Dynamic Ice-structure Interaction Research Articles

Peak loads during dynamic ice-structure interaction caused by rapid ice strengthening at near-zero relative velocity

A series of ice penetration tests with a rigid structure, with controlled oscillation, and with a single-degree-of-freedom structure were performed to investigate the peak load-velocity dependence for ethanol-doped model ice during a test campaign at the Aalto Ice and Wave Tank. For the rigid structure and controlled-oscillation tests, the ice drift speed ranged between 1 and 150 mm s−1. In the controlled-oscillation tests, amplitudes of oscillation between 0.40 and 15.90 mm and frequencies of oscillation between 0.143 and 4 Hz were prescribed such that the relative velocity between ice and structure never became negative. A constant ice drift deceleration experiment with a single-degree-of-freedom structure was performed to investigate the development of frequency lock-in and intermittent crushing in the model ice and compare the results with the rigid structure and controlled-oscillation tests. It was found that the peak load-velocity dependence identified in the rigid structure tests was not always uniquely defined as identified in the controlled-oscillation tests because the loading history affected the peak load at ice failure. A rapid strengthening of the ice developed at low relative velocity and carried over to high relative velocity until the ice failure dissipated the strengthening effect. The strengthening effect, observed in the rigid structure and controlled-oscillation tests, was also observed during frequency lock-in and intermittent crushing in the single-degree-of-freedom structure test. The observations in the present study indicate that the so-called velocity and compliance effects in ice-structure interaction originate from the same strengthening effect. It then follows that peak loads on compliant structures cannot exceed peak loads on rigid structures in the same ice conditions, with the only difference being that the peak loads on compliant structures occur at apparently higher far-field ice drift speeds due to the change in relative velocity.

Towards uncertainty and sensitivity analysis for modal parameters identification during ice–structure interaction

The integrity of offshore structures is prone to the threat of drifting sea ice. By exciting the natural frequencies of the ice–structure interaction system, severe vibrations may occur. For a better understanding of how dynamic ice–structure interaction affects the system properties, a stochastic subspace system identification technique, SSI-cov is introduced to identify modal parameters during dynamic ice–structure interactions. Due to the uncertain input variables of SSI-cov, the identified modal parameters suffer from the quantitative judgment of the analyst. To address this problem, this study proposes an uncertainty analysis (UA) framework to obtain estimates of modal parameters. This framework constitutes both a sensitivity analysis (SA) and a UA. First, SA is applied to pick up the input variables that contribute the most to the identified modal parameters. Next, the important variables are left free to vary over their range of existence to obtain the modal parameters’ uncertainties. The framework is applied to full-scale tests from Norströmsgrund lighthouse and model tests carried out in Hamburg Ship Model Basin (HSVA). The comparison with traditional automated modal parameter identification shows its superiority in terms of the accuracy of modal parameters.

Dynamic ice loads for offshore wind support structure design

For offshore wind farms which are planned in sub-arctic regions like the Baltic Sea and Bohai Bay, support structure design has to account for load effects from dynamic ice-structure interaction. There is relatively high uncertainty related to dynamic ice loads as little to no load- and response data of offshore wind turbines exposed to drifting ice exists. In the present study the potential for the development of ice-induced vibrations for an offshore wind turbine on monopile foundation is experimentally investigated. The experiments aimed to reproduce at scale the interaction of an idling and operational 14 MW turbine with ice representative of 50-year return period Southern Baltic Sea conditions. A real-time hybrid test setup was used to allow the incorporation of the specific modal properties of an offshore wind turbine at the ice action point, as well as virtual wind loading. The experiments showed that all known regimes of ice-induced vibrations develop depending on the magnitude of the ice drift speed. At low speed this is intermittent crushing and at intermediate speeds is ‘frequency lock-in’ in the second global bending mode of the turbine. For high ice speeds continuous brittle crushing was found. A new finding is the development of an interaction regime with a strongly amplified non-harmonic first-mode response of the structure, combined with higher modes after moments of global ice failure. The regime develops between speeds where intermittent crushing and frequency lock-in in the second global bending mode develop. The development of this regime can be related to the specific modal properties of the wind turbine, for which the second and third global bending mode can be easily excited at the ice action point. Preliminary numerical simulations with a phenomenological ice model coupled to a full wind turbine model show that intermittent crushing and the new regime result in the largest bending moments for a large part of the support structure. Frequency lock-in and continuous brittle crushing result in significantly smaller bending moments throughout the structure.

Ice rose diagrams for probabilistic characterization of the ice drift behavior in the Beaufort Sea

The purpose of the present paper is to investigate the viability of the so-called ice rose diagram, which is a graphic tool to provide a succinct view of the ice drift information such as speed and direction of ice drift events in terms of their relative distribution at a local observation station to support the dynamic ice-structure interaction in cold regions. The ice drift data is collected by using local subsurface measurements based on acoustic Doppler current profilers (ADCP) in the Beaufort Sea during 2006–2017. Probabilistic analyses are performed in order to characterize the time-series data from these ADCP measurements. Both the ice drift speed and direction are treated as random variables for which analytical probability density functions are fitted. It is found that sea ice in the Beaufort Sea tends to drift at a lower speed during the winter season (i.e. the growth season) than during the summer season (i.e. the melting season). The Weibull distribution provides the most appropriate fit to the data of ice drift speeds during the growth season. For the melting season, a mixed distribution provides the most appropriate fit for the drift speed. Regarding directionality of the ice drift, a mixed Von Mises distribution is applied in order to represent its statistical properties. In this work, the relationship between ice drift speed and wind speed is also studied. It is found that the magnitude of the ice drift speed is approximately 2.5% of the wind speed during the winter season.

A Dynamic Ice-structure Interaction Model for Prediction of Ice-induced Vibration

Sea ice crashing against offshore structures can cause strong ice-induced vibration and have a major impact on offshore structural safety and serviceability. This paper describes a numerical method for the prediction of ice-induced vibration when a vertical offshore structure is subjected to the impact of sea ice. In this approach, negative damping theory and fracture length theory are combined and, along with ice strength-stress rate curve and ice failure length, are coupled to model the internal fluctuating nature of ice load. Considering the elastic deformation of ice and the effect of non-simultaneous crushing failure of local contact between ice and structures, the present ice-induced vibration model is established, and the general features of the interaction process are captured. To verify its efficacy, the presented simulation methodology is subjected to a model test and two full-scale measurements based on referenced studies. Example calculations show good agreement with the results of the model test and full-scale measurements, which directly indicates the validity of the proposed simulation method. In addition, the numerical simulation method can be used in connection with FE programs to perform ice-induced vibration analysis of offshore structures.

Dynamic response of an offshore structure interacting with an ice floe failing in crushing

Interaction of sea or lake ice with vertically sided offshore structures may result in severe structural vibrations commonly referred to as ice-induced vibrations. With the surge in offshore wind developments in sub-arctic regions this problem has received increased attention over the last decade, whereas traditionally the topic has been mainly associated with lighthouses and structures for hydrocarbon extraction. It is important for the safe design of these offshore structures to have the ability to predict the interaction between ice and structure in an expected scenario. A model for simulation of the interaction between a drifting ice floe and a vertically sided offshore structure is presented. The nonlinear speed dependent ductile and brittle deformation and local crushing of ice are considered phenomenologically. A one-dimensional sea ice dynamics model is applied to incorporate the effects of floe size, wind and current. The structure is modelled by incorporating its modal properties obtained from a general-purpose finite element software package. Alternatively, the model can be coupled to in-house design software for fully coupled simulations. Examples of application of the model to simulate dynamic ice-structure interaction are provided. Simulation results are validated with public data from forced vibration experiments, small-scale intermittent crushing and frequency lock-in, and full-scale interaction with the Norströmsgrund lighthouse. Effects of floe size and environmental driving forces on the development of ice-induced vibrations in full-scale are studied. It is shown that sustained frequency lock-in vibrations of the structure can only develop for very specific combinations of environmental driving forces and ice floe size. In all other cases, the ice floe slows down and comes to a stop, or accelerates to a drift speed which exceeds the range where frequency lock-in develops. This results in only a few cycles of vibration per interaction event, such as observed for the Norströmsgrund lighthouse in the Baltic Sea.

A non-simultaneous dynamic ice-structure interaction model

To simulate non-simultaneous ice failure effects on ice-structure interaction, an extended dynamic Van der Pol based numerical model is developed. The concept of multiple ice failure zones is proposed to fulfil non-simultaneous crushing characteristics. Numerical results show that there is more simultaneous force acting on all segments at lower ice velocity and there is more non-simultaneous ice failure at higher velocity. Variations of force records show a decreasing trend with increasing ice velocity and structural width. These effects can be attributed to the assumption that the size of ice failure zone becomes smaller with increasing ice velocity, which increases the occurrence of non-simultaneous ice failures. Similarly, the decreasing size of ice failure zone as velocity increases is explained as the reason of different ice failure modes shifting from large-area ductile bending to small-area brittle crushing. The simulation results from a series of 134 demonstration cases show that the model is capable of predicting results at different ice velocities, structural widths and ice thicknesses. In addition, analysis of the ice indentation experiments indicates that the mean and minimum effective pressure have an approximately linear relationship with ice velocity, which testified the assumption on variations of ice failure zone in the model.

Experimental validation of a model for prediction of dynamic ice-structure interaction

Vertically sided offshore structures subjected to level ice are designed to withstand the effects of ice-induced vibrations. Such structures are, for example, offshore wind turbines on monopile foundations, multi-legged oil- and gas platforms or lighthouses. For the prediction of dynamic interaction between ice and structures, several phenomenological models exist. The main challenge with these models is the limited amount of data available for validation, which makes it difficult to determine their applicability. In this study, an attempt is made to validate one of the existing models. First, the parameters which define the ice in the model were derived from new model-scale experiments with a rigid rectangular structure. The model was subsequently applied to simulate the interaction between ice and two compliant rectangular structures with different structural properties. Finally, model-scale experiments were conducted for the two compliant structures. Results of the experiments and model were compared to assess the capability of the model to predict dynamic ice-structure interaction. Results show that the adopted approach allows for a definition of the input parameters of the model and accurate prediction of frequency lock-in and continuous brittle crushing for compliant structures. Intermittent crushing was not observed in the model-scale experiments due to the model-scale ice bending significantly during low ice speeds. As a consequence, the model could not be validated for this regime of interaction. The approach followed and challenges encountered during its application are discussed.

A dynamic ice-structure interaction model for ice-induced vibrations by using van der pol equation

A dynamic single degree-of-freedom ice-structure interaction model is developed based on a novel physical mechanism combination between self-excited vibration and forced vibration. Van der Pol equation, together with ice stress-strain rate curve and ice-velocity failure length are coupled to model the internal fluctuating nature of ice force in conjunction with the relative velocity caused by the structure as an external effect. Three distinct structural responses, quasi-static, steady-static and random modes have been captured during ice-structure interaction. And the key behaviour, ice force frequency lock-in phenomenon during ice-induced vibrations (IIV) is also observed. General results show good agreement with those in full-scale and small-scale experiments as well as in numerical models.

Numerical study of ice-induced loads and responses of a monopile-type offshore wind turbine in parked and operating conditions

Offshore wind is an attractive source of renewable energy. In regions with cold climates, such as Canada, the Baltic Sea, and Bohai Bay, a good understanding of ice loads is essential to design a reliable and cost effective support structure for an offshore wind turbine. This paper presents a study of the dynamic ice–structure interaction of a commonly used monopile-type offshore wind turbine in drifting level ice in both parked and operating conditions. A semi-empirical numerical model for ice–structure interaction was coupled to the aero-hydro-servo-elastic simulation tool HAWC2. A convergence study was performed to determine the proper time step and simulation length to obtain reliable response prediction. The simulated ice load was compared with the formulations in international standards. The coupling between the ice loads and the structural response of the monopile-type wind turbine was investigated and found to be important. The effects of the ice characteristics (e.g., ice thickness and drifting speed) were examined with the turbine being in parked and operating conditions. Compared with current numerical model, standards from International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) predict lower ice loads. 3-D effect of ice–monopile interaction and failure of local ice sheet are considered. More stochastic dynamic phenomena are captured for ice–structure interaction in our numerical model. The effect of the ice thickness on the response was found to be significant. A negligible drifting speed effect is found on the bending moment response in the fore-aft direction. There is a large increase in the fore-aft response as the inclination angle of the cone increases. Further sensitivity studies and validation against model tests will be performed for the conical waterline of the wind turbine.

Estimating Soil Liquefaction in Ice-Induced Vibration of Bucket Foundation

The two suction foundation platforms installed in the Bohai Sea have vertical narrow columns passing through the water level. The continuous ice crushing on the columns and thereby the dynamic ice-structure interaction may usually happen during the ice season resulting in sustained violent vibrations of the structures. The paper first introduces the ice-induced vibration analysis of the suction foundation platform by using the self-excited vibration theory. Then the maximum dynamic shear stresses in the soil induced by ice are obtained from the analyses. By comparing the dynamic stresses to the cyclic strength of soil, which can be determined according to soil characteristics and features of the dynamic loading, the potential soil liquefaction is finally assessed.

Crushing failure during ice–structure interaction

Small-scale indentation tests were conducted with compliant structures and freshwater ice sheets. Besides measuring forces and displacements, we installed grid-based tactile pressure sensors at the ice–structure interface to measure the pressure generated during an interaction. Similar to the results of earlier studies, the results of the present study with compliant structures show that there is ductile deformation of ice at low indentation speeds and continuous brittle crushing at high indentation speeds. During a typical cycle of the dynamic ice–structure interaction at intermediate speeds, the ice–structure interaction results in variable rates of indentation into the ice, and the tactile sensor data indicate that the ice deforms in a ductile manner at the low indentation rate (the loading phase), and fails in continuous brittle crushing at the high indentation rate (the ejection phase). Theoretical estimates of global force are given in terms of non-simultaneous local force per unit width during continuous brittle crushing. We find the effective pressure measured during small-scale indentation tests to be close to those measured on full-scale structures, when the indentation rate is high in both situations.

A numerical model for dynamic ice–structure interaction

This paper describes a numerical method for ice load determination in conditions where a drifting ice floe acts on a vertical offshore structure. The model is based on a substructure technique where separate dynamic models of the interacting media and components are combined by a computer model. First, FE technique is used to model the structure. Second, complex impedance functions are used to consider the stiffness and damping properties of the foundation. Third, the action of the drifting ice floe is considered in two phases by adopting separate models for the elastic far field area and a nonlinear near field area adjacent to the ice–structure interface.

Steady-state vibrations of offshore structures

Offshore structures subjected to the action of drifting ice floes may experience several kinds of interactions with the ice feature. This paper addresses the structural response to forces that arise when the ice floe fractures against a vertical structure. Special attention is paid to the steady-state vibrations which may arise at medium ice velocities. The paper describes a straightforward design procedure for the dynamic response of narrow structures. The applicability of simple design concepts is discussed in connection with the design problems of wide offshore structures. Reference is made to recent experimental findings to show that numerical models of dynamic ice-structure interaction provide a promising design tool for both quasi-static and dynamic response of wide offshore structures.

Laboratory tests for dynamic ice-structure interaction

The CRREL test basin is able to simulate dynamic ice-structure interaction with scale model tests which cover the whole range of structures that are considered here. For bottom-founded structure simulation, a test pile was designed so that its stiffness, natural frequencies and modes and damping could be varied. The ice movement against the pile was arranged to have constant acceleration in order to excite different modes with differing ice velocities. The flexibility of the drive system caused jerky ice movements with low velocities. Analysis of the recorded ice forces and acceleration include the refinement at measured ice forces by eliminating the response of the measuring system itself using the dynamic equilibrium or transfer function approach. The frequencies and the damping of vibrations were analysed using a Fourier signal analyser. Scaling laws are discussed and it is noted that full similitude cannot be achieved for both ice interaction force and pile and ice vibrations. Results so far show similarity with in-field ice force histograms: both random and saw-tooth like repetitious ice force fluctuations appear. The change from one interaction mode to the other occurs with increasing drive velocities. With high velocities relatively smooth random ice crushing occurs and in some cases natural modes are stable. The greatest energy content of the ice force does not always coincide with the natural frequencies of the structures.

Ice-Structure Interaction with Artificially Generated Force Records

AbstractThe paper describes the dynamic ice-structure interaction study of monopod platforms (fixed type) which are of considerable interest to the off-shore oil industry in cold regions. In view of the scantiness of continuous ice-force records, artificial-force records are generated from available small-scale field ice-force records using stochastic concepts. The method, based on the similarity between the fluctuating parts of randomly varying ice-force records and the seismic records uses a non-stationary random process obtained from filtering a shot noise through a second-order filter. The dynamic response analysis of a monopod structure located at Cook Inlet, Alaska, is carried out by hybridizing the relevant sub-routines of the computer programmes, SAP IV (Structural Analysis Programme) and EATSW (Earthquake response of Axisymmetric Tower Structures surrounded by Water). The work includes a study of the influence of soil properties on the frequencies and responses, and a comparison between the responses of fixed and elastically supported (by the soil foundation) monopod platforms.

Ice-Structure Interaction with Artificially Generated Force Records

Abstract The paper describes the dynamic ice-structure interaction study of monopod platforms (fixed type) which are of considerable interest to the off-shore oil industry in cold regions. In view of the scantiness of continuous ice-force records, artificial-force records are generated from available small-scale field ice-force records using stochastic concepts. The method, based on the similarity between the fluctuating parts of randomly varying ice-force records and the seismic records uses a non-stationary random process obtained from filtering a shot noise through a second-order filter. The dynamic response analysis of a monopod structure located at Cook Inlet, Alaska, is carried out by hybridizing the relevant sub-routines of the computer programmes, SAP IV (Structural Analysis Programme) and EATSW (Earthquake response of Axisymmetric Tower Structures surrounded by Water). The work includes a study of the influence of soil properties on the frequencies and responses, and a comparison between the responses of fixed and elastically supported (by the soil foundation) monopod platforms.