Coastal structures in ice covered waters as well as offshore structures for the exploration and production of hydrocarbons in the Arctic have to be designed to withstand the forces of moving ice. The magnitude of these forces are under heavy dispute because the forces predicted by various methods scatter by the factor of 10 and more. This situation as well as indications that the forces are indeed smaller than predicted by most of the scientists have stipulated a group of seven research organizations from six European countries to carry out the LOLEIF project "Validation of Low Level Ice Forces on Coastal Structures“.

The overall objective of the project has been to prove the validity of lower ice forces, by the following research program :

• Evaluation of existing ice force prediction methods.
• Full scale measurements of forces of level ice and ridges at the lighthouse Norstrømsgrund (Gulf of Bothnia).
• Development of new theoretical ice force models on the basis of the results of the previous tasks.
• Recommendations of ice forces on vertical structures from level ice and ridges in consultation with the classification societies.

Results

The present methods of predicting the ice forces on vertically sided marine structures were evaluated with special emphasis on the full scale measurements at the Molipaq caisson, when it was operated by GULF Canada in heavy ice in the Beaufort Sea.

Full scale measurements of ice forces were carried out at the lighthouse Norstrømsgrund in the northern part of the Gulf of Bothnia in 1999 and 2000. Eight large surface (1,6m x 1,2m) and eight small surface (0,5m x 0,36m) panels were specially designed, manufactured and mounted half way around the 7m diameter lighthouse. Together with the forces also the force effecting parameters such as ice properties, ice thickness, ice velocity were measured.
 

The results can be summarized as follows :

• The effective pressure decreases with ice thickness and nominal pressure area.
• The effective pressure is not significantly affected by the ice velocity nor by ridges if they are unconsolidated.
• The effective pressure is approx. 100 % higher in the quasi statue load case compared with the dynamic crushing failure.
• In the confined case of stress the ice failed by horizontal splitting (cleavage).

Numerical models for predicting ice forces on vertically sided marine structures were developed with the following results :

• An existing 2-D model was extended to a 3-D model and used to simulate loading conditions measured at the Norstrømsgrund lighthouse.
• Development of a fracture mechanics model of high pressure zones, which in part explains the decrease of the effective ice pressure with the nominal area of contact between ice and structure.
• A ridge material model was developed on the basis of the full scale tests. This will be the basis for a model to predict the forces of ridges.
• A probability model was developed and implemented by the computer program MATLAB, which allows purpose orientated analyses.

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Last update: 2001-07-30