Sediment Density Flow Impact Assessment and Mitigation Studies for Deepwater Developments: A Challenge on Multiple Scales
Authors: Clare Michael , Thomas Stephen , Spinewine Benoit 
Published: May 2013

A unified and verifiable approach is outlined for the assessment of submarine sediment density flows (debris flows, high density and dilute turbidity currents) providing diagnostic criteria of their previous occurrence to inform forward-looking analyses. The assessment outlined includes development of: Evolutionary geological model, summarizing temporal, geological and climatic controls on specific flow types, and estimations of recurrence intervals; Terrain model to understand regional to macro-scale spatial controls on flow dynamics, feasible geometries and predicted aspect of flows to structures; Sedimentary facies model to determine flow behaviour and evolution based on sedimentological analysis and interpretation of high resolution seismic data.

Crucial factors that influence the nature and magnitude of density flows include: the original initiation event (volume, sediment type and triggering mechanism); interrelationship of slope systems, such as downslope coalescence and densification of flows; degree of confinement caused by the seafloor relief; and the influence of local seafloor gradient. These factors may modify the downslope evolution of a flow from that predicted by conventional models. Examples are shown that may challenge preconceptions of flow dynamics, and outline how geological observations should be integrated to provide robust inputs prior to performing numerical modeling.

Case studies are presented from regional to millimetric scale, to summarize key observations that should be used to infer credible flow type, density, direction and magnitude, and are related to recommendations for modeling that may ultimately be translated to a vulnerability assessment. In particular, key metrics for flow characterization that can be derived from sedimentological analysis and interpretation of deposit morphology are discussed, as well as application and limitations of modeling techniques for different flow types.

Submarine sediment density flows occur in many environments worldwide that are being exploited for hydrocarbons and traversed by infrastructure associated with telecommunications and energy transport. Apparently subtle variations, on a variety of scales, may affect the nature of flows and, thus the type and magnitude of impact on subsea structures. Without detailed calibration and understanding of systems as a whole, the inputs that are fed in to geomechanical, hydrodynamic and vulnerability models may be misleading or unrepresentative of real-life conditions.

Introduction

The offshore continental slope may be affected by slope instability on a variety of scales and manifested in different manners which may require geohazard assessment in order for mitigation measures to be developed. These may range from huge slope-wide events, such as the well documented Storrega failure offshore Norway (Haflidason et al., 2004) and widespread blocky failure on the North-West Australian margin (Hengesh et al., 2011), to smaller events including canyon-flank slumping, localized shallow sliding (Thomas et al., 2010), and retrogressive failures within mini-basins located between salt diapirs (Kowsmann et al., 2002). In the same way, submarine sediment density flows may vary in magnitude, nature, direction of flow, velocity and hence the ultimate consequence to a field development, export pipeline or subsea cable placed on the continental slope (or in even deeper abyssal waters).