seconds, which insures that the platform will 
exhibit little heave motion in, say, a sea state 7. 
Figure 1. 
Elevated platform with circular 
cylindrical legs. 
The advantages of a hydrodynamically stable 
elevated platform are manifold. Firstly, because 
of stability, aircraft take-offs and landings are 
facilitated. This becomes an especially important 
consideration for handling large, heavily laden 
cargo aircraft that are designed to operate from 
terrestrial air terminals. 
Since elevated sections of modest plan dimen- 
sions are themselves hydrodynamically stable, the 
assembly of much larger floating units from these 
sections will be a simpler procedure than will be 
the case with the dynamically less stable shallow- 
draft sections. 
Habitation aboard the platform will be 
enhanced if it is hydrodynamically stable. The 
large platform may require the presence of consid- 
erable number of support personnel, having little 
or no nautical experience, who may be susceptible 
to motion sickness. Certainly, life will be more 
pleasant for everyone aboard a stable platform. 
Another advantage mentioned by proponents of 
elevated platforms is the favorable station-keep- 
ing properties of floating structures having 
minimal water plat and sail area. 
The elevated columnar platform could be 
designed so that damaged legs could be removed 
without recourse to dry-docking. Pumps could 
handle minor leaks which might develop from time- 
to-time, while water-tight compartments would 
isolate flooding due to localized failure to a leg. 
. rhe principal disadvantage of this type of 
platform is its inherent lack of static stability. 
C4 
A positive restoring moment can be assured only if 
enough ballast is added at the base of the legs (or 
1f the length and breadth of the platform is 
increased, thereby increasing the water plane resto; 
ing moment). Depending on the platform size and the 
weight distribution of the structural and buoyant 
elements, the ballast can assume an appreciable 
percentage of the total weight. 
Compared with the more conventional shallow- 
draft configurations, additional disadvantages of 
the columnar platform include (1) restriction to 
sites having a water depth greater than 300-400 feet 
because of the platform's large draft, and (2) high 
towing drag. 
Barge Platforms 
Barge platforms have several inherent attri- 
butes which command them for consideration in the 
MOBS program. It is apparent, for example, that 
there is a long and successful record established 
in the construction of ocean going concrete barges, 
ships and dry-docks. A 300x300-foot or even a 
400x1200-foot barge platform is certainly not 
beyond today's state-of-the-art in floating concrete 
structures. 
A 300x300-foot barge MOBS can be constructed 
' which has a considerable degree of positive static 
stability without the need of ballast, whereas the 
semi-submersible and elevated platforms must be 
ballasted to prevent capsizing. 
The shallow draft of the barge, and the use of 
fairings fore and aft, will result in a comparative- 
ly low hydrodynamic drag. This becomes an important 
consideration if the platform must be moved rapidly 
or for appreciable distances. The shallow draft 
will also allow operations at near-shore sites that 
are not possible with deeper draft configurations. 
The barge may also be an effective breakwater. 
It has been demonstrated, both analytically and 
experimentally, that a large, floating slab is an 
effective wave attenuator. One can conceive a 
sheltered area in the lee of the platform which 
could be used for docking all types of vessels, 
large and small. The relative motion between the 
barge-type platform and the vessels would be minimal 
and, as a result, cargo could be easily transferred. 
Compared to a 300x300-foot columar platform, 
a barge platform of the same size will tend to 
respond readily to the seaway. Helicopters can 
tolerate some deck movement - operation from air- 
craft carriers are routine - and future VIOL air- 
craft may eventually achieve a similar tolerance tO 
deck movement. It is questionable, therefore, 
whether the stability afforded by a semi-submersible 
or a columnar platform is really necessary for 
operations envolving these types of aircraft. 
Semi-Submersible Platform 
An early example of a semi-submersible platfor™ 
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-submersibles 
serving the offshore oil industry as exploration, 
development and work platforms. 
