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Assessment of Existing Jaket-Type Offshore Platforms in the Persian Gulf Region for Service Life Extension or Operative Conditions

Ebrahimian, Hossein | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42797 (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 platforms in Persian Gulf for life extension or operative conditions is a major concern for oil offshore companies in recent years. Significant damage to offshore platforms as well as their contents will induce significant losses such as financial losses, fatalities and casualties. On the other hand, extreme waves or earthquakes are low-probability, large-consequence and large-uncertainty hazards that can affect these critical facilities; therefore, the assessment of existing offshore platforms subjected to each of these sources can be successfully carried out by a probabilistic approach. In this dissertation, a unique probabilistic performance-based framework is utilized for assessment of existing jacket-type offshore platforms subjected to extreme waves or earthquake. Although the procedure for assessment under extreme waves and earthquake are of distinct categories, both approaches, however, are inherently consistent. In this study, a novel probabilistic framework named Probabilistic Incremental Wave Analysis (PIWA) is established in order to assess the performance of jacket structures under extreme waves. The PIWA is a robust procedure, which can properly estimate the Mean Annual Frequency (MAF) of exceeding a limit state as well as calculating the demand hazard. Nevertheless, the PIWA methodology encompasses complicated numerical calculations which make this procedure somewhat challenging for practical assessment of jacket structures. Hence alternative simple and approximate closed-form solutions are established which enable practical engineers to follow the PIWA framework, simply. Comparison between the exact PIWA methodology and the proposed approximate closed-form solutions indicate that the latter method can appropriately estimate the limit state frequency and the demand hazard curve. In addition, the probabilistic seismic demand analysis (PSDA) of offshore platforms in the Persian Gulf region has been investigated in terms of MAF of exceeding a given level of response. This approach combines the site-specific ground motion hazard information with the structural response of interest based on Incremental Dynamic Analysis (IDA) procedure. Accordingly, this study proposes a computationally efficient and theoretically consistent algorithm to select the ground motions that match the site-specific target response spectrum mean and variance. The target spectrum used in this research is the Conditional Spectrum (CS). On the other hand, the associated conditional spectra are conditioned on the geometric mean of the pseudo-spectral acceleration, Sa, over a certain range of periods, instead of the target Sa at a specific period. Hence, the pros and cons of the proposed conditioning Intensity Measure (IM) are compared with the conventional conditioning case by estimating the probability of collapse and the drift hazard. The results demonstrate that although the structural response of the offshore platform is sensitive to values at several periods, both record-selection strategies, however, lead to nearly identical probabilistic prediction of structural demands at different IM levels. While the traditional IDA uses the same ground motions for all IM levels to obtain the variation in structural response for the purpose of PSDA, the CS-based ground motions in this dissertation are re-selected at each IM level. Therefore, the adaptive IDA, which allows for the variation of the ground motions at different IM levels and maintains its continuity partially, is utilized as a new approach in the PSDA methodology.

  9. Keywords:
  10. Probabilistic Seismic Demand Analysis (PSDA) ; Extention Life Time ; Persian Gulf ; Jacket-type Offshore Platforms ; Probabilistic Incremental Wave Analysis ; Adaptive Incremental Dynamic Analysis ; Conditional Spectra

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