Requirements for Ice Performance of Large Ships with Shallow Draft

(By Loly Tsoy; INSROP Working Paper No. 30 – 1996, I.7.2)

An investigation was carried out on the effect of shallow draft and shallow water on ice performance of large ice ships. The necessity to construct such ships is due to the perspectives for shipping in the Arctic and in particular the necessity of transportation by sea of hydrocarbons from Arctic shelf areas.

A distinctive feature of the Russian Arctic is the vast shallow area along the Northern Sea Route which necessitates strict draft requirements for ships. To increase the cargo-carrying capacity of ships intended for the export of raw materials from the Arctic, non-traditional ratios of their dimensions will be required which may unfavourably affect the ship's icebreaking capability. At the same time no special investigations have yet been made to explore the effect of the shallow water and of a large breadth/draft ratio on the ice performance of ships.

In accordance with the task, the investigations were carried out in two directions. Consideration was given to the effect of shallow draft on the hull shape, value of efficiently processed power and accordingly on the icebreaking capability. The influence of shallow water (small depth clearance) on ice propulsion of ships was also studied. The principal results of the performed work may be summarized as follows.

In the case of large ships with restricted draft one may expect an increase in the breadth/draft ratio up to 4-5. Though the increase of a ship's breadth leads to more ice resistance, a large breadth/draft ratio favourably affects the traditional icebreaking shape of lines of the ship's forebody. This is associated with the increase of bow frame flare angles and the water line entrance angle. As model tests in the ice basin and also experience with the construction of shallow draft icebreakers have shown, the adverse effect of the ship's breadth on the icebreaking capability may be compensated by the favourable, from the point of view of ice propulsion, influence of a small draft on the hull shape at large breadth/draft ratios.

The most serious difficulties associated with the provision of the required icebreaking capability of shallow draft ships are to be expected relative to the rigid restriction of power which can be efficiently processed (without cavitation and aeration) at a small draft. Investigations have brought to light a substantial dependence of power on draft and accordingly on propeller diameter. Therefore the requirement for high power of large ships in active ice navigation with shallow draft, predetermines the application of multi-shaft propulsion plants.

The use on ships for Arctic navigation of non-traditional hull lines with the purpose of the reduction of required power (for instance, of conical lines as proposed by the company "Wartsila Marine" or of lines of the Thyssen-Waas system), as the experience with the conversion of domestic icebreakers has shown, does not seem promising.

As calculations show, the minimum safe, in a navigational respect, under-keel clearance of ship may be 1.5-2.0 m. This circumstance should be borne in mind while ensuring operational safety of large ships with a restricted draft in the process of their design. In particular, special attention should be paid to the design of the stern tube and of the power plant sea water cooling system. Taking into account higher ice damageability of bottom and bilge areas of the ship's hull under conditions of shallow water it is also necessary to take measures to ensure structural safety of shallow draft arctic navigation ships.

In accordance with theoretical studies the propeller thrust drops during sailing in shallow water. Calculations show that this drop is not significant, however. The deterioration of the icebreaking capability of icebreakers in shallow water is apparently to a greater extent associated with the increase of ice resistance to the ship's motion under conditions of small under-keel clearance, especially when this clearance is commensurable with the thickness of ice to be broken through.

As full-scale tests of icebreakers in water depths close to minimum admissible ones from the point of view of the safety of navigation have shown, the actual deterioration of the icebreaking capability in these conditions did not exceed 10-15%. Therefore the problem of deterioration of the icebreaking capability in shallow water is hardly of practical interest, considering the operation of icebreakers in limited depths is of episodical character.

Thus, on the basis of the investigations carried out one may come to the principal conclusion that as far as the provision of ice propulsion is concerned, the main problem is ensuring the efficient processing of high power when constructing large shallow draft ships. An expected small reduction of the icebreaking capability during the sailing in shallow water can scarcely be considered among the main problems requiring a solution when designing ships with restricted draft.

Present investigations have corroborated the feasibility in principle of the construction of large shallow draft ships for the Arctic. The main attention here should be paid to problems of the guarantee of ice strength of hull and reliable action of the screw-rudder system, stern tubes and sea water power plant cooling system.

Materials from the Project I.7.2 may be used for developing recommendations on the effect of draft restrictions upon the ice performance of shallow-draft ships in their design as well as for the evaluation of economic efficiency to use large shallow-draft ships in the Arctic.

Loly Tsoy, 1996, Requirements for Ice Performance of Large Ships with Shallow Draft, INSROP.©