mean decreased static stability unless compensated for by additional 
ballast at the base of the legs. 
Shape of Legs. The legs can assume any one of the configurations 
discussed earlier. Generally, legs are proposed which have either 
simple geometric shapes - e. g., circular, constant diameter cylinders 
which are relatively easy to design and analyze hydrodynamically - or 
exhibit one form or another of the bulbous type. 
The circular cylindrical leg can change only by increasing length 
and/or diameter”. The effects of these changes have been discussed 
previously. The bulbous leg can undergo certain transformations in 
shape which will improve the platform performance, and others which 
will detract from performance. For example, the size of the bulb at 
the base of each leg can be increased for the purpose of increasing 
the buoyancy. Increasing the buoyancy may result in a reduction of 
the total number of legs required. It may also mean that the narrow 
vertical support member between the deck and bulb can be reduced in 
diameter which results in a decrease of the water plane area per leg. 
However, increasing the buoyancy at the base of the legs will aggravate 
the problem of static stability and additional ballast must be added. 
Clearly, then, for the bulbous leg platform a distinct trade-off exists 
between an increase in the bulb size, ballast requirement and acceptable 
degree of static stability. 
Static Stability. Static stability is assured if a righting moment 
exists for all expected loading conditions and angles of platform list. 
Paradoxically, the elevated platforms which have good dynamic stability 
typically exhibit relatively poor static stability. As will be shown 
later, the ballast requirement for small platforms can amount to a 
significant portion of the total dead weight of the platform. Lateral 
and longitudinal stability can also be increased by enlarging the dimen- 
sions of the platform, i. e., increase the water plane restoring moment. 
A very large MOBS platform, say 1,000-feet x 4,000—feet is likely to 
have considerable stability, perhaps without the need for any ballast. 
Dynamic Stability. Dynamic stability will be investigated in more 
detail in a later section.of this report. Suffice it to say, this 
platform property is dependent on a number of factors including: 
Water plane area: stability is increased by a small value 
for this factor. 
Shape of legs: certain leg configurations lead to an 
increase in the added-mass and damping. 
Mass: the natural period of heave, pitch and 
roll increase as the mass increases. 
