JOURNAL OF MARINE ENGINEERING
Year:2015 | Volume:11 | Issue:22|Papers Count:12

In this paper a new method is suggested to calculate added mass coefficients of an underwater vehicle. In this method linear accelerated motion of the underwater vehicle is numerically simulated. Velocity and acceleration related forces, are extracted from the simulation results, from which the added mass coefficients are extracted. In order to validate the obtained results, initially, the added mass coefficients of an ellipsoid are calculated with the use of the present method and compared with the available corresponding analytical results. The obtained results from the suggested numerical method agreed well with those obtained from the analytical method, indicating that the present method can be used to obtain the added mass coefficients of more complicated geometries, including the underwater vehicle model constructed in Subsea Research and Development Centre. In addition, the numerically obtained results agreed well with the experimental results recently obtained, using the Planar Motion Mechanism tests conducted in Subsea Research and Development Centre.

Increased use of Autonomous Underwater Vehicles (AUVs) caused an increase in their design sensitivity. The subject of most of AUV design studies was on drag reduction, ease of handling and their stability. Adequate design and prediction of the behavior of these vehicles requires an accurate estimation of corresponding hydrodynamic derivative loads. In this study, Hydrodynamic derivatives of the AUV have been estimated using numerical and semi empirical methods. First, using a numerical method based on CFD, hydrodynamic derivatives related to the body of the Hydrolab500 have been estimated by applying pure heave and pure pitch oscillatory movements, and then, with the aid of semi empirical formulas, hydrodynamic derivative values of hydrofoils have been added. To validate the results, Planar Motion Mechanism (PMM) experimental test has been carried out using water tunnel tests. The validation results show that combining CFD method and semi empirical method is efficient in the calculation of hydrodynamic derivatives and is able to estimate these coefficients with reasonable accuracy. Using this method, the computational cost of detailed design has been reduced.

A number of experiments have been carried out in the current paper to investigate the effect of adding a longitudinal impermeable blade beneath the offshore pipelines on decreasing the scour around them. The main aim of this study was to reduce the hydraulics gradient beneath the pipe so that its magnitude recedes from the critical hydraulics gradient and therefore delays the onset of scour phenomenon or stops its process at all. Three pipes of different diameters were selected for the physical models of this study. Each pipe was tested using six blades with different relative widths. Also, a series of numerical simulations have been done using Flow3D software in which the similar boundary conditions as used for the physical models were applied. In both experimental and numerical investigations the reference sample that was an unprotected pipeline was first tested. Then blades with different relative widths (e.g, 0.05, 0.10, 0.15, 0.20, 0.25, 0.50) were added to the beneath of the pipeline and their performances were tested and analyzed against scour. The experimental and numerical results were in good agreement and they indicate a remarkable reduction in scour depth when a blade with the distinctive relative width was sticked to the pipeline.

As wind power continues its rapid growth worldwide, Offshore Wind Turbines (OWTs) are likely to comprise a significant portion of the total production of wind energy. These kinds of wind turbines cause of their placement environment should resist in great stormy conditions, which cause fatigue failures. Fatigue loads are one of the main failure reasons in offshore structures. One of the best ways for decreasing these kinds of loads is reducing the structure vibration. In this research application of a tuned mass damper with different masses, in an offshore Tension Leg Platform (TLP) is investigated. Tuned Mass Dampers (TMDs) modeled in a developed code FAST-SC. Results show that using TMD in the nacelle can reduce the moments in the base of the tower and turbine vibration. This reduction can also increase time until failure factor of the OWTs.

Wave overtopping on the breakwaters is an important issue in breakwaters’ design. Most of the previous studies focused on experimental researches and numerical modeling of irregular wave overtopping particularly on porous breakwaters has not been studied. In the present study, the verification between experimental studies and numerical modeling of irregular waves overtopping on the porous breakwater has shown 15% error for the wave height of 3 meters. Due to the existence error in accuracy of numerical model and software and the difference between numerical and experimental model, this error is in the acceptable range and usual in numerical studies. Moreover, this study presented the comparison between Owen formula and numerical modeling results of wave overtopping for two different conditions: the breakwater with the porosity of 0.15 and non-porous breakwater.

In this article, a rectangular cross section breakwater in water of finite depth and infinite domain is studied using the separation of the variables method in regular, sinusoidal waves. Determining the radiation potentials in three degrees of freedom e.g. sway, heave and roll, added mass and damping coefficients of sway, heave and roll motions is obtained. Diffraction problem is solved according to linear wave theory and resulting forces on structure for three degrees of freedom is obtained. As far as validation of this study is concerned, results are compared with other researches. Having the diffraction potentials in each region, the transmission and reflection coefficients and drift force are obtained. Furthermore, a parametric study on the effects of water depths, floating breakwater sizes as well as wave number on transmission and reflection coefficients carried out. Some new achievement is concluded by this study. The results show that mean drift force on the breakwater increases as the ratio of width to height of the breakwater, increases in different water depths. Also, increasing the depth of water results in increasing of mean drift force, similarly. However, the effect of the width of the breakwater is more pronounced than the effect of the depth of water.

Wave models or two-dimensional measurements of wave spectrum, determine the distribution of wave energy over frequency and different directions. However most of spectrum measurements are not directional. In this study, we are planning to determine how much energy in waves comes from storms taken place in distant areas and old waves have reached the study areas (swells), and how much energy is from the transfer of energy from the local wind to the waves near the measuring device (wind sea). For separating wave spectrum into wind sea and swell components, several methods have been proposed. In this article, first we reviewed different methods of separation, and then all methods were evaluated for a set of measured data. Based on comparisons done in this study, we concluded that the Earle and Hwang methods underestimate separation frequency. As a result, swell wave height is underestimated and mean period of swell components is over-estimated by these methods. Although the method of Portilla intrinsically chooses one one of local minimum of spectrum as separation frequency, it seems that the method is highly dependent on spectrum noise.

In the present paper, the effectiveness of Tuned Liquid Damper (TLD) in controlling the dynamic responses of offshore jacket platforms under earthquake excitation is investigated. This type of dampers consisting of one or more fluid (generally water) containers can be installed on the platform’s deck. Hydrodynamic actions induced by the tilting of the water surface in the container act as resistant forces against vibration controlling the structural response. In this research, using FE-based software package ANSYS, a jacket structure having dimensions appropriate for the Persian Gulf climate (case study: SPD1 platform) was modeled and then dynamically analyzed by the modal and time-history approaches subjected to the records of Tabas, El Centro, and Kobe earthquakes. The TLDs were optimally designed and after the verification of FE results, the dynamic responses of the jacket-type platform with and without TLDs were compared.

One of the most common methods to analyze and design monopiles under lateral load is load-displacement curves, particularly the p-y curves, at different locations of the structure. Thus, in order to investigate the soil-monopile behavior and interaction under lateral load is physically modeled and tested by utilizing the Geotechnical Centrifuge of Tehran University in the Ng space. In this study, five modeling tests are designed by considering the following two parameters with changes in their values and ratios: the penetration depth of the pile in soil (L) and the free length of the pile (e). The existing p-y curves in API are extracted and compared to experimental curves to observe differences and make the necessary modifications.

In this paper, flow separations at downstream of piggyback pipelines due to the steady current have experimentally been investigated for various Reynolds numbers, gap ratios and relative diameters of the two pipes. A number of experiments including 6 verification tests and 50 main tests were conducted in a channel with length of 10m, width of 0.3m and 0.5m depth using Teflon rods (PTFE) with different diameters. To visualize the separation pattern, the polystyrene particles with a density of 1.05gr/cm3 have been used and the whole processes of tests have been recorded using a digital camera. The experimental results have shown that with adding the secondary pipe stuck to the main pipe, the symmetrical separation pattern vanished and its length reduced up to 60%. Also, it was observed that with increasing the gap ratio to a certain extent, the separation lengths increased and decreased gradually for the main and secondary pipes, respectively. However, further increase in the gap ratio showed that the two pipes behaved separately and the separation or recirculation patterns became symmetrical and their effect to each other became negligible.

The most of wave spectrum analyzes in Persian Gulf were based on the existing field measurements, but the precision and applicability of the determined spectrum parameters were dependent on number of field wave stations, duration of field data and the spatial distribution of field wave stations. In this research, it is attempted to reanalysis again the wave spectrum parameters presented by previous researches by adding additional data gathered from two wave stations in Bushehr and Lavan to the previous existing data, in order to refine and extend Mazaheri-Ghaderi wave spectrum parameters. The outcome of this research showed that by adding new information and data to the existing ones, the wave spectrum parameters can be refined and modified. The new derived parameters showed better extendibility in the northern water areas of Persian Gulf in respect to previous wave spectrum parameters. In other words, by using the refined wave spectrum parameters, wave behavior can be presented more precisely in the northern water areas of Persian Gulf.