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

Paper in proceeding, 2016

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 study presents an analysis of a node connection 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 today. A 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. A 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%, hence, the cut-out design is recommended for the centre bulkhead connection.