Figure 32. Self-elevating (jack-up) mobile drill- 
ing rig. Open fabricated legs increase strength 
without substantially increasing resistance to 
waves. (The Offshore Co. photo) 
Figure 33. Submersible drilling rig. (Shell Oil 
photo) 
While floating platforms lack the stability of 
the bottom support type, they are not as restricted 
to a given depth of water and are cost-competitive 
at 200 feet or more. The floating platform 
includes two major categories: semi-submersibles 
and the ship-shaped type. 
VI-166 
The semisubmersibles are floating platforms 
supported on tall columns which rise from buoy- 
ant barge-like hulls or cylindrical torpedo-shaped 
tubes (Figure 34). Upon arrival on location they 
are ballasted so that approximately one-half the 
unit is below water. Their advantage over drilling 
ships or barges is that the major structure is 
located above or below the region of most severe 
wave action. This configuration provides improved 
stability by its large inertia, and by having a 
vertical natural frequency of movement which is 
affected little by wave forces. 
Figure 34. 
Semisubmersible rig (SEDCO 135) 
measures 300 feet on a side and in drilling posi- 
tion displaces 16,800 tons. 
ing photo) 
(Southeastern Drill- 
The semi-submersibles can be raised by pump- 
ing ballast water from the tubes and columns. 
Finally, they can be used to drill while resting on 
the sea floor if in sufficiently shallow water. 
Ship-shaped platforms consist of a drilling rig 
supported on a barge or self-propelled ship. A 
barge, because it is not self-propelled, must be 
moved by tugs to new assignments, but it has the 
advantage of low initial cost. The ship-shaped 
platform can transit at higher speeds and at less 
expense. A disadvantage of the ship-shaped vessel 
platforms is that much drilling time can be lost in 
bad weather due to vessel motion; however, this 
