I haven't given much thought to pipelaunchers lately.
Pipelauncher, definition: A pipe with one end capped that floats
vertically in the water. The pipelauncher remains vertical due to gravity
gradient. It is used to accelerate rockets vertically and does this by
changing it's buoyancy by increasing the volume of air inside the pipe
and thereby causing water to exit the uncapped end.

The usefulness of a pipelauncher is that it is a platform in which one
can launch rockets in the ocean. So it's rocket launch pad which adds
speed to a rocket before it blasts off from pad.
One advantage of a pipelauncher is it should be cheaper and in regards to crew launch perhaps in some ways safer. But because this way of launching rockets isn't done, there is that risk of unknown factors which could be substantial, thereby require testing and modest goals in terms in adding speed to a rocket.

The amount of speed that could added to a rocket depends upon the length of the pipelauncher and the amount gees one wishes to use to accelerate the rocket. Generally speaking a pipelauncher could deliver any amount gees as long as a rocket can structurally withstand it.
The power of a pipelauncher is related to it's buoyancy, which largely determined by it's diameter.
The dimension of a pipelauncher could be around 40' diameter by 800' long [tall]. This size could launch any rocket into space. A smaller size
pipelauncher could use for smaller rockets or sub-orbital rockets.

I noticed two things on the internet which could have something to do with building a pipelauncher.

"Future Pipe Industries produces large diameter FRP pipe in sizes up to 158” (13 ft) in diameter. Our Fiberstrong™ pipe are the largest commercially produced FRP pipe system in the United States; no other FRP pipe manufacturer comes even close to the large diameters produced by FPI."
...
Advantages of Fiberstrong large diameter FRP pipe

* Excellent corrosion resistance
* Electrically inert
* Long life and low life-cycle cost
* Light weight (1/4th of steel pipe, 1/10th of concrete pipe)
* Excellent hydraulic characteristics (Hazen Williams 'C' factor = 150)
* Long lengths (40 ft) & fast installation with coupling joints
* Complete piping system (pipe, fittings, flanges) in all diameters"
http://www.futurepipe.com/usa/inner.asp?P_SectionID=47&P_CategoryID=586

Now, 13' in diameter isn't really big enough, but maybe it could used in prototype testing or for small rockets. Or you could strap say 4 pipes together. This could be pretty and fast to do.

"This specification covers machine-made fiberglass pipe, 8 in. (200 mm) through 156 in. (4000 mm), intended for use in water conveyance systems which operate at internal gage pressures of 450 psi (3103 kPa) or less."
http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/D3517.htm?E+mystore

Another thing I saw:
"Pipelife offers PE pipes with diameters up to 2000 mm and long lengths up to 700 m. These
long length large diameter pipes are extruded directly into the water for transportation and installation. They are used for marine applications, like Marine Outfall, Submarine Potable Water Pipelines and Water Intakes, as well as for on-shore applications, like re-lining of old pipelines, Potable Water Transmission Lines and transportation of tailings"
http://www.pipelife.com/web/atinew/t2w_2_ati.nsf/webTop/A23FEA705C2F8B1AC1256D240029D7D7

These steel pipes can come lengths longer than you want, and be dragged to drydock without needing
to ship via truck and weld them into longer segments.