Oil and gas are present on the seabed and deep under the ocean water. These can be identified and extracted with the help of various oil rigs. The main difference in offshore and onshore rigs are about the kind of equipment used in each of them. Let us have a look at it in detail.

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What is Offshore Drilling?

A great amount of oil and natural gas reserves are located underwater. Offshore drilling is the process of extracting these from reserves located beneath the ocean surface, rather than reserves located on the mainland. In this process, a wellbore is drilled below the seabed. The drilling takes place on oil platforms.

This type of drilling relies heavily on various types of offshore rigs. It takes time for oil companies to set up equipment on offshore oil drilling sites but once set up, these can last 10 to 20 years. After the setup is complete, it takes a while before an oil drill reaches the oil well. This is because of the great distance from the surface to the deposits. Currently, there are approximately 500 offshore rigs worldwide.

What is Onshore Drilling?

Onshore drilling process is entirely land-based. It often happens at the seabed. In onshore drilling sites, the field has multiple wells that are close together and interconnected by carbon steel tubes.

There are two main types of onshore drilling:

Percussion or cable tool drilling: It consists of raising and dropping a heavy metal bit into the ground, thus punching a hole down through the earth. This method is usually used for shallow, low pressure formations.

Rotary drilling: It consists of a sharp, rotating metal bit used to drill through the earth’s crust. This type of drilling is used primarily for deeper wells, which may be under high pressure.

Offshore Drilling Rigs

There are many types of offshore platforms used for oil and gas drilling and exploration. Some of these are fixed to the ocean floor while others are floating platforms. Each is designed for different water depths and purposes.

Barge Rigs

Barge rigs work in shallower waters, usually less than 20 feet deep. They are floated to the drilling location and the hull is brought to rest on the ocean floor. This creates a stable platform for drilling. Many barge rigs operate in the Gulf of Mexico and around the world.

Submersible Rigs

A submersible rig is used in waters that are less than 50 to 70 feet deep. It is designed with a platform-type deck that contains drilling equipment and other machinery supported by pontoon-type columns that are submerged into the water. They operate in relatively shallow water and rest on the seafloor.

Submersible drilling rigs include the posted barge submersible, the bottle type submersible, and the arctic submersible.

Jack-Up Rigs

Jack-up rigs are used to drill wells in shallow waters of 400 feet or less. They belong to the class known as Mobile Offshore Drilling Unit (MODU). It can be used to drill wells to a total depth of 30,000 to 35,000 ft.  It is made up of a barge (or hull) and legs that can be lowered or raised onto the seafloor. This provides support for the main parts of the rig.

Jack-up rigs can be used to drill exploration wells, appraisal wells, and development wells. They are designed in such a way that there are three poles that enable the oil rig base to either get hoisted up or stay afloat in the water. The material used to create the jack up platform is made of steel. This is done to prevent threat of erosion when the rigs are placed in the water.

The first jack up rig was launched in the year 1954. The structure has been continuously improving with the advent of technology.

The types of jack-up rigs that exist are based on how they rest on the seafloor. There are two types of jack-up rigs:

• Cantilevered Jack-up: It has a drilling derrick mounted on an extending arm
• Keyway or Slot Jack-up: It has a drilling unit on the platform over an opening it drills through

Platform Rigs

A  platform rig is a large steel or concrete structure installed in water depths ranging from shallow to deep water, depending on the location and requirements of the project. This provides a secure foundation for drilling equipment and production facilities. They use a steel framework anchored to the ocean floor as a foundation for a surface drilling rig, equipment, and living quarters.

Types of Platform Rigs

Platform rigs are broken down into specific types suited for various depths. The different types of platform rigs are:

• Compliant Platforms
  • Compliant tower
  • Guyed Tower
  • Tension leg platform

• Fixed Platform
  • Steel Jacket Structure
  • Gravity structure

Floating Rigs or Floaters

In very deep water, it is not practical to attach an offshore rig to the bottom of the ocean. Floating oil rigs are kept in place by anchors or dynamic positioning systems that keep them over target. There are several types of floaters in use at various deep water drilling locations worldwide.

Types of Floating Oil Rigs

There are several types of floating oil rigs:

Semi-Submersibles

They are also known as semis. These rigs have columns or pontoons that are submerged underwater. They are connected to a deck that holds the drilling equipment. This provides stability by using the buoyant force of the submerged sections to counteract the weight above the waterline.

Drill ships

The drill ships have drilling equipment which is mounted in the center of the ship over a moon pool. It is a reinforced hole in the bottom of the ship through which the drill string is raised and lowered. The ship can be turned into the oncoming wind or currents for better stability, and it can operate in water too deep for anchors. Dynamic positioning systems or thrusters are used to maintain the position of the ship over the drilling site.

Onshore Drilling Rigs

Land rigs are also classified based on maximum drilled depth and mobility. There are two different types of land rigs, conventional and mobile drilling rigs.

Conventional Rigs

These are the most commonly used rigs in petroleum industry. They cannot be moved to the drill site as whole units.

Mobile Rigs

These are movable rigs in which the drilling systems are mounted on wheeled trucks. They are of two different types – jackknife and portable mast.

Jacknife Rig:

It is an onshore rig consisting of a mast that can raised or lowered according to drilling requirements. It can be assembled on the ground using pins and can be raised as a unit with the help of hoisting equipment.

Portable Mast:

This rig is used to drill wells that have moderate depth. They are usually carried to the drill site on wheeled trucks and can be raised vertically to a desired height.

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Similarities and Differences Between Onshore and Offshore Rigs

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