The behavior of a mat-supported mobile rig founded on very soft clay has been examined. The settlement of the mat into the clay during preload when setting up on the location is used to arrive at the ultimate bearing capacity of the soil. Analyses are then made of the bearing pressure of the mat on the soil when the mobile unit is subjected to storm loadings and these bearing pressures are compared to the ultimate soil beam pressures generated during settlements that accompanied preloading. This comparison provides guidelines for predictions of additional settlements that may occur under the storm loadings. The method putting bearing pressures under storm loadings is unique is that it considers elastic-plastic response of the soil resistance as contrasted to conventional elastic solutions or equivalent-area solutions.


The first mat-supported submersible mobile unit, as reported by Howe (1966), was the Barnsdall-Hayward Breton Rig 20, later known as Kerr-McGee Rig 40 that was completed in 1949. Many years before the advert of the submersible rig, there were units known as marine drilling rigs that operated in lakes, bayous and dredged canals. These marine rigs were used offshore beginning in the late 1940's, and they were often founded on shell pads to increase their operating water depth. The submersible mobile units and the marine drilling rigs share one common feature: the large area of the mat permits the units to operate with minimum settlement, even on some very soft clay bottoms.

Hirst et al (1976) have reported that the average seafloor contact pressure of mat-supported mobil units is approximately 50 psf when there is no wave loading. They also reported no significant settlement of the rigs as long as clay bottoms had shear strengths of 100 psf or greater. For very soft deltaic clays with shear strengths of 40 to 50 psf, settlements do occur under contact pressures of 500 psf. A handful of clay soil with 40 to 50 psf shear strength can easily be squeezed between the fingers.

To control settlement in very soft clay bottoms a planned preload is applied when setting up the unit on locations, Turner et al (1987). As the mat settles into the seafloor under the preload, additional bearing capacity is developed from three sources: the increasing strength of the soil as the mat goes deeper, the increased buoyancy from the displaced soil, and the increasing friction of the soil on the sides of the mat.

Young et al (1981) have presented an extensive discussion of foundation performance of mat-supported mobile units in soft clays.

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