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MAIN AND UTILITY PROCESSES
This page describes the offshore oil and gas main and utility processes and lists the accidental events that may occur.
Main processes
The operational phase of offshore operations is the time that a platform actually produces hydrocarbons. Most offshore UK fields are currently in their operational phase. Production from the UKCS started with gas production from the West Sole field in March 1967. First oil was produced from the Argyll field in 1975.
Producing wells
Hydrocarbons from the offshore oil and gas fields are produced via wells that are drilled into the underground fields or reservoirs. Most wells lead directly up to a platform, but some are tied-back to a platform via sub sea templates and sub sea flow lines. The reservoir fluids (oil, water and gas) are brought onto the platforms for treatment.
Oil and/or gas production starts by bringing the wells on line. See below for a description of drilling activities. Wells are capped by a set of safety valves, or 'christmas tree'. It is also common practice to fit a subsurface safety valve or SSSV which automatically closes a well in case of an emergency. The SSSVs are often operated by means of hydraulic oil, which is sometimes spilled.
All offshore platforms produce liquid hydrocarbons as well as gas. Gas platforms mainly produce gas with very small amounts of very light oil (or condensate). Fields that produce significant amounts of oil are referred to as oil fields, although they can also produce significant amounts of gas. In the UK more gas is actually produced by the oil platforms than is by the gas platforms. The biggest difference between oil platforms and gas platforms is oil platforms generally produce a lot more water.
Main processing equipment
On the oil platforms the oil, water and gas fractions are separated. The dehydrated oil and gas are transported to shore in separate pipeline systems and the produced water is most commonly discharged to sea (see Figure).
On the gas platforms, the gas, oil and water fractions are separated to protect the gas compressors, but are later recombined and transported to shore in the same pipeline (see Figure).
The above schematics show the basic process principles, but in reality, many variations and complications to these exist, depending on design preferences and field specific circumstances.
There are two main reasons for separating the oil, gas and water offshore. The first reason is to prevent corrosion in both the oil and gas export pipelines and to prevent the formation of hydrates in the export gas pipelines. Corrosion could cause oil spills and hydrates could block the gas pipelines. The second reason is that the market value of the oil and gas is a function of the amount of water in them.
Oil processing
On an oil platform, the principle piece of equipment for this dehydration process is the gravity oil separator. This is a fully enclosed vessel that separates the produced fluids into its primary components; oil, gas and water. A schematic of such a separator is shown below. The separation process is based on the principle that oil is lighter than water, whereas gas is lightest. The oil floats on top of the water and passes over a weir and is extracted on the right of the vessel, whereas the water sinks to the bottom of the vessel and is drained off on the left of the weir. The gas comes out of solution because the separator pressure is lower than the reservoir pressure and is extracted at the top of the separator. Since gravity separation works best when given time (about 2 minutes), gravity separators are among the largest pieces of equipment on an offshore oil platform.
A single gravity separator is often not sufficient, and a secondary gravity separator is often added downstream of the primary one. This second separator is operated at a lower pressure still. A test separator is often found parallel to the primary gravity separator to determine the performance of individual wells.
Further reduction of the water content of the oil is often achieved by adding an electrostatic coalescer, which uses an electrostatic field to encourage the smaller water droplets that would otherwise remain in the oil phase to separate out.
Even though many sophisticated separator designs exist, all seem to require the use of small amounts of chemicals to improve the oil and water separation process.
A considerable amount of energy is often required to separate the water from the oil. The oil is heated by using waste heat from the exhaust gasses from the turbines on the platform, by extracting heat from the already processed liquids or by using separate electrical or gas fired heaters. The fact that the oil is already warm when it surfaces is also exploited, i.e.;.making use of geothermal energy.
Gas processing
On a gas platform, the dehydration process is based on two principles. The first principle is cooling. Water and liquid hydrocarbons condense out of the gas-phase when the temperature of the gas is reduced. The easiest way of reducing gas temperature is by reducing its pressure. Gravity separators such as described above (but much smaller) are used to remove the bulk of the condensate and water from the gas. Reducing the gas pressure does, however, come at a cost. When the pressure of the gas is reduced below the pressure required to pump the gas to shore, then gas compressors are required to increase the pressure of the gas, which costs energy. Nevertheless, gas production does not require as much energy as the production of oil does. Cooling of the gas is also achieved with gas coolers, which use seawater to cool the gas.
The second principle that is used to dry the gas, is to pass it through a hydrophilic medium, which takes the water out of the gas and into solution of the water attracting medium. These systems are used to remove the final amounts of water from the gas. Both glycol and methanol based gas dehydration systems are used offshore. The hydrophilic medium is often recycled on the platform.
Oil and gas exporting
Offshore oil and gas is usually transported by sub sea pipeline to an onshore terminal. The oil pumps and gas compressors are driven by electric motors, reciprocating engines or gas turbines and account for a significant percentage of the offshore energy usage. An increasing number of offshore oil platforms export their oil via shuttle tankers rather than by sub sea pipeline.
On the UKCS there are over 10,000 km of sub sea pipeline. Pipelines of 14" and under are normally buried into the seabed given the smaller line’s inability to withstand the load applied by snagged fishing equipment. About 25% of UKCS pipeline kms are 14" or smaller. Sub sea pipelines are usually coated to protect them from corrosion or impact, to weigh them down or to keep the oil or gas warm. Internal flow improvement coatings are also used.
The main onshore terminals in the UK are located at: Sullom Voe, Flotta, Cruden Bay, Teesside, St Fergus, Bacton, Dimlington, Easington, Theddlethorpe, Point of Ayr and Burrow-in-Furness.
Utility processes
Exploration
Before any oil or gas can be produced, it needs to be found. This is the purpose of exploration. First a prospective area is mapped by means of seismic, but despite the advancements in seismic technology, the ultimate proof of the presence of hydrocarbons still lies in drilling a well.
Seismic
Offshore, seismic surveys are carried out by specialists vessels that tow arrays of air guns and microphones through the water. Seismic surveys generate atmospheric emissions and discharges typical of a vessel and can get in the way of fishing activity, but the noise generated by the air guns is the concern that stands out. Noise from the air guns used for seismic surveys is very loud and seismic surveys take place where dolphins, whales and seals live.
Air guns function by suddenly venting high-pressure air into the water. The sound thus generated penetrates the seabed and the echo of the sound is recorded in order to map underlying geological structures. Air gun arrays vary in size from about 10 to about 130 liters depending on the types of geological structures that are studied. The trend is for larger arrays of air guns, but also for combinations of larger and smaller guns. I am not aware of technological developments that are aimed at reducing their potential impact.
The offshore industry in the UK is observing a number of precautions in order to minimize the impact of seismic operations on marine mammals. When conducting a survey, operators are required to check for the presence of cetaceans before starting a survey line, and delay the start of shooting if cetaceans are present within 500 meters. Whenever possible, a "soft-start" procedure should be employed to warn cetaceans of the upcoming danger and lowest practicable energy levels should be used throughout the survey.
Drilling
Most wells are exploration wells, but what is described here also applies to development drilling. Exploration drilling is exclusively undertaken from mobile drilling rigs. Mobile drilling rigs are also used for development wells in shallow areas of the UKCS but most of the larger platforms in the central and northern North Sea have integrated drilling facilities. Mobile drilling rigs come in shapes and sizes, which jack-up drilling rigs being the most common in water depths of up to 100 meters. For deeper waters semi-submersible rigs or drill ships are used. There are a large number of information sources on the web that explain the art of drilling.
Again, drilling rigs generate atmospheric emissions and discharges typical of a vessel, but the environmental issues that dominate the drilling agenda are the use of chemicals, the risk of an oil spill and the interaction with other marine traffic.
Drilling chemicals
In the ‘rotary’ drilling technique that is used for most drilling programmes, a drill bit is suspended on the end of a tubular drill string (drill stem) which is supported on a cable/pulley system held up by a derrick. Drilling takes place when the drill bit is rotated while it bears down on the rock. To keep the bit cool and lubricated, and to remove the rock cuttings from the hole, drilling fluid (mud) is pumped down the inside of the drill string. When this mud reaches the bit, it passes through nozzles in the bit, impacts the bottom of the hole and then moves upward in the annulus (the space between the drill string and the well bore wall) with the cuttings suspended in it. At the surface, the mud is recycled by filtering through screens and other devices that remove the cuttings, and is then pumped back down the hole again.
Two main types of mud systems are used to drill a well: water based muds (WBMs) and Oil Based Muds (OBMs). WBMs are generally more acceptable in terms of toxic and environmental effects, although some wells can only be drilled with OBMs.
Generally, cuttings from wells drilled with WBM as well as any WBM that can no longer be used would be discharged to the sea. In the UK a formal risk assessment needs to be undertaken for all proposed usage and discharge of chemicals in the drilling programme.
In contrast, a total containment system would be used to prevent overboard discharge of any OBMs (and associated rock cuttings) if the use of OBM is necessary for technical reasons. Such containment system would be engineered for zero fluid or cuttings discharges to the marine environment. All used mud (and cuttings) associated with oil-based mud systems would either be taken to shore for treatment and disposal or injected into another well.
Oil spill risk
Drilling presents a spill risk through the handling of diesel fuel and OBMs, but the most significant spill risk is the risk of a blow-out while drilling for oil, where large amounts of crude oil could be spilled to the sea.
Usually the drilling mud is the primary pressure control method against this happening, with fluid weight adjusted to balance down hole pressures. If the fluid fails to balance these pressures, mechanical safety valves (blow-out preventers, BOP’s) are normally in place to control the well. Failure of both the BOP and the mud system would result in the formation fluids (i.e. oil or gas) flowing to the surface (a blow-out).
To mitigate the consequences of a major oil spill, authorities require operators to submit oil pollution emergency plans (OPEPs) for certain activities. An OPEP should be based on an assessment of the risk of an oil spill and consider all activities that might give rise to the spillage of hydrocarbons.
Design
Offshore oil and gas producing facilities are usually designed by one or more of the main engineering companies. The UK designs have evolved from early Gulf of Mexico designs. Oil is no longer transported in whisky barrels, and increasing safety and environmental awareness continue to drive design enhancements.
Offshore designs are often a balance between cost and performance, with the lack of space presenting a specific challenge. Performance standards that are considered normal for onshore processing installations, may not always be feasible for offshore facilities.
Water depth is also an important design consideration. As a rule, deeper waters make more expensive platforms. Deep water oil and gas fields fields are generally produced by means of large single installations, long reach wells and sub sea well clusters. Deep water platforms make for complex designs. Shallow water fields are more likely to be produced by a single main facility and a number of simple satellite platforms.
Construction, installation and commissioning
Offshore oil and gas production facilities are always constructed onshore. They are constructed in sections that are small enough to be lifted into position offshore. Once offshore, the individual sections are connected together. The process systems are tested prior to the treatment of any oil or gas being allowed.
The development of large lifting barges has made it possible to install very large facilities in single lifts, significantly reducing offshore hook-up and commissioning times.
Logistics/materials handling
The transport of personnel and materials to and from the offshore installations is by means of either helicopter or supply boat. Helicopters unload on elide, whereas supply boats are (off)loaded using cranes. Bulk materials such as potable water, diesel oil, drilling fluids and cement are offloaded by means of hoses.
Personnel travel from shore (via the company offices or directly) to the various heliports serving the offshore industry and from there by helicopter to the offshore installations.Suppliers deliver goods by road to the various operator shore bases dotted along the UK coast from where the goods are shipped to the offshore installations by boat. Urgent cargo is sometimes delivered by helicopter. Some bulk materials like drilling chemicals may be collected by boat from the supplier's quay side. Waste and excess materials are returned to the shore bases by boat and are sent by road to either licensed waste treatment plant or the original suppliers of the materials.
Power generation
Due to the large distances from the nearest shore, most offshore platforms generate their own power rather than importing it from the national grid. Usually a central power plant consisting of several reciprocating engine or turbine driven generators provide the electricity for the entire platform. Some large gas compressors and crude oil export pumps are driven by their own combustion engines. Emergency power is usually generated by smaller diesel generators. The engines are usually fueled by produced gas, although diesel oil is supplemented.
Power generation results in atmospheric emissions; mostly of carbon dioxide, although the emissions of NOX and SOX are also of concern. The energy efficiency of offshore power generation is much lower than that of onshore power generation. Offshore gas turbines have an energy efficiency as low as 25%, and the plant is not always operated at full capacity. Greater efficiencies are realized by installing waste heat recovery units, but this is only sensible on oil platforms when the recovered heat can help to separate the water from the oil.
Produced water treatment
Produced water is water that is produced with the oil and gas. Produced water is mainly associated with crude oil, whereas produced gas is relatively dry. With gas production, produced water always originates from the reservoir, whereas with oil production it is either reservoir water or seawater which was previously injected into the reservoir to maintain reservoir pressure and subsequently produced back with the oil. The composition of reservoir water differs from seawater and is often saltier.
Produced water needs to be treated before it can be discharged. The treatment is aimed at minimizing its oil content to below 30-40 parts per million. The treatment technology on the platforms differs greatly. Older installations may still have gas flotation units whereas newer designs incorporate hydroclyclones. Chemicals are often required to run the equipment effectively.
Accidental events
Although no deliberate offshore activities, the following are the typical things that can go wrong while producing oil and gas offshore.
- Collision
- Contained leak
- Emergency flaring
- Explosion
- Extinguished flare
- Fire
- Oil spill to sea
- Stuck in hole while drilling
- Well kick / blow-out
Risk sources 1 2 3 4
