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Constructing a pipeline is often called pipelay. It is done whenever there are large masses of fluids or gasses that needs transportation.

Pipelines have been successfully laid in a lot of different environments, and using several very different methods. There is no theoretical limit to pipeline sizes - but some factors are practical limits.

Length
From 0 m to 2000 km, as long as you can pay the bill, the construction companies will do as they are told.
Size
Two inches is perhaps the smallest I've heard of, and 48 inches is the current "standard" maximum. You can go for larger pipelines, but that means you have to develop the construction equipment as well.
Wall thickness
Everything goes - but the greater the thickness, the more time it will take to do the weld - and that will cost you money.
Depth
There are no limits. For heavy pipelines in great depth, you will need some non-standard equipment to be able to hold the pipeline while laying it, but that's no big deal.

Laying surface pipelines

When compared with subsea pipelines, land based pipelines are very easy to construct (cheating!). They are built from standard 12 m joints, which are lifted in-place and welded together.

Subsea pipelines using line pipe

This method uses small 12 m or 24 m long pipeline segments called pipeline joints, or line pipe. These segments are welded together onboard the lay vessel, and then lowered over the stringer and down to the seabed.

S-lay

At medium depths the pipeline forms the shape of an "S". To prevent it from buckling locally at the bends the vessel needs to pull the pipeline forward - it won't move, but the extra force in the pipeline will prevent it from collapsing on itself. This force increases with depth, and is the reason to why S-lay isn't used in all operations.
                  ======
^^^^^^^^^^^^^^^^^/^|_______/^^^(Waterline)^^
                / 
               /       (S-lay)
              /        
=============/
~~~~~~~~~~~~~~~~~~~~~~~~~(Seabed)~~
If the vessel doesn't push forward the pipeline the pipeline will tend to make 90 degree corners both at the vessel and the seabed. This is in most cases not good, and will need repair. For this reason, many lay vessels depend on anchors to hold them forward, and not the vessels' thrusters.

              *======
^^^^^^^^^^^^^^|^|_______/^^^(Waterline)^^
              |
              |   (S-lay gone horribly wrong)
              |
==============*  
~~~~~~~~~~~~~~~~~~~~~~~(Seabed)~~

J-lay

When the water becomes deep, the force required for holding the vessel in-place during an S-lay operation becomes to great, and J-lay is used instead. Here the pipeline is welded together in a vertical position, and then lowered down to the seabed in a J shape (big surprise?). The vessel still has to exert some forward pull on the pipeline or the bottom curve will buckle. The pipeline is constructed in either a vertical tower, or it is bent into a vertical shape if it's flexible enough. The reason J-lay isn't used universally is that it cannot lay pipe in shallow waters, and that things get cramped when trying to construct a pipeline in a vertical tower. The horizontal lay system used in S-lay can be 50-70 m long, and this is a bit too much for a vertical tower.
                  |
                  |      ____
                  |=_____| |
^^^^^^^^^^^^^^^^^^||_______/^^^(Waterline)^^
                  |
                  |
                  |
                  |
                  |     (j-lay)
                  |
                  |
                  |
                  /
=================/
~~~~~~~~~~~~~~~~~~~~~~~~~(Seabed)~~

Laying subsea pipelines using stalks

Tow-out

This method is used if the pipeline is supposed to be really short, or if it is to go in shallow water. Big pipeline stalks are fabricated on the shore, and then pulled out into the sea by very large winches. The stalk length is only limited to the amount of land you can spare, but they often are up to a kilometre long.

Reeling

If the pipeline is very flexible, it can be laid by reeling. The pipeline is then fabricated onshore, and then transferred onto the vessel. Onboard it is stored on a big reel. When laying the pipeline the vessel then slowly unreels the pipeline as it moves forward. A vertical tower is often used to control the process.
           ____
          /____O___
^^^^^^^^^/|_______/^^^
         |        
         |  (reeling)
         | 
=========/
~~~~~~~~~~~~~~~~
 

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