Text 3. Floating Units

Floating offshore drilling rigs include semi-submersibles and drill ships. Semi-submersibles, because of their design, are more stable than drill ships. Drill ships can carry more drilling equipment and supplies, which often make them the choice in remote waters.

Most semi-submersible rigs have two or more pontoons on which the rig floats. A pontoon is a long, relatively narrow, and hollow steel float with a rectangular or round cross section. When a semi-submersible is moved, the pontoons contain mostly air so that the rig floats on the water’s surface.

In most cases, towboats then tie onto the rig and move it to the drill site. Some semi-submersible rigs are self-propelled – they have built-in power units that drive the rig from one site to another. Semi-submersibles get their name from the fact that in the drilling mode the rig is not submerged to the point where its pontoons to make them submerge only a few metres below the water’s surface. The rig is “semi-submerged”. With its pontoons submerged below the waterline, waves do not affect the rig as much as they do when it floats on the surface. A semi-submersible rig offers a more stable drilling platform than a drill ship that drills while floating on the water’s surface.

Large cylindrical or square columns extend upward from the pontoons. The main deck of a semi is big and rests on top of the columns. Semis (short for submersibles) often use anchors to keep them on the drilling station. Workers release several large anchors from the deck of the rig. An anchor-handling boat crew sets the anchors on the seafloor. Semis are capable of drilling in water thousands of metres deep.

The latest semis are capable of drilling in water depths of 2,500 metres. Semis can drill holes up to 10,000 deep. Indeed, semi-submersibles are among the largest floating structures ever made. The biggest ones soar to over 30 metres tall and their main decks can be 2,500 square metres.

A drill ship is also a floater. Drill ships are very mobile because they are self-propelled and have a streamlined hull, much like a regular ocean-going ship. A company may choose a drill ship to make hole in remote waters, far from land. A drill ship is a good choice for drilling remote locations. It can move at reasonable speeds under its own power. Its ship-shaped hull can carry a large amount of the equipment and material required for drilling. Frequent resupplying from a shore base is not necessary.

The latest drill ships can drill in water depths approaching 3,000 metres, or nearly 3,2 kilometres. They can drill holes over 10,000 metres deep. These big drill ships are more than 250 metres long, which is almost as long as three football fields laid end to end. They measure some 30 metres wide, or a little wider than a football field.

Anchors keep some drill ships on station while drilling, but those drilling in deep water require dynamic positioning. Dynamically positioned drill ships use computer-controlled thrusters and sophisticated electronic sensors. Thrusters are power units with propellers that the builder mounts fore and aft on the drill ship’s hull below the waterline. Once the dynamic positioning operator tells a computer exactly where it should keep the rig positioned, the computer, using information transmitted by the sensors, automatically controls the thrusters. The thrusters offset wind, wave, and current forces that would move the rig away from the desired position.

Whether on land or offshore, and whether large, medium, or small, all rigs require personnel to operate them. The people who drill wells usually work for a company whose business involves drilling, either directly or indirectly. So, let’s look next at companies involved in drilling and the personnel who do the work.

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