Balanced Design Approach for Compact Icebreaking Vessels
July 14, 2022
by Dr. Oscar Lisagor, P.Eng.
Work boats capable of operation at low temperatures have many design challenges; they must be efficient in ice and in open water. Multifunctional ice capable tugs have at least two prime applications. The first is icebreaking and ice management when creating an ice channel for shipping, keeping this channel open and clearing the berth from broken ice when docking ships. The second prime application is escort and ship assist in any environmental conditions.
The hull shape must have a balanced design that combines good seakeeping in open water and at the same time having effective icebreaking capability. A vessel with a hull shape designed for the best icebreaking performance usually does not have good seakeeping ability in open water and vice versa. Similarly, it is likely not optimum for developing high escort steering and braking forces. With a balanced design approach, compromises are therefore necessary, according to the defined priorities of the vessel.
Open water performance can be determined with acceptable accuracy by modern computational methods including CFD analysis. However, it is difficult to predict icebreaking capability by existing analytic methodologies applied to a compact vessel with specific proportions. Analytic methodologies mostly were developed for large icebreakers or cargo ships. There is insufficient experimental data to define analytic equation coefficients used to verify the accuracy of the calculations. The most accurate and reliable approach to determine icebreaking performance for compact vessels and tugs is therefore physical ice model tests.
Robert Allan Ltd. has performed extensive ice model tests to develop our TundRA and Beaufort series icebreaking hull shapes. The tested models had variations of hull shape, particularly at the bow, and variations of specific appendages such as reamers and ice deflectors. The models were self-propelled and test series were carried out as both towed propulsion tests and free running tests. These model tests have defined the icebreaking performance when breaking ice going ahead, breaking ice going astern, maneuvering capability – turning on a spot, ridge penetration by ramming, and performance in a broken ice channel. In addition to the data measurements, these model tests give an excellent opportunity for a visual assessment from above and below the vessel of ice flow pattern along the hull and ice interaction with propellers, shaft lines, shaft brackets, nozzles, and Z-drives.
The successful operation of the numerous TundRA and Beaufort class vessels constructed to date have shown that a balanced hull design approach with ice model tests has proven to be effective and optimal.
