Advanced Materials Research Vols. 243-249

Abstract: The microstructure of cement test block and the C-S-H gel attacked by magnesium sulfate is observed by means of scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS)and X-ray diffraction(XRD).It is found that the presence of Mg²⁺ makes C-S-H gel decalcify, dissolve and decompose, which thereby increasing the erosion of SO₄^2- on the concrete. The decomposition of C-S-H and the generation of M-S-H result in the loss of concrete strength and the ultimate destruction of concrete.

Abstract: The water exchange with outer sea for the nearly closed bay is very weak, leading to the serious deterioration of the environment of the bay. Numerical experiments have been performed to find the effects of change of the water depth at the bay mouth on the water exchanging ability of the nearly closed bay. The results indicated, by decreasing the depth at the nearly closed bay mouth appropriately, the water exchanging ability can be improved.

Abstract: By the analyzing of Q cluster and characteristic factor matrix, coastal hydrological types and their distributions are found by selecting 7 characteristic factors in 26 districts of the China coastal zone. In the view of environmental hydrology and marine engineering, the distribution types of coastal zone are mainly divided into sedimentary coast (including sandy coast, muddy coast, strongly hydrodynamic coast) and rocky coast (including massive bedrock coast, grit bedrock coast, bank slope and platform coast). The formation, development and evolution of the coastal zone are mainly controlled by the land geologic actions, particularly by tectonism and hydraulic condition, and the ocean geologic actions only take part in the modification of the coastal features. The topography and lithology are mainly controlled by distribution of land water flow in coastal zone.

Abstract: The shear failure mechanism induced by wave of seabed at the Chengdao Sea was analyzed, one model to calculate the shear failure was established, and the factor of safety of shear failure at the different locations was calculated, some conclusions were gotten: (1) The shear stress at the seabed induced by wave and gravity along the submarine slope are the main triggers to cause shear failure, and the shear stress at the seabed surface induced by wave and currents can be ignored; (2) At extreme sea conditions, the shear failure regions are mainly distributed within the north-west and south-east. The shear failure depth of north-west area is within 4m, and that at south-east is relatively large, beyond 4m in depth. (3) The shear failure of Chengdao Sea was mainly controlled by wave stress in seabed. The shear failure region and depth calculated in this paper can be referred in oil engineering facilities design and construction.

Abstract: As a type of very large floating structures，the design concept of the grid structure is applied to the marine engineering floating pier structure. The grid structure has many merits such as high structural strength, stiffness, integrity, lower weight and high degree of industrialization. A finite element software named ABAQUS is used to analyze the stress and strain of the marine engineering floating pier structure in the marine environment. The analysis shows that the structure meets the engineering requirements in structural strength, stiffness, stability and wave resistance.

Abstract: In this paper, the stability and nonlinear dynamics are studied for a slender flexible cylinder subjected to axial flow. A nonlinear model is presented, based on the corresponding linear equation of motion, for dynamics of the cylinder supported at both ends. The nonlinear terms considered here are only the additional axial force induced by the lateral motions of the cylinder. Using six-mode discretized equation, numerical simulations are carried out for the dynamical behavior of the cylinder to explain, with this relatively simple nonlinear model, the flutter instability found in experiment. The results of numerical analysis show that at certain value of flow velocity the system loses stability by divergence, and the new equilibrium (the buckled configuration) becomes unstable at higher flow leading to post-divergence flutter. As the flow velocity increases further, the quasiperiodic motion around the buckled position occurs, and this evolves into chaotic motions at higher flow.

Abstract: Presented in this paper are the three-dimensional nonlinear finite element analyses of the failure envelopes of suction caisson under torsion, vertical and lateral pullout combined load in soft clay. The soft clay under undrained condition is simulated by perfectly elasto-plastic Tresca model. Through the numerical analyses, the failure envelopes for combined loading (V-T、H-T、V-H-T) of suction caisson is reviewed. And the mathematical expression of failure envelope is deduced. It is shown that (1) the circular plastic failure area is outward-extending. (2) The bearing capacity of suction caisson foundation in V-T、H-T load spaces is increasing with the aspect ratio L/D. (3) The equation of failure envelope can be used to evaluate the stability of suction caisson foundation for offshore floating wind turbine.

Abstract: Internal air pressure and water pressure of the buckets foundation, acceleration and dragging force of the 4-bucket foundation platform were determined to analyze the influences of wave height on towing when 4-Bucket foundation platform is towing in conditions of a certain towing velocity, mooring point position and draught in different regular waves based on the model test. Comparing platform towing in following and head waves, the heave motion of the latter is more strenuous than that of the former; the stability and seakeeping of the former is better than those of the latter. Roll motion, pitch motion and heave motion appear aggravate phenomenon with the increase of wave height.

Abstract: In this paper, the internal air pressure and bottom water pressure of the buckets, the acceleration of swaying, surging and heaving of platform were determined by the sensors fixed on the platform, as a naval bucket foundation platform for example. The motion rules of heave and sway of bucket foundations are studied with different drafts. The results of this test showed that the added mass coefficient of heave, roll and pitch are declined with the increase of the draft; the natural frequency of heave is constant with different drafts but the roll frequency and pitch frequency are consistent with the changes of the draft; the frequency of heave is larger than that of roll and pitch but the added mass coefficient of heave is lesser than that of roll and pitch.

Abstract: Taking a deep-water semi-submersible drilling platform and its mooring system which will be used in South China Sea area as an example, the motion responses of this platform in the frequency and time domains are analysed. In the frequency domain, the three-dimensional potential flow theory is emploied to calculate, the response function of six degrees of freedom and some hydrodynamic parameters, such as additional mass and additional damping. In the time domain, the non-linear time-domain coupling analytical method is used to calculate the motion responses of this platform under the action of wind and waves. Then according to the time series of motion response, the influence of the Jonswap spectrum’s parameters, including wave height and peak factors on the platform’s heave response is analysed.This numerical simulation results provides an reference for the target platform’s actual construction.