Underwater Spread Research Articles

Experimental study on the slump-flow underwater for anti-washout concrete

The workability of underwater anti-washout concrete (UWC), especially the flowability in particular application environments, was always a critical and controversial issue. In this paper, a new test device and approach have been proposed to study the behavior of underwater flowability of UWC and establish a relationship with laboratory slump-flow tests. Further, the effects of different mix proportions, such as aggregates, superplasticizer (SP), water-cement ratio (W/C), anti-washout agents (AWAs) type and dosage, beside yield stress and plastic viscosity of UWC were investigated. The differences (Δd) between the spread diameter of the samples in air and underwater were compared. The results indicate that the flowability of concrete underwater is less than that in air, and it was proved that there is significant association between the yield stress and Δd. Also, various factors that affect the spread diameter difference are prioritized to optimize the rheological properties and limit the underwater flowability loss as much as possible. Combined with the Roussel's flow model and stress equilibrium differential equations, the theoretical formula of interpreting UWC's underwater flowability loss by yield stress was deduced, which can certify that the buoyancy underwater counteracts part of the gravity that keeps the UWC flowing freely and reduces its maneuverability. Additionally, an empirical formula of underwater spread diameter of UWC was proposed according to the variation law that the spread diameter difference decreases due to the increase of yield stress. Finally, the outcomes provided basic theoretical reference to filling capacity of UWC when casting underwater.

Investigation on underwater spreading characteristics and migration law of oil leakage from damaged submarine pipelines

Great threats and damages to natural ecosystem and marine environment will be caused once oil and gas leakage occurs. It is crucial to develop a quick response strategy, including the accurate prediction of oil and gas migration path, oil droplet size distribution and other parameters. Volume of fluid (VOF) model and realizable k-ε turbulence model are employed to investigate the underwater spreading characteristics and migration law of oil leakage. Influencing factors such as wave length, leakage direction, current speed, wind speed, oil density, and leaking rate are analyzed by numerical simulation. The calculation results indicates that wave length affects both the underwater spread and drift process, while current speed and wind speed mainly influence the drift process. Leakage direction, oil density and leaking rate have significant influence on underwater spread process but limited effect on drift process. A formula is proposed to predict the oil diffusion distance at certain time. The results of investigation can offer a valuable guidance for formulation of emergency response.

Underwater spreading and surface drifting of oil spilled from a submarine pipeline under the combined action of wave and current

Tremendous economic loss and environmental damages are caused by oil-spilling accidents in sea. Accurate prediction of the underwater spreading and surface drifting of oil spills is important for the emergency response. In the present study, numerical investigation on the underwater spread and surface drift of oil spilled from a submarine pipeline under the combined action of wave and current was carried out to examine the effects of physical ocean environment, leaking flux and spilled oil density and viscosity. Reynolds-Averaged-Navier-Stokes (RANS) equations, realizable k-ε turbulence model and volume of fluid (VOF) model are employed to describe the multiphase flow, and velocity-boundary wave-making technique combined with the sponge layer damping absorber technique realizes the numerical wave flume. Oil spill experiments were conducted to validate the numerical model. The calculation results indicate that compared with the environmental conditions of still water, only current and only wave, a larger scope of underwater spreading and relatively slower rising rate and relatively faster drifting rate of oil droplets are observed under the combined action of wave and current. The leaking flux affects the floating time and dispersion concentration, while the ocean environment affects the horizontal migration and surface drifting. Under the specific conditions of present work, oil density has obvious effect on the underwater spread but limited effect on the surface drifting, while oil viscosity has little effect on both the two processes.

An experimental investigation of underwater spread of oil spill in a shear flow

The time taken for spilt oil to appear firstly at the sea surface and its location are two key issues for emergency response. The underwater spread of oil spill in a shear flow was studied experimentally in a re-circulating water channel. The high speed imaging technology was employed to record the whole transport process of oil spilt from a leak of a submarine pipe to the surface. Based on the experimental results, three underwater transport types are identified, which are single droplet pattern (model A), linear chain pattern (model B) and oil plume pattern (model C), respectively. The pressure difference inside and outside of the leak determines the underwater transport pattern. For single droplet pattern, the transport of oil droplet has two successive stage, namely the accumulation stage and the buoyant droplet stage. When it comes to linear chain pattern, the first stage changes to be the initial jet stage. Besides the initial jet stage and the buoyant droplet stage, oil plume pattern has an intermediate transition stage, namely the plume development stage. During the whole floating process, the pressure difference dominates the initial stage, while droplet buoyancy is the driven force in the rest. The required time for oil droplets to reach the surface is increased with the decreasing of the initial momentum and the increasing of the shear flow velocity. In the buoyance dominated stage, the floating rate of oil droplets is basically unchanged and the horizontal migration rate is similar with the shear flow velocity. Both the break-up and coalescence of oil droplets have two forms, which are single droplet splitting and droplet-column separation for break-up form and turbulence merging and pursuit merging for coalescence form, respectively.

Multipath of sound propagation in shallow water and underwater spread spectrum communication

Due to the influence of the sea bottom and the sound speed profile, the propagation of sound in shallow water appears to be a complicated multipath. The time spread and the amplitude fluctuation of different arriving paths cause strong fading and intersymbol interference (ISI). At the receiving point the multipath signal can be expressed as r(t)=A0s(t−τ0)+∑i=1N-1Ais(t−τi)+n(t), where the first term is arriving signal directly, the second term is arriving signals by different paths, the third term is noise. When the difference of time delays is over the duration of code chip, a strong ISI occurs. As we know, the pseudo-random signal has high resolution in both time and frequency domains. It is often used in spread spectrum communication. In this paper a diversity technique with spread spectrum is used for realizing coherent accumulation of the arriving paths. The results of the lake experiment and the sea experiment are shown in this paper. The efficiency of coherent accumulation is obvious.