Stress-Concentration Factor Analysis of Large-Diameter Stiffened Nodes

ABSTRACT Stress concentration factor (SCF) determination for large diameter, complex stiffened tubular joints remains a labor and computer intensive effort. This paper describes results obtained from finite element analysis of two such nodes from the Ninian Southern platform in the North Sea. Axial, in-plane moment, and out-of-plane moment SCF's are presented for the five braces of each node. The influence of boundary conditions, weld geometry, and load interaction are considered. SCF's are computed in accordance with U.K. Dept. of, Energy procedures. Results provide an indication of SCF values for large diameter, stiffened nodes. This, can be of value in comparing cast versus welded nodes for deepwater fixed platforms. INTRODUCTION The Ninian Southern structure is a fixed, steel jacket located in 140 meters of water in the 'North Sea. It was towed to location and installed in May 1977. While conducting a routine inspection in June 1984 it was discovered that a horizontal member at the -43 meter elevation was broken. Investigation by The Welding Institute showed that failure resulted from the propagation of a weld defect at the root of a single-sided weld at a temporary access manway. While no evidence of cracking was found at, the nodal connections, it was considered prudent to re-evaluate node fatigue lives. Consequently, SCF's were computed for the stiffened and unstiffened nodes on Ninian Southern platform. NODE DESCRIPTION This paper describes computational procedures and resulting SCF's for two of the nodes on the large (9.2 meter diameter) flotation legs. These nodes are at elevations -43 m and -12 m, and are representative of the remaining large leg nodes. Figure 1 shows the location of these nodes on the" Ninian Southern platform, as well as the structure's framing pattern. A cut away view of the -43/F4 node is shown in Figure 2. The node is heavily stiffened with ring stiffeners, horizontal and vertical diaphragms backing the braces, and circular or elliptical back-up tubes behind the "footprint" of all the braces. Except for the horizontal diagonal brace, the chord wall is locally thickened with a 63 mm insert plate. Brace member dimensions are given in Table 1, along with no dimensional joint geometry parameters. Circular and elliptical back-Up stiffeners are the same thickness as the corresponding brace stubs. The -12/F4 node is similar, with the following exceptions:Brace and stub sizes differ.Chord wall is generally 25 mm (vs 38 mm for -43/F4).The 63 mm insert plates are at all brace intersections.There is greater eccentricity in the line of action of the braces.The chord is stiffened by longitudinal stiffeners in addition to all previously cited stiffening. Table 2 contains brace member dimensions and joint geometry parameters for node -12/F4.