Deep water challenges: development of depositional models to support geohazard assessment for submarine facilities

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.