Types of Drilling Rigs and Structures

   There are many different types of drilling rigs. Which rig selected depends on the specific requirments of each drill site. Rigs are generally categorized as onshore(land) or offshore(marine).       

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      Onshore rigs are all similar, and many modrn rigs are of the cantilevered mast, or &#;Jackknife&#; derrick type. This type of rig allows the derrick to be assembled on the ground, and then raised to the vertical position using power from the drawworks, or hoisting system. These structures are made up of prefabricated sections that are moved onto the location by truck, barge, helicopter, etc.                                              

        Offshore drilling are divided into two types: Fixed structure types and Floating structure types.  

       Jack-up Rig. This is a self-elevating rig, and is used for smaller, shallower offshore deposits. The rig&#;s floating platform is towed into position by barges, then lowers its support legs down to the sea floor, raising the rig above the water&#;s surface.

       Concrete Platform. Concrete offshore structures show an excellent performance. They are highly durable, suitable for harsh and arctic environment and can carry heavy topsides. Often offer storage capacities and are very economical for water depths larger than 150m. Gravity type platforms need no additional fixing because of their large foundation dimensions and extremely high weight.               

        Compliant tower. Compliant tower rigs are similar to fixed platforms, since both are anchored to the seabed and hold most of their equipment above the surface. Since its design consists of a narrow and flexible tower, it can withstand large lateral forces by sustaining significant lateral deflections.

             Floating Offshore Structure(moored to the seabed).

            Barge Rig. Although Barge rigs are not moored to the sea floor, they are explained in this section. They are floating offshore drilling vessels but it is not self-propelled. The drilling equipment is on the barge. It is generally towed to the location and then has its hull filled with water. This type of rig is only used in relatively shallow, swampy areas and are generally capable of drilling in water depths of less than 12ft, or, in the case of a posted barge, perhaps to 20ft.

          Submersible Rig. A Submersible rig is a larger version of a posted barge and is capable of water depths of 18ft to 70ft. It has a floating drill unit that includes columns and pontoons that if flooded with water, will cause the pontoons to submerge to a depth that is predetermined.

           Semisubmersible Rig. They are the most common type of offshore drilling rigs, combining the advantages of submersible rigs with the ability to drill in deep water. The semisubmersible rig does not rest on the seafloor. This rig is a floating deck supported by submerged pontoons and kept stationary by a series of anchors and mooring lines, and, in some cases, position-keeping propellers. They have a water-depth operating range of 20ft to ft.

        Tension-leg Platform. It consists of a floating surface structure held in place by taut, vertical tendons connected to the seafloor. These long, flexible legs allow for significant side to side movement, with little vertical movement. TLP can operate as deep as ft.

           Spar Platform. Spar platforms are among the largest offshore platforms in use. These huge platforms consist of a large cylinder supporting a typical fixed rig platform. The cylinder however does not extend all the way to the seafloor, but instead tethered to the bottom by a series of cables and lines. The large cylinder serves to stabilize the platform in the water, and allows for movement to absorb the force of potential hurricanes.

            DrillShips. Drillships are most often utilized for extremely deep water drilling at remote locations. A &#;floater&#; like the semisubmersible, a drillship must maintain its position at the drilling position by anchors and mooring lines, or by computer-controlled dynamic positioning equipment. Most drillships have greater storage capacity than other types of rigs, allowing efficient operation at remote locations.

References:

Shadizadeh Seyyed Reza, Offshore Drilling Engineering, Course Note

Sadeghi Kabir, An Overview of Design, Analysis, Construction and Installation of Offshore Petroleum Platforms Suitable for Cyprus Oil/Gas Fields, Girne American University, Department of Industrial Engineering, Mersin 10, Turkey,

Ali Seyyedalangi- M.Sc in Drilling Engineering

Rotary drilling rigs are used for most drilling operations today. The hole is drilled by rotating a bit and applying a downward force. Generally, the bit is turned by rotating the entire drill string, using a rotary table or a Top drive at the surface, and the downward force is applied to the bit by using heavy thick walled pipe, called drill collars, in the drill string above the bit. The cuttings generated by drilling are lifted to the surface by circulating a drilling fluid down the drill string, through the bit, and up the annular space between the hole and the drill string. The cuttings are separated from the drilling fluid at the surface.

Fig 01- Land Rigs 

The main features of the marine rigs are portability and maximum water depth operation. Submersible drilling barges generally are used for inland water drilling where wave action is not severe and water depths are less than 20 ft. The entire rig is assembled on the barge, and the unit is towed to the location and sunk by flooding the barge. Once drilling is completed, the water is pumped out from the barge, allowing it to be moved to the next location. After the well is completed, a platform must be built to protect the wellhead and to support the surface production equipment.

Fig 02- Drilling Barges 

When water depth is less than about 350 ft, jackup can be used. The jackup is bottom supported rigs using metallic legs. On location, the legs are lowered to the bottom and the platform is jacked up above the wave action by means of hydraulic jacks.

Fig 03- Jack Up Rigs 

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Semisubmersible rigs are more expensive than jackup rigs and, thus, are used mostly in water depths too great for resting on bottom. Semisubmersible rigs employ large engines to position the rig over the hole dynamically. This can extend greatly the maximum operating water depth. The shape of semisubmersible rigs tends to dampen wave motion greatly regardless of wave direction. This allows its use in areas where wave action is severe.

Fig 04 -Semi-Submersible Rigs 

A second type of floating vessel used in offshore drilling is the drillship. Drillships are designed with the rig equipment and anchoring system mounted on the central turret. The ship is rotated about the central turret using thrusters so that the ship always faces incoming waves. This helps to dampen wave motion.

Fig 5- Drill Ship Rigs 

Offshore development drilling usually is done from fixed platforms. After the exploratory drilling program indicates the presence of sufficient petroleum reserves to justify construction costs, one or more platforms from which many directional wells can be drilled are built and placed on location. The platforms are placed so that wellbores fanning out in all directions from the platform can develop the reservoir fully. The various rig components usually are integrated into a few large modules that a derrick barge quickly can place on the platform.

Fig 7 - Platform Rigs 

Large platforms allow the use of a self-contained rig- all rig components are located on the platform. A platform /tender combination can be used for small platforms. The rig tender, which is a floating vessel anchored next to the platform, contains the living quarters and many of the rig components. The rip-up time and operating cost will be less for a platform/tender operation. However, some operating time may be lost during severe weather.

Although drilling rigs differ greatly in outward appearance and method of deployment, all rotary rigs have the same basic drilling equipment. The main component parts of a rotary rig are:

- Power system

- Hoisting system

- Circulating system

- Rotary system

- Well control system

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