Oil
and gas pipeline routes are pivotal pieces of information upon which pipeline
engineering depends. The route will define the pipeline size, terrain, soils,
and engineering analysis requirements. Engineering assessment based upon agreed
alignment selection criteria is an important part of a linear project. To be
able to reach the best construction line and optimise its components, the
phases – namely corridor, route, alignment, and construction line selection —
should be studied in the given order.
Selecting
the optimum route does not end with geotechnical challenges, as it also
requires interactive coordination between the owner, the engineer, the
regulator, the landowners, the construction contractor and a multitude of other
project stakeholders and interested parties.
In
North America, pipeline route selection is driven by regulatory requirements at
the federal, state and local levels and involves finding a route that minimises
the impact on the environment and archaeological artefacts and recognises the
concerns of the landowners while considering the geotechnical challenges which
affect the construction of the pipeline.
In
arctic regions like Siberia, the soil conditions are an important consideration
where areas of permafrost are interspersed with normal soils. In the permafrost
areas, the pipeline will be installed above ground on supports and the depth of
the permafrost determines the design of the supports, while in normal soil
areas the pipeline is buried in a trench in the conventional manner.
In
mountainous terrain, such as in Turkey, geotechnical considerations are a
significant aspect of pipeline route selection, as well as environmental and
landowner concerns. The pipeline design must address geohazard mitigation for
seismic areas and sections of the route which could be subject to landslides.
Geo-political
factors can also affect the route selection. Bringing Caspian Sea gas to Europe
requires, among other pipelines, a new pipeline in Europe. A northern route
requires a longer pipeline routed through environmentally sensitive areas, but
this route supports future expansion of the pipeline system’s capacity. A
southern route is shorter and reduces environmental concerns, but as this route
also involves a marine crossing, the future expansion of the pipeline system is
curtailed.
Primary
selection factors
The
detailed pipeline route selection is preceded by defining a broad area of
search between the two fixed start and end points. That is, possible pipeline
corridors. The route can then be filtered with consideration of public safety,
pipeline integrity, environmental impact, consequences of escape of fluid, and
based on social, economic, technical environmental grounds, constructability,
land ownership, access, regulatory requirements and cost.
Economic,
technical, environmental and safety considerations should be the primary
factors governing the choice of pipeline routes. The shortest route might not
be the most suitable, and physical obstacles, environmental constraints and
other factors, such as locations of intermediate offtake points to end users
along the pipeline route should be considered. Offtake points may dictate
mainline routing so as to minimise the need or impact of the offtake lines or
spurs.
Many
route constraints will have technical solutions (e.g. routing through flood
plains), and each will have an associated cost.
Corridor
selection in project key stages
Pipeline
routing is an iterative process, which starts with a wide ‘corridor of
interest’ and then narrows down to a more defined route at each design stage as
more data is acquired, to a final ‘right of way’ (ROW). Initially, a number of
alternative corridors with widths up to 10 km wide are reviewed. Each project
will have its own specific corridor-narrowing process depending on project size
and location.
Pipeline
corridors should initially be selected to avoid key constraints. The route can
then be further refined through an iterative process, involving consultation
with stakeholders and landowners and a review of the EIA criteria, to avoid
additional identified constraints. The ultimate aim is to achieve an
economically and environmentally-feasible route for construction.
Terrain,
subterranean conditions, geotechnical and hydrographical conditions
The
geography of the terrain traversed can generally be divided into surface
topography and subterranean geology. Both natural and man-made geographical
features can be considered under these two headings.
The
principal geographical features which are likely to be encountered and should
be taken into account include:
Surface:
·
Crops, livestock, woodlands;
·
Natural beauty, archaeological, ornamental rivers, mountains;
·
Water catchment areas, forestry;
·
Population, communications, services;
·
Contouring, soil or rock type, water, soil corrosivity;
·
Designated areas, protected habitats, flora and fauna.
Subterranean:
·Earthquake
zone;
·Geological
features;
·Infill land and
waste disposal sites, including those contaminated by disease, radioactivity or
chemicals;
·The proximity of
past, present and future mineral extractions, including uncharted workings,
pipelines and underground services;
·Areas
of geological instability, including faults, fissuring and earthquake zones;
·Existing
or potential areas of land slippage, subsidence and differential settlement;
·Tunnels;
·Ground
water hydrology, including flood plains.
Geo-hazards
Geo-hazards
are widespread phenomena that are influenced by geological and environmental
conditions and which involve both long-term and short-term processes. They
range in size, magnitude and effect. Many geo-hazards are naturally occurring
features and processes (e.g. landslides, debris flow, seismic activity, rock
falls, etc.) but there are also many geo-hazards that are caused by
anthropogenic processes (e.g. undermining, landfills, engineered fill,
chemistry and contamination, etc.), and these too need to be taken into account
during the pipeline routing exercise.
Geo-hazards
are identified as geological, hydro-geological or geomorphological events that
pose an immediate or potential risk that may lead to damage or uncontrolled
risk. The type, nature, magnitude, extent and rate of geological processes and
hazards directly influence pipeline route selection. Therefore, the process of
early-stage terrain evaluation and the identification and assessment of
geo-hazards and ground conditions are important as they can lead to extensive
cost and time savings in the design and construction of a pipeline.
The
process enables the routing of the pipeline through the most suitable terrain,
problem areas are identified, serious geo-hazards are avoided, where possible,
and risks are minimised and mitigated. In addition, terrain evaluation is
undertaken so that the need for expensive remedial measures or site restoration
works is limited or prevented and the operability of the pipeline is
safeguarded through a proper appreciation of the terrain conditions. By
minimizing the risk of damage to the pipeline the risk to human safety is
reduced.
Terrain evaluation
Terrain
evaluation along the pipeline corridor can be achieved using a variety of
low-cost techniques that include satellite imagery and aerial photography
interpretation, surface mapping and various other remote sensing techniques.
This data can be incorporated, together with historical data on seismic events,
geological features, meteorological processes and hydrological data, within a
geographic information system and detailed terrain and hazard models developed.
Terrain
evaluation supports the anticipation, identification and assessment of the
physical hazards and constraints within and outside of the pipeline corridor.
It is essential that features outside the corridor be evaluated, as hazardous
events outside of the corridor may be triggered by construction activity within
the corridor and the resultant event may impact upon the pipeline.
The
risks associated with geo-hazards or the likelihood of an event occurring and
its consequences can be qualitatively and quantitatively assessed using a
scoring system or by a quantitative risk assessment (QRA).
Safety
of the pipeline is paramount in the routing selection. The extreme effect of a
geological hazard on the pipeline is a rupture and it is this event that terrain
evaluation and risk analysis seeks to avoid by improving the decision-making
progress used in selecting the most appropriate route for the pipeline.
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