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Definition: “Semisubmersible” or “Semisub-” is referring to Offshore drilling rigs so called because they are combining the advantages of submersible rigs with the ability to drill in Semisubmersible drilling rig. The Semisubmersible drilling rig is made of a floating structure having vertical hollow legs connected to horizontal pontoons used for ballasting with seawater for stability. It also has numerous large heavy anchors which keep it secured to the seabed in turbulent offshore waters.

Comments: To be used in water depth beyond the reach of the Jack-up drilling rigs, 160 mters (492ft), the Semisubmersible rigs are the most common type of offshore drilling rigs,  With a Semisubmersible rig, the air is let out of the lower hull, the rig does not submerge to the sea floor. Instead, the rig is partially submerged, but still floats above the drill site. When drilling, the lower hull, filled with water, provides stability to the rig.

These Semisubmersible drilling rigs can be towed to their drilling area while floating, but this operation requests a lot of time and energy. So depending on the size of the Semisubmersible drilling rig, the most common practice is to dry tow them up to the site. Then the vessel carrying theSemisubmersible rig will lower the hull to let the Semisubmersible floating by itself.   

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