Compiled by :  Gp Capt Kumar Kirinde [retd], SLAF


An offshore drilling rig also known as oil platform and offshore platform is a large structure with facilities for well drilling to explore, extract, store, and process petroleum and natural gas that lies in rock formations beneath the seabed.

Most commonly, oil platforms engage in activities on the continental shelf, though they can also be used in lakes, inshore waters, and inland seas. Depending on the circumstances, the platform may be fixed to the ocean floor, consist of an artificial island, or float. Many oil platforms will also contain facilities to accommodate their workforce.

Offshore drilling presents environmental challenges, both from the produced hydrocarbons and the materials used during the drilling operation.

There are many different types of facilities from which offshore drilling operations take place. These include bottom-founded drilling rigs (jackup barges and swamp barges), combined drilling and production facilities, either bottom-founded or floating platforms, and deepwater mobile offshore drilling units (MODU), including semi-submersibles and drillships. These can operate in water depths up to 3,000 metres (9,800 ft). In shallower waters, the mobile units are anchored to the seabed. However, in deeper water [more than 1,500 metres (4,900 ft)], the semisubmersibles or drillships are maintained at the required drilling location using dynamic positioning. In summary it could be said that there are 5 different types of oil rigs as listed below.

Jackup rig
Barge Rig
Submersible Rig

Many offshore oil rigs are anchored platforms. They use a steel framework anchored to the ocean floor as a foundation for a surface drilling rig, equipment, and living quarters. Platforms may drill in many directions from this base, and they are broken down into specific types suited for various depths. Its of two types.

Fixed Platform – Fixed platforms are made of steel or cement and are intended to be permanent structures. They house large facilities, heavy equipment, and big crews. Most are located on the continental shelf in deep water up to 1700 feet. They can drill directionally with a radius of up to five miles. The platform is held in place by large steel or concrete legs that attach to the ocean floor.

Compliant Towers – When the oil or gas is even deeper, from 1500 up to 4900 feet, a compliant tower may be used. These structures are made of concrete and steel and are tall and narrow. They are intended to flex and sway with the wind and waves. Up to 3000 feet, they attach firmly to the seabed, but, for greater depths, a tension leg platform may be used. These towers are technically floating platforms that are secured with a series of anchors and cables. They are fixed but also floating, and they can reach depths of 7000 feet.

  1. Floating rig

A floating rig is used in very deep water as 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. It’s of two types.

Semi-Submersible Platform – For deeper waters up to 12,000 feet, the preferred method of exploration and drilling is to use semisubmersibles or drill ships. Semisubmersible work-decks float on top of the water, while most of their mass is contained below the water to help stabilize the platforms and keep them in place. As a result, the deck is rather stable and well-suited for drilling in rough waters 3000-10,000 feet deep. However, semisubmersibles are not easily moved.

Drill Ship – For new exploration and the drilling and capping of new wells, drill ships are typically used. Drill ships have the drilling equipment installed directly on the deck, typically in the middle of the deck. The well is drilled through the “moon pool,” an opening in the center of the ship. They are kept in place by dynamic positioning.

Sources: Images