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This study presents results from field data observed from Hangzhou Bay Subsea Pipeline (HBSP) to investigate the burial characteristics of subsea pipelines with spoiler on silty seabed. The study is based on large-scale seabed topography data and continuous ten years of pipeline route survey data conducted between 2005 and 2014. Analysis of surveyed data shows that along the pipeline route, seabed evolution around HBSP has three zones: continuous scour, scour followed by deposition, and continuous deposition zone. For continuous scour, pipeline remains exposed, and pipeline with spoiler induced full self-burial doesn't take effect. While for continuous deposition and scour followed by deposition, the combined effects of self-burial and seabed evolution increase average total burial depth from 1.5m in 2006 to 3.5m in 2014. Spoiler attached to pipeline is the main factor to increase total burial depth before 2006, and by 2014, the contribution of seabed deposition to burial depths far exceeds that of self-burial. The self-burial depth is over 2.0 D (D is pipeline diameter) under bidirectional tidal flow while it is no more than 1.0 D under unidirectional flow. These findings provide technical support for the implementation of spoiler in offshore pipelines under similar hydrodynamic and sediment conditions. • Burial state of the Hangzhou Bay Subsea Pipeline with a spoiler is investigated based on field surveys of ten years. • Both natural seabed changes and self-burial caused by the pipeline with spoiler affect the pipeline's burial state. • Self-burial depth is much greater than the local scour depth around a fixed pipeline in field engineering applications. [ABSTRACT FROM AUTHOR]

Abstract High resolution bathymetry combined with structural modelling is used to estimate changes in the on-bottom stability of an offshore pipeline due to scour and sedimentation over an 11 year period. Detailed observations of post-lay embedment changes have been combined with the pipeline structural characteristics and an elastic-plastic model of soil resistance to estimate the vertical and horizontal stability of the pipeline using a finite difference solution to the beam bending equation. Application of the design approach indicates that post-lay increases to the critical (break-out) velocity of 1–2 m/s occur along the full 19 km of surveyed pipeline due to scour and sedimentation, which act to reduce load and increase soil resistance. The rate at which this increase in stability occurs with time is found to vary along the pipeline, and is dependent on the mechanism of pipeline lowering (i.e. whether the pipe lowered due to sagging into widely spaced scour holes, or by sinking into the shoulders between many closely spaced scour holes). By incorporating sediment transport into the pipeline design, the present results suggest potential for significant improvements in pipeline on-bottom stability and associated reductions in minimum required specific gravity and/or secondary stabilisation. Highlights • Seabed mobility driven changes to pipeline embedment are observed in the field. • The resulting changes to pipeline on-bottom stability are quantified. • Increases in critical velocity of 1–2 m/s are calculated for a steady current. • The amount of increase is dependent on the mechanism of pipeline lowering. • Increased soil resistance and hydrodynamic shielding are both shown to be important. [ABSTRACT FROM AUTHOR]

The hazardous exposure and self-burial of a submarine pipeline were studied based on the investigation of a submarine pipeline in the sand wave area in the South China Sea, as well as on statistical data and mathematical calculations. The results indicate that the pipeline free span in the area was mainly induced by seabed erosion, sand wave movement, and human disturbance. Extreme weather events cause severe impacts on seabed topography and the pipeline condition. The burial status of the pipe is mainly controlled by the evolution of local seabed topographical conditions, i.e., the amount of sediment load and the movement of seabed sand waves. The migration of the continuous small-scale sand waves facilitates the self-burial of pipes, while the migration of isolated large-scale sand waves creates potential hazards for pipelines because of the existing free span continues to grow on both sides of the sand wave. [ABSTRACT FROM AUTHOR]