Ice Failure Research Articles

Numerical Study of the Interaction between Level Ice and Wind Turbine Tower for Estimation of Ice Crushing Loads on Structure

In this paper, the interaction between level ice and wind turbine tower is simulated by the explicit nonlinear code LS-DYNA. The isotropic elasto-plastic material model is used for the level ice, in which ice crushing failure is considered. The effects of ice mesh size and ice failure strain on ice forces are investigated. The results indicate that these parameters have a significant effect on the ice crushing loads. To validate and benchmark the numerical simulations, experimental data on level ice-wind turbine tower interactions are used. First, the failure strains of the ice models with different mesh sizes are calibrated using the measured maximum ice force from one test. Next, the calibrated ice models with different mesh sizes are applied for other tests, and the simulated results are compared to corresponding model test data. The effects of the impact speed and the size of wind turbine tower on the comparison between the simulated and measured results are studied. The comparison results show that the numerical simulations can capture the trend of the ice loads with the impact speed and the size of wind turbine tower. When a mesh size of ice model is 1.5 times the ice thickness, the simulations can give more accurate estimations in terms of maximum ice loads for all tests, i.e., good agreement between the simulated and measured results is achieved.

Analysis of shadowing effect of propeller-ice milling conditions with peridynamics

During research on the propeller−ice contact process, it was found that there is a shadowing effect that has a significant influence on the propeller loading. The shadowing effect was observed to occur with both staggered configurations of multi-propellers along with the separate blades of single propellers. The shadowing effect causes the load from the ice to decrease on the aft propellers or a following blade compared to the blade currently in contact with the ice. Two numerical simulation models based on the peridynamics method have been established in order to study shadowing effect for both forward and aft multi propeller-ice configurations as well as individual blades of a single propeller. The influence mechanism of the shadowing effect on the sea ice failure mode along with the loads acting on the propellers are examined in this paper. Additionally, a new analytical shadowing effect coefficient on the ratio of the front and back contacts divided by the axial length of the damaged ice is put forward based. The results show that the shadowing effects are well modeled by both peridynamic numerical simulations and the analytical shadowing coefficient.

Analysis of Offshore Structures Based on Response Spectrum of Ice Force

With the development of large-scale offshore projects, sea ice is a potential threat to the safety of offshore structures. The main forms of damage to bottom-fixed offshore structures under sea ice are crushing failure and bending failure. Referred to as the concept of seismic response spectrums, the design response spectrum of offshore structures induced by the crushing and bending ice failure is presented. Selecting the Bohai Sea in China as an example, the sea areas were divided into different ice zones due to the different sea ice parameters. Based on the crushing and bending failure power spectral densities of ice force, a large amount of ice force time-history samples are firstly generated for each ice zone. The time-history of the maximum responses of a series of single degree of freedom systems with different natural frequencies under the ice force are calculated and subsequently, a response spectrum curve is obtained. Finally, by fitting all the response spectrum curves from different samples, the design response spectrum is generated for each ice zone. The ice force influence coefficients for crushing and bending failure are obtained, which can be used to estimate the stochastic sea ice force acting on a structure conveniently in a static way. A comparison of the proposed response spectrum method with the Monte Carlo method by a numerical example shows good agreement.

A simple stress-based cliff-calving law

Abstract. Over large coastal regions in Greenland and Antarctica the ice sheet calves directly into the ocean. In contrast to ice-shelf calving, an increase in calving from grounded glaciers contributes directly to sea-level rise. Ice cliffs with a glacier freeboard larger than ≈100 m are currently not observed, but it has been shown that such ice cliffs are increasingly unstable with increasing ice thickness. This cliff calving can constitute a self-amplifying ice loss mechanism that may significantly alter sea-level projections both of Greenland and Antarctica. Here we seek to derive a minimalist stress-based parametrization for cliff calving from grounded glaciers whose freeboards exceed the 100 m stability limit derived in previous studies. This will be an extension of existing calving laws for tidewater glaciers to higher ice cliffs. To this end we compute the stress field for a glacier with a simplified two-dimensional geometry from the two-dimensional Stokes equation. First we assume a constant yield stress to derive the failure region at the glacier front from the stress field within the glacier. Secondly, we assume a constant response time of ice failure due to exceedance of the yield stress. With this strongly constraining but very simple set of assumptions we propose a cliff-calving law where the calving rate follows a power-law dependence on the freeboard of the ice with exponents between 2 and 3, depending on the relative water depth at the calving front. The critical freeboard below which the ice front is stable decreases with increasing relative water depth of the calving front. For a dry water front it is, for example, 75 m. The purpose of this study is not to provide a comprehensive calving law but to derive a particularly simple equation with a transparent and minimalist set of assumptions.

Peridynamic solution for submarine surfacing through ice

ABSTRACT This paper establishes a numerical model for simulating submarine surfacing through the ice. The numerical model is simplified to be the interaction between a submarine and an ice sheet. The ice sheet is discretized into a number of ice material particles, and the motion and the failure of them are calculated by the peridynamic method. A contact detection method is used to determine the interpenetration between ice material particles and the submarine. To verify the feasibility of peridynamics in modelling ice failure, the two cases of three-point bending of columnar ice and vertical penetration of ice sheet for a cylinder are simulated. Convergence studies are taken with respect to the particle spacing size for submarine surfacing through ice. The surfacing process of the submarine has been simulated, and the calculation results are discussed. The calculated ice failure feature is similar to the full-scale observation.

Prediction of the dynamic pressure distribution on a propeller blade under ice milling

During milling-type propeller-ice contact on ice-going ships, the propeller blades suffer from extreme ice loads, which can result in high deformation and damage. The dynamic pressure distribution on the propeller blade under ice milling directly controls the strength of the blades; therefore, a numerical method to predict the dynamic pressure distribution is developed using the propeller-ice contact model with peridynamics. A method for generating the propeller blade mesh is proposed; the propeller blade mesh is created according to its milling area. The effect of particle spacing size on the milling force curve, ice failure, and pressure distribution is then studied to analyze the convergence of the proposed method. The numerical method is validated by applying it to study the contact between a tool with a propeller-like profile and ice sheet. The dynamic pressure distribution and ice failure are simulated during propeller-ice milling at both the leading and trailing edges of the blade.

Stochastic subspace identification of modal parameters during ice-structure interaction.

Identifying the modal parameters of structures located in ice-infested waters may be challenging due to the interaction between the ice and structure. In this study, both simulated data from a state-of-the-art ice-structure interaction model and measured data of ice-structure interaction were both used in conjunction with a covariance-driven stochastic subspace identification method to identify the modal parameters and their corresponding variances. The variances can be used to assign confidence to the identified eigenfrequencies, and effectively eliminate the eigenfrequencies with large variances. This enables a comparison between the identified eigenfrequencies for different ice conditions. Simulated data were used to assess the accuracy of the identified modal parameters during ice-structure interactions, and they were further used to guide the choice of parameters for the subspace identification when applied to measured data. The measured data consisted of 150 recordings of ice actions against the Norströmsgrund lighthouse in the Northern Baltic Sea. The results were sorted into groups defined by the observed ice conditions and governing ice failure mechanisms during the ice-structure interaction. The identified eigenfrequencies varied within each individual group and between the groups. Based on identified modal parameters, we suggested which eigenmodes play an active role in the interaction processes at the ice-structure interface and discussed the possible sources of errors. This article is part of the theme issue 'Environmental loading of heritage structures'.

Evaluation and optimization of a shock load de-icing method for transmission lines with combined ice failure criteria

A mechanical de-icing process for overhead transmission lines using shock load is investigated numerically by finite element analysis, where a combination of three validated ice failure criteria is implemented, namely- the maximum bending strain criterion, the tensile detachment criterion and the shear detachment criterion. A total of 25 numerical scenarios are established and the ice shedding ratio and dynamic response of overhead lines are studied. The goal is to assess the relative importance of the leading parameters that affect the efficiency of the shock-load deicing method and the corresponding transient effects. The parameters under study include: the shock load characteristics (amplitude, time history and applied location along the span), the ice accretion characteristics (equivalent thickness and eccentricity ratio), and the line profile (height difference of span suspension points and adjacent spans). The study leads to suggestions to improve the de-icing efficiency of the shock-load method and reduce the adverse transient effects on the line components. The modeling approach used in this study provides a cost-effective tool and helpful reference for design, evaluation and optimization of a variety of mechanical de-icing methods, devices and procedures.

Mathematical Modelling of Ice-Structure Interaction

Determination of the force effect of ice formations on the marine structure is based on a model of interaction with the edge of the ice field. The development of a model of ice failure accounting for the results of studies of the failure mechanism of the ice field edge with the surface of the vertical shape structure is presented. The processes of cracking and shear of different nature with the formation of large and small fragments are described with mathematical model. A criterion of the limit value of the accumulated potential energy of elastic compression of the ice volume before structure is proposed as specific energy of mechanical failure of sea ice. The values of this criterion, which takes into account all types of ice failure, are obtained experimentally by testing large samples of ice on dynamic crushing.

Limit mechanisms for ice loads on inclined structures: Local crushing

This paper focuses on mechanisms that limit the sea ice loads on offshore structures. It introduces a probabilistic limit load model, which can be used to analyze peak ice load events and to estimate the maximum peak ice load values on a wide, inclined, offshore structure. The model is based on simple mechanical principles, and it accounts for a mixed-mode ice failure process that includes buckling and local crushing of ice. The model development is based on observations on two-dimensional combined finite-discrete element method simulations on the ice-structure interaction process. The paper also presents a numerical limit load algorithm, which is an extension of the probabilistic limit load model and capable of yielding a large number of stochastic peak ice load values. The algorithm is compared to simulation-based and full-scale observations. Analyzing peak ice load events is challenging as sea ice goes through a complex mixed-mode failure process during such events. The algorithm is an effective tool for this analysis, and it shows that distinguishing between the buckling and local crushing failure is virtually impossible if the only data available from a peak load event is the value of the peak ice load. The algorithm shows potential in improving estimates of maximum peak ice load values on offshore structures.

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.

An extended ice failure model to improve the fidelity of icebreaking pattern in numerical simulation of ship performance in level ice

The modelling of the ice failure including icebreaking pattern and ice bearing capacity is an important issue in numerical simulations of ships going through level ice, in order to predict ship performance and ice loads. Previous studies model the shape of ice cusps assuming a simplified geometry, e.g. circular or triangular. According to the observations during full-scale ship trials, the geometry of the ice cusps is more elliptical rather than circular, with larger breaking length at the edges than that at the center. In this paper, a new ice failure model is developed which results in more realistic cusp shapes compared to existing approaches. The model is based on an analytically-derived differential equation, which is solved numerically via the Finite Difference Method (FDM). The predictions of ice cusps geometry are validated against full-scale measurement of ice cusps, obtained with an on-board stereo camera system. Satisfying agreement is shown. The ice failure model is incorporated into a numerical model for the prediction of ship performance in level ice. The predictions are compared with ship speed record obtained from a full-scale trial. It is shown that the model gives reasonable results for ship speed.

A numerical study on ice failure process and ice-ship interactions by Smoothed Particle Hydrodynamics

In this paper, a Smoothed Particle Hydrodynamics (SPH) method is extended to simulate the ice failure process and ice-ship interactions. The softening elastoplastic model integrating Drucker-Prager yield criterion is embedded into the SPH method to simulate the failure progress of ice. To verify the accuracy of the proposed SPH method, two benchmarks are presented, which include the elastic vibration of a cantilever beam and three-point bending failure of the ice beam. The good agreement between the obtained numerical results and experimental data indicates that the presented SPH method can give the reliable and accurate results for simulating the ice failure progress. On this basis, the extended SPH method is employed to simulate level ice interacting with sloping structure and three-dimensional ice-ship interaction in level ice, and the numerical data is validated through comparing with experimental results of a 1:20 scaled Araon icebreaker model. It is shown the proposed SPH model can satisfactorily predict the ice breaking process and ice breaking resistance on ships in ice-ship interaction.

Scale Effect in Ice Flexural Strength

Ice flexural strength is an important parameter in the assessment of ice loads on the hulls of ice-class ships, sloped offshore structures, and sloped bridge piers. While scale effects in compressive ice strength are well known, there has been debate as to the extent of scale effects in ice flexural strength. To investigate scale effects during flexural failure of both freshwater and saline ice, a comprehensive up-to-date database of beam flexural strength measurements has been compiled. The database includes 2073 freshwater ice beam tests with beam volumes between 0.00016 and 2.197 m3, and 2843 sea ice beam tests with volumes between 0.00048 and 59.87 m3. The data show a considerable decrease in flexural strength as the specimen size increases, when examined over a large range of scales. Empirical models of freshwater ice flexural strength as a function of beam volume, and of saline ice as function of beam and brine volumes have been developed using regression analysis. For freshwater ice, the scale-dependent flexural strength is given as: σf=839(V/V1)−0.13 For sea ice, the dependence of flexural strength has been modeled as: σ=1324(V/V1)−0.054e−4.969vb. Probabilistic models based on the empirical data were developed based on an analysis of the residuals, and can be used to enhance probabilistic analysis of ice loads where ice flexural strength is an input.

A Probabilistic High-Pressure Zone Model for Local and Global Loads During Ice-Structure Interactions

For temperate ice regions, guidance provided by current design codes regarding ice load estimation for thin ice is unclear, particularly for local pressure estimation. This is in part due to the broader issue of having different recommended approaches for estimating local, global, and dynamic ice loads during level ice interactions with a given structure based on region, scenario type, and a variety of other conditions. It is essential from a design perspective that these three scenarios each be evaluated using appropriate definitions for local design areas, global interaction area, and other structural details. However, the need for use of different modeling approaches for ice loads associated with each of these scenarios is not based on ice mechanics but rather has largely evolved as a result of complexities in developing physics-based models of ice failure in combination with the need to achieve safe designs in the face of limited full-scale data and the need for implementation in a probabilistic framework that can be used for risk-based design assessments. During a given interaction, the ice is the same regardless of the design task at hand. In this paper, a new approach is proposed based on a probabilistic framework for modeling loads from individual high-pressure zones acting on local and global areas. The analysis presented herein considers the case of thin, first-year sea ice interacting with a bottom-founded structure based on an empirical high-pressure zone model derived from field measurements. Initial results indicate that this approach is promising for modeling local and global pressures.

Failure simulation of ice beam using a fully Lagrangian particle method

A realistic numerical simulation technology using a Lagrangian Fluid-Structure Interaction (FSI) model was combined with a fracture algorithm to predict the fluid-ice-structure interaction. The failure of ice was modeled as the tensile fracture of elastic material by applying a novel FSI model based on the Moving Particle Semi-implicit (MPS) method. To verify the developed fracture algorithm, a series of numerical simulations for 3-point bending tests with an ice beam were performed and compared with the experiments carried out in an ice room. For application of the developed FSI model, a dropping water droplet hitting a cantilever ice beam was simulated with and without the fracture algorithm. The simulation showed that the effects of fracture which can occur in the process of a FSI simulation can be studied.

A New Year’s Day icebreaker: icequakes on lakes in Alberta, Canada

Any process that causes a sudden brittle failure of material has the potential to cause earthquake-like seismic events. Cryoseisms represent an underreported class of seismic event due to their (often) small magnitudes. In this paper, we document the phenomenon of some of the largest magnitude lake-associated icequakes (ML2.0) yet reported. These events occurred nearly simultaneously (within ∼2 h) on geographically separate lakes in Alberta, Canada, starting 1 January 2018. We conjecture that these events were caused by the sudden brittle failure of lake ice due to thermal expansion; the effects of the thermal expansion were compounded by the lack of insulating snow cover, high lake water levels, and a rapid onset of atmospheric warming. These factors also contributed to ice-jacking — a repeating process in which thermal contraction produces tensile cracks (leads) in lake ice that are then filled with water that is frozen during the cooling cycle. Thus, any subsequent thermal expansion must be accommodated by new deformation or brittle failure. This ice-jacking process caused creeping ground deformation after the initial brittle failure and again two weeks later following a second warming period. In many cases, the resulting ground deformation was significant enough to cause property damage.

Simulation of Wave Induced Ice Brakeup Process Using Dilated Polyhedral Dem

An important issue for MIZ (Marginal Ice Zone) simulation is the correct prediction of ice-wave interaction. The energy of wave will be eliminated and at the same time induces the breakage and fragment of level ice. To study ice failure and the floe-size distribution (FSD) after fracture will help to improve the accuracy of further numerical simulation. We present a three-dimensional DPDEM (Dilated Polyhedral Discrete Element Method) in which element is similar to the real shape of single block to simulate the ice fracture process under the wave load. A large number of polyhedral ice elements generated with Voronoi tessellation are bonded together to form integral level ice, and each bond pair can resist tensile compressive and moment due to interaction. The tensile strength of the sea ice is calibrated by a uniaxial tensile simulation, and each bonded pair is given a random fluctuation that follows normal distribution. The wave force is calculated by adding up buoyant and drag force of each polyhedron elements, which are functions of wave height and velocity. The broken ice generated by the fracture will continues to collide with each other in the wave field. The influence of wave period and amplitude, thickness and bond strength of level ice on sea ice fracture process was studied. In addition, we analyzed the size distribution of broken ice with those properties.

An engineering method for simulating dynamic interaction of moored ship with first-year ice ridge

This paper aims to present a new tool to simulate dynamic ice ridge-structure interaction in 6 Dofs in time domain. The global forces caused by ice ridge as well as responses of the structure are simulated. The ice ridge loads are composed of loads due to sail, consolidated layer and unconsolidated keel. The sail load is neglected since it is small compared to the other two components. The consolidated layer could be taken as level ice. The related level ice force is simulated by integrating ice breaking force and ice accumulation force. Two ice failure modes, namely bending and crushing, are used when calculating the breaking force. A numerical model is used to calculating the ice accumulation force underwater by determining the relative horizontal positions and motions of ice rubble to the strip of the structure. The ice load due to the unconsolidated keel is calculated based on a modified theoretical model in probabilistic design. The motions of the moored ship are also applied to update the waterline of the hull at each time step to estimate precisely local contact areas between ice and hull. The simulation results are compared with model test data.

A temperature- and stress-controlled failure criterion for ice-filled permafrost rock joints

Abstract. Instability and failure of high mountain rock slopes have significantly increased since the 1990s coincident with climatic warming and are expected to rise further. Most of the observed failures in permafrost-affected rock walls are likely triggered by the mechanical destabilisation of warming bedrock permafrost including ice-filled joints. The failure of ice-filled rock joints has only been observed in a small number of experiments, often using concrete as a rock analogue. Here, we present a systematic study of the brittle shear failure of ice and rock–ice interfaces, simulating the accelerating phase of rock slope failure. For this, we performed 141 shearing experiments with rock–ice–rock “sandwich”' samples at constant strain rates (10−3 s−1) provoking ice fracturing, under normal stress conditions ranging from 100 to 800 kPa, representing 4–30 m of rock overburden, and at temperatures from −10 to −0.5 ∘C, typical for recent observed rock slope failures in alpine permafrost. To create close to natural but reproducible conditions, limestone sample surfaces were ground to international rock mechanical standard roughness. Acoustic emission (AE) was successfully applied to describe the fracturing behaviour, anticipating rock–ice failure as all failures are predated by an AE hit increase with peaks immediately prior to failure. We demonstrate that both the warming and unloading (i.e. reduced overburden) of ice-filled rock joints lead to a significant drop in shear resistance. With a temperature increase from −10 to −0.5 ∘C, the shear stress at failure reduces by 64 %–78 % for normal stresses of 100–400 kPa. At a given temperature, the shear resistance of rock–ice interfaces decreases with decreasing normal stress. This can lead to a self-enforced rock slope failure propagation: as soon as a first slab has detached, further slabs become unstable through progressive thermal propagation and possibly even faster by unloading. Here, we introduce a new Mohr–Coulomb failure criterion for ice-filled rock joints that is valid for joint surfaces, which we assume similar for all rock types, and which applies to temperatures from −8 to −0.5 ∘C and normal stresses from 100 to 400 kPa. It contains temperature-dependent friction and cohesion, which decrease by 12 % ∘C−1 and 10 % ∘C−1 respectively due to warming and it applies to temperature and stress conditions of more than 90 % of the recently documented accelerating failure phases in permafrost rock walls.