A Study of Different Flared Joint Configurations

ABSTRACT The paper deals with the stress distribution of flared tubular joints typically encountered in off-shore platforms and semisubmersible oil drilling structures. Stress results of an analytic computation treating the structure as a thin shell and using a finite element approach have been presented for an axial load condition and compared for the following four configurations: namely, a flared joint without any stiffeners, a flared joint with a ring stiffener at the junction between tube and flared portion, a flared joint with four cross-stiffeners (cruciform), and a flared joint with four cross-stiffeners and a ring stiffener. The analytical results for the non-ring stiffened joints have been compared with results obtained experimentally. The study indicates the effectiveness of the cruciform gusset plate in reducing the shell stresses in general. Ring stiffeners are instrumental in achieving a more uniform force transfer and in reducing the stresses in the critical joint regions. INTRODUCTION With the increasing emphasis on large Size diameter column members for both semisubmersible drilling units and drilling platforms, the design of the connection between the brace members and the column member has received considerable attention. A possible direct jacket-type connection is ruled out economically because of the requirement that the column wall thickness should preferably be equal to 4.5 per cent of the outer-diameter of the columns*. Amore appropriate solution is obtained by creating a load transfer through in-plane gusset plates. Another commonly used design incorporates external wing plates or so-called flared jointst2,3). In the latter cases, the intent is to spread' the branch member loads over a larger portion of the column member and to reduce the often critical local bending stresses in the column wall. This solution represents in effect an increase of the d/D ratio, i.e., ratio between the outer diameters d and 0 of the branch and column members respectively, and undoubtedly improves the state of stress in the column wall. However, unfortunately, both solutions inherently cause the development of stress concentrations in the branch member wall at either the tip of the wing plates or at the junction between the tubular section and the flared portion of the branch member. Because of the increasing use of flared connections, the paper deals with a study of the basic load transfer into the flared section as influenced by both ring and cruciform stiffeners. Because of the fundamental aspect of this study, a simple model consisting of a circular tubular section and a flared portion has been selected. The actual junction between the flare and the column has been omitted since the load transfer in that connection is in general less critical. The load condition selected for this basic model is a pure axial compressive load. The tubular joint under load approximates in structural behavior to a shell; i.e., the load is taken by the geometrical stiffness in addition to the material stiffness.