Dynamics of Continuous-Mode Icebreaking by a Polar-Class Hull—Part 2: Spectral Analysis

Abstract

Part 2 of this two-part paper presents additional results from an ice towing-tank study of a Polar-Class hull for two conditions: for one, the hull was free to pitch, heave, and undergo limited roll (free hull); for the other, the hull was restrained from motions (fixed hull). Measurements were made of resistance as well as motions and restraining forces and moments for the free-and fixed-hull conditions, respectively. In Part 1, the values of the mean and standard deviation for both conditions are compared and related to observed patterns of icebreaking. In Part 2, the spectral densities are examined with regard to the natural frequencies of hull motions and frequencies associated with icebreaking pattern to determine the dominant cycles of resistance and motions. A series of experiments was conducted to determine values of zero-speed natural frequencies and added-mass and damping coefficients for ice-covered conditions. The presence of an ice sheet significantly reduces the natural frequencies and increases the added-mass and damping coefficients. It was found that hull motions and icebreaking pattern determined the dominant cycles of resistance for the free hull. Icebreaking frequency, the frequency of individual breaking events, was significant only for thin ice such that significant heave and pitch did not occur. When w b was less than the natural frequencies of coupled heave and pitch amid ice, w the dominant frequency of resistance and motions occurred at integral fractions of usually b/2. The frequencies, (b/n, are associated with the icebreaking pattern, that is, cyclic manner by which hull trim changed as the hull broke and cleared a track through an ice sheet. When b equaled or exceeded to„, the dominant frequency of resistance coincided with The fixed hull responded at higher frequencies than the free hull, usually at u>0. Also, available full-scale data are shown to be in reasonable agreement with the present results for the free hull. Lastly, the principal conclusions of the overall study are summarized and the implications of the present work with regard to the development of prediction methods are discussed.