelerometer arrays, piezometers, extensometers, or other similar instrumentation. Ground instrumentation can be connected to emergency shut-down devices as required.

Increasingly, monitoring is conducted using differential LiDAR analysis and InSAR image stacks. These technologies allow much broader spatial coverage of the ground surface, and their use is likely to increase with time.

For ground that moves intermittently, monitored sites provide data that can be input into time to failure analysis (Voight 1989; Borsetto et al. 1991), with thresholds related to the actual strength of the pipe.

Watercourse monitoring may similarly include repeated ground investigations, the installation of erosion pins, and surveys of depth of cover compared to the predicted behavior of the river. Stream gauges may be installed and tied to emergency shut-down devices that close pipe valves in the event that flows are particularly high.

Mitigation options begin to fall into the exclusive realm of geotechnical and civil engineering, but engineering geologists are often part of the mitigation team and provide critical inputs and data about the ground conditions, the observations, and the processes that threaten the pipe.

In urban areas, where mitigation often means excavations, engineering geologists are still used to provide critical input and data about ground conditions.

Summary

Pipelines extend across millions of kilometers of ground, intersecting a wide variety of conditions, terrain, geological, and geomorphological processes along the way and are essential infrastructure in urban areas. Engineering geology provides fundamental understanding of the nature and characteristics of this ground, necessary to safely and effectively build, operate, and maintain a pipeline.

Cross-References