Table 3.3. Estimated Heave Periods for 
Selected MOBS Candidates 
Natural Period in Heave 
Platform Type Sec) 
(1) Elevated (330 ft-draft, 
muLlti-deck) 
Semi-submersible 
(multi-deck) 
Barge 
The added-mass for the elevated platform was assumed to be negli- 
gible and was neglected in arriving at the estimates in Table 3.3. 
This assumption makes sense only if the legs are slender, constant dia- 
meter cylinders without inter-connecting structural support. The ad- 
dition of supports between legs and the inclusion, especially, of damp- 
ing 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 dis- 
placed by the horizontal floats. The barge natural heave period is a 
gross estimate based upon the response of conventional ships with com- 
parable 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 magnitude is insurance against high platform response for all but 
extreme storm wave and swell conditions. 
An analysis, programmed to run on a computer and described in 
Appendix A, predicts the heave response for elevated platforms in regular 
seas. The heave response for various assumed irregular seas is also 
determined. Figure 3.10 represents a typical output from the program. 
In this case it is the heave response of the 300 x 300, single deck 
elevated platform with a heavy draft of 330-feet. Several values have 
been assumed for the damping coefficient. The principal effect of 
changing this coefficient is to alter the response at resonance. For 
all practical purposes the three curves concide to the left and to the 
“The estimate given by the equation compares favorably with the results 
of the analysis described in Appendix A. Since the latter accounts for 
the added-mass, there is further justification for concluding that this 
effect is small compared to the platform mass. 
