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RISS 인기검색어
- 검색키워드
- 저자 : Moazzam Aslam (검색결과 2 건)
- 무료
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- 유료
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Dynamic Analysis of an Articulated Offshore Tower Using Stokes Fifth Order Nonlinear Wave Theory
Moazzam Aslam,Mohd Moonis Zaheer,Nazrul Islam 대한토목학회 2023 KSCE Journal of Civil Engineering Vol.27 No.12
In offshore engineering, a compliant offshore structure accommodates dynamic forces by flexibility rather than rigidly withstanding load effects. As an alternative to conventional fixed platforms for loading/mooring in deep waters, articulated offshore towers are highly appealing because of their reduced structural weight. This study presents the results of a probabilistic random wave with and without current force on a 500 m high articulated tower with two hinges. By using the Lagrangian approach, governing nonlinear equations of motion are derived. The motion equation has been resolved in the time domain with the Houbolt approach. Based on Stokes fifth order nonlinear wave theory, wave forces have been assessed. Hydrodynamic forces are determined using a modified Morison equation with stretching modifications developed by Chakrabarti. The study examined two marine states, i.e., 10 m/10 sand 15 m/15 s. The current intensity of 2 m/sec has been taken into account. The salient statistical parameters such as maximum, minimum, mean, and standard deviation are presented under both the ocean environments. The proposed model is feasible for both the sea states. When comparing sea states, those with a greater wave height and period and those with a smaller wave height and period have more energy in their PSDs for all responses.
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Aerodynamic behaviour of double hinged articulated loading platforms
Zaheer, Mohd Moonis,Hasan, Syed Danish,Islam, Nazrul,Aslam, Moazzam Techno-Press 2021 Ocean systems engineering Vol.11 No.1
Articulated loading platforms (ALPs) belongs to a class of offshore structures known as compliant. ALP motions have time periods falling in the wind excitation frequency range due to their compliant behaviour. This paper deals with the dynamic behavior of a double hinged ALP subjected to low-frequency wind forces with random waves. Nonlinear effects due to variable submergence, fluctuating buoyancy, variable added mass, and hydrodynamic forces are considered in the analysis. The random sea state is characterized by the Pierson-Moskowitz (P-M) spectrum. The wave forces on the submerged elements of the platform's shaft are calculated using Morison's Equation with Airy's linear wave theory ignoring diffraction effects. The fluctuating wind load has been estimated using Ochi and Shin wind velocity spectrum for offshore structures. The nonlinear dynamic equation of motion is solved in the time domain by the Wilson-θ method. The wind-structure interactions, along with the effect of various other parameters on the platform response, are investigated. The effect of offset of aerodynamic center (A.C.) with the center of gravity (C.G.) of platform superstructure has also been investigated. The outcome of the analyses indicates that low-frequency wind forces affect the response of ALP to a large extent, which otherwise is not enhanced in the presence of only waves. The mean wind modifies the mean position of the platform surge response to the positive side, causing an offset. Various power spectral densities (PSDs) under high and moderate sea states show that apart from the significant peak occurring at the two natural frequencies, other prominent peaks also appear at very low frequencies showing the influence of wind on the response.
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