Most of these are for exploitation purposes and 
have been drilled from fixed platforms, artificial 
islands, or directionally from shore. 
The petroleum industry has more than $1 
billion in offshore drilling equipment presently at 
work. Drilling contractors, hired by operating oil 
firms, generally bear the burden of the capital 
investments for this phase of the operation. 
While the types of platforms used to support 
the drilling rigs vary greatly, the rigs are fairly 
standardized. They consist of: (1) a tall steel tower 
to hoist the bit, pipe, and other equipment in and 
out of the hole, (2) a system to rotate the pipe and 
bit, and (3) a system to circulate fluid to the 
bottom of the hole. 
Fixed platforms supported by pilings were 
constructed in shallow offshore waters as the 
drillers followed the seaward extension of oil 
fields. As the industry moved into deeper waters, 
it continued using this type of foundation for 
exploitation drilling. However, for exploratory 
drilling, where the incidence of dry holes is 
inherently higher, fixed platforms soon became 
too costly. 
The industry then began to develop mobile 
drilling platforms. This minimized the capital 
investment chargeable to each well site. The first 
mobile platforms were submersible barges for 
operation in 20 to 40 feet of water and evolved 
from the barge-mounted drilling rigs used in 
southern Louisiana marshlands. Later, jack-up 
mobile platforms were developed for greater water 
depths. 
Figure 31 shows the categories of offshore 
drilling platforms, their cost, daily operating rate, 
and depth capability. There are three general types 
of mobile platforms—submersible, jack-up, and 
floating. 
The jack-up rig is mounted on a buoyant hull to 
which extendable legs are attached. The legs are 
raised for moving the rig and lowered to the ocean 
floor to lift the platform above the ocean waves 
during drilling operations (Figure 32). The number 
of jack-up rigs has grown steadily, with about 75 
in operation in depths to 300 feet. Designs have 
been proposed for jack-up rigs for 600 feet of 
water. Recent innovations include a self-propelled, 
jack-up rig resembling a ship, to operate in 300 
feet of water. 
The submersible rig is mounted on a submers- 
ible hull that is ballasted with water and sunk to 
the ocean floor for support during drilling opera- 
tions (Figure 33). The largest submersible rig is 
designed to drill in 175 feet of water with 25-foot 
deck clearance; however, most submersible rigs are 
limited to 100 foot water depths. About 35 
submersible rigs are in use currently, a number 
almost constant since 1958 due to depth limita- 
tion and the increasing popularity of jack-up rigs. 
The advantage of the jack-up and submersible 
rigs is that they rest on the bottom while the 
platform stands clear of the highest waves, ena- 
bling them to operate in rough seas. The jack-up 
rig has more depth flexibility and capability while 
the submersible rig, a monolithic structure, can be 
towed more readily from one location to another. 
Figure 31 
COSTS AND DEPTH CAPABILITIES OF 
OFFSHORE DRILLING PLATFORMS 
Initial Cost! 
Category ($ million) 
Day Rate? Typical Operating 
($) Depths (Feet) 
Fixed Platforms 1.0 to 15.0 
Mobile Platforms . 
Bottom Supported 
Submersible 3.0to 5.0 
Jack-up . 4.0to 8.0 
Floating 
Semisubmersible . 7.0 to 10.0 
Ship-shaped 3.0to 7.0 
5,000- 7,000 0-300 
6,000-10,000 20-175 
6,000-15,000 20-300 
12,000-17,000 130-600 
10,000-15,000 50-600 
1 F a ; : A 
Depends upon soil conditions; wave, wind, and ice loading; and the number of wells supported by the platform. 
2 Includes rig, labor, transportation, routine daily services, and routine expendable materials. 
VI-165 
