I saw a National Geographic program about the complexity of laying down long pipes in the ocean.
I thought about it and it seems like pipes waste a lot of space as they are carried and stored on the ships. More than that some pipes need expensive assembly and accurate welding to create a single long pipe.
An idea I had that might be applied to plastic, glass (fiber optics), metal or other types of pipes is to make a special ship that would carry the raw material that is needed to make the pipes and a chamber that will melt this material and let it run down through special tubes into the ocean. The melted material would harden as it hits the cold water into a pipe.
This kind of pipe can be very long and without any joins which will save a lot of time and effort to produce and assemble. Also it should be much cheaper.
When a section of the pipe fails they can fix the section of the damaged pipe as they do with regular pipes.
I saw someone uses a similar method to create noodles as the dough is injected into boiling water. Why not use a more complex version to create very long pipes?
This can also be done in stages in several chambers creating multi-layered pipes from different materials. A special water container inside the ship can be used to harden the pipe as more layers are applied until it gets released into the sea.