Sammendrag
This paper presents challenges and solutions related to establishing structural reanalysis system models of existing offshore concrete platforms. The main challenges that will be described are establishing a global finite element model (GFEM) using solid elements for the complete reinforced concrete substructure, modelling of the response from post-tensioning, wave and hydrodynamic analyses, analyses of built-in stresses from the construction phase and capacity control of shell sections and critical nodal regions. The GFEM is established using a fully parameterized and modular workflow, allowing for distributed workloads within a group of analysts. Similarly, all load cases are considered separately, run in a tailor-made analysis system, and with further post-processing and combination into several thousand design load combinations. The system allows for an accurate inclusion of post-tensioning forces in the GFEM, by modelling the geometry of each cable based on as-built drawings, while accounting for short- and long-term post-tensioning losses. During construction and deck-mating, stresses from temporary phases are built into the structure. The method for analysing these effects is demonstrated. The governing variable loads on such structures are the wave loads. These are accounted for through a process of screening for critical sea states on simplified models, and further application of pressures for discrete wave hights, phase angles and wave directions on the GFEM. Capacity checks of the shell sections are performed considering interaction of membrane and plate forces, while critical nodal regions are controlled separately. Throughout the paper, a concrete gravity based structure (GBS) is used as a demonstration case.
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