PAB/99-02

E PL UR IBUS U NUM

NATIONAL TRA SPORTATIO

N

BOARD

SAFE T Y

N National Transportation Safety Board

Washington, D.C. 20594

Pipeline Accident Brief

Accident No.: DCA-97-FP-005

Type of System: Transmission pipeline

Type of Accident: Rupture and fire

Location: Indianapolis, Indiana

Date and Time: July 21, 1997; 2:33 p.m. CST

Owner/Operator: Citizens Gas & Coke Utility

Property Damage and Losses: In excess of $2 million

Fatalities: 1

Injuries: 1

Evacuated: 75

Material Released: Natural gas

Pressure: 310 psig

Type of Failure: Excavation damage

Component Affected: 20-inch-diameter buried steel transmission pipeline

The Accident

About 2:33 p.m. on July 21, 1997, a 20-inch-diameter steel natural gas

transmission pipeline owned and operated by Citizens Gas & Coke Utility Company

(Citizens Gas) ruptured and released natural gas near an intersection adjoining the Charter

Pointe subdivision in Indianapolis, Indiana. The gas ignited and burned, killing one

resident and injuring another. About 75 residents required temporary shelter. Six homes

were destroyed, and about 65 others sustained damage significant enough to be

documented by the local investigation team.

The pipeline had not been under full internal pressure since March 31, 1997, when

the pressure was reduced to approximately 30 pounds per square inch gauge (psig) in

anticipation of road construction work in the area.1 On May 19, 1997, a directional drilling

operation had been performed to install an 8-inch-diameter steel natural gas distribution

main parallel to the transmission pipeline. About 1 hour and 40 minutes before the rupture,

Citizens Gas had begun to return the 20-inch transmission pipeline to full service.

 

1

 The road construction work required that Citizens Gas relocate an adjacent section of its 20-inch

transmission pipeline. In view of the reduced seasonal demand for natural gas, the company decided to

reduce the pressure on the pipeline and discontinue using it to transport gas during the period of

construction.
2

Postaccident Inspections

On-site inspection of the ruptured pipe revealed a near-longitudinal gaping fracture

about 5.8 feet long. Along the entire length of the longitudinal fracture, the newly installed

8-inch distribution main was within approximately 4 inches of the 20-inch transmission

pipeline. (See figure 1.)

The ruptured segment of the 20-inch pipe was removed and sent to the National

Transportation Safety Board’s Materials Laboratory for examination. Safety Board

examination revealed that the pipeline fractures contained features typical of overstress

separation. The fractures contained no arrest marks, which is consistent with a crack in the

pipe that, once initiated, propagated continuously. The exterior pipeline surface along the

entire length of the longitudinal fracture contained circumferential gouge marks that

intersected both halves of the fracture. The gouge marks typically measured approximately

4 inches long in the circumferential direction. Wall thicknesses in the deepest area of two

gouge marks adjacent to the longitudinal fracture were 0.168 inch and 0.175 inch, which

were 60 and 62 percent, respectively, of the original 0.281-inch wall thickness.

Safety Board investigators obtained a tooth from the reaming tool that was used in

the drilling operation to install the 8-inch distribution main. Direct comparisons of the

tooth with the gouge marks revealed that the size and shape of the tooth tip were

Figure 1. Postaccident excavation revealing relative positions of 8-inch distribution main

and 20-inch transmission pipeline at site of rupture
3

consistent with the size and shape of the gouge marks in the vicinity of the rupture. (See

figure 2.)

Preaccident Pipeline Installation Project

To minimize damage to the roadway and to some adjoining landscaping features,

Citizens Gas decided to use directional drilling2

 to install an approximately 200-foot-long

section of new 8-inch distribution main parallel to the 20-inch transmission pipeline.

Citizens Gas contracted with Miller Pipeline Corporation (Miller), which carried out the

directional drilling operation on May 19, 1997.

Citizens Gas construction plans called for the 8-inch distribution main to be

installed parallel to the 20-inch pipeline with a horizontal centerline-to-centerline

separation of approximately 5 feet along most of its length. Before drilling began, Citizens

Gas personnel used paint to mark the approximate location of the 20-inch pipeline at

intervals of 15 to 20 feet.3

 Two excavations, one to verify the location of some utility

 

2 Directional drilling, also referred to as boring, is a pipeline construction technique in which a

roughly horizontal hole is drilled at the intended pipeline location and the pipeline is then pulled through

it. Normally, a relatively small pilot hole is drilled first. When the pilot drill reaches the exit location, the

pilot head is removed, and a larger reaming tool is attached. In this case, the pipeline was pulled back

through the hole during this back-reaming process.

3

 Indiana State Code (IC 8-1-26-2) defines approximate location as “a strip of land at least four (4)

feet wide but not wider than the width of the facility plus two (2) feet on either side of the outer limits of

the physical plant.” See Safety Study—Protecting Public Safety Through Excavation Damage Prevention

Figure 2. Comparison of cutting tool tooth with gouges found at rupture site
4

cable crossings and another to serve as an exit pit for the directional drilling operation,

exposed the 20-inch transmission pipeline and thus verified its location.

Miller drilling personnel stated that field measurements indicated that they could

maintain an approximate 5- to 7-foot horizontal separation between the new installation

and the paint marks used to indicate the location of the existing 20-inch transmission

pipeline. The drilling crew intended to maintain this separation throughout the bore except

for one location near the termination of the bore where, in order to avoid an underground

telephone duct, the pipeline separation would need to be reduced to an edge-to-edge

horizontal distance of about 1 foot.4

The Miller crew consisted of two persons, a foreman and a machine operator,

utilizing a Vermeer directional drilling rig with a 5-inch-diameter drill head and a 14-inchdiameter reaming tool. The foreman said that throughout the directional drilling operation,

he monitored the location and alignment of the pilot drill head every 5 to 10 feet with a

“walkover” detector. He said he marked the drill head’s location at these intervals as the

boring operation progressed to the east. If course corrections were needed, the foreman

said, he relayed instructions to the machine operator via a headset radio. The Miller

personnel reported that, because of the terrain at the entry point, the initial entry of the

drill head was angled slightly toward the 20-inch natural gas transmission pipeline and that

minor steering corrections were performed in the beginning portion of the drilling

operation. Miller personnel did not report any difficulty in performing the drilling

operation, nor did they note any other unplanned deviations from a straight line path.

During the back-reaming process, the location of the reaming tool was not tracked

as it was during drilling. The presence of rocks, the characteristics of the soil, the

alignment of the pilot hole, and the stiffness of the pipe material being pulled back through

the hole are some factors that can affect the accuracy with which the back-reaming

process follows the path of the pilot hole.

Actual Pipeline Locations

After the accident, the two pipelines were excavated at points along the bore path

to determine the exact horizontal centerline-to-centerline separation between the pipelines

and the depth of cover over each one. These excavations revealed that the 8-inch

distribution main was located at a horizontal centerline-to-centerline distance of 5.5 feet

north of the 20-inch transmission pipeline at the entrance point of the bore (west end).

Progressing eastward, the 8-inch distribution main veered sharply toward the 20-inch

transmission pipeline. Approximately 40 feet east of the entrance point of the bore, the 8-

inch distribution main was within a few inches of the existing 20-inch transmission

pipeline. At the rupture site, approximately 68 feet east of the entrance point of the bore,

the two pipelines were about 4 inches apart. As the 8-inch pipeline continued toward the

 

(NTSB/SS-97/01) for further discussion.

4

 The actual separation would be greater than 1 foot, since the pipelines were to be at different depths

at this particular location.
5

east, it veered away from the 20-inch pipeline to a horizontal centerline-to-centerline

separation of 3.3 feet about 113 feet east of the entrance point of the bore. From this point

eastward, the horizontal centerline-to-centerline separation ranged from 4 to 6 feet until

the 8-inch distribution main angled toward the 20-inch transmission pipeline at the east

end of the bore, as originally planned. (See figure 3.)

During the postaccident excavations conducted to determine the locations of the

pipelines, rocks were discovered east of the rupture site. One rock, located directly

beneath the 8-inch distribution main, had been scored in a manner consistent with having

been struck by the reaming tool.

Regulatory Requirements

Title 49 Code of Federal Regulations (CFR) Part 192.6145 requires gas pipeline

operators to develop and follow a written program to prevent damage to their pipelines

from excavation activities. Excavation activities includes boring operations. Paragraph

(c)(6) specifically requires operators, whenever they have reason to believe their pipelines

could be damaged by excavation activities, to provide for inspections of those pipelines as

frequently as necessary during and after the excavation to verify the integrity of the

pipelines.

The Gas Piping Technology Committee (GPTC) publishes the GPTC Guide for

Gas Transmission and Distribution Piping Systems to provide guidance for natural gas

operators to follow to help ensure regulatory compliance.6 The GPTC has informed the

Safety Board that it is in the process of adding material to the guide to address safety

issues specific to directional drilling and other trenchless technologies.

Actions Taken Since the Accident

Citizens Gas had procedures in place at the time of the accident that were

applicable to general construction activities in proximity to its pipelines, but it did not have

procedures specific to directional drilling operations. Citizens Gas has provided to the

Safety Board copies of procedures that it has now adopted that are specific to directional

drilling in proximity to existing pipelines. These procedures include a requirement that,

whenever directional drilling operations are carried out parallel to and within 10 feet of an

existing transmission pipeline, spot holes will be excavated at 25-foot intervals so that the

exact location of the drill head can be visually determined. These spot holes will also allow

personnel to verify pipe alignment as the pipe is installed during the back-reaming process.

 

5

 Title 49 CFR Part 195.442 contains similar provisions for operators of pipelines transporting

hazardous liquids.

6

 The GPTC is an American National Standards Institute technical committee made up of technical

specialists from both industry and government.
20“ Transmission Pipeline

8“ Distribution Main

Rupture 

 Site

96th Street

Landscaped Area

Charter Driv

e

Scale in Feet

0 100

Area of Cable Crossings Area of Exit Pit

Figure 3. Relative locations of new 8-inch and existing 20-inch pipelines
7

Miller has also provided the Safety Board with procedures it has adopted to

provide additional protection for existing facilities during the company’s directional

drilling operations.

Probable Cause

The National Transportation Safety Board determines that the probable cause of

this accident was the failure of Citizens Gas & Coke Utility and Miller Pipeline

Corporation to have adequate controls in place to ensure that directional drilling

operations carried out in the proximity of existing underground facilities would not cause

damage to those facilities.

Adopted: April 20, 1999