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The 2010 Chilean earthquake: gas
distribution system resilience
by Enrique Acuna C
Dandilion Ingeniería Ltda, Santiago, Chile
T
HIS ARTICLE provides a quick review of the gas distribution pipeline systems’ behaviour during and
after the Cobquecura earthquake; with an 8.8Mw1 magnitude, this was the fifth biggest earthquake since
1900 and the seventh (by magnitude) ever recorded in human history.
Modern transmission and distribution piping systems endured the earthquake effects with no fatalities,
serious injuries, or property damage related to gas pipe failures. Some marginal supply loss occurred due to
customers’ decisions to cut-off the gas supply and some under-pressure-protection actuation. Gas service
was restored within two to three weeks.
In the city of Concepcion and the port of Talcahuano, the area most affected by the Cobquecura earthquake
and following tsunami, an old cast iron gas network collapsed because of earth settlement and subsequent
flooding. On one hand, service restoration and cast iron pipe insertion took between 7 and 11 weeks to
get maximum penetration; on the other hand, 97km were impossible to rehabilitate, and 500 customer
services have had to be converted to LPG.
There are no available official figures and evaluation of the effect of the Cobquecura earthquake on gas
transmission and distribution systems. Information is not available either from companies or authorities.
The figures presented in this article are general information acquired from public sources and unofficial
discussions, and are not from an official damage report.
Cobquecura earthquake data
The 2010 Chilean earthquake happened at 03.34 on
Saturday, 27 February, with a magnitude of 8.8Mw1. The
epicentre was in the Chilean sea offshore Cobquecura village,
at a depth of 47.4km below the seabed. The earthquake had
a duration of 2mins and 45secs, and was felt in most of the
Southern Cone of South America from Ica in Peru in the
north, to Buenos Aires and Sao Paulo in the east.
The earthquake was generated by the subduction energy
accumulation between the Nazca and South American Plates
during a period of over 100 years. The fracture length was
453km, with a plate displacement of 10m.
With an 8.8Mw magnitude this was the fifth biggest
earthquake in magnitude since 1960, and the second biggest
in Chile after the Valdivia earthquake of 1960, which had a
magnitude of 9.5ML1. The areas most affected were Regions
1 * Mw: Seismic moment magnitude which represents earthquake energy liberation.
This magnitude scale is coincident with Richter scale (ML).
Author’s contact details:
tel: +56 2 42 80 800
email: [email protected]
V to IX, with 13 million inhabitants (80% of the country’s
population). There were 521 fatalities and 56 disappearances
reported, and a preliminary figure of US$30,000 million
in infrastructure damage and loss.
Cobquecura tsunami and
Pichilemu earthquake
After the Cobquecura earthquake, a tsunami hit the
epicentre area. Over 200 aftershocks with magnitudes of
4.9Mw or greater also hit the area during four days after the
earthquake. The maximum aftershock magnitudes were 6.9,
6.7, and 6.6Mw, in a group of 17 aftershocks of a magnitude
greater than 6.0Mw.
Two weeks after the Cobquecura earthquake, on 11 March
at 11.39, a new earthquake of 7.2Mw magnitude occurred
at the coastal town of Pichilemu. Its epicentre location
was 9km offshore at a depth below the seabed of 35km.
Immediately following the Pichilemu earthquake three
aftershocks occurred with intensities of 6.9, 6.7, and 6.0Mw,
the last at 12.06. This new earthquake hit the same rupture
area as the first earthquake.
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Fig.2. Largest (Richter magnitude ML ≥ 8) earthquakes
since 1900.
Fig.1. Cobquecura earthquake location and intensity.
Gas pipelines hit by the
Cobquecura earthquake
Four major cities with gas distribution pipelines were
in the influence area of the Cobquecura and Pichilemu
earthquakes.
The nearest city with gas distribution pipelines is
Concepcion, 105km SW of the epicentre, then Temuco
at 285km to the south, Santiago at 335km to the NE, and
Valparaiso at 375km to the north. In the following sections
a brief description of the Cobquecura earthquake’s effects
on each city will be given, in reverse order of their distance
from the epicentre. Reference is made to the Mercalli scale
of earthquake-intensity measurement (which uses Roman
numerals) – see box. Some comments will also be made
about the tsunami and the Pichilemu earthquake.
Fig.3. Fracture area and intensity.
Santiago area (Mercalli intensity:VIII)
Valparaiso area (Mercalli intensity:VIII)
Valparaiso’s natural gas distribution system consists of
1,700km of newly-installed pipe, 12 years old or less. The
distribution system, serving 84,000 customers, is mainly
composed of 4-bar polyethylene pipe and a around 10km
and 30-bar API 5L steel pipelines of 4in to 12in diameter.
No fatalities or injuries from gas emergencies were reported
during or after the two earthquakes. Some landslides in
hilly areas occurred, but with no risk to pipeline integrity.
There was a gas supply interruption to approximately 15%
of customers, and most of them voluntarily proceeded
to cut-off their own gas supplies. A minimum amount of
cut-offs were produced service regulator action. The gas
service was restored within three weeks of the interruption,
although this does not include service to damaged buildings
or interior networks, where gas restoration could mean a
risk to customers.
Santiago, the Chilean capital, has an extended gas
distribution network serving more than 450,000 customers.
Two different distribution systems coexist:
• The natural gas system, built in 1997-2000, is a
network of more than 4,500km length. The majority
of the network is composed of 4-bar polyethylene
pipe, complemented with 10-bar and 35-bar API 5L
steel pipeline in 6in to 20in diameters.
• The manufactured gas system is mainly composed
of 100-year-old low-pressure (20-mbar) cast iron pipe
and 300-mbar higher-pressure steel pipelines.
No fatalities or injuries were reported due to gas system
emergencies or explosions or fires. There was an increase
in emergency calls driven by customers’ perception of threat
rather than real gas releases.
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As in the Valparaiso area, gas supplies were cut off by
customers themselves and a number of service regulators
were actuated. Service interruption affected about 13% of
Santiago’s customers and the service was restored within
three weeks. Service to a minimum number of customers
was not restored because of damaged buildings or internal
network leaks.
Parallel to the restoration process, a contingency procedure
was activated by the local distributor in order to detect
leaks in the manufactured gas system area. There was no
significant amount of new leaks found.
Temuco area (Mercalli intensity:VIII)
Photo 1. Concepción multi-story building collapse.
Temuco has a propane-air gas-distribution system, serving
about 15,000 customers. The 480-km long network
is composed of new 4-bar polyethylene pipelines. No
emergencies or major service interruptions were reported
by the local distributor.
Concepción area (Mercalli intensity: IX)
The pipelines in the city of Concepción and the port of
Talcahuano were those most affected by the Cobquecura
Earthquake and tsunami.
The Concepción distribution system serves 35,000 customers
and has a total network length of 700km. The system is
composed by two subsystems:
Photo 2. Concepción residential area street settlement.
• a natural gas distribution system, built in 1998,
composed of 4-bar polyethylene and 10-bar API 5L
steel pipelines
• a methane-air gas-distribution system composed of
a 200-km cast iron network, previously transporting
manufactured gas, and converted to this new service
in 1999. Operation pressure was maintained at
20mbar.
No fatalities or major injuries were reported due to gas
system failures, and neither were there any explosions or
fires. A cast iron pipe failed because of soil settlement,
subsidence, and vibration.
Photo 3. Concepción residential area sidewalk settlement.
In the Talcahuano area many pipelines were flooded and
meter assemblies were washed away by the tsunami.
A massive interruption of service was decided upon by
Concepción LDC Operation Group, cutting-off the mixing
plant delivery. This action affected approximately to 35%
of the customers.
The service restoration process faced many difficulties,
among which were power supply and telecommunications
cut-off, difficult site accessibility and poor safety, huge
amounts of debris accumulation due to building collapses
Photo 4. Concepción downtown debris.
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The Modified Mercalli Intensity Scale
Photo 5. Concepción street settlement.
Photo 6.Talcahuano coastal avenue debris after the tsunami.
The effect of an earthquake on the Earth’s surface is
called the intensity, and the Mercalli intensity scale
consists of a series of certain key responses graduating
from people awakening to total destruction. Although
numerous intensity scales have been developed over
the last several hundred years to evaluate the effects
of earthquakes, the one currently used in the United
States is the Modified Mercalli Intensity (MMI) scale.
Developed in 1931 by the American seismologists
Harry Wood and Frank Neumann, the MMI scale is
composed of 12 increasing levels of intensity that range
from imperceptible shaking to catastrophic destruction,
designated by Roman numerals. It does not have a
mathematical basis; instead it is an arbitrary ranking
based on observed effects.
The MMI value assigned to a specific site after an
earthquake has a more meaningful measure of
severity to the non-scientist than the Richter (or other)
magnitude because intensity refers to the effects actually
experienced at that place. The lower numbers of the
MMI scale generally deal with the manner in which the
earthquake is felt by people, while the higher numbers
are based on observed structural damage. Structural
engineers usually contribute information for assigning
intensity values of VIII or above.
The following is an abbreviated description of the 12
levels of the MMI scale
I. Not felt except by a very few under especially
favourable conditions.
II. Felt only by a few people at rest, especially on
upper floors of buildings.
Photo 7. North Talcahuano tsunami debris.
and street ruptures. When potable water service was restored,
some water pipe ruptures produced bellhole flooding, etc.
All these aggravating circumstances hindered data collection,
analysis, and response planning.
In the Concepción – Talcahuano area service restoration
was different from the other cities. Not only were meter
assemblies involved, but also cast iron pipeline damage,
which subsequently needed repair by polyethylene-pipe
insertion. Fortunately, very early in the process, during the
second week after the earthquake, emergency crews came
from Valparaiso and Punta Arenas to reinforce local crews.
Also many gas contractors arrived from the Santiago to the
Concepción areas.
III. Felt quite noticeably indoors, especially on upper
floors of buildings. Many people do not recognize
it as an earthquake. Standing motor cars may
rock slightly. Vibrations similar to the passing of
a truck. Duration estimated.
IV. Felt indoors by many, outdoors by few during the
day. At night, some awakened. Dishes, windows,
doors, disturbed; walls make cracking sound.
Sensation like heavy truck striking building.
Standing motor cars rocked noticeably.
V. Felt by nearly everyone; many awakened. Some
dishes, windows broken. Unstable objects
overturned. Pendulum clocks may stop.
VI. Felt by all, many frightened. Some heavy furniture
moved; a few instances of fallen plaster. Damage
slight.
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VII. Damage negligible in buildings of good design
and construction; slight to moderate in well-built
ordinary structures; considerable damage in
poorly-built or badly-designed structures; some
chimneys broken.
VIII.Damage slight in specially-designed structures;
considerable damage to ordinary substantial
buildings with partial collapse. Damage great in
poorly-built structures. Fall of chimneys, factory
stacks, columns, monuments, walls. Heavy
furniture overturned.
IX. Damage considerable in specially-designed
structures; well-designed frame structures thrown
out of plumb. Damage great in substantial
buildings, with partial collapse. Buildings shifted
off foundations.
X. Some well-built wooden structures destroyed;
most masonry and frame structures destroyed
with foundations. Rails bent.
XI. Few, if any (masonry) structures remain standing.
Bridges destroyed. Rails bent greatly.
XII. Damage total. Lines of sight and level are
distorted. Objects thrown into the air.
Source: The Severity of an earthquake, a US Geological
Survey General Interest Publication.
Nevertheless, restoration was still slow: 65% by the second
week, 75% by the sixth week, 80% by the eighth week, and
95% by the eleventh week. At this point the local distributor
realized that 90km of cast iron pipe and 500 customers in
the Talcahuano area were impossible to restore because of
massive pipeline destruction and/or flood. The final solution
to these customers was to be converted to LPG gas supply,
and this conversion process was expected to end in early
June (the 14th week).
Transmission pipelines Pipeline operators Gas Andes, Electrogas, Gasoducto del
Pacífico, and Innergy reported no damages to their systems
due to the Cobquecura or Pichilemu earthquakes. Some
minor landslides occurred in the Andes mountains, which
did not jeopardize pipeline integrity.
Central Chile’s pipeline network, comprising around
1,000 km of 8-in to 30-in diameter API 5L steel pipe, was
constructed between 1996 and 2000.
Photo 8. Downtown Concepción damaged network repair by
pipe insertion.
Conclusions
The Cobquecura earthquake with its historic 8.8Mw
magnitude has demonstrated the resilience of new steel
and polyethylene pipeline systems built with modern
techniques. It also demonstrated that old cast iron
systems are not capable of withstanding earthquakes of
this magnitude and Mercalli scale IX intensities. It is not
clear yet how the soil characteristics in Concepción and
Talcahuano influenced the pipeline ruptures. Nevertheless,
cast iron systems faced with lower Mercalli intensities, such
as theVIII in Santiago, have had a better performance with
no loss of integrity.
Service continuity has not been significantly affected.
Only 15% of customers were subject to supply
interruption, and service was restored within two or three
weeks. An important issue is that supply interruption
was produced by the customers themselves, based on
a perception of lack of safety. Probably an educational
campaign is required.
The service-restoration process in Concepción faced the
problem of cast iron network failure, which made the process
very slow. This situation was aggravated by the chaos that
the earthquake produced.
It is not yet clear if a rapid analysis and planning process
and more manpower availability would have allowed a faster
service-restoration process.
Finally, the lack of official information from both the
authorities and the distribution companies makes it harder to
answer pending questions and know exactly how to improve
the integrity-management processes under catastrophic
earthquake scenarios.