Performance of pile foundations for fixed-type platforms during Hurricane Katrina
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According to increasing demand for oil, numerous offshore oil producing structures have been constructed in the Gulf of Mexico. A jacket-type platform is one of the oil producing structures, which has deck structures, jacket frames and foundation structures. For foundation structures, driven steel pipe piles are typically used. From Hurricane Katrina, no known collapses of offshore oil jacket platforms occurred because of foundation failure even though foundations were subjected to environmental loads greater than their design capacity. Conservatism in pile design significantly impacts on the cost of the jacket platforms. This research intends to study the bias between the predicted and the observed capacity of pile foundations. To investigate the bias, a plastic model of pile system collapse is adapted and extended. The model is useful to account for an interaction between axial and lateral capacities of individual piles and also to consider interactions between multiple piles. Analyses of foundation capacity for 8-leg and 6-leg jacket platforms using the plastic model result in an interaction collapse diagram for the foundation. Compared with the loads from Katrina the plastic model indicates that the pile systems should have collapsed under the loads. Finally, sensitivity studies regarding pile geometry, loading, soil properties and pile-soil interaction are conducted to identify and study the potential sources of bias in the pile design. The potential sources of the bias include (1) uncertainty from soil sampling and field/lab testing, (2) the contributions of mudline elements to vertical resistance of pile system, (3) rate of loading, (4) reverse end bearing, (5) hardening of sands at pile tips and (6) set-up effect. When the effects of these potential sources are considered to increase axial and lateral capacity of individual piles, the interaction diagram for the two example platforms expands beyond the estimated environmental loads from Hurricane Katrina and therefore the survival of the two platforms from the hurricane may be explained.