Offshore Towers and Platforms 



Offshore towers and platforms differ from totally submerged structures 

 in three major ways: 



1 . They are currently used only in the shallow portion of the 

 continental shelf. 



2. They extend through the air— water interface and are thus 

 subjected to large wave forces. 



3. They are often large and massive because of the magnitude 

 of environmental factors encountered. 



Several hundred offshore platforms are in existence (Howe, 1967). 

 These structures are located in water depths of up to 370 feet, have total 

 weights in excess of 3,500,000 pounds, and use pile or caisson foundations 

 almost exclusively as their permanent foundation systems (Figure 34). 



Platforms in shallower waters are often constructed on site, beginning 

 with pile driving and continuing upward. For the larger offshore platform, the 

 underwater substructure, which doubles as a guide for the pile driving, is usually 

 prefabricated, towed to the site, and positioned on the bottom. The substruc- 

 ture typically utilizes a spread footing or shallow caisson configuration for 

 temporary support while the piles are being driven and grouted. These plat- 

 forms are founded on soils ranging from sand to soft clayey silt. As much as 

 300 feet of pile penetration and as many as sixteen 56-inch-diameter piles may 

 be required to resist the loads of larger platforms. 



In addition to these relatively permanent structures, there are about 

 100 drill rigs of the jack-up variety (Howe, 1969a). These rigs use large caissons, 

 pads, or mats as their foundation system. The structures are movable and have 

 been used in water depths to 300 feet. Total weights of the jack-up rigs range 

 from 1,000,000 to 10,000,000 pounds. Maximum lateral dimensions may 

 exceed 240 feet. Foundation pads or mats range from 20 to 120 feet in 

 major lateral dimension (Figure 35). 



Specific information is available on a limited number of offshore 

 platforms and towers (Table 4) . Most information is considered proprietary 

 and is, therefore, available only in generalized form. Generalizations concern- 

 ing performance of petroleum structures (information collected from a number 

 of sources) are summarized in the following paragraphs along with available 

 specific performance information. 



Argus Island. Argus Island was constructed in the summer of 1960 

 as a Navy research platform (McDermott, 1960). This structure, which is 

 similar to oil well drilling platforms, supports a two-story, 85- by 85- by 



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