Abstract
In nearshore regions, bidirectional tidal flow is the main hydrodynamic factor, which induces local scour around submarine pipelines. So far, most studies on scour around submarine pipelines only consider the action of unidirectional, steady currents and little attention has been paid to the situation of bidirectional tidal currents. To deeply understand scour characteristics and produce a more accurate prediction method in bidirectional tidal currents for engineering application, a series of laboratory scale experiments were conducted in a bidirectional current flume. The experiments were carried out at a length scale of 1:20 and the tidal currents were scaled with field measurements from Cezhen pipeline in Hangzhou Bay, China. The experimental results showed that under bidirectional tidal currents, the scour depth increased significantly during the first half of the tidal cycle and it only increased slightly when the flow of the tidal velocity was near maximum flood or ebb in the following tidal cycle. Compared with scour under a unidirectional steady current, the scour profile under a bidirectional tidal current was more symmetrical, and the scour depth in a bidirectional tidal current was on average 80% of that under a unidirectional, steady current based on maximum peak velocity. Based on previous research and the present experimental data, a more accurate fitted equation to predict the tidally induced live-bed scour depth around submarine pipelines was proposed and has been verified using field data from the Cezhen pipeline.
Highlights
With the development and exploitation of offshore oil and gas resources, submarine pipelines are widely used to transport oil and gas from offshore to onshore areas
It was evident that the scour depth development was much slower for the bidirectional tidal current compared to that under unidirectional steady current, whose velocity equaled the maximum velocity of the tide
This implied that the effective flow intensity, ur, under bidirectional tidal currentd with varied flow velocity was smaller than that of the unidirectional steady current based on umax7
Summary
With the development and exploitation of offshore oil and gas resources, submarine pipelines are widely used to transport oil and gas from offshore to onshore areas. The tidal period was nearly 12 h, which was much longer than common is no study on the scour around submarine pipelines under bidirectional tidal currents. The objective of this study was to gain further insights into the scour characteristics around submarine pipelines under bidirectional tidal currents. According to Froude’s law, the length scale between the model experiment and prototype was Experimental runs were carried out under bidirectional tidal current and unidirectional current conditions, respectively. The time history of the experimentally generated tidal velocity was in good agreement with the theoretical value, indicating that the multi-pump frequency control system could generate an excellent bidirectional tidal current. The unidirectional velocity u0 was equal to the maximum peak velocity umax in a tide period and the water depth was equal to the value of the bidirectional tidal current
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