How are we able to build oil rigs and other infrastructure under the ocean?

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How are we able to build oil rigs and other infrastructure under the ocean?

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Subsea stuff is fabricated and put together in segments on land then lowered via a crane on a ship to the ocean floor and maybe assembled by divers and or an ROV. Interesting topic to look up: saturation diving

There are a number of different types of ‘oil rigs’ in the ocean. In relatively shallow water (less than 200 feet, or so) we can use ‘jack up rigs’. These are triangular shaped floating vessels with long legs on the three corners. When the rig is where we want it to be, we jack the legs down to the bottom of the ocean and lift the vessel above the water 25 to 50 feet. We then drill wells from that platform. When we are finished we jack the legs back up, lowering the vessel back to the water where it can be floated to a new location. 
Drill ships are ships that have drilling rigs on them. They can work in very deep water. The ship is floated to the location where the wells are to be drilled then anchors are taken out from it in all directions and dropped. The anchor cables are attached to machines that pull or slack the cables as necessary to keep the ship in one place, even in high winds or heavy seas (big waves). This is called dynamically positioning.
Some wells are drilled from platforms that are giant structures that are floated out to the spot where the wells are to be drilled and then stood up and anchored to the sea floor. These can be as tall or taller than the largest buildings on earth. 
There are other methods that are a hybrid of these or similar in some ways. All of them are very expensive and require lots of engineering. 

There isn’t a lot of actual building that happens underwater. We’re certainly capable of it, but it’s expensive and time-consuming.

Depending on what “other infrastructure” you’re talking about, most of it is built above-ground and sunk, or we build a dam of some sort to keep the water out while we build, then let it back in when we’re done.

Some “oil rigs”, in fact, aren’t even “underwater”. For example, as can be quite amazingly seen in the Mark Wahlberg movie of the same name, the infamous Deepwater Horizon exploration rig is actually a giant boat.

So mostly, we don’t if we can get away with it. When we can’t we try to keep it down to just assembling large pieces to keep the costs down.

When we do build underwater, it’s mostly limited to welding or concrete. Welding actually works better in the absence of oxygen, so most welding techniques already use some form of protective gas to protect the weld. At that point, it’s more about the logistics of finding welders who can dive (or divers who can weld) and getting the resources down to them. Concrete doesn’t “dry” like most would expect, it’s actually cured via a chemical reaction. That chemical reaction is exothermic (gives off heat) so curing concrete underwater where that heat is easily carried off actually creates a stronger structure–although it does slow the cure time down some.

Some links you might like:

* https://en.wikipedia.org/wiki/Cofferdam
* https://en.wikipedia.org/wiki/Oil_platform

I like that concrete collars that are such to the sea bed and then all the water is pumped out creating a beach type scene. Minus water of course 👍