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Experiments by Invited Researchers

 

Model Tests of Iceberg Towing

Project acronym: HyIII-HSVA-08
Name of Group Leader: Kenneth Johannessen Eik
User-Project Title: Model Tests of Iceberg Towing
Facility: Large Ice Model Basin (LIMB)
Proceedings TA Project: Model tests of iceberg towing
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Icebergs may cause a threat to installations, vessels and operations in a number of Arctic and Antarctic regions. If icebergs are detected and considered to be a threat, it has been documented that they in open water can be deflected into a safe direction in approximately 75% of the events. The preferred method for iceberg deflection is single vessel towing.

While all successful iceberg towing operations so far has taken place in open water, future oil and gas developments are expected to take place in regions with occurrence of icebergs embedded in sea ice. The possibility to manage icebergs in such conditions will contribute to increased safety in future operations. The potential for handling icebergs in sea ice may also directly influence the design of offshore structures in Arctic waters.

In order to increase the understanding of what happens when an iceberg tow is started, models describing tow line configuration and tension as well as iceberg displacement and rotations are required. The present model tests should be considered as a first step in order to provide data which can be used as a basis when developing such models.

The primary objectives of this project were as follows:

  • Gain knowledge regarding possibilities and complications by iceberg towing in ice covered
    waters.
  • Provide data for validation and further development of existing mathematical models for
    iceberg displacement, rotation and line tension.
  • Gain experience regarding physical model tests of iceberg towing.

Short description of the work carried out

Achievements
Model tests were performed in April 2009. Tests were carried out with two different iceberg models and at four different ice concentrations. For each model and concentration, two types of tests were carried out. In total, 16 runs were completed during the test period. From all tests, tow line forces, iceberg displacements and rotations were recorded. All tests were documented by video recordings.

In addition, a number of friction tests were performed during the test period. The objective with these tests was to identify maximum static friction between the tow rope and the iceberg model. Data on static friction was achieved for five levels of initial tow line tension.

Difficulties
The main concern was limitation with respect to available time. Initially, totally 24 tows were planned. However, during the test period, it was decided to give priority to the basic 16 tests while the remaining 8 tests, which were planned with a different tow rope, were omitted.

From the first crude evaluations of test results, it is evident that some of the load cells in some of the tests were “drifting”. This can be seen from either the start or the end of the recorded data (i.e. negative loads in one of the sensors from the start). However, since records from three sensors are available, it has been possible to manually correct these offsets. The time series seems reasonable after these corrections, but nevertheless, this cause some unfortunate uncertainties in the load results.

The models were stored in the water tank during the tests. However, during the test period, there was a significant mass loss (1% mass reduction per day) in the models due to melting. This was somewhat surprising as the water temperature was below 0˚C during this period. The mass reduction took only place under the water line while the surface volumes were more or less constant even if the air temperature in some periods were as high as +2˚C. The mass loss is however, well explained by the theory on buoyant vertical convection and the observations will actually provide useful data for comparisons between theory and observations within this subject.

Highlights of important research results

It should be emphasized that only some crude analyses of the measured data have been carried out at present stage. However, some conclusions may already be indicated.

By considering the total load required for a supply vessel (or an icebreaker) to perform a tow, it seems obvious that towing in 80% ice concentration is not realistic. Maximum loads in the tow line may be as high as 10 times the maximum bollard pull from a standard supply vessel. During the tows in high concentrations, ice was breaking in flexural mode, crushing, rafting and ridging continuously. As the ice was piling up in front of the icebergs, the loads were growing. With respect to the tow line, this was fully extended and lifted up from the water during tests with 50% and higher concentrations. 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 was at the same level as maximum bollard pull for powerful nuclear icebreakers indicating that towing in such conditions not is feasible. With respect to tows in 20% concentration and open water, loads are significantly lower indicating that towing in low ice concentrations should be feasible. However, from observations, it can be seen that the tow line will experience significant wear from ice floes during towing making even this more complicated than in open water. With respect to tow line configuration, the tow line will generally be submerged both in open water tows and low concentration tows. However, even smaller floes may bring parts of the tow line up from the water.

Stability was not a concern in the present tests. Roll and Pitch movements were always less than 0.5 degrees. With respect to yaw, both models experienced rotations up to approximately 20 degrees during some of the tests. However, no sliding was observed between the iceberg and tow line and the rotations initiated by a change in tow course were rapidly damped.


Cubature of iceberg model

Cylindrical shaped iceberg model in tow


Launching of rectangular shaped iceberg model

Rectangular shaped iceberg model in tow

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