have a greater draft than a comparable flat hull, they offer greater 
structural resistance to sagging and hogging stresses. In addition, 
the decreased water plane area of the twin and triple hull barges 
augers well for reducing the dynamic coupling between these barges 
and the seaway. 
The barges depicted could be constructed as monoliths in dry docks 
(or in specially excavated shore sites), or assembled at sea from pre- 
cast modules and/or sections (Part 5 discusses the construction alter- 
natives). The modular approach could result in any one of the configu- 
rations shown in Figure 3.4 or the finished product shown in Figure 3.5. 
Semi-Submersible Platform 
An early example of a semi-submersible platform was the steel 
structure designed (but never built) for use in Project MOHOLE. The 
MOHOLE platform was to consist of twin submersible hulls, measuring 
350-—feet in length, which were to support an elevated deck structure upon 
six large diameter, vertical cylinders. Today, there are many semi- 
busmersibles serving the offshore oil industry as exploration, develop- 
ment and work platforms. 
During tow (the platform could be self-propelled) the semi-submersi- 
ble rides high out of the water in a shallow draft condition thus reducing 
hydrodynamic drag to a minimum. On station the platform is ballasted 
into a stable, deep-draft mode. Hydrodynamic stability on station re- 
sults from (1) the relatively low water plane area of the vertical sup- 
ports, (2) the large added-mass resulting from oscillation of the 
horizontal pontoons, and (3) fluid drag on the pontoons and connecting 
struts. 
The semi-submersible shares some of the best features of the other 
two concepts. A properly designed semi-submersible has the dynamic 
stability of the elevated platform and the favorable drag characteristics 
of the barge. Conceivably, like the barge, a semi-submersible could be 
constructed with a propulsion system. It is difficult to imagine any 
type of elevated platform having this capability. 
Figure 3.6 pictures a possible semi-submersible configuration. The 
platform has horizontal pontoons that support a multi-level deck. The 
vertical supports could be circular in cross-section as shown, or they 
could be streamlined for reducing the form and wave drag during tow or 
cruise. Several platforms like the one depicted could be joined to 
form large floating complexes. 
If the ballast penalty for static stability is not considered ex- 
cessive, if the design and assembly complexities involved in forming this 
type of platform from concrete can be resolved, and if a propulsion 
system is determined to be compatible with a submerged concrete hull, 
then a semi-submersible platform can be considered a strong contender 
in the MOBS program. 
