نشریه مهندسی دریا
پیاپی 3 (زمستان 1384)

Monopile platforms are rather slim structures which are sensitive to wave loadings and the natural period of them are within the range of sea waves’ periods. Therefore, suitable dynamic analysis and wave modeling for these platforms is necessary. There are two methods for analyzing such flexible structures under wave loading, i.e., deterministic and random methods. In deterministic method, a regular wave representing the sea state is used while random analysis considers irregular wave including spectral and time history analysis. A comprehensive research has been carried out by applying the above mentioned methods for several monopile platforms to evaluate the validity of these methods. Results show that time history random method has the most reliable responses because of accurate modeling of waves and considering nonlinearities in the analysis. However, spectral method accuracy is acceptable if inertia term of wave loading is dominated and natural period of platform is far from wave period. Deterministic method can be used when platform has high rigidity.

In this paper, a new computational model with appropriate specifications is presented for finite element analysis of local stresses at the intersections of longitudinal stiffeners with transverse frames under lateral loads in ship’s side structures. Because of the importance of such locations for fatigue analyses and also the dependence of these analyses on the maximum stress values at critical locations, performing the complete finite element analyses is to be a time consuming and expensive work for different details and locations. This is a special concern in the early stages of the design that the exact dimensions and scantlings of structural members have not been determined yet. In the proposed computational model, maximum stress concentration factors at structural details of intersections can be predicted by a simple finite element analysis. The results of this model is compared to that of the complete finite element analysis of a mid-side of a single side tanker, which have been done using ANSYS code, and it shows that the values of maximum stress concentration factors from simple model is about 10% to 15% greater than the complete analysis. This difference seems to be adopted as a proper safety factor in the early stages of design.

In this paper, several models of structure-fluid coupling are employed to investigate on the vibration behavior of the structure. Using basic equations of vibration and employing a mathematical model, a single degree of freedom system is analyzed. Some parameters of the mathematical model are obtained from test. To examine structure-flow interaction, coupled system of nonlinear second-order differential equations, including vibration equation of structure and van der Pol's equation, are solved synchronously. Lift coefficient is obtained by solving the coupled equations. Numerical solution is accomplished for three coupling models: displacement coupling, velocity coupling and acceleration coupling in each of which force is function of displacement, velocity and acceleration respectively. Time response, lift coefficient and vibration amplitude in steady state are obtained and plotted. Phase angle between the structure motion and lift coefficient change considerably when passing the locking zone which is well coincident with experimental results. Steady state vibration amplitudes for mentioned models are verified by comparing with experimental results.

Development of advanced spectral wind wave models has been the subject of comprehensive researches which has led to reliable wave predictions for assessing the impact of waves on the natural environment, coastal protection, offshore structures and harbor over the past two decades. On the other hand, the Geographical Information Systems (GIS) are developed for working with geographical data which as standard tools are able to manage and represent such data. In the present work, linking the third-generation wind wave model of SWAN and GIS has been implemented to ease data management and representation of inputs and outputs of the model that are geo-reference data. Then the effect of computational time step has been investigated using available data for Urmia Lake situated in the Northwest of Iran. As SWAN has the ability to perform the options of 1st, 2nd and 3rd generation modeling, these options have been used to evaluate results obtained by applying these models. Results indicate that the 3rd generation wave model is more sensitive to higher computational time steps than other modes of wave generation models. However, the convergence behavior of the third-generation model is much faster than that of other models.

The drag coefficients on discs rotating in a cylindrical tank has studied in relation to the nature of a composite coating attached to the underside of the disc, and direct comparisons were made with refrence discs of identical geometry which were completely rigid. The composite coating was formed by: (1) bondig of velvet fabric to the metal disc with velour face outward, (2) casting a premixed silicone composition over the velvet fabric, (3) covering the silicone layer with a thin, stiff polymer film.As a result of the tests in certain cases drag coefficient was reduced by 20% or more as compared to refrence rigid discs. Two conclusions are possible: (1)True reduction of energy exchange in the turbulent boundary layer was achived, perhaps by damping of the turbulent bursting process (2) A significant increase in the laminar region at the center of disc was obtainded (transition of laminar to turbulent delayed). The stiffiness of the top film influenced the performance of the coating. The change of drag coefficient of discs to be strongly influenced by the dynamic mechanical properties of the silicone elastomer network.

The sea wave causes excess resistance which is out of scope of calm water resistance. The total wave force in horizontal plane is divided into “Added Resistance” and “Drift Force”. In this study, based on Gerritsma and Beuckelman[3] hypothesis a computer program has been developed for calculation of added resistance and drift force at various ship speeds and various heading angles in oblique seas. This program calculates wave force both in regular and irregular waves. After validation of the program, effects of different parameters on wave force imposed on vessel have been studied.

This paper presents the results of hydraulic model tests on the stability of rubble mound breakwaters with armour layers of antifer cubes. The tests were performed using irregular waves. The effect of wave parameters and the structure slope on the stability of these artificial armour units has been investigated and the stability formulae have been derived. Finally a comparison between these formulae and the stability relationships derived by van der Meer (1988) for cubes has been made.