Storage and Transportation of Crude Oil, Natural Gases, Liquid Petroleum Products and Other Chemicals

 

Pipelines, marine vessels, tank trucks, rail tank cars and so forth are used to transport crude oils, compressed and liquefied hydrocarbon gases, liquid petroleum products and other chemicals from their point of origin to pipeline terminals, refineries, distributors and consumers.

Crude oils and liquid petroleum products are transported, handled and stored in their natural liquid state. Hydrocarbon gases are transported, handled and stored in both the gaseous and liquid states and must be completely confined in pipelines, tanks, cylinders or other containers prior to use. The most important characteristic of liquefied hydrocarbon gases (LHGs) is that they are stored, handled and shipped as liquids, taking up a relatively small amount of space and then expanding into a gas when used. For example, liquefied natural gas (LNG) is stored at –162°C, and when it is released the difference in storage and atmospheric temperatures causes the liquid to expand and gasify. One gallon (3.8 l) of LNG converts to approximately 2.5 m3 of natural gas at normal temperature and pressure. Because liquefied gas is much more “concentrated” than compressed gas, more useable gas can be transported and provided in the same size container.

Pipelines

It is generally the case that all crude oils, natural gas, liquefied natural gas, liquefied petroleum gas (LPG) and petroleum products flow through pipelines at some time in their migration from the well to a refinery or gas plant, then to a terminal and eventually to the consumer. Aboveground, underwater and underground pipelines, varying in size from several centimetres to a metre or more in diameter, move vast amounts of crude oil, natural gas, LHGs and liquid petroleum products. Pipelines run throughout the world, from the frozen tundra of Alaska and Siberia to the hot deserts of the Middle East, across rivers, lakes, seas, swamps and forests, over and through mountains and under cities and towns. Although the initial construction of pipelines is difficult and expensive, once they are built, properly maintained and operated, they provide one of the safest and most economical means of transporting these products.

The first successful crude-oil pipeline, a 5-cm-diameter wrought iron pipe 9 km long with a capacity of about 800 barrels a day, was opened in Pennsylvania (US) in 1865. Today, crude oil, compressed natural gas and liquid petroleum products are moved long distances through pipelines at speeds from 5.5 to 9 km per hour by large pumps or compressors located along the route of the pipeline at intervals ranging from 90 km to over 270 km. The distance between pumping or compressor stations is determined by the pump capacity, viscosity of the product, size of the pipeline and the type of terrain crossed. Regardless of these factors, pipeline pumping pressures and flow rates are controlled throughout the system to maintain a constant movement of product within the pipeline.

Types of pipelines

The four basic types of pipelines in the oil and gas industry are flow lines, gathering lines, crude trunk pipelines and petroleum product trunk pipelines.

 

Regulations and standards

Pipelines are constructed and operated to meet safety and environmental standards established by regulatory agencies and industry associations. Within the United States, the Department of Transportation (DOT) regulates the operation of pipelines, the Environmental Protection Agency (EPA) regulates spills and releases, the Occupational Safety and Health Administration (OSHA) promulgates standards covering worker health and safety, and the Interstate Commerce Commission (ICC) regulates common carrier pipelines. A number of industry organizations, such as the American Petroleum Institute and the American Gas Association, also publish recommended practices covering pipeline operations.

Pipeline construction

Pipeline routes are planned using topographic maps developed from aerial photogrammetric surveys, followed by actual ground surveying. After planning the route, obtaining right-of-way and permission to proceed, base camps are established and a means of access for construction equipment is required. Pipelines can be constructed working from one end to another or simultaneously in sections which are then connected.

The first step in laying pipeline is to construct a 15- to 30-m-wide service road along the planned route to provide a stable base for the pipe-laying and pipe-joining equipment and for underground pipeline excavation and backfill equipment. The pipe sections are laid on the ground alongside the service road. The ends of the pipe are cleaned, the pipe is bent horizontally or vertically, as necessary, and the sections are held in position by chocks above the ground and joined by multi-pass electrical arc-welding. The welds are checked visually and then with gamma radiation to assure that no defects are present. Each connected section is then coated with liquid soap and air-pressure tested to detect leaks.

The pipeline is cleaned, primed and coated with a hot, tar-like material to prevent corrosion and wrapped in an outer layer of heavy paper, mineral wool or plastic. If the pipe is to be buried, the bottom of the trench is prepared with a sand or gravel bed. The pipe may be weighed down by short, concrete sleeves to prevent its lifting out of the trench from groundwater pressure. After the underground pipeline is placed in the trench, the trench is backfilled and the surface of the ground returned to normal appearance. After coating and wrapping, aboveground piping is lifted up onto prepared stanchions or casements, which may have various design features such as anti-earthquake shock absorption. Pipelines may be insulated or have heat trace capabilities to keep products at desired temperatures throughout transport. All pipeline sections are hydrostatically tested prior to entering gas or liquid hydrocarbon service.

Pipeline operations

Pipelines may be either privately owned and operated, carrying only the owner’s products, or they may be common carriers, required to carry any company’s products provided that the pipeline’s product requirements and tariffs are met. The three major pipeline operations are pipeline control, pumping or compressor stations and delivery terminals. Storage, cleaning, communication and shipment are also important functions.

·         Pipeline control. Regardless of the product being transported, the size and length of the pipeline or the terrain, pipeline pumping stations, pressures and flow rates are completely controlled in order to ensure appropriate flow rates and continuous operations. Typically an operator and computer control the pumps, valves, regulators and compressors throughout the pipeline system from a central location.

·         Oil pumping and gas compressor stations. Crude oil and petroleum products pumping stations and gas compressor stations are located at wellheads and along the pipeline route as needed to maintain pressure and volume. Pumps are driven by electric motors or diesel engines, and turbines may be powered by fuel oil, gas or steam. Many of these stations are automatically controlled and not staffed at most times. Pumps, with and without vapour return lines or pressure equalizing lines, are commonly used in smaller pipelines for transport of LNG, LPG and compressed natural gas (CNG). Pressure drop detectors are installed to signal any leaks in pipelines, and excess flow valves or other flow limiting devices are used to minimize the rate of flow in case of a pipeline leak. Storage vessels and reservoirs may be isolated from main pipelines by manually operated or remote control valves or fusible link valves.

·         Pipeline product storage. Crude and petroleum product pipeline terminals have breakout storage tanks to which shipments may be diverted, where they are held until required by a refinery, terminal or user (see figure 1). Other tanks at pipeline pumping stations contain fuel for operating diesel-driven pump motors or for running electrical generators. Because gas fields produce continuously and gas pipelines operate continuously, during times of reduced demand, such as summertime, liquefied natural and petroleum gases are stored underground in natural caverns or salt domes until needed.

·         Pipeline cleaning. Pipelines are cleaned on a scheduled basis or as necessary in order to continue flow by reducing friction and maintaining as large a diameter interior as possible. A special cleaning device, called a pig or go-devil, is placed into the pipeline and pushed along by the flow of oil from one pumping station to the next. As the pig passes through the pipeline it scrapes off any dirt, wax or other deposits which have built up inside the pipeline walls. When it reaches a pumping station, the pig is removed, cleaned and reinserted into the pipeline to travel to the next station.

·         Communications. It is important that there be communication and agreement concerning schedules, pumping rates and pressures and emergency procedures between pipeline stations and operators and those shipping and receiving crude oil, gas and petroleum products. Some pipeline companies have private telephone systems which transmit the signal along the pipeline, while others use radios or public telephones. Many pipelines use ultra-high-frequency microwave transmitter systems for computer communications between control centres and pumping stations.

·         Petroleum product shipment. Petroleum products may be shipped a number of different ways on pipelines. A company operating a refinery may blend a specific grade of its own gasoline with appropriate additives (additize) and ship a batch through a pipeline directly to its own terminal for distribution to its customers. Another method is for a refinery to produce a batch of gasoline, called a frangible or specification product, which is blended to meet a common carrier pipeline company’s product specifications. The gasoline is placed into the pipeline for delivery to any company’s terminals which are connected to the pipeline system. In a third method, products are shipped by companies to each other’s terminals and exchanged in order to avoid extra transportation and handling. Frangible and exchange products are usually blended and additized at the terminal which receives the product from the pipeline, to meet the specific requirements of each company operating from the terminal. Finally, some products are delivered by pipeline from terminals and refineries direct to large commercial consumers—jet fuel to airports, gas to gas distribution companies and fuel oil to electric generating plants.

·         Product receipt and delivery. Pipeline operators and terminal operators should jointly establish programmes to provide for the safe receipt and transfer of products and to coordinate actions in case an emergency occurs on the pipeline or at the terminal during shipment which requires shutdown or diversion of product.

 

Figure 1. A terminal operator transfers product the Pasagoula Refinery into holding tanks in the Deraville Terminal near Atlanta, Georgia, US.

American Petroleum Institute

Instructions for receiving pipeline deliveries should include verification of the availability of the storage tanks to hold the shipment, opening and aligning tank and terminal valves in anticipation of delivery, checking to assure that the proper tank is receiving product immediately after the start of delivery, conducting required sampling and testing of batches at the start of delivery, performing batch changes and tank switches as required, monitoring receipts to assure that overfills do not occur and maintaining communications between the pipeline and the terminal. The use of written communications between terminal workers, especially when shift changes occur during product transfer, should be considered.

Batch shipments and interface

Although pipelines originally were used to move only crude oil, they evolved into carrying all types and different grades of liquid petroleum products. Because petroleum products are transported in pipelines by batches, in succession, there is commingling or mixing of the products at the interfaces. The product intermix is controlled by one of three methods: downgrading (derating), using liquid and solid spacers for separation or reprocessing the intermix. Radioactive tracers, colour dyes and spacers may be placed into the pipeline to identify where the interfaces occur. Radioactive sensors, visual observation or gravity tests are conducted at the receiving facility to identify different pipeline batches.

Petroleum products are normally transported through pipelines in batch sequences with compatible crude oils or products adjoining one another. One method of maintaining product quality and integrity, downgrading or derating, is accomplished by lowering the interface between the two batches to the level of the least affected product. For example, a batch of high-octane premium gasoline is typically shipped immediately before or after a batch of lower-octane regular gasoline. The small quantity of the two products which has intermixed will be downgraded to the lower octane rating regular gasoline. When shipping gasoline before or after diesel fuel, a small amount of diesel interface is allowed to blend into the gasoline, rather than blending gasoline into the diesel fuel, which could lower its flashpoint. Batch interfaces are typically detected by visual observation, gravitometers or sampling.

Liquid and solid spacers or cleaning pigs may be used to physically separate and identify different batches of products. The solid spacers are detected by a radioactive signal and diverted from the pipeline into a special receiver at the terminal when the batch changes from one product to another. Liquid separators may be water or another product that does not commingle with either of the batches it is separating and is later removed and reprocessed. Kerosene, which is downgraded (derated) to another product in storage or is recycled, can also be used to separate batches.  A third method of controlling the interface, often used at the refinery ends of pipelines, is to return the interface to be reprocessed. Products and interfaces which have been contaminated with water may also be returned for reprocessing.

Environmental protection

Because of the large volumes of products which are transported by pipelines on a continuous basis, there is opportunity for environmental damage from releases. Depending on company and regulatory safety requirements and the pipeline’s construction, location, weather, accessibility and operation, a considerable amount of product may be released should a break in the line or leak occur. Pipeline operators should have emergency response and spill contingency plans prepared and have containment and clean-up materials, personnel and equipment available or on call. Simple field solutions such as building earth dykes and drainage ditches can be quickly implemented by trained operators to contain and divert spilled product.

Maintaining pipelines and worker health and safety

The first pipelines were made of cast iron. Modern trunk pipelines are constructed of welded, high-strength steel, which can withstand high pressures. Pipe walls are periodically tested for thickness to determine whether internal corrosion or deposits have occurred. Welds are checked visually and with gamma radiation to assure that no defects are present.  Plastic pipe may be used for low-pressure, small-diameter flow lines and gathering lines in gas and crude-oil-producing fields, since plastic is light in weight and easy to handle, assemble and move.

When a pipeline is separated by cutting, spreading flanges, removing a valve or opening the line, an electrostatic arc may be created by impressed cathodic protection voltage, corrosion, sacrificial anodes, nearby high-voltage power lines or stray ground currents. This should be minimized by grounding (earthing) the pipe, de-energizing the cathodic rectifiers closest to both sides of the separation and connecting a bonding cable to each side of the piping prior to starting work. As additional pipeline sections, valves and so on are added to an existing line, or during construction, they should first be bonded to the pipelines in place. Work on pipelines should cease during electrical storms. Equipment used to lift and place pipe should not be operated within 3 m of high-voltage electric lines. Any vehicles or equipment working in the vicinity of high-voltage lines should have trailing grounding straps attached to the frames. Temporary metal buildings should also be grounded.  Pipelines are specially coated and wrapped to prevent corrosion. Cathodic electrical protection may also be required. After the pipeline sections are coated and insulated, they are joined by special clamps connected to metallic anodes. The pipeline is subjected to a grounded source of direct current of sufficient capacity so that the pipeline acts as a cathode and does not corrode.

All pipeline sections are hydrostatically tested prior to entering gas or liquid hydrocarbon service and, depending on regulatory and company requirements, at regular intervals during the life of the pipeline. Air must be eliminated from pipelines prior to hydrostatic testing, and hydrostatic pressure built up and reduced at safe rates. Pipelines are regularly patrolled, usually by aerial surveillance, to visually detect leaks, or monitored from the control centre to detect a drop in flow rate or pressure, which would signify that a break in the pipeline has occurred.

Pipeline systems are provided with warning and signalling systems to alert operators so they may take corrective action in an emergency. Pipelines may have automatic shutdown systems which activate emergency pressure valves upon sensing increased or reduced pipeline pressure. Manually or automatically operated isolation valves are typically located at strategic intervals along pipelines, such as at pumping stations and at both sides of river crossings.

An important consideration when operating pipelines is to provide a means of warning contractors and others who may be working or conducting excavations along the pipeline route, so that the pipeline is not inadvertently ruptured, breached or punctured, resulting in a vapour or gas explosion and fire. This is usually done by regulations which require construction permits or by pipeline companies and associations providing a central number which contractors can call prior to excavation.

Because crude oil and flammable petroleum products are transported in pipelines, the possibility exists for fire or explosion in case of a line break or release of vapour or liquid. Pressure should be reduced to a safe level before working on high-pressure pipelines. Combustible gas testing should be conducted and a permit issued prior to repair or maintenance involving hot work or hot tapping on pipelines. The pipeline should be cleared of flammable liquids and vapours or gas prior to starting work. If a pipeline cannot be cleared and an approved plug is used, safe work procedures should be established and followed by qualified workers. The line should be vented a safe distance from the hot work area to relieve any build-up of pressure behind the plug.  Proper safety procedures should be established and followed by qualified workers when hot tapping pipelines. If welding or hot tapping is conducted in an area where a spill or leak has occurred, the outside of the pipe should be cleaned of liquid, and contaminated soil should be removed or covered to prevent ignition.  It is very important to notify operators at the nearest pumping stations on each side of the operating pipeline where maintenance or repair is to be performed, in case shutdown is required. When crude oil or gas is being pumped into pipelines by producers, the pipeline operators must provide specific instructions to the producers as to actions to take during repair, maintenance or in an emergency. For example, prior to tie-in of production tanks and lines to pipelines, all gate valves and bleeders for the tanks and lines involved in the tie-in should be closed and locked or sealed until the operation is completed.

Normal safety precautions concerning pipe and materials handling, toxic and hazardous exposures, welding and excavation apply during pipeline construction. Workers clearing right-of-way should protect themselves from climatic conditions; poisonous plants, insects and snakes; falling trees and rocks; and so on. Excavations and trenches should be sloped or shored to prevent collapse during underground pipeline construction or repair (see the article “Trenching” in the chapter Construction). Workers should follow safe work practices when opening and de-energizing electrical transformers and switches.  Pipeline operating and maintenance personnel often work alone and are responsible for long stretches of pipeline. Atmospheric testing and the use of personal and respiratory protective equipment is needed to determine oxygen and flammable vapour levels and protect against toxic exposures to hydrogen sulphide and benzene when gauging tanks, opening lines, cleaning spills, sampling and testing, shipping, receiving and performing other pipeline activities. Workers should wear dosimeters or film badges and avoid exposure when working with density gauges, source holders or other radioactive materials. The use of personal and respiratory protective equipment should be considered for exposure to burns from the hot protective tar used in pipe-coating operations and from toxic vapours which contain polynuclear aromatic hydrocarbons.

Marine Tankers and Barges

The majority of the world’s crude oil is transported by tankers from producing areas such as the Middle East and Africa to refineries in consumer areas such as Europe, Japan and the United States. Oil products were originally transported in large barrels on cargo ships. The first tanker ship, which was built in 1886, carried about 2,300 SDWT (2,240 pounds per ton) of oil. Today’s supertankers can be over 300 m long and carry almost 200 times as much oil (see figure 2). Gathering and feeder pipelines often end at marine terminals or offshore platform loading facilities, where the crude oil is loaded into tankers or barges for transport to crude trunk pipelines or refineries. Petroleum products also are transported from refineries to distribution terminals by tanker and barge. After delivering their cargoes, the vessels return in ballast to loading facilities to repeat the sequence.

Figure 2. SS Paul L. Fahrney oil tanker.

American Petroleum Institute

Liquefied natural gas is shipped as a cryogenic gas in specialized marine vessels with heavily insulated compartments or reservoirs (see figure 3). At the delivery port, the LNG is off-loaded to storage facilities or regasification plants. Liquefied petroleum gas may be shipped both as a liquid in uninsulated marine vessels and barges and as a cryogenic in insulated marine vessels. Additionally, LPG in containers (bottled gas) may be shipped as cargo on marine vessels and barges.

Figure 3. LNG Leo tanker loading at Arun, Sumatra, Indonesia.

American Petroleum Institute

LPG and LNG marine vessels

The three types of marine vessels used for transport of LPG and LNG are:

·         vessels with reservoirs pressurized up to 2 mPa (LPG only)

·         vessels with heat-insulated reservoirs and a reduced pressure of 0.3 to 0.6 mPa (LPG only)

·         cryogenic vessels with heat-insulated reservoirs pressurized close to atmospheric pressure (LPG and LNG).

 

Shipment of LHGs on marine vessels requires constant safety awareness. Transfer hoses must be suitable for the correct temperatures and pressures of the LHGs being handled. To prevent a flammable mixture of gas vapour and air, inert gas (nitrogen) blanketing is provided around reservoirs, and the area is continually monitored to detect leaks. Before loading, storage reservoirs should be inspected to ensure that they are free of contaminants. If reservoirs contain inert gas or air, they should be purged with LHG vapour prior to loading the LHG. Reservoirs should be constantly inspected to ensure integrity, and safety valves should be installed to relieve the LHG vapour generated at maximum heat load. Marine vessels are provided with fire suppression systems and have comprehensive emergency response procedures in place.