Production and Transport of Oil and Gas [antikvár]

chapter 6 GATHERING AND SEPARATION OF OIL AND GAS 6.1. Line pipes 6.1.1. Steel pipes Steel pipes used in transporting oil and gas are either hotrolled seamless pipes or spiral or axially welded pipes, respectively. Pipes manufactured according to API Standards are used mostly by the oil- and gas-industry all over the world. On the basis of API Spec. 5L-1978 Table 6.1 — 1 gives the main characteristics of the plain-end pipes of 1/8"—1 1/2" nominal diameters. The main parameters of the pipes exceeding these sizes are given in Table 6.1—2. The table was constructed on the basis of API Spec. 5L- 1978, 5LX- 1978, 5LS- 1978, and 5LU- 1972. For each OD of the pipes the following data are given: the smallest and largest mass per length of the pipes (Column 3); the wall thickness (Column 4); the corresponding smallest and largest IDs (Column 5); and, besides. Table 6.1 -2 shows that how many kinds of pipes of standard sizes manufactured according to the above specifications are available within the range of the given size limits (Columns 6 — 9). The tolerance of the OD depends both on the pipe-size and its mode of fabrication. The maximum admissible tolerance is ± 1 percent. The tolerance of the wall-thickness varies also depending on the pipe-size mode of fabrication. The maximum admissible tolerances range between + 20 and — 12-5 percents. Pipe ends are bevelled to facilitate butt welding. Unless there is an agreement to the contrary the bevel angle is 30° (tolerance -1-5° —0°) as measured from a plane perpendicular to the pipe axis. The height of the unbevelled pipeface, perpendicular to the axis should be 1-59 mm with a tolerance of +0-79 mm. Some characteristic data of these pipe materials and their strength are listed in Table 6.1—3. Threaded-end pipes for joining with couplings of 20 in (508 0 mm) or smaller nominal sizes are also made. These pipes are made however exclusively of steels of A —25, A and B grades. During the past two decades experts did their best to produce weldable steels of the highest possible strength for the oil- and gas-industry. Figure 6.1 — 1 shows, after Forst and Schuster (1975), in a simplified form, that how the strength of the new standard pipe-materials increased in the course of the years. On the left side of the ordinate axis the standard grade can be seen while on its right the yield strength can be read in MPa units. The quality improvement during Period I is mainly due to the application of the micro-alloys, in Period II it was facilitated by a new type of thermo-mechanical treatment of the pipe steel, while in the third phase it was made