Arctic Ice Capable Ships
Discovery of oil fields and natural gas in the artic has led to an increased interest in the development of ice-breaking cargo vessels and/or tankers for use in transporting these resources to refineries and consumers at remotely situated markets. The cargo and/or tanker ships must operate efficiently during the transportation of their cargo. In order to operate efficiently, they must maintain a satisfactory speed with a relatively low fuel consumption. In order to meet these efficiency requirements, conventional ship designs have been developed. Such conventional designs have a low value of ship-ice resistance per unit cargo capacity. Such conventional designs are generally characterized by a relatively large length-to-beam ratio, fine bow forms and long parallel middle-body sections. Such hull designs allow the ship to perform efficiently during normal travel through non-ice-covered waters, and to perform well during straight travel through ice-covered waters.
However, due to their relatively long parallel middle body sections, these conventional ships have poor maneuverability in ice-covered waters. The poor maneuverability of the conventional design has presented serious problems when attempting to turn these ships in order to change course in ice-covered waters to avoid objects, such as a major ice ridge or for maneuvering the ship into a docking facility. Accordingly, the poor maneuverability of such conventional designs within ice-covered waters deterimentally affects the safe operation and time required to effectively dock and position the vessel.
The progress of a conventional vessel through ice is dependent mainly on the thickness and type of ice; the thrust of the propeller or propellers; the shape of the hull, with particular emphasis on the forward section; and friction between the hull of the vessel and the ice. Should any of the above factors change or be changed then the vessel's performance would change. The ability of a vessel to steer, when operating in ice, is dependent principally on the thickness and characteristics of the ice; the shape of the bow section; the shape of the stern section; the thrust of the propeller and size of the rudder; and possibly of greatest influence, the length of parallel body (straight ship sides). So, although most vessels, other than ice breakers, have trouble in navigating in ice covered waterways, some have a lot more trouble than others, and this difficulty is proportionately increased with ship length.
In order to avoid safety hazards and attempt to minimize the transit time required to specifically maneuver the vessel, some ships have been designed to serve as the primary ice-breaking vessels. Such vessels escort the conventional cargo ships, clearing the path in front of the cargo ship. Such ice-breaking ships must have both a high maneuverability in the ice and cut a wide channel for the cargo vessel in which to follow. The necessary maneuverability, and ability to form a wide channel are made possible by providing such ice-breaking vessels with a stocky, rounded hull with a relatively low length-to-beam ratio, typically in the range of 4.0 to 5.5. The water plane-shape of this type of hull enables a certain degree of turning within the confines of the channel cut by the ship's beam. However, such a high beam-to-displacement ratio makes such a vessel configuration unsuitable as a cargo vessel. The high beam-to-displacement ratio results in a relatively high power requirement per unit cargo capacity which is moved. Furthermore, this high beam-to-displacement ratio results in an increased open water resistance per unit displacement. Therefore, such vessels do not travel efficiently through ice-covered or non-ice-covered waters.
Another design which has been developed in order to increase the maneuverability of a cargo ship in ice-covered waters includes a wide beam forward configuration. The object of the wide beam forward design is to cause the ship's bow to cut a sufficiently wide channel through the ice to allow a relatively narrow middle body and stern to swing outward to either side during a turning maneuver. This concept was embodied in a converted tanker SS Manhattan. While the wide beam forward design does provide a certain degree of improved turning capability in ice-covered waters, it suffers to some extent from the same effects as the stocky, rounded hull escort vessel discussed above. The wide beam forward configuration requires greater propulsion power per unit displacement in order to break through the ice than is required by an equivalent sized ship having a relatively high length-to-beam ratio. Therefore, although the wide beam forward configuration allows for greater maneuverability during turns in ice-covered waters, the design is inefficient for straight forward travel through ice-covered or non-ice-covered waters. The conventional fine hull shape with a long, parallel middle body section is a fuel efficient design. The fuel efficiency of this design is sacrificed to achieve improved maneuverability when the wide beam forward design is utilized.
When the ship is moving straight ahead, the ice is broken by its bow and the unbroken ice tends to hug the sides and develop considerable friction, impeding forward movement. The condition is complicated by the fact that the ice is often "uneven", as a result of channels having been broken and rebroken, with the pieces of ice thrown up into uneven mounds and refreezing in that form. This uneven structure increases the friction and resistance to movement. If straightline movement is difficult, the problem is compounded when the ship tries to turn. In making a turn, under the action of the rudder, the ship pivots about a point about a third of the way from the bow to the stern (this will vary somewhat depending on the design of the vessel and its draft forward as against its draft aft). Bow thrusters are sometimes used to move the bow laterally, but these tend to become fouled in ice and so are not usually employed for winter navigation.
The US Navy has not recently designed surface ships, other than ice breakers, to operate in the Arctic. The problems of ice damage and topside icing when surface ships were operated in high latitudes were handled on an ad hoc basis. From time to time during the design of a new class of surface ships, the issue of ice hardening has arisen. One example was during the design of the Perry (DDG-7) class guided missile frigates. While high latitude operations were envisioned, these ships were heavily cost constrained and the ice hardening characteristic was dropped from consideration during cost tradeoffs.
The US Navy and Coast Guard, however, have designed icebreakers, as have commercial interests. Other commercial ships have been designed for ice hardening. Most major classification societies who govern the details of commercial ship hull design have established rules for the design of ship hulls for operations in ice. The American Bureau of Shipping (ABS) would be the relevant classification society for US ship design. ABS Rules require strengthening of the bow and stern areas. Bow mounted sonar domes and arrays in particular would require careful attention. Propellers, rudders, fin stabilizers, and sea chests are also affected by ice operation. The effect of topside icing and a provision to de-ice must also be considered.
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