COLLAPSE OF TEXAS TOWER NO. 4 9 
velocities it is not probable that waves over 40 feet will occur. It is, however, 
definitely possible under these high wind conditions that the waves will be 
unstable and will be breaking due to wind forces and independently of bottom 
drag conditions.’ 
These considerations gave rise to design criteria of 125 miles per hour 
wind in combination with a breaking wave of 35 feet in height and 
this was concurred in by the scientists of the Woods Hole Oceano- 
graphic Institution. 
Preliminary evaluations indicated that a height for the lowest elements of the 
platform above mean sea level of 67 feet was desirable. In view of all uncer- 
tainties concerning maximum waves, a single wave with a height of 90 feet 
seems possible in deep water. If the bottom of the platform is at elevation 67 
feet, a 4.5-foot clearance for the crest of such a wave is provided. Further, * * * 
bottom of platform at elevation 67 provides clearance for a 96-foot wave.® 
(0) Number and size of legs—Minimum. interference to the passage 
of waves led to a design configuration of a three-legged structure, 
the minimum number to support a platform, and to the use of legs 
of minimum diameter consistent with load-carrying capacity and the 
elimination of any auxiliary legs since “additional supporting mem- 
bers increase wave forces as rapidly as increased resistance is ob- 
tained.” ® <A three-legged configuration for an offshore structure is 
rather unique in that most, if not all, the oil-drilling rigs in the Gulf 
of Mexico use legs in multiples of two. <A three-legged structure con- 
tains no factor of safety as— 
an inherent characteristic of the three-legged structure is that loss of one leg 
by any accidental means will result in complete and immediate loss of the 
platform and personnel. Therefore, accident protection in the form of increased 
size and strength of the main legs or supplementary legs which will not contribute 
to vertical support of the platform under normal conditions, but which will pre- 
vent complete loss in case of an accident is strongly recommended.!° 
The recommendation was not adopted on the ground that evacuation 
of the tower could take place in advance of any vessel, which might 
be out of control, striking the platform supports and toppling the 
tower. 
Testimony by Mr. L. B. DeLong, president of the DeLong Corp., 
a pioneer in the design and construction of offshore platforms, sup- 
ported the use of the three-legged structure as proper for the purpose 
of keeping the resistance to wave passage to the minimum consistent 
with load-bearing capacity. 
The hydrodynamic forces of the waves were found to be of far 
greater significance than the aerodynamic forces of winds acting 
against the tower. Keeping the resistance to these forces to a minimum 
outweighed the safety factor inherent in the incorporation of a greater 
number of legs and the recommendation for the installation of auxil- 
lary legs. 
(ce) Braced underwater foundation.—A. water depth of 100 feet is 
treated as the point, generally speaking, where legs must be braced 
to insure integrity of support of an offshore platform. It was found 
in the feasibility study that— 
1. The bending moments under design wave forces in cylindrical legs approxi- 
mately 250 feet in length between ocean bottom and bottom of platform without 
bracing required unreasonably large diameter legs. 
8 Ibid., p. 23. 
®Tbid.. p. 40. 
10 Tbid., p. 7. 
