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Assessment of Existing Jacket Type Offshore Platform against Environmental Wave Loading Utilizing Theory of Reliability

Bagheri Baygi, Vahid | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41164 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Golafshani, Ali Akbar
  7. Abstract:
  8. Assessment of existing jacket type offshore platform against environmental wave loading hazard is the central component of this work. Two general approaches exist in this context, deterministic and probabilistic assessment. In this study, a new probabilistic method (reliability analysis) to assess offshore structures under environmental loading (extreme wave) is introduced. Before describing this approach, several necessary elements such as collapse analysis, sampling methods and wave hazard analysis should be investigated. First of all, deterministic assessment of SPD2 platform located in Persian Gulf, which is the case study considered in this dissertation, is conducted utilizing procedures proposed by offshore codes and standards. This deterministic evaluation discloses the general behavior of platform. Despite the fact that current pushover approach proposed by offshore codes for assessment of jacket platform is imperfect, a novel approach named Incremental Wave Analysis (IWA) is introduced, which is able to evaluate the nonlinear static and dynamic behavior of jacket type platforms against environmental wave loading, accurately. Static and Dynamic Incremental Wave Analyses (SIWA and DIWA) can bring about collapse prevention limit state of jackets as well as a novel parameter called Collapse Wave Height (CWH). CWH is a unique parameter which can estimate the collapse probability of jackets using a deterministic procedure. In addition, CWH plays an important role in the proposed probabilistic approach. Three main categories of uncertainty and variability are considered in this research including variability in sea state parameters, uncertainties in the prediction of the wave force on jacket structure and design/modeling uncertainties. In this work, a combination of LHS and Simulated Annealing (SA) is utilized which can diminish the number of simulations, exceptionally. As a result of this approach, 15 simulations are taken into account for 7 random variables. Sensitivity analysis of platform’s response to different random variables is also conducted to distinct the influential parameters. This sensitivity analysis is carried out by two different approaches, and the results are compared. Despite the fact that uncertainty in sea state parameters is another necessary element of proposed probabilistic approach, wave hazard analysis for Persian Gulf (South Pars Gas Field Development Phase-1) is carried out. After completing the mentioned procedures, the new probabilistic method (reliability analysis) is introduced. In this approach, the mean annual frequency (MAF) of exceeding a structural performance level is defined as specified structural parameters (e.g., base shear, deck displacement, etc) reaching a structural limit state (e.g. collapse prevention limit state). The expression for mean annual frequency of exceedance is derived by taking into account the aforementioned uncertainties (due to inherent randomness and limited knowledge). Within this approach, the assigned probabilities are a measure of the uncertainty about the structure, and not an element of the structure itself. Dynamic and Static Incremental Wave Analyses as well as improved LHS are utilized to calculate different limit states such as ultimate capacity, collapse displacement and Collapse Wave Height (CWH). In order to simplify the proposed probabilistic approach, a closed form solution is presented. This approximate approach can be implemented in offshore codes and standards, properly. Comparison between exact solution (proposed probabilistic approach) and closed form solution illustrates that this approximate method can estimate the probability of failure, accurately. The closed form solution based on collapse wave height indicator also results in a remarkably simple equation, which can be used easily
  9. Keywords:
  10. Reliability Analysis ; Incremental Wave Analysis ; Offshore Platforms ; Performance Evaluation ; Wave Loading

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