Reliability Based Approach to Determine Design Ice Condition for Dynamic Performance Assessment of Jacket Platforms

In cold region, the ice load is normally the dominant environmental force, and the offshore structures have to resist the static and dynamic ice forces. However, the current design codes of ice-resist offshore platforms only account for the static ice force. The field observations of the offshore platforms in Bohai Gulf in recent years showed that dynamic ice-induced significant vibrations resulted in some harmful consequences, such as fatigue failure of the jacket platform, human discomfort and gas leak caused by flange loosing of pipelines on the deck. Thus, the assessment of the dynamic performance of ice-resistant platform is very important, in which the determination of design ice condition is the key problem. In the past decade, the research of dynamic ice force has undergone a lot of developments, and the simplified deterministic dynamic ice force formulation and the random ice spectrum were proposed, which are closely related to the ice velocity and ice thickness. However, how to select the proper ice parameters (velocity and thickness) for the design of ice-resistant platforms is still an open problem, because the ice velocity and thickness are randomly distributed, and the ice force is very complicated in nature that is structure-dependent. In this paper, a reliability based approach to determine the design ice condition for dynamic performance assessment of jacket platforms is proposed. We focus on the ice-induced deck acceleration that plays a significant role in the dynamic performance, such as human discomfort, failure of pipeline and facility, etc. Based on two rational assumptions that the ice thickness and velocity are independent random variables and the ice-induced peak deck acceleration follows Rayleigh distribution, we use the random ice spectrum and PEM (Pseudo Excitation Method) to calculate the failure probability corresponding to the dynamic performance with respect to peak deck acceleration efficiently. The selection of ice thickness and velocity of the design ice condition with respect to different types of offshore structure properties in Bohai Gulf is provided. Finally, The applications on different types of jacket platforms in Bohai Gulf demonstrate the efficiency and feasibility of the proposed approach.