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Papers by ahmad reza mostafa gharabaghi

Research paper thumbnail of FOSM-based updated consecutive modal pushover procedure (FUCMP) for seismic uncertainty quantification of jacket offshore platforms

Research paper thumbnail of Introducing a simple and reliable multi-objective optimization method to estimate hull dimensions of a semi-submersible rig

Research paper thumbnail of Application of Artificial Neural Network and Fuzzy Inference System in Prediction of Breaking Wave Characteristics

Research paper thumbnail of Introducing a simple and reliable multi-objective optimization method to estimate hull dimensions of a semi-submersible rig

مهندسی دریا, 2020

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...

Research paper thumbnail of Application of Artificial Neural Network and Fuzzy Inference System in Prediction of Breaking Wave Characteristics

Wave height as well as water depth at the breaking point are two basic parameters which are neces... more Wave height as well as water depth at the breaking point are two basic parameters which are necessary for studying coastal processes. In this study, the application of soft computing-based methods such as artificial neural network (ANN), fuzzy inference system (FIS), adaptive neuro fuzzy inference system (ANFIS) and semi-empirical models for prediction of these parameters are investigated. The data sets used in this study are published laboratory and field data obtained from wave breaking on plane and barred, impermeable slopes collected from 24 sources. The comparison of results reveals that, the ANN model is more accurate in predicting both breaking wave height and water depth at the breaking point compared to the other methods.

Research paper thumbnail of Simulation of wave-induced motions of a moored floating breakwater using SPH method

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.

Research paper thumbnail of Simulating wave-structure interactions using a modified WCSPH scheme

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.

Research paper thumbnail of A multi-mode N2 (MN2) pushover procedure for ductility level seismic performance evaluation of jacket type offshore platforms

Ocean Engineering

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.

Research paper thumbnail of Application of poroelastic layers in a semi-submersible platform: Devising an efficient heave motion response reduction method

Ocean Engineering

Abstract This paper describes an efficient and applicable approach to decrease the heave motion r... more Abstract This paper describes an efficient and applicable approach to decrease the heave motion response of a semi-submersible platform by introducing poroelastic layers (PELs) at the bottom of its pontoons. This study was carried out utilizing a semi-analytical method based on eigenfunction expansion, which involves determining the added mass, damping, related hydrodynamic force and heave motion response. The adequacy of the semi-analytical method was verified by applying it to a typical GVA4000 drilling semi-submersible platform, whose heave response amplitude operator was extracted and compared with available experimental data. The extracted value and available data showed close agreement. Furthermore, the effectiveness of the PELs attached at the bottom of the GVA4000 drilling semi-submersible platform and the effect of PEL characteristics such as the thickness, intrinsic permeability, and elasticity on reducing heave motion response were evaluated. Comparing the results with the studied GVA4000 drilling semi-submersible platform without PELs indicated that the application of PELs could considerably reduce the heave motion response and nearly eliminate the risk of creating a resonance phenomenon in the semi-submersible platforms in different sea wave conditions. The approach introduced in this study could be a suitable pathway to improve the performance of semi-submersible platforms, particularly their heave motion responses.

Research paper thumbnail of Simulating Regular Wave Effects on a Pile-Moored Floating Breakwater Using a Modified WCSPH Method

Journal of Waterway, Port, Coastal, and Ocean Engineering

Research paper thumbnail of Pitch Motion Response of an Equipped Semi-Submersible Platform with Tuned Sloshing Dampers

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...

Research paper thumbnail of Effect of Sediment on Dynamic Pressure of Gravity Dams Using an Analytical Solution

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 ...

Research paper thumbnail of Effect of Wave Loading on the Seismic Behavior of a Typical Fixed Jacket-Type Platform

Volume 1: Offshore Technology, 2008

ABSTRACT Design of fixed offshore platforms to resist earthquakes involves many of the same probl... more ABSTRACT 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.

Research paper thumbnail of Study of Nonlinear Dynamic Behavior of a Typical Jacket Type Platform Under Environmental Loading (Wave and Current) and Blast Overpressure

23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B, 2004

ABSTRACT

Research paper thumbnail of Nonlinear Dynamic Behavior of Fixed Jacket-Type Offshore Platforms Subjected to Simultaneously Acting Wave and Earthquake Loads

23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B, 2004

ABSTRACT

Research paper thumbnail of A survey of passive wave absorbers

Research paper thumbnail of Dam-Reservoir Interaction Analysis Using Finite Element Model

Research paper thumbnail of Dynamic Behaviour of Sediment Deposits in Front of Concrete Gravity Dams

Research paper thumbnail of Study the Effect of Wave Directionality on Dynamic Nonlinear Behavior of Jack-Up Subjected to Wave and Earthquake Loading

Research paper thumbnail of Experimental investigation of the effect of splitter plate angle on the under-scouring of submarine pipeline due to steady current and clear water condition

The combustion process in diesel engines is under constant study. A major research effort is bein... more The combustion process in diesel engines is under constant study. A major research effort is being made to establish a better understanding of diesel combustion. Many issues regarding diesel combustion remain unresolved, and these issues may prevent the use of ...

Research paper thumbnail of FOSM-based updated consecutive modal pushover procedure (FUCMP) for seismic uncertainty quantification of jacket offshore platforms

Research paper thumbnail of Introducing a simple and reliable multi-objective optimization method to estimate hull dimensions of a semi-submersible rig

Research paper thumbnail of Application of Artificial Neural Network and Fuzzy Inference System in Prediction of Breaking Wave Characteristics

Research paper thumbnail of Introducing a simple and reliable multi-objective optimization method to estimate hull dimensions of a semi-submersible rig

مهندسی دریا, 2020

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...

Research paper thumbnail of Application of Artificial Neural Network and Fuzzy Inference System in Prediction of Breaking Wave Characteristics

Wave height as well as water depth at the breaking point are two basic parameters which are neces... more Wave height as well as water depth at the breaking point are two basic parameters which are necessary for studying coastal processes. In this study, the application of soft computing-based methods such as artificial neural network (ANN), fuzzy inference system (FIS), adaptive neuro fuzzy inference system (ANFIS) and semi-empirical models for prediction of these parameters are investigated. The data sets used in this study are published laboratory and field data obtained from wave breaking on plane and barred, impermeable slopes collected from 24 sources. The comparison of results reveals that, the ANN model is more accurate in predicting both breaking wave height and water depth at the breaking point compared to the other methods.

Research paper thumbnail of Simulation of wave-induced motions of a moored floating breakwater using SPH method

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.

Research paper thumbnail of Simulating wave-structure interactions using a modified WCSPH scheme

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.

Research paper thumbnail of A multi-mode N2 (MN2) pushover procedure for ductility level seismic performance evaluation of jacket type offshore platforms

Ocean Engineering

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.

Research paper thumbnail of Application of poroelastic layers in a semi-submersible platform: Devising an efficient heave motion response reduction method

Ocean Engineering

Abstract This paper describes an efficient and applicable approach to decrease the heave motion r... more Abstract This paper describes an efficient and applicable approach to decrease the heave motion response of a semi-submersible platform by introducing poroelastic layers (PELs) at the bottom of its pontoons. This study was carried out utilizing a semi-analytical method based on eigenfunction expansion, which involves determining the added mass, damping, related hydrodynamic force and heave motion response. The adequacy of the semi-analytical method was verified by applying it to a typical GVA4000 drilling semi-submersible platform, whose heave response amplitude operator was extracted and compared with available experimental data. The extracted value and available data showed close agreement. Furthermore, the effectiveness of the PELs attached at the bottom of the GVA4000 drilling semi-submersible platform and the effect of PEL characteristics such as the thickness, intrinsic permeability, and elasticity on reducing heave motion response were evaluated. Comparing the results with the studied GVA4000 drilling semi-submersible platform without PELs indicated that the application of PELs could considerably reduce the heave motion response and nearly eliminate the risk of creating a resonance phenomenon in the semi-submersible platforms in different sea wave conditions. The approach introduced in this study could be a suitable pathway to improve the performance of semi-submersible platforms, particularly their heave motion responses.

Research paper thumbnail of Simulating Regular Wave Effects on a Pile-Moored Floating Breakwater Using a Modified WCSPH Method

Journal of Waterway, Port, Coastal, and Ocean Engineering

Research paper thumbnail of Pitch Motion Response of an Equipped Semi-Submersible Platform with Tuned Sloshing Dampers

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...

Research paper thumbnail of Effect of Sediment on Dynamic Pressure of Gravity Dams Using an Analytical Solution

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 ...

Research paper thumbnail of Effect of Wave Loading on the Seismic Behavior of a Typical Fixed Jacket-Type Platform

Volume 1: Offshore Technology, 2008

ABSTRACT Design of fixed offshore platforms to resist earthquakes involves many of the same probl... more ABSTRACT 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.

Research paper thumbnail of Study of Nonlinear Dynamic Behavior of a Typical Jacket Type Platform Under Environmental Loading (Wave and Current) and Blast Overpressure

23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B, 2004

ABSTRACT

Research paper thumbnail of Nonlinear Dynamic Behavior of Fixed Jacket-Type Offshore Platforms Subjected to Simultaneously Acting Wave and Earthquake Loads

23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B, 2004

ABSTRACT

Research paper thumbnail of A survey of passive wave absorbers

Research paper thumbnail of Dam-Reservoir Interaction Analysis Using Finite Element Model

Research paper thumbnail of Dynamic Behaviour of Sediment Deposits in Front of Concrete Gravity Dams

Research paper thumbnail of Study the Effect of Wave Directionality on Dynamic Nonlinear Behavior of Jack-Up Subjected to Wave and Earthquake Loading

Research paper thumbnail of Experimental investigation of the effect of splitter plate angle on the under-scouring of submarine pipeline due to steady current and clear water condition

The combustion process in diesel engines is under constant study. A major research effort is bein... more The combustion process in diesel engines is under constant study. A major research effort is being made to establish a better understanding of diesel combustion. Many issues regarding diesel combustion remain unresolved, and these issues may prevent the use of ...