7 Fragility Functions Of Gas And Oil Networks Engineering Essay

Published: 2021-06-30 23:20:05
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P. Gehl1, N. Desramaut1, A. Réveillère1,2, H. Modaressi1,3
1 : BRGM, 3 avenue Claude-Guillemin BP 36009, 45060 Orléans Cedex 2, France
2 : Now at Geostock, 2 rue des Martinets CS 70030, 92569 Rueil-Malmaison Cedex, France
3 : Now at Saunier & Associés, 205 avenue Georges Clemenceau, 92024 Nanterre Cedex, France
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7.1 Introduction
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7.2 Identification of the Main Typologies
The various elements composing the gas and oil transportation and distribution networks can be roughly broken down into three categories, i.e. the actual edges of the network (pipelines), the storage tanks and finally the different facilities that perform specific operations such as pressure control or pumping.
7.2.1 Pipelines
The first typological distinction that can be made for pipelines is whether they are buried or elevated above ground, usually on a steel or concrete support. Since, buried pipelines are the most typical means of transportation for hydrocarbon products – especially around inhabited areas –, the present chapter will mostly emphasize on this typology.
Natural gas networks are operating at various pressures, depending on their scale:
supra-regional transmission pipelines: these pipelines operate at very high pressures (~100 bar) and present large diameters (up to 1.40 m). Such pipelines can cover large areas (e.g. from west Siberia to Europe, from Norway to France…);
regional transmission/distribution pipelines: these pipes still operate at high pressure (from 1 to 70 bar) and are used to connect local distribution systems;
local distribution pipelines: these smaller pipelines usually operate in the medium (0.1 – 4 bar) or low-pressure (< 0.1 bar) range.
Therefore the pressure that the pipelines must withstand while in operation will condition a set of typological features that are linked to the mechanical standards they must meet, namely:
material type,
material strength,
diameter,
wall thickness,
smoothness coating,
type of connection,
design flow.
Among the criteria listed above, the material and the connection types are of crucial importance, since they govern the behaviour and the potential failure mode of buried pipelines in the case of an earthquake. Reports from American Lifeline Alliance (ALA 2001) and HAZUS (NIBS 2004) have detailed some of the most common types of materials and connections used for buried pipelines (see Table 7.1).
Table 7.1. Most common types of materials and connections used in the design of buried pipelines
Material type
Connection type
asbestos-cement (AC)
arc welded
cast iron (CI)
bell and spigot
ductile iron (DI)
cement
concrete (C)
riveted
polyvinyl chloride (PVC)
rubber gasket
welded steel (WS)
welded
While Table 1 details a series of materials that are mostly suitable for water or waste-water transport, pipelines specifically designed for oil and gas are more likely to be made of ductile materials such as steel or PVC. Also, another material type that is absent from Table 1 is polyethylene (medium or high density, i.e. MDPE or HDPE), which is used in more recent networks due to their high ductility.
Finally, another relevant criterion to classify pipelines might be the age or the corrosion state, as shown by the poor performance of ancient pipelines in past earthquake events (ALA 2001).
TYPOLOGY OF CASE-STUDY PIPES
7.2.2 Storage Facilities
A first distinction can be made between underground and surface storage facilities. Sub-facilities for natural gas storage are usually used to balance seasonal variations in demand (i.e. between the heating and non-heating periods). These facilities are located hundred meters below the surface and they are usually natural geological reservoirs, such as depleted oil or gas fields or salt cavern reservoirs.
Aside from underground storage facilities, natural gas is usually stored while in its liquefied state (LNG) in specific LNG tanks: these facilities are designed to insulate the gas from any heat ingress, using "auto-refrigeration" techniques (vaporization of part of the stored liquefied gas counteracts the unavoidable heat flow coming from the outside, keeping the two phases system at the equilibrium, which implies a constant temperature for a given pressure). This technique requires an inner tank (which contains the stored liquid) and an outer tank (which provides security in the event of any loss of containment from the inner tank). Inner shells are usually made of a nickel-steel alloy, whereas the outer shell is a pre-stressed concrete construction. The LNG tanks are usually vertical cylinders and can represent huge facilities, making them too specific object for a statistical fragility analysis.
In the case of gas networks, storage tanks are mostly air-tight pressured cylindrical or spherical containers that are either partially buried, located at grade or elevated on steel frames.
On the other hand, oil storage tanks are atmospheric reservoirs (i.e. vertical cylindrical tanks), which are often categorized by the following features:
material: steel or reinforced concrete,
construction type: at grade or elevated,
anchored or unanchored,
roof type,
capacity,
shape factor: height-on-diameter ratio,
amount of content in the tank: full, half-full, empty.
The most common typologies are usually based on the material type, the construction type and the anchorage of components. Finally, it should be noted that tanks are just a part of the storages facilities, which include also components like inlet/outlet pipelines or mechanical equipment.
7.2.3 Processing Plants / Stations
In the case of the gas network, processing stations can be first classified according to their function within the system, i.e. compression, metering or pressure reduction.
+ Compressor stations:
Compressor stations are used to supply the gas with a given amount of pressure or energy to keep it flowing. They are located along the transmission lines to ensure the transport over long distances and around the storage facilities in order to inject the gas into the distribution network.
This type of station usually includes one or more compressor units, auxiliary equipment for secondary functions (i.e. power generation or cooling of discharge gas) and a SCADA (Supervisory Control And Data Acquisition) system. Most of these stations are housed in proper low-rise buildings and the following features are often used to identify the typologies:
with anchored or unanchored components,
within low-buildings, made of masonry or reinforced concrete.
For instance, in some European countries, like Greece, compressor stations are usually housed in low RC buildings with anchored components.
+ Metering / Pressure reduction stations (M/R stations):
Metering stations are used to control the amount and quality of the gas flowing through the lines. They usually include a pressure reduction facility in order to set the gas pressure at the required level for industrial or commercial use. Usually such stations include the following features:
gas pre-heating,
gas-pressure reduction and regulation,
gas odorizing,
gas-pressure measure,
control through a SCADA system.
These stations have very strong specificities depending on the area they are located. For instance, in central Italy (i.e. the L’Aquila area), these M/R stations are referred to as RE.MI cabins (i.e. "REgolazione e MIsura" in Italian) and they are housed in one-storey masonry buildings with steel roofs, without any SCADA system.
These large disparities and the specificities of the operations performed within M/R stations prevent them from being included in the same typologies as the compressor stations.
+ Reduction groups:
Reduction groups are very local stations that reduce the gas pressure to the level of the distribution network for individual houses. They are the last step of the transmission-distribution chain. They consist in small equipment that can be either buried, sheltered in a kiosk or housed within a building. Again, in central Italy, these reduction groups are referred to as GRF stations (i.e. "Gruppi di Riduzione Finale").
Pumping stations along oil pipelines can be assimilated to the same typology as gas compressor stations, pumps and compressors possessing very similar characteristics and functionalities.
7.3 Description of Damage Mechanisms and Failures Modes
The various elements composing gas and oil networks are sensitive to very different seismic manifestations (e.g. acceleration or displacement), depending on their very nature (e.g. buried or at grade elements, ductile or fragile materials…).
7.3.1 Damage mechanisms of buried pipelines
Like many other underground components, buried pipelines are very sensitive to permanent ground deformation (resulting from various types of ground failures), in addition to transient ground deformation due to seismic wave propagation: the characteristics of these physical phenomena are summed up in Table 7.2. Indeed, according to Eguchi (1987), past earthquakes have caused significant damages to underground pipelines throughout the world: yet inertia forces are not the main issue for buried components, whereas faulting, landslides or liquefaction pose the most problems Hall (1987).
Table 7.2. Overview of the two main types of pipelines affecting buried pipelines
Ground failure
Transient ground deformation
Hazard
surface faulting, liquefaction, landslides
R-waves, S-waves
Usual descriptor
PGD
PGA, PGV, strain
Spatial impact
local and very site-specific
large and distributed
7.3.1.2 Damage from Permanent Ground Deformation
The first sign of damages to buried pipelines is the 1906 San Francisco earthquake, which resulted in significant fires through the city, due to the rupture of water lines needed by fire-hydrants. Regarding the causes of damage, according to O’Rourke and Liu (1999), the zones of lateral spreading accounted for only 5% of the built-up area affected by strong ground shaking, yet approximately 50% of all pipeline breaks occurred within one city block of these zones: this fact demonstrate the high impact of ground failure on pipelines damage.
Damage to buried pipelines induced by permanent ground deformation is usually the main source of failure, as shown by numerous examples of past earthquakes: 1952 Kern County, 1964 Niigata, 1964 Alaska, 1971 San Fernando, 1978 Miyagi-ken-oki and 1983 Nihonkai-Chuba earthquakes. During the 1971 San Fernando earthquake, the steel pipeline system withstood significant ground shaking, yet it was damaged by abrupt vertical or lateral dislocations or ground ruptures: lateral spreading (Figure 7.1) induced severe damages during that earthquake (EERI 1986, O’Rourke and Trautmann 1981, O’Rourke 1988), and one of the most severe damage was observed in a pipeline that was deformed by a differential lateral movement up to 1.7 m. Regarding liquefaction, a good example is the 1964 Niigata earthquake, where the average failure ratio for one of the pipeline systems was as high as 0.97 per km, with all kinds of failure types (pipe body breaks, weld breaks, joint separations).
[Figure 7.1 about here]
Fig. 7.1. Pipeline damage in (a) perpendicular and (b) parallel crossings of a lateral spread, adapted from Rauch (1997)
7.3.1.1 Damage from Transient Ground Motion
O’Rourke and Ayala (1990) report that a few earthquakes have induced damages to pipelines only by the effect of seismic wave propagation:
1985 Michoacan earthquake, which damaged a large corrosion-free modern continuous steel pipeline,
1964 Puget Sound, 1969 Santa Rosa, 1983 Coalinga and 1989 Loma Prieta earthquakes.
Yet, in most cases, it appears that seismic wave propagation damaged mainly pipelines that were previously weakened either by corrosion or welds of poor quality (EERI 1986). More recent events, like the 1994 Northridge, 1995 Kobe, 1999 Kocaeli or 1999 Chi-Chi earthquakes, confirmed the relative vulnerability of piping systems to strong ground motions and the somewhat good performance of welded-steel pipes with respect to seismic wave propagation.
As a result, the emphasis is put on the ductility of pipes and the quality of welds, when building earthquake resistant piping systems: still, pipe welds or joints seem to be the most vulnerable parts of this component. Also, corrosion is an aggravating factor of the pipeline vulnerability (Young and Pardon 1983, Ogawa 1983).
7.3.2 Damage mechanisms of storage tanks
Damage to atmospheric storage tanks (i.e. vertical cylinders) has been quite common for past earthquakes, as shown by the examples below (EERI, 1986):
7.3.3 Damage mechanisms of processing plants / stations
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7.3.3 Key Modelling Issues
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7.4 Review of Existing Functions and Gaps
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7.4.1 Definition of Adequate Intensity Measures
7.4.1.1 Pipelines
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7.4.1.2 Pumping plants
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7.4.1.3 Processing plants / Stations
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7.4.2 Definition of Damage Scales
7.4.2.1 Pipelines
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7.4.2.2 Pumping plants
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7.4.2.3 Processing plants / Stations
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7.4.3 Description of Existing Functions
7.4.3.1 Pipelines
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7.4.3.2 Pumping plants
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7.4.3.3 Processing plants / Stations
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7.4.4 Comparative Analysis and Limitations
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7.5 Recommendations
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