Açık deniz uzay kafes sistemin çevresel yükler altında akışkan-yapı etkileşimli analizi
Öz
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.
Anahtar Kelimeler
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
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