سال:1392 | دوره: | شماره: |تعداد مقالات:8

Offshore pipelines used for oil and gas transportation are often buried to avoid damage from fishing activities and to provide thermal insulation. Thermal expansion and contraction of the pipeline during operation can lead to lateral or upheaval buckling. A safe buried pipeline design must take into account a reliable evaluation of soil uplift resistance and pipe embedment depth. While the cost of burying a pipeline for tens or hundreds of kilometer is significant, it is important to optimize the required soil cover depth. In this paper a parametric study of pipeline upheaval buckling in clayey backfill has been conducted using finite element analysis. Three different embedment depths are considered. Uplift resistance is calculated and failure mechanism is obtained. To simulate the large penetration of the pipe into clayey backfill a novel Arbitrary Lagrangian Eulerian (ALE) finite element technique was employed in this paper. The results reveal that as embedment depth increases, uplift resistance increases and also uplift mechanism changes. However, uplift resistance differ less than 5% for deep embedment case. In addition, the amount of pore pressure is investigated beneath the pipe for deep embedment cases and it reveals that negative excess pore pressure occurs under the pipeline.

This paper presents an experimental study the influence of wave parameter on the damage of reshaping seawall, model tests have been performed in several water level conditions. The experiments of physical modeling of this research have been done in the flume of the Soil Conservation and Watershed Management Research Institute (SCWMRI). The waves applied to the structure model are irregular and the energy spectrum of the applied waves is JONSWAP. The material of arm our layer have been regarded with the scale of 1: 25 model and grading class of D85/D15=1.82. A number of 3000 waves were applied on the structure over the research in general and some 60 experiments have been totally accomplished. The results of tests after drawing graphs, has been analyzed. That it can be harvested with %82 increasing in wave steepness, damage parameter %85 decreased and also with %57 increasing in wave height, the damage parameter %87 increases and with %57 increasing in wave length, the damage will %83 increase.

Sedimentation is one of the most important problems in harbors that results in considerable economic costs. Harbor planforms affects the flow pattern in the harbor basin and consequently, plays an important role in sediment transport and sedimentation. In the present study, a two dimensional depth-averaged hydrodynamic and sediment transport model has been developed to investigate the effect of harbor planform on sedimentation and sediment transport in harbor basin. Various planforms have been examined by the numerical model and results are plotted for alternative harbor planform geometry parameters, namely, planform aspect ratio, entrance location and entrance width which are presented in a dimensionless form. Results suggest that planform aspect ratios less than unity, leads to less sedimentation Inside the harbor basin. In addition, it can be observed that, while entrance location dose not seriously affect suspended sediment transport rate through the harbor entrance, entrance locations closer to the corner of the basin lead to less sedimentation.

In this study, the effect of cement content on concrete performance in corrosive environments is investigated on concrete mixtures having the same water/cement (w/c) ratio. Three w/c ratios (0.40, 0.45 and 0.50) were used and for each w/c ratio three mixtures were prepared with cement contents 350, 400 and 450 kg/m3. The compressive strength, electrical resistivity and chloride diffusivity of the samples without steel reinforcement were determined. In addition, the half-cell potential of samples with steel reinforcement in 5% NaCl solution was measured to evaluate the rebar corrosion. It is found that although w/c ratio is kept constant, strength increases and corrosion probability of steel reinforcements decreases when cement content in the mixture decreases from 450 to 350 kg/m3.

The Sirri jacket offshore platform was designed and installed in Persian gulf of Iran in 1975 and has been in service since that time. The importance of offshore structure's stability and the failure of this kind of structure during an earthquake in different active zones revealed that earthquake loading has to be considered. In this paper, Magneto rheological (MR) fluid dampers have been proposed as a powerful tool to control seismic vibration of platform. A combination of four MR dampers and six friction pendulum isolators on the joints of cellar deck is applied for dynamic control of an existing steel jacket. In order to accurately evaluate the performance of Sirri jacket with MR dampers under Kobe seismic excitation, the platform numerically modeled in SACS software. The size of generated model with 198 degree-of-freedoms (DOFs) was dynamically reduced so as to be utilized in semi-active control algorithm. To this end, the stiffness, mass and damping matrices of the model has been reduced to the 25 DOFs model by programming in MATLAB software. The algorithm is used in this study for semi-active control of platform was H2/LQG. Comparison between dynamic response of the jacket with and without using semi-active controller showed a great difference in quantity of joints displacement and acceleration. As a consequence, cellar deck joints displacement and acceleration reduced about 50% in average. The results of using semi-active MR dampers showed that the earthquake-induced vibrations can be effectively suppressed by the isolation layer with MR dampers.

In order to present a quantitative indicator for the onset of instability, in this paper, the critical points of a stratified gravitational flow on a slope are found and analyzed. These points are obtained by means of the solution of the two-dimensional Navier-Stokes equations via the standard Arakawa-C finite-difference method. Results show that in the marginal Richardson numbers, the critical points begin to originate. Also, the cyclic evolution in the temporal differenced density field in the vicinity of the critical points is used as a quantitative criterion of the onset of mixing. Therefore, it is possible to predict the beginning of the mixing phenomenon via analysis of only a limited number of critical points.

Because of fluctuation in leg tension, pitch motion is very effective fatigue and life safety of leg elements in tension leg structures (TLSs). In this paper an exact solution for pitch vibration of a TLS interacting with ocean wave is presented. The legs of TLP are considered as elastic springs. The flow is assumed to be irrotational and single-valued velocity potentials are defined. The effects of radiation and scattering are considered in the boundary value problem. Because of linear behavior of legs during wave excitation, ignoring coupling effects with other degrees of freedom, the analytical solution of pitch response has good agreement with the real behavior of the structure.

Bioremediation, the process by which microorganisms degrade organic compounds to non-toxic or less toxic substances, has been widely used for cleanup of coastal ecosystems after oil spills. In this study, the hydrocarbon degradation rate and half lives in three bioremediation strategies (natural attenuation, biostimulation, and bioaugmentation) were compared in weathered crude oil (WCO) contaminated sediment samples. Three initial WCO concentrations of 3, 30 and 60 g WCO per kg sediment were investigated. Kinetic evaluations were carried out in a 90-day laboratory scale experiment. All oiled sediments showed decreasing WCO concentrations over time. After two weeks, the rate of degradation in the natural attenuation experiments stayed approximately constant. Bioaugmentation demonstrated higher oil removal than biostimulation or natural attenuation. The results indicated that first order kinetics can describe bioremediation of crude oil in sediments. The values of R2 (coefficient of determination) varied from 0.9552 to 0.9965. The first order kinetic constant for the reactors at different WCO concentrations was between 0.0014 and 0.0159/day. The half life of WCO in sediment reactors was different for each applied method. The minimum WCO half life for natural attenuation, biostimulation and bioaugmentation was 408, 69 and 44 days respectively.