NTSB Safety Recommendation Letter P-80-012 through P-80-013

Page 1Official PDFPresident Northwestern Public Service Company Northwestern National Bank Building Huron. South Dakota 57350 SAFETY RECOMMENDAT I ON (5) P-80-12 and -13 A t 10:25 a.m., c.d.t., on August 28, 1978, natural gas, which had escaped from a circumferential fracture in a socket heat-fusion coupling on a 2-in. polyethylene (PE) main, operating at 40-psig pressure, migrated beneath a one-story house in Grand Island, Nebraska, exploded, and then burned. One person w a s injured; the house at 2517 Apache Road was destroyed; and three adjacent houses were damaged. The gas was ignited by an unknown sowce. The 2-in. PE main was installed by a contractor for the Northwestern Public Service Company at a depth of 30 in. in a northsouth utility easement along a lot line, terminating just south of 2517 Apache Road. The main was inspected and tested on November 12, 1971. On December 6, 1971, a 4-in. acrylonitrile butadiene styrene (ABS) sewer service was installed at a depth of 52 in. perpendicular to and just beyond t h e end of the gas main and was inspected. On August 22-24, 1972, the gas main was extended 495 ft., inspected, and tested. The failed coupling was 23 in. south of the coupling used to tie in the extension. When the flexible, 2-in. PE gas main was exposed after the accident, it w a s laying on the semirigid, 4411. ABS sewer pipe at a depth of 52 in. The coupling that failed was located at a depth of 48 in. A 64-in. section of pipe, from a point 27 in. north of the sewer line to a point 37 in. south of t h e sewer line, was displaced from its original depth of 30 in. This sharp displacement, caused by the settling of improperly compacted backfill material, initiated a stress in t h e coupling which, in time, resulted in the circumferential fracture on the bottom of the coupling. The downward force which caused the displacement might have been prevented if t h e company had followed the construction procedure outlined in the Northwestern Public Service Company specifications for plastic pipe installation. Section 18, BACKFILL, of these specifications contains the following instructions: The Contractor shall make sure the line is properly supported by undisturbed earth in the trench hottoin before the back-fill operation begins. Any voids under the h e shall be filled by hand. Special care shall be taken to assure proper support at transition points from steel to plastic. Where bell-holes have been dug for any reason, the back-fill shall be compacted below the line before mechanical back-fill operation begins. 2839#

Page 2-2- Other sections of the specifications make damage t o the pipe during the construction Services Company's construction procedures we G, apparently the procedures were not followed carefully during t h e construction of the main extension in August 1972. Therefore, the National Transportatio Northwestern Public Service Company: Emphasize the enforcement of company construction procedures b project engineers and inspectors through training programs an supervisory followup. (Class 11, Priority Action) (P-80-12) Examine sample locations of utility crossings and tie-ins, and take the necessary appropriate corrective measures to insure that mains are properly supported. (Class 11, Priority Action) (P-80-13) KING, Chairman, DRIVER, Vice Chairm concurred in these recommendations. GOLDM#