dc.description.abstract | 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. | en |