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Deflected sand wave migration due to severe wind events along the BBL-pipeline : a 3D-numerical modelling study using eight years of bathymetrical data

Krewinkel, B.C. (2017) Deflected sand wave migration due to severe wind events along the BBL-pipeline : a 3D-numerical modelling study using eight years of bathymetrical data.

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Abstract:Migrating sand waves may pose risks to subsea pipelines. As a result of migrating sand waves free spans can occur. Free spans may initiate vibrations resulting in pipe fatigue damage. Therefore, it is vital to understand the behaviour of these sand waves for pipe maintenance. This study investigates an irregular and dynamic sand wave field which covers part of the BBL-pipeline, a pipeline from Balgzand (NL) to Bacton (UK). Regular surveys along the BBL show sand waves migrating inconsistent spatially and temporally. This indicates that sand wave migration may be caused by both the regular tide and (severe) wind events, making the migration difficult to predict. The research objective of this study is therefore to improve the understanding in sand wave migration and related wind influences for future pipeline maintenance. The first part focuses on the survey data. Crests and troughs are selected after a low pass Fourier filter is applied. This shows spatial and temporal inconsistencies in the sand wave migration, resulting in four spatial migration patterns. The wind data subsequently shows anomaly magnitudes and angles during 2013 and 2015, which link to the temporal inconsistency of the sand wave migration. This raises the hypothesis that wind contributes to sand wave migration. In the second part a 3D-hydrodynamic Delft3D-FLOW model is setup to investigate the tide residual currents and southern wind influences in the sand wave area. Results show a tide-induced residual circulation induced by sand banks. The circulation caused by the Winterton Ridge sand bank likely causes the four observed spatial migration trends. The idealized severe wind scenarios, including wind waves, show a factor of order 10 for wind driven currents compared to the tide residual currents near the pipe. For sediment transport this factor increases to an order 100 comparing a severe wind scenario to a no-wind case, initiating transport to the north-west. For south and south-eastern wind this increase is higher than for wind from the south-west. Coupling these ratios of sediment increase per wind direction and magnitude to the wind data, the years 2013 and 2015 show a wind induced sediment transport deviation to the north-west. This resembles the field observations indicating a north-west sand wave (crest) migration increase during the years 2013 and 2015. Lastly, a case where the pipe is in free span is investigated. This shows that for local analysis of sand wave migration the model uncertainty in the position of the residual circulation is too large, and these cases require an additional bathymetrical analysis for sand wave migration prediction. It is concluded that the model is able to explain the general spatial sand wave migration trends along the BBL. The temporal trend during 2013 and 2015 is better understood, since sediment transport is enhanced during the severe (southern) wind in these years. This results in similar deviations compared to the migration patterns retrieved from the eight years of field data. The main challenge for the future is to define when asymmetry (crest migration), and when migration is the result of the wind events.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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