Performance Based Risk Design Research Of Ice-resistant Jacket Structures

Performance-based design is a new concept and methodology for seismic engineering in recent years. The reasonable design is to consider structural performance, safety, economy, etc. Cost - effectiveness criterion and "individuation" performance of ice-resistant platforms are critical principle in performance-based design, considering not only engineering technology factors but also economical, social and political consequences. In order to improve the original design of ice-resistant jacket structures, only based on extrenme static ice load and not considering the significant risks induced by ice vibrations, in the marginal oil field of Bohai Sea, with financial supports of the National high-tech research development project (No. 2001AA602015), a study on performance based risk design research of ice-resistant jacket structures is conducted in this paper. The major contents are summarized as follows:In section 1, the history of design idea of ice-resistant structures in marginal oil field, the background and aim of this paper are introduced and the main work is drawing out.In section 2, from cost-effectiveness, life-cycle cost model, initial cost, and damage costs, some relevant questions of performance-based design of ice-resistant jacket structures are discussed.In section 3, based on the full-scale field tests, the current achievements of ice load are summarized, and the affection of ice-resistant structures induced by ice vibrations is established. This research provides reliable theory basis for failure modes and dynamic analysis in performance-based design of ice-resistant jacket structures.In section 4, based on the full-scale field tests and ice load research, it is pointed out that the phenomenon of dynamic magnifying is obvious on both of vertical structures and conical structures. The failure modes analysis for ice-resistant platforms in Bohai Bay is discussed and the evaluation criteria are given from the monitoring information, numerical simulation, vibration experimentation, etc. The results indicate that under extreme static ice load the safety reserves of ice-resistant jacket platforms are greater, and the structures could withstand the push-over ice force. But the ice-induced vibrations not only cause significant cyclical stress of tube nodes but also great acceleratory response, which can endanger the pipeline systems on the platform and discomfort the crew members, even affecting work efficiency.In section 5, based on the failure modes analysis in the pre-chapter, failure quantization evaluation of the ice-resistant structure induced by ice vibrations is built, including fatigue life estimation based on the data monitored, risk assesment of crew member induced by dynamic ice, dynamic response analysis of pipeline system exposure to ice-induced vibration on offshore platforms. This work implements shortages of ice-resistant structure design and risk assesment.In section 6, a central issue in performance based design is different reliability analysis, such as the global reliability under extreme ice load, structure dynamic reliability and fatigue life induced by ice vibration. Fistly, the statistical properties of the global resistance and extreme responses of the jacket platforms in Bohai Bay are studied, considering the randomness of ice load, dead load, steel elastic modulus, yield strength and structural member dimensions. Then, an efficient approximate method of the global reliability analysis for the offshore platforms is provided, which converts the implicit nonlinear performance function in the conventional reliability analysis to linear explicit one. Secondly, dynamic reliability based on fist passage breakage mechanism is analyzed. Lastly, based on ice fatigue load and ice force spectrum, the approximate explicit formula of nodes stress with respect to the ice velocity and thickness is established. The fatigue life includes two stages, crack growth and propagation, whose time -dependent reliabilities are analyzed based on the S-N formula and Paris formula, considering the resistance degradation of tubular joints.In section 7, the optimum design model of minimization expected lifecycle cost for ice-resistant platforms based on cost-effectiveness criterion is proposed. Multiple performance demands, such as structure, facilities and crew members, associated failure judgement criterion, costs of construction, consequences of structural failure modes including damage, revenue loss, death and injury as well as discounting cost over time are considered. In order to demonstrate the life cycle cost-effectiveness for the design of ice-resistant platforms, illustrative design example of a typical platform against ice loads in Bohai Sea is discussed. The optimal design is proposed and assessed from the generation of cost/benefit relationships. From the result of the numerical investigation, it may be positively stated that the life cycle cost-effective optimum design model proposed will lead to a more rational, economical and safer design compared with the conventional static design according to standard demand and the optimum design only considering dynamic demand.