On-Bottom Stability Without Significant Hydrodynamic Forcing
The seabed is the pipeline's resting place, literally speaking. The pipeline can be laying on the surface of the seabed or buried in the seabed, or it may be lying on the seabed surface covered with non-local material (rocks, grout bags, etc.). But the seabed is not a hard surface so the pipeline will not necessarily stay where it is initially positioned even when there is no hydrodynamic forcing present. The pipeline may sink into the soft if the downward force exceeds the beating capacity of the bed and may move laterally by sliding if the pipe is installed on a non-horizontal seabed. The movement of the pipeline by seabed failure (slips and slides) is not considered here.
Another hazard for pipelines placed in trenches and covered with backfill material is that they may "float" up through the backfill. The processes responsible for flotation of pipelines have been studied for many years but recently new advances are being made in understanding some of the mechanisms leading to flotation, namely static liquefaction and upheaval buckling. Both these processes are potentially more important now that the pipes are becoming lighter and the hotter products can lead to greater thermal gradients and mechanical stresses.
Subsea developments require the installation of (flexible) pipelines to connect the wellhead to the distribution system and this installation is undertaken by surface vessels ,with diver support or by diverless ROV (Remote Operated Vehicle) technology. In these situations, pipelines are often pulled along the seabed or dragged sideways. Ensuring reliable connection operations requires an adequate knowledge of the seabed soils and configuration of the system being installed. This will help provide accurate predictions of the pulling forces required. Recent advances in the scaling of laboratory scale experiments within the framework of soil mechanics has produced a better understanding of the processes acting and generated better predictions of the loads required to move pipelines over the seabed.