Design Radius of Curvature
Determining the entry and exit points, the depths that must be achieved, and direction changes for the drill path are all major parts of the planning process. One of the keys to these calculations is the bend radius of the drill rod and/or the product pipe. Bend radius is often misunderstood. It is often confused with the number of feet needed to make a 90-degree turn. Bend radius is defined as the forward distance required for a drill string to make a 90-degree turn (see Figure 4-2).
Understanding bend radius and how to use it is very important to the success of a HDD crossing and of the utmost importance to the engineer and driller. HDD
- 314-feet of pipe required
HDD Rig
FIGURE 4-2 Bend Radius for 90-Degree Bend contractors often only think of radius as it applies to the drill rod, and owners/engineers often think of it as it applies to the product pipe. Both must be considered during the HDD design, as well as the allowable bend radius of mud motors and wireline systems.
Drill Rod. Bend radius is a key factor in any HDD design because it is an indication of how much bending the drill rod can handle without significant and possibly harmful stresses. Exceeding the allowable bend radius can result in significant damage to the pipe or drill rod. While drill-rod manufacturers may use different methods to establish the bend-radius rating for their pipe, SAE standard J2022 provides a voluntary guideline for evaluating drill rods using the same baseline. It is necessary to know what method of calculation is being used by a manufacturer when comparing the bend radius of drill rods from different manufacturers.
Regardless of the design, drill-rod wear and ultimate failure will happen. However, effective designs with proper bend-radius ranges will help extend the useful life of the drill rod. Drill-rig manuals provide data concerning the bend radius for the drill rod and recommendations and allowable steering changes for each section of the rod. The recommended allowable steering changes provided apply to both up or down pitch changes as well as left or right corrections. Allowable bend-radius information is used to determine the setback distance required by the drill unit at the entry point. Shallower bore depths require smaller entry angles and greater setback distances, while deeper bore depths allow for steeper entry angles. Any changes and corrections that are made during the drill should not exceed the recommended bend radius of the drill rod. When the bend radius is exceeded, the useful life of the drill rod decreases dramatically. The damage that is done to the drill rod is subtle and most likely will not be noticed immediately. The failure of the drill rod usually occurs several jobs after the damage is done.
Product Pipe or Conduit. Steel product pipe usually has a much larger bend radius than the drill rod. The largest bend radius of either the drill rod or product pipe is the number that must be used for planning and completing the bore. A common industry standard for determining the design radius of curvature for bends used in HDD installations is to multiply the nominal diameter of the pipe in inches by 100 to determine the allowable radius in feet. This relationship has been developed over the years in the HDD industry and is based on experience, not theoretical analysis. Bending radius and stress calculations are important for HDD installations. Methods for calculating bending stresses in steel pipe have been presented in many publications and are covered in this chapter and in Chapter 5 of this book.
In general the bending radius ratings for plastic, generally PVC or PE, pipe will not exceed that of the drill rod. The exception may be the larger diameters of these types of pipe, where the integrity of the pipe and the joints becomes a concern if they are pulled through severe bends. Bending stresses and bend radius are usually not critical factors for PVC, PE, and ductile-iron products; however, they should be calculated and compared to the manufacturer's specifications for allowable bending radius. There is much data from the Plastics Pipe Institute and pipe manufacturers on this subject. The general equations for calculating bending stresses are similar for steel, PE, and other materials, but the material properties and behavior are different and must be accounted for during any calculations.
Exceeding the recommended bend radius of the product pipe has consequences that may be apparent during installation or not show up for years, reducing the life of the product pipe. The main result during installation is a harder than expected pullback. The product pipe must go through tight bends and curves, and there is a much greater chance of getting stuck in the hole. If the drill path is smooth and gradual, the pullback loads are smaller and the pull will be much easier and quicker.
An area of concern, often overlooked, when considering the allowable bend radius involves the use of mud motors and wireline systems. The mud motor is a very stiff section, and depending upon the length, it generally has a bend radius greater than the drill rod. The stainless-steel monel section that houses the steering tool is also very stiff and more brittle than the drill rod. The suppliers of the mud motor and steering tools can provide information and recommendations concerning the allowable bend radius of their products. When using these tools, the designer should always factor their limits into the bore plan.
Pipe Stress Criteria
Typical products installed by HDD construction are composed of steel, polyethylene (HDPE and PE), polyvinyl chloride (PVC), and ductile iron, as well as direct buried cables. During installation the pipeline products experience a combination of tensile, bending, and compressive stresses. The magnitude of these stresses is a function of the approach angle, bending radius, product diameter, length of the borehole, and the soil properties at the site. Proper selection of the radius of curvature and type of product will ensure that these stresses do not exceed the product capacity during the installation. The HDD design should require the minimum number of product-pipe joints. Flush-type joints, such as butt fusion or welding, are preferable to glued or treaded joints, which tend to increase the drag on the product in the bore hole. Other considerations include minimum cover, minimum separation from existing utilities, tolerances for deviation in the vertical and horizontal profiles, and maximum true depth.
During the design, the stresses imposed on the HDD product pipe during construction and operation must be calculated and checked to be within allowable limits for the material being used. The stresses at each stage must be considered both individually and in combination. Stresses come from the spanning between rollers prior to pullback, the hydrostatic testing pressures, pulling forces during installation, radius of curvature as the pipe enters the ground, the drilling profile curvature, external pressures in the drilled hole, and the working pressure. The load and stress analysis for a HDD pipeline is different from similar analyses of conventionally buried pipelines because of the high-tension loads, occasional severe bending, and external fluid pressures endured by the pipeline during the installation process. In some cases the installation loads may be higher than the design service loads. Pipeline properties, such as wall thickness and material grade, and pilot-hole bore path must be selected so the pipeline can be installed and operated without risk of damage.
During design the preinstallation, installation, and operating stresses of the product pipe must be considered and analyzed. Chapters 5 and 6 of this book cover the methods for estimating these stresses. The principal stresses of concern are listed below.
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