How Reliable Are Numerical Simulation Models to Predict Ship Performance in Lake Ice?


Introduction

For the design of inland waterway ships capable of navigation through ice-covered area, designers need to estimate the resistance and speed of a ship in ice since these are important performance indices. The most reliable way for this aim is to carry out model tests where ships in reduced scale are tested in model test basin with artificial ice. However, model test is expensive and time consuming, thus not applicable at early design stage when the hull form needs to be determined. An alternative approach is to carry out numerical simulation, which simulates the physical icebreaking process and provides the desired outputs with a computer. The marine technology group of Aalto University has developed a numerical simulation program which is capable of simulating ship motion in ice and predicts ice resistance and attainable speed. In INFUTURE (Future Potential for Inland Waterways) project, the program has been deployed to predict the performance of a ship concept developed through this project. The results are compared with model scale test results of the ship which was carried out at Aalto Ice Tank.


Numerical Simulation Program ASOIS

The Aalto Ship Operation in Ice Simulator (ASOIS) is an in-house simulation software which helps ship designers to assess ship performance at early design stage in order to optimise the hull form and achieve the desired ice-going capability. The program is developed based on extensive ship-ice interaction modelling, which has led to a doctoral thesis at Aalto University. The program can simulate various ship operations such as going straight ahead or turning in ice. Figure 1 presents an example of the user interface of the program. Researchers from Aalto University have tested this program with a polar supply and research vessel S.A. Agulhas II which conducted its full-scale ice trial on the Baltic Sea in 2012. Good agreement on the turning performance of the ship has been reached between the simulation results and full-scale test.

Figure 1. User interface of ASOIS


Simulation of INFUTURE Ship Concepts in Lake Ice

One of the tasks of INFUTURE project is to develop a vessel concept for cargo transportation between Finnish Saimaa and Russian Volgo-Balt regions. The concept is a 3560DWT double-acting general cargo ship which operates bow ahead in open water and stern ahead in ice, see Figure 2. The main specifications of this ship concept are summarised in Table 1. The ship is designed to break 0.6m level ice at the speed of 2knots.

Table 1. Main specifications of INFUTURE ship concept


Figure 2. Ship concept developed by INFUTURE project


Parameters of the ship are established in ASOIS. Two different ice thicknesses, 40cm and 60cm, have been used to represent Saimaa lake ice thickness in typical winter and severe winter. The flexural strength of ice in the first set of simulations is set to be 800kPa, which is stronger than Baltic Sea ice because lake ice is formed from freshwater. The flexural strength of ice in the second set is then set to be 500kPa to represent Baltic Sea ice. Simulations are carried out at different ship speeds. Figure 3 presents a snapshot of the simulation program which shows the contact between the ship and ice sheet. The time history of icebreaking resistance is also shown in this figure.


Figure 3. Snapshot of the simulation and time history of ice breaking force


According to the predicted ice resistance and estimated thrust of the ship with ASOIS, the attainable speed in 40cm thick ice and maximum ice thickness the ship can break at 2knots speed can be calculated. According to the simulation, the ship can sail at 4kn speed in 40cm ice and breaks 0.6m level ice in lake ice.


Compare Numerical Simulation with Model Tests

To validate the simulation results and draw solid conclusions on the performance of the project ship, model scale tests have been implemented at Aalto Ice Tank, see Figure 4. The model ship was towed through artificial ice, which was made by spraying cold water onto the water surface to the target thickness and strength by controlling the spraying time and room temperature. The targeting thicknesses corresponds to 40cm and 60cm ice in full-scale, which are the same as the settings in ASOIS. By measuring the towing force, ice resistance can be extracted and then used for further analysis.


Table 2 and 3 compares the attainable speed in 40cm thick ice and icebreaking capability at 2knots speed based on model test and numerical simulation results. The resistance measured from model test is larger than numerical simulation, therefore the attainable speed and icebreaking capability is lower than predicted. Model test results indicate that the ship can break 0.6m sea ice or 0.5m lake ice at the speed of 2knots, while simulation results indicate 0.7m and 0.6m respectively. The different is about 1kn in attainable speed and 0.1m in icebreaking capability, which is in the acceptable range for engineering purpose. This indicates that the simulation program ASOIS can fulfil the requirement as a supplementary design tool to optimise ship hull form before model test is carried out.

Table 2. Comparison of attainable speed in 40cm thick ice

Table 3. Comparison of icebreaking capability at 2knots

Figure 4. Images of the 3560 DWT ship in Aalto ice tank.


Conclusions

Based on the comparison, it can be concluded that the numerical simulation program ASOIS developed at Aalto University can serve as a supplementary design tool which helps designers to choose the best hull form in early design stage. Model test is still necessary in later design stage to validate the concept and confirm the required engine power to be installed on the ship.



Authors: Professor Pentti Kujala and Li Fang, Postdoctoral Researcher, Aalto University, Helsinki, Finland.


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This study has been conducted in the frames of the INFUTURE, Future Potential of Inland Waterways project, funded from the South-East Finland – Russia CBC 2014–2020 programme. The total budget for the project is EUR 1,251,538.