If the subsurface is hard and compact, as in the North Sea, a 

 gravity platform can be used. However, known geologic characteristics 

 of United States frontier areas indicate that soft sediments predominate. 

 Therefore, fixed-pile platforms will likely be used in all frontier 

 areas. 



Construction/Installation (Drilling) 



Determining the number of platforms that will be required, their 

 location, and the number of wells per platform is based on a careful 

 analysis of the data obtained during exploratory and appraisal drilling. 

 This analysis involves such factors as the number and thickness of 

 productive horizons, geographic extent, water depth, formation depths, 

 well pressures, etc. Marketing factors will also have a bearing in 

 setting production rates, transport modes, and time frame for recovery. 



Production platforms are not standardized. They are custom designed 

 and engineered for a specific location. While many components, such as 

 motors, derricks, cranes, and housing modules are standard items, the 

 structure on which they are housed may have to stand in water depths 

 ranging from 50 to 1,000 feet (Figure 17). Platform engineering must 

 take into account depth, sea floor soil conditions, wave action (including 

 consideration of the 50 to 100 year wave), winds, sea floor stability, 

 and the weight of the structure. 



In the Gulf of Mexico, the trend is to construct a master platform, 

 from which wells are drilled, and several satellite platforms on which 

 crew quarters, separators, or compressors, etc. are mounted. Each of 

 the satellites is connected to the main platform by a foot bridge. In 

 the North Sea where weather conditions are more severe and the water 

 depths are greater, thus increasing the cost of platforms, the trend is 

 to locate the wells and all direct support facilities on a single 

 structure. 



Production drilling differs somewhat from exploratory drilling. 

 Exploratory rigs are readily moved from one location to another, but a 

 production platform is fixed in place for the life of the field. Modern 

 platforms are designed for drilling multiple wells. The largest platforms 

 have slots to accommodate as many as sixty wells. Exploratory wells are 

 usually drilled vertically; production wells may be drilled either 

 vertically or directionally. Directional or slant drilling requires the 

 deployment of special production rigs (that are mounted on the platform) 

 which can rotate the drill strings through the drive pipe or conductor 

 pipe that may be set at angles up to 30° in the sea floor. (See Figure 

 16) The bottom of a slant well may be more than a mile measured in the 

 horizontal direction, from the platform on which it was drilled. 

 Production rigs are usually designed with the derrick mounted on rails 

 so that after each well is completed, the derrick can be readily moved 

 over a new hole. The pace of drilling is slower for production than 



80 



