Claude Daley

The role of discrete failures in local ice loads

Abstract A conceptual model of ice failure is proposed. The model describes ice failure as a nested hierarchy of discrete failure events. In this model, each failure event changes the geometry of the problem, and thus, sets one of the important initial conditions of the next failure event. The ice load, along with other results of the ice–structure interaction, depends on how the sequence of failure events progresses. It is argued that the best method of understanding ice loads is to treat ice failure as a process of discrete failure events. The literature on discrete failure events in ice is reviewed. The experimental background and specific solutions using discrete failure processes are described. Specific attention is given to those local failure processes that take place at an ice–structure interface. The mechanics of pulverisation, showing possible alternative discrete mechanisms are presented. Finally, an event tree for local ice failure is given.

Hyper-Real-Time Ice Simulation and Modeling Using GPGPU

This paper describes the design of an efficient parallel implementation of an ice simulator that simulates the behaviour of a ship operating in pack ice. The main idea of the method is to treat ice as a set of discrete objects with very simple properties, and to model the system mechanics mainly as a set of discrete contact and failure events. In this way it becomes possible to parallelize the problem, so that a very large number of ice floes can be modeled. This approach is called the Ice Event Mechanics Modeling (IEMM) method which builds a system solution from a large set of discrete events occurring between a large set of discrete objects. The simulator is developed using the NVIDIA Compute Unified Device Architecture (CUDA). This paper also describes the execution of experiments to evaluate the performance of the simulator and to validate the numerical modeling of ship operations in pack ice. Our results show speed up of 11 times, reducing simulation time for a large ice field (9,801 floes) from over 2 hours to about 12 minutes.

Overload response of flatbar frames to ice loads

ABSTRACT The paper presents the results of a focused study into the capacity and overload response of a wide range of simple flatbar frames. The work addresses a number of issues of current concern for ice class vessel design. While ice class rules have no formal requirements for overload response, concern for safety often leads designers and regulators to evaluate the full range of response to ice. This study examines the elasto-plastic response of simple frames to ice-like loads. This study is preliminary in nature, in that it only examines one frame type in one loading pattern and only examines the load-deflection response. Future work would need to examine a fuller range of parameters and issues. Nevertheless, the investigation explores a critical aspect of ice class frame design and takes one step towards a more comprehensive approach. A novel simple approach to the load is employed, in the hopes of creating a consistent and effective way to assess overload capacity. Typically, such investigations either apply a simple load patch or they use a much more realistic but complex technique of modelling the ice material as a failing solid in contact with the structure. In contrast to these approaches, this study employs a rigid indenter as a way to create an ice-like load, while maintaining simplicity and allowing the structure to exhibit realistic behaviour. The study uses LS-DYNA®, but could have used any explicit time-stepping finite element (FE) code. The results of the study show, not unexpectedly, that the overload response consists of a sequence of local plastic mechanisms. The results show that there are alternative failure paths, that depend on the specific local failures, that themselves depend on structural dimensions and material parameters.

Ice Collision Forces Considering Structural Deformation

With increasing interest in resource exploitation and shipping in the Arctic, the focus on ice class ships and offshore structures is growing every year. In Arctic waters collision with ice is a major concern. The standard analytical ship-ice collision load model is based on the assumption that the ship structure is perfectly rigid body and the crushing energy of ice is equated to the available kinetic energy to calculate the ice load. This paper extends the standard approach by including the local plastic deformation. By considering the plastic work done to the structure in the balance of energy, a model is developed that can be used to help specify the levels of structural damage that may occur at various speeds. A simple unit side shell of ship structure is modeled to evaluate the absorbed energy and permanent deformation, with elasto-plastic response including linear strain hardening. A simple patch load is used. The purpose of this model is to provide a practical evaluation method of ice loads with the consideration of ship structure’s deformation. While there have been similar issues tackled numerically by several researchers, this work takes a more analytical approach, and will hopefully enable designers to more rapidly assess potential designs. Furthermore, this approach may provide a tool for regulation that is more related to actual risk levels and consequences. To illustrate the issues and practical application, the paper presents an assessment of the bow structure of a high ice class 150kT arctic tanker involved in an iceberg collision.Copyright

Evaluation of spatial pressure distribution during ice-structure interaction using pressure indicating film

Abstract Understanding of ‘spatial’ pressure distribution is required to determine design loads on local structures, such as plating and framing. However, obtaining a practical ‘spatial’ pressure distribution is a hard task due to the sensitivity of the data acquisition frequency and resolution. High-resolution Pessure-Idicating Flm (PIF) was applied to obtain pressure distribution and pressure magnitude using stepped crushing method. Different types of PIF were stacked at each test to creating a pressure distribution plot at specific time steps. Two different concepts of plotting ‘spatial’ pressure-area curve was introduced and evaluated. Diverse unit pixel size was chosen to investigate the effect of the resolution in data analysis. Activated area was not significantly affected by unit pixel size; however, total force was highly sensitive

GPU-Event-Mechanics Evaluation of Ice Impact Load Statistics

The paper explores the use of a GPU-Event-Mechanics (GEM) simulation to assess local ice loads on a vessel operating in pack ice. The methodology uses an event mechanics concept implemented using massively parallel programming on a GPU enabled workstation. The simulation domain contains hundreds of discrete and interacting ice floes. A simple vessel is modeled as it navigates through the domain. Each ship-ice collision is modeled, as is every ice-ice contact. Each ship-ice collision event is logged, along with all relevant ice and ship data. Thousands of collisions are logged as the vessel transits many tens of kilometers of ice pack. The GEM methodology allows the simulations to be performed much faster than real time. The resulting impact load statistics are qualitatively evaluated and compared to published field data. The analysis provides insight into the nature of loads in pack ice. The work is part of a large research project at Memorial University called STePS2 (Sustainable Technology for Polar Ships and Structures). Introduction Ice class vessels are unique in a number of ways in comparison to non-ice class vessels. Hull strength, power, hull form and winterization aspects are all issues that raise special challenges in the design of ice class ships. This paper focuses on matters of local ice loads which pertain to hull strength in ice class vessels. More specifically, the paper examines the parametric causes of local ice loads and statistics that result as a ship transits through open pack ice. The issue of pack ice transit is of interest to those wishing to operate safely in such conditions. One key question is that of safe operational speeds. Consider the special case of open pack ice, where floes are relatively small, numerous and resting in calm water. A vessel moving through such an ice cover would experience a series of discrete collisions. As long as a vessel moved very slowly, the loads would be very low. In such a case the vessel could make safe and steady progress, even if it had a relatively low ice class. However, if the vessel attempted to operate more aggressively, impact speeds would increase and a higher ice class would be needed for safe operations. The investigation below provides some insight into the factors that influence the loads in this situation. These factors include hull form, speed, floe size and concentration, ice thickness, strength and edge shape. Most prior studies have tended to focus on ice thickness and strength as the primary determinants of load. This study shows that ice edge shape and mass, along with hull form and locations are also strong determinants of loads, and especially the load statistics. The simulations provide some interesting data, especially when compared to field trials data. A related focus for the study is to explore the use of the GPU-Event-Mechanics (GEM) simulation approach. The GEM approach represents the integration of a number of concepts. The physical space is described as a set of bodies. The movement (kinematics) of the bodies is tracked using simple equations of motion. Time is divided into relatively long ‘moments’, during which events occur. All variables in the simulation; forces, movements, fractures and other changes, are considered to be aspects of events. Some events are momentary, while others are continuing. Some events involve a single body and are termed solo events. Motion, for example, is treated as a solo event. Some events are two-body events. Impact is an example of a two-body event. The GEM approach lends itself to parallel implementation, which in this case is accomplished in a GPU environment. A GPU (Graphics Processing Unit) is a common element found in modern computer graphics cards. The GPU is primarily intended for making rapid calculations associated with the display. However, special software can access the GPU and enhance the computing power available to the user. See (Daley et.al. 2012) for further discussion of GPUs. The event models are the analytical solutions of specific scenarios. As a result, the events do not require solution (in the numerical sense) during the GEM simulation. The

Direct Design of Large Ice Class Ships With Emphasis on the Midbody Ice Belt

The future development of oil and gas reserves in remote Polar Regions areas will require a new generation of highly ice-capable vessels. Many may need to be capable of operating at all times of the year. These ships will need to be able to travel faster in heavy ice than all but the largest icebreakers, which poses challenges for both hull and machinery design. The American Bureau of Shipping (ABS), BMT Fleet Technology Limited (BMT) and Hyundai Heavy Industries (HHI) are currently undertaking a joint project aimed at addressing these design challenges. Because of the unique and innovative aspects of large fast ships for Polar ice development, new methodologies for direct calculation of loads on all areas of the hull are needed. The project is also addressing the need for new techniques for the analysis of the outer hull, double hull and gas containment systems of these ships under design and accidental loads; areas in which ‘rule design’ can only provide a starting point. This paper focuses on the midbody ice loads that may results from both ice pressures and from glancing collisions in the midbody area. The paper highlights some of the challenges of direct design.© 2008 ASME

Cone ice crushing tests and simulations associated with various yield and fracture criteria

ABSTRACT To determine the most reliable numerical simulation parameters affecting the ice resistances, such as yield functions with relevant ice-breaking criteria, compressive volumetric hardening intensities and element sizes, numerical simulations for different parameters were compared with the crushing tests of cone-shaped ice specimens. The yield functions of crushable foam, Drucker–Prager and Mohr–Coulomb were taken into consideration with three different volumetric hardening intensities. The ice resistance variation was also studied for two finite element models with different element sizes. To realize ice breaking in numerical simulations, a damage variable was introduced to trigger fracture initiation and evolution. The nominal contact area-dependent energy per unit area as a damage to fracture was newly proposed from the test results composed of a crushing force versus indenter stroke. This new concept could be realized by applying the different nominal contact area-dependent fracture energies to finite elements along the vertical location.

Ice Loads in Dry and Submerged Conditions

This paper describes a study of the effects of submergence on ice crushing loads. Thirty three small scale indentation tests have been performed on cone-shaped ice samples in dry and submerged conditions using a material testing system (MTS machine) located in a cold room at −7°C. The indenter was a flat aluminum plate at the bottom of a container that was attached to the actuator of the MTS machine. Transparent windows facilitated visual observations and recordings using a high speed camera. In the submerged tests the container was partly filled with salt water. Testing was performed at rates of 1 mm/s and 100 mm/s. The specimens were ice cones with 25 cm in diameter and with 20° and 30° angles. Data recordings comprised time-penetration and time-force histories. Generally higher forces were obtained in submerged tests. Furthermore, the difference between dry and submerged condition was more pronounced at the high indentation rate.Copyright

Guidelines for the nonlinear finite element analysis of hull response to moving loads on ships and offshore structures

ABSTRACT The structural hull response of a ship or offshore structure to moving (or sliding) loads has been shown to be significantly different than that of stationary loads of the same magnitude; when those loads cause plastic damage. A standard hull grillage structures capacity to resist a moving load may be as little as half its capacity to resist a similar stationary load. Real hull structures most often experience operational loads in a way better modelled as moving loads; particularly for the case of operational ice loads. Many accidental loads are also moving loads. This paper provides guidelines for the nonlinear finite element analysis (FEA) of moving loads on hull structures, where the moving load is not expected to induce hull puncture or subsequent tearing of the hull plating.