Structural analysis of node cut-outs in a semi-submersible offshore platform

Abstract

The connection between column and deck-box on a semi-submersible platform is often subjected to high stress concentrations that can be governing for the structural strength. One common design used to withstand the high stresses is a cast integral in high tensile material, which is both heavy and expensive to manufacture, which is why an alternative cut-out design is investigated. This thesis aims at analysing the current node connection, with a finite element analysis (FEA), with regard to yield stresses, buckling, fatigue damage, weight and cost in order to propose an alternative cut-out design using the same criteria. The cast integral, which is analysed as a reference case and used for comparison of designs, is of a typical design used by GVA Consultants AB. The alternative cut-out design is first studied in a simplified two-dimensional (2D) case, developed with applied loads and boundary conditions following recommendations from the classification society DNV-GL. Different geometrical cut-out shapes used in other applications are tested and evaluated. The cut-out design with the lowest yield stresses is implemented and further evaluated in a case study of a complete semisubmersible production platform. The cut-out design is investigated with a parametric study for the complete loading condition, which is defined from previous calculations performed by GVA. The cut-out is analysed with both the simplified fatigue assessment and with a stochastic approach, in order to investigate differences and inaccuracies in the methods. The parametric study shows that the use of a cut-out design may be advantageous in the centre bulkhead connection between the column and the deck-box. The adjacent geometry is of simple character and the cut-out does not affect the overall structural design. The cut-out is found to be a feasible alternative to the cast integral, with yield stresses and fatigue damage in the same range. Further refinements of the cut-out design could, however, decrease both yield stresses and fatigue damage, which is why the cutout is recommended in future work. The suggested cut-out solution leads to a decrease of 4.5 % in weight for a reference structure. For the same reference structure the production cost is estimated to decrease by 9.5 %, hence, the cut-out design is recommended for the centre bulkhead connection. However, it is concluded that the cut-out design is only developed for one specific semisubmersible platform in specific load conditions, for which reason further analysis and adjustments are needed for other applications and structures.