On the mechanics of submerged vertical slender structures subjected to varying axial tension

Abstract

This paper presents a mathematical basis for determining the structural and hydroelastic behaviour of submerged vertical slender steel structures operating at low tension. These structures have significant bending stiffness combined with significant self-weight so that the axial force varies greatly from one end to the other and may even change sign. Such structures form key components of drilling and production platforms used for exploitation of hydrocarbons under the world’s oceans. An understanding of their structural behaviour under reduced tension offers opportunities for cost reduction and further optimization. The buckling behaviour at low tension is evaluated by retaining nonlinear curvature terms and using an expansion in series to reduce the governing equation to a set of linear ordinary differential equations, which are then solved sequentially by employing Galerkin’s technique. This approach is also extended to lateral oscillation of the structure when excited by forced horizontal oscillatory motions at the top end. The paper uses these solution techniques to explore the behaviour at low tension of typical slender structures used in offshore developments.