Sediment Density Flow Impact Assessment and Mitigation Studies for Deepwater Developments: A Challenge on Multiple Scales

Abstract

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.

Entrainment, transport and deposition of sediment by saline gravity currents

“…The importance of studying the dynamic of gravity currents and associated sediment transport mechanisms, is a matter of natural hazard protection. Great topography changes and seafloor irregularities caused by the passage of gravity currents, can be an obstacle for the installation of submarine facilities [13]. Density flows are considered a credible and high risk geohazard, especially in case of direct impact causing the rupture of cables or dislocation of pipelines and flowlines [13].…”

“…Great topography changes and seafloor irregularities caused by the passage of gravity currents, can be an obstacle for the installation of submarine facilities [13]. Density flows are considered a credible and high risk geohazard, especially in case of direct impact causing the rupture of cables or dislocation of pipelines and flowlines [13]. Major concerns are related to the potential impact on offshore installations and infrastructures located in the ocean and sea floors and to the ability of density flows to erode sediment from the bed over which they propagate, leading to scour undermining underwater structures.…”

Numerical Modelling of Runout and Velocity for Slide-Induced Submarine Density Flows: A Building Block of an Integrated Geohazards Assessment for Deepwater Developments

“…Finally, one should stress the importance of using credible input parameters, which should be based on sound geological and engineering judgment (Clare et al, 2013). The initial slide metrics need to be based on an integral study of slope stability rather than simply determined from seismic data.…”

“…We refer to the companion paper by Clare et al (2013) for a more general and in-depth discussion of the types and settings of submarine sediment density flows relevant to geohazards assessment for deepwater developments. In this paper, we focus on the presentation of a numerical model that does not a priori discriminate between debris flows and turbidity currents, but instead relies on general mass and momentum conservation principles supplemented with constitutive relations that may easily be adapted for a case or the other.…”

“…A sensible geohazards risk assessment methodology will therefore need to address these consecutive elements in an integrated manner (see also the companion paper by Clare et al, 2013). A sensible geohazards risk assessment methodology will therefore need to address these consecutive elements in an integrated manner (see also the companion paper by Clare et al, 2013).…”