NTSB Safety Recommendation Letter P-98-014 through P-98-015

Page 1Official PDFNational Transportation Safety Board Washington, D.C. 20594 Safety Recommendation Date: A p r i l 3 0 , 1998 In reply refer to: P-98-14 and -15 Mr. Stanley .J. Bright Chairman, President, and Chief Executive Officer MidAmerican Energy Corporation 666 Grand Avenue Post Office Box 657 Des Moines, Iowa 50303-0657 At 10:07 a.m. central daylight savings time on Monday, October 17, 1994, a natural gas explosion and fire destroyed a one-story, wood frame building in Waterloo, Iowa. The force of the explosion scattered debris over a 200-foot radius. The Safety Board investigation determined that natural gas had been released from a plastic service pipe that had failed in a brittle-like manner at a connection to a steel main.' Six persons inside the building died, and one person sustained serious injuries. Three persons working in an adjacent building sustained minor injuries when a wall of the building collapsed inward from the force of the explosion. The explosion also damaged nine parked cars. A person in a vehicle who had ,just exited the adjacent building suffered minor injuries. Additionally, two firefighters sustained minor injuries during the emergency response, Two other nearby buildings also sustained structural damage and broken windows. The National Transportation Safety Board determined that the probable cause of the explosion and fire was stress intensification, primarily generated by soil settlement at a connection to a steel main, on a 1/2-inch polyethylene pipe that had poor resistance to brittle-like cracking. Safety Board examination of the fracture surface and failed pipe from the Waterloo accident revealed evidence of stress intensification. For example, the upper portion of the inside of the failed pipe showed the impression of the edge of the tee stiffener, indicating that the top of the pipe had been pressed down. The failure of the pipe can be directly associated with this stressed area, which was characterized by several brittle-like slow crack growth fractures that 'For more information, see appendix A (Pipeline Accident Brief of Waterloo, Iowa, accident) to National Transportation Safety Board Pipeline Special Investigation Report--Brirrle-like Cr ackirig in Plastic Pipe f i r Gas Setvice (NTSBISIR-98IO I). 6984#

Page 22 originated on or near the pipe inner wall just outside the depression associated with the tip of the tee stiffener, These slow crack fractues propagated through the wall of the pipe The stress intensification noted in the Waterloo pipe was consistent with the pipe’s having been subjected to shear and bending forces generated primarily by soil settlement.’ Soil settlement is a common source of stress intensification for buried plastic pipelines, and it can occur and contribute to a piping fiilure even though no observable voids are noted during a subsequent excavation. Ultimate settlement of backfill can take many years, and sometimes it only occurs after periods of heavy rains (such as the area experienced the previous year) or under additional external loading (such as that represented by truck traffic over the connection). The accident investigation could not determine whether the ground settlement at Waterloo occurred because of inadequate compaction and support under the connection at the time it was installed, or whether it occurred despite initial adequate compaction and support. Nor could it be conclusively determined whether the amount of soil settlement was slight and generated relatively low stresses over a long period of time, or whether the soil settlement was large and generated relatively high stresses over a relatively short period of time. Because of these uncertainties, investigators could not determine how much more resistance to crack initiation and slow crack growth the pipe would have needed to have successfully resisted the stresses to which it was subjected. An examination of MidAmerican Energy’s recent constmction standards for minimizing shear and bending forces at plastic service connection points to steel mains revealed that MidAmerican Energy had no standard that called for firm compacted support under these connections. MidAmerican Energy connected plastic service pipe to mains with steel tapping tees welded at the factory to factory-joined plastic-to-steel transition fittings, Although MidAmerican Energy designed its own protective sleeves for this application, it did so without a design criteria for length or inner diameter, or for positioning the protective sleeves,. Without such criteria, MidAmerican Energy may reduce the sleeve’s effectiveness in limiting stress intensification. The Safety Board concluded that, because MidAmerican Energy’s gas construction standards do not establish well-defined criteria for supporting plastic pipe connections to steel mains or for designing or installing its protective sleeves at these connections, these standards do not ensure that connections will be adequately protected from stress intensification. Federal regulations require that gas pipeline system operators have procedures for monitoring gas system failures and leakage histories, analyzing failures, and submitting failed samples for laboratory examination, all intended to help determine the causes of failures so that action can be taken to minimize the possibility of recunences. Before the Waterloo accident, Midwest Gas developed only a limited capability for monitoring and analyzing the condition of its gas system. For example, the company did not statistically conelate failure rates to the 2 The failed pipe also showed signs that the installed horizontal curve may have generated horizontal bending forces Other factors contributing to stress at the connection included the pipe’s internal pressure and may have included residual stresses inside the wall of the pipe resulting from the manufacturing process f 1#

Page 33 amounts of installed pipe or components provided by specific manufacturers. The design of the program meant that the relatively few areas with high failure rates (for example, those with Century pipe) were aggregated with and therefore masked by the large number of plastic piping installations that had low failure rates. Thus, the Midwest Gas surveillance program did not reveal the high failure rates associated with Century pipe. Only after the accident did Midwest Gas identify the Century pipe within its pipeline system as having high failure rates, even though the company could have collected and processed the same type of data and reached the same detetmination before the accident. If Midwest Gas had further correlated its data to years of installation, it may have also been able to examine the effects of its changing installation methods or changes in performance with different manufacturers through the years. The Safety Board concluded that, before the Waterloo accident, the systems used by Midwest Gas Company for tracking, identifying, and statistically characterizing plastic piping failures did not permit an effective analysis of system failures and leakage history. The Safety Board further concluded that if, before the Waterloo accident, Midwest Gas bad had an effective surveillance program that tracked and identified the high leakage rates associated with Century piping when subjected to stress intensification, the company could have implemented a replacement program for the pipe and may have replaced the failed service connection before the accident. Since the accident, MidAmerican Energy has revised its systems, adding parameters to provide the company with added capability to sort failures. However, MidAmerican Energy did not chose parameters that will allow an adequate analysis of its plastic piping system failures and leakage history. For example, the generic “improper installation” is a parameter to be linked to leaks; however, no parameters were added for the presence, lack, improper design, or improper placement of a protective sleeve. And no parameters were added to link leaks to squeeze locations, improper joining, or items to differentiate between insufficient support and excessive installed bending. The Safety Board therefore concluded that MidAmerican E,nergy’s current systems for tracking, identifying, and statistically characterizing plastic piping failures do not enable an effective analysis of system failures and leakage history. An effective surveillance program would include the data base inputs that would allow the company to adequately monitor and characterize the types and causes of plastic piping field failures. The A.G.A. Plusric Pipe Manual for Gus Service recommends the use of a form for recording necessary information on plastic piping failures; this form may be helpful to MidAmerican Energy as it decides which data fields would be necessary to provide for an adequate analysis of its plastic piping system failures and leakage history. The A. G.A Plusric Pipe Manual .for Gus Service further recommends collecting this information, then performing visual examinations of the type and cause of failure and, in some instances, a laboratory analysis. The above steps may help MidAmerican Energy comprehensively monitor and address parts of its plastic pipeline system--ather than those installations with Century pipe-that may also indicate unacceptable performance. The National Transportation Safety Board thexefore makes the following safety recommendations to Mid American Energy Corporation:#

Page 44 Modify your gas construction standards to require (1) firm compacted support under plastic service connections to steel mains, and (2) the proper design and positioning of protective sleeves at these connections. (P-98-14) As a basis for the timely replacement of your plastic piping systems that indicate unacceptable performance, review your existing plastic piping surveillance and analysis program and make the changes necessary to ensure that the program is based on sufficiently precise factors such as piping manufacturer, installation date, pipe diameter, geographical location, and conditions and locations of failures. (P-98-15) Also, the National ‘Transportation Safety Board issued Safety Recommendations P-98- 1 through -5 to the Research and Special Programs Administration; P-98-6 to the Gas Research Institute; P-98-7 through -9 to the Plastics Pipe Institute; P-98-10 to the Gas Piping ‘Technology Committee; P-98-11 and -12 to the American Society for Testing and Materials; P-98-13 to the American Gas Association; P-98-16 and -1 7 to Continental Industries, Iric”; P-98-18 to Dresser Industries, Inc.; P-98-19 to Inner-Tite C.orporation; and P-98-20 to Mueller Company. The National Transportation Safety Board is an independent Federal agency with the statutory responsibility “to promote transportation safety by conducting independent accident investigations and by formulating safety improvement recommendations” (Public Law 93-633). ‘The Safety Board is vitally interested in any action taken as a result of its safety recommendations. ‘Therefore, it would appreciate a response from you regarding action taken or contemplated with respect to the recommendations in this letter. Please refer to Safety Recommendations P-98-14 and -15 in your reply. If you need additional information, you may call (202) 314-6469. Chairman HALL, Vice Chairman FRANCIS, and Members HAMMERSCHMIDT, GOGLIA, and BLACK concurred in these recommendations. By:#