Icebreakers – escorts in ice-covered Canadian waters
According to the International Maritime Organization (IMO), an icebreaker specifically refers to “… any ship whose operational profile may include escort or ice management functions, whose powering and dimensions allow it to undertake aggressive operations in ice-covered waters” (International Maritime Organization, 2010). Expanding from Canada’s proximity to the north, icebreakers represent an important subdivision of the Canadian Coast Guard, performing icebreaking services through a host of fundamental roles and supporting economic and commercial development:
Each year, from late June to Early November… icebreakers are deployed… to the Arctic, where they perform a broad range of important tasks, such as icebreaking, search and rescue, the placing of navigational aids, and vessel support… (Standing Senate Committee on Fisheries and Oceans, 2009)
Historically, the use of icebreakers in Canada was born out of Governor General Earl Grey’s desire to construct a terminal harbour for the Hudson Bay Route, a project that began in 1910 and “intended to provide short rail transport to seaboard… ” (Fraser, 1963). Following the Governor General’s tour of the proposed site in 1910 aboard the Canadian Government Ship Earl Grey, appropriately regarded as the “first Canadian ice-fighting machine”, the first Canadian icebreakers to be sent north for service, the Stanley and the Minto, built in 1888 and 1899, respectively, commenced the inaugural charting of northern waters charged with ice conditions (Fraser, 1963); this endeavor continued for several more seasons before the outbreak of the Great War disrupted the northern activities. Up until the onset of the Great War, however, the icebreakers Stanley and Minto had limited icebreaking capabilities, and were often stymied by relatively light ice, as well as the more dangerous ice fields encountered in the Hudson Strait (Fraser, 1963).
Contemporary icebreakers operate as escorts in ice-covered waters, specifically referring to “… polar waters where local ice conditions present a structural risk to a ship” (International Maritime Organization, 2010). Operations from an icebreakers perspective within Canada include providing an escort to Canadian Naval and Coast Guard vessels and ensuring a Canadian presence in the Northwest Passage; as sea ice in the circumpolar region continues to recede through a cycle known as the ice-albedo feedback loop, whereby retreating sea ice exposes darker, heat-absorbing seawater, navigation shortcuts will subsequently open earlier each year to regular shipping. The effects of the ice-albedo feedback loop have generated considerable political dialogue regarding the future of the Northwest Passage; will the disputed waterways become open to international shipping, or will Canada affirm sovereignty?
The range of contemporary icebreakers in Canada extends 60 degrees north of the Canadian Arctic Archipelago, further including the southern scope of Ungava Bay, Hudson Bay, and James Bay (Standing Senate Committee on Fisheries and Oceans, 2009). Concerning the microcosm of challenges faced in the circumpolar region, the Canadian Coast Guard’s largest geographical area of operation, the suitability for materials must be acknowledged when addressing the challenge of strengthening, manoeuverability and propeller-ice interaction considering the anticipated temperatures (International Maritime Organization, 2010).
Considerations in structure, hullform and propulsion
In the practice of contemporary icebreaker design, “… the strength requirement of icebreakers is strongly influenced by ship geometry and size… ” (Johnston, 1994). Canadian icebreakers are activated seasonally acknowledging appropriate polar class designations and structural requirements, the latter encompassing: shell plating, framing, web frames, stiffened plates, as well as stem and stem frames. Longitudinally, hull areas are subdivided into four sectors: bow, bow intermediate, mid-body and stern; all but the former are further divided as bottom, lower and ice belt regions, respectively (Abraham, 2008). Specific to the function of icebreakers, ice classes are employed by the International Association of Classification Societies (IACS), also known as polar classes (PC); ranging from PC1 to PC7, ice classes acknowledge the wavering operational seasons of icebreakers, depending on their structure and framing. PC1 represent vessels that are operational year-round and PC7 represent vessels with reduced operational periods, usually during the late summer months when the ice is thinnest.
|Polar Class||Ice Description|
|PC1||Year-round operation in all polar waters.|
|PC2||Year-round operation in moderate multi-year ice conditions.|
|PC3||Year-round operation in second-year ice which may include multi-year
|PC4||Year-round operation in thick first-year ice which may include old
|PC5||Year-round operation in medium first-year ice which may include
old ice inclusions.
|PC6||Summer/autumn operation in medium first-year ice which may
include old ice inclusions.
|PC7||Summer/autumn operation in thin first-year ice which may include
old ice inclusions.
(International Association of Classification Societies, 2011)
Between the mid-1970’s and late-1980’s, numerous bow forms had been developed, the most frequented designs being the “pontoon” type bow form which “reduces ice resistance by the use of a low stem angle and a rectangular section, [exploiting] the weakness of ice in shear and bending… ” and the Thyssen/Waas bow which employ “side runners to assist in shearing”; the major limitations of the latter include poor seakeeping properties as well as poor open water performance (Paterson, 1989). The advent of the S-bow reduced ice resistance by shearing, but required offsetting the icebreakers ability to maintain performance over a given route charged with ice conditions; despite maintaining an upward-acting icebreaking bow, as achieved in contemporary structural designs, testing concluded that it was recommended that the S-bow should not be used for PC1 vessels, or vessels that would be relied upon for “heavy” icebreaking (Paterson, 1989).
An upward-acting icebreaking bow offers certain advantages over conventional, downward-acting icebreaking designs. These advantages, designated “operational” advantages, may not affect ice resistance directly, but would affect the utility or overall efficiency of a vessel. As the roles of icebreaking vessels become more diverse, these advantages can be expected to receive a higher priority (Paterson, 1989)
The presence of a bulbous bow, referred to as an “ice bow”, is particularly ineffectual in breaking ice. Although appropriate in functioning on an ice strengthened vessel, the performance of a bulbous bow displaces broken ice along an icebreakers hull, resembling the hydrodynamic flow; icebreakers must break free of the ice, therefore a bulbous bow is not necessary (Riska, 2010). Due to the effects of ice resistance, hullform and propulsion considerations are intimately connected in the design process and performance of an icebreaker:
Due to their increased manoeuvrability, ships having propulsion arrangements with azimuthing thruster(s) or “podded” propellers shall have specially considered Stern Icebelt (Si) and Stern Lower (Sl) hull area factors (International Association of Classification Societies, 2011)
Performance is determined by manoeuverability in ice; reducing propeller-ice interaction is decisively offset through the design of the hullform, specifically by the employment of a bow plough. The bow plough is specifically designed to redirect submerged ice floes, allowing broken ice to float to the surface (Riska, 2010). Principally acting to protect the vessels propellers from wandering ice through the redirection of submerged ice floes, the bow plough is a major aspect of both hullform and propulsion considerations. The performance of the bow plough protects the vessels propellers from wandering ice, preventing propeller-ice interaction and a decrease in propulsion efficiency; propeller-ice interaction can cause severe damage to a vessels propeller blades; therefore, preventing propeller-ice interaction prevents a decrease in propulsion efficiency.
Furthering the relationship between hullform and propulsion considerations, the use of a bow propeller decreased ice resistance through decreasing breaking resistance, “reducing the forces required to break ice”, therefore reducing friction (Riska, 2010). The bow propeller would become obsolete, however, since replaced by the advent of controllable pitch propellers with heightened manoeuvring capabilities; known for strengthened controllable pitch propellers, azimuthing thrusters, or “podded” propellers, enhance manoeuvrability as “thrust can be directed in any direction”, discharging icebreakers from ice (Riska, 2010).
Looking to the future
Against recent developments made in icebreaker performance, several aspects of the design process remain controversial, specifically the effects of ice interaction; the current approach used with ice interaction is a microcosm of different design methods which “do not have a single methodological background” synonymous to hydrodynamic design (Riska, 2010).
The framework of an icebreaker is surrounded by wavering data, specifically the unknown effects of ice interaction. To determine the effects of ice interaction, data is evaluated and incorporated into specific formulas considering the mission profile of the vessel (Brown, 1993). The wavering interactions of ice loads, ice resistance, propeller-ice interaction, and ice pressure represent large-scale unknowns in the design process (Riska, 2010); with the performance of an icebreaker dependent on the evaluation of such large-scale unknowns, opportunities for further development in the design process remain untouched. The author of this report contends that with such opportunities, coupled with the effects of the ice-albedo feedback loop and heightened levels of economic interest in the Canadian Arctic Archipelago, icebreakers will experience a period of enhanced design research and an expanding role in Canada.
Abraham, Jacob. (2008). Plastic response of ship structure subject to ice loading. (Masters dissertation). Memorial University of Newfoundland, St. John’s, NL.
Brown, Peter Wilfred. (1993). Optimal design of bow plating ships operating in ice. (Masters dissertation). Memorial University of Newfoundland, St. John’s, NL.
Johnston, Michelle. (1994). Variation of local pressures during ice-structure interaction. (Masters dissertation). Memorial University of Newfoundland, St. John’s, NL.
Fraser, R. J. (1963). Commentary: Early Canadian Icebreakers. Arctic, 16(1), 2-7.
International Association of Classification Societies. (2011). Requirements concerning polar class. London: International Association of Classification Societies.
International Maritime Organization. (2010). Guidelines for ships operating in polar waters. London: International Maritime Organization.
Paterson, Robert Bruce. (1989). Toward the development of an effective upward-acting icebreaking bow: The preliminary design and testing of the S-bow. (Masters dissertation). Memorial University of Newfoundland, St. John’s, NL.
Riska, Kaj. (2010). Design of ice breaking ships. Oxford, UK: Encyclopedia of Life Support Systems.
Standing Committee on Fisheries and Oceans. (2009). Controlling Canada’s Arctic Waters: Role of the Canadian Coast Guard. Canada, Senate Committee Reports.