Moment‐Curvature Expressions for Fabricated Tubes

Abstract

The behavior of fabricated tubes is far more complicated than that of hot‐rolled tubes because of out‐of‐roundness and residual stresses introduced during the process of fabrication. In deepwater platforms, the capacity of fabricated tubular members may be reduced significantly by external hydrostatic pressure. A rigorous numerical analysis of even an individual member requires considerable computer time, which can be reduced significantly by using closed‐form curve‐fitting expressions for the moment‐curvature relationship. In this paper, the results of rigorous numerical analysis are used to develop closed‐form moment‐curvature expressions for a fabricated tubular crosssection subjected to a given value of axial force and external pressure. As an example, these closed‐form expressions, combined with similar expressions for axial stiffness, are used to analyze a fabricated tubular cantilever beam at a depth of 800 ft of water. The finite element approach, coupled with the tangent stiffness procedure, is employed in the analysis. Due to external pressure, the capacity of the beam is reduced by 15%.