فهرست مطالب

Coastal and Offshore Engineering - Volume:4 Issue:13, 2019
  • Volume:4 Issue:13, 2019
  • تاریخ انتشار: 1397/11/12
  • تعداد عناوین: 6
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  • Milad Zabihi, Said Mazaheri*, Masoud Montazeri Namin Pages 1-8

    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.

    Keywords: OWC, Numerical Simulation, Wave Energy, CFD, Experimental
  • Akbar Rashidi Ebrahim Hesari*, Sajad Andi, Hosein Farjami Pages 9-16

    In this research, density, temperature and salinity fields were investigated in different seasons using observational data of ROPME Marine Cruise in the Persian Gulf (PG). Based on in-situ measurements, areas with density stratification were identified. Having analyzed Landsat and SAR satellite images, internal waves (IW) were detected in different regions of the Persian Gulf and more frequently in the eastern part of the PG related to seawater stratification. Based on analysis of satellite images, it is shown that the length of internal waves crest detected in the north-eastern part of Al-Zhahirah (Qatar) was more than 120 km; while it’s in range of 5 to 20 km in the south and east of Larak Island, 15 to 40 km in the north-east of Abu Musa Island, and 3 to 65 km in the south-east and south of Hondurabi Island. Moreover, IWs with shorter crest’s wide were recognized near Lavan, Siri, Farur, Halul, Khark Islands and Bandar Lengeh, as well. In addition, studying satellite images in the above mentioned areas for a longer time period from 2000 to 2017 showed that IWs mostly occur in the eastern part of the PG in summer and disappear in other seasons.

    Keywords: Internal Wave, Persian Gulf, Satellite Images
  • Abdolrahim Taheri*, Mahdi Tasdighi, Mohammad Faraji Pages 17-23

    he importance of oil transportation in the maritime industry has increased in recent years due to increased oil and gas production. According to technical and financial aspects, on hydrocarbon transfer methods, the pipelines are the best option for the transfer of oil and gas in the maritime industry. High temperature and high pressure in the pipeline can lead to the buckling. Buckling can either be in the direction of vertical (upheaval) and horizontally (lateral). The uncertainty in the buckling parameters of the pipeline increases error in the uplift and the effective axial compressive force calculation. The existence of these errors in the pipeline design is costly for the project. So reducing the errors can be very important. This paper presents the reliability analyses for studying and quantifying the variation of the reliability index (β) with the main parameters involved during the upheaval buckling of submarine buried pipes caused by high temperature and pressure conditions (HTHP). In this paper, uncertainty is considered in the geometric parameters of the pipeline. PDF and reliability index (β) can be determined by FORM and other. FORM, FOSM and sampling methods are three main methods which are used to account the PDF and reliability index (β). This research shows that among these three methods, for a fixed state, the sampling method has the lowest beta and the highest probability of buckle, which has a higher accuracy than the other methods. For soil cover with a thickness of more than 1000, it is worth noting that by increasing the thickness of the soil cover, more force is required for the upheaval buckling in the pipeline.

    Keywords: Upheaval Buckling, Reliability, Subsea Pipeline, Monte Carlo Simulation
  • Mohammad Reza Ghasemi*, Naser Shabakhty, Mohammad Hadi Enferadi Pages 25-35

    Undesired oscillations of jacket platform may influence the structural functionality and sometimes fatigue occurs. The main objective of this research is to control wave-induced vibrations of fixed jacket platforms with the use of optimized shape memory alloys dampers. To model the hysteretic behavior of SMA elements and performing dynamic analysis an efficient isothermal idealized constitutive model is developed in this research and direct integration time history analysis is carried out. Dynamic responses of multi-degree of freedom system of jacket platform, with 90 m height and equipped with SMA dampers, is estimated and compared with the bare jacket. Furthermore, an optimization algorithm such as Ideal Gas Molecules Movements (IGMM) is implemented in this research to improve the efficiency of the dampers and minimize the deck displacements under the action of extreme wave. The results show that the optimized SMA dampers can improve the structural response by decreasing 47.5 percent of deck displacement, 56.5 percent of deck acceleration and finally 28 percent of base shear. In an SMA damper-equipped platform, reduced wave intensity will reduce the damper efficiency.

    Keywords: Vibration Control Steel, Jacket Platforms, Shape Memory Alloys (SMA)‎, Idealized Constitutive Model
  • Mohsen Soltani, Rouhollah Amirabadi* Pages 37-46

    This study aims to investigate the capability of two common numerical methods, Homotopy Analysis Method (HAM) and Variational Iteration Method (VIM), and to suggest more efficient approximate solution method to the governing equations of nonlinear surface wave propagation in shallow water. To do so, semi-flat, moderate, and sharp slope of shore which are connected to an open ocean with a uniform depth are exposed to a solitary wave with initial wave height H=2 and stationary elevation d=20. Then, the surface elevation and velocity curves for these profiles are determined and compared by HAM and VIM.  To verify the numerical modeling, two slopes i.e. semi-flat and moderate slope are considered and modeled in Flow-3D. Afterwards, the results of surface elevations are compared to each other by using correlation coefficient. The correlation coefficients for the slopes represent that the results coincide well. Ultimately, although the results of both methods are quite similar, using HAM is highly recommend rather than VIM since it makes solution procedure fast-converging and more abridged.

    Keywords: Homotopy Analysis Method (HAM), Variational Iteration Method (VIM), Shallow water equations
  • Reza Dezvareh* Pages 47-57

    Tuned liquid column gas damper is a new type of energy absorber that can mitigate the vibrations of structures if their frequency and mass parameters are well tuned. Since this damper has recently been introduced and its behaviour in certain structures such as offshore oil platforms and wind turbines has already been tested, a suitable and accurate method is required to identify these optimal parameters. Therefore, considering the complexity of loads exerted on wind turbines in seas (wave and wind loads), in present study attempts are made to use a new artificial neural network approach to obtain optimal tuned liquid column–gas damper (TLCGD) parameters for mitigation of wind turbine vibrations. First fixed offshore wind turbines at various depths are designed in the MATLAB coding environment. After obtaining the stiffness, damping and mass matrices of the structures, the program enters the Simulink, and the wind turbine structure along with the TLCGD is exposed to different wave-wind load combinations within reasonable range of damper parameters. The neural network training is launched based on available statistical data of the offshore wind turbine with different heights as well as different frequency and mass ratios of the damper. According to this method, the percentage of errors found in the neural network outputs was negligible compared to the actual results obtained from the analysis in Simulink (even for inputs that stood outside the training range of the neural network). The mean error percentage, the standard deviation and the effective value of the neural network with actual values are below 10% for all three types of the structure. Finally, the method presented in this study can be used to obtain optimal parameters of the TLCGD for all kinds of offshore wind turbines at different depths of the sea, which leads to the optimal design of this damper to reduce the vibrations of wind turbines under wave and wind load pressures.

    Keywords: offshore wind turbine, tuned liquid column gas damper, soft computing, neural network, Simulink model