Experimental and numerical study of natural backfill of pipeline trenches

[Truncated] After a sub-sea pipeline is laid in a trench excavated on a sandy sea bed, sediments outside the trench will be washed into the trench by the flow under certain flow conditions, leading to natural backfill of the trench. Natural backfill of pipeline trenches can be beneficial to the stability of the pipeline and reduce engineering costs related to mechanical backfill of the trench. In this study, natural backfill of pipeline trench under steady currents, wave only and combined currents and waves conditions was investigated experimentally and numerically.

Experimental tests were carried out to investigate the backfill process of pipeline trench. The evolutions of the pipeline trench profile were measured with a topography scanner by stopping the tests at set intervals. The model pipeline with a diameter of 5cm was placed in a pre-dredged trench. The direction of flow was perpendicular to the axis of the pipeline trench. Effects of various parameters such as incoming flow velocity, wave height and wave period, ratio of pipeline trench depth to pipeline diameter, and slope of trench on backfill rate were investigated. Tests were carried out in both clear-water scour and live-bed scour conditions. Pipe Reynolds number and trench depth Reynolds number under steady currents conditions were within the ranges 1.0×10⁴≤Re_p≤1.4×10⁴ and 1.0×10⁴≤Re_t≤2.6×10⁴, respectively. In these experiments backfill depth showed stronger dependency on the Shields parameter θ, while backfill-time scales seemed to be influenced by both θ and Re_t. In general, a larger Shields parameter induced a quicker backfill rate; the gentler the trench slope, the slower the backfill; the larger the depth of the trench, compared to the pipeline diameter, the larger the backfill depth. In later stage, the geometry dimensions of the ripples formed generally at the upstream side of the trench are also a determinant factor in the backfill process of the trench. When the ripples are comparable in size to the upstream shoulder of the trench, the pipeline undergoes periodic burial due to the passage of the ripples The effect of parameter U_c/(U_c+U_w) on backfill process was also investigated for those cases under combined currents and waves conditions. Experimental evidence showed that the growth of the imposed current does not necessarily imply an increase in the backfill depth and backfill rate. As U_c/(U_c+U_w) goes from wave alone, i.e. U_c/(U_c+U_w)=0, to current alone, i.e. U_c/(U_c+U_w)=1, the relative backfill just near the pipeline increases firstly, and then decreases. For U_c/(U_c+U_w)<0.403 in present test condition, the superimposed flow acts as an assistance to wave to transport sediments on two shoulders into trench. Yet, for U_c/(U_c+U_w) going from 0.403 to 0.443, the current become relatively strong that it is able to transport the sediment stirred up to water by wave far away from the trench, and then the backfill process was hindered somehow.