Deterministic and probabilistic analyses of offshore pile systems

Chen, Jinbo, Ph. D.
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The offshore pile system capacity and the pile capacity model biases are important aspects in the assessment of existing offshore platforms and in the performance reliability that is achieved using the state of practice. The objectives of this research are to improve understanding of the pile system behavior, to calibrate the pile system capacity model bias factors, and to evaluate the reliabilities of offshore pile systems. A simplified single pile failure surface in terms of three dimensional pile head loads is proposed based on the analytical lower and upper solutions, and is verified through finite element analyses. Numerical lower and upper bound models are then proposed for the ultimate capacity of a pile system, and are shown to be efficient and be effective in considering global torsion and out-of-plane failures. The evidence from the survival of offshore platforms indicates that (1) well conductors should be included in assessing the pile system ultimate capacity; (2) static p-y curves should be used which increases the pile system lateral capacity by 10 to 20%; (3) the mean value of the steel yield strength should be used; (4) jacket leg stubs should be included; and (5) site-specific geotechnical information is important. The model bias factors in the API load and resistance design recipe are calibrated through Bayes’ Theorem based on the predicted and observed performance of eighteen offshore platforms in recent Gulf of Mexico hurricanes. The API load and resistance design recipe is calibrated to be close to unbiased for predicting the jacket system performance; be slightly conservative for predicting a foundation overturning failure in clay; and be conservative for predicting a lateral failure in clay and a foundation overturning failure in sand. The reliability of a pile system is shown to be insensitive to water depths and locations in the Gulf of Mexico, but depends on the pile layout, number of piles, loading direction, and expected failure mode. The pile system redundancy (a measure of capacity beyond failure of the first element) and robustness (a measure of capacity when the system is damaged) depend on the failure mode, pile geometry and layout, and loading directions. In general, the 8-leg pile system is more redundant and more robust than the 3-leg and 4-leg pile systems. The complexity (a measure of the how well the most critically-loaded element represents all elements) depends on the pile layout, the expected failure mode of a single pile and the pile capacity uncertainty. The complexity is generally small, indicating that the failure probability of the most critically-loaded pile is representative of the failure probabilities for all piles.