Partha Chakrabarti, Ibrahim Abu-Odeh, Adinarayana Mukkamala and Bidyut Majumdar (ZENTECH Inc.)
Michael H. Faber and Daniel Straub (ETH Zurich)
Juan de Dios de la O (Pemex Mexico)
Pemex Exploration y Producción owns and operates several fields in the Bay of Campeche, located in the south Gulf of Mexico, for oil and gas production. Many of these fixed offshore platforms were built during the 70s and 80s and have already exceeded their design service life. In order to meet the growing demand for oil and natural gas it is necessary to extend the service life of these platforms by at least another 15 to 30 years. To meet this extended service life, thorough and systematic reassessment studies need to be conducted leading to identification of any structural weakness and possible locations of fatigue problems. To extend the fatigue life of the welded joints, inspections are required to be performed according to a risk based inspection planning procedure.
In the present paper, an overview of the reassessment study procedure is outlined and pertinent results are presented for more than twenty platforms which were studied in a recent project. The most important engineering considerations and various analyses involved in the study are discussed in detail. The platforms cover the categories of Drilling, Production, Gathering and Habitation. Depending on the category, Pemex specifications assign different levels of acceptable reliabilities and reserve strength ratios. The ultimate strength of the platforms is determined using a detailed finite element model of jacket, piles and deck structures and a state-of-the-art non-linear progressive collapse analysis technique commonly known as ‘pushover’ analysis. The analytical structural models include local joint flexibility of the jacket joints, soil-pile interaction, geometric non-linearity and material plasticity. They also include the information of damages and deterioration obtained from inspection reports, such as dents, bents, cracks etc. The joint strength modeling is performed using the latest available procedures that use non-linear load-deformation curves. Fatigue analyses are based on spectral analysis technique and include the dynamic response of the structure to wave loads. The structural models for fatigue analysis include the effects of local joint flexibility (LJF) and soil-pile interaction. Results for one typical platform are presented in complete detail to facilitate the understanding of the reassessment study procedure. A typical risk based inspection planning for extending fatigue service life is also presented. Finally, the summary of results for 28 platforms is presented to appreciate the importance of the various analytical parameters. It is hoped that this very wide database of results for platforms of different configurations can serve as a useful resource for the offshore industry in general.
Proceedings OMAE2005, 24th International Conference on Offshore Mechanics and Arctic Engineering, Halkidiki, Greece, June 12-17, 2005, [OMAE2005-67051].
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