Paskin, Sandra; (1993) The self-burial of seabed pipelines. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Recent experience in the Dutch North Sea sector has demonstrated that given conducive environmental conditions, oil and gas pipelines laid directly on the surface of a loose sandy seabed will bury themselves. The incentives to exploit this natural phenomenon where possible are considerable, arising from the large cost savings and potential reductions in installation damage risks by avoiding expensive mechanical trenching. This has sensibly attracted great interest from offshore operators principally in the Dutch sector where a flexible approach by the authorities has led to the installation of eighteen pipelines subjected to the natural self-burial since 1977. Pipeline self-burial is an extremely complex phenomenon dependent on many variables associated with local environmental, pipeline and seabed soil conditions. At the present time this rules out effective theoretical modelling. Predictions have therefore to be based on empirical correlations of measured data preferably from actual installed pipelines. This thesis describes an experimental programme investigating pipeline self-burial utilizing a unique reversing flow channel in conjunction with a non-intrusive scour depth measurement technique using a flat, fine sand bed. Two-dimensional scour holes were studied under both uni-directional and reversing flow conditions for varying sediment transport rates and pipeline diameter. Flow visualisation techniques were employed to gain further insight into the flow regimes. The hypothesis that lee scour is intrinsic to deep pipeline burial was further investigated as was the suitability of a previously proposed time scale parameter involving the sediment transport rate and pipeline diameter squared. A further experimental programme was carried out to investigate the three-dimensional aspects of pipeline scour. The growth of an artificially initiated pipeline free-span was studied which represents the first of its kind due to the great experimental difficulties. A computer model was also developed, which, based on experimental data, simulates the growth of pipeline free-spans. A discussion is included of the interactive processes of sagging and scouring and their relevance to deep self-burial. A number of important conclusions emanate from the findings of the research. Firstly, and perhaps most crucially, the presence of either reversing tidal flow or large amplitude wave induced oscillatory flow across the pipeline appears to represent an essential condition for self-burial to occur. A new scouring mechanism, peculiar to reversing flow, has been identified which provides more rapid, deep scour hole development than in the corresponding uni-directional flow case. The scour time scale parameter was found to be relatively successful in correlating the scour hole data for the different pipeline diameters and sediment transport rates used in the tests. Other correlating variables were also identified and systematically investigated. The three-dimensional studies revealed a constant span length development rate. The scour hole end angles were found to be very steep, in excess of the angle of repose of the sand due to local frictional and flow effects. A vortex was seen to pass through the span ends, identified by a ridge formation with a much increased local sediment carrying capacity. Results from a simple numerical model developed to predict the rate of increase of the spanwise length of the scour hole were in good agreement with the corresponding experimental measurements. An analytical version of the numerical model has recently been developed for use in design studies for a major North Sea pipeline project. Calculated sag rates were found to be very large indicating pipeline touchdown, emphasising the relevance of the interactive processes of sagging and scouring.

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