During tow (the platform could be self- 
propelled) the semi-submersible rides high out of 
the water in a shallow draft condition thus reduc- 
ing hydrodynamic drag to a minimum. On station 
the platform is ballasted into a stable, deep-draft 
mode. Hydrodynamic stability on station results 
from (1) the relatively low water plane area of the 
vertical supports, (2) the large added-mass result- 
ing 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-sumbersible has the dynamic stability 
of the columnar 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 columnar platform having this 
capability. 
Figure 2 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. 
Figure 2. 
If the ballast penalty for static stability is 
Not considered excessive, if the design and assembly 
complexities involved in forming this type of plat- 
form from concrete can be resolved, and if a 
Propulsion system is determined to be compatible 
with a submerged concrete hull, then a semi-submers- 
ible platform can be considered a strong contender 
in the MOBS program. 
300x300 semi-submersible platform section. 
Preliminary Desi 
Several candidate platform configurations were 
considered in the basic study with particular . 
emphasis on the columnar, semi-submersible and barge 
type platforms. Optimization was found to depend 
primarily on considerations of static and dynamic 
stability, material requirements, and design 
complexity. 
This section summarizes the resulting estimates 
of candidate platform size, weight and hydrodynamic 
response. It is emphasized that the results are 
preliminary estimates. However, such approximations 
are sufficiently accurate for relative comparisons 
and determining the order of magnitude of concrete 
qualities involved. 
Structural Design Assumptions and Criteria 
The design calculations were based on simplifi- 
ed geometries of each basic configuration (Table 1). 
In addition the following assumptions were applied: 
1. Both the single slab and multi-level 
decks for the columnar and semi-submersible plat- 
forms were considered as separate structural 
components resting on buoyant support elements. 
2. The vertical legs for both the columnar 
and semi-submersible platform were considered to 
have sufficient lateral bracing to prevent failure 
due to buckling. 
3. All structural elements were designed 
according to ACI standards for reinforced concrete 
constructions. 
4. Design live loads for the platforms 
were: 
(a) with multiple decks 
£lightideck ss) 1) le tele - 250 psf 
aircraft storage deck ...... - 250 psf 
personnel deck .....2-e+-. - - LOO psf 
(b) with single slab deck 
flight/storage deck .... . - 400 psf 
5. Design live load was considered distri- 
buted uniformly throught; no allowance was made for 
partial loading. 
6. Concrete having a density of 150 lb/ft? 
and a compressive strength of 6,000 psi was used in 
all design estimates. 
7. All columnar and semi-submersible plat- 
forms were held to a minimum clearance of 50 feet 
between the bottom deck slab and the mean water 
surface when the platform was loaded with the full 
design dead load plus live load. This specification 
insured that wave uplift on the deck will be pre- 
vented in all but exceptionally high sea states. 
8. All platforms were designed with a 
minimum free-board of 60 feet to insure that deck 
washing does not impede aircraft landings/take-offs 
as well as cargo transfer and storage operations. 
C-5 
