Hysteretic Damping of Piles in Sands for Offshore Wind Jacket Structures

Abstract

This paper presents a study on the hysteretic damping of cyclic axially loaded piles in sand that would be of particular interest to the design and analysis of offshore wind turbine (OWT) jacket structures. Foundation damping can help to reduce fatigue loads thereby leading to leaner designs and/or extended design life. Damping has been studied in laterally loaded OWT monopiles, but there is little information and guidance for estimating damping for axially loaded piles in an OWT jacket structure. The objective of this study was to (1) document hysteretic damping in axially loaded pipe piles in sands and (2) develop a simple and practical numerical approach for predicting the hysteretic damping (and stiffness) properties for dynamic numerical analysis of the OWT structure. First, existing low-frequency pile load test data are compiled from the literature and analyzed to evaluate the hysteretic damping. Second, a simple quasi-static equivalent linear modeling approach is described that can be used to predict hysteretic damping at the pile head using a conventional static t-z analysis. The axial stiffness and damping ratios predicted for the test piles at the pile head compared very well to the observed pile data when the maximum interface friction was accurately predicted.