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NGL TRANSPORTATION
Natural Gas Liquids (NGLs) are byproducts of oil and gas production.
The figure below illustrates the components of a raw gas stream, including
methane, the various components of raw NGL, and water, carbon dioxide,
nitrogen, and other non-hydrocarbons. Liquified Petroleum Gases (LPGs),
such as propane and butanes, are a part of the NGL stream. LPG is
composed predominantly of the following hydrocarbons, either in pure
state or as mixtures: propane (C3H8), propylene (C3H6), butane (C4H10),
and butylene (C4H8). Heavier NGLs, such as pentanes, hexanes, and
heptanes, are further refined and produce products such as kerosene,
naphtha, gasoline and light fuel oil.
Relationship of LPG to NGL and Raw Natural Gas
Product Description
| METHANE (CH4) is the natural gas that we use in our
homes to cook and to heat water and air. It exists as a gas
at ambient pressure and temperature and is lighter than air.
It is usually shipped via pipeline.
PROPANE (C3H8) also exists as a gas at ambient pressure
and temperature, but is heavier than air with 1.52 specific
gravity as a vapor at 1 atmosphere and 60º F. (air =
1.0). It has many residential and commercial uses such as
fueling BBQs, camping stoves and lanterns. It is also used
as an alternative motor fuel, including places such as City
of Lompoc. For more remote areas (including some areas in
Santa Barbara County) it substitutes for natural gas. It is
generally shipped by pipeline, truck or train, always in a
liquified form to increase volume per shipment (liquification
occurs via compression or refrigeration).
BUTANE (C4H10) exists in both gaseous and liquid forms
at ambient pressure and temperature, being less stable than
propane in a gaseous form. Butane is heavier than air and
heavier than propane. Its main use these days is industrial
as a chemical feedstock (mostly after conversion to iso-butane),
and is otherwise more waste than product.
HEAVIER NGLs (C5H12+), such as pentanes, hexanes,
heptanes exist as liquids at ambient pressure and temperature
and are shipped in liquid form. NGLs usually are removed from
natural gas during processing. Lighter ends, including propane
and butane may be removed, stored and shipped separately.
Alternatively, the operator may simply ship raw NGLs to a
NGL plant for separation. Heavier NGLs often are used as a
diluent for shipping crude oil via pipeline; it reduces the
viscosity of heavier crude oils.
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TRUCK TRANSPORT OF NGLS
Weight, rather than volume, is the controlling factor in determining
container capacity. A semi-truck with tank can carry 10,238 gallons
of propane, which constitutes 89% of the container volumetric capacity
and 100% of the trucks "on-road" weight limitation of
80,000 pounds. The same tanker truck can carry only 9,149 gallons
of butane because butane is heavier than propane. For heavier NGL
(i.e., C5+), the same truck can carry only 6,500 to 8,000 gallons.
The volumes described above are even less for a truck and trailer
(propane = 10,000 gallons, butane = 8,900 gallons, and heavier NGLs
= 6,400 to 7,800 gallons).
When shipped by truck or train, propane is almost always odorized
with mercaptans as a safety precaution to warn many residential
and commercial users in the event of a release. The odor is the
same odor associated with retail natural gas. Butane, however, is
largely shipped by trucks in an unodorized state because its consumers
are largely industrial users that do not want the butane contaminated.
HAZARDS OF TRANSPORTING NGLS
Transporting NGLs has been identified as the highest risk to public
safety associated with offshore oil and gas development. This high
ranking largely stems from the risk of transporting these products
via highway, through populated areas, combined with heightened probability
of human error. An accidental release of NGL, while in transport,
may result in the following hazards:
- POOL FIRES - Upon release, heavier NGLs are flammable
and, therefore, pose the hazard of thermal radiation.
- FIREBALLS - A large, pressurized release of a liquefied
hydrocarbon such as propane or butane may burn in the form of
a fireball, which grows larger and also moves upwards. Thermal
radiation is the related hazard.
- VAPOR CLOUD FIRES - Upon release, propane or butane can
form a vapor cloud that spreads horizontally. If little or no
wind is present and atmospheric conditions are very stable, the
spreading cloud mixes slowly with oxygen. It can burst into flames
if ignited and flash back to the source of the release. Thermal
radiation is the hazard.
- VAPOR CLOUD EXPLOSIONS - NGL vapor clouds are potentially
explosive both in unconfined and confined situations. The hazard
of such an explosion generally results from building damage or
breaking windows.
- BLEVE (Boiling Liquid, Expanding Vapor Explosion) - NGL
containers, when exposed to an external source of fire, are vulnerable
to explosion. The external fire weakens the container while also
heating the liquid inside well above its boiling point.
MEASURES TO INCREASING SAFE TRANSPORT OF NGLS
- MODE OF TRANSPORTATION - Pipeline is generally a superior
mode compared to rail or highway. Pipeline transport of NGL can
be accomplished by (1) construction of a dedicated products pipeline,
(2) blending of NGLs with crude oil for shipment via pipeline
(or marine tanker if such is the case), or (3) batching in a crude
oil pipeline. Of these three options, the second one is the most
feasible in the current County context. Comparing rail to highway,
there is no significant difference in risk between these two modes
for shipments destined for the Bay area or Los Angeles area; however,
highway is significantly safer compared to rail for shipments
destined for Kern County area.
- ROUTING - Some NGLs are shipped via highway. The particular
route can and does significantly influence safety; usually routes
that are less densely populated prevail over those routes through
more densely populated areas, although route-specific accident
rates can influence a different conclusion.
- ROUTE-SPECIFIC IMPROVEMENTS - Specific highway routes
can be further mitigated in several ways, including enhanced patrolling
by the California Highway Patrol (CHP), physical improvements
in design for areas of particularly high risk, restrictions on
transporting hazardous materials on a route during adverse conditions
(e.g., flooding). The County has contracted with the CHP over
several years to enhance patrolling along State Route 166, between
Santa Maria and Cuyama.
- USE OF SAFE CARRIERS - The particular carrier contributes
significantly to the safety of shipments; those with very good
hiring and/or training programs, strong maintenance and safety
programs, positive reinforcement, and pay drivers on an hourly
basis (as opposed to a shipment basis) are likely to have less
accidents. The Energy Division applies this mitigation through
a Transportation Risk Management and Prevention Program (TRMPP).
- TYPE OF CONTAINER WITH REGARD TO HEAVIER NGLS - Heavier
NGLs should not be shipped in relatively thin-shelled tanks (M-307),
almost identical to those used for shipment of gasoline to retail
outlets. These shells often rupture in an accident. The LPG-rated
tank (MC-331) is a much sturdier, thicker shell that often withstands
impacts such as rollovers.
LOCAL GOVERNANCE OF NGL TRANSPORTATION
The County hired Arthur D. Little, Inc. in 1989 to conduct a comprehensive
risk assessment of transporting locally produced NGL to three major
destinations: the greater Los Angeles area, the Bay area, and the
Bakersfield area. Acting on the findings of the assessment, the
County adopted Resolution 93-480 in 1993. This Resolution requires
the following measures to reduce the risk of transporting NGL to
public safety.
- MODE OF TRANSPORTION:
- Blend NGLs into the crude oil stream for shipment via pipeline
or marine barge
- Transport NGLs destined for Kern County that cannot be blended
with crude oil and ship them via highway rather than rail
- Report quarterly on volumes of NGLs transported and mode
of transportation employed
- HIGHWAY ROUTES
· Utilize State Route 166 for shipments to Kern County
- CONTAINERS
· Transport heavier NGLs in LPG containers when transported
by highway
- TRANSPORTATION RISK MANAGEMENT AND PREVENTION PROGRAM
· Establish and implement comprehensive surveys of carriers
biennually to ensure use of safest carriers only
· Establish and implement truck-loading procedures that
bring the facility operator and truck driver together to visually
inspect truck and container
· Establish and implement requirements that carrier use
Vehicle Monitoring Systems for governing vehicle speed
· Establish and implement requirements that carriers use
cellular phones or other effective means of communicating with
its dispatch and CHP when traversing State Route 166
- ROUTE MITIGATION
· Contribute annually the full cost of enhancing CHP enforcement
on State Route 166
The new resolution has been incorporated into the relevant permit
conditions for the Point Arguello, Point Pedernales, Santa Ynez
Unit (including POPCO), and Molino projects.
Board
Resolution (Adobe .pdf)
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