How would they assemble oil rigs in the middle of sea?

For profound water (we're talking 3000 feet, 1000 meters) they assemble a coasting stage in a port city, and tow it out to the ideal position, at that point attach it with at least four securing lines to secures set in the ocean depths. The securing lines are fixed to where the coasting stage is pulled a piece beneath its light point so the lines are all in steady pressure and the stages are not swaying here and there yet at the same time. 

These are called strain leg stages. 

Most importantly they don't collect "OIL RIGS" out in the sea. Oil rigs are basically portable and drill a well and afterward proceed onward to the following area to bore more wells. Since they will consume their whole working time on earth moving around it is simpler to construct them in a shipyard and afterward move them to profound water. 

Oil creation stages are likely what you mean in the inquiry. These designs are the ones for all time situated over the highest point of the repository and which really produce the unrefined. 

There are a few techniques for introducing a stage, for example, having a drifting boat secured to the ocean bed however again I expect you mean a jacketed stage in your inquiry. 

These designs are implicit segments, called modules, coastal in a shipyard. The biggest module is the coat which is a tall grid structure coming to from the seabed to numerous feet above ocean level. Coats are built lying level and afterward glided into area. The precarious part is in turning the coat vertical without hitting the seabed. As the coat is developed from empty steel tubulars it is feasible to specifically flood different areas of the design to control the turning of the coat and to keep it light until it tends to be drifted into position upward. 

At this stage an extremely enormous drifting crane is important to lift the wide range of various modules into place on top of the coat. The modules are accumulated on top of one another very much like blocks while a development team weld every module down. 

In the event that the stage planners have taken care of their work effectively and the module manufacturers have been exact every one of the interconnections between the different modules will arrange. That implies the development group can interface the electrical wiring, the air conduits and an unending measure of channeling running between modules in speedy time. That won't ever occur. The interfaces between modules never line up appropriately and that is the reason gathering of a seaward stage is a gifted work. 

The last stage is to interface the stage to the wells which have been bored by a penetrating apparatus while the stage was being constructed and association with the fare pipeline which was additionally introduced while the stage was being assembled. 

Everything sounds straightforward however the entire interaction can require quite a long while and cost a huge number of dollars.

These are called tension leg platforms.