In a riser design, the assessment traditionally adopted by the industry for the attainment of platform motions applied to the top of the riser consists in the use of de-coupled methodologies. Those formulations consider the static environmental loads over the platform (current and wind) through a static offset and the dynamic environmental loads due to wave through imposition of top riser displacement calculated from cross spectral response of sea spectrum and vessel's RAOs (Response Amplitude Operators). Nowadays due to shift of oil and gas exploitation to deeper waters more accurate methodologies, based on coupled analysis, have been introduced. The coupled analysis considers the interaction between the hydrodynamic behavior of the hull and the structural behavior of mooring lines and risers submitted to environmental loads. For deep waters the coupling effects of lines over platform motions can be especially significant and it is expected a reduction of the amplification of platform motions if compared to the platform motions obtained from de-coupled analysis. This paper presents a typical Steel Catenary Riser design, connected to a semi-submersible platform, where the motions applied to the top of riser are obtained from the "traditional" way (de-coupled) and from the coupled analysis. A numerical application will be presented in order to assess the comparison of the two presented methods in terms of SCR results. The coupled model studied herein is composed of approximately 80 lines connected to the platform, which requires an excessive computational effort. In order to reduce the CPU time some additional studies are performed considering the variation of line mesh discretization and time step size. The objective of this study is the adoption of an optimized model keeping the required accuracy of platform motions results to be applied on top of SCR. The CPU time consuming and platform motions are presented. The significance of this work is the conclusion that a Steel Catenary Riser design adopting prescribed displacements from coupled analyses will provide more realistic and optimum results.
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