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Seven years of field survey measurements of a subsea pipeline obtained using sonar profilers and underwater video indicate significant lowering of the pipeline into the seabed due to sediment mobility and scour. The majority of the lowering occurs within 2 years of pipeline laying and appears to result from sustained ambient tidal and soliton currents as opposed to large storms. The lowering results in an increase in pipeline embedment relative to the far field seabed of up to 0.8 times the pipe diameter (referenced at a distance ± 8 D from the pipeline). At most locations along the pipeline, this increase in far-field embedment is uniform and occurs after the formation of many closely spaced scour holes. This suggests that the pipeline lowered mainly through sinking into the seabed at span shoulders, rather than sagging into widely spaced scour holes, for much of the pipeline length. A beam bending analysis confirmed the dominance of sinking, but did show some evidence of pipeline sagging, calculating deflections of up to 0.3 pipeline diameters at the time of surveying. In contrast to the traditional conception of span growth and self-burial, which conceives of complete pipeline burial as an endpoint, this pipeline primarily appears to exhibit ‘self-lowering’ towards a mature state that consists of a pseudo-static profile of alternating spanning and embedded sections that are distributed at regular intervals. The observed changes appear to be predictable given sufficient pipeline setting data, which suggests that they can be quantified in the stability design of new pipelines. This opens up the possibility of more efficient on-bottom stability design, as the beneficial shielding and support provided by the self-lowering process is not usually accounted for. |
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