Papers by Ahmad Reza Mostafa Gharabaghi
Design of fixed offshore platforms to resist earthquakes involves many of the same problems and i... more Design of fixed offshore platforms to resist earthquakes involves many of the same problems and issues as other onshore structures. However, there are some important differences that the most important one is the presence of seawater. It influences the mass, stiffness, damping and strength characteristics of the platform, which can have important effects on the structure response characteristics. The entrained water inside the platform elements and the water that is accelerated by the motions of the structure, have very important effects on the mass and damping characteristics of the platform. The presence of other environmental loads, particularly waves can also influence the response of these structures. In this paper, the nonlinear response of a typical jacket type platform, which has been installed in Persian Gulf, under wave and earthquake loadings are studied. The structure is modeled by ANSYS, finite element software. Time histories of different earthquake loadings are applied. Wave characteristics are based on local information. These loadings are applied separately and simultaneously. Moreover, they are applied in the same direction and different directions. The results are compared with each other and the most sever cases are extracted. 1-INTRODUCTION Among different types of offshore platforms, fixed jacket type is the most common platform used on the continental shelves all around the world. These platforms are subjected to different environmental loads during their lifetime. These loads are imposed on platforms through the natural phenomena such as wind, current, wave, earthquake, earth movement, snow, and ice. Among various types of environmental loading,
Marine Engineering
In this study, the damping sheets were attached to a semi-submersible platform to mitigate the he... more In this study, the damping sheets were attached to a semi-submersible platform to mitigate the heave motions. We first examined a typical GVA4000 semi-sub under a monochromatic linear wave train in deep water. An analytical method based on diffraction theory was used to determine the heave motion response. The results revealed that the analytical results show good agreement with available experimental data. Then, the damping sheets were attached to the pontoons of the GVA4000 semisub under study. The heave motion response was evaluated with the changing numbers and widths of sheets under constant thickness in different sea waves conditions. The results showed that with increasing damping sheet width, the heave motion response is improved. Moreover, the heave natural frequency of semi-sub is shifted far away from the monochromatic wave frequency, so, the resonant phenomenon significantly decreases. Based on the proposed method, it is concluded that the solution to reducing heave motion response of the semi-submersible platform is simple, effective, economical and practicable.
AIP Conference Proceedings, 2008
This paper presents an analytical solution to get a reliable estimation of the earthquake‐induced... more This paper presents an analytical solution to get a reliable estimation of the earthquake‐induced hydrodynamic pressure on gravity dams by proposing closed‐form formulas for the eigenvalues involved when solving the fluid and dam interaction problem. A new analytical technique ...
مهندسی دریا
For optimal design of the hull dimensions of a semi-submersible platform, a method is required th... more For optimal design of the hull dimensions of a semi-submersible platform, a method is required that calculates optimal answers in a short time according to the certain objectives. In this study, the grid search (GS) algorithm was introduced in the form of triple-objective function in order to minimize the hull weight and reduce the heave and pitch motion response. For this purpose, first the equations of the hull weight and heave and pitch motion response were discretized as the parametric equations. The design variables and constraints of the semi-submersible platform were considered. Then, this algorithm was implemented as a single-objective function with the objective of minimizing hull weight of a semi-submersible platform and its efficiency was evaluated by comparing the results with the results from previous studies. Subsequently, the GS algorithm was developed as the triple-objective function, which gives acceptable results in the shortest computational time. This study shows that the GS algorithm is an appropriate method for multi-objective optimization and can contribute to the more economical and efficient design of floating platforms.
Article History: Received: 15 Jan. 2019 Accepted: 15 Jun. 2019 Tuned sloshing dampers (TSDs) are ... more Article History: Received: 15 Jan. 2019 Accepted: 15 Jun. 2019 Tuned sloshing dampers (TSDs) are applied to dissipate and absorb vibrational energy in structures. They can become an appropriate candidate for damping vibration in rotating offshore structures. In this study, the TSD systems are utilized in a semi-submersible platform in order to suppress its pitch motion response. First, the hydrodynamic behaviors of two different types of vessels are evaluated including a typical GVA4000 semi-submersible rig, and a floating oil storage tank using a finite element analysis. The results are compared with the available data from previous research, which the agreement is good. Subsequently, the semi-submersible platform equipped with four TSDs, which are located inside the bilge of pontoons and filled with 20% water is analyzed. It is analyzed in the frequency domain by considering the effect of internal fluid sloshing of TSDs. The results show that TSDs play a significant role for reduc...
In this study, the dynamic motions of a rectangular chain-moored floating breakwater with three d... more In this study, the dynamic motions of a rectangular chain-moored floating breakwater with three degrees of freedom (heave, sway, and roll motions) are simulated using a weakly-compressible smoothed particle hydrodynamic (WCSPH) scheme. A regular wave is simulated using a piston type wave maker inside a numerical wave flume. Then, a rectangular moored floating breakwater is placed inside the developed numerical wave flume and the dynamic motions of the structure are recorded. In the developed numerical scheme, the wave-structure and mooring-structure interactions are evaluated by solving new SPH-based equations. In addition, the mooring system is simulated by the SPH particles. Comparing the obtained results from the developed SPH scheme with the experimental data reveals that the modified smoothedparticle hydrodynamic scheme is able to simulate the dynamic motions of a chain-moored floating breakwater with a reasonable accuracy.
Article History: Received: 9 Jan. 2017 Accepted: 22 Jul. 2017 A weakly compressible SPH (WCSPH) s... more Article History: Received: 9 Jan. 2017 Accepted: 22 Jul. 2017 A weakly compressible SPH (WCSPH) scheme has been developed to simulate interaction between waves and rigid bodies. The developed WCSPH scheme is improved by applying a modified equation to calculate the wavestructure interaction, in order to increase its accuracy. The effects of relative fluid/solid particles’ acceleration are considered in the modified equation. To evaluate the efficiency of developed model, the dynamics of structural movements and related pressure fields are investigated for several test cases and the results are compared with the experimental data. It seems that the modified algorithm is able to improve the accuracy of simulated wavestructure interactions.
Ocean Engineering, 2021
Abstract The static pushover analysis can be employed to check the adequate ductility of jacket t... more Abstract The static pushover analysis can be employed to check the adequate ductility of jacket type offshore platforms (JTOPs) under the abnormal level earthquake (ALE) in the design phase. The conventional pushover analysis suffers from different inherent limitations such as disregarding the higher mode effects. In this paper, it is intended to develop an advanced pushover procedure for the seismic design of JTOPs. To this end, a multi-mode N2 (MN2) pushover procedure is proposed that modifies the SRSS modal combination rule in the extended N2 procedure. The proposed procedure utilizes several modes of vibration to increase the accuracy of the method. The presented procedure is applied to two case study JTOP structures for the ductility level seismic performance evaluation. The seismic demands obtained by the MN2 procedure is compared with the average results of the benchmark nonlinear response history (NRH) analyses conducted using a suite of far-field ground motion records scaled to the ALE design response spectrum. The results of a statistical study show that there is a strong correlation between the spectral values of the records at the shorter periods of JTOPs and the responses of the foundation and the lower braced story levels of the jacket, which emphasize the important effects of the higher modes on the seismic demands of JTOPs if considering soil-pile-structure interaction. The utilized formulation for the modal combination is shown to improve the accuracy of the MN2 method at the pile foundation and at the lower braced story levels of the jacket. Moreover, the seismic drift demands predicted by the proposed MN2 method are in excellent agreement with the benchmark NRH results along the height of the platforms. Considering the simplicity and ease of application, this method can be utilized as a useful tool for checking the design ductility level earthquake.
Journal of Waterway, Port, Coastal, and Ocean Engineering, 2017
Volume 1: Offshore Technology, 2008
Design of fixed offshore platforms to resist earthquakes involves many of the same problems and i... more Design of fixed offshore platforms to resist earthquakes involves many of the same problems and issues as other onshore structures. However, there are some important differences that the most important one is the presence of seawater. It influences the mass, stiffness, damping and strength characteristics of the platform, which can have important effects on the structure response characteristics. The entrained water inside the platform elements and the water that is accelerated by the motions of the structure, have very important effects on the mass and damping characteristics of the platform. The presence of other environmental loads, particularly waves can also influence the response of these structures. In this paper, the nonlinear response of a typical jacket type platform, which has been installed in Persian Gulf, under wave and earthquake loadings are studied. The structure is modeled by ANSYS, finite element software. Time histories of different earthquake loadings are applie...
23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B, 2004
ABSTRACT
23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B, 2004
The nonlinear dynamic response of jacket-type offshore platform (which has been installed in Pers... more The nonlinear dynamic response of jacket-type offshore platform (which has been installed in Persian Gulf) under simultaneously wave and earthquake loads is conducted. The interaction between soil and piles is modeled by Konagai-Nogami model. The structure is modeled by finite element method. The analyses include models with the longitudinal component of earthquake and wave in the same direction and in different directions. The results indicate that when the longitudinal component of earthquake and wave are in the same direction, wave may reduce the response of studied platform and when they are in different directions, in some cases there is an increase in the response of platform.
Volume 2: Ocean Engineering and Polar and Arctic Sciences and Technology, 2006
The dynamic response of gravity type quay wall during earthquake including soil-sea-structure int... more The dynamic response of gravity type quay wall during earthquake including soil-sea-structure interaction is calculated using ADINA finite element techniques. The main objective of this study is to investigate the effects of fluid-structure interaction on the residual displacement of wall after a real earthquake. A direct symmetric coupled formulation based on the fluid velocity potential is used to calculate the nonlinear hydrodynamic pressure of sea water acting on the wall. The doubly asymptotic approximation (DAA) is used to account for the effects of outer fluid on the inner region. The non-associated Mohr-Coulomb material behavior is applied to model the failure of soil. The full nonlinear effective stress analysis is performed in this study and the soil-pore fluid interaction effects are modeled using porous media formulation. Viscous boundary condition is implemented to model the artificial boundary in direct method analysis of soil-structure interaction system and sliding c...
The most popular offshore structures, jacket platforms, are made of tubular members that welded t... more The most popular offshore structures, jacket platforms, are made of tubular members that welded to gather. Due to dynamic and harsh environment, fatigue analysis and assessment of these structures is an essential problem. The S-N curve method is an accepted procedure for estimating fatigue life of jackets. In this method the maximum range of stress occurred during loading is needed. In consequence of geometry, stress distribution in tubular joints under various loadings is complicated and they have some points of concentrated stress; thus these regions are the most critical places of jackets in any loading. The general method to calculate the maximum stress of tubular joints is employing equations that give factors multiplied in brace's stresses and lead to considered values. At the present there are no such equations for tubular KK-joints. In present research a wide data bank of stress concentration factors (SCFs) is produced by parametric study on these joints under balanced a...
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Papers by Ahmad Reza Mostafa Gharabaghi