Design of Concrete Gravity Structures to Withstand Concentrated Ridge and Flow Impact Loads

Abstract

ABSTRACT This paper presents a method of designing concrete gravity structures to withstand concentrated ridge and floe impact loads. The philosophy to be adopted and major considerations in the design of these structures is presented. The methods discussed include local and global effects due to concentrated loads, soil structure interaction, multi-dimensional stress states and considerations for out-of-plane shear. The approach is general in nature and can also be used for less hostile environments. INTRODUCTION Hydrocarbon exploration and production activities in ice infested areas is continuing at a steady pace. While the potential for commercial oil and gas discoveries is high for these areas, so are the challenges and the cost of exploration and development. One of the key cost centers in these systems is the fixed platform from which offshore operations can take place. This paper describes the design of fixed gravity concrete platforms for use in these hostile areas. The approach discussed in this paper can also be used for steel-concrete hybrid systems. The design of structural systems is generally based on recognized codes of practice, rules or specifications, such as the AISC (I), API RP 2A (2), ACI 318-83 (3), DNV (4), etc. However, at the present time, the designer will find only limited guidance in the existing codes, rules or specifications to assist him in designing arctic concrete structures. This paper will outline a rational design method for these structures. DESIGN CONSIDERATIONS Some of the principal factors governing the design process of a concrete structure for the Arctic are the same as those for other structures, namely:PlanningEnvironmental ConsiderationsMaterial SelectionGeotechnical and Structural ConsiderationsConstructionTransportation and Marine OperationsInstallation and Removal Some of these factors will be examined in this paper. DESIGN CRITERIA The design of concrete structures for ice infested environments is controlled by several factors, but we shall limit our discussion to ice loads and "hard-spot" foundation loads. Traditionally, deterministic approaches have been used to date to predict global ice loads and the associated contact area relationship (6). Recently, a probabilistic method for predicting global ice loads and contact pressures was described by Vivatrat and Slomski (7, 8). Their method is general in nature and results in the development of cumulative probability distributions for ice load events. The model accounts for a full suite of ice features ranging from first year sheet ice, rafted sheet ice, rubble pile accumulations and consolidated rubble fields through to multiyear floes. The development of the load estimate allows for variations in ice feature size and thickness, temperature, age, strain-rate, occurrence frequency, etc. This method is used in predicting ice loads and contact ice pressures for a vertical sided structure shown in Figure 1. The predicted global load for a 20-year and 100-year return interval is 130,000 kips and 200,000 kips, respectively. The design ice-pressure loaded area relationship is shown in Figure 2.