Experimental and numerical investigations of monopile ringing in irregular finite-depth water waves

Abstract

In storm conditions, nonlinear wave loads on monopile offshore wind turbines can induce resonant ringing-type responses. Efficient, validated methods which capture such events in irregular waves in intermediate or shallow water depth conditions are needed for design. Dedicated experiments and numerical studies were performed toward this goal. The extensive experimental campaign at 1:48 scale was carried out for Statoil related to the development of the Dudgeon wind farm, and included both a rigid model and a flexible, pitching-type, single degree-of-freedom model. Twenty 3-hour duration realizations for 4 sea states and 2 water depths were tested for each model. A high level of repeatability in ringing events was observed. Uncertainties in the experimental results were critically examined.The stochastic variation in the 3-hour maximum bending moment at the sea bed was significantly larger than the random variation in repetition tests, and highlighted the need for a good statistical basis in design. Numerical simulations using a beam element model with a modified Morison wave load model and 2nd order wave kinematics gave reasonable prediction of the ringing response of the flexible model, and of the measured excitation forces on the rigid model in the absence of slamming. The numerical model was also used to investigate the sensitivity of the responses with respect to damping and natural period. A simple single degree-of-freedom model was shown to behave similarly to a fully flexible model when considering changes in natural frequency and damping.

Effects of Simulation Length and Flexible Foundation on Long-Term Response Extrapolation of a Bottom-Fixed Offshore Wind Turbine

“…The usual ranges for these values are: 1.5≤ CP + CA≤ 2 and, 0.6≤ CD≤ 1.2, according to [34]. For this work, the coefficients have been taken as C D =0.9, C A =0.75, CP=1, for a detailed explanation about the selection of these values the reader is referred to [35] which is also supported by [36]. Other input parameters have been left…”

Experimental and numerical investigation of a jacket structure subject to steep and breaking regular waves

“…In order to identify the wave forcing levels directly from the dynamic response force measurements, it is necessary to filter the structural dynamics. Several methods have been proposed previously using either low pass filtering technique [8,27,28] or inverse force identification algorithms based on dynamic properties of the structure [24,29].…”

Efficient Calculation of Spatial and Temporal Evolution of Hydrodynamic Loads on Offshore Wind Substructures

“…The Morison equation is generally used in the design of slender structures. If the undisturbed wave kinematics, such as the water particle acceleration and velocity field under waves are known it is possible to achieve good prediction of the hydrodynamical load with empirical force coefficients [11].…”