Search

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.

Passive Tuned Liquid Column Dampers, which are cost-effective, easy to implement, and reliable structural vibration control devices, have received significant attention from designers and researchers. Tuned liquid column dampers are divided into four categories based upon their configuration, i.e. the U-shaped TLCD (UTLCD), V-shaped TLCD (VTLCD), TLCD with no Uniform Section (LCVA) and gas TLCD or with spring (TLCGD) to organize this review systematically. In this study, first, the seismic performance of a jacket-type platform equipped with UTLCD under three earthquake records is studied numerically with MATLAB software. Then, according to the value of the platform natural frequency, the design of a variety of TLCDs has been explored. To optimize the dynamic behavior of the damper-platform system, a general flow chart for the design of TLCDs was presented. Furthermore, discussions and investigations have been conducted on the effects of a fluid sloshing phenomenon on damper design. It has become apparent that using LCVA or TLCGD is the best solution to provide a reliable and effective damper design if the natural frequencies of the platform are below 0.5 Hz. TLCGD has a very small limit in terms of optimum design between these two dampers.

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.

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 wave-structure 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 wave-structure interactions.