Page 66 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Riskvariables 3/45
Risk variables
their burial 2.5 to 3 feet deep. However, encroachments ofpop-
ulation and land development activities are routinely threaten- Many mitigation measures are in place in most Western coun-
ing many pipelines today. tries to reduce the threat of third-party damages to pipelines.
In the period from 1983 through 1987, eight deaths, 25 Nonetheless, recent experience in most countries shows that
injuries, and more than $14 million in property damage this remains a major threat, despite often mandatory systems
occurred in the hazardous liquid pipeline industry due solely to such as one-call systems. Reasons for continued third-party
excavation damage by others. These types of pipeline failures damage, especially in urban areas, include
represent 259 accidents out of a total of 969 accidents from all
causes. This means that 26.7% of all hazardous liquid pipeline
accidents were caused by excavation damage 1871. Smaller contractors ignorant of permit or notification
In the gas pipeline industry, a similar story emerges: 430 process
incidents from excavation damage were reported in the No incentive for excavators to avoid damaging the lines
1984-1987 period. These accidents resulted in 26 deaths, 148 when repair cost (to damaging party) is smaller than avoid-
injuries, and more than $18 million in property damage. ance cost
Excavation damage is thought to be responsible for 10.5% of Inaccurate mapshecords
incidents reported for distribution systems, 22.7% of incidents Imprecise locations by operator.
reported for transmissiodgathering pipelines, and 14.6% of all
incidents in gas pipelines [87]. European gas pipeline experi- Many of these situations are evaluated as variables in the
ence, based on almost 1.2 million mile-years of operations in suggested risk assessment model.
nine Western European countries, shows that third-party inter- The pipeline designer an4 perhaps to an even greater extent,
ference represents approximately 50% of all pipeline failures the operator can affect the probability of damage from third-
[441. party activities. As an element ofthe total risk picture, the prob-
ability of accidental third-party damage to a facility depends on

Exposure The ease with which the facility can be reached by a third
party
To quantify the risk exposure from excavation damage, an esti- The frequency and type ofthird-party activities nearby.
mate of the total number of excavations that present a chance
for damage can be made. Reference 1641 discusses the Gas Possible offenders include
Research Institute’s (GRI’s) 1995 study that makes an effort to
determine risk exposure for the gas industry. The study sur- Excavating equipment
veyed 65 local distribution companies and 35 transmission Projectiles
companies regarding line hits. The accuracy of the analysis was Vehicular traffic
limited by the response-less than half (41 %) of the companies Trains
responded, and several major gas-producing states were poorly Farming equipment
represented (only one respondent from Texas and one from Seismic charges
Oklahoma). The GRI estimate was determined by extrapolation Fenceposts
and may be subject to a large degree of error because the data Telephone posts
sample was not representative. Wildlife (cattle, elephants, birds, etc.)
Based on survey responses, however, GFU calculated an Anchors
approximate magnitude of exposure. For those companies Dredges.
that responded, a total of25,123 hits to gas lines were recorded
in 1993; from that, the GRI estimated total U.S. pipeline Factors that affect the susceptibility of the facility include
hits in 1993 to be 104,128. For a rate of exposure, this number
can be compared to pipeline miles: For 1993, using a reported Depth of cover
1,778,600 miles of gas transmission, main, and service Nature of cover (earth, rock, concrete, paving, etc.)
lines, the calculated exposure rate was 58 hits per 1000 line Man-made barriers (fences, barricades, levees, ditches. etc.)
miles. Transmission lines had a substantially lower experience: Natural barriers (trees, rivers, ditches, rocks, etc.)
a rate of 5.5 hits per 1000 miles, with distribution lines suffer- Presence of pipeline markers
ing 71 hits per 1000 miles [64]. All rates are based on limited Condition of right ofway (ROW)
data. Frequency and thoroughness of patrolling
Because the risk of excavation damage is associated with Response time to reported threats.
digging activity rather than system size, “hits per digs” is a
useful measure of risk exposure. For the same year that GRI The activity level is often judged by items such as:
conducted its survey, one-call systems collectively received
more than an estimated 20 million calls from excavators. Population density
(These calls generated 300 million work-site notifications Construction activities nearby
for participating members to mark many different types of Proximity and volume of rail or vehicular traffic
underground systems.) Using GRI’s estimate of hits. the risk
exposure rate for 1993 was 5 hits per 1000 notifications to dig
~41. Number of other buried utilities in the area.