فهرست مطالب said mazaheri

Supplying world future energy is tied with renewable energies and wave energy is one of the biggest resources of renewable energy which is somehow untapped. Oscillating Water Column (OWC), one of the most familiar devices in harnessing wave energy, is still not being properly commercialized due to the complicated hydrodynamic behavior. Offshore OWCs are exposed to higher wave energy; however, the researches on this kind of OWCs is limited. Hence, in this paper, a fully nonlinear two phase flow model of a fixed offshore OWC is developed using Ansys Fluent. Unlike the previous studies, the developed numerical model has the merit of being validated against a relatively large scale physical model (1:15). The results of the model are compared by those obtained in experimental campaign conducted by the authors. Results of both free surface elevation and air pressure in the OWC chamber are compared. Generally, the results showed an admissible accordance between numerical and experimental model. Some discrepancies could be detected in the free surface elevation in the chamber especially for short wave period. This can be attributed to the increase of nonlinear effects in the chamber by increase of wave steepness. The developed model can be applied for further researches on OWCs such as optimization or improving OWC performance.

Three-dimensional RANS simulations are employed numerically to study flow characteristics around a near-wall circular cylinder for varying gap-to-diameter (G/D) ratios (Where G is the gap between the cylinder and the wall and D is the cylinder diameter) and at Reynolds numbers from 100 to 3900. Pressure distribution around the circular cylinder, base pressure magnitude, separation and stagnation angles, force coefficients and Strouhal numbers were calculated and compared for all of the cases. Inception of vortex shedding can be seen when a sudden decrease in the maximum of positive pressure coefficient occurs. Vortex shedding mechanism and possibility of suppression further investigated via comparison of swirling strength in upper and lower vortex regions through parameter Λ, which signifies vortical activity and balance with respect to the wake center-line and also the flow type parameter, λ, representing the extensional strain dominance in the wake flow and gap flow. Vortex shedding suppression observed for the cases with the high unbalance vorticity content in the vortex regions, namely for Λ ≥ 2.

Increasing problems due to supplying energy demand conveyed researchers to find a solution in renewable energy resources and consequently marine engineers drew attentions towards wave energy which has the merit of higher energy density than the other resources. Oscillating Water Column (OWC) is one of the most propitious devices for capturing wave energy. Researchers have studied the device under different wave height and period conditions and they investigated various geometric parameters such as front wall draft and the chamber length. However, the effects of wave spectrum type or shape has not been investigated deeply yet. Different wave spectra have been developed for different places around the world but the focus of this study is on the two well-known spectra called JONSWAP and Pierson-Moskowitz to see how the type of the spectrum can impact on inner chamber fluctuation, pressure variation and reflection response of an offshore OWC. To achieve this goal, a 1:15 scale model of an offshore OWC was constructed in National Iranian Marine Laboratory. The results show that inner chamber free surface spectrum is affected by the type of incident wave spectrum. In another word, energy content at peak frequency was approximately 50% higher when the incident wave spectrum is of JONSWAP type. However, energy corresponding to sloshing frequency and total energy content in the chamber were almost the same for both types of the spectra. Pressure spectra inside the chamber showed a similar trend as free surface elevation. Although there was a little difference in reflection response of an OWC influenced by the type of spectra, this discrepancy was more pronounced in high frequency waves.

Developing numerical tanks to study wave structure interaction drew engineers’ attention in last decade. Numerical wave tanks are absolutely essential for investigating wave-structure interaction. This paper presents two different numerical software capabilities to generate regular gravity waves in a wave tank. The wave generation was performed using the FLUENT package and Flow-3D. Both models are based on Navier-Stokes and VoF equations. The results of the mentioned models were compared with theoretical results. Free surface elevation and horizontal component of wave particle velocity were the two parameters which have been considered for comparison. Results indicate that Flow-3D in some cases is a bit more accurate than Fluent in capturing free surface elevation. In numerical models it is important to dissipate wave energy and prevent wave reflection. In this way four different slopes were evaluated to determine the minimum slope needed for wave energy dissipation. The results showed that a minimum slope of 1V:35H is needed to avoid wave reflection. The variation of streamlines and velocity potential are also studied. The pattern of horizontal and vertical velocity variation in the fluid domain is similar to stream function and potential velocity function variation, respectively.

Existing methods presented in references and standards for calculation of berth occupancy ratio, are more concentrated on container terminals. However, some empirical formulas can be found to predict the berth occupancy, but these methods are neither accurate enough nor economically wise to be employed for the prediction of berth occupancy ratio in large and complex bulk liquid terminals. Therefore, it is attempted in this paper to introduce a method for more precise prediction of berth occupancy ratio for bulk liquid terminals. In the proposed method for calculation of berth occupancy ratio in bulk liquid terminals, times which is not used for loading/unloading operation is investigated. The loading/unloading lost factor is considered in calculations in order to calculate the operational value for loading/unloading rate. Next, practical values for these items are given based on previous authors’ experiences. Finally, a case study is performed for a liquid bulk petrochemical terminal to specify the presented method. Results show 24% difference between new method and old empirical formulas.

Regarding the important role of droplet size distribution for determination of time, location and trajectory of oil droplets in subsea oil and gas releases, in this research a model for prediction of oil droplet size distribution was presented. This paper dealed with the theoretical basis for the model first and then the model specifications were presented. The model parameters were used based on available experimental data. Effects of gas void fraction and buoyancy were also discussed. Using this research model, oil specific diameter can be calculated and then based on Rosin-Rammler (k=0.693, α=1.8) or Lognormal (S=0.78) distributions, oil droplet size distribution was determined. Surfacing time, location and trajectory of oil droplets can be calculated based on oil spill models. This research results can be extended to full scale oil and gas release conditions given good agreement between this research results and experimental data (Deep Spill).

Many procedures suggest for reduction of responses of riser to Vortex Induced Vibrations (VIV). Natural frequencies of marine risers is an important parameter that can affect the responses of riser to VIV. Change of riser properties such as top tension and bending stiffness can alter natural frequencies. In this study effects of riser specifications on the responses and fatigue damage of marine risers were investigated analytically and numerically. For numerically analysis 2D wake-structure coupled model is used for modeling of VIV of riser in two directions of Cross Flow (CF) and In Line (IL). The wake dynamics, including IL and CF vibrations, is represented using a pair of non-linear Van der Pol equations that solved using modified Euler method. The Palmgren–Miner Rule is used for evaluation of fatigue damage. Riser of Amir-Kabir semisubmersible placed in Caspian sea is used for case study. Because VIV is self-limiting, it is showed that lower modes have lower curvature, that in some cases this is lead to lesser stress and also fatigue damage. The results show that for tension dominant modes of vibration, natural frequencies was increased with top tension and for a certain Strouhal frequency, dominant modes of vibration was reduced which leads to reduction of stress and fatigue damage. The results show that stress and fatigue damage increased with module of elasticity of riser and reduction of this leads to reducing of stress and fatigue damage. Therefore suitable procedure for reduction of VIV responses of riser should be selected based on the current velocity.

The major problems in modeling of different oceanographic and meteorological parameters are limitations in numerical methods and human incomplete knowledge in physical processes involved. As a result, significant differences between the results of these models and in situ observations of these parameters might exist. One of the powerful solutions for decreasing the forecast errors in the models is to use data assimilation technique. In this study the optimal interpolation data assimilation method is employed which is based on statistical rules. Moreover, the quick Canadian method is used to optimize the data assimilation method used in model. Model assessment is performed by comparison between running wave model with and without using data assimilation and SAR wave data in Persian Gulf. It shows that using data assimilation in WAVEWATCH-III model reduces the error in wave height predictions significantly.

Because of environmental sensitivity of the Caspian Sea, any subsea oil spills and blowouts can inflict destructive impacts on marine environment and cause catastrophic damage in the area. Employing mathematical simulation is one of the easiest and most effective prediction tools for understanding the oil spill behavior under various environmental forces. In this regard, Mike3D simulation package including 3D Flow and Ecolab-Oil Spill Modules were setup, verified and employed. Based on the environmental history of Caspian Sea area, it was decided to simulate the hydrodynamic behavior of the basin for the actual force driving parameters occurred in 1982. Then, according to various oil spill scenarios, it was attempted to simulate oil spill distribution in the basin for every season of the year. Results show how seasonal met-ocean parameters play as important key factors and can influence the oil spill transport and distribution in the area.

For the purpose of having a safety navigation, the transportation path, deployment of carrier, physical specifications, locations and … have high level of importance. In this research, along with the above mentioned factors, we have compared the result with technical standard and commentaries for port and harbor facilities in Japan, and to analysis both advantage and disadvantages of them. The IRAN LNG project (ILC), will product the overall amount of 10/80 million tons of LNG per annum. Based on the type and capacity of LNG ship (Membrane Type,150000 m3 ), 13 vessels and 6 tug boat are estimated, mostly considered as high capacity LNG carriers. Since the IRAN LNG ships enter to terminal on ballast and after loading departure and also with considering number and marine traffic on South pars, one way channel with 0.5 L (L is length of ship) considered. On this base, we need Maximum width of 157.5 m and minimum 122m (for LNG Ships), and Maximum width 120m and minimum 89m (for LPG Ships). Area, circumference and diameter of basin with anchorage & buoys on general jetty that in collusive general Cargo, Sulphar & Ro-Ro berth respectively are 154610 m2, 1830m and 320m that covered Japanese’s standard completely but about turning circle, we have area, circumference and diameter respectively 5896 m2, 250m and 708m which doesn’t coordinate with Japan’s standard except only one subject (Sulphar berth with aids tug boat).

There are several global marine forecast systems based on large scale regional data covering Iranian seas which are freely available on their websites. They are also several local marine forecast systems in Iranian waters. This paper performs an inter-comparison among different marine forecasting systems in Iranian seas in order to evaluate the accuracy and difference between them and give a proper ground for further investigation and research in this crucial issue. For this purpose, the results of two forecasting system in several points in the Persian are compared and the reason of differences are investigated.

LNG chain value consists of gas exploration, gas liquefaction, transportation, Re-gasification and finally gas distribution to the end user network. Transportation part of this chain normally consists of 25 to 30% of the total vale and is the most lucrative part of it. As proved Iranian gas reserves is about 29 trillions cubic meter and nearly half of it is South PARS gas field shared between Iran and Qatar ,there is an increasing concern on entering into gas export market in near future.
This enthusiasm enquires a thorough financial study on economical feasibility study on this industry. In this study by emphasizing on key parameters of LNG shipping industry like, technical vessel specification distance between exporting and importing terminals, cost of LNG carriers , boil of ratio of vessels, oil and gas future price prediction vessels heeling portions and most importantly the difference between vessel purchase and hiring daily rate of vessels.
We are to model this industry economically and by doing an economical sensitivity analysis on the model some economical indexes are extracted which are of most benefit for countries decision makers.
Two scenarios of purchasing and renting vessel have been investigated and it is shown than in different financial condition and oil and gas market there are at least minimum internal rate of return of 4 to 18% and at last these finding have been demonstrated in a 3D diagram showing net in come oil price (as an important parameters) and rent daily rate of vessels to show the profitability of this industry to the reader.

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 re-analysis 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.

Free spans or suspended spans normally occur in subsea pipelines due to the irregularity of seabed and by scouring phenomena around the installed non-buried pipeline. The hydrodynamic around the pipeline can lead to a vibration of the pipeline.This kind of vibration may cause fatigue damage to the pipeline. In order to study the hydrodynamic around the pipeline, calculating the natural frequency of the marine pipeline in free spans accurately is very important. Resonance phenomenon may occur when the frequency of the hydrodynamic forces induced by a vortex shedding approach the natural frequency of pipelines. A resonating span can experience significant deflections and associated stresses. Several parameters such as pipeline profile, axial forces, seabed soil and boundary conditions influence the natural frequency of the pipeline. The soil characteristic is an important factor which should be considered in determining the natural frequency of pipeline in free span. In this paper, attempts have been made to investigate the natural frequency of free spanning pipelines and influence of soil characteristic in support of pipeline in free span. In this regard, various boundary conditions were considered and the results were analyzed.

The effects of wave direction, frequency and the waterline shape of floating structures on the wave mean drift force formula have been considered separately by several authors but there isnt a general formula to take into account all of these effects. In this regard, Faltinsen’s wave drift force formula has been modified by adding finite draft coefficient. The results obtained from this formula which is dependent on wave frequencies, has been compared with Helvacioglu’s experiments favorably in high wave frequencies. Moreover, the influence of the current on the wave mean drift force has been taken into account by considering the current coefficient derived from the ship added resistance formula. In addition, the formula for the calculation of the wave drift damping has been extended to cover high wave frequencies as well as low wave frequencies. The results compared with asymptotic formula showed good agreement in the high frequency band.