Natural Gas Liquids (NGLs) are products of oil and gas production. The figure below illustrates the components of a raw gas stream, including methane, ethane, the various components of raw NGLs, and water, carbon dioxide, nitrogen, and other non-hydrocarbons. Liquefied 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), butanes (C4H10), and butylenes (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
METHANE (CH4) is the major component (predominant) 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.
ETHANE (C2H6) is a component of the natural gas. It also exists as a gas at ambient pressure and temperature. It is slightly heavier than air.
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 liquefied form to increase volume per shipment (liquefaction occurs via compression and refrigeration).
BUTANES (C4H10) exists in both gaseous and liquid forms at ambient pressure and temperature, being less stable than propane in a gaseous form. Butanes exist as normal butane and isobutane. Butane is heavier than air and heavier than propane. Its main use these days is industrial as a chemical feedstock (mostly after conversion to isobutane). It is used to boost the crude oil API gravity by blending it with crude oil.
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. Heavier NGLs (Naphtha) could also be used as a feedstock for gasoline.
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 NGLs (i.e., C5+), the same truck can carry only 6400 to 8,000 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. Butanes, however, are largely shipped by trucks in an unodorized state because their consumers are largely industrial users that do not want the butanes 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 air. 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 in 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 liquids inside well above their boiling points.
MEASURES TO INCREASING SAFE TRANSPORT OF NGLS
- MODE OF TRANSPORTATION -
- Pipeline is generally a superior mode compared to rail or highway. Pipeline transport of NGLs 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 tanks (MC-331/MC-330) are much sturdier, with thicker shells 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 NGLs 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 NGLs 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
· Transport heavier NGLs in LPG containers when transported
- 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 and Santa Ynez Unit (including POPCO).
Resolution (Adobe .pdf)