Comparison of Finite-Element Stress Solutions of Multi-Plane Tubular Joint With and Without Ring and Longitudinal Stiffeners
Authors: Rhee H.C. 
Published: October 2008

The effects of stiffeners on the stress state of a tubular joint are investigated. Several aspects of stiffener modelling for finite element analyses are first discussed, using a two-plane T-joint. This joint was modelled with and without internal ring and longitudinal stiffeners. The stiffened joint has two ring and two longitudinal stiffeners. Both stiffened and unstiffened joint models have an internal ring stiffener at each chord end to stiffen the chord. The finite element stress solutions along the brace-chord intersection lines of the joint with and without internal stiffeners are compared to understand the effects of stiffeners on the stress solutions.

INTRODUCTION

Stiffening by ring or longitudinal stiffeners is one of the most efficient methods to improve the resistance of an offshore structural tubular joint against static collapse or fatigue failure. Such stiffened joints are frequently used in the offshore industry. The analysis method used for stiffened joint is primarily the experimental approach. Since a test on a full scale tubular joint, whether it is stiffened or unstiffened, is expensive and time consuming, there are not many data available for the engineering of stiffened tubular joints in the industry. I The data required for the local design and integrity assessment for stiffened tubular joints can be developed economically using numerical analysis methods, especially the finite element method. Although other numerical analysis methods, such as the boundary element method, can also be used, the finite element method is the most efficient for tubular joint analyses because of its structural characteristics.

Once it is developed, a finite element model can be used repeatedly for different boundary and loading conditions to make a parametric study efficient for the joint design. Further, the recent development of computer hardware technologies makes numerical analyses of large and complex structural models inexpensive. For these reasons, the finite element method is becoming the main stream analysis method for offshore structural tubular joint analyses. However, the use of the finite element method for stiffened tubular joint analyses is rare in the industry. One of the reasons for the present infrequent use of finite element analysis for stiffened tubular joints is the difficulty in the development of the finite element models, due to the geometric complexity. Thus, an efficient finite element modelling capability is essential for the future engineering of stiffened tubular joints.

The objectives of this paper are to demonstrate that the development of the finite element models is practical for complex tubular joints with stiffeners when a well-conceived finite element preprocessing program is used and to present the advantageous effects of stiffeners in tubular joint design. A two-plane T-joint, which originated from a common caisson support joint of an offshore platform, was used for this study. This joint was modelled with and without stiffeners using PRETUBE. 2 For the stiffened joint, two different models were developed. One is with shell elements only, and the other with solid elements for the welds along the brace-chord intersections and with shell elements for other tubular parts of the joint.