Change Point Problem for Censored Data

Department of Probability and Mathematical StatisticsKatedra pravděpodobnosti a matematické statistikyMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Unlocking the benefits of long-term pipeline-embedment processes: Image analysis-based processing of historic survey data

An image analysis-based method for retrieving high-resolution bathymetry data from historic pipeline inspection video is detailed. The resulting time-dependant bathymetric datasets are used to inform an improved understanding of pipe-soil and pipe-soil-fluid interaction. This allows better management of existing pipelines and improved design of new pipelines. The pipeline and seabed positions are extracted automatically from the survey video, spurious points removed, the data scaled, and interpolation applied. The data can then be projected in various forms, for different applications. Drawing on field data from Australia's North West Shelf, applications of the method are described. For pipeline design, it is shown how improvements in on-bottom stability and changes in seabed friction can be quantified for mobile seabeds and through buckling sections of pipeline. This approach unlocks advances in existing design practice by providing quantification of the effects of seabed mobility. The processes and mechanisms that lead to through-life changes in pipeline embedment, soil support, and hydrodynamic shielding can be quantified, allowing design to move beyond the usual assumptions of a pipeline embedment that is invariant in time and space.</p

Drained breakout resistance of a pipeline on a mobile seabed

This paper describes a numerical study investigating the effect of sediment transport and associated changes in the local seabed profile on the drained breakout resistance of subsea pipelines. Limit analyses were conducted assessing the breakout response of a pipeline placed on a cohesionless Mohr-Coulomb material considering different seabed profiles around the pipeline. These profiles were determined from surveys of a pipeline on an erodible seabed. The parametric study shows the relative importance of various parameters describing the seabed profile geometry, including the local pipe embedment and the adjacent slope of the seabed. Significant changes in drained resistance occur due to changes in local pipeline embedment resulting from scour induced pipeline lowering and/or sedimentation. The seabed slope local to the pipeline also has a strong impact. The assumption of a flat seabed can lead to predicted seabed resistance that differs significantly from the actual value, accounting for a more natural seabed profile.</p

Observed changes to the stability of a subsea pipeline caused by seabed mobility

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

Sedimentation-induced burial of subsea pipelines: observations from field data and laboratory experiments

Sediment transport-induced changes to the embedment of three 26 km long sections of subsea pipeline are analysed and subsequently explained using model scale experiments. Rather than the scour and scour-induced sinking and sagging traditionally thought to dominate post-lay pipeline spanning and embedment change, the change for these pipelines is shown to be caused by sedimentation. The pipelines traverse a range of metocean and soil conditions; the variation in embedment correlates well with the variation in metocean conditions, with most change occurring in an area where multidirectional high-velocity short-duration flows associated with internal waves propagate at near-perpendicular angles to the pipeline. To understand the mechanism driving these changes, a series of model scale tests in O-tube flumes have been completed under flow conditions mimicking those recorded in the field. Good agreement is found between the field and laboratory results, both in terms of the process timescale and the post-sedimentation profile. The consistency of the embedment changes between the pipelines, their correlation with metocean conditions, and the ability to replicate these changes in model scale tests suggests that such changes can be accounted for in more effective pipeline design. Spans are relatively rare along the pipelines but where they do occur fish rather than scour are shown to be the principal agent of span formation.</p