Hydrodynamics of High-Speed Ship Transit near Level Ice

Abstract

The annual average extent of Arctic sea ice has declined steadily in recent decades and the diminishing is expected to continue. Expansion of the duration and spatial extent of seasonal ice-free water will increase the accessibility of maritime activities in this polar region. Much of previous research for ship activities in the Arctic region has focused on ice-breaking operations that are limited to low speeds, where wave-making resistance is less important. Ships will seek to travel at higher speeds in the Arctic. Yet there is little research on high-speed ship transits in icy conditions. The problem of high-speed ship transit in the open water between large ice sheets (lead) is becoming an increasingly common scenario but has not been studied previously.

This dissertation presents a combined theoretical and numerical analysis of a ship traveling in a lead with the objective to understand how the ice sheets influence the ship hydrodynamics, and how the ice responds to the ship-generated waves. The first part of the analysis uses a mathematical model to evaluate the wave resistance in deep-water canals (or ice sheets of infinite thickness). The model is able to separate the contributions of the transverse and divergent waves to the total wave resistance. Significant influence of both the ship speed and canal width is observed to increase the wave resistance by as much as 129% or decrease by up to 82% relative to the open-water conditions.

The second part of the analysis uses high-resolution computational fluid dynamics (CFD) on a contemporary ship that is traveling between two rigid ice sheets of finite thickness. The CFD simulations identify the critical ice thickness that corresponds to the condition in which the ice sheets function nearly as canal walls. It is found that the effect on the wave resistance is noticeable when the ice is 5% of the fundamental wavelength, and when the ice sheets are thicker than 20% of the fundamental wavelength, the resistance change due to the sheet ice is nearly that of canal walls. CFD simulations are also performed for the same idealized canal from the first part, which demonstrate strong agreement in wave resistance with the theoretical analysis. The more accurate CFD confirms a 74% increase and 31% decrease of wave resistance in leads.

The last part of the analysis investigates the problem of a ship traveling in a lead between flexible ice sheets by using an adapted fluid-structure-interaction (FSI) solver. The FSI solver couples a moving CFD domain and a static ice sheet that is modeled as a thin elastic plate. The thin ice sheets are generally compliant with the water waves, where the ice deflections are found to correspond to the ship-generated waves. The maximum ice deflections always occur at the ice edge, which increases with ship speed and decreases with ice thickness. Overwash on top of the ice is found to be a significant phenomenon and greatly affects ice behavior. Higher overwash flux rates unexpectedly occur on thicker ice, which is explained by the smaller relative wave elevations for the thinner ice due to its compliance. Potential ice fractures are identified by using the Mohr-Coulomb yield criterion for sea ice, where both tensile and compressive failures are found for the higher ship speeds and most of them occur on the ice edge.