Investigations on Internally Ring-Stiffened Joints of Offshore Platforms

Abstract

The paper presents in detail the experimental and numerical investigations on tubular joints that are stiffened internally with three annular rings in order to study their behaviors and evaluate their strengths under axial brace compression loading. The nominal chord and brace diameters of the tested T and Y-joints were 324 and 219 mm and their thicknesses were 12 and 8 mm, respectively. The tested joints are approximately quarter-size when compared to the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field. Some of the joints were actually fabricated by a leading offshore agency that is directly involved in the fabrication of prototype structures. Bending of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened joints in contrast to ovaling and punching shear of the unstiffened joints. Strengths of the internally ring-stiffened joints were found to be almost twice that of unstiffened joints of the same dimensions. A tri-linear stress-strain model that takes into account the strain-hardening characteristics of the material was developed to assess the strength of the internally ring-stiffened joints. Finite element analysis (FEA) was performed to predict the bending deflection of the internally ring-stiffened joints under axial brace compression loading. This FEA used NISA II software package and was based on the tri-linear model proposed in this study. [S0892-7219(00)00104-7]