Compressor and Pumping Stations

To ensure that the fluid flowing through any one pipeline remains pressurized, compression

(for gas) or pumping (for liquids) is required periodically along the pipe. This is

accomplished by stations, usually placed at 40 to 100 km intervals along the pipeline.  

On gas pipelines, turbine compressors gain their energy by using up a small proportion of the

natural gas that they compress. The turbine itself serves to operate a centrifugal compressor

that compresses and pumps the natural gas through the pipeline. Some compressor stations are

operated by using an electric motor to turn the same type of centrifugal compressor/pump.

This type of compression does not require the use of any of the natural gas from the pipe,

however it does require a reliable source of electricity nearby. Reciprocating natural gas

engines are also used to power some compressor stations. On gas pipelines, in addition to compressing natural gas, compressor stations also usually contain some type of liquid separator, much like the ones used to dehydrate natural gas during its processing. Usually, these separators consist of scrubbers and filters that capture any liquids or other undesirable particles (rust for instance) from the natural gas in the pipeline. Although natural gas in pipelines is considered ‘dry’ gas, it is not uncommon for a certain amount of water and hydrocarbons to condense out of the gas stream while in transit. The liquid separators at compressor stations ensure that the natural gas in the pipeline is as pure as possible, and usually filters the gas prior to compression. 

 

Metering Stations

Metering stations are placed at take offs and at the boundary between states. These stations

allow pipeline companies to monitor and manage the products in their pipes. Essentially, these

metering stations measure the flow of product along the pipeline, and allow pipeline

companies to ‘track’ the product as it flows along the pipeline. These metering stations

employ specialized flow meters to measure the product as it flows through the pipeline,

without impeding its movement. Metering stations may be controlled by the state authorities for fiscal duties purpose. 

 

What to look for?

This section describes briefly each pipeline construction step and for each one gives

indications for its inspection.

 

Design

As for any other inspection, the inspector uses the approved design and specifications as a

reference for his inspection. Non conformities may only be raised against these requirements.

Before starting any inspection it is important to examine carefully all the requirements of the

design and verify that the contractor uses the last revision of these documents.

Among the many drawings issued for a pipeline construction, a special attention shall be

brought to the typical drawings.

 

The yards

Pipeline construction starts from the yards. This is where the construction equipment and the

pipes arrive, are stored and sometimes prefabricated and from where they are distributed.

Pre-construction survey

Before construction begins, surveyors check for environmental features along proposed

pipeline segments. Utility lines and agricultural drainages are located and marked to prevent

accidental damage during pipeline construction. This is done not only on the pipeline route

but also on all the site accesses and any other working area. 

 

Surveying and stacking the right-of-way (ROW) 

The right-of-way is a narrow strip of land that contains the pipeline(s) and is where all

construction activities occur. Prior to any construction, it is surveyed, cleared of brush and

trees, and levelled to give workers and equipment access to build, inspect and maintain the

pipeline. The route is surveyed and the proposed centreline staked. The outer boundaries of the

construction corridor are staked also (stacks have different colours, typically with a 50m

spacing). The proposed centreline of the pipeline is not in the centre of the construction right-

of-way, but offset to one side. The overburden (excavated material) will be placed on the narrow side of the construction corridor. On the wider side, there is room for two vehicles to pass and a work area for laying and welding the pipe. Depending the type of terrain crossed by the pipeline, the ROW does not have the same width along all the route. The width also largely depends on the diameter of the pipe, as this diameter conditions the size of the construction equipment. Typical widths are from 6m for a 3” pipeline to 20m for a 24” line. At the end of the construction, a narrow band of the ROW may be kept for the maintenance of the pipeline. This is generally not the case in valuable agricultural land. 

 

Right-of-way Preparation 

 

Temporary Fence installation 

A fencing crew follows the staked centreline of the pipeline, and installs temporary gates and

fencing where the right-of-way crosses a landowner’s fence. Prior to cutting a fence, it should

be braced at the boundaries of the construction corridor for a minimum width that would

allow construction equipment to pass prior. The bracing allows a fence to be cut and still

maintain integrity of the overall fence. Temporary gates are installed across the width of the

construction right-of-way to allow the ditch to be excavated, as well as provide room for the

pipe and construction equipment to pass. 

 

Timber Clearing 

The right-of-way crew clears the right-of-way of all shrubs and trees. Smaller timber are

properly disposed of or cut and stacked on the right-of-way for use by the landowner for

firewood, if appropriate. Special attention is brought to merchantable timber if this has not

been taken care of by the landowner prior the construction begins.

 

Clearing and grading

After temporary fencing and timber clearing have been accomplished, a crew removes

stumps, shrubs, topsoil and small trees. When the work is done along a hillside, the topsoil

should normally be placed on the uphill side to prevent mixing with other excavated material

during later stages of construction. The right-of-way is then be levelled to allow construction

equipment room to work. In areas along the sides of hills (“side hilling”), two levels may be

necessary. One level would contain the ditch and material removed from it. The second level

would accommodate the pipe fabrication area, as well as construction equipment and passing

lanes. This technique reduces the amount of material that must be displaced during the

temporary construction phase of the work. Access points to the site require a special attention as a lot of heavy equipment will use this passage. Damage to the public road network should be avoided as well, many small country roads or tracks being not used to see so much traffic.

 

  Trenching

 After the construction zone is cleared and levelled, trenching machines begin digging the

trench where the pipeline will be buried. In agricultural areas and before trenching, the

topsoil is removed from the work area and stockpiled separately. This is generally part of the

ROW preparation. Materials removed from the ditch would normally be placed adjacent to

the topsoil pile or on the opposite side of the ROW, depending the availability of space. 

The depth of the trench can vary, the minimum cover being defined by the regulation (for

hydrocarbon pipelines at least) or the owner’s specifications. The total depth of a trench

depends on:

- the diameter of the pipe,

- the minimum cover required by the regulation (typically 0.8m),

- additional cover required in special locations (road, river, railway crossings) to protect

the pipeline,

- increased depth to accommodate soft backfill before the pipe is laid (in rocky soil),

- to decrease slope on top of hills.

Pipeline layouts will define for each location the minimum design depth of the trench.

The most effective and economical solution is to use a trenching machine. A trenching

machine is capable of cutting through all types of soils except areas that have very large

boulders or rocks. It cuts the trench and side casts the soil. Track-mounted excavators are normally be used to dig the ditch in hilly or mountainous terrain. Extra ditch depth is dug to ease the transition of the pipeline at the bottoms and tops of hills, at water crossings, road crossings and railroad crossings. This requires additional temporary right-of-way width to accommodate the extra material excavated for the ditch. Trenching machines equipped with special blades are also capable of trenching through soft rock. Other equipment called rock saws can cut through rock to the desired depth. Only in rare occasions would blasting be used to trench through rock. If blasting is required, the charges are shaped to limit the amount of outward explosion. To limit the amount of debris spread, heavy mats can be placed over the charges. Special crushers are used to transform the rocks in coarse sand that can be used to prepare the bottom of the trench and the backfill material in contact with the pipe.