Buckling and Ultimate Strength of Offshore Framed Structures

Abstract

The development of oil and gas is increasing in deeper and severer waters. Accordingly, it is getting very important to estimate the buckling and ultimate strength of offshore framed structures such as jackup rigs and platforms.
The following methods are introduced in this paper from the design point of view to estimate the buckling strength of braces and ultimate strength of offshore framed structures as both accurately and simply as possible. The method to estimate the buckling strength uses the simple column buckling equation with initial deflection in combination with the elastic buckling stress obtained from eigen value analysis of framed structures. Simplified non linear analysis method which consists of iterative procedure of structural model modification/linear analysis/ buckling and plastification criteria check is used to estimate the ultimate strength.
Four pieces of 3-dimentional 1/7 scale leg model of actual jackup rig were tested and compared with the above mentioned methods and obtained the following results. (1) The calculated buckling strength is within the error of 8% comparing to experimental results and the method to estimate the buckling strength seems accurate in spite of the simplicity of the buckling equation. (2) Simplified non linear method is found useful for design work by comparing the calculated and experimental results. (3) With using two kinds of brace material, one is high tensile steel and the other is mild, the effect of material on the buckling, the ultimate strength and the mechanism of failure is confirmed both by experiments and calculations.