Effect of Joint Behavior on the Reassessment of Fixed Offshore Platforms in the Bay of Campeche, Mexico

Pemex Exploration y Produccio´n 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. To meet the growing demand for oil and natural gas it was found economic to extend the service life for 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. As a part of the reassessment study non-linear pushover and spectral fatigue analyses are conducted. The effect of joint behavior, viz. the local joint flexibility and strength, on the structural ultimate capacity and fatigue life is discussed in this paper. In conventional analysis the tubular joints are assumed to be rigid and the flexibility effects due to shell deformations are ignored. In this present paper, the effect of the joint behavior is included in the analysis and its implications on the results are discussed. For the ultimate strength analysis both API and MSL formulations for the load-deformation behavior of the joint are studied and compared. For the fatigue analyses, local joint flexibility modeling using Buitrago’s formulation is used. Results including and excluding these effects are compared. Effect of grouting of the joint is also studied. Comprehensive results of the study for a number of platforms, which cover the categories of Drilling, Production, Gathering and Habitation, are presented. The effects of local joint flexibility and joint strength on structural behavior have been recognized to be important in the recent publications of the recommended practices and the codes such as the API RP 2A. However, comprehensive discussions and the results of application of these aspects are rare in the published literature. This paper addresses these issues and presents the results of a large number of platforms of different configurations, indicates some noticeable trends and suggests some general conclusions.