Numerical study on effects of the cofferdam area in liquefied natural gas storage tank on the leakage and diffusion characteristics of natural gas

The leakage and diffusion characteristics of natural gas were investigated in the condition of the leakage of liquefied natural gas(LNG) in the storage tank.Fluent was adopted to simulate the process in a series of three-dimension unsteady state calculations.The effects of different heights of the cofferdam(1.0 m, 2.0 m and 3.0 m),wind directions,ambient temperature,leakage location,leakage volume on the diffusion process of natural gas were investigated.The diffusion characteristics of the natural gas clouds over cofferdam were found.Under windless condition,when the gas clouds met,the gas clouds rose due to the collision,which made them easier to cross the cofferdam and spread out.The higher the ambient temperature was,the higher the gas concentration around the cofferdam was,and the smaller the gas concentration difference was.When the leakage occurred,the higher coffe rdam was more beneficial to delay the outward diffusion of gas clouds.However,when the leaka ge stopped,the higher cofferdam went against the dissipation of gas clouds.Under windy condition,the time to form stable leakage flow field was faster than that of windless,and the lower cofferdam further reduced this time.Therefore,considering the effect of barrier and dissipation,it was suggested that the rational height of cofferdam should be designed in the range of 1.0 m to 2.0 m.In case of emergency,the leakage of gas should be deduced reasonably by combining the measurement of gas concentration with the rolling of gas clouds.When windless,the leakage area should be entered between the overflows of gas clouds.

Influence of Artificial Cofferdam and Spartina alterniflora Expansion on Evolution of Suaeda salsa Marsh in Yancheng Coastal Wetland of East China

Suaeda salsa is an important local species in the intertidal beach of the Western Pacific coast. However,under the artificial cofferdam and Spartina alterniflora expansion,Suaeda marsh has degraded seriously. Therefore,using Yancheng Nature Reserve as a case study area,taking ETM+images in 2000,2006 and 2011 as the basic data sources,we revealed the evolution characteristics of Salsa marsh which was impacted. The research results are as follows. From 2000 to 2011,Salsa marsh area in the artificial area tempestuously degraded,decreasing by 87. 158%,more than 22% than those in the natural area. The landscape was fragmentized. Landscape polymerization degree index dropped from 95. 780 to 65. 455,more than 16% than those in the natural area. The mean patch area fell down to 21. 429 ha from 389. 333 ha,more than 11% that in the natural area. Compared to the steady change in natural conditions,the area was reduced by 118. 167 ha/a from 2000 to 2006,while during 2006-2011,it was only 51. 500 ha/a in artificial area. As for spatial change of landscape,in artificial area,the Salsa marsh centroid moved forward to the southeast with 666. 350 m,but that in natural area moved forward to the north with 1 042. 710 m from 2000 to 2006. From 2006 to 2011,the centroid moved forward to east and north respectively. Artificial cofferdam transformed the area into freshwater ecosystem,and meanwhile the freshwater was beneficial to Reed marsh. During 2000 to 2006,in the artificial area,539 ha Salsa marsh controlled by cofferdam transferred into reed marsh and aquaculture ponds,of which the transformation rate was nearly 4% higher than that in natural area. From 2006 to 2011,178 ha Salsa marsh was transferred into reed marsh,the transformation rate was 20% higher than that in natural area. With rapid spreading and strong competition of Spartina species,the coastal wetland has formed the pattern of " Salsa – Spartina marshes". From 2000 to 2006,in artificial area,15. 24% of Salsa marsh was transferred into Spartina marsh,of which the transformation rate was13% higher than that in natural area. And from 2006 to 2011,30. 07% Salsa marsh was replaced by the Spartina marsh in artificial area,the rate was almost 10% higher than that in the natural area.

Theory of Combined Seepage Applied to Dewatering Systems

In the present study, an analytical solution is presented to solve the problem of combined seepage, under a sheet piling cofferdam, applied to dewatering systems. Existence of the sheet pile creates a confined seepage followed by an unconfined seepage in the same field, which presents a combined seepage problem. Two equations were developed to analyze the combined seepage underneath a sheet piling wall. Using such equations, both the maximum height of the free surface just behind the sheet piling cofferdam (Ho) and the quantity of seepage discharge to be pumped out from the construction site (q) can be determined. The main parameters affecting the combined seepage characteristics underneath a sheet piling wall are: The depth of permeable foundation layer (T), the horizontal distance behind the sheet pile (X), the depth of excavation in the construction site (D), the embedded depth of sheet pile (S), the retained water head (H1), the accumulated seepage water depth (H2), and the side slope factor of excavation line (M). Study showed that, the above parameters have a great effect on the combined seepage, but with different extents.

Structural Analysis of Offshore Cofferdam Subjected to Wave Load and Suction Pressure

This study presented a novel circular cofferdam for offshore bridges consisting of ring segments and investigated its structural behaviors using ANSYS Mechanical. Because the bottom segment of the cofferdam which has a double sleeve cross-section was installed by suction, contact behaviors of the cofferdam wall and the lid plate during installation are important for design and the behaviors were also analyzed. Prior to numerical investigation of the bottom segment and complete cofferdam after dewatering, a suction modeling for structural analysis was proposed and evaluate by seepage analysis. Hydrodynamic loads applied to the cofferdam were also evaluated using panel method based on the potential flow theory. Through numerical analyses, structural behaviors of the cofferdam during installation were then investigated. First, contact behaviors between the lid plate and the wall were analyzed using different contact conditions imposed on the interface. Sharp stress increases were shown while the stress jumps were limited to contact area. Next, structural behaviors were investigated by considering seepage pressure. Using an axisymmetric seepage model, the total water pressure considering seepage was estimated and applied to structural analysis. The analysis results showed that strong effects of seepage on the stress change in cofferdam occurred and the seepage effects are necessary to be considered in design of the cofferdam induced by suction.

Extension of Giurgiu Port on Lower Danube

The paper deals with the construction works devoted to extending and upgrading the port functions to the highest European Standards. The Port of Giurgiu is located on the left bank of the Danube River, at the 493 km, and also at 65 km south of Bucharest, the Capital of Romania. Giurgiu is one of the Romanian largest Danubian Ports. Advanced computer programs were used in design. An active monitoring system, regarding the pre-existing constructions on the site, directly influenced by the excavations was also used. The adopted solutions for infrastructure were dependent by the monitoring results of construction site. The weak foundation conditions as well as the proximity of some existing buildings arouse unexpected obstacles difficult to overpass. However, by a perfect human cooperation all targets were finally reached.

Phase Ⅲ Rolled Compacted Concrete Cofferdam Construction

Phase Ⅲ Rolled Compacted Concrete(RCC) cofferdam construction belongs to Phase Ⅲ diversion works. It is a key item to ensure an achievement in the reservoir impoundment, navigation and power generation in 2003 and a guarantee to the safety construction of the follow up right bank intake dam section and the powerhouse. Restrained by conditions for construction, navigation, the worksite and the duration, both the concreting intensity and the height of Phase Ⅲ RCC cofferdam exceed the current construction level. In view of these difficulties, a great deal of research work performed by relevant departments is described in the paper.

COUPLING EFFECT BETWEEN RELIABILITY OF BEDDING LAYER AND STABILITY OF DOWNSTREAM CONCRETE SLAB OF OVERFLOW EARTH-ROCK COFFERDAM

The protection of downstream concrete slab is a key issue for the stability of overflow earth-rock cofferdam. The coupling effect between bedding layer and concrete slab was taken into account when the stability of downstream concrete slab was researched. The characteristics of overflow and seepage over the downstream concrete slab were investigated when floodwater passes over the cofferdam. Firstly a limit equation of seepage failure for the bedding layer was derived with the consideration of geometric and mechanical factors, and a reliability model was established and numerically simulated. Then based on the reliability calculation for the bedding layer, the coupling effect between bedding layer and downstream concrete slab was analyzed. Under the most unfavorable pressure condition for the concrete slab, its instability criterion was put forward, which offers a structural design tool of downstream concrete slab for overflow earth-rock cofferdam. Compared with model tests, it shows that the model of reliability calculation of bedding layer and the stability analysis of downstream concrete slab are effective.

Behavior of Plastic Concrete Diaphragm Walls in Three Gorges Project

The high earth and rockfill cofferdam of Three Gorges Project employs plastic concrete diaphragm walls as seepage barrier. In view of the importance and technical difficulty of the cofferdams, the behavior of plastic concrete and the diaphragm walls within the cofferdams was studied. Plastic concrete samples taken from concrete mixer in the dam site were tested by using large triaxial testing apparatus in Tsinghua University. Mechanical properties and parameters of Duncan Chang nonlinear elastic model were obtained. Test results indicated that, comparing with ordinary concrete, the materials have the features of low modulus of deformation and favorable impermeability. The analysis of stresses and deformations of the diaphragm walls was performed by means of finite element method (FEM), using parameters obtained from the result of triaxial tests. Calculation results were discussed.