Abstract: We present a numerical model to predict the trajectory, run-out distance and frontal velocity of pulsed submarine mass gravity flows, and illustrate how it forms an essential building block of geohazards assessments for deep offshore pipeline and cable developments. The model is inspired from Center-of-Mass approaches initially proposed for pulsed snow avalanches, but specifically adapted to the submarine sediment density flows. Because it writes as a system of ordinary differential equations, it is more amenable to repeated simulations than depth-integrated Finite Element models which may rapidly become CPU-prohibitive for field applications.
The approach is therefore advantageously used at the stage of a geohazards risk assessment. The sensitivity of model outputs to differing trigger locations, failed volumes, and soil parameters, can be readily investigated, and model outputs combined into maps of maximum velocities and runout distances. These parameters allow estimating the magnitude and duration of forces impacting on submarine pipelines and cables, which can then be used for a verification of structural integrity based on a pipe/soil interaction model.
The adequacy of the approach is first tested on small-scale laboratory experiments of pulsed debris flows down a slope. Then, its application at field scale is illustrated for a hypothetical case of a localized slope failure on the steep flank of a submarine canyon.
At a period when the Oil & Gas offshore industry is pushing to deeper waters where the proximity with the continental slope makes these regions prone to mass gravity flows, the quantification of the associated risks becomes increasingly important for the decision-making process when such flows may jeopardize the integrity of seafloor structures. Combined with a site-wide assessment of trigger mechanisms and potential slope instabilities, this approach may help providing a more consistent and robust quantification of the potential impact loads imposed on existing or planned infrastructure.