فهرست مطالب mohammadhadi erfani

The ultimate capacity of offshore structures including jacket type offshore platforms is used to achieve structural performance levels and determine their ductility. Accurate estimation of this parameter 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 change of the ultimate capacity of these structures over time of their life. These cases should be considered in calculating the ultimate capacity of the offshore platforms at any given time of their life. However, 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. The purpose of this paper is to achieve the correct compressive behavior of compression members. ABAQUS finite element computer software is used for this purpose. The buckling envelope derives from 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 capacity curve obtained from the pushover analysis of jacket-type offshore platforms gives their structural performance levels, ultimate capacity and ductility. Accurate estimation of structural capacity curve is of great importance. Accurate modeling of the global and local buckling of compression tubular members in a correct form is an effective part of studying the behavior of offshore jackets under all various types of loading conditions at any given time of their life. Modeling of compressive braces by shell or solid elements when the imperfections are applied leads to deformations due to local buckling based on buckling modes. This paper aims to achieve more accurate compressive behavior of compression members. The ABAQUS finite element software has been used for this purpose. Regarding to the results achieved from investigation of buckling in tubular members proper elements have been introduced to investigate the global and local buckling phenomena. Then pushovers results of Ressalat jacket with conventional modeling versus more accurate modeling proposed in this paper for compressive members have been compared as a case study. According to the results applying improper mesh size for compressive members can under-predict the ductility by 33% and under-estimate the lateral loading capacity up to 8%. Finally, ISO equations and Marshall strut theory have been applied to investigate critical buckling load and post-buckling response of tubular braces. The innovation of this paper is investigating the interaction of global and local buckling in the braces of jacket with 1-Dimentional elements using ISO equations and buckling envelope derived from the solid element results, which results in low computational costs.