Ice Induced Vibrations

The level-ice forces exerted on a scale model of a compliant bottom founded structure are identified from noncollocated strain and acceleration measurements by means of a joint input-state estimation algorithm. The identification is performed based on two different finite element models: one entirely based on the blueprints of the structure, and an updated one which predicts the first natural frequency more accurately. Results are presented for two different excitation scenarios characterized by the ice failure process and ice velocity, and known as the intermittent crushing and the continuous brittle crushing regimes. The accuracy of the identified forces is assessed by comparing them with those obtained by a frequency domain deconvolution on the basis of experimentally obtained frequency response functions. Results show a successful identification of the level-ice forces for both the intermittent and continuous brittle crushing regimes, even when significant modeling errors are present. The ice-induced displacements of the structure identified in conjunction with the forces are also compared to those measured during the experiment. These are found to be sensitive to the modelling errors in the blueprint model. By a simple tuning of the model, however, the estimated response is seen to match the measured one with high accuracy.

The problem of level ice interacting with compliant structures is addressed, where the ice loads can depend on the dynamical behavior of the structures. We are interested in a special type of ice-induced vibration, known as frequency lock-in, and characterized by having the dominant frequency of the ice forces near a natural frequency of the structure. It is shown that accurate estimates of the model parameters for the well-known Määttänen's model for ice-induced vibrations can be obtained from measurements of the structural vibrations and the ice velocity. Määttänen's model uses a state-dependent piecewise nonlinear function for the ice crushing strength, which leads to nonlinear negative damping in the equations of motion of the considered structure. The identification is achieved by means of an Unscented Kalman Filter using simulated noisy measurements of the structural behavior.

Pressures at the ice-structure interface during model-scale ice-structure interaction are often measured with tactile sensors. Resulting datasets usually include large volume of data along with some measurement error and noise; therefore, it is inherently hard to extract the hidden fluctuating pressures in the system. Identifying the deterministic pressure fluctuation in ice-induced vibrations is essential to understand this complex phenomenon better. In this paper, we discuss the use of two different multivariate analysis techniques to decompose an ensemble of measured pressure data into spatiotemporal modes that gives insights into pressure distributions in ice-induced vibrations. In particular, we use proper-orthogonal decomposition (POD) and inexact robust principal component analysis (IRPCA) in conjunction with measurements of intermittent crushing at different ice speeds. Both decompositions show that most of the energy is captured in a ten-dimensional space; however, the corresponding eigenvalues are different between the decompositions. While POD-based modes have

Interest to understand dynamic ice structure interaction and to develop theoretical simulation models has increased significantly lately. However, the background data on both the full-scale or even scale-model measurements is scarce and in some cases incompletely documented. To cover this pitfall the Norwegian University of Science and Technology initiated a project DIIV, Deciphering Ice Induced Vibrations, at the beginning of 2011. The objectives were defined to design and manufacture an adaptable test setup , to conduct scale-model tests in the EUHYDRALAB Transnational Access Program, and analyze results. Systematic variation of the most important structure and ice parameters allows finding out their contribution to the dynamic ice-structure interaction, and especially to the frequency lock-in vibrations. The measurement program included a test matrix with varying ice mechanical properties, ice velocity, structure waterline diameter, surface roughness, structure compliance, natural frequencies, and as a fresh approachan analogy to vortex shedding testinga forced sinusoidal ice crusher movement superposed to the ice velocity. This paper describes the design of the test setup , instrumentation and calibration, conducted tests, and general findings. Deciphering Ice Induced Vibrations Parts 2 and 3 in this 21 st IAHR Symposium on Ice present more detailed analysis procedures and some initial results.

M. Maattanen, S. Loset, A. Metrikine, K.-U. Evers, H. Hendrikse, C. Lonoy, I. Metrikin, T. Nord, and S. Sukhorukov 1. Sustainable Arctic Marine and Coastal Technology (SAMCoT), Centre for Research-based Innovation (CRI), Norwegian University of Science and Technology, Trondheim, Norway 2. Delft University of Technology, Delft, The Netherlands 3. Hamburg Ship Model Basin (HSVA), Hamburg, Germany 4. DNV, Nora, Norway * mauri.maattanen@ntnu.no

Granular flows in vibrated bed exhibit various physical phenomena primarily driven by vibrating base. As the vibrating surface is the only source of energy in an otherwise dissipative flow, most of the theoretical models relate the steady state energy input to the RMS velocity of vibration. Here variation of heat flux is studied at varying frequency of vibration while keeping the RMS vibration velocity and the cell loading constant. Using single particle analysis and MD simulations, an extended version of grain-base collision is observed resulting in the reduction of heat flux at lower frequencies (<50 Hz) of vibration. The presented findings are important as most experimental studies are reported at these frequencies of excitation.

An offshore wind turbine (OWT) analysis relies on aero-hydro-servo-elastic simulation tools, which take into account an interaction of various environmental conditions and the entire structural assembly of the turbine, including its control system. For the more confident analysis of loads imposed on OWTs located in ice infested regions, an additional interface between sea ice loading and a support structure is necessary.

The non-stationary process of ice breaking at the contact of the edge of a drifting ice field (IF) and the sea ice-resistant platform (IRP) can lead to dangerous vibrations and potentially dangerous dynamic loads on this offshore structure. Extreme resonant oscillations of the platform base can cause not only violations of the regular functioning of the object, but also significantly reduce the reliability of the structure and its durability, causing fatigue fracture in the structure of the IRP or its equipment, also such process can change the bearing capacity of the soil under the platform foundation. Dynamic ice destruction is a complex process, and the development of models of this phenomenon requires a well defined methodology and research procedure. The dynamic reaction of the structure on impact of the ice field depends on a combination of many factors: the size and flexibility of the impacted leg of the platform; the ice loading velocity, temperature and physical-mechanical parameters of ice, and others. The object of this research is the physical processes involved in the real system "IF-IRP” - the energy transfer from the moving ice fields to the control volume of ice in the contact area, accumulates the elastic energy received to its critical level in this volume and causes its destruction with a certain frequency. The most important property of the object of study, i.e. the subject of the research, is the mechanism of ice fracture in the zone of interaction of two basic elements of the system: the ice field and IRP. The aim of the study is to identify and describe the regularities of formation of cyclic ice loads on the structureand describe the process, taking into account the phenomenological features of sea ice fracture as a mechanism for converting the kinetic energy of the ice field into the elastic energy spent on to deviations leg of the platform and the energy spent on destructing the ice.

The present paper deals with ice loads on wind turbine foundations from large floating ice floes. Some newly developed procedures to estimate the extreme ice load and combined loads from wind with loads from ice are described. The procedures have been developed as a part of the design basis for two of the first large offshore wind farms in Denmark, and they have been further developed in a research project including ice model tests with cones and vertical cylinders at the National Research Council (NRC), Canada. The principles are included in the new bDanish Recommendation to Technical Approval of Offshore Wind TurbinesQ. The design procedures are an integrated part of a selected design philosophy, which also includes a procedure for defining a consistent set of partial safety factors for the various extreme load cases. For a partial safety factor for wind loads of 1.4 to 1.5 (as defined in the Danish standards), the partial coefficients for ice load should be 2.0 to 2.5, depending on the determining ice load case. D

In this paper, we consider a two degree-of-freedom model where the focus is on analyzing the vibrations of a fixed but flexible structure that is struck repeatedly by a second object. The repetitive impacts due to the second mass are driven by a flowing
fluid. Morison’s equation is used to model the effect of the fluid on the properties of the structure. The model is developed based on both linearized and quadratic fluid drag forces, both of which are analyzed analytically and simulated numerically. Conservation
of linear momentum and the coefficient of restitution are used to characterize the nature of the impacts between the two masses. A resonance condition of the model is analyzed with a Fourier transform. This model is proposed to explain the nature
of ice-induced vibrations, without the need for a model of the ice-failure mechanism. The predictions of the model are compared to ice-induced vibrations data that are available in the open literature and found to be in good agreement. Therefore, the use
of a repetitive impact model that does not require modelling the ice-failure mechanism can be used to explain some of the observed behaviour of ice-induced vibrations.

Several modelling approaches have been proposed for the modelling of ice-induced vibrations. Part of the differences in these approaches stem from the difference in the failure mode of the ice, whether in compression or bending. Within the spectre of crushing-failure models, different analytical models have been proposed. To investigate the differences, similarities and ease of implementation of models based on the crushing failure, two specific crushing failure frequency models are chosen for solution, simulation and comparison. These models are solved and then simulated with the same set of parameters. Corrections to one of the model are proposed and it is shown that although based on a similar modelling approach, the models do not predict the same behaviour.