Leakage diffusion of underwater oil pipelines is affected by many external factors. To narrow these influences down, this paper focused on only four factors, including water-surface wind velocity, leakage hole diameter, water flow velocity, and initial oil leakage velocity. In this chapter, the parameters which is not as a variable all refer the data in Table 2.
Water-surface wind velocity
Referring to “Preliminary simulation results” section, the diffusion behaviors of spilled oil under different wind velocities were compared. The volume distribution of spilled oil under different wind speeds and leakage durations are shown in Figs. 4, 5 and 6.
As shown in the images above, the spilled oil mostly diffused along the river-bottom water flow direction in a horizontal manner under the combined action of wind and water, and the frontier oil droplets were dispersed quickly by the water flow. The smaller the wind speed, the smaller the horizontal displacement of the spilled oil. In the vertical direction, oil columns formed around the leakage hole under the action of buoyancy.
The variation regularities of the maximum horizontal and vertical displacement of underwater oil diffusion are presented in Figs. 7 and 8, respectively.
The following conclusions can be drawn from Figs. 7 and 8:
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1.
The maximum diffusion displacements of the spilled oil in both the horizontal and vertical directions increased linearly under all wind velocity conditions. The reason for this phenomenon is that the existence of a wind field on the water’s surface can accelerated the leeward diffusion of oil films and the faster the wind speed is, the faster the oil film expands. Therefore, the vertical displacement of leakage oil will be reduced in the same time period.
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2.
The higher the water-surface wind velocity, the greater the maximum vertical displacement. The water-surface wind velocity had no significant influence on the horizontal displacement of the spilled oil.
Leakage hole diameter
By referring to Sect. "Preliminary simulation results", a leakage diffusion model with hole diameters of 0.05 and 0.01 m was constructed. Volume distribution diagrams of oil diffusion under different leakage hole diameters and diffusion durations were compared to these results, as presented in Figs. 9, 10 and 11.
As the above images demonstrate, the greater the leakage hole diameter, the higher the leakage amount within the same period. In a wind field, the smaller the leakage hole, the more difficult the formation of a continuous jet-flow, and the easier the diffusion of the frontier oil straying from the main oil body. Under the disturbance of wind, spilled oil transformed into individual droplets and drifted with the wind. Under the condition of a leakage hole diameter of 0.01 m, the spilled oil usually diffused in the river-bottom along the flow direction, which was difficult to notice.
Variation diagrams of the maximum horizontal and vertical displacements during underwater oil diffusion are presented in Figs. 12 and 13, respectively.
According to the above images, the following implications can be drawn:
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1.
With various leakage hole diameters, both the maximum horizontal and vertical displacements of the spilled oil followed linearly increasing tendencies. The greater the leakage hole diameter, the greater the diffusion displacement increase rate.
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2.
When the leakage hole diameter was 0.01 m, the vertical displacement of the spilled oil was small and remained almost constant during diffusion.
Water flow velocity
By referring to Sect. "Preliminary simulation results", the leakage behaviors under water flow velocities of 2, 4, and 7 m/s were compared, and the results of the volume diffusion under different water flow velocity and diffusion durations are displayed in Figs. 14, 15 and 16.
As displayed in the above images, the faster the water flow, the more difficult the formation of a continuous jet-flow, the lower the oil column, and the greater the horizontal displacement of the spilled oil. With a decrease of water flow velocity, the oil column height increased, and the frontier oil droplets presented an obvious increasing tendency under the action of buoyancy, i.e. the influence of water buoyancy force on the spilled oil became more obvious.
Variations of the maximum horizontal and vertical displacements during oil diffusion are presented in Figs. 17 and 18, respectively.
Figures 17 and 18 demonstrate the following conclusions:
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1.
Under different water flow velocities, the maximum value of the horizontal displacement of the spilled oil increased linearly. The faster the water flow, the more significant the horizontal displacement variation.
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2.
The growth of the vertical displacement could be divided into two parts. Within 1 s of diffusion, the vertical displacement increased sharply, but slowed after 1 s of diffusion. During a water flow velocity of 2 m/s, the vertical displacement increase was the fastest. The faster the water flow, the smaller the vertical displacement of the spilled oil.
Leakage initial velocity
Based on “Preliminary simulation results” section, the initial leakage velocity of the oil was considered as a variable. By comparing the leakage conditions under various initial leakage velocities, the volume distribution diagrams under different leakage initial velocities and diffusion durations were obtained, as shown in Figs. 19, 20 and 21.
According to the above figures, when the initial leakage velocity was relatively low (i.e. 5 m/s), it was impossible for a jet-flow to form, and the spilled oil only settled along the horizontal direction. With an increase of leakage velocity, the oil gradually rose vertically under the combined influence of buoyancy and the wind field, and the diffusion frontier droplets demonstrated a distribution pattern.
Variation regularities of maximum horizontal and vertical displacement during diffusion are shown in Figs. 22 and 23, respectively.
According to the above figures, the following conclusions could be drawn:
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1.
When the initial leakage velocity was different, the maximum horizontal displacement of the spilled oil increased linearly. The greater the initial leakage velocity, the larger the horizontal displacement increase rate.
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2.
The increase of vertical displacement was divided into two parts. Within 1 s of leakage, the vertical displacement increased quickly and then slowed. When the initial leakage velocity was relatively low (under 10 m/s), the vertical displacement of the spilled oil remained almost constant after 1 s.