Level ice interaction with sloping and conical offshore structures

Bruce, Jonathon E.F. (Jonathon Edward F.) (2009) Level ice interaction with sloping and conical offshore structures. Masters thesis, Memorial University of Newfoundland.

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Abstract

The use of sloping sided or conical structures is often a favorable design option for structures placed in ice covered waters. An understanding of the mechanics involved during level ice interaction with conical or sloping sided structures is necessary for safe structural design in environments where ice cover is present. This work provides a review of the failure mechanics involved during an ice interaction with a conical or sloping sided structure and the methods which have been developed to model these types of interactions. -- The sensitivity of the ice loads, estimated by the Croasdale Model, to the variation in input parameters has been studied in this work. From this analysis, it was determined that if a rubble pile was present on the structure, the flexural strength of ice was not a significant factor affecting the ice load. There were however a number of scenarios which were outlined for which the flexural strength of ice was of significance. A ship ramming event is one such scenario for which the flexural strength of ice plays a significant role in limiting the maximum ice load. The maximum ice load occurs as a crushing failure on the bow of the ship, which is limited by flexural failure due to the weight of the vessel on the ice feature. Another scenario for which the flexural strength of the ice may dominate involves the use of conical structures in the Arctic. Here, designers are concerned with thick multiyear floes interacting with large conical structures. In this scenario, ride up is likely to occur with limited rubble formation due to the dissipation of kinetic energy, thus making the flexural strength of the ice a critical component affecting the design load. Further to this, the scale of the interaction has been found in this work to be a critical component affecting the flexural strength of ice, which is due to the presence of a size effect. -- The results presented in Chapter 3 show that the methodology used to predict the flexural strength of ice based on brine volume alone may well lead to an over estimation of the flexural strength of ice for full scale interactions. This is achieved by using full scale data from the icebreaker Oden during the International Arctic Ocean Expedition in 1991 where the icebreaker Oden was part of a three vessel expedition to the central Arctic Basin. The results of the work show a significant reduction in flexural strength when compared to the methodology which considers brine volume only. This result is due to the size effect present in the flexural strength of ice. The author recommends the use of the methodology presented by Williams and Parsons (1994), which includes the scale of the interaction, when calculating the flexural strength of ice for full scale ice structure interactions. -- In Chapter 4 a probabilistic model was developed to determine extreme level ice loads acting on the conical Confederation Bridge piers in the Northumberland Strait. A Monte Carlo technique was utilized to simulate the ice environment and to derive the annual maximum ice loads on the structure. In order to achieve this, full scale data was obtained from public sources and fitted with probability distributions to model the input parameters. -- The model developed in this work simulates the total number of ice floes interacting with a bridge pier in the Northumberland Strait for a given season, as well as individual parameters for each ice floe. Each ice floe is assigned a diameter, an ice thickness, and an ice-structure friction coefficient. Each floe is then broken up into intervals which are individually assigned a flexural strength, along with a rubble height and the angle that the rubble pile makes with the horizontal axis. The Croasdale model is utilized to calculate the horizontal and vertical ice forces acting on the bridge pier for every interval in each floe. The maximum force acting on the bridge pier for each floe is stored and the annual maximum ice force is obtained from these. The model was then run for 4000 years worth of ice structure interactions, resulting in an estimated 100 year ice load of 10.7 MN and a 10,000 year ice load of 16.0 MN acting on a 52°, conical bridge pier with a diameter of 14m at the waterline located in the Northumberland Strait. -- In Chapter 5 the author has used data published by the Confederation Bridge Ice Monitoring Program and the National Research Council to validate the probabilistic model developed in Chapter 4. The model developed is believed to provide an appropriate representation of the level ice loads acting on the Confederation Bridge piers. The model which was developed in this work produced results which suggest that the 10 year ice load is 8.6MN, whilst the maximum load published by the Confederation Bridge Ice Monitoring Program is 8.4MN for the first 10 years of operation.

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