Mark W. Mucek
Amoco Corp.
Naperville, III.
In light of increased concerns about petroleum transportation operations, Amoco Corp. has issued specifications for line pipe which exceed minimum standards in an effort to prevent pipeline failures.
A number of recent pipeline failures in the U. S. and Canada have prompted increased attention from Congress, regulatory agencies, the media, and the population at large.
Causes of past failures include splitting along the longitudinal electric resistance weld (ERW) seam, cracking of full encirclement repair sleeve welds, corrosion, mechanical damage, and other causes. 1-4
While it is understood that no single effort can eliminate all possible causes of pipe failure, Amoco feels that imposing additional requirements on newly ordered line pipe, beyond those set forth in API 5L, will address some of the concerns.
MANUFACTURING
All pipe manufactured for Amoco must be API monogramed and meet all requirements of the current edition of API 5L, plus additional requirements listed in the Amoco specification. Before production, the manufacturer is asked to provide evidence of capability to produce the size and grade requested, such as a successful commercial or test run.
Amoco and its designated representatives, including independent inspection agents, must have access to the manufacturer's facility during manufacturing, testing, and inspection of the pipe. Advance notice of 10 days (30 days outside the U.S.) is requested before start of production.
Manufacturers are asked to submit with their first quotation a description of their melting, rolling, forming, welding, and heat-treating practices. It is required that Amoco be notified of changes in these practices on subsequent quotations.
In order to ensure steel of high inherent toughness, pipe made for Amoco must be produced from fine grain, fully killed steel, with a final ferrite grain size of ASTM 8 or finer.
The influence of grain size on impact toughness is shown in Fig. 1, and it is clearly evident that toughness increases as grain size decreases.' Toughness is discussed later.
Welded pipe must be manufactured to length as a one-piece fabricated product having a single, longitudinal weld seam. Double weld, spiral weld, or jointers are not permitted.
Amoco does not allow center slit skelp to be used to make welded pipe. During the steel-making process, nonmetallic inclusions and other impurities can be segregated to the center of the ingot or billet and subsequently end up at the centerline of the skelp. Slitting the skelp at or near its mid-width and welding this edge can potentially lead to defects in the ERW seam, such as hook cracks.
Amoco requires that electric weld pipe be seam normalized through the full wall thickness. This heat treatment refines the weld microstructure and improves the toughness while reducing the hardness of the weld zone.
Normalizing is defined as heating the full weld to a temperature sufficient to ensure complete austenitization and then cooling the steel at an appropriate rate to produce a refined microstructure (ASTM 8 or finer ferrite grain size).
Examples of acceptable and rejectable pipe are given in Fig. 2. Full-body normalizing is an acceptable alternative to seam normalizing.
Seamless pipe must be full-body normalized unless prior approval has been obtained from Amoco not to normalize. Approval depends on the documented capabilities of the mill to produce as-rolled pipe with specified toughness and grain size.
Weld repairs to ERW and seamless pipe are not permitted.
Weld repairs to submerged-arc welded (SAW) pipe are permissible only on the weld seam of the pipe before cold expansion.
Repair welds are not permitted within 8 in. of the pipe ends nor are back-to-back (ID to OD) weld repairs permitted. Weld procedure specifications (WPS) with supporting documentation (PQR's) must be submitted to Amoco for approval prior to repairs.
CHEMICAL REQUIREMENTS
Sulfur and phosphorous contents are restricted to 0.012% and 0.015%, respectively. These restrictions are aimed at improving pipe toughness.
The influence of sulfur and phosphorus on steel toughness is shown in Figs. 3 and 1 7 4, respectively . It is apparent that toughness improves with decreasing concentrations of these elements.
The major benefit of lowering sulfur is to raise the upper shelf energy of the steel. In the case of phosphorus, the primary influence is on the ductile-to-brittle transition temperature.
For fully deoxidized steels,
each 0.01% increase in phosphorus increases the transition temperature about 13 F. and slightly lowers the upper shelf energy.8
An additional benefit of reduced sulfur is that the steel cleanliness is improved by lowering its inclusion content.
This increased cleanliness can result in a substantial increase in resistance to stepwise cracking.'
Carbon-equivalent (CE) calculations are made to assess the susceptibility of the steel to weld cracking during fabrication or repair, For steels with a carbon content greater than 0.12%, the carbon equivalent is calculated using the following formula:
CE = %C + Mn/6 +
(Cr + Mo + V)/5 +
(Ni + Cu)/l 5
Amoco's acceptable limit for CE is 0.35% maximum.
For steels with a CE less than or equal to 0.12%, the composition parameter (P.) has been found to be a better indicator of crack susceptibility.
The P,m is calculated using the following formula:
Pcm = %C + (Mn + Si +
Cu + Cr)/20 + Ni/60 +
Mo/15 + V/10 + 5B
The acceptable limit using this equation is 0.20% maximum.
The concentration of the following elements must be reported in the mill test report in order that the CE and Pcm calculations can be made: C, Mn, P, S, Si, Ni, Cr, Cu, Y, and Mo.
The concentration of any other intentionally added element, such as Ti, Nb, or B, must also be reported. All chemistry requirements and calculations are based on product analyses.
MECHANICAL PROPERTIES
Flattening tests for ERW pipe are performed in accordance with API 5L, with the exceptions that O' and 901 coupons must be taken from each test location and that in the 90 test, no opening in the weld can take place when the distance between the plates is one half of the original outside diameter.
This restriction is aimed at opening up any ERW seam discontinuities that a less severe test may miss.
In order to obtain pipe with high toughness, Amoco requires Charpy V-notch impact tests on each lot of 1 00 lengths of pipe or fewer with a lot not to contain more than one heat, size, weight, or grade. Transverse specimens of full, three-fourths, two-thirds, or one-half size are tested.
The largest standard specimen size obtainable is used based on pipe diameter and wall thickness. Where the wall thickness or pipe diameter does not permit the use of transverse specimens, longitudinal specimens of the largest standard size possible are used.
All tests are performed at 32 F. (O' C.), which was chosen because it is slightly lower than any temperature the pipe is likely to see in service. The minimum acceptable shear fracture area is 75% for an average of three specimens, with no single value below 60%.
The exception to this is pipe intended for CO2 service in which 90% minimum shear is required. The minimum acceptable absorbed energy values are shown in Table 1.
The hardness of the pipe cannot exceed HRC 22 (HB 235) to reduce susceptibility to stress corrosion cracking in sour environments. An average of three tests constitutes a single hardness value. Hardness tests are to be performed on a single length of pipe in each lot of 100 lengths or fewer.
In order to minimize arc blow during construction, the residual magnetism at the end of any single joint of pipe cannot exceed 25 Gauss. This measurement must be made with a Hall-effect gaussmeter.
HYDROSTATIC, NDT
Each length of pipe must withstand, without leakage, a hydrostatic test in which the stress in the circumferential direction is 1 00% of the specified minimum yield strength (SMYS). The holding time is 1 0 sec.
In the event of excessive deformation, the test pressure may be lowered but not below 95% SMYS. Calculation of the test pressure is to be based on nominal wall thickness, and end loading may be considered in the calculation.
Notification must be given to Amoco if the hydrostatic test pressure is less than 100% SMYS.
Amoco hydrostatically tests all pipe in the field. Stresses during these tests are up to and including 100% SMYS. The costs to locate, replace, and retest any pipe which fails a field hydrostatic test due to defective pipe are charged back to the pipe manufacturer.
For ERW pipe, 100% ultrasonic examination of the longitudinal weld seam is required after hydrostatic testing. Calibration must be based on an N-1 0 notch or a 1/8-in. drilled hole.
Pipe with indications greater-than 80% of the reference signal are rejected. "Prove up," or manual characterization of indications, is not allowed.
SAW pipe is radiographed in accordance with API 5L. Seamless pipe is also tested in accordance with API 5L, except that paragraph 10.6b, regarding weld repair, does not apply.
All repair welds must be 100% magnetic particle tested and either radiographed or ultrasonically tested. The radiographs are to be available for review by Amoco, and Amoco reserves the right of final interpretation of all radiographs.
Pipe containing cracks or crack-like defects are to be rejected. Crack-like defects are defined as any feature in the pipe that cannot be distinguished from a crack with the typical NDE methods used to evaluate line pipe.
NDE operators must be certified to ASNT-TC-1A, Level 1, or equivalent. All personnel who interpret ultrasonic or radiographic test results must be certified to ASNT-TC-1A, Level 11 or equivalent. NDE procedures are to be the responsibility of personnel qualified in accordance with ASNT-TC-1A, Level III or equivalent.
DIMENSIONS, COMMITMENT
All pipe must be essentially straight. Deviation from a straight line is not to exceed 0.15% of the length. For a distance of 4 in. from each pipe end, the outside diameter cannot be more than -0.5% of nominal diameter.
The permitted wall-thickness variation for ERW and SAW pipe is + 10% to 4% of nominal wall. The wall-thickness tolerance for seamless pipe is + 15% to - 4% of nominal wall.
Amoco is committed to quality and the need to eliminate defects before they reach the field. Imposing requirements in excess of those of API 5L is intended to ensure that all new pipe put into the ground is of the highest quality commercially available.
Amoco's nondestructive examination, flattening test, and hydrostatic test requirements are intended to find and reject defective pipe at the mill before it gets to the job site.
The grain size and Charpy-impact requirements are designed to result in pipelines that have steel of high inherent toughness or resistance to crack propagation. The restrictions on sulfur and phosphorus also aid in this regard.
With pipe of high toughness, if a crack were to develop, the toughness of the pipe would resist its propagation, and a small leak, rather than a long, brittle fracture would be the result.
The restrictions on steel chemistry and carbon equivalent are intended to increase the weld-ability of the pipe. For new construction, this aspect is not of critical importance because preheat can be used to decrease the likelihood of cracking.
For repair welding, such as that during the installation of full encirclement sleeves with the pipe full of flowing liquid, preheating is not possible and hence the carbon equivalent of the pipe is a critical factor in determining whether the weld is likely to crack.
Thus, the restrictions on carbon equivalent are intended to ensure that Amoco installs pipe that is weld repairable in the future.
The tightened dimensional requirements, on the other hand, are intended to provide for better fit-up during the initial weld fabrication of the pipeline. Better fit-up will aid the welder in making high quality welds, with fewer repairs. The restriction on sulfur improves the steel cleanliness by lowering its inclusion content. In doing so, the pipe's resistance to stepwise cracking is enhanced.
While API 5L provides the basis or foundation for pipe quality, Amoco feels that imposing additional requirements, beyond those set forth in API 5L, is necessary to obtain the highest quality pipe commercially available.
By using only high quality pipe, Amoco hopes that some future pipeline failures involving ERW seam splits, hydrogen cracks at repair welds, and long running brittle fractures, can be avoided.
REFERENCES
- Kiefner, J. F., and Eiber, R. J. "Study shows shift in line pipe service problems," OGJ, Mar. 30, 1987, p. 98.
- Kiefner, J. F.. and Eiber, R. J., "Effects of hydrogen evident in recent pipeline failures," OGJ, Apr. 13, 1987, p. 38.
- Eiber, R. J., Jones, D. J., and Kramer, G. S., "Outside forces cause most natural gas pipeline failures," OGJ, Mar. 16, 1987, p. 52.
- Williams, D. N., and Eiber, R. J., "Test measures ERW line pipe weld ductility," OGJ, Apr. 8, 1985, p. 96.
- Patch, N. J., "The Ductile-Cleavage Transition in Alpha-iron," Fracture, Technology Press, Cambridge, Mass., 1959, pp. 54-67.
- Metals Handbook (9th edition), Vol. 1, p. 694.
- Parker. E.R., Brittle Fracture of Engineering Structures, 1957, p. 162.
- Metals Handbook (9th edition), Vol. 1, p. 693.
- Charles, J., Lemoine, Y., and Pressouyre, G. M., "Metallurgical Parameters Governing HIC and SSG Resistance of Low Alloy Steels," Paper 164, NACE Corrosion/86.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.