Pipeline Routing /Routing Selection

Route selection is a complex procedure, which can be governed by several variables. Clearly,
the shortest distance between the terminal points is likely to be the most economic from a
material standpoint, but possible overriding factors must be considered. The key principles to take into account when performing route selection are

Safety of the public and personnel - the route must provide a safe and secure environment for the pipeline during construction and over its operational life and ideally be routed away from populated areas
Economic – the route should meet the project’s economic objectives, without compromising safety and environment and minimizing local economy impact on communities that the pipeline passes through, and have the smallest footprint feasible (ideally the shortest distance between pipeline start and end points).
Land ownership related factors e.g. the number of landowners, anticipated ease and cost to obtain/purchase consents
Easement width
Contents of the pipeline and operating conditions, e.g. consideration of leakage of a high vapour pressure liquids.
Environmental impact – the route must have a minimum negative impact on the environment and minimum land use
Terrain and subterranean conditions, including geotechnical, hydrographical, and meteorological conditions. This includes ground stability, including other land uses which may create instability (e.g. mine subsidence, land development/excavation)
Cultural heritage sites
Existing and future land plan usage. This can be determined by research of public records and consultation with land planning agencies which should identify:
- third-party activities
- agricultural practice
- existing facilities and services
- future developments
Existing and planned transport facilities and buried/above ground utility services
Construction, testing, operation and maintenance - the pipeline must be installable along the route
Permanent access – the pipeline must ideally be accessible for inspection and maintenance all year round over its operational life
Security – The pipeline system should be routed to minimise security concerns, particularly due to trespass and sabotage, during both construction and operation.
Other hazards
Follow existing linear disturbances where possible (roads). Use of existing linear routes (e.g. roads or power-lines) may avoid or reduce impact to sensitive areas. Although using routes occupied by other infrastructure may affect safety and corrosion potential from for example electrical interference.

Pipeline route selection

Source: http://www.piping-engineering.com/wp-content/uploads/2014/07/pipeline-route-selection.jpg

Fabrication, Installation and Operational Cost Considerations

A significant proportion of the total cost to install a pipeline which is directly affected by the chosen route is incurred during fabrication and installation. The associated activities are:

Length of fabricated pipeline pipe (coated);
Pre sweeping of route;
Pre lay installed freespan correction supports;
Post lay installed freespan correction supports;
Trenching, burying or rock dumping.

Some or all of these activities will be present within the selected pipeline route. As a general rule the design should be performed to:

Minimize length of pipeline required;
Avoid requirement for presweeping;
Avoid pre-lay installed freespan correction supports;
Minimize post-lay freespan correction supports;
Minimize trenching, burying and rock dumping.

Route Optimization
Optimization of pipeline routing is usually not performed as the route probably has no obstruction, is in an accessible water depth and the seabed topography is flat: Hence a straight line between the two termination points would suffice. However, on seabeds with onerous terrain significant savings on fabrication and installation costs can be made if route optimization is performed.

To perform a route optimization, reasonably accurate costs for the following activities are required:

Supply of additional pipeline pipe/unit length;
Presweeping a corridor/unit length, including cost of reduced lay rate due to a smaller lay corridor;
Prelay freespan correction supports (each), again including cost of reduced layrate due to smaller lay corridor;
Post lay freespan correction supports (each);
Trenching, burying and rockdumping/unit length (for each).

Based on the derived costs, a total cost for each route can be derived.It is worth noting that the optimization cannot be completed until all the pipeline design parameters are finalized (for instance the number of freespan correction supports will not be known until the allowable freespan has been determined).

Public safety, content of the pipeline, operating conditions and location class

The main operating conditions in pipelines that can affect route selection are:

The internal fluid
Operational envelope
Location
Pipeline material, diameter and thickness

Various codes categorise fluid as to their hazard potential, and the most hazardous flammable and toxic fluid should, where practicable, avoid built-up areas or areas with frequent human activity. Consideration should be given to routing that minimises the possibility of external damage in these areas.

The pipeline route should be an appropriate distance from buildings in accordance with the codes being used. Codes also use a system of area or location classification based on population densities or number of buildings. Design factors are stipulated relevant to the classification levels.
Pipeline material, diameter and content, affect the probability of failure and associated consequences:

Pipe fracture
Maximum rate of release of contents
Change of state of the fluid under atmospheric conditions
Total volume that can escape under emergency conditions

The consequential impact of the above should be considered in the routing process, and ensuing QRA and risk and pipeline threat assessments .

Source:
Subsea-Pipelines and Risers - Yong Bai
http://wiki.iploca.com/display/rtswiki/Appendix+5.1.1+-+Pipeline+Route+Selection+Process

Subsea Pipeline Engineering