Sharif Digital Repository

Offshore jacket platforms are one of the most motivating structures for damage detection due to their importance and productivity. In this study, the application of finite element model updating in damage detection of an offshore jacket platform is investigated. The objective function of this method is based on the measured and analytical modal data, including natural frequencies and mode shapes. However, the measured data is expected to be noisy. Also, to avoid obtaining false damage results, a penalty term is added to the objective function. To update the model, genetic algorithm is utilized as a robust global searching tool. Afterward, the efficiency of this method is evaluated on several... 

The development of damage detection techniques for offshore jacket structures is vital to prevent catastrophic events. This paper applies a frequency response based method for the purpose of structural healthCgreater detection efficiency. In addition, the performance of the proposed method was evaluated in relation to multiple damages. The aforementioned points are illustrated using the numerical study of a two dimensional jacket platform, and the results proved to be satisfactory utilizing the proposed methodology  

Structural systems will normally fail as a consequence of a chain of different components failure es. In this paper, fatigue reliability of fixed offshore platforms is investigated by analyzing different failure scenarios. In order to evaluate the occurrence probability of a special scenario, it is divided into a finite number of sub-scenarios. All combinations of time sequences are generated for a given sequence of failures, using a specially developed program. In order to calculate the occurrence probability of each scenario, a massive reliability analysis should be done for each of corresponding sub-scenarios. A large number of sub-scenarios should be analyzed, therefore implementing time... 

In this paper a new ribbed bracing system (RBS) is proposed capable of performing as a variable stiffness system that can be used for controlling structural deformations and frequency shifting to compensate seismic energy. RBS has two important advantages. Because of ribbed system, the compressive member is rigidly moved like a piston and a cylinder and therefore it is a buckling prevented system. Also it is possible to use this system as a semi-active system by considering the story drifts and global structural damage and control the system if it is necessary to be open or closed based on the operational criteria assigned in the system. RBS has no need to any actuator and large power... 

In this paper, application of Endurance Time (ET) method in nonlinear seismic analysis of offshore pile supported systems has been studied. The ET method is a time-history analysis in which structures are subjected to intensifying artificial acceleration functions. The ET method reduces complexity and computational demand of conventional nonlinear seismic analysis, and it provides response at different seismic levels in a single ET analysis. The aforementioned methodology has been applied to a typical model of single pile and then to a functional jacket offshore platform in Persian Gulf region. Seismic response of aforesaid models by ET method has been compared with conventional time-history... 

Structural control using energy dissipater devices is emerging as a heavily researched strategy in earthquake engineering. Among several control systems, semiactive control is usually possible and efficient. In this research, a semiactive energy dissipating bracing system based on a viscous damper is proposed. In the conventional bracing systems, it is assumed that the braces can buckle under compression. Therefore, a semiactive on-off brace strategy is implemented to improve the conventional brace performance. Further, an energy absorbing mechanism is implemented. In the proposed system, the buckling of the member is prevented by implementing a one-way valve device. The permanent story... 

Total motion in waves can be considered as a superposition of the motion of the body in still water and the forces on the restrained body. In this study the effect of added mass fluctuation on vertical vibration of tension leg platforms (TLP) in the case of vibration in still water for both free and forced vibration subjected to axial load at the top of the leg is presented. This effect is more important when the amplitude of vibration is large. Also this is important in fatigue life study of tethers. The structural model used here is very simple. Perturbation method is used to formulate and solve the problem. First- and second-order perturbations are used to solve the free and forced... 

Compliant offshore structures are used for oil exploitation in deep water. Tension leg platform (TLP) is a suitable type for very deep water. The nonlinear dynamic response of TLP under random sea wave load is necessary for determining the maximum deformations and stresses. Accurate and reliable responses are needed for optimum design and control of the structure. In this paper nonlinear dynamic analysis of TLP is carried out in both time and frequency domains. The time history of random wave is generated based on Pierson-Moskowitz spectrum and acts on the structure in arbitrary direction. The hydrodynamic forces are calculated using the modified Morison equation according to Airy's linear... 

In this paper the dynamic response of the leg of a tension leg platform (tether) subjected to the load simulated as ocean wave at the top of the leg is presented. The structural model is very simple but several complicated factors such as foundation effect, buoyancy and simulated ocean wave load are considered. Two continuous models are proposed to present the structural system and the mentioned effects. The problem is solved by means of non-harmonic Fourier expansion in terms of eigenfunctions obtained from a non-regular Sturm-Liouville system  

The resulting motion in waves can be considered as a superposition of the motion of the body in still water and the forces on the restrained body. In this study the effect of added mass fluctuation on vertical vibration of TLP in the case of vibration in still water for both free and forced vibration subjected to axial load at the top of the leg is presented. This effect is more important when the amplitude of vibration is large. Also this is important in fatigue life study of tethers. The structural model used here is very simple. Perturbation method is used to formulate and solve the problem. First and second order perturbations arc used to solve the free and forced vibration respectively  

Employing dampers to control wave-induced and seismic vibrations of offshore jacket platforms is an attractive method in order to mitigate fatigue and seismic damage. However, adjustable parameters of a damper are designed by considering only one type of environmental loads; either normal-condition load or extreme-condition load. So, it is important to investigate effectiveness of damping system, for both of two main categories of environmental loads. Also it is ideal for the system to have an acceptable performance in both normal and extreme conditions. The idea investigated in the current study is to use a friction damper device (FDD) and a tuned mass damper (TMD) simultaneously in... 

Many existing buildings are irregular in plan or elevation because of asymmetric placement of masonry infills. The stiffness of masonry infill is a considerable value relating to that of the structure. Produced torsion from eccentricity because of infill stiffness leads to extra forces and deformations in structural members and diaphragms. An appropriate alternative to solve this problem especially in existing buildings is using dampers. Dampers can enhance structural performance by reducing seismically induced lateral displacements and by reducing inelastic behavior of beams and columns. In this paper some simple models are used to show structural modeling and a conceptual discussion is... 

A friction damper device (FDD) is used for vibration control of an existing steel jacket platform under seismic excitation. First, the damping is presented for vibration mitigation of structures located in seismically active zones. A new method for quick design of friction or yielding damping devices is presented. The effectiveness of the damping system employing such FDDs in a jacket platform is evaluated numerically. The influence of key parameters of the damping system on the vibration suppression of the offshore structure is studied in detail. To examine the vibration control effectiveness of the FDD for the jacket platform, performance of the controlled structure under the seismic... 

In this article, a novel passive control system, ribbed bracing system (RBS), has been proposed to deal with the buckling problem. RBS is a bracing system with a simple mechanism that can be installed in braces as a supplemental part. The behavior of RBS is similar to that of conventional braces under tensile loading. However, under compressive force, it endures an insignificant force and prevents the braces from buckling through length reduction. In addition, seismic damage is concentrated in the bracing system of the structures equipped with RBS, decreasing the dissipated hysteretic energy in other structural members. There are two different mechanisms for RBS: 1) completely-closed RBS... 

This study presents the experimental investigation of half-scale, one-story, one-bay pinned frames equipped with a ribbed bracing system (RBS). The RBS is a newly developed passive control system designed to eliminate buckling and enhance the seismic behavior of structures. Here, mechanical models of this bracing system were designed and constructed. Pinned frames equipped with the RBS were cyclically tested. The hysteretic behavior and energy absorbing capacities of the frames were evaluated. Based on the results, the full plastic capacity of the brace was achieved and no buckling occurred. The RBS frame illustrated proper hysteretic behavior and energy dissipation capacity up to 4% story... 

In this paper, an innovative seismic lateral force resisting system for tall buildings is introduced. In this system, a novel supplemental part, ribbed bracing system (RBSyst), is attached to Braced Tube System, creating a modified BTS. RBSyst is a supplemental part which is attached to the conventional bracing members to eliminate buckling problem. The behavior of RBSyst under tensile force is similar to that of the conventional braces. However, in compression, it prevents the braces from buckling by length reduction. In order to evaluate the efficiency of this new BTS system by performance-based assessment, two typical 40-story tall buildings with different story modules equipped with this... 

In this paper a new approach is introduced for structural health monitoring of offshore jacket platforms. The procedure uses the measured ambient vibration responses and the corresponding readable natural frequencies and mode shapes of the structural system. Since offshore platforms are composed of heavy topsides supported by jacket structures, participation of the first mode is dominant in each direction in the response of the structure under field excitations. Moreover, ambient vibrations such as wave loads and boat impacts only excite the first modes of the structure. Therefore, it is difficult to find higher modes and the pertinent frequencies by use of accelerometers data. The... 

In this paper, an innovative seismic lateral force resisting system for tall buildings is introduced. In this system, a novel supplemental part, ribbed bracing system (RBSyst), is attached to Braced Tube System, creating a modified BTS. RBSyst is a supplemental part which is attached to the conventional bracing members to eliminate buckling problem. The behavior of RBSyst under tensile force is similar to that of the conventional braces. However, in compression, it prevents the braces from buckling by length reduction. In order to evaluate the efficiency of this new BTS system by performance- based assessment, two typical 40-story tall buildings with different story modules equipped with... 

This study presents the experimental investigation of half-scale, one-story, one-bay pinned frames equipped with a ribbed bracing system (RBS). The RBS is a newly developed passive control system designed to eliminate buckling and enhance the seismic behavior of structures. Here, mechanical models of this bracing system were designed and constructed. Pinned frames equipped with the RBS were cyclically tested. The hysteretic behavior and energy absorbing capacities of the frames were evaluated. Based on the results, the full plastic capacity of the brace was achieved and no buckling occurred. The RBS frame illustrated proper hysteretic behavior and energy dissipation capacity up to 4% story... 

The harmonic displacement response of a beam is utilized as the input signal function in wavelet analysis. Sudden changes in the spatial variation of transformed response identify the location of damages and defects. The damage incurred causes a change in the stiffness or mass of the beam. This causes a localized singularity which can be identified by a wavelet analysis of the displacement response. In this article, it is shown that using harmonic response is superior to the static deflection response and this approach is more effective in the presence of noise and more sensitive to the versatility of the applied harmonic loads. © 2007 SAGE Publications