c Response. The natural heave periods 
for three selected 300x300 platforms are presented 
in Table 3. The estimates for the columnar and 
sgemi-submersible platforms were determined from the 
following expression: 
2 
1, VE 
where T., is the natural period in heave, k is the 
restoring force per foot of submergence and M is 
the total mass of the platform (including the mass 
of the live load and the added-mass). 
Table 3. Estimated Heave Periods for 
Selected MOBS Candidates 
Platform Type 
(1) Columnar (330 ft-draft 
multi-deck) 
Natural Period 
in Heave (sec 
(2) Semi-submersible 
(multi-deck) 
(3) Barge 
The added-mass for the columnar platform was 
assumed to be negligible and was neglected in 
arriving at the estimates in Table 3. This assump- 
tion makes sense only if the legs are slender, 
constant diameter cylinders without inter-connect- 
ing structural support. The addition of supports 
between legs and the inclusion, especially, of 
damping plates at the base of the legs will add 
considerably to the vitural mass of the elevated 
platform. The heave period, in this case, would 
be greater than that shown in the Table. The added 
mass for the semi-submersible platform was assumed 
equal to the mass of the water displaced by the 
horizontal floats. The barge natural heave period 
is a gross estimate based upon the response of 
conventional ships with comparable displacement. 
For a platform to be considered "stable" in 
heave, it should have a natural heave period of at 
least 20 seconds. A natural period of this magni- 
tude is insurance against high platform response 
for all but extreme storm wave and swell conditions. 
Construction Quantities, Time and Cost 
Concrete quantities for the various platforms 
are given in Table 4. From the standpoint of 
volume, one may compare a large platform to that of 
a medium size dam. Mass concrete of 2,000,000 
cubic yards or more will be required. Currently 
there are 17 plants routinely producing in excess 
of 500,000 cubic yards per year. Any number and/or 
combination of similar plants can be assembled at 
the construction site to obtain virtually any 
production rate - and the rate can be scaled up or 
down to meet demands. The only restriction appears 
to be the problem of adequate manpower for excep- 
tionally rapid construction. 
C-7 
rasioe 
Size (ft 
Table 4. Concrete Quantities (in million cu 
yds) for Type of Platform Indicated® °” 
 astore [5 ee 
Size (ft 
Semi-Submersible 
Multiple}|Single | Multiple 
fae deck deck deck 
300x300 0.05 0.08 0.04 -04 
400x1200 | 0.28 0.42 0.21 0.27 
1000x4000} 2.34 1.73 2.28 
"The table values also represent time in years when 
production rate is one million cubic yards per 
year. 
Cost estimates for bare hull structures at or 
near the assembly/launch site are given in Table 5. 
These are based on the cost per cubic yard of con- 
crete required for construction. Conservative 
estimates of $150/cubic yard for structural 
concrete and $75/cubic yard for ballast concrete 
were used in the calculations. The $150 per cubic 
yard chosen for MOBS is about 50% greater than the 
national average for buildings and 25% greater than 
the prevalent estimate of $120/cubic yard for float- 
ing concrete airports. 
a/ 
Estimated Bare-Hull Construction 
Costs (in millions of dollars) 
Semi-Submersible 
Single| Multiple|Single | Multiple Barce 
deck deck deck deck 8 
7.0 10.7 5.3 7.4 5.0 
37.0 57.5 28.3 30.2 26.7 
1000x4000) 312 481 236 328 223 
a/ 
“Excludes such items as power systems, machinery, 
mission equipment and personnel support facilities. 
Table 5. 
300x300 
400x1200 
Semi-Submersible Scale Model 
The model, shown in Figure 4, is a 1/10th 
scale twin-hull semi-submersible platform with 
hulls spaced on 20-foot centers. The model was 
constructed to demonstrate the feasibility of 
assemblying available concrete products elements 
into a platform, to evaluate construction tech- 
niques, and to study means of linking the platform 
modules together to form large platforms. 
The basic elements of the hull and columns are 
precast pipe sections conforming to ASTM Designation 
C76-69. They were fabricated by the Ameron Pipe 
Products Division of South Gate, California. The 
deck is of steel, consisting of open floor grating 
supported by 8-inch channels that also serve as 
the main deck beams. The deck in plan is 27 feet 
by 32 feet. 
