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Tidal flow and large amplitude wave motion are examples of reversing flow in nature. As pointed out by Grass and Hosseinzadeh, (1995), the most obvious feature o f the offshore environment which has to date been largely ignored in scour research studies, possibly because o f the need for non-standard laboratory facilities, is reversing tidal flow. Therefore few research studies have been carried out using reversing tidal flow conditions. Mao (1986), employed a laboratory flume to simulate tidal flow and from his study concluded that, in the reversing flow condition wake induced erosion plays a significant role in the final profile o f the scour hole. Scour depth in reversing flow was also observed to be larger than that produced by uni-directional flow. Mao's reversing flow tests also showed that as in the uni-directional flow tests, the normalised scour depth for smaller diameter pipelines appears to be greater than that obtained with larger diameter pipeline.

It is noteworthy to mention that scour holes generated by uni-directional flow are highly asymmetrical, with the point o f maximum scour depth located well downstream of the pipeline axis. In nature, however depending on the pipeline orientation, reversing tidal flow with its reversing wake scour, has been observed to produce very wide and often relatively symmetrical scour hole (Hulsbergen 1982). Mao's experiments do suffer from lack o f symmetry in to positioning of the model pipeline inside the test flume which produced an asymmetrical scour hole. Effective study o f the self-burial process by the action o f reversing tidal flow requires special facility simulating natural tidal flow condition to produce symmetrical scour hole. Paskin's (1993) work is the most intensive study o f this type conducted to date involving use o f a unique, specially designed reversing flow channel at UCL, simulating tidal conditions.

Paskin's reversing flow tests examined the effect o f different sediment transport rates on the final scour depth. This work demonstrated that the final scour depth is largely insensitive to the prevailing sediment transport rates, whereas the scour

hole width is sensitive to the sediment transport rates. In other words higher flow rates produce wider scour widths. In relation to pipeline diameter, despite the fact that Paskin's tests were actually run to very different dimensionless test times, making comparison particularly difficult, close inspection o f the data suggests that there is a decrease in the final equilibrium scour depth with increasing pipeline diameter. However the relative width o f the scour hole (W/D) was observed to increase with decreasing pipeline diameter. These experimental observations are entirely consistent with the present potential flow model predictions which indicate that wider scour holes produce larger maximum scour depths (see chapter four). Paskin's tests produced very deep scour holes, in some cases greater than twice the pipeline diameter.

Owing to the fact that in the field scouring takes place under the pipeline in a three dimensional manner, it is necessary to address this aspect o f the problem. Scour develops along the length o f the pipeline and grows with time giving rise to free pipeline spans o f progressively increasing length. Depending on the pipeline stiffness, the pipeline will sag by varying degrees into the scour hole. From an engineering point o f view the dimensions of the scour hole under the pipeline are thus very important. In particular, the maximum scour depth and length are o f paramount importance. Whilst data is available on the maximum scour depth under pipelines^ corresponding data for free span length is scarce.

Development o f free span length determines the rate o f centre span sagging which in turn affects the scouring rate and ultimately the scour depth at which the centre span point on the pipeline first settles onto the scoured bed. Recent experiments have shown that (see for example Grass et al., 1992 and Paskin, 1993) very high span length development rates were achieved with corresponding pipeline sagging rate increasing exponentially with the total free span length. An attempt was also made by Mao (1986), to assess the maximum span length for a pipeline. He postulates that when there is no free span under the pipeline, the load on the pipeline is balanced by the support o f the plane bed. But in the case o f a free

span, provided the length o f span is long enough the pipe deflects at the mid span as it does in the case o f girder. Mao argues that when the span length is long enough the pipeline puts larger pressure on the sustaining part of the bed, namely the span shoulders. Consequently, it is more difficult for sand particles on the shoulder slope to slide down.

It should be noted that pipeline sagging at the span shoulder is closely related to the three-dimensional scour. Depending on the pipeline relative weight and sand failure, the pipeline may sink into the sand, since as the scour progresses, the load due to the weight of the pipeline will be acting on a smaller area o f the supporting ridge. Sumer and Fredsoe (1994) suggested that when the length o f a span shoulder is decreased to a critical value («5D), the pipe begins to sink into the sand owing to soil failure. They used different density pipeline models exposed to steady currents and came to the conclusion that the section of pipeline at the span shoulders may sink up to 50-80 % o f the pipeline diameter below the initial bed level. Bruschi et al. (1986), tried to model the flow field in relation to three dimensional scouring below pipelines. They paid due attention to the scour induced free span length and reached simiW conclusion as Mao, discussed previously. Bruschi et al. concluded that a theoretical model can provide approximate information on maximum scour depth and maximum free span length, but it is unable to predict the time scale and evolution o f the scouring phenomenon.