The effect of rack/rib plate on the stress concentration factors in jack-up chords

Abstract

It is well known that stiffener plates can have a large effect on the stress concentration factors found at the intersection of welded tubular joints. The strong relationship between the fatigue life of these tubular joints and the stresses experienced at the intersection make the accurate knowledge of the SCFs in this region of critical importance. Previous stress analysis work on SCFs in stiffened joints is briefly reviewed here. The results of almost 400 finite element models are presented in this paper, covering several popular jack-up chord designs. These results cover axial, in-plane bending and out-of-plane bending loading modes. This study has quantified the effect of rack/rib plates on SCF across a range of joint parameters representative of common SCF parametric equations for unstiffened tubular joints. The results of this FE study show that for every rack plate configuration investigated the hot spot SCF was reduced compared to the corresponding unstiffened joint. For axial loading, reductions of 50% of the unstiffened joint SCF were approached. The reduction under OPB was shown to be less but was still significant at around 20%. The peak SCF was unaffected under IPB loading as would be expected due to the different position of chord wall deformation. The mechanism by which the stiffener affects the changes in stress distribution has been investigated using evidence gained from applying various translational and rotational restraints to the chord surface of an unstiffened joint along the line where the central rack plate would intersect if it were present.