فهرست مطالب ahmadreza mostafa gharabaghi

Cylindrical piles are often used in most of coastal and offshore structures such as bridges, wharfs, offshore wind turbine foundations and Oil platforms. Most of these structures are installed in shallow water and exposed to strong currents, waves and broken waves. These phenomena can cause scour around them which can damage their structural integrity and stability. In literature, scour process around the cylindrical piles under currents and waves have been studies frequently. However, there are very little knowledge about the bed forms due to the broken waves. In this paper, the effect of broken waves on the characteristics of bed forms around a cylindrical pile has been studied experimentally in a large wave flume. The three-dimensional bed topography was measured by Close Range Photogrammetry. Vortex ripples and truncated cone scour with vortex ripples were the main observed scour pattern. Shields parameter as well as Keulegan-Carpenter (KC) number were used as the non-dimensional parameters for bed form classification. It is noticed that the ripple height and ripple steepness for broken regular waves is less than non-breaking regular waves. However, the relative equilibrium scour depth for broken waves is larger than non-breaking waves.

Waves propagating to the shore can experience breaking in the near-shore zone which can cause sediment transport due to the large vortices and turbulence generated by broken waves. The pattern of sediment transport can be affected by the presence of slender cylindrical members forming the major components of many coastal and offshore structures. In this paper an experimental investigation was performed to measure the bed morphology due to the breaking wave impact on a slender vertical cylinder. Exact and rapid measurement of bed topography is very important in experimental hydraulics as it helps understanding of these complex processes and measured bed mapping. This can substantially aid in the design of particular projects. This paper gives a brief description of the close-range photogrammetry method that is currently available for bed mapping in hydraulic modeling. Digital Elevation Models (DEMs) and 3D maps are created; thus 3D bed figures and scouring patterns are determined.

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 semi-sub 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.

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.

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 reducing the pitch motion response of the semi-submersible platform.