Oil Production Facilities
Introduction
For many thousand years, oil has been utilized for myriad purposes, especially lighting, production of electricity, and the provision of combustion power in locomotives. In areas where oil deposits exist in reservoirs that are shallow, seeps of gas and oil may develop, and some of the oil could be collected from seepage and tar ponds. Historically, tales of eternal fires emanating from oil and gas seeps are widely known. A prime example is an incidence that occurred in 1000 B.C, whereby the oracle of Delphi was set to be built (Kelland 33). Also, the Chinese are known to have been using natural for purposes of boiling water during the 500 B.C.
It was not until 1858 that Edwin Drake, a colonel, drilled the first oil well for the sole aim of finding oil. The well was at the epicenter of a quiet farm in North Pennsylvania, and it was from this well that the international search for and industrial use of petroleum was incepted (Kelland 45). Notably, these wells were too shallow by modern standards. However, these shallow wells could give quite large oil production. Simply put, the ancient exploration of oil and gas was insufficient in regards to the intricateness of facilities. However, with time, advances in technology have led to the development of better facilities that have sufficed the exploration and production of oil. In the light of the above synopsis, this paper explores various oil production facilities.
Onshore Facilities
Generally speaking, offshore production of oil is economically feasible from a few tens of barrels and upwards on a daily basis. In the globe, there are several millions of wells from which oil is being produced. In essence, gas gathering networks can gain an appreciable large size, having production from hundreds of wells, several miles apart, feeding through gathering networks into processing plants (James et al. 942). One facility prevalent in onshore oil production is the sucker rod pump, also known as the donkey pump. For significantly small reservoirs, oil is gathered in a tank and then collected at regular intervals by the use of a tanker truck or railcar.
Onshore wells existing in areas rich in oil comprise of high capacity wells with many thousand barrels on a daily basis, linked to a 1,000,000 barrel oil-gas separation plant (James et al. 943). Then, the product is conveyed from the plant through pipeline and tankers. Important tasks in such operations may include the logging and metering well-streams leading to the gathering networks.
Offshore Facilities
Depending on the size and water depth, multi-faceted structures are employed in the offshore production of oil. In the past few years, sea pure bottom installations consisting of multiphase piping to both onshore and offshore topside structures have been observed. In order to reach various parts of a reservoir, outlying wellhead towers have been replaced, and deviation drilling has been introduced. The following are some of the commonplace offshore structures:
Shallow Water Complex
It comprises of several independent platforms characterized by different parts of utilities and process connected by gangway bridges. The individual platforms will be explored in details in the latter sections of this paper. They include the rise, wellhead, power generation platforms, and processing accommodations.
Gravity Base
These are vast concrete structures placed at the sea bottom, typically in the oil storage cells located in the “skirt” resting on the sea bottom. There is a large deck that receives all the process and utility parts in large modules. This facility was typical for the 80s and 90s fields, which went to 100-500 m water depths (Dennis 22). At the time, some fluid was poured at the location at the shore containing sufficient air in the storage units to aid in keeping the structure afloat until tow and lowering into the seabed.
Compliant Towers
These resemble fixed platforms, and they have of a narrow tower, which is attached to a foundation located on the floor of the sea and extending to the platform. It is worth noting that this type of tower is flexible, unlike the relatively rigid legs exhibited by a fixed platform. Because of the flexibility, it is possible to operate in appreciably deeper waters because of the fact that a high level of the pressure it experiences from the sea and wind can be ‘absorbed’. In practice, the compliant towers are utilized for water depths between 500 to 1000 meters (Dennis 66).
Floating Production
This form of production involves the location of all topside systems on an afloat structure with subsea or dry wells. The first type of floaters employed in oil production is the FPSO, which involves storage, floating production, and offloading. In principle, a tanker type hull contains wellheads on the turret in order for the ship to freely rotate around in order to point towards the wind, waves, or current. Notably, the turret consists of chain and wire rope connections to many anchors, or it can be positioned dynamically by the use of thrusters. Typically, this facility is used for the production of oil in water depths of 200 to 1000 meters and is commonplace with subsea wells (Benemann et al. 68). The central process is located on the deck whereas the hull usually utilized for the storage and offloading of the product onto shuttle tankers.
Another floating facility used onshore is the Tension Leg Platform (TLP), which comprises of a structure maintained on location by vertical tendons linked to the sea floor through pile-secured templates. It is worth noting that the structure is usually maintained in a particular position by the use of tensioned tendons, which serve to facilitate the base of TLP in broad water depth ranges of up to 2000m (Benemann 68). One point worthy of consideration is that this equipment exhibits restricted vertical motion. Also, the tendons are normally constructed in the form of hollow steel pipes carrying the structure’s buoyancy. A variant of this platform is the Seawater type, which is simply a miniature floating leg platform, resembling the semi-submersible type.
SPAR
It is characterized by a single floating cylinder hull, which supports a fixed deck. However, the cylinder does not extend to the floor of the sea, but it is tethered to the sea bottom by the use of a series of lines and cables. It is important to note that the large cylinder stabilizes the platform in water and permits movement, which facilitates the absorption of the force of hurricanes. Notably, SPARS can be significantly large and are utilized for water depths ranging from 300 m to 3000 m (Gieg et al. 264). The term SPAR refers to its likeliness in regards to a ship’s spar, and spars have the ability to offer support to dry completion wells.
Subsea Production Systems
These are wells positioned on the seafloor and just like floating production systems, petroleum is extracted from the floor of the sea, and is then ‘tied-back’ to an existing production platform or onshore facility. Usually, the well gets drilled by the use of a moveable rig, and the natural gas and oil that is extracted is delivered through pipeline and the underground riser to the processing facilities. This arrangement makes it possible for one strategically positioned platform to serve many walls over a large area. Typically, subsea systems are used at depths of 8,000 feet or even more, but there is no possibility of drilling; they only allow for extraction and transportation (Gieg et al. 264). In a normal setup, drilling and completion is carried out from a surface rig. Further, the horizontal offsets span up to 250 kilometers.
Semi-Submersible Platforms
These are platforms with legs of adequate buoyancy to make the structures float. They also have sufficient heights that serve to keep the structures upright. Notably, semi-submersible rigs can be transported from one point to another; they can be ballasted up or down through the alteration of the amount of flooding occurring in the buoyancy tanks. Generally, these rigs are anchored by the use of a wire and polyester rope or a chain amidst drilling operations. However, they can be kept in place by using dynamic positioning. It is noteworthy that sub-submersible rigs are employed in water depths ranging from 100 to 10,000 feet (Armaroli, Nicola, and Balzani 66).
Jack-Up Platforms
As their name suggests, these are platforms with the ability to be jacked up above the sea by the use of legs that can be lowered, just like jacks. Typically, these platforms are used in water depths reaching 400 feet (Armaroli, Nicola, and Balzani 66). However, some designs can be employed up to 550 feet depths. It is also noteworthy that the platforms are designed to be moved from one location to another, and then provide anchorage by themselves through the deployment of legs to the ocean bottom. They make use of a rack and pinion gear system constituted by each leg.
Compliant Towers
These are platforms consisting of a pile foundation and narrow, flexible towers that serve to support a conventional deck for the production and drilling operations. Notably, compliant towers have been designed to withstand multiple lateral deflections and forces. Further, they are typically employed in water depths ranging from 1600 and 3,100 feet (Havard 44).
Drillships
These are maritime vessels fitted with drilling apparatus. Mostly, they employed in the exploratory drilling of new gas or oil wells lying in deep water. However, they can be used for scientific drilling as well. In order to maintain their position over the well, most drillships are usually fitted with dynamic positioning systems.
Main Processing Sections
Wellheads
The wellhead is normally positioned on top of the gas or oil well, leading to the reservoir. Also, a wellhead may be an injection well, which is employed for the injection of water o gas back into the reserve for the purpose of maintaining pressure levels in order to maximize production. At the instant when an oil or natural gas well has been drilled, and has been ascertained that the quantities of oil or gas available are commercially viable, the well has to be ‘completed’ to facilitate natural gas or petroleum flow out of the formation to the surface (Mega et al. 1866). It is noteworthy that this process involves the strengthening of the wellhole by the use of a casting, the evaluation of the temperature and pressure of the formation, and, finally, the installation of proper equipment to see into it that there is an adequate natural gas and oil flow out of the well. A choke comes in handy for the purpose of controlling the well flow.
Usually, the wellhead comprises of pieces of equipment that are mounted at the well opening in order to control and oversee hydrocarbon extraction from the oil and gas formations. Also, the pieces of equipment serve to prevent the leakages of natural gas and oil out of the well. Even more, they prevent blowouts occurring because of formations exhibiting very high pressure. Typically, formations under high pressure require wellheads that are able to withstand an appreciable amount of upward pressure emanating from the escaping liquids and gases. It is noteworthy that the wellheads must have the ability to bear pressures reaching 150 MPa (1500 Bar) (Havard et al. 45). The wellhead has three fundamental components: the tubing head, the casing head, and the ‘Christmas tree”.
A classic Christmas tree has a master gate valve, pressure gauge, a wing valve, a choke, and a swab valve. Also, a number of check valves may be contained by a Christmas tree. The roles of these devices are delineated in the subsequent paragraphs.
Casing-Head and Casing Hangers
Usually, the casing bolted, screwed or welded to a hanger. Further, several plugs and valves are normally fitted to facilitate access to the hanger, permitting the opening, closing or bleeding of the casing. In some cases, it allows for the production of a flowing well through the casing. The valve comes in handy during the determination of leaks in the tubing, casing or the packer.
The Tubing Hanger
It is also referred to as the donut, and it is utilized for correct positioning of the tubing in the well. In like manner, the sealing allows the removal of the Christmas tree.
Meter Gate Valve
This is a valve of high quality, which serves to provide full opening, connoting that it opens to an inside diameter equal to that of the tubing, an essential operation for running specialized tools through the tubing. It is noteworthy that this valve must be able to withstand the full-throttle pressure of the well safety for all the imminent conditions. Usually, this valve is left open and, hence, not used to regulate flow.
The Pressure Gauge
This is a minimum instrumentation, which is positioned above the master gate valve, just before the wing valve. It serves to sense the existent pressure, which is essential in regulating the flow.
The Wing Valve
It can be a ball or gate valve. When well shutting is being carried out, the wing valve is used to ensure that the tubing pressure is read easily.
The Swab Valve
This is a valve employed to facilitate the access to the well in order to conduct wireline operations, interventions, and several other workover procedures. An adapter and cap are located on top of it in order to mate with various equipment.
Variable Flow Choke Valve
This is a typically large needle valve. Notably, it has an adjustable calibrated opening, which can be varied in 1/64 increments (George et al. 33). It is made of high-quality steel to ensure that it can bear the high-velocity flow of abrading materials passing through the choke.
Manifolds/Gathering
In practice, each well stream is conveyed into the primary production facilities via a network of manifold systems and gathering pipelines. The core purpose of this undertaking is to permit the setup of production ‘well sets’ so that for a certain level of production, the cost, efficacious reservoir utilization, and well flow composition (water, oil, and gas) can be chosen from the available wells.
In regards to gas gathering systems, a common practice is to meter gathering lines to the manifold. For the multiphase (combination of water, oil, and gas) flows, the rather high costs of multiphase meters mostly prompts the use of software flow rate calculators that make use of well test data to compute the actual flows.
When employed offshore, the completion wells located at the center of the main field feed directly into the production manifolds. On the other hand, subsea installations and outlying wellhead towers feed through the multiphase pipelines to the production risers, which are systems facilitating pipeline “rise” up towards the top of the structure (George et al. 56). In regards to the flowing structures, the process entails the provision of a method to take up movement and weight.
Separation
Some oil production wells are characterized by gas production, which is pure in nature, and can be directly conveyed to gas treatment and compression. Oftentimes, the well provides a combination of oil, gas, and water and other contaminants, which must be subjected to separation and processing. In this regard, it is noteworthy that the production separators may be provided in various designs and forms, with the typical version being the gravity separator.
In the process of gravity separation, a horizontal vessel serves to receive the well flow. The most typical retention period is usually five minutes, which allows bubbling out of gases, settling of water at the bottom, and, finally, the taking out of oil from the middle. In severe stages (low-pressure separator, high-pressure separator), the pressure is usually reduced in order to allow for the regulated separation of volatile elements. It is noteworthy that an instantaneous pressure reduction may permit flash vaporization, causing safety hazards ad instabilities.
Metering, Storage, and Export
Typically, most oil production plants do not provide for local oil and gas storage, but often, oil and gas are stored before loading the products to a vessel, which may be a shuttle tanker transporting the product direct to a crude oil carrier or into a larger tanker terminal. It is noteworthy that the offshore production facilities lacking a direct pipeline connection mainly rely on crude oil storage in a hull or base in order to ensure that a shuttle tanker is offloaded approximately once a week (Havard 89). In contrast, larger production complexes consist of an associated tank far terminal that allow for the storage of myriad grades of crude.
In order to allow operators to manage and monitor the oil and natural gas obtained from the production installations, metering stations are provided. These stations make use of meters to measure oil or natural gas as it continues to flow through the pipeline. It is worth noting that those metered volumes show a transfer of ownership from a particular producer to their client and, hence, referred to as Custody Transfer Metering. Notably, it forms the basis for production taxes invoicing sold products and sharing revenue between partners.
In a typical setup, the installation comprises of several meter runs such that one meter does not have to serve full capacity and rover loops in order to ensure that the meter accuracy cab be adequately tested and calibrated at certain intervals.
Utility Systems
These are systems that do not deal with the hydrocarbon process flows. Rather, they serve to provide some utility to the primary process safety. Depending on the installation location, many functions can be available from nearby structures, such as electricity. However, many remote installations have to be fully self-sufficient and, hence, generate their own water and power.
Rod Pumps
These are sucker rod pumps, which are also known as Donkey pumps (James et al. 950). They are the most prevalent artificial lift systems employed in land-based operations. They have a motor that provides drive to a reciprocating system linked to a polished road that passes into a tubing through a stuffing box. The Donkey pump usually proceeds down to level of oil and is linked to a plunger containing a valve.
The plunger lifts a certain volume of oil up during each upward stroke. On the downward stroke, the plunger sinks, with oil flowing through the valve. It is important to note that the torque and motor speed are controlled to ensure efficient and minimal wear of the Pump off Controller (PoC). However, the use of sucker rod pumps is limited to a meters (hundreds) and a maximum of flows of up to 10 gallons per stroke (James et al. 953).
Downhole Pumps
These pumps serve to insert the entire mechanism of pumping into the well. In the current setups, an electrical Submerged Pump is usually inserted into the well. In this setup, a well assembly is comprised of a narrow motor and a multiphase pump (for example, a progressive cavity pump) or a centrifugal pump. The motor hangs by the use of an electrical cable consisting of tension members.
Gas Lift
The purpose of the gas lift lids to introduce gas into the flow of the well. Normally, the pressure of the downhole reservoir falls of the wellhead because of the counter pressure created by the oil column weighting the tubing. Hence, a 160 MPa reservoir at around 1600 meters becomes zero wellhead pressure at instances the specific gravity if 8 g/cm3 (Kelland 88). Specific gravity is cut down through the injection of the gas into the oil and, hence, the well starts to slow. In practice, the gas that is injected between the tubing and the casing, and a release valve located at the gas lift mandrels inserted in the tubing. The valve opens at a particular set pressure in order to fill the tubing with lift. In order to ameliorate the lifting and start up, several mandrels containing valves are set at various temperature ranges.
Plunger Lift
This is a facility normally utilized in low-pressure gas wells with some oil, condensate or water, or high oil gas ratio wells. In these scenarios, well flow conditions are set in such a way that the liquid begins to collect downhole and finally blocks the gas such that the well production comes to a stop. In such a case, a plunger containing an auxiliary valve can be placed in the tubing. At the top, a plunger catcher is used to open the valve and hold the plunger. Usually, the cycle of operation begins with the plunger entering the well while the valve is open (Dennis 77). The condensate, gas and oil can have their way through the plunger till they reach the bottom. At this point, the valve is normally closed, with a volume of condensate oil or water on the top. Notably, gas begins accumulating below the plunger and after some time, it pushes the plunger in an upward direction, with the liquid lying on the top, which finally flows out through the wellhead discharge.
Conclusion
There is a myriad of facilities that make the production of oil and natural gas possible. These facilities are needed to exhibit a variety of characteristics that ensure they suffice the drilling and oil operations. While some equipment is used onshore, some are used offshore, and each type is designed in a manner that suits its area of use. Further, it has been realized oil production is a rather intricate process that calls for the use of sophisticated and adequate facilities. The equipment utilized for the production of oil has been noted to fall into various categories. However, the principal ones include subsea production systems, semi-submersible platforms, jack-up platforms, compliant towers, drillships, main processing systems, manifolds, separation facilities, and metering systems to mention but a few.
Works Cited
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