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

Examensarbete för masterexamen
Master Thesis
Karlsson, Daniel
Forser, Mårten
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.
Materialvetenskap, Vattenteknik, Konstruktionsteknik, Materials Science, Water Engineering, Construction engineering
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