Types of Drilling Rigs and Structures

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

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Onshore rigs share various similarities. Many modern rigs feature a cantilevered mast or 'Jackknife' derrick design. This rig type allows for assembly on the ground before raising it to a vertical position using power from the drawworks or hoisting system. These structures consist of prefabricated sections transported to the location via truck, barge, helicopter, etc.

Offshore drilling rigs are divided into two main types: fixed structure types and floating structure types.

Fixed Structures

Jack-up Rig: This self-elevating rig is utilized 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 demonstrate excellent performance. They are highly durable, suitable for harsh and arctic environments, and capable of supporting heavy topside equipment. They often have storage capacities and are economically viable for water depths greater than 150 meters. Gravity-type platforms require no additional fixing due to their extensive foundation dimensions and substantial weight.

Compliant Tower: Compliant tower rigs resemble fixed platforms as both types are anchored to the seabed, with most equipment stationed above the surface. Their narrow and flexible design allows them to withstand significant lateral forces through considerable lateral deflections.

Floating Structures

Barge Rig: While not anchored to the seafloor, barge rigs are considered here. These floating drilling vessels are non-self-propelled and have drilling equipment mounted on the barge. They are generally towed to the location and then filled with water to settle. Barge rigs are confined to relatively shallow, swampy areas, typically capable of drilling in water depths of less than 12 feet, or up to 20 feet for a posted barge.

Submersible Rig: A submersible rig is a larger version of a posted barge, capable of drilling in water depths ranging from 18 feet to 70 feet. It features a floating drill unit that includes columns and pontoons. When flooded, these pontoons submerge to a predetermined depth.

Semi-submersible Rig: These are the most common type of offshore drilling rigs. They combine the advantages of submersible rigs with the capability to drill in deep water. Unlike resting on the seafloor, a semi-submersible rig features a floating deck supported by submerged pontoons and maintained in a stable position by anchors and mooring lines, and in some cases, dynamic positioning propellers.

Tension-leg Platform: A tension-leg platform 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 minimal vertical motion.

Spar Platform: Spar platforms rank among the largest offshore platforms in use today. They consist of a large cylinder supporting a fixed rig platform. The cylinder does not extend entirely to the seafloor but is tethered to the bottom by cables and lines, providing stabilization and flexibility against wave forces, particularly during hurricanes.

Drillships: Drillships are primarily employed for deep-water drilling in remote locations. As a 'floater' similar to the semi-submersible rig, a drillship maintains its position over the drilling site utilizing anchors, mooring lines, or computer-controlled dynamic positioning systems. Most drillships offer greater storage capacity than other rig types, facilitating efficient operation in isolated areas.

Drilling Rig Types

Rotary drilling rigs are prevalent in today's drilling operations, where the hole is created by rotating a bit while applying a downward force. This action is generally achieved via a rotary table or top drive at the surface. The bit is turned by rotating the entire drill string, while the downward force is exerted using heavy, thick-walled pipes known as drill collars above the bit. The generated cuttings are lifted to the surface by circulating drilling fluid down the drill string, through the bit, and up the annular space between the hole and the drill string, where cuttings are separated at the surface.

Rotary drilling rigs can be categorized into land rigs and marine rigs. The salient features of land rigs are portability and maximum operating depth. Land rigs are designed for easy movement and reusability for new drill holes. Various rig components are skid-mounted for convenient transport and connection. The jackknife or cantilever derrick is assembled on the ground with pins and then elevated as a unit utilizing rig hoisting equipment. The portable mast, suitable for moderate depth wells, is typically mounted on wheeled trucks or trailers incorporating the hoisting machinery, engines, and derricks as a single unit. Hydraulic pistons raise and extend the telescoped portable mast to its full height.

Marine rigs feature similar aspects of portability and maximum operational water depth. Submersible drilling barges are typically designated for inland water drilling where wave action is limited and water depths do not exceed 20 feet. The rig structure assembles on the barge, which is towed to the location and submerged by flooding. After completing drilling, water is pumped out of the barge for relocation. Once the well is finished, a platform must be established to safeguard the wellhead and support surface production equipment.

When water depth is below approximately 350 feet, jack-up rigs are applicable. These bottom-supported rigs utilize metallic legs, which lower to the bottom and elevate the platform above wave action through hydraulic jacks.

Semisubmersible rigs, being more costly than jack-up rigs, are predominantly utilized in deeper waters where resting on the bottom is untenable. These rigs utilize substantial engines for dynamic positioning over the drilling location, permitting deeper operational water capabilities. Their design dampens wave motion significantly, allowing their use in areas with strong wave action.

The second type of floating vessel in offshore drilling operations is the drillship. Drillships are engineered with the rig's equipment and anchoring system mounted on a central turret. The ship rotates around this turret using thrusters, ensuring the vessel constantly faces incoming waves, thus diminishing wave motion.

Offshore development drilling is chiefly undertaken from fixed platforms. When exploratory drilling indicates sufficient petroleum reserves to warrant construction costs, multiple platforms from which directional wells can be drilled are constructed and placed on site. These platforms allow wellbores to extend in all directions, optimizing reservoir development. The various rig components typically integrate into several large modules that a derrick barge can quickly position on the platform.

Large platforms facilitate the use of self-contained rigs, where all components reside on the platform. Alternatively, a platform/tender combination can serve smaller platforms. The rig tender, a floating vessel anchored next to the platform, houses living quarters and many rig components, resulting in lower rip-up time and operating costs compared to standalone setups. However, some operating time may be lost during adverse weather conditions.

Despite the significant differences in appearance and deployment methods, all rotary rigs share common essential drilling equipment, including:

• Power system
• Hoisting system
• Circulating system
• Rotary system
• Well control system

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