Prevention of pipeline floatation during dredge-based backfilling

Underwater pipelines have become a vital part of modern civilisation. Transport by pipeline is relatively inflexible compared to other means of transport, however it consumes less energy. Most offshore pipelines carry oil or gas, but they can also transport water or other fluids. More and more oil and gas is produced from offshore fields. The product has to be carried onshore and that is usually done by pipeline. Pipelines can be laid uncovered on the seabed, or can be embedded in the seafloor when stabilisation or protection is required. Pipelines are primarily covered when they are located in shallow waters. The material used to refill an excavated trench is called backfill. The typical material used as artificial backfill is coarse granular soil (rock, gravel, or coarse sand). When there is no suitable backfill material available close to the pipeline route, then material has to be transported from another location. A subsea pipeline should remain stable during its entire lifetime. Normal engineering practice focuses on the operational phase, while the installation phase is almost neglected. However, this study focuses on the installation phase and particularly on dredge-based backfilling. Dredge-based backfilling means using the suction pipe of a Trailing Suction Hopper Dredger (TSHD) as discharge pipe. It is not always clear if a particular soil can be used as backfill material. In the past this lack of knowledge resulted a few times in pipeline floatation during the installation phase. This lack of knowledge can also result in the disposal of perfectly suitable backfill material. A pipeline can be lifted up from the bottom of the trench if the weight of the pipeline is lower than the weight of the liquefied backfill. Pipeline floatation occurs if a soil-water mixture remains liquefied over a too large distance. In general, coarse granular soils remain liquefied for only a short period of time. If the backfill is composed of fine material, it will take a relati
Geo-engineering
Geoscience & Engineering
Civil Engineering and Geosciences