Sammendrag
The objective of the paper is to study the effect of different parameters regarding on-bottom stability of subsea pipelines under combined irregular waves and currents. The effect of friction coefficient is first investigated. The development of lateral displacement and penetration for three different friction coefficients are compared for sandy and clayey seabed respectively when applied wave and current conditions are kept same for all the cases. The friction coefficient affects the soil resistance force and further changes the initial time when the pipeline starts to move in the lateral direction. The accumulated displacement reduces for large friction coefficient and it results in less penetration. The total effect of the increasing friction coefficient depends on the competition between the increased friction force and the reduced passive soil resistance force. The pipeline usually crosses different types of soil along the route. Hence, different combinations of soil types along the route are applied in the analysis. The soil property at middle of the pipeline is found to be important when the boundary conditions at both ends are fixed. Three analyzing procedures, namely standard 3-hour procedure, the procedure recommended by PONDUS and the procedure recommended by DNV, could be used for on-bottom stability analysis under storm conditions. The comparison of these procedures shows that the procedure recommended by PONDUS is the most appropriate for the storm conditions. The procedure recommended by DNV considers the build-up of initial penetration before the storm; and it could be applied in the analysis when the penetration is stabilized after the start -up time (20% of 3-hour).