may not be an important consideration in pro- 
tected drilling locations (Figure 35). 
The floating platforms can drill in depths 
exceeding 600 feet, with some of the latest 
exceeding 1,000 feet. The floating drilling vessels 
normally are held over the drill hole by a system 
of anchors; however, some of the newer vessels are 
held by various types of multidirectional thrust 
systems (Figure 36). Most companies plan on 
conventional anchoring in depths to 1,300 feet. 
Drilling exploration wells often requires being on 
station for long periods (100 days, for example) 
and excessive fuel would be consumed if dynamic 
positioning were used. However, in comparatively 
sheltered waters with moderate winds and sea 
states, the costs of dynamic positioning systems 
compare favorably with conventional mooring 
systems. 
Figure 35. Self-propelled drilling ship Glomar 
Sirte measures 380 feet x 64 feet, displaces 9,500 
tons, and is one of the largest drilling ships in 
operation. (Global Marine photo) 
Should a drill ship be compelled to abandon its 
station, upon return it must be able to relocate the 
seafloor wellhead and reinsert the drill string. 
Recently, acoustic systems have been designed and 
tested for hole re-entry. Sea floor pingers or 
transducers will be increasingly used for precise 
repositioning, for effectively relocating the well- 
head, and for accurately guiding the drill string. 
: =< 
4 Hii. t 
1 iT % ce 
couse oe 
Figure 36. Artist's concept of dynamically posi- 
tioned drilling vessel designed to maintain a fixed 
position without anchors. (Esso Production 
photo) 
— 
3. Delineation Drilling 
Drilling a delineation or appraisal well (combi- 
nation exploratory and development well) is be- 
coming a common practice in the United Kingdom 
sector of the North Sea. The operators use the 
wells to define or appraise a geological structure 
once it has been confirmed by a successful wildcat 
operation. The practice has been to re-enter 
previously suspended delineation wells and equip 
them for gas production, thus realizing consider- 
able capital savings. 
D. Production 
1. Present Capability 
Recently a fixed platform was installed in 340 
feet of water in the Gulf of Mexico. At the time of 
installation this depth was a world record. This 
2,900-ton structure, towering more than 550 feet 
VI-167 
