Unexpected ice-induced vibrations of a conical structure in model scale

Abstract

Offshore wind turbines in cold sea areas can be fitted with ice cones to reduce static and dynamic loads from drifting sea ice. The effectiveness of ice cones in reducing static loads has been tested in model-scale ice basin experiments. However, only a few experiments used compliant test setups to study ice-induced vibrations on conical structures. This study explores the dynamic interaction between level ice and a downward-bending cone with a 60° slope angle through ice basin tests with a hardware-in-the-loop system based on a hybrid technique, combining a physical indenter with a numerical structure model of an offshore wind turbine. Two types of periodic ice-induced vibrations were observed for the first time in an ice basin: bending failure-induced vibrations and unexpected vibrations caused by local failure at the ice-structure interface. The local failure had characteristics of both shear failure and crushing failure and occurred at low ice-structure interaction speeds during tests. Local failure-induced vibrations were significant in the dynamic test with an ice-drift speed of 5 mm s^-1, however they also contributed to the dynamic response of the structure at higher ice-drift speeds. Bending failure-induced vibrations occurred at critical ice-drift speeds (30 mm s^-1 to 40 mm s^-1 and 70 mm s^-1 to 100 mm s^-1) where the bending failure frequency matched the 1st or 2nd natural frequency of the structure model. The results show that ice-induced vibrations on conical structures occur at various ice-drift velocities for both previously known and unexpected ice failure modes. Furthermore, the results provide new insight into conducting ice basin tests on ice-structure interaction with compliant conical structures.