Journal of Advances in Solid and Fluid Mechanics (ASFM) 1 (2019) 9-17 Study on Ultimate Capacity of Offshore Jacket Platforms Considering the Effects of Global and Local Buckling of the Elements M.R. Tabeshpour 1* , M.H. Erfani 2 , H. Sayyaadi 3 1 Assistant Professor, Department of Mechanical Engineering 2 Ph.D. Student, Department of Mechanical Engineering 3 Professor, Department of Mechanical Engineering (Manuscript Submitted January 06, 2019; Accepted March 22, 2019) Abstract The capacity curve of offshore structures including jacket-type offshore platforms is used to achieve structural performance levels and determine their ultimate capacity and ductility. This curve is obtained from the pushover analysis and accurate estimation of that is of great importance. Formation of fatigue cracks at the joints, corrosion of members, environmental loads and damages caused by accidental dynamic loads such as impacts of vessels and floating bodies will result in the change of the ultimate capacity of these structures over the course of their life. These cases should be considered in calculating the ultimate capacity of the offshore platforms at any given time of their life. In all of these cases, accurate modeling of the global and local buckling of compression members is important at any time of calculating the ultimate capacity. Buckling modes and deformations due to local buckling will be considered if the compressive braces are modeled by Shell or Solid elements and the imperfections are applied. This paper aims to achieve the correct compressive behavior of compression members. The ABAQUS finite element software suite was used for this purpose. The buckling envelope derived from the Marshall strut theory defines the post-buckling damaged elasticity model and the hysteretic loop response. Finally, by using this modified behavior in Frame elements, the effects of local buckling in compressive braces can be considered. The innovation of this paper is investigating the interaction of local and global buckling in the braces of jacket using ISO equations with 1-Dimentional Frame elements, which results in low computational costs. Keywords: Jacket type offshore platforms, buckling, Marshall Strut Theory, compressive behavior, ultimate capacity. 1. INTRODUCTION Critical buckling loads can be calculated for tubular members using empirical equations. These equations are employed by design codes such as API RP 2A- LRFD: (Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms) for achieving load and resistance design factors in order to assure that design loads in normal and extreme operational conditions do not meet buckling loads. Also design codes cover both global elastic buckling and inelastic buckling and include decreasing of loads to prevent local buckling of walls in tubular members as well as imperfections. Hence, structural configuration has been explained by codes in such a way that applied loads are smaller than buckling loads in normal operations. Some relevant studies have been conducted on the buckling of the tubular members. Yasseri and Skinner (2006) conducted a detailed study in the form of a technical report on the global and local buckling and their interaction for tubular members under concentrated force and moment. They categorized the compressive members based on the ratio of length to gyration radius (L/r) and the diameter to thickness ratio (D/t) and investigated the suggested API curve for elastic and non-elastic buckling based on the slenderness. The effects of taking into account the * Corresponding author, E-mail address:tabeshpour@sharif.edu - Ductility Level Earthquake imperfection of these members and finally, the introduction of the modified properties of steel material which can be used for beam elements to achieve the same response of shell elements, are also presented in that report. Wenjing and Hoogenboom (2011) conducted a buckling check involving manual determination of the buckling lengths of the frame member. They estimated that 5 to 10% of the man- hours in structural analysis of removal projects is spent on checking and correcting buckling lengths. An alternative method is available that does not require determining buckling lengths. In their paper it was shown how this method can be derived from the NORSOK standard for tubular steel frame structures. The method has been demonstrated in the removal analysis of an offshore jacket. They concluded that this method can be successfully applied. In Extreme load conditions such as DLE 1 the consequences of this event for structures are needed to be considered when some members compress higher than their buckling limit. It is not always necessary to design structures in a way that local failure does not occur in members under critical loads. Subsequently, analysis concentrates on whether the level of structural failure remains within acceptable limits. For this analysis, load factors and capacity factors are equal to 1 and the actual response of the structure is calculated. It is particularly important that local buckling of Downloaded from journals.hsu.ac.ir at 14:58 +0330 on Friday December 10th 2021