Ice-structure Interaction Process Research Articles

Development of a Numerical Ice Tank Based on DEM and Physical Model Testing: Methods, Validations and Applications

The determination of ice loads on polar vessels and offshore structures is important for ice-resistant design, safe operation, and management of structural integrity in ice-infested waters. Physical model testing carried out in an ice tank/basin is usually an important technical approach for evaluating the ice loads. However, the high cost and time consumption make it difficult to perform multiple repetitions or numerous trials. Recently, the rapid development of high-performance computation techniques provides a usable alternative where the numerical methods represented by the discrete element method (DEM) have made remarkable contributions to the ice load predictions. Based on DEM simulations validated by physical model tests, numerical ice tanks can be developed as an effective complement to their counterparts. In this paper, a numerical ice tank based on 3D spherical DEM was established with respect to the small ice model basin of China Ship Scientific Research Center (CSSRC-SIMB). Based on spherical DEM with parallel bond model, the model tests of typical structures (vertical cylinder and inclined plate) in level ice sheets were established in the numerical ice tank, and the ice–structure interaction process under the same initial conditions was simulated. The accuracy of the simulations is verified by comparing the simulated ice loads with the measured ice loads from the model tests in the CSSRC-SIMB. Furthermore, the application of the numerical ice tank was extended to simulate the navigation of a Wass bow in level ice and broken ice conditions. The value of the break resistance of the Wass bow in level ice was evaluated, and the numerical ice tank produced results that were found to be consistent with those obtained from Lindqvist’s formula. The statistical properties of the bow load for different broken ice fields with the same initial physical conditions are analyzed by performing a repeatability test on the broken ice fields.

Mechanism of Phase-Locked Ice Crushing against Offshore Structures

This paper addresses a detailed analysis of the ice–structure interaction process of the phase-locked ice crushing (PLC) against offshore structures. Directly measured ice load, structure response data, and in situ observation from the field measurements on the Molikpaq lighthouse and jacket platform were used in the study. This paper summarizes a new ductile damage-collapse (DDC) failure mechanism for the PLC process. The DDC mechanism shows that the ice failure is a discrete ductile crushing process rather than a ductile–brittle transition process. The analysis identifies that the ice has a failure length in PLC and this failure length plays an important role in understanding the interaction. It reveals that PLC can occur on most vertical-sided offshore structures when the velocity of the ice sheet falls within the range of the failure length divided by the natural period of the structure. This paper proposes that this relationship between ice failure length and the natural period of the structure can be used as one of the PLC occurrence conditions. The DDC failure mechanism provides a basis for another technical route to solve the PLC problem.

Load distributions in the ice-structure interaction process in shallow water

Model-scale experiments were performed to explore the process of level ice interacting with a, in model-scale, ten-meter-wide inclined structure in shallow water. During each experiment, an initially intact ice sheet was pushed against the sloping structure, failed against it and accumulated into a grounded rubble pile in front of it. The strength of the model ice used in the experiments was varied. The spatial and temporal ice load distributions were measured at two scales: the structure was divided into ten identical segments along its waterline with the horizontal load on each segment measured, while the local pressure distributions were simultaneously measured with tactile sensors from two of the segments. This paper focuses on the relationships between the loads on different segments and the correlation between global and local loads. The load records of the individual segments during the whole experiment correlated with each other. The Pearson correlation coefficient for the load records of two neighboring segments was about 0.8, and for segments having a distance of 5 m or more, about 0.4. The magnitudes of both the maximum and the mean load on each segment were approximately equal. Even if the segment loads correlated during the whole experiment, the load records of the individual segments showed also transient ice load peaks, which did not correlate. These were used to define local peak load events and related local line loads, defined by dividing the magnitude of a peak load by its width in space. Roughly 2/3 of the local peak load events concentrated on one segment and only about 5% of them covered more than three segments. Narrow local peak load events caused the local line loads of the highest magnitude, with the magnitude and the relative frequency of the events increasing with the ice strength. Narrow events resulted in local line loads up to four times higher than the global line load, which was defined by dividing the total load on the structure by its width. The investigation of the local peak load events also showed that an inclined structure having a width of approximately 5 m (the ice thickness to structure width ratio of 1:100) appeared infinitely wide and gave similar results as a wider structure. Ice pressure measurements showed a line-like pressure distribution at the waterline of the structure with the weakest ice, whereas with stronger ice, zones of high ice pressure were observed at upper parts of the segments.

Determination of initial breaking pattern in the bending failure of a semi-infinite ice sheet

There is a growing interest in the development of real-time ice-ship or ice-structure interaction simulators based on theoretical ice sheet bending failure models. The initial ice-structure interaction process is commonly idealized as a bending failure of semi-infinite ice sheets. Existing theoretical models either assume a breaking pattern of radial-before-circumferential cracking, or that of a circumferential-before-radial cracking. However, field observations indicate that the breaking pattern depends on various factors. For generic applications of real-time simulators, the determination of breaking patterns is thus an essential first step towards reasonable ice load predictions. This paper addresses the current research gap by considering a normalized formulation of the ice sheet bending failure. The locations and orientations of the maximum principal stresses at the upper and bottom surfaces of a semi-infinite ice sheet are obtained, from which the criterion for breaking pattern determinations is determined. It is found that factors such as large structure waterline curvature radii, small ice thicknesses, large ice flexural strengths, and small ice compressive strengths induce a breaking pattern of circumferential-before-radial cracking. Conversely, small structure waterline curvature radii, large ice thicknesses, small ice flexural strengths, and large ice compressive strengths result in a breaking pattern of radial-before-circumferential cracking.

A Finite Element Model for Compressive Ice Loads Based on a Mohr-Coulomb Material and the Node Splitting Technique

Abstract This paper presents a finite element model for the simulation of ice–structure interaction problems, which are dominated by crushing. The failure mode of ice depends significantly on the strain rate. At low strain rates, the ice behaves ductile, whereas at high strain rates it reacts in brittle mode. This paper focuses on the brittle mode, which is the dominating mode for ship–ice interactions. A multitude of numerical approaches for the simulation of ice can be found in the literature. Nevertheless, the literature approaches do not seem suitable for the simulation of continuous ice–structure interaction processes at low and high confinement ratios in brittle mode. Therefore, this paper seeks to simulate the ice–structure interaction with the finite element method (FEM). The objective of the here introduced Mohr-Coulomb Nodal Split (MCNS) model is to represent the essential material behavior of ice in an efficient formulation. To preserve mass and energy as much as possible, the node splitting technique is applied, instead of the frequently used element erosion technique. The intention of the presented model is not to reproduce individual cracks with high accuracy, because this is not possible with a reasonable element size, due to the large number of crack fronts forming during the ice–structure interaction process. To validate the findings of the model, the simulated maximum ice forces and contact pressures are compared with ice extrusion and double pendulum tests. During validation, the MCNS model shows a very good agreement with these experimental values.

Model-scale tests on ice-structure interaction in shallow water: Global ice loads and the ice loading process

Laboratory-scale experiments on ice-structure interaction process in shallow water were performed by pushing a ten-meter-wide ice sheet against an inclined structure of the same width. Seven experiments were performed in three series: In one of the series, the compressive and flexural strengths were both about 50kPa, in the two other test series the ice strength was two and four times higher. The ice thickness was about 50 mm in all experiments. The loading process showed two phases: the ice load on the structure (1) first increased linearly with a rate that was constant for all experiments, after which (2) the loading process reached a steady-state phase with approximately constant load. The magnitude of ice loads was not proportional to ice strength, as the weakest ice yielded higher loads than the ice having twice its strength. The ice rubble grounded in all experiments, but the bottom carried only a small portion of the load. The load records could be normalized by a factor combining the weight and the characteristic length of the intact ice. Based on the normalization, a model explaining the loading process was derived; the weight of the incoming ice has a dominant role during phase (1), while buckling explains the change in the process to phase (2) when the ice is strong enough. The loading process for the weakest ice was different from that for the other two ice types used. For example, instead of forming a rubble pile consisting of distinct ice blocks, weakest ice formed a dense pile of slush. The normalized ice load data highlighted the differences in the loading process.

Laboratory experiments on floating saline ice block breakage in ice-to-ice contact

This study uses a unique experimental system to explore the breakage of laboratory-grown, floating saline ice blocks under ice-to-ice contacts. This topic is important to the understanding and modeling of force transmission through ice rubble fields. In a collection of ice blocks subjected to compressive loading, force is transmitted via force chains, and their stability plays a crucial role in ice-structure interaction processes. Peak loads are limited by the buckling of these force chains or breakage at the ice block contacts. Lack of information on the breakage mechanism motivates the present effort. In the experiments, three ice blocks were set up to form two ice-to-ice contacts, after which the three-block system was compressed to failure. The force transmitted through each contact and the failure process of the blocks were recorded. In total 32 tests with varying contact areas were performed. In about 75% of the cases, the load transmitted by an ice-to-ice contact was limited by shear failure. The key property limiting the magnitude of the force transmitted by an ice-to-ice contact was found to be shear strength; blocks typically failed in shear on planes having the characteristics of ‘Coulombic shear faults’. Quasi-static force equilibrium analysis of these shear failures showed that the floating ice blocks with a naturally occurring temperature gradient used in this study had a shear strength of 279 kPa. Other failure modes, including crushing, splitting and ‘Y-shaped’ conjugate failure, were occasionally observed.

In-Plane and Out-of Plane Failure of an Ice Sheet using Peridynamics

ABSTRACTWhen dealing with ice structure interaction modeling, such as designs for offshore structures/icebreakers or predicting ice cover’s bearing capacity for transportation, it is essential to determine the most important failure modes of ice. Structural properties, ice material properties, ice-structure interaction processes, and ice sheet geometries have significant effect on failure modes. In this paper two most frequently observed failure modes are studied; splitting failure mode for in-plane failure of finite ice sheet and out-of-plane failure of semi-infinite ice sheet. Peridynamic theory was used to determine the load necessary for inplane failure of a finite ice sheet. Moreover, the relationship between radial crack initiation load and measured out-of-plane failure load for a semi-infinite ice sheet is established. To achieve this, two peridynamic models are developed. First model is a 2 dimensional bond based peridynamic model of a plate with initial crack used for the in-plane case. Second model is based on a Mindlin plate resting on a Winkler elastic foundation formulation for out-of-plane case. Numerical results obtained using peridynamics are compared against experimental results and a good agreement between the two approaches is obtained confirming capability of peridynamics for predicting in-plane and out-of-plane failure of ice sheets.

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.

Scatter and error estimates in ice loads – Results from virtual experiments

This paper analyses peak ice loads on an inclined, rigid marine structure by using data from 2D combined finite-discrete element method simulations. The aim is look for answers to the questions: what type of distributions do peak ice loads follow, what type of data is needed for their analysis, and how should the data on peak ice loads be collected? In the simulations, an initially continuous ice sheet, modeled as a floating beam, breaks into smaller ice blocks as the sheet is pushed against the structure. Statistical tools were used to analyse the peak ice load distributions, error estimates, and peak ice load occurrences. Load distributions appeared to be right-skewed and thus non-normal. Gumbel distribution appeared to describe the data well. The results show that the large scatter in the ice load data is due to the ice-structure interaction process itself. Due to the scatter, a large number of observations are needed for studying peak ice load statistics: To reliably observe a 15% difference in peak loads due to a single parameter would require more than 80 observations in total. The results showed that high peak loads may occur even in the early stage of the ice-structure interaction process.

Limit mechanisms for ice loads on inclined structures: Buckling

This paper focuses on the mechanisms and limits for the peak ice load values on inclined marine structures, and presents a buckling model, which explains well the phenomena behind the maximum peak ice loads. The study is based on two-dimensional combined finite-discrete element method simulations of the failure process of level ice against a structure. The simulations yield ice load records, which show consecutive peak ice load events. The complexity of the failure process makes the analysis on the mechanical phenomena behind the peak ice load events extremely challenging. The introduced buckling model assumes that the ice sheet breaks into separate ice floes in front of the structure before the maximum peak ice loads occur. The model is demonstrated to be able to quantify the effect of force chains, which have an important role in the ice-structure interaction process.

The statistical analysis of peak ice loads in a simulated ice-structure interaction process

This paper analyses peak ice load data from 2D combined finite-discrete element method simulations. In these simulations, an initially continuous ice sheet, modeled as a floating beam, breaks into smaller ice blocks as the sheet is pushed against an inclined structure. Multivariate linear regression modeling and the variable elimination method were used in the analysis of the data. The analysis gave valuable insight into the peak ice load data in a simulated ice-structure interaction process. It was found that the peak ice load data can be estimated with good accuracy using only five parameters: the ice thickness, the inclination angle of the structure, the shear strength of ice, the ice-structure, and ice–ice friction coefficients. The ice thickness and the structure's inclination angle had the strongest relative effects, with their importance changing during the process. The results also showed that the stage of the ice-structure interaction process should be taken into account in ice load models. The results of this paper underline the importance of the statistical analysis of ice load data and give valuable suggestions for future work.

Discussion of assumptions behind rule-based ice loads due to crushing

Forecasts and trends indicate an increase in marine operations in polar waters. The design of new ships for severe polar conditions is usually solved via theoretical considerations that are combined with previous experience and engineering judgment. A deeper understanding of the theoretical considerations that underlie rule-based formulations is required for designing safe and efficient structures. This paper focuses on the assumptions that are hidden in the ice load formulations of the International Association of Classification Societies (IACS) Unified Requirements for Polar Ships (IACS, 2011) and of the Russian Maritime Register of Shipping (RMRS) Rules for the Classification and Construction of Sea-going Ships (RMRS, 2014), particularly the Daley ice load model and the Kurdyumov and Kheisin hydrodynamic model for ice crushing. A qualitative comparison of the two models is presented. The assumptions that underlie rule-based ice loads in the bow area are placed in the context of current understanding of ice–structure interaction process. The comparison of the models demonstrates that the underlying assumptions regarding the pressure–area relationship, ice edge spalling characteristics, dynamic viscosity and strength of the crushed ice are the most important assumptions, although they are highly contentious.

Results of Field Monitoring on Ice Actions on Conical Structures

Ice-structure interaction plays a central part in determining ice loads and ice-induced vibrations. This is a controversial research issue, and many factors make the problem more complicated. The authors have been monitoring several ice resistant structures in the Bohai Sea for 20 years and have measured ice forces and simultaneously observed ice-structure interaction processes. This paper describes typical physical ice sheet–conical structure interaction processes, field data, and theoretical explanations for different ice conditions and structure dimensions. The conclusions are more widely applicable, and we relate them to field work on ice resistant conical structures in other ice-covered regions. Further work will quantify ice loads on conical structures once the interaction process is understood.