Line pipe is manufactured by several different processes. Chevron commonly uses seamless (SMLS), electric weld (ERW or HFI), and submerged arc welded (SAW) pipe. There is also helical or spiral welded submerged arc welded pipe, however its use has not been common in Chevron’s operations. Each process has its inherent advantages, disadvantages and suitability for different sizes of pipe. Refer to Figure 300-1.
Seamless Pipe
Manufacturing of seamless (SMLS) pipe begins with a solid round billet that is heated to about 2200°F and pierced to make a hollow cylinder. The cylinder passes through several hot (1800-2200°F) rolling steps to make a pipe with the desired size and wall thickness. Seamless pipe may be supplied as-rolled, or it may be heat treated after rolling to improve its properties. Either normalizing or quenching and tempering heat treatments may be used. Straightening if required is done either hot or cold depending upon the mill practice.
Seamless pipe has greater variation in wall thickness that welded pipe. Also the length variation in a particular lot or mill run is greater than welded pipe. The engi-neer is advised to clearly specify the acceptable length variations on the purchase order.
(1) The range represents the capacity variations for different manufacturers.
(2) Above 1.25 in. refer to ANSI/ASME B31.4 and B31.8 for stress relief requirements.
Electric Welded Pipe (ERW or HFI)
Electric welded pipe is manufactured from a long, flat coiled strip called skelp that has been rolled to the desired wall thickness of the finished pipe. The strip has a width equal to the circumference of the pipe. In the pipe mill the skelp is fed through a series of rolls which form it into a cylinder. The edges are welded together using electric resistance (ERW) or induction (HFI) heating and pressure from the rolls to make a longitudinal seam. No filler metal is added to the weld, and after the “flash” from the weld is trimmed off it is difficult to visually locate the weld on the OD. At the ID the flash trimming operation creates a small depression which makes the weld line distinguishable in many cases. The narrow heat affected zone along the seam is heat treated (seam normalized) after welding using localized induction heating coils. EW pipe is usually not given an additional heat treatment Fig. 300-1 Typical Availability and Usage for Types of Pipe
Seamless EW (ERW or HFI) SAW Spiral Weld
Minimum
Relative Cost More expensive than EW. Cost premium
and its mechanical properties are determined by the original properties of the skelp.
Forming, final sizing, and straightening are all done cold.
ERW pipe has a better surface finish and can be more uniform in length than seam-less. The advantage of better surface quality is that for FBE coated pipe there are fewer problems with holidays in the coating.
Note The term ERW is used in this manual to refer to two processes for manufac-turing electric welded (EW) pipe, and includes electric resistance welded (ERW) and high frequency induction welded (HFI). The latter is the newer process. The basic difference between the two processes is: the ERW process is conductive where the heating of the vee formed by bringing the edges of the skelp together is
produced by flowing a high frequency current between the edges of the skelp prior to pressing the edges together to form the weld; whereas in the HFI process, the heat is generated by an induction coil placed around the formed skelp cylinder. HFI is claimed to have the advantage of producing a higher heat flux across the weld during the manufacturing and therefore is claimed to be more suitable for thicker walls. Chevron has not made a quality distinction between the two processes.
ERW Weld Quality. Over 25 years ago, ERW pipe gained a reputation as poor quality pipe. Most of the performance problems were associated with frequent field leaks during field hydrotesting and operations caused by manufacturing defects in the weld. Advances in skelp material quality, manufacturing processes, particularly high frequency resistance and high frequency induction welding, and more accurate and reliable NDE equipment especially ultrasonic testing have virtually eliminated these problems. ERW pipe made today in a modern mill can be manufactured to be equal in performance to seamless. Recommendations for specifying and ordering ERW pipe are found in Section 316 and Figure 300-2.
Submerged Arc Welded Pipe
Submerged Arc Welded (SAW) longitudinal seam pipe is manufactured by forming a plate into a cylinder, then making a longitudinal seam using the submerged arc welding process with filler metal. The most common forming process is called UOE, which stands for the three main forming steps: bending the plate into a U, pressing into an O, and then (after welding the seam) expanding the pipe to final size. All of the forming is done cold, including the expansion step. In addition to final sizing, cold expansion also improves roundness, redistributes the residual stresses from forming, and acts as a severe proof test of the weld. Forming processes other than UOE, such as pyramid rolling and press breaking, may also be used to make SAW pipe.
Spiral Weld (Helical Weld) Submerged Arc Welded Pipe is manufactured from skelp which is twisted in a helix. The spiral seam is made with the submerged arc welding process. Mechanical properties are determined by the original plate proper-ties. The finished pipe and weld seams are not heat treated. Spiral weld pipe is not usually cold expanded.
Spiral welded pipe is not included in PPL-MS-1050 or PPL-MS-4041 because Chevron has very limited experience with the process. API 5L spiral welded pipe is not manufactured in the U.S. There is, however, extensive use in Canada and
Fig. 300-2 Specification Decision Tree for Mill Runs of ERW (1 of 2)
Fig. 300-2 Specification Decision Tree for Mill Runs of ERW (2 of 2)
Europe. Spiral weld pipe should be considered for large diameter lines ( greater than 36 inches) in sweet service where it may be more economical than longitu-dinal seam SAW. It can be purchased to a modified version of PPL-MS-1050. A detailed review of the supplier is mandatory.
Other Processes
Corrosion resistant alloy (CRA) line pipe is essentially line pipe made from chro-mium (13 Cr) and duplex stainless steel, nickel-chrochro-mium stainless steels (316) or nickel alloys (Incoloy 825). It is covered in API Specification 5LC. CRA line pipe is seeing increased applications as an alternative to chemical inhibition for corro-sion control. It may be cost effective for high temperature streams >350°F (176C) where inhibition is not feasible or in deep offshore applications whenever the total cost of ownership is considered. This product is about 3 to 10 times the cost of conventional carbon steel depending upon alloy grade and size. Consult a materials engineer for assistance in selecting the appropriate alloy and specifications for the environment.
Clad or Bimetallic pipe is a new technology for flowlines and gathering lines. This line pipe product consists of a conventional steel line pipe backing to contain the pressure and a “liner” of corrosion resistant alloy. This combination of materials provides a high strength, cost competitive (with solid CRA) pipe up to grade X-65 in sizes over about 6 NPS. The lining material is selected on the basis of the envi-ronment in the pipe. The lining may either be metallurgically bonded or hydrauli-cally fitted into the steel pipe. For diameters of NPS 16 and larger this product is made from clad plate. API specification 5LD contains the basic requirements for the purchasing and inspection of clad pipe. In sizes over about 12 NPS it is the preferred way to employ CRA. Justification for selection is usually based on the total cost of ownership of the installation including operating expenses for corro-sion control and monitoring. The CRTC Materials and Equipment Engineering Unit can assist in developing the proper ordering specification.
Coiled tubing refers to a continuously manufactured length (1000s of feet in length) of electric welded tubing spooled on to a reel. Sizes range from about 3/4 to 5 inches. This product differs from reeled line pipe which is conventional API line pipe of 40 to 60 foot lengths which have been welded together and rolled onto a spool. Because coiled tubing is manufactured as a long length, thousands of feet, it does not conform to all of the API 5L line pipe requirements, especially the weld testing frequency and the prove up of NDE weld line indications. Coiled tubing was developed and used for downhole workovers and is recently (early 90’s) being considered for flowlines. Before coiled tubing is used for pressure flow line applica-tions specialists in design, materials and quality assurance in CPTC and CRTC should be consulted.
Furnace butt-welded pipe is similar to ERW pipe, except that the weld is made by heating the edges of the plate in a furnace and then pressing them together. This process does not produce good quality welds, and API 5L only permits it for grade A25. The Company does not use grade A25 for pipelines.
Lap welded pipe is no longer made. However, there are still significant quantities of this pipe in the ground.