Rocky Taylor

Development of a Probabilistic Ice Load Model Based on Empirical Descriptions of High Pressure Zone Attributes

During an ice-structure interaction, the localization of contact into high pressure zones (hpzs) has important implications for the manner in which loads are transmitted to the structure. In a companion paper, new methods for extracting empirical descriptions of the attributes of individual hpzs from tactile sensor field data for thin first-year sea ice have been presented. In the present paper these new empirical hpz relationships have been incorporated into a probabilistic ice load model, which has been used to simulate ice loads during level ice interactions with a rigid structure. Additional aspects of the ice failure process, such as relationships between individual hpzs and the spatial-temporal distribution of hpzs during an interaction have also been explored. Preliminary results from the empirical hpz ice load model have been compared with existing empirical models and are discussed in the context of both local and global loads acting on offshore structures.Copyright

A Response Comparison of a Stiffened Panel Subjected to Rule-Based and Measured Ice Loads

Ships operating in ice-covered waters are exposed to intense loads from ice features. Thus, their structures have to be designed to resist these ice loads. To achieve compliance with classification societies’ rules, analysis of these ice loads is achieved through the introduction of a uniform pressure patch applied to the hull surface. This uniform pressure approach does not account for the high degree of spatial and temporal variations observed in ice load measurements, which are inherent to the ice failure process. Thus, this paper will compare the response of a stiffened panel to ice loading by applying a rule-based uniform pressure patch as well as instantaneous non-uniform pressures based on measured spatial distributions of loads from field tests in order to investigate the effect of spatially localized loads due to high pressure zones on local plastic deformation of the hull.Copyright

Analysis of High Pressure Zone Attributes From Tactile Pressure Sensor Field Data

Tactile sensor data collected during the Japan Ocean Industries Association (JOIA) medium-scale field indentation test program provide detailed information about spatial and temporal distributions of contact pressures during ice crushing. The localization of contact into high pressure zones (hpzs) through which the majority of loads are transmitted to the structure is an important feature of these data. For all but the slowest interaction rates, non-simultaneous failure is observed, with linear distributions of hpzs comprising a total contact area on the order of 10% of the nominal interaction area (structure width × ice thickness). To improve understanding of the nature of individual hpzs during compressive ice failure, a new approach to analyzing tactile sensor data has been developed. Analysis algorithms developed for automatic hpz detection and tracking are discussed. Issues associated with pressure threshold value definition and selection are considered. Probabilistic descriptions of high pressure zone attributes based on analysis of JOIA field measurements are presented. The development of a probabilistic ice load model based on these hpz data is detailed in a companion paper.Copyright

EXPERIMENTAL INVESTIGATION OF COMPRESSIVE FAILURE OF TRUNCATED CONICAL ICE SPECIMENS

In total, twenty-eight (28) small-scale ice indentation tests have been carried out to study the compressive failure of polycrystalline ice during indentation and to explore the link between various parameters that influence the ice failure processes, using ice specimens having a truncated conical geometry. Taper angle, temperature, indentation rate, indenter shape and grain size are considered as controlled variables in this research program. For the experiments, three geometric configurations (with taper angles of 13°, 21°, 30°) have been used, conducted at temperatures of -10°C and -5°C. Indentation rates of 0.1 mm/s, 1 mm/s and 10 mm/s have been considered using two indenter shapes (a flat plate and a spherical indenter). Two grain size ranges were considered for these tests. The total force and pressure were found to show dependencies on the indentation rate. The force becomes higher and failure process changes from brittle to ductile as indentation rate decreases. For example, in case of the 21o taper angle ice sample, maximum ice loads were 20 kN and 145 kN and peak pressures were 8 MPa and 18 MPa for indentation speeds of 10 mm/s and 0.1 mm/s respectively. The total force also depends on the taper angle of ice sample. The loads increase as the ice samples become flatter. So, the 13° ice sample was stronger than the 30° ice sample. Different shaped indenters also observed to have distinct experimental outputs. Tests that were done using the spherical indenter show lower forces than the tests that were done using the flat indenter. Effects of temperature reveal that the warm tests show a greater tendency to ductile failure than cold tests having same parameters. The ice samples with smaller ice seeds need more force to fail compared to ice samples with bigger ice seeds. To observe the microstructural modification, horizontal and vertical thin-sections of the damaged ice adjacent to the indenter have been examined. Ice particles were collected from the testing area following each experiment to observe the influence of different factors on the particle size distributions. The effect of each variable on observed failure processes and associated loads are presented in the thesis.

Scaling of Flexural and Compressive Ice Failure

Physical model tests are a powerful means of obtaining solutions to a variety of engineering problems. The applications in hydraulics and aerospace engineering are prominent, where the use of similitude and dimensionless numbers is well developed. The first step is to understand the mechanics of the process. In the case of ice, the theory has not been developed to the same degree as in fluid mechanics. The use of scale models in test basins has often focused on resistance to ship motion and on flexural failure of the ice. This has been reasonably well addressed. The properties of the model ice have often been modified to permit scaling of flexural strength as well as elastic modulus to achieve appropriate behaviour.Extension of testing to situations where ice fails in compression or combined flexure and crushing leads to additional complication. At low rates of loading, ice creeps and also demonstrates enhanced rates of creep if the stress is sufficient to cause damage (microstructural change) in the ice. At higher rates of loading, fracture processes result in a localization of loading, and in the formation of high-pressure zones, which have their own special failure process.In the paper a review of scaled ice testing is given, with associated mechanics including flexural failure. This is followed by a discussion of the failure processes in compression and related mechanics such as creep, damage and fracture. Suggestions as to scaling of these processes are made. An important aspect that is considered is the randomness of ice loads as measured in the full scale. Modelling this aspect and determination of appropriate extreme values is discussed. The Weibull modulus is suggested as an appropriate parameter.Copyright

EXPERIMENTAL STUDY OF DYNAMICS DURING CRUSHING OF FRESHWATER TRUNCATED CONICAL ICE SPECIMENS

Ice crushing dynamics in ice structure interactions can result in hazardous vibrations and potentially damaging loads on offshore structures. Ice cone crushing experiments were conducted in the lab to characterize loading and dynamics processes for compressive failure. The indentation rate, temperature and shape of the ice specimens were varied in control tests so that the sensitivity of the resultant dynamic ice load frequency and amplitude could be determined. The results indicate that all control variables had a marked effect on both the frequency and amplitude of load fluctuations. Indentation rates varying from 0.1 mm/s to 10 mm/s and ice taper angles from 13° to 30° had drastic effects. The effects of temperature also demonstrated variations in force, pressure and dynamic behavior. In addition to load measurements, video was used to observe failure mechanisms and in particular spalling and crushing. In the present paper observations are described, though a thorough quantitative assessment has been published elsewhere. Tactile pressure sensors were also used in the experiments, allowing for the correlation of loads and processes to pressure distributions. Finally, the forensic examination of crushed specimens also provided insights into the behavior of ice under various compressive failure scenarios. On the surfaces of intact specimens and revealed within through cross-polarized views of thin sections were signs of ice damage and recrystallization zones of varying extents. The effects of the variables on the dynamic processes and failure behaviors are discussed.Copyright

Numerical Simulation of Ice Ridge Gouging

In coastal regions throughout the Arctic, the seabed is frequently scoured or gouged by sea ice ridges and icebergs. This presents an environmental hazard to pipelines or subsea infrastructures operating in the area and therefore a greater understanding of these processes is needed. This paper describes a three dimensional (3D), numerical model that was developed to simulate the failure behavior of a ridge keel as it interacts with the seabed. The simulation was conducted in Yade, an open-source code, which uses the Discrete Element Method (DEM) to model particle motions. The ice blocks in the ridge keel are modeled as spheres, which are initially bonded to contacting blocks via freeze-bonds. A Cohesive Frictional Model (CFM) which has cohesive bonds in tension and shear was used to simulate the freeze-bonds between ice blocks. In addition to normal and shear bonds, the model features springs which resist compression, shear, bending and torsion. Once the bonds are broken the material is assumed to behave like a Mohr-Coulomb material with a constant friction angle. Since the main focus of this paper is the failure behavior of the keel, the seabed is simplified as being rigid. Numerical simulation is compared with data collected from the Pipeline Ice risk Assessment and Mitigation (PIRAM) test program.Copyright