Turbidity Current Hazard Assessment For Field Layout Planning

Meyer, V. (ed.), Frontiers in Offshore Geotechnics III: proceedings of the Third International Symposium on Frontiers in Offshore Geotechnics (ISFOG 2015), Oslo, Norway, 10-12 June 2015, CRC Press, Boca Raton, pp. 945-950.

10 Jun 2015
Clare, M.A., Thomas, S., Cartigny, M.J.B. and Mansour, M.
DOI: 10.1201/b18442-137

The west Ghanaian continental slope is incised at the shelf break by multiple gullies that coalesce downslope to formbroader canyons. Evidence for past turbidity current activity includes the presence of upslope migrating, kilometer-scale bedforms within such canyons, and smaller bedforms on the interfluves. The larger bedforms are inferred to relate to high density turbidity current activity focused within a canyon thalweg, with more dilute plumes of sediment overspilling onto the interfluves. These are interpreted to have been formed by supercritical turbidity currents which can pose a credible threat to seafloor structures – particularly pipelines. Piston cores acquired within canyons show evidence of past high density (sand-rich) and low density (mudrich) turbidity currents. The results of geochronological analyses show that turbidity current magnitude appears to have been controlled by climatic changes over the past 25 thousand years. High density turbidity current activity “switched off” approximately 17,000 years ago, coincident with sea level rise and the restriction of sediment delivery to the shelf. Hence, the canyons can be accepted as a viable route for pipelines or placement of structures. Bedforms on the interfluves are interpreted as static features during the present day and only present a topographic consideration. The geotechnical implications of laterally and vertically variable turbidity current-controlled deposition are illustrated, including sand-rich deposits within canyon bases, heterogeneous facies on the flanks, and clay-rich sediment on the interfluves. We document how an integrated approach has reduced the perceived risk of high density turbidity currents to a proposed field development and has also been used to understand the field-wide variability in soil conditions.

 

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