Academic literature on the topic 'Turbidity currents, Geohazard, Numerical simulations'

Turbidity currents may be a relevant lever to manage the accumulation of fine sediments in reservoirs. In this paper, we propose to show how two different numerical codes simulate the propagation of turbidity currents. Telemac 3D and Ansys CFX 17.1 solver were chosen as they are commonly used by many research and engineering teams. The simulations are performed on two configurations. The first case aims at modeling the plunging of a turbidity current. The second model is validated based on an experimental work performed at EPFL. The latter consisted on testing turbidity current venting as a solution to manage reservoir sedimentation. A long and narrow flume was used to simulate the reservoir where a turbidity current was triggered. The advantages and limits of both approaches are discussed in order to supply guidelines for the modeling of turbidity currents in real reservoirs.

Breaching flow slides result in a turbidity current running over and directly interacting with the eroding, submarine slope surface, thereby promoting further sediment erosion. The investigation and understanding of this current are crucial, as it is the main parameter influencing the failure evolution and fate of sediment during the breaching phenomenon. In contrast to previous numerical studies dealing with this specific type of turbidity currents, we present a 3D numerical model that simulates the flow structure and hydrodynamics of breaching-generated turbidity currents. The turbulent behavior in the model is captured by large eddy simulation (LES). We present a set of numerical simulations that reproduce particular, previously published experimental results. Through these simulations, we show the validity, applicability, and advantage of the proposed numerical model for the investigation of the flow characteristics. The principal characteristics of the turbidity current are reproduced well, apart from the layer thickness. We also propose a breaching erosion model and validate it using the same series of experimental data. Quite good agreement is observed between the experimental data and the computed erosion rates. The numerical results confirm that breaching-generated turbidity currents are self-accelerating and indicate that they evolve in a self-similar manner.

Water stratification commonly exists in nature, such as thermocline in lakes and oceans and halocline in estuaries and oceans (He et al. 2017). Turbidity currents in estuary often encounter stratified sea water, which may significantly influence their propagation and deposition. This study presents high-resolution numerical simulations of lock-exchange gravity and turbidity currents in linear stratifications on a flat bed. Laboratory experiments are conducted to validate the numerical model and good agreements between numerical results and measurements are found. The evolution process, front velocity, internal wave, and entrainment ratio are analyzed based on the numerical results. For a gravity current in a strong stratification, its front velocity can be maintained as a near constant state for a long time after an initial acceleration period because of interactions between the current and internal waves. However, sedimentation of suspended particles due to the damping effect of ambient stratification on turbulence makes a turbidity current quickly lose its structure so the maintaining effect of the internal waves on its front velocity is quite weak. During the evolution process of a turbidity current, the ambient stratification is found to damp the turbulent structures, and front velocity. Stratification can also decrease the entrainment ratios between a gravity current and ambient water after the initial period, but it has an insignificant influence on the entrainment ratios of a turbidity current. This study provides a better understanding of gravity and turbidity currents in estuary stratifications.

Abstract. High resolution direct numerical simulations (DNS) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier–Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two, and three-dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring mesh performance in capturing the range of dynamics. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. Use of discontinuous discretisations and adaptive unstructured meshing technologies, which reduce the required element count by approximately two orders of magnitude, results in high resolution DNS models of turbidity currents at a fraction of the cost of traditional FE models. The benefits of this technique will enable simulation of turbidity currents in complex and large domains where DNS modelling was previously unachievable.

Abstract. High-resolution direct numerical simulations (DNSs) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier–Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two and three dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring model performance in capturing the range of dynamics on a range of meshes. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. The use of adaptive mesh optimisation is shown to reduce the required element count by approximately two orders of magnitude in comparison with fixed, uniform mesh simulations. This leads to a substantial reduction in computational cost. The computational savings and flexibility afforded by adaptivity along with the flexibility of FE methods make this model well suited to simulating turbidity currents in complex domains.

AbstractTurbidity currents occur widely in submarine environments, but field-scale numerical simulations of the flow features have not been applied extensively. Here, we present a two-dimensional layer-averaged numerical model to simulate turbidity currents over an erodible sediment bed, and taking into consideration deposition, entrainment and friction. The numerical model was developed based on the open-source code, Basilisk, ensuring well-balanced and positivity-preserving properties. An adaptive spatial discretization was used, which allows multi-level refinement. The adaptive criterion is based on the dynamic features of the flow and sediment concentrations. The numerical scheme has a relatively high computational efficiency compared with models based on the Cartesian mesh. A hypothetical case based on a true large-scale landform (the Moroccan Turbidite System, offshore NW Africa) was studied. Compared with previous models, the current model accounted for the coupling between flow, sediment transportation and bed evolution. This approach may improve simulation results and also allow the simulation of complex field-scale landforms, while preserving the flow details.

Abstract. Although in situ measurements in modern frequently occurring turbidity currents have been performed, the flow characteristics of turbidity currents that occur only once every 100 years and deposit turbidites over a large area have not yet been elucidated. In this study, we propose a method for estimating the paleo-hydraulic conditions of turbidity currents from ancient turbidites by using machine learning. In this method, we hypothesize that turbidity currents result from suspended sediment clouds that flow down a steep slope in a submarine canyon and into a gently sloping basin plain. Using inverse modeling, we reconstruct seven model input parameters including the initial flow depth, the sediment concentration, and the basin slope. A reasonable number (3500) of repetitions of numerical simulations using a one-dimensional layer-averaged model under various input parameters generates a dataset of the characteristic features of turbidites. This artificial dataset is then used for supervised training of a deep-learning neural network (NN) to produce an inverse model capable of estimating paleo-hydraulic conditions from data on the ancient turbidites. The performance of the inverse model is tested using independently generated datasets. Consequently, the NN successfully reconstructs the flow conditions of the test datasets. In addition, the proposed inverse model is quite robust to random errors in the input data. Judging from the results of subsampling tests, inversion of turbidity currents can be conducted if an individual turbidite can be correlated over 10 km at approximately 1 km intervals. These results suggest that the proposed method can sufficiently analyze field-scale turbidity currents.

Turbidity currents deliver sediment rapidly from the continental shelf to the slope and beyond; and can be triggered by processes such as shelf resuspension during oceanic storms; mass failure of slope deposits due to sediment- and wave-pressure loadings; and localized events that grow into sustained currents via self-amplifying ignition. Because these operate over multiple spatial and temporal scales, ranging from the eddy-scale to continental-scale; coupled numerical models that represent the full transport pathway have proved elusive though individual models have been developed to describe each of these processes. Toward a more holistic tool, a numerical workflow was developed to address pathways for sediment routing from terrestrial and coastal sources, across the continental shelf and ultimately down continental slope canyons of the northern Gulf of Mexico, where offshore infrastructure is susceptible to damage by turbidity currents. Workflow components included: (1) a calibrated simulator for fluvial discharge (Water Balance Model - Sediment; WBMsed); (2) domain grids for seabed sediment textures (dbSEABED); bathymetry, and channelization; (3) a simulator for ocean dynamics and resuspension (the Regional Ocean Modeling System; ROMS); (4) A simulator (HurriSlip) of seafloor failure and flow ignition; and (5) A Reynolds-averaged Navier–Stokes (RANS) turbidity current model (TURBINS). Model simulations explored physical oceanic conditions that might generate turbidity currents, and allowed the workflow to be tested for a year that included two hurricanes. Results showed that extreme storms were especially effective at delivering sediment from coastal source areas to the deep sea, at timescales that ranged from individual wave events (~hours), to the settling lag of fine sediment (~days).

ABSTRACT Intraslope basins, or minibasins, are topographic features of the continental slope that can be filled with sediment transported by submarine flows. These deposits may contain important hydrocarbon reservoirs. Here we present results of two-dimensional numerical simulations of multiple turbidity currents entering two linked minibasins. The numerical model accounts for the non-uniformity of sediment grain size in the flow and the resulting deposit. Model results reasonably reproduce the evolution of linked minibasins illustrated in the field based “fill-and-spill” conceptual model. The conceptual model was developed for the Brazos–Trinity system from field observations. Further, simulations of two linked minibasins show that the upstream basin traps most of the coarse sediment. This material is deposited in the proximal zone of the basin and fine sediment is transported farther downslope, resulting in the formation of a weak pattern of downstream fining. Model results with different initial and boundary conditions reveal that minibasin geometry and turbidity-current characteristics are important controls on the deposit shape and grain-size distribution.

Turbidity currents occur in many submarine settings, from shallow to deep water, and may transport large volumes of sediment over low angle (<0.01°) slopes, reaching speeds of ~20 m/s. These flows pose a serious risk to offshore seafloor oil and gas infrastructure. A great number of uncertainties exists in terms of their triggers, frequency and behaviour: most of the present understanding comes from outcrops studies, cores and flume tank experiments, but there are significant limitations related to scaling issues. Large and fast turbidity currents may break pipelines with catastrophic hydrocarbons losses into the marine environment, but also relatively dilute and low impact turbidity currents may generate scour around seafloor structures, causing structural or operational issues which can be technically challenging to remedy in ultra-deep water settings. A better understanding of potential impacts and consequences of turbidity currents is required to improve risk assessment and mitigation strategies. The ability to model properly the gravity flows, in order to evaluate the potential impacts against submarine facilities, represents a strong improvement in risk reduction within the exploration and production activities, as well as in facility engineering. Eni S.p.A. (Upstream & Technical Services) owns a developed in-house forward modelling software, through a customization of the partly open-source solver of the commercial software FLOW-3D®. The software is able to simulate hydrodynamics, geometry and internal characteristics of sediment gravity flows and related deposits (turbidites). The direct monitoring of real-world flows can provide new information about the hydrodynamics of turbidite flows but is restricted to few measurement points, while flume tank experiments are limited to reproduce small-scale very fine-grained sediment flows. Outcrop studies, on the other hand, are difficult, expensive and time-consuming. Given the limited state of knowledge, a step change in the understanding of turbidite flows behaviour can be obtained through the calibration of numerical models with data acquired from outcrop, direct monitoring and flume tank experiments. The thesis was focused on the simulation of sediment gravity flows and on the estimation of their possible impacts on submarine infrastructures, contributing to the development of a new module for geohazard assessment within the available proprietary software. The geohazard module will contribute to the construction of geological risk maps with the evaluation of the impact of potential gravity-driven flows on subsea structures (i.e. pipelines). The research work represents a collaboration among groups in Eni E&P Headquarters and Eni UK/OPU (i.e. Sedimentology, Engineering, R&D Units) and XC Engineering Srl., the service company which is developing the proprietary software. Consequently, the material diffused with this thesis elaboration took into account all the necessary confidentiality issues. The main aim of thesis was to investigate the dynamics of sediment gravity flows, both from sedimentological and engineering viewpoints; some important aspects related to the development of the geohazard module have been explored. A sensitivity analysis was performed: different scenarios were investigated to evaluate the potential magnitude of turbidity flows, varying and combining flow bulk volumes, flow concentrations (% of grain size populations in the flow) and sediment/water ratio. The results of simulations have illustrated both very catastrophic (rare but reliable) events and lower magnitude (more frequent) events, whose impact magnitude velocities are within ranges commonly used for engineering calculations (1 to 10 m/s, Bruschi et al., 2006). The thesis begins with an overview of the concept of turbidity currents and turbidites (Chapter 2), followed in Chapter 3 by a literature review about numerical modelling of turbidity currents, from first numerical approaches to more complex numerical models and computational fluid dynamics. A summary of the industry geohazard risk assessment framework for offshore oil and gas production facilities is reported in Chapter 4, describing the different stages of geohazard risk assessment process: system definition, geohazard identification, geohazard estimation, geohazard risk evaluation and geohazard risk management. The main impacts of turbidity currents on pipelines are outlined in Chapter 5, focusing on loads acting on a subsea pipeline and on vortex induced vibration (VIV) phenomenon. The development of the geohazard module, without specifications regarding software algorithms is discussed in Chapter 6. The main results of the numerical simulations with the application to a real case study are presented and discussed in Chapters 7 and 8, without specifications regarding strategic data for the company, followed by concluding remarks in Chapter 9. The thesis closes with a literature Reference list.