Research On Dynamic Performance-Oriented Design Optimization For Ice-Resistant Jacket Platforms

In view of characteristics of the enviroments and the oil and gas recourses in the Bohai Sea, where there are many ice-infested marginal fields, the economic steel fixed jacket platforms with the ice-beaking cone have been used widely. For some in-service plalforms in the Bohai Sea, in-field observations show that there are considerable ice-induced vibrations in some cases. However, the current codes do not speicify how to consider the ice-induced vibration. The dynamic problem of the economic ice-resistant jacket platform becomes a key issue. Under the framework of 863 Program (No. 2001AA602015-1), this paper introduced the concepts of performance-based design and structural optimization into the research of the novel ice-resistant platform, and focused on the dynamic design optimization for ice-resistant jacket platform, and discussed key issues, such as the definitions of performances of the ice-resistant jacket platform, the determination of dynamic ice force for dynamic design, the analysises of ice-induced vibrations and the formulation of ice-resistant design optimization.Related studies of this paper are listed as follows:1. In framework of the performance-based design, the definitions of performances of the ice-resistant jacket platform has been specified. It is suggested that the extreme static performance requirments and the dynamic performance requrirments should be considered separately in design optimization. For ice-resistant jacket platmorms with ice-breaking cone, two forms of dynamic performance-oriented design optimization for ice-resistant jacket platform are developed.2. Severe ice-induced vibrations can cause significant dynamic stress of tubular joints, which could evoke fatigue of tubular joints. In order to overcome the conflict beteen the accuracy and the efficiency of fatigue analysis for complex tubular joints, an efficient spectral method of ice-induced fatigue analysis for complex tubular joints has been proposed, which employs the pseudo exitation method and a mixed finite element model that can reflect the global and local characters of the jacket platform.3. Two detailed optimization formulations of ice-resistant jacket platforms have been developed from different views to consider the ice-induced fatigue of tubular joint: optimum design considering fatigue life of tubular joints and optimum design considering fatigue reliability of tubular joints, respectively. These proposed optimization models can meet the requirements of the platform such as the wellhead platform without crews, which should consider ice-induced fatigue and weight reduction,4. The definition of short-term dynamic ice cases has been specified and an approach to assessing the conditional exceedance probability of ice-induced acceleration is proposed. Based on these, the framework of determination of the short-term dynamic ice case for dynamic analysis of ice-resistant jacket platform is developed, including two approaches: the failure probability-based approach and the expected loss-based approach. This framework can cover the influence of the uncertainty of ice environments and the distinction between different ice-resistant structures.5. The acceleration-oriented design optimization of ice-resistant jacket platforms has been developed, which could emphasize ice-induced deck acceleration and need controlling weight. This model can treat the requirements of the platform such as accommodation and power platform with crews. The objective function is the root mean square value of ice-induced deck acceleration caculated by the proposed approach to determination of the short-term dynamic ice force. In this way the uncertainty of ice environments and the variability of ice-resistant structure could be reflected during design optimization.6. A simplified conceptual model of ice-resistant jacket platform is developed and employed to make some analytical discussions. The influence of conceptual parameters on conceptual performances of ice-resistant jacket platform is studied in analytical form. Subsequently, a statement of performance-oriented conceptual parameter optimization is proposed. The characteristics of the rigid ice-reistant design and the flexible ice-resistant design are discussed. It is pointed out that in order to meet the requirements of the Bohai Sea the ice-resistant jacket platform design should follow the flexible ice-resistant design and pay more attention to ice-induced vibrations, and this intent could be implemented by the proposed dynamic performance-oriented ice-resistant jacket platform optimization as one of important approaches.