(c) Currents. Not significant. 



(d) Winds. Up to 150 miles per hour during hurricane season, usually in late 

 summer and fall. 



(e) Storm Surge and Tide. Normal tidal range, 4 feet (approximate); storm tide, 

 10 to 12 feet. 



(f) Littoral Transport. None, due to offshore location. 



(g) Water Depth at Structure. Bottom at 327 feet below mean water level, 

 (h) Bottom Conditions. Soft clay soil. 



(5) Structural Features (Fig. 60). Drilling and production platform of steel 

 construction, 85 by 165 feet was supported on 8- to 48-inch-diameter main piles and 8- to 

 60-inch-diameter skirt piles, driven by means of template placed on bottom. 



(6) Design Data. The design wave as recommended by oceanographer consultants 

 was that forecast for a 100-year storm. Due to the owner's knowledge of the area, an 

 extensive soil investigation program was carried out, with the 100-year criteria used for 

 foundation design in connection with sea floor soil movements. 



The high cost of a platform installed in over 300-foot depths requires a design close to 

 minimum specifications; for a platform in shallower water, a more conservative design is still 

 economically feasible. 



(7) Structural Performance. In August 1969, Shell Oil Comany's Platform B was lost 

 during Hurricane CamiUe (Fig. 62). As the result of an extensive investigation, data have been 

 collected showing conclusively that the primary cause of failure was movement of the sea 

 floor. 



Although the existence of weak clay soils in the area was known, the weakness and the 

 depth to which it extended were both underestimated. It is felt now that the exceptionally 

 high wave which was experienced induced soil movements fatal to the foundation piUng. 



During the hurricane, 72-foot waves were recorded on a wave staff within 5 miles of the 

 platform. Although this may not have been excessive for the 100-year storm, the effect on 

 the sea bottom was more than anticipated. Frequency estimates for such combined 

 conditions in the area vary from once in 75 years to once in 400 years, with current 

 estimates being once in 250 years. Post-hurricane studies indicate that forces exerted on the 

 structure by water alone were only 60 to 70 percent of the design force. 



The above opinion is strengthened by the results of soil samples taken within weeks after 

 the storm and a year later, and comparing these with the samples taken prior to installation. 

 The samples show a drastic change in bottom level and soil strengths taking place during the 

 storm, and then remaining without appreciable change for the following year. 



Topographic surveys and side-scan sonar runs, in addition to the soil borings, show as 

 much as 8 feet of subsidence in the immediate area of the structure and a 35-foot buildup in 

 nearby areas, all of which falls into a general pattern of down-slope movement throughout 

 Block 70 (Sterhng and Strohbeck, 1973). 



Ill 



