Types of oil rigs

The Phases of Offshore Drilling


​PHASE 1: EXPLORATION
A mobile drilling platform  is sent to the site to obtain a core sample, which geologists analyse for signs of petroleum. The exploratory drilling rig will typically drill several temporary wells, each taking a few months to complete. A positive find (called "a show") is followed by more exploratory wells to verify the quality before taking next step: Drilling the much more elaborate production well.

PHASE 2: DRILLING BEGINS
A drill bit is attached to a "drill string" made of segments of drill pipe, each about nine meters long. The drill bit is lowered into the sea – sometimes to a depth of several kilometres – until it meets the seabed. The drill is rotated by a turntable at the platform floor, and as the drill grinds its way downwards (sometimes horizontally as well) extra lengths of drill pipe are attached.
For cooling, cleaning and stabilising pressure, a "drilling fluid" is pumped through the drill pipe and out through nozzles in the drill bit at high velocity. The fluid is usually a mixture of water, clay, barite and chemicals. Back at the platform, the fluid is recycled through a circulation system that disperses the crushed rock and reuses the fluid.
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PHASE 3: FINALISING THE WELL
When the drill reaches target depth, production casing is lowered into the well and cemented into place. At the bottom, small explosive charges make small holes that allow the oil or gas to flow into the well while controlling pressure.
Once a well is "live", the drilling rig is typically replaced by a production platform, assembled at the site using a barge equipped with heavy lift cranes. An average well can last decades, so offshore production platforms are built with a long stay in mind.

Drilling Rig Types

The type of oil rig used in offshore drilling depends on the depth at the location, the type of oil, and prevailing conditions. The general categories include.
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  1. Drilling Barges
  2. Jack Up Rigs
  3. Submersible Rigs
  4. Semi Submersible Rigs
  5. Drill Ships

1. Drilling Barges

  •  Utilized for shallow water drilling in calm water conditions. Drilling equipment is placed on to the barges’ decks and towed to the site by tugs. Anchors hold the barges in position where drilling is to be carried out.

2. Jack Up Rigs

  • These drilling structures are positioned onto the site by lowering and rooting their three or four giant legs on the sea floor. The barges are then lifted up above water. They are more stable than the floating rigs and are therefore able to drill up to 100 meters.

3. Submersible Rig

These rigs are pontoon based which allow for water ballasting and de-ballasting in order to position, anchor and re-float the massive structure. All necessary drilling equipment is fitted on the platform’s deck, similar to the drilling barges and jack-up rigs. Their operations are limited to shallow waters due to their submersible design.
4. Semi-Submersible Rig
A Semi-Submersible rig is a Mobile Offshore Drilling Unit better known as MODU, which is designed for offshore drilling in ultra-deep waters of the oil and gas rich areas across the globe. They are partially submerged in water during drilling operations and are normally moored to the sea bed by anchors.
5. Drill Ship
  •   Maritime vessels modified to drill wells in oil and gas fields. They are adapted to  provide complete offshore drilling solutions. Drill ships are fitted with expensive machinery and drilling equipment, which are similar to the ones fitted on any other drilling unit. Since they are required to work in ultra deep water areas ranging up to 3000 meters, all supplies and equipment are catered by the offshore work boats. The drill ships have a drilling deck and are equipped with a moon pool, and are somewhat similar in design as the other drilling units. The drilling equipment that passes the moon pool is connected to the ‘sub-sea well’ via the ‘riser pipe’ (which is flexible to some extent).