Pipeline design for installation by horizontal directional drilling
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GPR is most useful at depths of less than 20 ft when the density of the object or utility in question contrasts greatly with the surrounding ground. GPR is more effective in dry sands than in wet soils and does not work well in clay soils and soils that are salt contaminated, or in identifying pipe made of clay. Unlike test pitting, which is performed by means of excavation equipment such as a backhoe, vacuum excavation removes the soils by means of high-pressure air or water jetting. This method reduces the risk of damage to existing utility lines. The water or air loosens the soil, which in turn is vacuumed into a truck for replacement upon completion of the survey.
Conventional vacuum excavation is limited to depths of approxi- mately 20 ft and is most effective in unsaturated, medium density, gravel size, or less granular soils. Excavation holes must be large enough to allow for visual inspection of the uncovered utility lines. A time recorder is used to record the time of origin of the wave and the time of arrival at each detector. Seismic surveys are generally used in uncongested areas or locations where deep utility installations exist.
The geotech- nical investigation should be tailored to suit the complexity of the instal- lation being designed. Investigations for complex installations should consist of two phases: a general geologic review and a geotechnical survey. A general geologic review involves examining existing geological data to determine what conditions might be encountered in the vicinity of the installation.
Existing data may be available from construction project records in the area of the HDD buildings, piers, bridges, levees, etc. It also allows the geotechnical survey to be tailored to the anticipated conditions at the site, thus enhancing the effectiveness of the survey. Methods utilized in the survey of underground utilities, as described previously, can also be incorporated into the geotechnical survey. Typically, split spoon samples are taken in soil overburden at 5-ft depth intervals in accordance with ASTM D Where rock is encountered, it should be cored in accordance with ASTM D , to the maximum depth of the boring.
A mixture con- taining cement grout and a bentonite product to promote expansion is recommended.
An example of this is shown in Figure If establishing a contractual statement of subsurface geotechnical condi- tions that may be encountered during the directional drilling is desired, a GBR or GDR may be included in the contract documents. For more complex projects, consideration may be given to preparing a GBR for inclusion in the documents. The GBR is typically limited to inter- pretive discussion and baseline statements and refers to the information contained in the GDR. However, in establishing a contractual statement of subsurface geo- technical conditions for an HDD project, it should be remembered that the conditions along a drilled path are rarely visible.
Verifying actual subsurface conditions encountered versus the established baseline condi- tions is generally not possible. During the geotechni- cal and utility excavation programs, soils and groundwater should be examined by both visual and olfactory means to determine if potential hazardous materials exist. Samples should be taken and analyzed in accordance with applicable state and EPA regulations and methods.
When hazardous materials or contaminated soils are encountered, special consideration should be given to selecting an appropriate pipe material for these conditions. This can be illustrated by considering a river crossing. The water body is the obvious obstacle; however, a river is a dynamic entity.
Channels can migrate vertically and horizontally. Additional obstacles can be associated with a river. A riparian barrier of trees may need to be preserved and thus included in the drilled path. An environmentally sensitive wetland may be associated with the river and included in the drilled path. Conversely, the actual bank-to-bank distance of a river may exceed that which is technically or economically feasible for an HDD installation. In this case the drilled segment may be designed to cross the deep channel of the waterway using marine equipment to support the rig and construct approaches through shallower water where cut-and-cover construction is more economical.
A consideration in designing the drilled path is the minimization of drilled length. Minimizing the drilled length of an HDD crossing reduces installation costs. However, the design must also consider availability of workspace at the entry and exit locations such that the HDD can be fea- sibly constructed within the physical site constraints. A typical designed drilled path is shown in Figure The typical drilled path shown follows a straight alignment in the horizontal plane.
The three- dimensional combined curvature in both the vertical and horizontal planes should also be considered. Therefore, horizontal curves should only be used after due consideration and analysis have been given to their potential negative effect on constructability. Exit angles should be designed to provide ease in breakover support of the pull section. High exit angles require the pull section breakover bend to be supported at an elevated position during pull back.
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As part of a general constructability review, the design engineer should check pull section handling requirements to evaluate the constructability of the design. A minimum of 15 ft of separation beneath the obstacle should be maintained Directional Crossing Contractors Association , Hair and Hair This minimum provides a margin for error in surveying methods both before and during construc- tion.
It should be noted that permit requirements may exceed these values. This connec- tion between pipe diameter and radius of curvature is derived from estab- lished practice for steel pipe rather than from theoretical analysis. Reduction of the design radius from this standard is possible, particularly for crossings utilizing alternate pipe materials such as high-density poly- ethylene HDPE pipe, fusible polyvinyl chloride pipe FPVC , or ductile iron pipe DIP. For instance, the cold bending radius for HDPE pipe in HDD and other pull-in applications is usually limited to 40 to 50 times the diameter.
In these cases, the lower limit of radius is generally controlled by the capabilities of the drill pipe being used. However, reduction in radius increases bending stress and pulling load on steel pipe. These factors are discussed in more detail in Chapter 4. This is particularly critical where HDD is being used to install a gravity sewer. The required line and grade tolerances may not be achievable or may be achievable only after multiple pilot holes have been attempted.
D-Geo Pipeline -Horizontal Directional Drilling - Deltares
A reasonable target at the pilot-hole exit location is 10 ft left or right and minus 10 ft to plus 30 ft in length Directional Crossing Contractors Asso- ciation This method consists of drilling pilot holes from each side of the installation and intersecting the pilot holes.
Intersecting pilot holes require a great deal of drilling precision. Multiple-line installa- tions can be achieved by placing individual pipes in individual holes along roughly parallel paths or by placing a bundle of lines in one drilled hole. Where multiple lines are to be placed in individual holes, decisions must be made with respect to vertical and horizontal spacing. Ranging involves placing a guide wire within an installed pipeline, as opposed to above ground along the centerline, and steering subsequent crossings in relation to the known position of the wire.
Downhole surveying and as-built documentation are discussed in Chapter 6. Multiple lines may be placed in a single drilled hole by joining them to a common pulling head and installing them as a bundle American Gas Association If separation of steel lines is required for cathodic protection reasons, pipe spacers can be used. However, spacers should be avoided if possible because they can increase drag. Pipe bundles may roll during installation. This should be taken into account in planning for tie-ins to approach piping at each end of the drilled segment. Where casings are employed, it is usually to provide strength to resist installation loads as in the case of an HDPE within a steel casing.
HDPE may have been selected because of its resistance to corrosion during operation, but it may not have the tensile capacity to resist installation loads over a long drilled segment. The steel casing provides the structural strength needed for HDD installation. From an HDD design standpoint, no differentiation is made between a casing and carrier or product pipe. HDD operations are essentially the same. This is not to be confused with surface casing, which is temporarily installed to stabilize near-surface soils.
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Surface casing is usually removed after the HDD installation. Installation of pipelines by horizontal directional drilling, an engineering design guide. Directional Crossing Contractors Association. In some cases these loads may be higher than the design service loads American Gas Association Pipe properties such as strength and wall thickness must be selected such that the pipeline can be both installed and operated within custom- ary risks of failure. The stresses and failure potential of the pipe are a result of the inter- action of these loads American Gas Association The purpose of this section is to describe the loads that act on a pipeline during installation by HDD and to present methods for estimating these loads.
In addition to these forces that act within the drilled hole, frictional drag from the portion of the pull section remaining on the surface typically supported on rollers also contributes to the tensile load on the pipe. Additional loads that the horizontal drilling rig must overcome during pull back result from the length of the drill string in the hole and the reaming assembly that precedes the pull section. Nonetheless, if a direct correlation with the overall rig force is desired, loads resulting from the reaming assembly and drill string must be estimated and added to the tensile force acting on the pull section.
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A theoretical pulling load may be calculated by hand or with the aid of one of several commercially available software packages. Such variables cannot easily be accounted for in a theoretical calculation method designed for use over a broad range of applications. Bearing in mind that most pilot holes are drilled longer, deeper, and to tighter radii than designed, a conservative approach in the absence of as-built pilot-hole data is to evaluate a worst-case drilled path that accounts for potential deviations from the design. The individual loads acting on each segment are then resolved to determine a resultant tensile load for each segment.
However, the component of the tensile load resulting from the effective weight of the pipe may either be positive, negative, or zero depending on the buoyancy of the pipe and whether the pipe segment being evaluated is being pulled upward, downward, or horizontally. However, it should be noted that this value can vary with soil conditions. For curved segments, calculation of the bearing force is more complicated because additional geometric variables must be considered along with the stiffness of the pipe.
Pipeline Design for Installation by Horizontal Directional Drilling: Asce Manual of Practice
Puckett The unit weight of the pull section includes not only the product pipe, but also its contents ducts, internal water used for ballast, etc. Hydrokinetic pressure due to surge or plunger action and hole wall bearing pressure cannot be readily calculated and must be estimated using engineering judgment and experience. As a result, existing procedures for calculating and limiting stresses can be applied. However, unlike a cut-and-cover installation in which the pipe is bent to conform to the trench, a continually welded or fused pipeline installed by HDD contains elastic bends.
Flexural stresses imposed by elastic bending should be checked in combination with other longitudinal and hoop stresses to evaluate if acceptable limits are exceeded. The operating loads imposed on a pipeline installed by HDD are described below. The internal hydro- static pressure from the depth of the HDD installation should be consid- ered when determining the maximum internal pressure. These bends are typically approximated as circular curves having a radius of curvature that is determined from as-built pilot- hole data.
One common method of calculating the radius of an approxi- mate circular curve in a single plane i. It should be noted that horizontal curvature typically exists to some extent during drilling, even in crossings designed to be straight. Various methods are available to calculate combined radius, one of which is shown below. Therefore, stress is induced by a change in temperature from that existing when the line was constructed to that present during operation.