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.

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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 rightof-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-WaterPipeline 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 Approach2nd 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., PrenticeHall, London

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