نشریه مهندسی دریا
پیاپی 21 (بهار و تابستان 1394)

Tension leg platform is a compliant structure with vertical moorings and added buoyancy force. In this paper، effective parameters on experimental modeling of TLPs have been investigat. experimental modeling with correct scale factor has been done in Sharif University of Technology marine engineering laboratory test basin. deep water simulation capability were discussed and finally the model، launched in the basin. Pretension modeling is another conceptual component that could be easily analyzed. Ultimately Sea keeping results of ISSC TLP model duo to the regular laboratory waves with different heading angles and two amplitudes were extracted. The main purpose is ISSC TLP frequency response and surge beating analysis by model testing.

Flows of natural hydro-environments are usually turbulent and mostly stratified، like the flows in lakes، reservoirs، estuaries and atmosphere to name a few. In stratified flows due to the buoyancy forces، the turbulent stresses are usually non-isotropic. Therefore the accuracy of the numerical simulations for such flows is highly dependent on the turbulence model and the implementation of non-isotropic stresses. The hydrodynamic model deployed herein is the 2DV WISE (Width Integrated Stratified Environments) numerical model. In this paper an explicit algebraic Reynolds stress model together with an explicit algebraic scalar flux model have been developed and implemented in the original hydrodynamic model. To validate the EARSM turbulence model، lock release tests have been simulated for saline and dense stratified flows. The predicted values of the evolution and the front head position of the stratified flow، and the velocity profiles simulated by EARSM model have been compared against the measured values reported in the literature and the predicted results of the buoyant k-ε model originally included in the hydrodynamic model. The comparisons indicated good performance of the EARSM turbulence model and showed its superiority compared to the k-ε turbulence model in these regions.

Ship maneuvering performance evaluation is essential for primary design stages. Ship maneuvering calculations، horizontal plane motion control and development of maneuvering simulators need a mathematical description of ship maneuvering. In the recent years، different mathematical models are suggested for maneuvering of displacement vessels that are capable of estimation of vessel maneuvers with acceptable precision. These mathematical models are based on determined hydrodynamic coefficients and their accuracy depends on the known coefficients used to solve the mathematical model. System identification methods are developed to calculate these coefficients utilizing input and output data obtained from different sources. In this paper different maneuvers are executed through free running model tests of a container ship. The hydrodynamic coefficients in the mathematical model are determined by the Extended Kalman filter method. Then the mathematical model is solved and different maneuvers are simulated by coefficients calculated from the experiments. Simulations are validated by model tests. Finally the influence of rudder angle and advance speed on turning circle is studied. The mathematical model and hydrodynamic coefficients presented in this paper can be applied for optimization of ship maneuvering performance and course control purposes.

Structural health monitoring is essential for ensuring the structural safety performance during the service life. The process is of paramount importance in case of the floating wind turbine due to the structural parts subjected to the marine environmental risky conditions. In this paper a fuzzy-based damage identification method using dynamic response of the Spar floating wind turbine has been proposed. In the first step، the nonlinear equations of motion of the floating wind turbine system derived using the theorem of conservation of angular momentum and Newton’s second law. Then the variation values of the frequency characteristics of the structure in each DOFs due to stiffness changes of mooring lines (simulated damage) are considered as input features to the fuzzy system. Also the fuzzy system was trained based on calibrating of the membership functions by defining the damage classes properly. For validating the proposed method، noise with different SNRs was contaminated to the measured features and the success rate of the damage detection was calculated. The results showed that the proposed method is able to identify the damage classes with acceptable success rate.

Experimental tests on superplastic forming of alloy Ti-6Al-4V with large dimensions can be quite difficult and expensive. In order to avoid the expense of manufacturing alloy sections، numerical modelling is used to simulate the alloy’s superplastic behaviour. However the results obtained from the numerical modelling need to be verified against experimental tests. In order to reduce the expense of the experiments، tests are conducted on smaller samples first and then alloy forming is simulated with larger dimensions numerically. In this article، for superplastic hydrostatic forming purposes، design procedures of Ti-6Al-4V plates as well as their experimental parameters have been studied. Superplastic constitutive equation of Ti-6Al-4V alloy at 970 C has been determined and implemented in the finite element software Abaqus. The results obtained from the numerical simulations show an excellent agreement with the existing experimental results indicating that the new equation reliably predicts the alloy’s grain growth، stress، strain and the strain rate. Investigation on the effective parameters in the superplastic forming process of the alloy provided the possibility of varying these parameters and their ratios accordingly. Thereafter the superplastic forming process was conducted for both sub and full scale alloy sections with scaling ratio of 1:20 and the effective parameters obtained from these two numerical simulations were compared. Results comparisons of experimental and numerical tests demonstrated that the conducted tests on the proposed design of alloy sections is able to overcome the difficulties associated with superplastic behaviour of Ti-6Al-4V alloy after forming.

Topsides of offshore platforms contain different equipments، therefore earthquake excitation may impose large forces on them. In designing these topsides، the interaction of the secondary systems and primary structure is not usually considered، which is not realistic. In order to study this subject a 3D model of a typical platform in the Persian Gulf is constructed and analyzed، using OpenSees software، when subjected to different records. These records are scaled for two earthquake levels. In the first level the structure is in elastic condition but for the second level it behaves inelastically. According to the results، in secondary systems with low periods of vibration، structure and secondary system interactions can be neglected. But when the period of secondary system is increased، it is necessary to consider interactions effects.

Digital Shoreline Analysis System (DSAS) is the most widely used tool by researchers and experts to shoreline change rate measurements. Several factors may cause uncertain data in the measured values by this tool. Understanding these errors and fix them if possible، improve the accuracy of the results. The DSAS does not have this capability. The purpose of this paper is to present a new computational module for DSAS to identify suspected data errors. This module is an executable file that is written using MATLAB software and communicate through XML files with DSAS. Suspected data error detection is performed using statistical methods include: Box plot، Hampel’s test، Median Absolute Deviation (MAD) and Z-score test. The results show the good performance of this module to identify possible errors in data.

In this article by evaluating the performance of standard Directional Spreading Functions (DSFs) with those of field DSs at the Strait of Hormuz، such standard DSFs have been calibrated. Here، field DSs have been extracted considering the region extreme events based on up to 20 months field measurements at four different 25m stations. Statistical assessment of standard DSFs clearly showed their lack of accuracy. So، a calibration procedure considering their introduced coefficients as variables using generalized reduced gradient method was used. Such procedure has been implemented with and without considering the event's direction for Cosine-power، Poisson and hyperbolic DSFs. Results show a great improvement up to 60% in terms of their conformity. So، such calibrated DSFs could be used as a better approximation of wave directional distribution based on available measurements at the region.

The principle included in construction of submerged breakwater is to protect beach from morphological changes and the sediment transport against incoming waves. In the present study، boundary element method (BEM) is employed for solving the scattering problem of incident wave passing the vertical and inclined submerged breakwaters with rigid boundaries. The boundary element integral equation with constant and linear elements is used to solve the Laplace equation (for the incompressible fluids) in boundary value problems by development of a FORTRAN computer program. The diffraction of water waves has been investigated in floating and submerged breakwaters so that to gain this goal، a parametric study on the water wave reflection and transmission coefficients and the water surface elevation has been performed considering the influence of various sea states such as wave number، obliquely of waves and wave stippness، and several structural configurations such as width، depth and angle of inclination.