Estimation of Ice Resistance of Ships based on Measurements of Ice Thickness, Speed and Power
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- Institutt for marin teknikk 
The activity in the polar marine areas is increasing from offshore and shipping activities supporting both commercial and tourist operations. Particular of interest is the expansion in offshore oil and gas exploration and productions activities in ice covered waters in the northern hemisphere. The presence of sea ice is the main factor for the complexity for operations in these regions. The presence of sea ice is the main factor hindering the operations in Arctic. Sea ice is a complex material, and induces high pressures in contact with ships or structures. Different types of sea ice and their mechanical and physical properties are briefly described. There exist different models for calculation of contact pressure acting on a vessel or a structure due to ice. This thesis will briefly present three different and popular approaches to predict ice pressure on structures and ships, namely; empirical pressure relationships, physical models and stochastic models. The best approach to predict ice pressure depends much on the problem and what kind of data that is available. Ice class rules for vessels operating in ice infested waters are reviewed. The ice-class rules developed by IACS and DNV are summarized, and both principles behind the rules and the numerical values have been compared. The main difference is that IACS us a plastic method of approach, while DNV uses an elastic method. Despite the difference in method of analyses, the numerical comparison shows they are relative similar. The IACS rules are typically most conservative for larger vessels with large displacement, while the DNV rules are conservative for smaller vessels with small displacement. A review of two different formulations for estimation of ice resistance for ship is given. This is inexpensive analytical models that can give an early estimation of the ice resistance and power requirement. Using main properties from KV Svalbard the two different formulations is compared, and they seem two compare quite well for thin ice (hi < 1m) when the vessel speed is low. For higher vessels speeds the results differs more from each other. The increasing activity in polar areas is the main motivation for Det Norske Veritas project Ice Load and Monitoring (ILM). The overall aim of the ILM-project is to increase the knowledge about the actual ice conditions a vessel meets and its effect on the hull. As a part of the project a prototype of a monitoring system was mounted on the coast guard vessel KV Svalbard, which is described. During research work with KV Svalbard for a total of two weeks in late March 2007 operating in ice covered waters around Spitsbergen, measurement from the ILM-system was stored for later usage. Based on conservation of energy a formulation is outlined to describe how the ice resistance can be estimated for a particular case. In this thesis KV Svalbard is used with available measurements of ice thickness, speed and power from the voyage in late March 2007. The estimated resistance is compared with the two reviewed formulations from literature, and the trend in the results seems to agree well. In the final task, a regression analysis was used to find the "best fit" line or curve to the estimated resistance of KV Svalbard. Different Least Square curves was evaluated and discussed and it was found out that an exponential curve fitted best to the estimated data.