Ice forces on a multifaceted conical structure

Wang, Zhiguo (1997) Ice forces on a multifaceted conical structure. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (31MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

To simplify fabrication and reduce costs of conical structures for arctic offshore development, a multifaceted conical shape was proposed to replace the conventional smooth cone. This raised a number of concerns about the mechanisms for ice interaction with this multifaceted conical structure (MCS) and the validity of analytical models which were developed for the smooth conical structure (SCS). A vertical neck at the top of the MCS was proposed for a prototype and industry has desired a large size for this neck, i.e., its diameter to be only slightly smaller than water-line diameter. This raised another concern: what was the effect of this vertical neck on ice loads ? -- To address these concerns, a university-industry joint program (NSERC file # 661- 119/88) was initiated to carry out a series of test program. The program involved three series of tests carried out in three Canadian test facilities (ESSO Resources Canada, Calgary; NRCC's Institute for Mechanical Engineering, Ottawa; and NRCC's Institute for Marine Dynamics, St. John's) with structural models at scales of 1:50 to 1:10 and at a cost about 1.3 million Canadian dollars. The results of these tests were presented in test reports published by each facility; while presenting these test results no detailed analysis was carried out to understand the ice/structure interaction in a comprehensive manner. The data contained in these test reports have been used in this study to understand in depth the various interaction scenarios possible between a multi- year ice ridge and the MCS. -- The direct analysis of the test data, presented in this study, covers answers to most of the concerns raised by the offshore industry but is not limited to them. Besides the ice failure mechanisms involved in the process of ice interaction with the MCS models, the parameters analyzed include neck size, structural orientation, ridge width, and the events that caused the maximum ridge loads. In the analysis of the ice failure mechanisms, three ridge failure patterns are identified. Both ridge cracking and ridge segment ride up processes are recognized to be events causing the maximum ridge loads. The influence of a number of factors on ice cracking pattern and ice loads exerted on the MCSs are considered in the data analysis. -- To provide an insight into the interaction process and the ice failure mechanisms, a series of numerical simulations are carried out using a commercial discrete element code (DECICE). DECICE is capable of realistically simulating the ice breaking processes accompanied by broken ice pieces riding up on the structural surface. This overcomes the disadvantage of the conventional finite element analysis in which the ride-up forces are to be approximately computed under an unrealistic assumption that only one layer of ice rides up. The simulations using DECICE show the broken ice pieces to be actively involved in the breaking process of impinging ice. The effect of neck size on ridge and sheet ice loads is also studied using DECICE. -- An analytical model is developed which takes the particular feature of the MCSs and ridge length into account; this model should provide designers with a simple estimation of ridge cracking loads. This analytical model is given in the form of a set of equations covering the initial crack event and hinge crack event for both finite length (short) and infinite length (long) ridges. Three loading conditions for hinge cracks in an infinite ridge are considered in the equations. The most conservative loading condition for the hinge cracks is chosen for short ridges to give a conservative estimation of the maximum ridge loads. The equations for long ridges are expressed in a general form with different coefficients for various crack events and loading conditions. -- An extensive comparison of the experimental results given in this thesis, for level ice fields, has been made with the analytical models that were develped for prediction of level ice loads on SCSs. The results show Nevel's analytical model for sheet ice load estimation to be fairly valid for use in estimation of sheet ice loads on MCSs though it was developed for smooth cones. Ralston's model is also acceptable for MCSs if appropriate parameters are chosen for inputs to this model. -- Of the various analytical models available for ridge load estimation, the model developed in this thesis gives the best prediction (closest to the measured loads). As a second choice, Wang's plasticity model which has been widely accepted for smooth cones is also applicable to the case of MCSs.

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