Slope instabilities and consequent mass gravity flows are major hazards for offshore pipelines, flowlines, cables and umbilicals crossing submarine slopes. Although several methods are available to evaluate the stability of submarine slopes, uncertainty remains over the evolution of the failed soil mass from slide initiation to complete run-out, which may lead to impact and damage of the assets even at significant distances from the initial failure. This paper is intended to apply in-situ knowledge required for the evaluation of the risk of damage and displacements of offshore pipelines caused by the impact of forecasted submarine mass transport events, typically defined as debris flows.
A step-by-step methodology is proposed to define design event scenarios tied to site specific geological and geotechnical characterization, calibrate numerical models aimed to ultimately assess mass flow trajectories and impact forces on a given pipeline segment, and to drive pipeline structural response analysis. A fluid dynamics debris flow model, accounting for the seabed 3D bathymetry, has been developed to predict the propagation of debris flows from inception to runout. The model is calibrated against recent well-identified local debris flow deposits, mapped and characterized in terms of preconditioning factors, triggering mechanisms, resulting morphologies, flow dynamics and ages through the use of a combination of geophysical, geotechnical and geological tools. Specific laboratory tests are performed at relevant location and depths, to evaluate the expected rheological behavior and ranges of yield strength and viscosity of the failed soil mass.
Design scenarios are then selected based on local morphology and tied to dynamic slope stability evaluations which define the likelihood of the event and initial failed volume. Model input parameters are critically evaluated based on relevant calibration cases. For each simulated scenario, results include the trajectory and run out distance of the mass flow events, along with the distribution of flow velocity and thickness in time and at each location along the flow path. Finally, the main parameters governing the effect of debris flow events on pipelines exposed on seabed are evaluated for incorporation in spatio-temporal pipeline structural response analysis.
As with any engineering analysis, results are only as reliable as input. In the case of debris flow impact on pipelines, it is critical that analyses are carefully constrained with field and laboratory data. At each step in the process engineers and geologists must assess the site conditions, and insure that results are meaningful. This applies to all steps in the process, from the initial site characterization and identification of credible geohazard scenarios, selection of modeling methodology, evaluation of rheological parameters (preferably measured in the laboratory), model calibration and matching to field observations, through the final numerical modeling of the pipeline response to impact. The methodology outlined below identifies the main steps in the analysis, and highlights the need to verify engineering results with field observations.