Soil-Spring Model for Fatigue Evaluation of Cyclic-Loaded Offshore Conductors
Ilupeju, Olusola Ayandire
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As oil exploration projects move further into deep and ultra-deep waters, where severe environmental conditions persist, having safe and functional structures, at optimal cost, by optimizing designs of offshore structures and foundations becomes more important. Design considerations and methodology for dealing with conductors and piles subjected to cyclic lateral loads, have been based on modifications to formulations for monotonic loads. Soil-structure interaction problems involving offshore conductors are nonlinear. A convenient and computationally efficient approach to modeling this behavior uses lateral transfer curves (P-y curves) from which deflections resulting from applied loads can be estimated. P-y curves can be back-calculated from instrumented laterally loaded test piles, either full-scale field tests or small-scale laboratory model tests. Centrifuge tests, which permit small scale model tests at stress levels representative of those occurring in situ, are particularly useful for this purpose. This research involves the back-analysis of centrifuge test data on piles subjected to cyclic lateral loads to obtain P-y curves applicable to soft to medium clays. The tests were conducted in a kaolin test bed in an overconsolidated stress state. Instrumentation data included strain gage measurements along the length of the pile, and displacement, force, and tilt measurements at the pile head. The test interpretation involved deducing equivalent soil resistance (P) and pile deflection (y) measurements from the strain gage data. The former is particularly challenging, because it requires obtaining numerical second derivatives from a spatial array of strain gages. For this purpose a local least squares regression analysis was developed. For convenient implementation into an analytical model the resulting P-y curves were fitted to two alternative model forms: power law and Ramberg-Osgood. A 0.91m conductor was subjected to small lateral displacements (0.01D to 0.02D), for which simplified expressions for secant stiffness and equivalent damping ratio has been presented. The back calculated moments from the Power law and Ramberg-Osgood equations, compared very well with measured bending moments. This study has provided a framework for interpreting and generating P-y curves for cyclic load on offshore conductors. It has also provided design parameters, the stiffness modulus, and damping ratio that can be used as input for pile deflection and fatigue analysis of cyclic loaded offshore conductors. The results of this study will contribute towards understanding the behavior of offshore conductors installed below the ocean floor in harsh environmental conditions.