Oil and Gas- Offshore Drilling Rigs

Modern and advanced drilling techniques and methods have increased Companies’ ability to find hydrocarbons and the speed at which their projects are developed.

There are technology advances for exploration and development, better control of environmental impacts, increased efficiency, and improved understanding of the marine environment.

The techniques and equipment for drilling offshore wells (offshore drilling) are very similar to those used for onshore drilling.

The main differences consist in the arrangement of the drilling rig and of the equipment and in certain particular methods of carrying out the operations, which have to be adapted to the requirements dictated by far more difficult and often extreme environmental conditions.

This entails considerably higher costs, which must also be added considerable investments to provide the facilities and plants for subsequent field development.

An offshore drilling rig has to create the same working conditions as for onshore rigs which can move from one point to another without any difficulty, and they have several additional features to adapt them to the marine environment, including:

Heliport

• a large deck area that is placed high and to the side of offshore rigs;
• It is an important feature since helicopters are often the primary means of transportation.

Living quarters

• usually comprise bedrooms, a dining hall, a recreation room, office space, and an infirmary.

Cranes

• used to move equipment and material from workboats onto the rig and to shift the loads around on the rig.

Risers

• used to extend the wellhead from the mudline to the surface.

These conditions increase the complexity of the offshore rigs and justify their higher daily rate compared with an onshore drilling rig of the same capacity.

From an operational point of view, offshore drilling may be subdivided into two main categories, depending on the water depth.

1.  Bottom supported drilling rigs

• The safety equipment, i.e. ordinary Blow-Out Preventers (BOPs) located permanently above sea level and accessible from the supporting structure;
• drilling operations are identical to those carried out in onshore drilling.

2.  Floating drilling rigs

• The wellhead and the subsea BOPs are placed on the seabed and are not therefore directly accessible from the supporting structure.
• Drilling operations differ from onshore ones, as the rig floats, subjected to the action of the wind, currents, and waves, which cause it to make small horizontal and vertical movements.

The use of floating rigs is necessary for exploratory drilling in water depths of more than 100 m (this refers solely to exploratory drilling operations, and not to the subsequent development drilling).

Exploration Drilling Rigs

1.  Bottom supported rigs

Barges

• They are designed to work in shallow water (less than20 ft deep).
• The rig is floated to the drill site, and the lower hull is sunk to rest on the sea bottom.
• The large surface area of the lower hull keeps the rig from sinking into the soft mud and provides a stable drilling platform.

Drilling Barge

Submersibles

• They are barge that is designed to work in deeper water (to 50 ft deep);
• They have extensions that allow it to raise its upper hull above the water level.

Submersible Drilling Rig

Platforms

• Platforms use a jacket (a steel tubular framework anchored to the ocean bottom) to support the surface production equipment, living quarters, and drilling rig.
• Multiple directional wells are drilled from the platform by using a rig with a movable substructure.

Offshore Drilling Platform

Jackups

• Backups are similar to platforms except that the support legs are not permanently attached to the seafloor – The weight of the rig is sufficient to keep it on location.

Jack Up Drilling Rig

2. Floating drilling rigs

They are not attached or resting on the ocean bottom and have special features to facilitate this:

                    (i)  They are held on location by anchors or dynamic positioning
                    (ii)  The drill string and riser are isolated from wave motion by motion compensators.

There are two categories of drilling floating rigs

Semi-submersible rigs

• They consist of a large triangular, rectangular or pentangular platform, connected with submerged hulls by means of large columns which vary in number from 3 to 8, according to the shape of the vessel
• They are kept vertical over the site by means of mooring or dynamic positioning systems
• Semis are subjected to wave motion, their drilling apparatus is located in the center where wave motion is minimal.

Drill ships

• Drilling ships are used for operating in deep waters, often under extreme environmental conditions, such as drilling in arctic areas
• Drilling ships represent an optimal solution of drilling exploratory wells in remote areas, far removed from supply points, as it can carry all the material necessary for drilling even a particularly difficult well.
• Drilling ships are kept in a vertical position over the well by means of mooring or dynamic positioning systems

Drilling Ship

Offshore drilling operations

• Drilling starts after the rig has been positioned vertically over the well, by means of mooring or dynamic positioning systems.

Subsea BOPs

• In offshore drilling, BOPs have the same function as those used in onshore wells but are connected in a single complex (the BOP stack) before being mounted on the wellhead, so as to reduce assembly times at the sea bottom.
• They are lodged in a square-section cage structure with female columns at the apexes, into which the male tubular columns of the permanent guide structure fit.
• The BOP stack is lowered and fastened to the wellhead by means of a hydraulically controlled connection, ensuring hydraulic sealing.

A drilling ship and a subsea BOP stack

A semisub drilling rig and a subsea BOP Stack

                        

Credits: Oil and Gas portal