Iceberg towing

Trials in the Barents and Kara seas

Map of iceberg towings

The possibility of changing an iceberg’s drift trajectory in order to prevent its collision with offshore facilities is an important factor for ensuring safety in polar regions. This approach is well-known and has been used for several decades by energy companies operating offshore Newfoundland and Labrador. Timely detection and towing of such giant and unsymmetrical objects as icebergs is associated with a number of specific features.

Correct understanding of this process requires field tests and proper data analysis.

In order to test the technology of iceberg management under conditions of the Russian Arctic we performed field iceberg experiments with various technical means in the autumn periods for several years. Experiments were carried out in the vicinity of the archipelagoes Novaya Zemlya, Severnaya Zemlya, and Franz Josef Land with different diesel icebreakers, equipped with all necessary deck equipment.

During these field trials, the following types of physical impact on icebergs of different sizes were implemented in a variety of weather conditions:

  • iceberg towing using a floating rope
  • iceberg towing using an ice net
  • directed water jet from vessel’s fire monitor
  • vessel’s circulation around iceberg

Two towing systems were used — floating synthetic rope and ice net. The synthetic rope was used for towing relatively big icebergs with the largest linear size of over 50 m. The net was used for small icebergs and debris in a size range from 8 to 50 m. In both cases tow load did not exceed the maximum rope loading of 115 tons. Typically, the working load varied up to 100 tons, the working range of iceberg towing loads in Canada.

Most of the experiments were performed using the floating rope towing system, which is proved to be the most effective, applicable, and flexible method of physical impact on an iceberg.

Each of the iceberg towing experiments started with a determination of the initial parameters of an iceberg’s drift with the help of the ice radar. The second step was installation of underwater equipment for measurements of sea currents and waves. The third step was above water (helicopter stereo photography or unmanned aerial vehicle) and underwater (multi-beam echo sounder from a survey boat) iceberg surveys, resulting in a 3D model of the object.

Several drifting satellite and/or radio buoys were also installed on the iceberg in order to monitor its drift and rotation.

Towing was carried out for icebergs of various shapes and sizes in a wide range of environmental conditions:

  • maximum mass of the towed iceberg was 1.1 million tons
  • maximum iceberg movement per day was 50 miles
  • low visibility when maneuvering of 300 meters
  • maximum wind speed during towing was 20 m/s
Iceberg in ice towing

Iceberg towing in ice conditions

We also performed iceberg towing experiments in ice conditions, that are unique and first in the world in their real-scale tests with monitoring of all parameters.

Numerical simulations and ice basin modeling showed a fundamental possibility of iceberg towing in ice conditions, as well as its significant difficulties. The lack of information about the actual behavior of the iceberg and towing system under the influence of ice cover seriously complicated interpretations of the modelling results. These experiments of iceberg towing under conditions of newly formed ice helped to understand ice effects and thus provided unique data for further research.

The choice of a maneuverable and powerful diesel icebreaker made it possible to tow icebergs in a wide range of ice conditions, including breaking out icebergs frozen in ice fields. The vessel was equipped with the necessary deck equipment: towing and mooring winches, and hydraulic cranes of various capacities located in the fore and aft parts of the vessel.

Operating conditions of the experiments allowed to determine the icebergs towing force on speed dependence, and identify the technological aspects of towing in ice.

The described trials allowed us to determine extreme ice conditions when iceberg towing is possible with the vessels used for towing in open water. Based on experiments results, optimal tactics for towing icebergs of different sizes under conditions of early ice formation with the means of icebreaking fleet were proposed. The studied aspects are important for an extension of the safe operation season in the Arctic. The experiments confirmed the possibility of iceberg towing in ice fields 10-15 cm thick (with concentration up to 10), which corresponds to 2 weeks of ice formation in the Kara and Laptev seas. Results can be used for development of new and update of existing standards of ice management system, as well as for effective and safe development of the Arctic shelf.

Physical modeling

In order to determine the hydrodynamic characteristics of icebergs besides numerical modeling, we carried physical modeling in a wind tunnel and in a towing tank.

Iceberg in wave basin

Wave basin

The purpose of the tests was to experimentally determine the positional components of the iceberg’s hydrodynamic characteristics, as well as to determine the average component of the additional towing resistance when icebergs move in waves. The position of the towing angles coincided with the drift angles used during the experiment in the wind tunnel

Experiments were carried out in the towing tank of 35.7 m length and 5.5 m width, with the maximum water depth of 2.5 m. To study the parameters of the resistance of the models in the towing tank, a gravity system and a system of the Froude type were provided. To create waves, a plunger wave generator was used – the plunger makes reciprocating vertical movements which causes wave generation. By adjusting the amplitude and frequency of oscillations of the plunger, it made possible to obtain a system of waves of different lengths, heights and frequencies.

As a result of the work, the directions of towing icebergs were determined, corresponding to the least resistance, as well as zero value regions of the torque. Derivative of the torque coefficient, inertial characteristics of translational and rotational motions were obtained. It was shown that the amplitudes of pulsations of the additional added waves resistance have a quadratic dependency on the dimensionless wavelength.

Instantaneous trajectories of motion of fluid particles and vector fields in various planes were constructed. The results of numerical modeling, both qualitatively and quantitatively, are in good agreement with physical modeling. An analysis of the numerical values of the hydrodynamic characteristics allowed us to conclude that when towing an iceberg, the main contribution to the resistance is made by the positional characteristics determined for translational motion at constant values of velocity and acceleration.

Iceberg model in air tube

Wind-tunnel

The purpose of the tests was to determine the positional components of the hydro-aerodynamic characteristics of the iceberg when moving in an arbitrary direction, as well as to assess the dependency of the aerodynamic characteristics on the incoming flow velocity.

The test site was a wind tunnel – a closed tunnel with open test section – with a diameter of 1.85 m and a test section length of 2.4 m. The flow velocity was up to 50 m / s. The flow core had a diameter of 1.6 m. The turbulence intensity did not exceed 1%. The experimental setup for determining the positional characteristics was placed on a stand that rotated the model in a horizontal plane by 360° degrees and allowed the model to be installed in a flow with a given drift or trim angle. The positioning accuracy of the model is ±0.5° degrees.

Related Publications

Details of this research and further discussion can be found in the papers:

*These are Accepted Manuscripts of articles published by International Society of Offshore & Polar Engineers ISOPE available online: http://www.isope.org/publications” © 2019-2020 by the International Society of Offshore and Polar Engineers (ISOPE).