Bottom-Supported Units

Submersibles and jackups contact the seafloor when drilling. The lower part of a submersible’s structure rests on the sea-floor. In the case of jackups, only the legs contact the seafloor.

A submersible MODU floats on the water’s surface when moved from one drilling site to another. When it reaches the site, crew members flood compartments that submerge the lower part of the rig to the seafloor. With the base of the rig in contact with the ocean bottom, wind, waves, and currents have little effect on it. The first MODU was a submersible. It drilled its initial well in 1949 off the Gulf Coast of Louisiana in 5, 5 metres of water. It was a posted-barge submersible – a barge hull and steel columns supported a deck and drilling equipment. It proved that mobile rigs could drill offshore. Posted barges are now virtually obsolete, because newer and better designs have replaced them.

About 1954, drilling moved into water depths beyond the posted barge’s capabilities, which was about 9 metres. So, naval architects designed bottle-type submersibles. A bottle-type rig has four tall steel cylinders (bottles) at each corner of the structure. The main deck lies across several steel supports and the bottles. The rig and other equipment are placed on the main deck. When flooded, the bottles cause the rig to submerge to the seafloor. In the early 1960s, the biggest bottle-type submersibles drilled in 45-metre water depths. Today, jackups have largely replaced them; jackups are less expensive to build than bottle-types and can drill in deeper water.

A special type of a submersible rig is an arctic submersible. During the arctic winter, massive chunks of ice form and then move with currents on the water’s surface. Called “floes”, these moving ice blocks exert tremendous force on any object they contact. The force is great enough to destroy the legs of a jackup or the hull of a conventional ship or a barge. Arctic submersibles have a reinforced hull, a caisson. One type of caisson has a reinforced concrete base on which the drilling rig is installed. When the sea is ice-free in the brief arctic summer, boats tow the submersible to the drilling site. When ice floes form and begin to move, the arctic submersible’s strong caisson hull deflects the floes, enabling operations to continue.

A fourth submersible is an inland barge rig. It has a barge hull – a flat-bottomed, flat-sided, rectangular steel box. The rig builder places a drilling rig and other equipment on the barge deck. Inland barge rigs normally drill in marshes, bays, swamps, or other shallow inland waters. By definition, barges are not self-propelled; they have no built-in power to move them from one site to another. Boats tow them to the drilling location. When being moved, the barge floats on the water’s surface; then, when positioned at the drilling site, the barge is flooded so that it rests on the bottom ooze. Since they often drill in swampy shallow waters, drilling people often call inland barges “swamp barges”.

A jackup rig is a widely used mobile offshore drilling unit. It floats on a barge hull when towed to the drilling location. Most modern jackups have three legs with a triangular-shaped barge hull; others have four or more legs with rectangular hulls. A jackup’s legs can be cylindrical columns, somewhat like pillars, or they can be open-truss structures, which resemble a mast or a derrick. Whether it has columnar or open-truss legs, when a jackup’s barge hull is poisoned on the drilling site, the crew jacks down the legs until they contact the seafloor. They then raise, or jack up, the hull above the height of the highest anticipated waves. The drilling equipment is on top of the hull. The largest jackups can drill in water depths up to about 120 metres, and are capable of drilling holes up to 10000 metres deep.

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