Production Facilities

The oil industry uses maritime drilling platforms or drilling rigs, and production platforms that are classified according to their intended purpose and to the depth of the water in which they can function, as listed below:

1. Fixed Plataforms

Jackets

Jackets are formed by large modular steel structures that are installed in the operation site through unmoving legs anchored into the seabed. The structure may be made of either metal or concrete. The water depth at the position site must not exceed 400 meters, as installation costs increase exponentially with depth. Jackets are only used worldwide in fields where the water depth does not exceed 300 meters.  

The jackets are designed to receive drilling equipment, store materials, staff lodging, as well as all the necessary installations for the wells’ production. In them, a Christmas tree (an assembly of valves that controls the well’s production) is what is referred to as dry completion.

A process/utility plant is installed in the deck where its main function is to separate and treat the fluids produced by the wells. The oil is exported through oil pipelines, while part of the compressed gas is offloaded onto land through gas pipelines, and the other part is used to generate the energy of the platform. The water is discarded in the sea or injected back into the reservoir.  

This technology was developed between the 1930s and 1950s in Venezuela, and was then used in the Gulf of Mexico.

Fixed steel platforms

Fixed steel platforms do not have the capacity to store oil and gas, and, consequently, the production is done onshore, or in another platform, interconnected through oil and gas pipelines.

Fixed concrete platforms

Fixed concrete platforms, or fixed gravity platforms, are made up of a steel upper deck supported by three or more legs over a lower base, which rests directly over the seabed. This base is not fixed on the seabed by stakes, it maintains its stability through to its enormous weight and low center of gravity, situated close to the base.     

This type of platform shares similar characteristics and applications as the fixed steel platforms, except that they have the capacity to store oil in compartments situated in the base. Thus, the oil does not need to be exported through an oil pipeline, since the production can be stored and, from time to time, transferred to a shuttle tanker.   

Most of the concrete platforms are installed in the Norwegian sector of the North Sea, due to the favorable conditions for building through the use of Fjords.

Compliant Towers

Compliant towers are fixed structures that are similar to traditional jackets, used in water depths between 350 and 800 meters. Their behavior is compliant to wave power, they are lighter than jackets and tolerate greater horizontal movement.  

Some of these towers are already operational in the Gulf of Mexico.

2. Semi-Sub Platforms

Semi-sub platforms are floating units widely used both in the drilling and the production. Its structure can include one or more decks, which are supported by columns (or legs) on pontoons.

The floating units are subject to movement due to wave, current and wind action. Two types of system are responsible for the positioning of the floating unit: the mooring system and the dynamic positioning system.

The mooring system is built of 8 to 12 anchors, as well as cables and/or chains that act as coils that can restore the position of the floating unit when it is dislocated. In the dynamic positioning system, there is no physical connection between the platform and the seabed, except for the drilling equipment. Acoustic sensors determine the drift and propellers in the hull restore the position of the platform, activated by computers. 

Semi-sub platforms can have their own propulsion system or can be towed to the location. Because of its great mobility, they are the preferred units for the drilling of exploratory wells.

One of the disadvantages of this type of platform is that it is not suitable for the storage of produced oil during the exploratory process, and therefore requires oil and gas pipelines.  

These platforms were first used in Brazil by Petrobras for its production systems. They are also widely used in the Gulf of Mexico and in Africa.

3. FPSO Platforms

FPSOs (Floating, Production, Storage and Offloading) are floating systems that can process and store the oil and provide the unloading of the oil and/or natural gas.

FPSOs are large oil tankers, either new or used, that were converted, such as VLCCs (Very Large Crude Carrier) or ULCCs (Ultra Large Crude Carrier). The hulls of made-to-order FPSOs are specifically designed and built to operate as a FPSO.

The deck contains a process plant to separate and treat the fluids produced by the wells. After the coil is separated from the water and gas, it is stored in tanks on the actual vessel and then offloaded from time to time to a shuttle tanker. 

FPSOs are stationary units that are anchored through mooring systems that depend primarily on: i) the number of risers connected to the unit; ii) the position of the wells in relation to the platform; and iii) the environmental conditions (waves, currents, wind, etc.).

The mooring can be a Single Point Mooring or a Spread Mooring, which allows for the installation of a great number of production risers.

In Single Point Mooring, a Turret is installed, which allows the vessel to move around the connection of the risers to the platform. This movement is provided by a huge bearing, while the transfer of fluids from the fixed part to the mobile part of the Turret is provided by a system called a Swivel.   

The offloading of the oil to a shuttle tanker is done through floating hoses, which usually measure between 16 and 24 inches in internal diameter.

Some FPSOs are maintained in position by a dynamic positioning system and are mainly used for long duration well testing during the exploratory phase.

FPSOs are widely used in Brazil and in Africa. FPSO is the production unit used in the Atlanta field, located in the Santos basin, where QGEP holds 30% of the shares.

4. Tension-Leg Platforms

Tension-Leg Platforms (TLPs) are an evolution in the concept of semi-subs. They present a hybrid behavior: on one hand, they are compliant with regard to the horizontal degrees of freedom, like a floating system, and, on the other hand, they are more rigid with regard to the vertical degrees of freedom, like the fixed platforms. 

The main conception behind its construction is the use of legs as a base, which are secured to the seabed with tethers tensioned in the platform and connected to the legs.

The platform is maintained in a position where the thrust is a lot larger than the weight. Thus, the vertical balance of the platform is achieved through a set of tensioned tethers with an enormous vertical stiffness, to prevent disconnection from the seabed. The reduced stroke (the movement between a reference point of the rigid production riser and the deck) allows the TLP to offer dry completion to several wells or operate in great water depths with rigid risers (steel catenary unit), due to the reduced vertical movement.

TLPs are comprised of:

  • Deck structure (similar to the ones used in semi-sub platforms), which may include drilling rigs and process/utility plants. Some TLPs are used for wet completion (without a drilling rig) or are a type of wellhead (drilling rig, without production/utility plant);
  • Hull structure (designed to bear the deck and the traction from the tethers);
  • Vertical tubular steel tethers, responsible for the mooring (they may have constant diameters in deep waters and variable diameters in ultra deep waters);
  • Base consisting of secured legs (designed to keep the platform in place, resisting the loads originating from the mooring system).

These platforms are developed mainly for the Gulf of Mexico. They are also used in the North Sea, and have more recently been installed in Brazil, in 2014, at the Papaterra field.

5. SPAR Platforms

Spar is a type of drilling or production platform used in ultra deep waters. Its structure consists of a transversal circular hull section that is positioned vertically in the water, supported by the floatation tanks located in the upper part of the structure. If needed, its stability can be increased through the addition of a solid ballast in the compartment located in its keel. A deck is built over the hull, containing a drilling rig and a production/utility plant.  

Spar type platforms consist of a single vertical cylinder with a large diameter, supporting a platform. Attached to it is a typical onshore platform topside, containing drilling equipment and a production/utility plant.

Spar platforms are kept in place through a taut-leg mooring system. This type of unit has reduced movement amplitude, making it possible to use dry completion wells with tensioned risers, so that several wells can operate in great water depths with rigid risers (steel catenary unit), because of reduced vertical movement. The dry completion wells are usually drilled from the Spar itself and connected to the deck through vertical rigid risers, similar to the ones used during the drilling phase.  

There are three types of Spar platforms: 

  • Classic Spar;
  • Truss Spar and
  • Cell Spar

Spar platforms can be found in the Gulf of Mexico.

Last updated on 2015-05-07T16:23:09

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