مهندسی عمران و معماری | مصالح ساختمانی

A Technical Framework for Probabilistic Assessment of Existing Offshore Platforms in Persian Gulf under Extreme Environmental (Wave) Loading

- حسین ابراهیمیان- علی اکبر گل افشان- , 5th National Congress on Civil EngineeringAssessment of existing jacket type offshore platforms in Persian Gulf for life extension is of great importance in the field of offshore platforms in recent years. The main question to answer is whether an existing platform is safe enough to be used beyond the design life. Evaluation of existing jacket type offshore platforms under extreme environmental (wave) loading in Persian Gulf is a part of this major assessment. Two general approaches exist in this context, deterministic and probabilistic assessment. In this paper, a new probabilistic method (reliability analysis) based on demand and capacity factored design method (DCFD) for assessing offshore structures under environmental loading (extreme wave) is introduced. DCFD is a probability-based load and resistance factor (LRFD)-like format used for performance based design and assessment of structures, which was first introduced for probabilistic seismic demand analysis (PSDA) of buildings. The DCFD format is based on a technical framework that provides a closed-form analytical expression for the mean annual frequency (MAF) of exceeding a structural performance level, which is usually defined as specified structural parameters (e.g., strength) reaching a structural limit state (e.g., onset of yield, collapse). The expression for mean annual frequency of exceedance is derived by taking into account the aleatory uncertainty (due to inherent randomness) and the epistemic uncertainty (due to limited knowledge) in three main elements consisting environmental hazard, structural response (as a function of wave loading) and capacity. Static non-linear system strength analysis (push-over) is the common method to calculate the capacity of the system. In the present study, a more realistic approach used to calculate the system capacity called “Incremental Wave Analysis” (IWA) will be discussed. In the next step, the uncertainty propagation and reliability analyses are evaluated using a relatively new approach by combining the FOSM method and Monte Carlo simulation. Within this approach, the assigned probabilities are a measure of the uncertainty about the structure, and not an element of the structure itself. An existing jacket in Persian Gulf is assessed by using this new approach and relevant analysis methods are presented