pipeline. There are two different production techniques, one for small diameter pipes and one
for large diameter pipes. For large diameter pipes, from 20 to 60 inches in diameter, the pipes
are produced from sheets of metal which are folded or rolled (spiral) into a tube shape, with
the ends welded together to form a pipe section. Small diameter pipe, on the other hand, can
be produced seamlessly. This involves heating a metal bar to very high temperatures, then
punching a hole through the middle of the bar to produce a hollow tube. In either case, the
pipe is 100% tested before being shipped from the steel mill, to ensure that it can meet the
pressure and strength standards for transporting natural gas. Line pipe is also covered with a specialized coating to ensure that it does not corrode once placed in the ground. The purpose of the coating is to protect the pipe from moisture, which causes corrosion and rusting. Coating can also be applied on the inside of the pipes to protect them against corrosion and to reduce the friction loss. There are a number of different coating techniques. In the past, pipelines were coated with a specialized coal tar enamel. Today, pipes are often protected with what is known as a fusion bond epoxy, which gives the pipe a noticeable light blue colour.
In addition, as the coating is never totally perfect and will deteriorate with time, cathodic
protection is often used. This is a technique of running an electric current through the pipe to
ward off corrosion and rusting.
Valves
International pipelines include a great number of valves along their entire length. These valves work like gateways; they are usually open and allow the product to flow freely, or they can be used to stop the flow along a certain section of pipe. There are many reasons why a pipeline may need to restrict flow in certain areas. For example, if a section of pipe requires replacement or maintenance, valves on either end of that section of pipe can be closed to allow engineers and work crews safe access. Some valves are equipped with actuator that can be triggered by a sudden drop of pressure resulting from a major leak. These large valves can be placed every 10 to 30 km along the pipeline, and are subject to regulation by safety codes.
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.
Pipe hauling and stringing
Once the pipeline’s path has been cleared sufficiently to allow construction equipment to gain access, sections of pipes are laid out along the right-of-way, ready for welding, a process called ‘stringing’. The pipe sections are commonly 12m or 24m long. The 24m long sections are made at the yard by welding together two 12m standard section; this is called double-joining. The joints made at the yard may be coated at the yard or left as such for subsequent coating with the line joints. Pipes are specific to their destination and the hauling contractor uses the pipeline layout drawings to string the right pipes at the right place. Indeed, certain areas have different
requirements for coating material, pipe thickness and even pipe material and diameter.
Pre-coated pipe joints are hauled to the right-of-way on stringing trucks. The pipe are
unloaded from the trucks with a side-boom tractor or the crane mounted on the truck and
placed end-to-end alongside the ditch so they are accessible to construction personnel.
Pipe stringing may occur concurrently with trenching. In some cases, pipe stringing may
occur ahead of the trenching, but it normally follows the trenching crew (to avoid damages to
the pipe by the heavy earth moving equipment). In cases where the trench is to remain open for extended periods (over night or longer) it is sometimes barricaded to reduce any safety hazard.
Where water and road crossings are to be accomplished, the appropriate pipes are stockpiled
on one or both sides of the crossing so they are to the construction crews that would follow.
Depending on access and terrain, the trucks off-load the pipe and then turn around and return
to the pipe lay-down area. In cases where there is a narrow construction corridor, the trucks
have to make a continuous loop by driving a significant distance up the corridor, then off-load
the pipe and follow the corridor a significant distance to exit the corridor.
Bending the pipe
A pipeline must cross over hills and curve around special places. A hydraulic pipe-bending
machine bends individual joints of pipe to the desired angle at locations where there are
significant changes in the natural ground contours or where the pipeline route changes
direction. The pipes to be bent, the location of the centre of the bend on the pipe joint and the
angle are defined by the surveyor after stringing. Bending data are marked on the pipe joints.
The bending is limited to making many small bends along the length of a pipe joint until the
desired bend angle is obtained. Doing so, the pipe retains its strength and remains circular
where it is bent because of the characteristics of steel and the bending techniques used.
The bending technique should be qualified before production (maximum angular deviation for
each stroke, distance between strokes, distance from the ends of the pipe, for each
diameter/thickness, material).
Welding
After the stringing and bending are complete, the pipe sections are aligned, welded together,
and placed on temporary supports along the edge of the trench, waiting for NDT and lowering
in. This step is repeated a number of times until multiple pipe joints are joined to form a pipeline
section which will be lowered in in one shot. Such sections may be as long as 2 km or more in
clear areas. Automatic welding machines (external and internal) are used where possible and some hand welding also takes place, generally for repairs and special points. As for mass production on an assembly line, the various welding phases are performed by specialized crews, having each their specific equipment. The purpose of this organization is to produce as many welds per day as possible (expensive operation). The equipment and staffing of the welding spread depends on the diameter, thickness and welding procedure. A typical set-up looks like the following: the pipe gang follows the pipe stringing and bending crews. The pipe gang uses one or two side booms to set the pipe up on wooden frames (skids) to support the pipe off the ground and line up the pipe with the contour of the trench. Pipe ends are aligned and clamped in place,
using an internal pneumatic clamp or an external mechanical one. Once the pipe is fixed and proper alignment has been achieved, two welders perform the first pass (root pass). Immediately after the first pass, the next group of welders applies the second bead or hot pass. The hot passers can number anywhere from 2 to 4 welders with their helpers. The completion of the root pass (1st pass) and the hot pass (2nd pass) is the primary function of the pipe gang. Production from the pipe gang is expected to be 500m to 2000m per day, depending on weather and terrain. The firing crew is the largest crew, and consists of 8 to 12 welders using welding machines mounted on pickup trucks. The welding crews follow and place the remainder of the weld material into the joint, including the final “cap” weld. When completed there are a total of four or five layers of welds on the pipe joint. All welders that work on a joint have unique identifying codes. The codes are marked on the area adjacent to the pipe so complete records of the welding are maintained.
Joint Coating
Line pipe, normally mill-coated or yard-coated prior to stringing, requires a coating at the