Offshore rigs

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The offshore rigs generally carry out the same operations as the onshore ones. They just differ in the place of drilling depending on the water depth, and the desired mission.
The Drill ship is one of the offshore rigs. It is mainly used in the exploration phase, but it can be also used in the drilling process. Recently, the companies equipped them enough to be able to carry out dual missions such as drilling and casing handling.

On each Drillship, there is an opening located on the base of the hull. This opening is simply called “Moon Pool”as shown in the below figure. The drilling equipment can pass through it.
The below figure shows the main components of the Drillship.

The drillships should be maintained fixed on the desired location on the sea surface. There are two mechanisms to be well positioned:
1) Multiple Anchors: The ship is positioned by heavy supporters”Anchors” in the sea bed floor.
2) Dynamic Propulsion “Thrusters”: They are 2 large engines that position the drillship in the required place by computer system.
A combination of these two ways can be used.

The main advantages :-
1) They can carry heavy loads that may reach 45,000 tons of displacements.
2) Facility of transportation between different wells.
3) They can drill for very deep water depths reaching 5000 ft.
4)Multiple wells can be drilled from the same drillship.

Source: Speight, J. (2015). ” Handbook of offshore oil and gas operations”. Gulf Professional Publisher