نشریه مهندسی دریا
پیاپی 10 (پاییز و زمستان 1388)

Development of a compatible computational fluid dynamics procedure to investigate rigid and fixed/free coastal and offshore structures hydrodynamics in a time-dependent one/two phase flow of viscous incompressible fluids is presented. Differential governing equations are discretised using finite volume approach based on a colocated arrangement. The conservation equations for mass and momentum are solved using a fractional step method. In conjunction, free surface is simulated using a volume method. An overlapping mesh system is implemented to deal with a two-dimensional computational domain including a non-uniform background mesh of Cartesian cells as well as an overset mesh of quad cells. According to the complete assessment of the code developed based on the algorithm, there is a robust tool to deal with hydrodynamics where extension to three dimensions is straightforward, as well. Although many two-dimensional problems can be solved, but the code must be developed for three dimensions. In addition, turbulence is not considered in the present formulation and left for future researches. A simple non-conservative approach to transfer all two-phase flow variables is introduced. Besides, Algebraic sets of equations extracted from different meshes for each variable are solved simultaneously to construct a strong coupling. The final algorithm is a new and appropriate combination, as well.

Offshore oil/gas pipelines are usually installed unburied. Free spans in the line may occur initially by an uneven seabed or be created later due to under-scouring. Free spans are usually unfavourable spots in the submarine pipeline. They are vulnerable to vortex induced vibrations, excessive stresses in the pipe wall or may exhibit undesirable responses to strong ground excitations. In this paper, the simplified/conventional added mass method for simulation of water-pipe interactions has been evaluated against a more elaborate coupled acoustic-structural model. With the coupled system the surrounding sea, the pipe body, the sea bed and the free-spanning have all been incorporated in the model. Effects from both irregular (earthquake) and harmonic excitations have been investigated. The validation of numerical models has been achieved by comparison of their dynamic characteristics to some available analytical-experimental relationships. From the results of the current investigation it was noticed that the added mass method generally predicted more conservative free spanning responses in comparison to those from the acoustic model. Under harmonic excitations, the predictions from the two models for the maximum longitudinal strain in the pipeline deviated around 60%. For irregular (seismic) excitations, this deviation came down to about 15%. The disagreement between predictions from the two methods was increasing by the decrease in the span length. The difference was more remarkable for vertical excitations as compared to the horizontal ones.

Using dead zones of rivers for berthing purpose may involve some problems of which the most serious are sedimentation and silting. These processes depend on water flow pattern in these zones. In this paper, a two dimensional depth averaged model is explained for flow modeling in river dead zones. Governing equations of fluid flow are discretised using the “Control Volume” method and solved by applying the “SIMPLER” algorithm. To reduce some instability problems such as oscillation and non-convergence of the results, two dimensional results are compared with one dimensional data and some justifications are made where necessary. To verify the model, results of the flow modeling in backward facing step have been compared with some validated data and proved efficiency of the technique. The model was then applied to simulate a river dead zone flow. A rectangular geometrical section was applied and several Reynolds numbers were considered for flow modeling. Velocity profiles and vortices are given in some positions of the zone for several Reynolds numbers. Non-dimensional vortex length variations are drawn for several Reynolds number. The locations where sedimentation may occur are predicted and discussed based on velocity profile.

Development of a simple fast-solving method based on the popular Morrison approach for prediction of semi-submersible's motion response is the main purpose of the current research. The oblique sea's hydrodynamic specifications are modeled using small amplitude wave theory. The velocity forces and moments have been shown to be small and are therefore neglected. The damping coefficients are obtained from experimental results while added-mass is evaluated using known experimental and theoretical data. Also the interference effects of the various volume elements of the semi-submersible on the flow field are neglected. Comparison of this approximate method shows quite a good agreement with those carried out by other researcher or softwares except for pitch motion which gives a much higher prediction. Spectral analysis is discussed in detail and is applied in predicting the short-term and long-term extreme motion response. The short-term extreme response is based on 50 and 100 years return design sea states while long-term prediction is based on the anticipated life of the semi-submersible. At the end for verification of the model and for showing the method capabilities, "AKER H-4-2 P" semisubmersible is analyzed. The approximate method results are in relatively good agreement with the output results of MOSES Ultramarine software as shown.

The intensity of damage in concrete structures has a vital relationship to the position of concrete respect to sea level. A hydraulic model consists of a channel and a wave maker machine capable of making regular waves with various heights and periods, has been designed and constructed in hydraulic laboratory to simulate the marine environment and conditions in this research study. The Oroomiyeh lake water and controlling the temperature of channel in 30-35 °c range were used to reproduce severe corrosive marine environment. Different water/ cement ratios and different cement contents were selected for reinforcement and plain concrete specimens. Furthermore, silica fume was used as supplementary cementing material. After standard curing, these specimens were kept in different conditions: submerged, tidal, splash, atmospheric and outdoors. A series of tests such as corrosion potential measurement, electrical resistivity and reinforcement corrosion intensity, chloride ion concentration and compressive strength were carried out at different ages. In this paper, specimens, durability in different zones have been investigated. Results show that in view of corrosion the splashing zone has the worst conditions and in submerged condition, because of thin air, although corrosion potential of reinforcement is high, the rate of corrosion intensity is low.

In this study, the boundary element method is formulated to evaluate hydrodynamic characteristics of bodies including free surface effect. The method is based on two equations, the perturbation potential boundary integral and the pressure Kutta condition, which are solved simultaneously. The method uses isoparametric elements for both quantity and geometric on the boundary. The method is first applied to two three-dimensional bodies (hydrofoil and vertical strut) of the NACA4412 profiles and symmetric Joukowski with 12% thickness. It is assumed that hydrofoil moves in constant speed. Some results of the pressure distribution, lift, wave-making drag and wave pattern are presented. It is shown that the computational results are in good agreement with the experimental measurements and other calculated values.

Incremental Dynamic Analysis (IDA) is a parametric analysis method that has recently emerged in several different forms to estimate more thoroughly structural performance under seismic loads. It involves subjecting a structural model to one (or more) ground motion record(s), each scaled to multiple levels of intensity, thus producing one (or more) curve(s) of response parameterized versus intensity level. Of great interest in Performance-Based Earthquake Engineering (PBEE) is the accurate estimation of the seismic performance of structures, and in particular, the mean annual frequency (MAF) of exceeding a specified structural demand or a certain limit-state capacity. In this study the behavior of a jacket type offshore platform with different characteristics in its two directions, separately and with 3D modeling considering pile-soil-structure interaction is investigated. By obtaining the IDA curves and summarizing the results, the behavior of the jacket is studied. All analyses are performed using OpenSees software. It is observed that the difference of geometry of jacket in two directions due to employing Float Over Deck (FOD) installation method, does not cause similar behavior in both directions and finally in direction with float over installation design requirement is not satisfied.

For investigation of natural phenomenon, numerical simulation has attracted considerable attention from many sciences in the engineering field. In this paper, the parameters of hydrodynamics around cylindrical structures in steady current (specially, vortex-shedding frequency which results oscillating lift force and also Reynolds number) have been described then flow regime around specified piles and its characteristics have been investigated by analysis of experimental data and power spectral density (in frequency domain) of the time series of the recorded lift forces which exerted on the piles. Then, the experimental condition of the supposed data, have been modeled numerically by CFD methods and fluent package. Output results which have been obtain form software compared with foregoing experimental data and simulating proceed have been validated. Furthermore, position of the separation point (as a function of Re), has been estimated by output of the fluent.