What a Semi-Submersible Drilling Rig Is

A semi-submersible drilling rig (often called a "semi") is a drilling rig that floats. It is used to drill wells in water too deep for a rig that stands on the seabed. Its hull has three main parts:

• Pontoons: two long, watertight hulls that lie deep under the water when the rig is working.
• Columns: usually four to eight, standing on the pontoons and holding up the deck.
• The deck box: the upper hull, with the decks, the machinery, the mud system, the stores, the accommodation and the drill floor.

When the rig works, it takes in sea water as ballast and sinks to its operating draft. The pontoons are then far below the waves, out of most of the wave action, and the sea's surface is part of the way up the columns. Because only the slim columns cut the surface, the waves push on the rig much less than they would push on a ship. This is why a semi moves gently, even in a rough sea.

Staying over the well

A floating rig must stay almost exactly over the well, because a pipe, the marine riser, joins it to the seabed. There are two ways to do this:

• Moored: anchors on the seabed, joined to the rig by chain or wire through winches or windlasses on the deck. Moored semis often have eight or more lines.
• Dynamically positioned (DP): computer-controlled thrusters push the rig back into position, using satellite and seabed reference signals. Used in deep water, where anchors are hard to lay.

On a floating rig, the blowout preventer (BOP) stack sits on the wellhead at the seabed, not under the drill floor. The riser carries the drill string and the returning mud between the BOP and the rig. Module 4 looks at this in detail.

Other kinds of offshore installation, such as fixed platforms, jack-ups, tension leg platforms and spars, are compared in the course Offshore Operations Fundamentals. This course stays on one kind of rig, a moored semi-submersible, and on the equipment people work with every day. The rig used in the examples belongs to an invented company, Delvane Offshore.