Saimaa is an important inland waterway area for Finland, as industry products are exported through waterways from different parts of Saimaa. For vessels sailing through this area during wintertime, icebreaking is needed to be able to go through the frozen waterway. Ice thickness and strength properties are the major parameters affecting the resistance vessels encounter; therefore they are important design parameters for ship design. Unlike Baltic Sea ice whose properties have been extensively measured and studied, flexural strengths in the Saimaa area are scarcely present in the literature. To fill the gap and contribute in ship design, researchers from Aalto University carried out measurement campaign in the Saimaa region during winter 2021.
The measurement campaign includes three expeditions, the first from 25 January to 1 February, the second from 22 to 26 in February and the third from 22 to 26 in March. This covers a relatively large time span so that the seasonal variation in ice properties can be revealed. Three different areas are covered in the campaign, including the southern Saimaa around Imatra, Northeast Saimaa around Joensuu and Northwest Saimaa around Kuopio. Figure 1 shows the locations where measurement have been conducted. The measurement sites are chosen to be close to actual waterway, so that the measurements are representative to ice which vessels meet.
Figure 1. Measurement locations
The content of measurement program includes:
1. Ice thickness, temperature and density measurement
2. Ice flexural strength measured by different techniques
3. Ice grain structure and contaminants
Figure 2 shows two different measurement devices for flexural strength test. The devices break the ice by bending and measure the force and displacement to calculate flexural strength. Figure 3 presents an example of the ice structure. The photo is taken on a thin layer of ice placed between polarisers, different colors showing different ice grains.
Figure 2. Measurement of ice flexural strength by cantilever beam test (left) and three-point bending test (right)
Figure 3. Three typical grain structures observed from different ice samples
The measurement from the first voyage has been processed and summarised in a conference paper, which has been accepted by the 26th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC2021). Measurements from the other two voyages are under processing. The plan is to summarise the scientific and engineering findings of the three voyages in a journal paper.
During the first voyage, ice thickness in different locations vary from 14cm to 37cm. The density varied from 884 to 914 kg/m3 the average being 890 kg/m3. The ice and water look clear in the southern measurement sites but are brownish and have clear impurities in most of the northern sites. The average flexural strength across all the measurement sites is 593.4kPa by cantilever beam test and 783.0kPa by three-point bending test. As a reference, sea ice is formed by saline water and the flexural strength is usually below 500kPa by three-point bending test. Lake ice, as shown by the measurement, is clearly stronger since it is formed by freshwater ice.
Three measurement expeditions have been carried out by the Aalto research team as a part of INFUTURE project. The results and findings have both engineering value for the aim of ship design and scientific value for better understanding of freshwater ice mechanics.
Author and Photos: Fang Li, Postdoctoral Researcher, Marine Technology unit, Aalto University