Controlling Factors And Mechanisms Of Sunken And Submerged Oil Formation Based On Wave Tank Simulation

The sunken and submerged oil(SSO)is mainly submerged in the marine environment by the formation of relatively stable oil-suspended particulate matter aggregates(OSAs)due to colliding between oil droplets and suspended sediments,and its formation mechanism is complex.The motion characteristics of SSO are different from those of surface oil film.Due to the limited monitoring techniques and lack of efficient recycling equipment,SSO may cause long-term damage to marine organisms and the environment.In the present research,the batch-scale experiments,as a screening basis,have been conducted to study the time-scale effects of different kinds of suspended sediments on the formation of SSO based on the hydrodynamic condition and hydrochemical environment in the Bohai Sea.Also the comprehensive effects of various controlling factors on the formation of SSO,such as oil type,suspended sediment type,temperature and dispersant,were systematically carried out.Additionally,based on the mesoscale wave tank experiments and numerical fitting,the time-scale numerical model of SSO formation was further verified.Through screening the typical oil type and suspended sediment,the wave tank experiments with different conditions were also further carried out to investigate the formation law and size distribution of SSO under the breaking waves.Furthermore,the mechanism of the dispersant on the formation of SSO was also discussed.Based on the results of mecoscale wave tank experiments,the generation model of SSO in the presence of the dispersant and suspended sediment was preliminarily established.The main results obtained are as follows:(1)In the bench-scale experiments,Merey crude oil and three kinds of suspended sediments(including sand,silt and quartz)were tested.The results showed that the oil sinking and submerging efficiency(OSSE)increased quickly with the mixing time until reaching and keeping a stable plate,and the formed sunken oil accounted for 40-95%of the SSO,which could pose a huge potential risk to the benthic organisms living on the seabed.According to the time series of OSSE,a prediction model of SSO formation as a function of time was established to determine the equilibrium time of SSO formation.Increasing the concentration of suspended sediments within a certain range could significantly shorten the equilibrium time of SSO formation.Besides,the formation rate of SSO with sand was the fastest corresponding to the shortest equilibrium time,followed by the quartz and the silt,which mainly attributed to the difference of the suspended sediment size and hydrophobicity.(2)Based on the determined equilibrium time of SSO formation,batch-scale experiments have been developed to study the formation of SSO under various controlling factors including oil type,suspended sediment type and concentration,temperature and dispersant.The results showed that a large amount of SSO was generated with Oman crude oil and Merey crude oil in the presence of sand,silt and quartz,and there existed an optimal suspended sediment concentration to maximize the OSSE Also sunken oils with negative buoyance were generated with Merey crude oil,and more submerged oils were formed with Oman crude oil,while almost no SSO was generated with 380#fuel oil.This is mainly caused by the combination of oil viscosity and asphaltene content,and the temperature is also based on this mechanism to affect the formation of SSO.The OSSE increased with the increasing temperature.Based on the existing data,an empirical formula between OSSE,oil viscosity and asphaltene content was established to predict the effects of different oil type and temperature on the formation of SSO.It was noted that the formation of SSO was enhanced in the presence of the dispersant and sediment,and a higher viscosity of test oil was correponding to more significant synergistic effect in the presence of the dispersant and the sediment.However,due to the complex mechanisms of the dispersant,as the dispersant dosage(dispersant to oil ratio,DOR)increased,the OSSE was not always with an increasing trend in the presence of the dispersant and sediment When the DOR increased to 1:5,the OSSE reached the maximum.(3)Based on the bench-scale experiments,the meso-scale wave tank experiments were further carried out.Under the breaking waves,the OSSE exhibited the similar trends to the bench-scale experiments with the mixing time increasing,and the prediction model for the formation of SSO as a function of time in the latter was also applicable.The equilibrium time of SSO formation in the wave tank experiments was much shorter compared with the bench-scale due to the difference of the model scale and the mixing energy.As a result the parameter correction was performed.Besides,the mean size of sunken oils in the wave tank first increased with increasing mixing time and then decreased,followed by a slight fluctuation.(4)Under the breaking waves,there also exists an optimal suspended sediment concentration to maximize the OSSE,and the OSSE increased with the suspended sediment size decreasing,while the oil sinking efficacy(OSE)showed a decrease with the small size sediment Furthermore,the addition of suspended sediment could significantly refine the size of oil droplets in the water column,and a smaller mean size of the sediment was corresponding to the more obvious refining effect.Based on the existing data analysis,an empirical relationship between the concentration and size of suspended sediment and OSSE was established,and the generation model of SSO in the presence of suspended sediment was initially constructed.(5)Under the breaking waves,the combination of dispersant with different DOR and sediment could also significantly promote the physical OSSE and refine the dispersed oil droplets in the water column,which was greater than that of dispersant and sediment alone.The size ratio between oil droplet and sediment was resized,but the adhesion of oil droplet was also weakened.The OSSE increased linearly with the DOR increasing,while the OSE had a decrease with high DOR,even lower than that of the sediment alone.Based on the existing data analysis,an empirical relationship between the dispersant and OSSE was established,and the generation model of SSO in the presence of the dispersant and sediment was also initially constructed.(6)It is a reversible process of dynamic transform between sunken oil and submerged oils in the water column.Under the various controlling factors,the submerged particles in the wave tank showed a multi-peak distribution,while the sunken oils exhibited a single-peak distribution It is noted that the Log Normal distribution function is more suitable for the size distribution fitting of the submerged oils,but the Rosin-Rammler is more suitable for the sunken oils with no more than 7%simulation error.As the size distribution of SSO is closely related to its transport and microbial degradation rate,the present research could provide the theoretical support for improving the transport and fate model of SSO,and provide data for the prediction of ecological risk of SSO.