Hydrodynamic Drag. The force required to tow a platform in the 
ocean depends on the fluid drag of the buoyant support. Generally the 
drag will increase with an increase in leg length, diameter, or drag 
coefficient. The drag can be minimized by employing streamlined leg 
shapes, e. g., teardrop or "airfoil'' shaped legs, and will be maximized 
for such non-streamline configurations as the cubical float leg of 
Figure 3.2. The addition of struts or other lateral support members 
between legs, which may be necessary for structural integrity and de- 
sirable for increasing the platform damping, will tend to increase the 
drag. Since the drag on a properly streamlined strut at certain Reynold's 
numbers may be as little as 1/10 the drag on a circular cylinder with 
the same projected area, serious consideration should be given to 
employing streamlined components whenever possible (Shapiro, 1961). 
Mass. The total mass and its distribution is important, of course, 
in studying the static and dynamic stability of candidate platforms. 
For elevated platforms with modest plan dimensions, e. g., the 300 x 
300-foot platforms, each added pound of dead weight in the deck must be 
compensated for by additional ballast in the legs if the same degree 
of static stability is to be preserved. It behooves the designer to 
maintain the deck weight at a minimum. The use of prestressed structural 
elements and lightweight concrete are two ways of achieving this. 
Freeboard. The freeboard must be sufficiently high to prevent deck 
washing and wave uplift on the underside of the deck. Raising the free- 
board will have the effect of increasing the displacement and reducing 
static stability. To preserve stability, ballast, once again, must be 
added to increase the restoring moment. An increase in the freeboard 
also effects the aerodynamic and hydrodynamic drag, since both the sail 
area and submerged leg length are increased. For an efficient design, 
then, it is imperative that the freeboard be kept to a minimum, com- 
mensurate with the operational requirements for the platform. 
Station Keeping. One of the inherent advantages of an elevated 
platform is its apparently favorable drift response in a seaway. Pilot 
tests at NCEL (Naval Civil Engineering Laboratory) have shown (somewhat 
inconclusively) that certain leg cross-sectional shapes, e. g., a cre- 
scent section with cusps aligned in the direction of wave travel, may 
actually result in the platform's drifting into the advancing sea. It 
should be emphasized, however, that keeping the platform ''on station" 
is dependent on factors other than wave induced drift. If the platform 
is located at a site where steady currents and winds prevail, then no 
variety of exotically shaped leg is likely to prevent the platform from 
drifting. 
