Natural Backfilling Of Pipeline Trenches

Abstract

ABSTRACT Sea pipelines are for various reasons frequently laid in trenches in the seabed rather than on the seabed itself. In certain areas it is requested, for security reasons, that the pipeline should be covered to a certain extent, either through natural backfilling or through artificial backfilling. For the planning of the pipeline protection, it is therefore important to be able to calculate the rate of natural backfilling. The present paper describes how the natural backfilling may be calculated on the basis of the following parameters: currents, waves, bed material, shape of trench and water depth. The backfilling (sedimentation) in the trench is partly caused by a gravity effect on the bed load transport and partly by settling of the suspended load due to decreasing transport capacity over the trench. The physics behind these two sedimentation mechanisms are described. A theoretical calculation of the sedimentation, due to gravity effects on the bed load transport is developed in two steps. The first step contains only current, crossing the trench at an arbitrary angle. In the second step, it is shown how the same solution may be applied to bed load transport caused by wave action. The sedimentation due to settling of suspended load is also calculated for currents alone and for combined currents and waves. From the developed mathematical model, it is possible to calculate which portion of the suspended load will be able to settle as the current passes the trench. Four examples of how to calculate natural backfilling are presented. Two examples in current only and two examples in combined currents and waves. INTRODUCTION The aim of the studies presented in the present paper has been to provide mathematical models for calculation of natural backfilling in pipeline trenches under the assumption that the sea bed consists of non-cohesive sediments and that the depth of the trench is small compared to the water depth, see Fig. 1. The local scour around the pipeline is not considered in the present paper. The transportation of non-cohesive sediment is normally separated into the transportation of bed load and suspended load. In many contexts, such a distinction is of minor importance, but for the present purpose it will be demonstrated that such a distinction is necessary. As an illustrative example, the especially simple case where the alignment of the dredged trench is parallel to the direction of the current is considered. In this case, no suspended sediment is carried from the undredged regions to the trench by the current, if secondary currents are neglected. As long as the variations in water depth are small, the magnitude of secondary currents is negligible. However, while the suspended load is carried in the current direction, the direction of the transport of bed load deviates from the current direction due to gravity effect on the bed load moving on the slopes of the dredged trench.