Effects Of Wave Energy And Suspended Sediment Characteristics On Oil-sediment Aggregation And Settlement

With the continuous expansion of the scale of marine oil transportation and exploitation,the occurrence of oil spill accidents is inevitable.In the water area with enough suspended particles,the spilled oil will break up and disperse into oil droplets under the action of waves,further collide with suspended particles to form the oil particle aggregate(OPA).After the formation of OPA,it will sink to the seabed and harm the marine benthos and ecological environment seriously.Therefore,it is of great significance to explore the mechanism of OPA formation for oil spill risk prediction and clean-up.In order to simulate the actual sea conditions,a wave flume was used to expand the scale of hydrodynamic condition in this paper.Roncador crude oil was selected to firstly investigate the effect of wave energy dissipation rate on the dispersion process.In order to assess the effects of wave energy,sediment concentration and size on the oil settlement,OPA morphological characteristics and settlement rate,the OPA formation experiment was conducted by selecting the natural sediment in Bohai Sea.On this basis,an empirical model for predicting oil trapping efficiency under different mixed energy and suspended sediment concentration is established.The main findings are as follows:(1)When the wave energy dissipation rate was high,the oil dispersion efficiency(ODE)of Roncador crude oil firstly increased sharply over time and then tended to be stable.After the wave energy decayed,the ODE showed a trend of "increasing first,then decreasing and finally stable" over time.The ODE under different wave energy all reached equilibrium within 120 min.At the same time,the ODE increased in turn with the increase of wave energy.Regardless of the energy energy,the size of dispersed oil droplets all dropped sharply at the initial stage and stabilized after 60 min.At the same time,the average size and distribution of oil droplets decreased and narrowed sequentially with the increase of wave energy.(2)Under the three wave energy,the oil trapping efficiency(OTE)all increased with time and eventually stabilized.In the same oscillation time,OTE increased with the increase of wave energy dissipation rate.With the continuous collision and aggregation of oil-sediment,the small-size dispersed oil was firstly captured by the sediment,and the maximum size of the coated oil droplets gradually increased.However,the proportion of large-size oil droplets gradually decreased.With the enhancement of the energy dissipation rate,the sedimentation rate of OPA increased,the overall size became larger,and the linear fit between the equivalent diameter and the sedimentation rate improved.(3)Increasing the concentration of suspended sediment in a certain range can improve the sedimentation of spilled oil and the sediment content of OPA,OTE and sedimentation rate increased accordingly.At the same time,the addition of suspended particles can refine the size of coated oil droplets and prevent the coalescence of dispersed oil.Large-size sediment has a poor capture effect on oil droplets,but the size of formed OPAs were relatively large,and the time required for OTE to stabilize was significantly shortened.(4)Through the fitting analysis of the time series of OTE,it was found that the Lagrange first-order kinetic equation is applicable to the prediction of OTE under different mixed energy,sediment concentration and size.The values of the goodness of fits are all above 90%.The results of this paper reveal the dispersion of oil spill and the mechanism of OPA formation,further provide theoretical basis and technical support for improving the existing oil spill models and the prediction ability of oil spill environmental behavior.