MOBILE OCEAN BASING SYSTEM 
D. A. Davis and J. J. Hromadik 
Naval Civil Engineering Laboratory 
Port Hueneme, California 
Abstract 
This paper deals with an investigation into 
the feasibility and practicability of concrete as 
the construction material for large ocean platforms 
which are envisioned as satisfying basing require- 
ments of the Navy in the mid 80's. The floating 
platforms would consist of structural components 
mass-produced ashore, constructed into modules, 
launched, towed to the site and assembled into 
platforms. Three platform sizes were investigated: 
300x300, 400x1200, and 1000x4000, with dimensions 
given in feet. Various configurations of three 
basic types were considered: (a) elevated decks 
on columnar, vertical supports for providing 
buoyancy, (b) elevated decks with semi-submersible 
type horizontal hulls and (c) elevated decks with 
barge-type hulls for floatation. Concrete produc- 
tion quantities and costs are estimated for all 
platforms investigated. The construction, assembly, 
launch and testing of a 1/10 scale model twin hull, 
semi-submersible platform is also described. The 
model was constructed to verify the findings of the 
platform feasibility study. 
Introduction 
A vast real estate potential exists in Mobile 
Ocean Basing Systems (MOBS); large floating plat- 
forms that can essentially occupy any ocean site. 
The floating platforms are seen as consisting of 
components mass produced ashore, constructed in 
modules, launched, towed to the site and assembled. 
Such a capability to support occupancy of a 
particular ocean region for the performance of 
specified operational tasks exists. It does not 
appear to require major scientific discoveries or 
technical breakthroughs; it does require systematic 
development with accompanying RDT&E to update and 
extend current technology. While available 
materials of construction provide designers with a 
choice, concrete does appear to stand out. It is 
readily available, economical, can be mass produced 
and lends itself to repetitive large-scale construc- 
tions. With concrete, it is not necessary to bring 
the project to the industrial plant, the production 
processes can go to the site. Moreover, the 
history of concrete in a marine environment speaks 
for itself. 
Concrete is an exotic material, not by itself, 
but through applications that have evolved as a 
result of recent developments ... improvements in 
handling and placing, and in the design and control 
of concrete mixtures ... higher strengths with 
improved cement formulations and reinforcing 
techniques ... innovations in thin-shell construc- 
tion, longer spans, and now entirely precast 
systems. These developments are leading not only 
to improvements in quality, but also to techniques 
that are ever broadening the applications, enabling 
an efficiency in modern concrete structures never 
before realized. 
C-3 
The use of large floating concrete platforms 
to satisfy basing requirements of the Navy in the 
mid 80's has been under investigation at the Naval 
Civil Engineering Laboratory since 1970. (1, 2 
This paper summarizes investigations into the 
feasibility of the MOBS concept and concludes with 
a description of the design, construction and test- 
ing of a 1/10 scale twin-hull concrete semi-submers- 
ible platform. 
Description of Concepts 
Candidates are classified according to their 
buoyancy elements into the three basic types defined 
below. 
Definition 
single or multi-story decks 
supported on vertical, 
hollow buoyant columns (also 
called legs) or piles. 
single or multi-story decks 
supported on barge-type 
hulls. 
Semi-Submersible single or multi-story decks 
supported on vertical legs 
atop submerged horizontal 
pontoons. 
All suggested configurations not falling into one 
of the above were grouped into a separate classifi- 
cation, which is beyond the scope of this 
presentation. 
Columnar Platforms 
The most obvious feature of the columnar 
concept (Figure 1) is the many possible geometries 
of the vertical buoyant elements for supporting the 
deck. De-coupling from the sea is achieved by 
reduction of the water plane area relative to the 
mass of the platform. This idea is not new. In 
1924 Armstrong patented a concept for a floating 
airdrome that he envisioned as a refueling station 
for trans-Atlantic aircraft. His platforn, 
constructed of steel, resembled that depicted in 
Figure 1. 
An elevated columnar platform can be designed 
to have a minimum heave, pitch, and roll response 
for practically any sea condition. For a platform 
having cylindrical legs of constant diameter and 
length, and uniform spacing in both plan dimensions, 
the natural heave period is: 
ey oh 
where S. is the wetted length of one of the legs. 
Thus, an elevated columar platform with a draft of 
330-feet would have a natural heave period of 20 
