Improvement in static and fatigue performance of offshore tubular joints is often achieved by stiffening the chord wall at the joint using internal ring stiffeners. This has been attributed mainly to the lessening of stress concentration and an even distribution of stresses around the joint. This aspect has been dealt with in detail in this paper. The performance of joints with varying number of stiffeners has been studied using the finite element approach. Wherever possible, comparison has been made with experimental results.
It is well recognised that the region of tubular intersections in offshore structures are characterised by severe stress concentration (UEG. 1985). On account of this, the ultimate strength and fatigue strength are considerably reduced. The stress field around a tubular Intersection is generally a function of the given geometry and type of loading. It is highly complex with local regions of very high stresses. When a tubular joint is subjected to brace loading, the chord member deforms in order to satisfy compatibility requirements at the intersection, thereby introducing bending and membrane stresses in the chord wall. Thus, the radial flexibility of the chord has a direct influence on the stress concentration and hence on the fatigue life of the joint. Of the two types of stresses induced in the chord wall, it is the magnitude of bending stresses that contribute to the high concentration of stresses at the tubular intersections. Stress concentration in tubular joints can be reduced to a large extent by the strengthening of the chord member. In practice, this Is often achieved either by increasing the chord thickness at the joint (known as ‘joint can’) or by providing stiffeners. The provision of joint can is beset with problems such as non-availability of plates with increased thickness, welding and fabrication of joints With low γ ratio and reduction in fatigue life due to thickness effect.
