are somewhere in the range of $50 to $60 per ton (or $100 to $120 per 
cubic yard of concrete). At this rate, a 20-foot O. D. pipe would cost 
from $7,500 to $9,000 per 20 feet of length. These sections are con- 
servatively designed to withstand an internal pressure of approximately 
IR7/5) -oesabe 
High quality concrete pipe sections from 30 inches to 25 feet in 
diameter can be constructed routinely using equipment of the above 
description. Larger diameters are feasible, but most equipment used 
in the process would have to be strengthened and enlarged; this is not 
a problem. Since the cylindrical sections for the MOBS platforms are 
on the order of 30 feet, this process is nearly capable of immediately 
supplying the required construction elements. 
If desired, such things as longitudinal holes for post-tensioning 
cable, an internal reinforcing net, or access holes can be made a part 
of a section. These and other reasonable design requirements pose no 
problems for the technique. 
Vertical Slip Form Casting. Vertical slip form casting is a 
patented method of fabricating long cylindrical sections in deep water. 
Developed in Europe, the process involves a floating work platform which 
is furnished with all the equipment needed to mix and place concrete, 
store and transport materials and provide living quarters for the crew. 
The method is well-suited for mass production. As shown in Figure 
5.5(a), the casting begins with a precast end piece (in this case a 
hemispherical section) which can be made locally, perhaps on a nearby 
floating dry dock, or shipped to the scene of the slip-forming operation. 
If necessary, water ballast can be added to the casting for stability. 
The casting is done in a floating form, as shown in Figure 5.5(b). 
The unit can be trimmed by periodically adding water to the completed 
portion below the waterline. Using this procedure, it is possible to 
maintain the top of the form at a constant height. Usually this height 
is about 20 feet, so at a pour rate of 8 inches per hour, the concrete 
will be 30 hours old when it reaches the waterline. Concrete at this 
age will be strong enough to survive the water pressure. Curing will 
continue under water. 
When sections are completed, the open end can be capped, perhaps 
with another concrete end-piece. By pumping the water out, the section 
will slowly rotate until it is in a horizontal position, Figure 5.5(c), 
ready for towing. If desirable, the completed section can be maintained 
vertically for final positioning under the deck of an elevated 
platform. 
The vertical orientation of slide form casting is favorable to 
longitudinal prestressing. Good quality concrete, combined with pre- 
stressing will ensure water tightness. Prestressing also markedly 
increases the capability of the section to resist longitudinal bending 
moments. 
