Ring expansion testing could provide a cost-efficient solution to the need to comply with a new regulation that will potentially require American utility operators to evaluate the integrity of their gas pipelines. Ring testing also could help prevent the re-occurrence of ruptures such as the disastrous San Bruno, CA explosion last year.
U.S. Transportation Department Secretary Ray LaHood recently admitted that a full review of the country’s gas transmission network was urgently needed in a bid to “fix America’s pipeline system.” His bold statement came at a press conference in May during which the Department of Transportation announced that it will issue new rules for the nation’s oil and gas pipeline operators this summer, responding to one of the worst gas pipeline explosions the nation has experienced in decades.
The San Bruno explosion occurred on Sept. 9 in a suburb of San Francisco, killing eight people and leveling 37 houses. The blast was so forceful that it created a crater 72 feet long, 26 feet wide and 40 feet deep along the sidewalk of one of the streets it affected. The immediate reaction from respondents to the incident was that an earthquake had hit, or that a jetliner from nearby San Francisco International Airport had crashed. It took rescue crews nearly an hour to establish that the cause was the rupture of a 30-inch steel natural gas pipeline, owned and operated by Pacific Gas & Electric.
According to as-built drawings, the affected pipeline was constructed using API 5L Grade X42 seamless pipe. However, initial investigations into the incident discovered that the pipeline in the area of the rupture was constructed with longitudinal seam-welded pipe. An examination of the ruptured area revealed that it was constructed from five sections of pipe, some of which were short pieces measuring about four feet long. These pieces contained different longitudinal seam welds of various types, including single- and double-sided welds. It is believed that inaccurate records such as these may lead to potentially unsafe Maximum Allowable Operating Pressures (MAOPs).
Urgent recommendations for the safe operation of America’s gas pipelines were published by the National Transportation Safety Board (NTSB) in January as a result of the preliminary findings of the investigation. The proposal urged pipeline owners to: 1) conduct an intensive records search to identify gas transmission lines that had not undergone testing designed to validate a safe operating pressure; 2) determine the maximum operating pressure based on the weakest section of pipeline or component identified in the records search; and 3) if operators are unable to determine a safe operating pressure via these methods, then a safe operating pressure should be determined by a specified testing regimen.
While the nature of the pipeline test outlined by the NTSB has not yet been proposed, the California Public Utilities Commission, which regulates pipeline operators in the state, has already planned new laws requiring operators to investigate the condition of older networks or replace them. It is only a matter of time before similar directives are introduced across the nation.
The anticipation of new regulation on the operating pressures and damage tolerance of gas pipelines has caused debate among transmission and distribution professionals over the most effective method to evaluate the maximum operating pressure of pipelines. One answer lies in a technology that has been practiced by the industry since the 1960s, consistently producing accurate and reliable results.
Ring expansion testing – also known as ring tension testing – has been used by technical advisors to the gas industry, such as GL Noble Denton, for more than three decades as a mid-scale test for the evaluation of the yield strength and damage tolerance behavior of steel transmission pipelines.
A 76-mm-wide ring is cut from the pipe, sandwiched between two steel platens and pressurized with hydraulic oil (Figure 1). The yield strength of the pipe is then determined by the relationship between the internal pressure and circumferential expansion of the ring, while damage tolerance can be assessed by measuring the failure pressure of rings with artificial defects, such as dents, fatigue cracks and reduced ligament thickness. A schematic of the rig used to perform the test is outlined in Figure 2.
Figure 2: Schematic of test rig.
Ring expansion testing is trusted by the gas industry because it provides a cost-effective reliable approach to evaluating the condition of a pipeline. Only a segment of pipeline is needed to perform the test, as opposed to other methods of testing such as a full-scale burst test. During the test, the ring specimen experiences radial expansion and, in effect, behaves as an infinitely long pipeline. As such, defects across the width of the ring can also be regarded as infinitely long axial defects along the length of a pipeline.
A major benefit of ring tension testing over conventional tensile testing (such as round bar or flattened strap) is that both the full pipe circumference and wall thickness are assessed. The flattening process reduces yield stress on the outer radius of the specimen where the material is put into compression (as a result of the Bauschinger effect) and, on the inner radius, the yield stress is increased as a result of cold working the material in tension. As such, a residual strain gradient is set up in the material which can falsely lower the yield point, depending on the grade of material and degree of flattening required.
Round bar specimens give a more realistic measure of material behavior but they only sample a part of the pipe wall thickness. Ring expansion testing comes closer to the actual pipe situation.
The cost-effective nature and flexibility of ring expansion testing makes it an ideal candidate to support LaHood’s recent pledge to conduct a “top-down review” of America’s pipeline system, in which he expects to see the immediate replacement of any pipelines in a “critical condition.” Ring expansion testing is useful for assessing the integrity of a pipeline that’s already in service, and test data can also be used to assess the specifications of material used when purchasing replacement lines, in addition to evaluating new pipeline designs against conventional tensile specimen data.
GL Noble Denton’s Asset Management team came across an example of this recently when a client discovered instances of low seam weld toughness in the latter stages of constructing a major offshore gas pipeline. A program of ring expansion tests established the potential for failure in the event of damage from mechanical interference in the vicinity of the seam weld (Figure 3), and the results defined the most critical operating conditions for the asset. These were subsequently evaluated using full-scale tests, and the client was able to accept the pipe, avoiding lengthy project construction delays and many costly full-scale tests.
Figure 3: Detail showing dent/gouge defect on 1,220-mm OD X80 grade pipe ring.
The San Bruno incident has uncovered some significant issues with the way that the operating pressures in America’s gas pipelines are tested and monitored and it is likely that the regulation that will be introduced by the Department of Transport in August will address this.
While there are a number of methods for testing pipes – from full-scale burst tests to high-pressure hydrotests – it is in the interests of both regulator and operator to focus on a solution that is both cost-effective and flexible in its application. It is suggested that the frequent use of ring expansion testing could make a significant contribution in the avoidance of further devastating incidents such as San Bruno. Contact: www.gl-nobledenton.com
Neil Bramley, FIMechE, is manager for the Engineering Analysis team at GL Noble Denton, based in Loughborough, UK. The team is multidisciplinary, able to combine expertise in materials, corrosion, chemistry, testing, vibration and finite element analyses. He has been in his current role for five years and has been a chartered engineer for over 16 years. He started his career in 1987 after graduating with honors in mechanical engineering. He began by undertaking engineering analysis specializing in the assessment of nuclear industry pressure systems. He joined GL Noble Denton in 1998 as a senior engineer continuing to work in engineering analysis with an emphasis on pipelines and pressure vessels.
Andrew Brealey is a consultant with GL Noble Denton’s Vibration and Mechanical Engineering Services team in Loughborough, UK. He has been with GL for nine years, and specializes in mid-scale and full-scale pressure testing of pipes and fittings along with developmental and investigative projects associated with oil and gas transmission and distribution.