Açık deniz uzay kafes sistemin çevresel yükler altında akışkan-yapı etkileşimli analizi

Abstract

Bu çalışma kapsamında, açık deniz platformunun kafes tipi altyapısına iki yönlü akışkan-yapı etkileşim analizi yapılarak, yapının dinamik ve modal davranışı incelenmiştir. İncelenen model 62.5 m yüksekliğinde, dört katlı, silindirik elemanlardan oluşan, açılı-dört ayaklı, açık deniz kafes sistemdir. Yapı operasyonel yüklerin yanında rüzgar ve dalga kuvvetlerini içeren çevresel kuvvetlerin etkisi altındadır. Rüzgar kuvvetinin hesaplanmasında Eurocode hız profili kullanılırken, dalga kuvvetlerinin hesaplanmasında Lineer dalga hız profili kullanılmıştır. Yapının, çevresel kuvvetlerin ve bunların etkileşiminin modellenmesinde ABAQUS sonlu elemanlar analiz programı kullanılmıştır. Yapı ABAUS/Explicit çözücüsünde modellenirken, çevresel kuvvetleri meydana getiren rüzgar ve dalga ABAUS/CFD çözücüsünde modellenmiştir. Bu iki çözücünün etkileşimi, akışkan-yapı etkileşim modülü ile sağlanarak, yapının her katında meydana gelen maksimum deplasmanlara ve gerilmelere ulaşılmıştır. Gerilme ve deplasmanların yanında yapının ilk dört modu ve ilgili mod şekilleri elde edilmiştir. Numerik modelleme ile sayısal ve görsel sonuçlar elde edilmiştir. Nümerik sonuçlar yarı analitik modelleme yapılarak kontrol edilmiştir. Yarı analitik yöntemde, yapı çok serbestlik dereceli sistem olarak modellenirken, çevresel yükler ayrıca hesaplanarak yapıya etki ettirilmiştir.

Keywords

• Açık deniz kafes sistem, çevresel yükler, akışkan-yapı etkileşimi, nümerik analiz

Fluid structure interaction analyses of offshore lattice tower under environmental loads

Abstract

In the scope of this study, bidirectional fluidstructure interaction (FSI) analysis is performed to investigate dynamic and modal behaviors of lattice substructure of offshore tower. The studied model is a jacket type offshore structure with four stories having 65 m height and formed of cylindrical members with four angled legs. The structure is under the effect of environmental loads as wind and wave loads as well as operational ones. While Eurocode velocity profile is used to calculate wind forces, Airy wave velocity profile is utilized to determine wave forces. Abaqus finite elements analysis program is performed to model the structure with environmental loads. In the first step of this study, three dimensional offshore lattice tower with its environments are modeled in the finite elements analysis program to investigate the dynamic behavior under environmental loads. Secondly, the structure is also modeled as MDOF to determine the accuracy of the analysis results. The structure is created by Abaqus/Explicit solver. On the other hand, wind and wave forces are modeled by Abaqus/Cfd solver. The interaction of the solvers is provided by FSI module to determine the maximum displacement and stress values in each story of the structure. Co-simulation module of Abaqus analysis program is used to determine the exact interaction with solid (Abaqus/Explicit) and fluid models (Abaqus/Cfd).In Co-simulation module the boundary conditions on surfaces should be specified. The inlet velocities are parallel to the inlet flow axis without cross stream components. In the outlet boundary, zero pressure value and no streamwise variation for velocity components are set. Surfaces in z direction and top surface in y direction are set far field boundary conditions whose velocities are assumed to be equal to inlet velocity. On the other hand, base surface is defined as non-slip wall boundary condition. Fixed support conditions to sea floor are assigned on the lattice tower. Meshing operation is the last step of finite elements modeling. 10-node modified tetrahedron elements (C3D10M) are used in ABAQUS/Explict model. Besides, 4-node modified tetrahedron elements (FC3D4) are utilized in wave and wind environments in ABAQUS/CFD model. In finite elements program, the models are divided into nodes to perform and make complex analysis more simple analyze complex models. Distance between nodes in lattice tower is 0.01 m which is less than profile thickness. In wave and wind models, while node distance is 0.01 m between lattice tower and contact surfaces, the distance is 0.10 m in other parts. While complex structure is modeled in FEM analysis, the structure is modeled as a single vertical line in MDOF method. Investigated model is quite applicable in terms of its geometry. For this reason, the structure is idealized as a lumped mass tower. Lattice type substructure is idealized a multi degrees of freedom (MDOF) system that is subjected to time varying horizontal wave force and static wind force. In addition, first four modes with related mode shapes are obtained as well. Numerical results are controlled by semi-analytic modeling. The structure is modeled as multi-degree of freedom system in semi-analytic model and environmental loads are also effected to the structure. First of all, natural frequency results are investigated after analyses. It is seen that the difference between two analysis types change between %3.76 and %3.95. Fourth mode appears to be torsion one when mode shapes are investigated. Compatibility of peak point displacements is detected. Difference in displacement values from first to top story is calculated as %2.18, %4.11, %4.20 and %4.32 respectively. Environmental forces calculated by CFD module of finite elements analysis program is transferred to the structure by FSI and structural analysis is completed. The accuracy of MDOF and finite elements analyses results are determined in the end. Because, node and element numbers make FEM analyses more difficult when structure height increases, MDOF analysis can be used as an alternative method when fluid outputs are not necessary other than structural ones

Keywords

• Offshore lattice tower, environmental loads, fluid-structure interaction, numerical analysis