1. Introduction

Pipelines are large, long distance welded steel structures. They are constructed and laid onshore or subsea, and transport many fluids such as crude oil, crude oil products (e.g., diesel), and natural gas. The subsea pipelines reside on or below the seabed, whilst the onshore pipelines are usually buried below ground, in a ‘right of way’, Figure 1.

Figure 1. Pipelines are usually buried onshore, or are subsea.

2. Pipeline ‘Right of Way’

A ‘right of way’ is the right to pass over property owned by another party. Private rights of way are also called ‘easements’. An easement is a limited right to use another’s land for a specified purpose. An easement for a pipeline is a right of passage over land or waterway.

A right of way provides an area of access to the holder of the easement. The holder (beneficiary or grantee) holds certain rights regarding usage of the property described in an agreement.  The holder’s rights of use are described and restricted by the agreement.  The landowner usually continues to own the land, and has only given up defined rights on the portion of land used for the right of way, but the ROW is needed for the life of the pipeline.

Accordingly, the right of way (ROW) for a pipeline is the strip of land (usually 8 to 50 metres (26 feet to 66 feet) wide) containing the pipeline, and allowing pipeline workers to gain access for inspection, testing, emergencies, etc.. It also identifies the land as of special interest (i.e., pipeline activities), and should help protect the line. The ROW also maintains an unobstructed view for frequent surveillance.

The width needed for the ROW is partly dependent on the size of pipe and the type of pipeline to be constructed. The ROW should be (Figure 2):

  • wide enough to allow for normal operation and maintenance (including replacement) of the pipeline; and,
  • consider the potential for neighbouring developments.
Figure 2. A pipeline right of way (ROW) (top), and maintenance work in a ROW (bottom).

Photograph copyright and courtesy of Penspen Ltd., UK.

3. Constructing Pipelines Today

Pipelines are major construction projects. All engineering constructions create hazards; therefore, care should be taken at all stages of construction. Safety is the key, and continuous issue throughout the construction process.

There are many types of pipelines (gas, liquids, slurries (solids contained in a liquid)) in many types of terrain (onshore/subsea, swamps/deserts, mountains/plains, etc.). This section cannot cover construction in all these terrains/conditions; therefore, it will end with a summary of the steps in constructing a cross-country pipeline.

Note that this section does not cover the construction of pipeline facilities such as compressor stations, nor components such as valves, Figure 3.

Figure 3. Compressor station for a natural gas pipeline (left), and a valve arrangement in a pipeline (right).

Photographs copyright and courtesy of Penspen Ltd., UK.

3.1. History

Pipelines transporting crude oil in large quantities were first built in the 1860’s in the USA. The quality of the pipe materials, welding, etc., has changed with time, as has the construction practices (handling the pipe, bending, laying, etc.); for example, the first pipelines, built in the mid-1800’s, used wooden pipes, then cast iron, but they regularly leaked, until wrought iron was used.

Iron

Wrought iron (almost pure iron, containing less than 0.5% carbon) is very malleable, but low strength. Cast iron (iron with a relatively high-carbon content of 2 to 4%) is higher strength, but brittle.

Pipeline materials need to be ductile to withstand the bending and stretching that occurs during construction and operation. This means that neither wrought iron, nor cast iron, function well as a pipeline material.

Wrought iron was replaced by ‘mild steel’. Steel is a mixture of iron and carbon, but mild steel has a very low (‘mild’) content of carbon (typically less than 0.25%), making it strong, ductile, malleable.

Pipeline materials need to be ductile to withstand the bending and stretching that occurs during construction and operation. This means that neither wrought iron, nor cast iron, function well as a pipeline material.

Wrought iron was replaced by ‘mild steel’. Steel is a mixture of iron and carbon, but mild steel has a very low (‘mild’) content of carbon (typically less than 0.25%), making it strong, ductile, malleable, and weldable.

The early pipelines carried crude oil, onshore. There was a big market for crude oil in the mid-1800s, as a replacement for whale oil. Whale oil (oil obtained from whales) was used as a source of light; however, the high demand for whale oil decimated whale populations and as their numbers dropped the price of whale oil rose rapidly. The demand for ‘oil’ was then far higher than the supply.

Many companies and individuals were looking for an alternative and longer lasting source to replace whale oil. Apart from a brief period of coal oil, the answer came with the development of drilling for crude oil, and the use of ‘black gold’.

Crude oil pipelines were rapidly and extensively built throughout the latter half of the 1800s, but natural gas also emerged in the 1800s. The first successful American natural gas well (8.2 metres (27 feet) deep) was in Fredonia in 1821. The Fredonia Gas Light company formed in 1858, and was the world’s first recorded natural gas company, but throughout the 19th century the use of natural gas was restricted because there was no safe or efficient way to:

  • transport large quantities of it over long distances; or,
  • store it.

Long-distance gas transmission became practical during the late 1920’s: from 1927 to 1931 more than 10 major transmission systems were constructed in the USA alone. Natural gas was also competing against ‘manufactured’ gas (gas from burning coal). Manufactured gas had an advantage due to coal’s established distribution system, and this delayed the expansion of natural gas

The early years of the petroleum business were onshore-based, and dominated by crude oil. The oil was plentiful onshore, and the technology was not available to go ‘offshore’. In the late 1800’s, engineers in California erected wharfs to tap oil and gas reserves close to shore; but the first oil well structures to be built in open waters were in the Gulf of Mexico. Kerr-McGee Corporation drilled the first well from a fixed platform offshore out-of-sight of land in 1947, and Brown and Root built the offshore pipeline. 90% of the world’s offshore structures are in relatively shallow waters (less than 76 metres (250 feet) deep), but oil and gas is now being developed at deeper than 2000 metres (6562 feet).

Today, all types of pipelines are constructed by modern equipment and technologies. A complete set of equipment – for preparing the pipeline right of way, excavating the trench for the pipeline, welding the pipe together, lowering into the trench, and backfilling the trench – is called a ‘spread’. The pipeline design standard BSI PD 8010-1 [1] defines a ‘spread’ as: ‘continuous length of sequential pipeline installation on which a contractor is currently working’.

3.2 The ‘Spread’ Method for Onshore Pipeline Construction

The spread method was developed in USA in the 1930s and 1940s: it is a production line, but the pipeline is static, and the workforce moves along the line. Each spread is composed of various crews, each with its own set of responsibilities. As one crew completes its work, the next crew moves into position to complete its piece of the construction process.

A spread can contain all aspects of the construction: from clearing the right of way, to testing the pipeline, Figure 4. It involves several groups of workers and equipment conducting stages of the construction: each group completes their activity (excavation, pipe laying, welding, inspection and reburying), and picks up where the previous group left off. There may be several sets of construction equipment operational along the pipeline route at any given time.

Figure 4. Constructing a pipeline using the ‘spread’ method.

The distance along the pipeline over which this equipment is deployed is typically less than a mile. Special construction crews will be used for fence building, road, railroad, and waterway bored crossings, and valve installations. Most onshore pipeline construction use this spread method to build a pipeline, and major pipeline projects may use multiple spreads, Figure 4.

A spread can consist of 250 to 300 people, moving at between 1 and 3 km /day. A major spread could have 500 people. A smaller project will have much less.

3.3 The Working Width

The pipeline is constructed within a ‘working width’, Figure 5. This is the area of land, leased by the pipeline construction company while they lay the pipeline. After the pipeline has been constructed, the company will return the land to its original condition.

The working width is usually determined and documented with the land owners and occupiers as part of the overall land acquisition process for the pipeline. Any later amendments to the working width will be agreed with the occupier before work is started, or the working width is modified.

Trenching
Figure 5. Example of a pipeline being constructed in a pipeline working width.

Photograph copyright and courtesy of Penspen Ltd., UK.

4. Steps in the Construction of an Onshore Pipeline in a Rural Region

This section will cover the construction of a rural pipeline. For simplicity, it will divide the construction into the steps usually adopted by construction companies, but these steps are not necessarily followed as a sequence. Different construction companies follow differing sequences. Similarly, different companies use differing definitions and terminologies.

4.1 Clearing the Working Width

The working width will be clearly marked/staked by surveyors before any construction work begins. The working width needs to be prepared for the construction traffic, and laying the pipe in its trench. This is the start of the construction, and requires ‘clearing’ the right of way to create the working width. ‘Clearing’ means remove fences, cut down small trees, clear bushes and growing crops, etc., to allow the first part of the ‘spread’ to move in, Figure 6.

Figure 6. Clearing and grading a working width.

Photograph copyright and courtesy of Penspen Ltd., UK.

4.2 Grading

The terrain is ‘graded’ (levelled, and cleared to the correct width), and any crop, timber, or other obstacles in the path of the pipeline removed. The grading will create a continuous ‘roadway’ capable of supporting all equipment to be used, and wide enough to accept all equipment, Figure. 6

4.3 Stringing Out

The line pipe to be used in the pipeline will have been delivered and stored in a nearby, secure location. The contractor brings the pipe to the working width along access roads. The contractor ‘strings out’ the sections of steel line pipe along the working width, Figure 7.

Figure 7. Line pipe being ‘stringed-out’ along the working width.

Photographs copyright and courtesy of Penspen Ltd., UK.

4.4 Bending Pipe

Pipelines – particularly onshore – will need to avoid obstacles. This will mean some line pipe will need to be bent, or bent pipe will need to be procured.

Line pipe is elastic and flexible and can accommodate gentle turns and changes in elevation; therefore, the line pipe can naturally ‘flex’ to fit gentle bends, but for more severe turns a bending machine is used to bend straight sections of line pipe, to create the bend, Figure 8.

Figure 8. Line pipe bending machine (left) and bent line pipe (right).

Photographs copyright and courtesy of Penspen Ltd., UK.

A bending machine (Figure 8) is used to tailor the shape of the pipe to conform to the contours of the terrain, or to make changes in the direction of the line. Slight bends can be made in the field. Sharp turns require special, made-to-measure, bends, delivered to site:

4.5 Welding the Line Pipe

The line pipe and the bends are now ready for joining together, by welding, on the construction site (‘in the field’). The use of steel pipes and the introduction of electric arc welding in the 1920’s established the techniques for modern pipe laying, and today there is sophisticated welding for both on and offshore pipelines. Welders join the line pipe together, using a welding rod. The rod is a ‘consumable’, which melts to join the line pipe together, Figure 9.

Figure 9. Welding the line pipe together.

Photograph copyright and courtesy of Penspen Ltd., UK.

The weld that joins the line pipe is called the ‘girth’ (or ‘circumferential’, or ‘field’) weld, Figure 9. The completed girth welds are inspected using X-ray machines, to ensure pipe joints have been welded together properly, and they do not contain any unacceptable defects.

4.6 Coating the Girth Welds

Most line pipe is delivered to site having been coated at a coating plant to prevent corrosion of the steel, but a small section on each end (75 mm (3”) to 150 mm (6”)) is left uncoated, to allow the field welding, Figure 10. Consequently, the ends of each section of line pipe do not contain coating. These ends need cleaning after the welding, then a corrosion protection coating must be placed over them and the weld.

Figure 10. Coating the girth weld.

Photograph copyright and courtesy of Penspen Ltd., UK.

4.7 Crossing Barriers

A pipeline route will inevitably face barriers (‘obstacles’) such as highways, streams, and other pipelines, Figure 11. The barriers will need to be ‘crossed’. There are various ways to cross these barriers:

  • placing the pipeline above (e.g., on a bridge) the obstacle (this ‘aerial’ crossing creates an obstacle for the obstacle…);
  • cutting a trench across the obstacle (‘open cut’) and placing the pipe in the trench; or,
  • use drilling, ramming, jacking, or tunnelling methods to go under the obstacle.

Trenching or ‘open cutting’ in avoided in areas where these methods cause disruption (e.g., highways) or controversy (e.g., environmentally-sensitive areas).

‘Trenchless’ techniques involve digging pits at either side of the barrier to be crossed, and then drilling, jacking, ramming, or tunnelling under the barrier.

Figure 11. Boring under a highway (left), and crossing under existing pipelines (right).

Photographs copyright and courtesy of Penspen Ltd., UK.

4.8 Tie-ins

The pipeline will have many components (valves, fittings and instruments), Figure 12. Many of these components will be ‘tied-in’ to the pipeline; i.e., welding into the pipeline.

Figure 12. Valve arrangement ready to be tied into the pipeline.

Photograph copyright and courtesy of Penspen Ltd., UK.

4.9 Laying in the Trench

The welded and coated pipe is ready for laying in its trench after it has been welded and the welded area coated, Figure 13. A trench is a long, narrow ditch. The trench should be excavated in previously undisturbed soil, so that the pipe is left on undisturbed soil. The trench must be deep enough (usually no less than 1.2 metres deep) to prevent damage to the pipe from farm implements, and freezing. The time the trench is left open is kept to a minimum to minimise associated risks, earth slip, etc.

Figure 13. Trenching machine.

Photograph copyright and courtesy of Penspen Ltd., UK.

4.10 Lowering the Pipe

The welded line pipe is lowered into the trench using cranes called ‘side-booms’, Figure 14.

Figure 14. Lowering the pipeline into the trench.

Photograph copyright and courtesy of Penspen Ltd., UK.

4.11 Backfilling the Trench

The trench is backfilled with its original subsoil, using specialised equipment designed to pad the pipe and protect it from large (say, >200 mm (8”) diameter) or sharp rocks and abrasion. The trench should be backfilled with material salvaged from the excavation to preserve, as far as is possible, the original soil sequence. Backfilling is conducted as soon as possible after laying, to reduce the risk of damage to the pipeline and trench, Figure 15.

Figure 15. Backfilling the pipeline.

Photograph copyright and courtesy of Penspen Ltd., UK.

4.12 Reinstatement

The backfilling is followed by reinstatement of the subsoil, Figure 16. This is the start of the reinstatement process of restoring landscape, then marking the pipeline. The intention is to regrade and restore the working width as closely as possible to the original condition. Reinstatement should generally be completed prior to the pressure test (see below), although topsoil may be laid after the pressure test [1].

http://intranet/Collaboration/EA/Integrity/TrainingEPM/Documents/30162%20South%20North%20Ireland%202007/2007-09/Top%20Soil%20Replacement-04.JPG
Figure 16. Grading the working width after construction.

Photograph copyright and courtesy of Penspen Ltd., UK.

The working width is then cleared, the subsoil ripped to relieve compaction, and the topsoil re-laid and cultivated. Sometimes the topsoil is not replaced immediately, as the subsoil can settle unevenly and leave small holes; hence, the topsoil might be replaced six months later to allow for settling. This allows better topsoil replacement and smoothing. The working width will be graded to restore pre-construction topography.

4.13 Pre-commissioning the Pipeline

Pre-commissioning usually involves:

  • filling the pipeline (with water);
  • cleaning and gauging the pipeline;
  • pressure testing;
  • dewatering; and,
  • drying the pipeline.

4.13.1 Cleaning the Pipeline

The pipeline needs to be cleaned of any debris left inside. ‘Pigs’ (tools that are inserted into the pipeline, and pumped along it) are usually used to clean the line, although chemical cleaning, purging with air or gas followed by a liquid flush, etc., can be used.

The pigs (often put down the pipeline in ‘trains’, Figure 17) clear debris from the line; for example, ‘brush’ pigs (Figure 17) will rub the internal surface and clean it, while the pig discs and following water flow, will move any debris along the line.

Figure 17. Cleaning a pipeline (top), and a brush pig (bottom).

Photograph copyright and courtesy of Penspen Ltd., UK.

The pigs can be driven (pumped) by clean water, at a speed of about 0.5 m/sec (1.5 ft/sec) to 1.0 m/sec (3 ft/sec).

4.13.2 Gauging the Pipeline

The pipeline needs to be clean, but it also needs to be checked for blockages, such as dents in the pipe body. The pipeline is checked for blockages using a ‘gauging’ pig, Figure 18. These gauging pigs have special plates mounted on board that deform if they collide with a restriction, Figure 18. The gauge pig is a standard pig, but with a soft metal plate (e.g., aluminium) mounted on it. The gauge plates are machined to a pre-set tolerance diameter (e.g., 95 to 97% of the pipeline internal diameter). The gauge pig will indicate problems in the pipe bore by emerging with the plates dented or damaged (Figure 18); however, they will not usually contain equipment that can determine where damage is, the number of damage locations, or dimensions.

Figure 18. Gauging pig (left), and a damaged gauge plate (right).

Photographs copyright and courtesy of Penspen Ltd., UK.

A damaged gauge plate indicates a blockage, or pipe wall damage. The standard gauge pig has no locating equipment on board; i.e., it will indicate damage in the pipeline, but will not be able to give its location. Where there is damage, a ‘calliper’ pig, or a ‘deformation’ pig, can then be run to size and locate the damage. These are ‘smarter’ pigs, as they can locate and size the damage/blockage, and are often used to check the pipeline for damage during its service.

These pigs have arms (callipers) on board that deflect when they pass through, or in, a restriction, that can size the damage. These pigs also have odometers that track and record the progress of the pig along the pipeline; hence, these pigs can locate damage and size damage.

4.13.3 Testing the Pipeline

A requirement of pipeline design standards is that the completed pipeline is subjected to a ‘pressure test’. A definition of ‘pressure test’ for pipelines is [2]:

‘… means by which the integrity of a piece of equipment (pipe) is assessed, in which the item is filled with a fluid, sealed, and subjected to pressure. It is used to validate integrity and detect construction defects and defective materials.’.

The test is called a ‘hydrotest’ when water is used at the test medium, and a ‘pneumatic’ test when air or an inert gas (for example, nitrogen) is used. Gas testing is usually only applied at low pressures.

Most high pressure pipelines use water as the test medium; therefore, they are ‘hydrotested’. This means that after cleaning the pipeline, the pipeline has to be filled with water, to prepare it for hydrotesting. Often the pipeline cleaning and filling with water for the hydrotest is completed in one operation.

Figure 19. Pressure testing a pipeline.

Pressure testing of pipelines involves (Figure 19):

  • sealing the pipeline at both ends (using ‘test ends’);
  • providing a fluid (‘medium’) to fill the pipeline;
  • fixing air vents, depending on the length and size of the pipeline;
  • providing pumps to create the pressure;
  • disposing of the fluid afterwards.

The pressure of the pressure test is higher than the pipeline’s design pressure, Figure 19. The test is a strength and leak test on the completed pipeline, and demonstrates that it can withstand its design pressure. The pressure is held for several hours, Figure 19, according to requirements of the design standard.

4.13.4 Drying the Pipeline

The pipeline will need to be ‘dewatered’, or dried if water has been used for the pressure test. This is necessary as pipelines which transport fluids have to meet a dryness requirement and minimum water content limits (e.g., natural gas has strict limits on water content); additionally, other pipelines may need drying to prevent corrosion resulting from any leftover water. Crude oil and some product pipelines can have the hydrotest water removed by the first flow of the product: with oil lines, a small quantities of water in the first production is unlikely to cause problems, and dewatering can be simple

Line drying can be achieved by:

  • swabbing;
  • using super dry air, or blowing hot air into the line;
  • vacuum drying;
  • passing methanol (which absorbs water) up the line, between pigs;
  • purging the line with dry nitrogen (which absorbs the water);
  • etc..
Figure 19. Examples of dewatering and drying a natural gas pipeline using pigs.

4.14 As-built Surveys and Drawings

As-built surveys are conducted after construction has been completed. These surveys are a way to verify that:

  • design specifications were met; or,
  • to capture the changes to original design specifications that were required to be made to adjust to field conditions.

An as-built survey is presented in as-built drawings containing: alignment sheet number; plot reference; position and extent of bends; depth of cover, and where changes occur; chainage; weld number; marker posts; existing services and drainage; CP connections and test posts; etc.

When the pipeline is constructed and tested, the constructor will hand-over all the necessary documentation to the pipeline owner/operator. These documents will include pipeline route, details of crossings, land drainage, services close to pipeline, ancillary facilities, hydrotest certificates, welding records, etc.

4.15 Commissioning the Pipeline

‘Commissioning’ usually refers to the work required to bring the pipeline system into operation, after completion of the construction and hydrotesting; i.e., the introduction of product into the pipeline, ready for operation. The purpose of commissioning is to check the facilities to ensure they are capable of transporting the products, and preparing the facilities to perform their function.

Before commissioning, a certificate of completion is usually handed to the pipeline operator. Commissioning should not take place until all operating, maintenance, and emergency procedures are established and in place. Commissioning is complete when the pipeline is on-stream and all measurement facilities have been calibrated and shown to be operating correctly, and all valves, etc., are operational.

References

  1. Anon., ‘Pipeline systems. Steel pipelines on land. Code of practice’, British Standards Institution. PD 8010-1:2015+A1:2016. UK, 2016.
  2. Anon., ‘Pipeline Transportation Systems for Liquids and Slurries’, American Society of Mechanical Engineers’, ASME B31.4-2016. New York, USA. 2016.