An elevated 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: 
Ty = 1.11 ./S, 
where S. is the wetted length of one of the legs. Thus, an elevated 
platform with a draft of 330-feet would have a natural heave period of 
20 seconds, which insures that the platform will exhibit little heave 
motion in, say, a sea state 7 where the significant wave period is 
about 10 to 11 seconds. 
Advantages. 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 impor- 
tant consideration for handling large, heavily laden cargo aircraft, 
such as the C-130, that are designed to operate from terrestrial air 
terminals. 
Since elevated sections of modest plan dimensions are themselves 
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. 
Assume a heavy-lift role for the 300 x 300 platform. The raising 
and lowering of large acoustic arrays or other heavy gear (weighing 
up to, say 400 - 600 tons) through a center-well is greatly simplified 
by elimination or reduction of dynamic loads induced by the oscillation 
of the platform. According to Davis and Wolfe (1969) this becomes an 
especially critical factor if cable is the suspension medium. 
Habitation aboard the platform will be enhanced if it is hydro- 
dynamically stable. The large platforms may require the presence of 
considerable number of support personnel, having little or no nautical 
experience, who may be susceptible to motion sickness. Certainly, life 
will be more pleasent for everyone aboard a stable platform. 
Another advantage mentioned by proponents of elevated platforms, is 
the favorable station keeping properties of floating structures having 
minimal water plan and sail area. Pilot tests conducted in a wave tank 
tend to support the belief that an elevated platform with suitably 
shaped legs may actually remain stationary or advance into an incident 
train of regular waves. Additional tests are needed to support this 
finding, including tests with random seas which are more representative 
of actual sea conditions. It seems unlikely, however, that hydrodynamic 
form will ever be a suitable answer for some sea and wind conditions. 
For example, at locales were steady currents prevail, it will have no 
real effect. The platform will drift with the current. Steady winds 
of 20, 30 or more knots intensity can induce an appreciable thrust on 
floating structure having a large sail area. This latter effect sug- 
gests limiting the deck thickness and the diameter of the legs above 
the sea surface. 
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