Pipeline Bend and Elbow

The need to use segmented induction bends and elbows can arise for a variety of reasons during construction of new pipelines or during pipeline repair and maintenance activities. There are often instances where bends with a tighter radius than can be accomplished by cold field bending are required to accommodate abrupt directional changes; either points of inflection, changes in topography, or both (Figure 1).

Figure 1. INDUCTION BEND BEING USED DURING CONSTRUCTION OF A CROSS-COUNTRY PIPELINE

Some tight-radius points of inflection or changes in topography can be accommodated by ordering induction bends or elbows with specific bend angles. This is generally true for points of inflection which can be surveyed in detail prior to construction. However, the specific bend angles required are not always known prior to construction, particularly for changes in topography during pipeline construction in challenging and hilly terrain. The use of segmentable induction bends and elbows may also be required during pipeline repair activities. Often times the pipeline has to be taken out of service during these activities, and due to time constraints, purchasing a precise bend angle from a supplier would be logistically impossible. While some bend angles can be accommodated using a combination of standard (pre-manufactured) bend angle fittings and field bends, it is often useful to order segmentable induction bends and/or elbows that can be cut to the required bend angle in the field.

1. Field Bends

The radius of curvature for cold field bends is generally limited to 40 times the pipe diameter (1.5 degrees per pipe diameter of length) to minimize damage to fusion bonded epoxy (FBE) coatings, although cold field bends with a radius of curvature as small as 15 to 8D may be achievable in some pipe diameter and wall thickness combinations. Beyond this, wrinkling along the intrados tends to occur as well as excessive strains and wall thinning along the extrados. The practice of field bending is also heavily reliant on equipment availability and operator knowledge and experience. While field bends should be used where practical, they are not always an option.

2. Induction Bends

Induction bends are formed in a factory by passing a length of straight pipe through an induction bending machine (Figure 2). This machine uses an induction coil to heat a narrow band of the pipe material (Figure 3). The leading end of the pipe is clamped to a pivot arm. As the pipe is pushed through the machine, a bend with the desired radius of curvature is produced. The heated material just beyond the induction coil is quenched with a water spray on the outside surface of the pipe. Thermal expansion of the narrow heated section of pipe is restrained due to the unheated pipe on either side, which causes diameter shrinkage upon cooling. The induction bending process also causes wall thickening on the intrados and thinning on the extrados. The severity of thickening/thinning is dependant on the bending temperature, the speed at which the pipe is pushed through the induction coil, the placement of the induction coil relative to the pipe (closer to the intrados or extrados), and other factors.

Figure 2. INDUCTION BENDING MACHINE

Figure 3. HEATED PORTION OF PIPE MATERIAL DURING INDUCTION BENDING PROCESS

Most induction bends are manufactured with tangent ends (straight sections) that are not affected by the induction bending process. Field welds are made or pipe pup sections are attached to the unaffected tangent ends (Figure 4), allowing for fitup similar to that found when welding straight sections of pipe together.

Figure 4. PIPE PUP SECTION BEING ATTACHED TO FACTORY-SEGMENTED INDUCTION BEND

Induction bends come in standard bend angles (e.g. 45°, 90°, etc.) or can be custom made to specific bend angles. Compound bends (out-of-plane) bends in a single joint of pipe can also be produced. The bend radius is specified as a function of the diameter. For example, common bend radii for induction bends are 3D, 5D and 7D, where D is the nominal pipe diameter.

3. Elbows

Elbows are formed in a factory using one of several manufacturing methods. The first method involves the use of plate material that is heated and forged into two halves (clam shells) using a press and a die that will produced the desired radius and diameter (Figure 5). The edges of each half are trimmed (Figure 6) and the two halves are then assembled and welded together (Figures 7 and 8) using two submerged-arc welds (one along the intrados and the other along the extrados). The weld reinforcement is ground flush for the entire length of the elbow or for only a few inches at each end (Figure 9). Following radiographic inspection of the seam welds (Figure 10), the ends of the elbow are trimmed and prepared for field welding. Dimensional checks are then performed on the end preparations (Figure 11) and throughout the length of the elbow for diameter and out-of-roundness (Figure 12).

Figure 5. HOT FORMING OF ELBOW HALVES

Figure 6. TRIMMING OF ELBOW HALVES

Figure 7. ASSEMBLY OF ELBOW HALVES

Figure 8. SEAM WELDING USING SUBMERGED ARC WELDING

Figure 9. GRINDING OF LONGITUDINAL SEAM WELDS

Figure 10. RADIOGRAPHY

Figure 11. DIMENSIONAL CHECK OF END PREPARATIONS

Figure 12. DIMENSIONAL CHECK OF DIAMETER AND OUT-OF-ROUNDNESS

Elbows can also be manufactured using the “bend over mandrel” process (Figure 13). Pipe material is heated and bent while a mandrel is drawn through. The mandrel prevents ovalization and maintains a constant inside diameter throughout the length of the elbow. Elbows also come in standard bend angles (e.g. 45°, 90°, etc.). Elbows can be custom made (i.e., cut in the factory) to specific bend angles. The bend radius is specified as a function of the diameter. For the purposes of this project, “long radius” when used to describe an elbow refers to a radius of curvature equal to three times the pipe diameter (i.e., a 3D elbow).

Figure 13. ILLUSTRATION OF DRAWN-OVER-MANDREL PROCESS

Source :

DNV Phase 1 Final Report, Guidance for Specification and Purchase of Segmentable Induction Bends and Elbows   http://www.ingaa.org/File.aspx?id=18182