geotechnical considerations in pipeline design

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GEOTECHNICAL CONSIDERATIONS IN THE DESIGN OF PIPELINES 1.0 Introduction Pipelines are a very important part of modern civilization. And pipeline transport has become the most important way of moving fluids from one point to the other. Pipelines have been used for millennia for the movement of water and pipeline technology was able to revolutionise petroleum exploration in the 1800’s (Antaki, 2003). These days pipelines are used to move substances ranging from water, oil or natural gas, ethanol, hydrogen gas, to beverages and pneumatically driven particulate solids (Shukov, 2009). Pipelines typically cost more than roads or open channels. But they can offer reductions in cost based on shorter more direct routes than roads or open channels (Linsley et al, 1992). Construction of pipelines, especially for large scale water supply or petroleum projects are large multi-disciplinary activities which involves the investment of large amounts of cash and other resources. Because of this, and the fact that safety is 1

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Page 1: Geotechnical Considerations in Pipeline Design

GEOTECHNICAL CONSIDERATIONS IN THE DESIGN OF PIPELINES

1.0 Introduction

Pipelines are a very important part of modern civilization. And pipeline transport has

become the most important way of moving fluids from one point to the other. Pipelines

have been used for millennia for the movement of water and pipeline technology was

able to revolutionise petroleum exploration in the 1800’s (Antaki, 2003). These days

pipelines are used to move substances ranging from water, oil or natural gas, ethanol,

hydrogen gas, to beverages and pneumatically driven particulate solids (Shukov, 2009).

Pipelines typically cost more than roads or open channels. But they can offer reductions

in cost based on shorter more direct routes than roads or open channels (Linsley et al,

1992).

Construction of pipelines, especially for large scale water supply or petroleum projects

are large multi-disciplinary activities which involves the investment of large amounts of

cash and other resources. Because of this, and the fact that safety is of high essence in the

construction and operation of pipelines (Kuryla, 2009) environmental factors including

the soil that it will be laid upon or buried underneath should be taken account during the

design process.

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1.1 Defining Terms

Consideration: something to be taken into account when weighing the pros and cons of a

situation before making a decision

Design (Pipeline): The process of creating detailed plans and drawing of the nature of the

pipeline with a view to solving problems that might occur in the construction and

operation of the pipeline these problems may be hydraulic, structural or geotechnical.

Geotechnical: Application of technical knowledge and skills to some aspect of earth

material, usually earth materials found at or near the earth’s surface (Holtz, 1981).

Pipeline: A pipe or system of pipes designed to carry something such as oil, natural gas,

or other petroleum-based products over long distances, often underground.

1.2 Types of Pipelines

Pipelines maybe classified based on different criteria (Shukov, 2009). These criteria

include

1. Material Made out of: Pipelines are made out of various materials such as steel,

cast iron, plastic, non-ferrous metals such as aluminium; concrete, vitrified clay

and even wood.

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2. Substance Transported: As earlier mentioned, Pipelines could be used to transport

substances such as water, waste water, petroleum oil, gas, beverages and

particulate matter such as cement and flour.

3. Method of Construction/Environment: Pipelines are classified as either seamless,

seam-welded or flange jointed, depending on method of joining. They are also

classified as underground, above ground, elevated, offshore and underwater

(submarine type)

4. Function: Pipelines can be classified under this heading as transmission,

distribution, or collection pipelines, based on the function of that line in relation

to the larger system of pipelines.

1.3 Pipeline Design

The procedure for designing a pipeline depends on several factors which include: type of

material transported, length of the pipeline, the environment of the pipeline, whether the

pipeline is on land or offshore and the whether the climate is warm or cold. Liu (2003)

puts across that the similarities in designing all pipeline types are more than the

dissimilarities and hence, once a person understands how a pipeline was designed and

built, it should not be difficult for him to design and built any other of any type

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1.3.1 Pipeline Design and Build Process

Liu (2003) divides the planning and construction of pipelines into the following phases:

1. Preliminary planning: Determining the origin and destination of the pipeline, the

approximate length of the pipeline, the product to be transported, diameter and

type of pipe used, hydraulic factors such as type of flows expected in a pipeline,

approximate capital cost and running expenses.

2. Route selection: The route selection being from a highway map and/or a

topographical map. Aerial photography should be undertaken to obtain data

needed for the design and preparation of route maps and property plats, which are

requires for right-of-way acquisition.

3. Right-of-way Acquisition: This acquisition may be done in the either by the

landowners voluntarily negotiating with the pipeline owners for the sale, lease or

easement of their plots. Also, for public pipelines, the procedure for

condemnation, which is an involuntary legal process may be explored to acquire

land.

4. Soil borings, testing of soils and data collection: Once the acquisition of the right-

of-way has been completed, the pipeline developer can undertake necessary

geotechnical investigations and determine whether groundwater and/or hard rock

will be encountered, and collect other data along the route needed for the design

of the pipeline.

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5. Design: This involves structural design (in relation to loading and stresses),

hydraulic analysis and design and also designing of a job schedule or scheme.

6. Seeking of Legal Permits: Permits from different agencies including the Federal

Environmental Protection Agency, the forestry services of the various states and

the Ministry of Transportation.

7. Pipeline Construction: This involves the actual work the lay the pipes and the

appurtenances needed for the smooth economical operation of the pipelines. This

involves right-of-way preparation, ditching and trenching, boring, tunnelling,

river crossing; welding, coating and wrapping; backfill and restoration of land.

Plate 1. Laying of land outfall pipeline (Source: Julius Berger Plc)

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1.4 Geotechnical Considerations

Whether the pipeline is buried underground, exposed on the earth’s surface, offshore or

onshore the interaction between the pipeline structure and the soil is an important factor

in the design of the pipeline. Soil movements in the pipeline may cause leakages in the

pipeline or outright failure (Liu, 2003).

The decision whether to bury or not to bury depends on several factors (Antaki, 2003). A

buried pipeline offers better protection against the effects of temperature changes, allows

for shorter routes, is better protected form wind loads, avoids existing above ground

obstructions, is difficult to vandalise and if deeply buried, is protected from the effects of

above ground traffic. On the other hand, a buried pipe has unique corrosion challenges,

requires more elaborate repairs, has to be backfilled to prevent excessive settlement and

has to be designed for soil and surface loads.

Some of the considerations include:

1.4.1 Soil Load: According to Antaki (2003), the study of soil loads in pipes dates back

to the beginning of the last century. At the time large scale irrigation projects were just

being started, relying on underground clay tiles to distribute water to farms. Matson

(1913) in a study experimentally presented a formula for the soil load in pipelines, He

was of the idea that the soil loads on the pipe can be gotten from a prism formula which

states that the weight of the soil prism above the pipe is equal to the soil load on the pipe

.Pv=γ H

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Where:

Pv= Earth load pressure on buried pipe

γ= Unit weight of backfill material

H= Burial depth

Subsequent publications in this field have verified the wisdom in this proposition. If the

pipe is below the water table, then the effects of buoyant forces and weight of water have

to be added then the prism formula becomes:

Pv=γ H-0.33h/H γH+ γwH

Where:

h= height of water above pipe

γw= Specific weight of water

Fig.1 Soil prism above pipe

If instead of placing in a ditch with backfill, the pipe is tunneled into place, the soil load

is reduced by a factor of 2c (H/D), where c is the cohesion of the soil (Moser, 1990). The

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reduction in soil loads is one of the factors encouraging the increased use of Horizontal

Directional Drilling (HDD) in pipeline construction (McGuire, 2009)

1.4.2 Earth Movement: Earth movements could be a gradual settlement/spread or a

sudden failure due to a landslide earthquake or mining operation. Ground movement

assessment consists of two parts: prediction of deformed pipe profile and estimation of

resultant stresses on the pipeline due to the deformations. The American Petroleum

Institute standard (API, 2009) for a deflection X is that may be allowed to happen over a

distance of pipe L at least equal to

L= (3.87x107DX+7.774xX2/(FDSγ-SE))0.5

L=minimum required length, ft

D=outside pipe diameter, in

X=mid-span deflection, ft

FD=design factor

SY=minimum yield stress of pipe material, psi

SE=longitudinal stress in pipe prior to ground movement, psi

Fig. 2 Mid-span deflection (Source, Liu)

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1.4.3 Seismic Consideration: In the event of earthquake, seismic forces could cause

failure in a number of ways: a large ground movement could cause failure by tension,

particularly in corroded joint sections; the pipeline could undergo failure due to the large

cyclic movement caused by the passage of the seismic wave.

1.4.4 Slope Stability: It is important to avoid unstable or potentially unstable slopes

during pipeline design (Mohitpour, 2003). Landslides are varied in distribution and

characteristics and they depend on local soil and groundwater conditions and landforms.

In addition, as a result of the pipeline construction slopes that were stable could become

unstable. The worst type of slope failure is, according to Mohitpour (2003) is a deep

seated failure where the failure plane passes well beneath the pipe. However, pipelines

can be designed to traverse potentially unstable slopes without initiating renewed soil

movement. To do this slope stabilization must be undertaken.

Fig. 3 Pipeline deformation due to landslides (Source: Mohitpour)

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Drainage and Erosion Control: Pipelines, like all structures on land need to be adequately

drained to prevent sensitive slopes from being inundated with water. Severe erosion

problems can be avoided by the use of suitable drainage and erosion control measures.

Diversion berms, gabions, ditch plugs and subdrains are usually installed in pipelines for

this purpose (Liu, 2003).

1.4.5 Diversion Berms: A diversion berm is a shallow earth filled dyke that is placed at

intervals on a slope to collect and direct surface runoff flow away from the pipeline

(Mohitpour, 2003). Construction of berms have been standard practice in the pipeline

construction industry for many years (Antaki, 2003). Problems that may occur in the

construction and operation of these berms are:

1. Berms of insufficient height will permit flows to breach and allow flows over the

ditch

2. Berms constructed with an excessive downhill gradient can result in erosion of the

uphill side of the berm.

To solve these problems, the following steps should be taken (Kuryla, 2009):

1. The down slope of the berm should be approximately 5% to limit erosion from

surface runoff.

2. The berms should extend across the full right-of-way to prevent the flow of water

back onto the right of way.

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3. Berm spacing should be reduces as slope increases. For example, a slope in

excess of 30o should have a spacing of 10 meters while a slope of 15o may only

require a spacing of 60 meters

Fig 4. Diversion berms

1.4.6 Gabions: In areas subjected to severe erosion or concentrated surface drainage a

more robust type of robust form of erosion control maybe required. A typical form of

construction involves the construction of gabion baskets, fabricated from wire mesh,

which are filled with stones and are placed in the uphill side of a diversion berm.

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Gabions are used along stream banks to prevent toe erosion. Generally, gabions are used

to protect the bank or the diversion berm where the stones available are too small to be

used as rip-rap. A gravel blanket or filter cloth is typically laid under the gabion to

prevent erosion of fines and to create a flat surface for tying the baskets (Mohitpour,

2003).

1.4.7 Ditch Plugs: When designing pipelines in sloping terrain, it is important to

recognize the potential of subsurface seepage to collect and flow within loose pipe

backfill (Mohitpour, 2003). If this seepage is poorly controlled, it might lead to backfill

erosion and subsequent. The installation of ditch plugs or impervious seepage barriers

will effectively block subsurface seepage within the pipe surface and force it to the

surface where it would be effectively channeled by a diversion berm (Liu, 2003).

A ditch plug is typically made up of a dry mixture of bentonite clay with fine gravel or

concrete sand (API, 2009). When bentonite comes in contact with water, it swells on

saturation and forms an impervious barrier (Holtz, 1981). In many cases, it is less costly

to use pure bentonite, thereby eliminating mixing equipment and easing installation

(Mohitpour, 2003).

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Fig. 5 Typical bentonite ditch plug

1.4.8 Subdrains: In some cases, where the land is inundated with water, it may be

necessary to lover water levels to improve soil stability and prevent erosion. The

installation of subdrains along the right-of-way has proven effective in lowering water

levels and controlling shallow groundwater flows.

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The subdrain consists of a perforated, galvanized, corrugated metal pipe placed in a

trench across the right-of-way the upper portion of the subdrain trench is backfilled with

fine grained soil to prevent the infiltration of surface water.

Fig. 6 Section through a typical subdrain

1.5 Conclusion

The construction and maintenance of pipelines is essential for any economy, especially a

developing oil producing economy like Nigeria. It is essential that Nigerian engineers

master the concepts involved in construction and maintaining pipelines. Soil-Water-

Pipeline interaction may be one of the causes of pipeline failure.

I would like to recommend that Nigerian engineers take up an interest in pipeline

construction to increase the local content in this field of construction.

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References

American Petroleum Institute, (2009) RP 1111, 4th Edition, Design, Construction, Operation and Maintenance of Offshore Hydrocarbon Pipelines and Risers, American Petroleum Institute, Austin

Antaki, G.A. (2003) Pipeline and Pipeline Engineering-1st ed., Mercel Dekker, New York

Holtz, R.D and Kovacs, W.D. (1981) An Introduction to Geotechnical Engineering-1st ed., Prentice-Hall, New-Jersey

Kuryla, C (2009) API Standards Plan-2009, American Petroleum Institute, Houston

Linsley R.K, Franzini J.B, Freyberg, D.K and Tchobanoglous, G (1992) Water Resource Engineering-4th ed., McGraw-Hill, New York

Liu, H (2003) Pipeline Engineering-1st ed., CRC Press Company, Boca Raton.

Marston, A., and Anderson, A.O. (1913) The Theory of Loads on Pipes in Ditches, and Tests of Cement Clay Drain Tile and Sewer Pipe, Bulletin 31, Iowa Engineering Experiment Station, Iowa State University, Ames, Iowa,

McGuire, T. (2009) Directional Drilling Tackles Tricky Terrain, North American Pipelines, Volume 2, Issue 2, September/October 2009, Benjamin Media, Peninsula

Mohitpour H., Golsan, H. and Murray, A. (2003) Pipeline Design, A practical Approach-2nd ed., TransCanada, Windsor

Moser, A.P. (1990) Buried Pipe Design, McGraw Hill, New York.

Shukov, V (2009) Commercial Transport and Distribution by pipeline-3rd ed., Prentice-Hall, London

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