Model tests of iceberg towing

Icebergs may cause a threat to offshore installations, vessels and operations in a number of Arctic regions. In order to increase the understanding of what happens when an iceberg tow is started in ice covered waters; physical tank model tests have been carried out in various concentrations of sea ice. The objectives with these tests have been to evaluate the practical arrangements for iceberg towing and to collect data regarding tow loads and iceberg behaviour during the tow. The tank model tests were carried out in scale 1:40 in the ice tank at Hamburg Ship Model Basin (HSVA), Germany. Two different iceberg models were used and each towed in four different ice concentrations. From all tests, tow line forces, iceberg displacements and rotations were recorded. It was concluded that towing in 50% ice concentrations and higher were not realistic due to high resistance. During the tows in high concentrations, ice was breaking in flexural mode, crushing, rafting and ridging continuously in front of the iceberg models. With respect to the tow line, the line was fully extended and lifted up from the water/ice. In real operations this may increase the risk for tow line rupture and subsequent "snapping". In 50% ice concentration, total loads in the tow line will most of the time be lower than maximum bollard pull for powerful diesel electric icebreakers indicating that towing up to this concentration may be feasible. However, tow lines will have to resist even the highest peak loads during a tow and it is unclear whether sufficiently strong tow lines can be produced. With respect to tows in 20% concentration and open water, loads are significantly lower indicating that towing in low ice concentrations should be feasible. Measured loads seem to be reasonable well described by a log-normal distribution. The concentrations of surrounding sea ice are found to be most important for the load magnitude while variations in speed, acceleration, course and iceberg shape seem to be less important. A log-normal distribution, in which the parameters are functions of the sea ice concentration, has been fitted to recorded data. Combined with information regarding expected tow length, this distribution may be applied in order to provide crude estimate on extreme loads during an iceberg tow. By performing additional model tows in different ice conditions and with larger variations in iceberg size, this model may be further developed to be applicable in a wide range of scenarios. (C) 2009 Elsevier By. All rights reserved.