Title: Numerical analysis of wind induced response of spar mooring-riser system / Abdulrahman Eyada Ibrahim
Abstract: The search for more sources of energy has become highly urgent given the excessive oil
and gas consumption in the 20th century and the depletion of most hydrocarbon
reservoirs in the world. In recent decades, the exploitation of hydrocarbon reservoirs
under the seas and oceans has been regarded a viable alternative. Several types of
floating structures are used in deep water oil and gas production. Spar platforms are the
latest type of platforms to be designed and utilized in deep water environments.
Coupled behaviour of spar–mooring–riser system under random wave, current and wind
loads have been studied using coupled analysis method. This study conducts nonlinear
analysis of a spar–mooring–riser system. The spar hull is modelled as a rigid beam
element with six degrees of freedom (DOF), with catenary mooring lines and riser pipes
treated as hybrid beam elements. The mooring lines are connected to the spar hull at the
fairlead on one end and anchored onto the seabed at the other end. In a similar manner,
the riser pipe is connected at the spar keel, and its lowest end is hinged to the seabed,
which is modelled as a rigid flat surface with a large area capable of mooring contact
behaviour simulation. Instantaneous damping and tension fluctuation forces of the
mooring–riser system, as well as other properties that vary with time have also been
considered. Finite element analysis of the spar–mooring–riser system simulated as a
single fully coupled integrated model using the ABAQUS/AQUA code was conducted
to obtain system responses under a long crested random wave with and without current
and wind inclusion. Some of the obtained results from free vibration, static and dynamic
analysis are compared with well–established published experimental results. Time
histories, power spectral density functions, and statistical analyses were used in evaluating the system responses. Riser inclusion caused more damping to the coupled
spar–mooring–riser system. The maximum values of the surge, heave and pitching
motions as well as mooring top tension are decreased by 22.9%, 63.3%, 20.7% and
3.1% respectively. Current load induces a significant shift in the spar position away
from its original place in the random sea state in addition to a notable reduction in the
heave, pitch and riser top tension responses. The diminishing fluctuation highlights the
firmness and controlled oscillations of the spar platform relative to its new mean
position. Aerodynamic loading induces greater lateral shifting of the spar hull with 88%
due to total hydro – aerodynamic loading with respect to wave and current induced
surge while the heave motion reduced by 13.5% for the wind and current inclusion
comparing with the case of current only. This fact shows the firmness of the Spar-mooringriser
system. The tension fluctuation caused by the wind force does not increased
because of the high pretension in mooring system; however, the top tension magnitude
of the mooring, riser top tension and Von Mises stress are significantly increased by
4.77%, 27.1% and 26.3% respectively.
Publication Year: 2017
Publication Date: 2017-12-01
Language: en
Type: dissertation
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