During placement of the platform substructures, which typically utilize 

 footing or caisson configurations for temporary support, penetrations of up to 

 8 feet have been recorded. In some instances the actual penetration, or imme- 

 diate settlement, has been as much as 3 feet more than expected. 



After the piles have been driven and grouted in, settlements have 

 occurred in many instances. At several locations where the soil profile was 

 considered competent and rather firm, total settlements of up to 3/4 inch 

 and differential settlements of up to 1/2 inch occurred over periods ranging 

 from 1/2 to 2 years (Busher, 1969). 



Of the more than 1 00 mobile jack-up rigs fabricated, at least 30 

 have been involved in major mishaps (Howe, 1968). Six of these mishaps 

 have been attributed to foundation or soil problems. In one incident, a rig 

 utilizing a 65- by 97-foot mat and applying a pressure of 180 psf to the sedi- 

 ments was involved in a bearing capacity failure and was subsequently lost. 

 At the time of this failure (1958), it was apparently the accepted practice of 

 many operators to forego detailed soil investigations at each specific work 

 site. Since most operators worked primarily in one area, for which their rigs 

 and foundation systems had in many cases been specifically designed, the 

 expense of additional soil investigations was considered unjustifiable. The 

 usual practice was to move onto a site, jack the rig up, and then preload the 

 foundation for a period of time before commencing actual drilling operations. 

 The rig involved in this failure penetrated as much as 9 feet into the soft Gulf 

 of Mexico soils during preloading (Beaupre, 1969). 



For other jack-up rigs, some of which use caisson configurations 

 with diameters ranging from 20 to 50 feet, penetrations of up to 30 feet have 

 been experienced before adequate bearing capacity was achieved. Pilelike con- 

 figurations have been used on some similar jack-up rigs, but penetrations in 

 several cases were considered excessive. Two rig losses have been attributed 

 to excessive penetration of piles. To overcome excessive penetration, spud 

 cans have been added to the piles on some of these rigs to increase their 

 bearing area (Howe, 1968). 



As a result of these and other incidents, more attention is being paid 

 to in-situ investigations and foundation analyses for such rigs. It is known 

 that in at least one case the insurance underwriters now require sampling at 

 the site, laboratory evaluation of the unconfined compressive strength of the 

 soil, and a satisfactory calculation of a suitable factor of safety. 



Another problem for such rigs is scouring action caused by bottom 

 currents or surge. Scouring is assumed to be a major problem only in water 

 depths of 400 feet or less. Massive steps are sometimes necessary to prevent 

 undermining of the foundations by this phenomenon. Rip-rap and other 

 materials are used to form a protective blanket in some instances. In other 

 cases, scour curtains have been built into the periphery of mat foundations. 



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