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Quantification of near-bed dense layers and implications for seafloor structures: new insights into the most hazardous aspects of turbidity currents

Quantification of near-bed dense layers and implications for seafloor structures: new insights into the most hazardous aspects of turbidity currents
Quantification of near-bed dense layers and implications for seafloor structures: new insights into the most hazardous aspects of turbidity currents
Turbidity currents pose a serious hazard to expensive oil and gas seafloor installations, especially in deep-water where mitigation, re-routing or repair is costly and logistically challenging. These sediment-laden flows are hazardous because they can be exceptionally powerful (up to 20 m/s), and can flow for long distances (>100s km) over several days duration, causing damage over vast areas of seafloor. Even less powerful flows (~1-2 m/s) can damage seafloor equipment, or break strategically important submarine telecommunication cables. The consequences of turbidity currents impacting seafloor structures depends on the velocity, duration, direction of impact and, perhaps most crucially, the sediment concentration (or density) of the flow. While some recent studies have successfully monitored turbidity currents in deep-water, imaging flow properties close to the seafloor has proven problematic. We present innovative approaches to the quantification of the velocity and sediment concentration of dense near-bed layers that provide new insights into this important aspect of turbidity current flow.

Firstly, we describe a novel experimental setup that is capable of measuring near-bed sediment concentration in dense (>10% volume by concentration) flows. Density contrasts are measured using Electrical Resistivity Tomography – a technique initially developed for geophysical characterisation of subsurface reservoirs. Velocity is measured using Ultrasonic Doppler Velocity Profiling and concentration is characterized using an Ultra High Concentration Meter. Secondly, we outline some recently developed geophysical approaches for the quantification of sediment concentration and velocity for real-world flows based on recent work in fjords, estuaries and deep-sea canyons. This includes integrated moored deployments of Acoustic Doppler Current Profilers, Multibeam Sonars, and a novel Chirp array. We outline some limitations and advantages of these methods. Finally, we underline the value and importance of establishing multiple field-scale test sites in a variety of settings, including deep-water, that will enhance the industry's understanding of turbidity current hazards.

Our results demonstrate the importance of near-bed dense layers for turbidity current interaction with seafloor structures. Density contrasts and pressure build up at the base of a flow may lead to uplift, undermining and loss of support, dragging, or pipeline rupture; hence quantification of this layer is crucial for hazard assessment.

Measurements of sediment concentration within turbidity currents are incredibly rare, and yet are a vital input for any numerical model that aims to predict sediment transport by turbidity currents in deep-water settings. Currently it is necessary to infer densities and velocities; however, such inferences are poorly calibrated against experimental or real world data. Our measurements underline the importance of understanding near-bed dense layers.
978-1-61399-379-8
OTC-25705-MS
OnePetro/Society of Petroleum Engineers
Clare, M.A.
2b5fe9d9-a4fa-4c54-b7ed-337d15b1c4cf
Cartigny, M.J.B.
d252d7b1-16c6-47b1-bf86-8087070934ce
North, L.J.
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Talling, P.J.
1cbac5ec-a9f8-4868-94fe-6203f30b47cf
Vardy, M.E.
8dd019dc-e57d-4b49-8f23-0fa6d246e69d
Hizzett, J.L.
e653991a-457e-4bd7-b261-f0539d736f34
Sumner, E.J.
dbba4b92-89cc-45d9-888e-d0e87e5c10ac
Hughes Clarke, J.E.
81b97b31-58a8-474e-b693-c4f20a54d992
Spinewine, B.
f699f45a-8429-46a5-9437-7c3ee94fefe7
Clare, M.A.
2b5fe9d9-a4fa-4c54-b7ed-337d15b1c4cf
Cartigny, M.J.B.
d252d7b1-16c6-47b1-bf86-8087070934ce
North, L.J.
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Talling, P.J.
1cbac5ec-a9f8-4868-94fe-6203f30b47cf
Vardy, M.E.
8dd019dc-e57d-4b49-8f23-0fa6d246e69d
Hizzett, J.L.
e653991a-457e-4bd7-b261-f0539d736f34
Sumner, E.J.
dbba4b92-89cc-45d9-888e-d0e87e5c10ac
Hughes Clarke, J.E.
81b97b31-58a8-474e-b693-c4f20a54d992
Spinewine, B.
f699f45a-8429-46a5-9437-7c3ee94fefe7

Clare, M.A., Cartigny, M.J.B., North, L.J., Talling, P.J., Vardy, M.E., Hizzett, J.L., Sumner, E.J., Hughes Clarke, J.E. and Spinewine, B. (2015) Quantification of near-bed dense layers and implications for seafloor structures: new insights into the most hazardous aspects of turbidity currents. In Offshore Technology Conference 2015. OnePetro/Society of Petroleum Engineers. OTC-25705-MS . (doi:10.4043/25705-MS).

Record type: Conference or Workshop Item (Paper)

Abstract

Turbidity currents pose a serious hazard to expensive oil and gas seafloor installations, especially in deep-water where mitigation, re-routing or repair is costly and logistically challenging. These sediment-laden flows are hazardous because they can be exceptionally powerful (up to 20 m/s), and can flow for long distances (>100s km) over several days duration, causing damage over vast areas of seafloor. Even less powerful flows (~1-2 m/s) can damage seafloor equipment, or break strategically important submarine telecommunication cables. The consequences of turbidity currents impacting seafloor structures depends on the velocity, duration, direction of impact and, perhaps most crucially, the sediment concentration (or density) of the flow. While some recent studies have successfully monitored turbidity currents in deep-water, imaging flow properties close to the seafloor has proven problematic. We present innovative approaches to the quantification of the velocity and sediment concentration of dense near-bed layers that provide new insights into this important aspect of turbidity current flow.

Firstly, we describe a novel experimental setup that is capable of measuring near-bed sediment concentration in dense (>10% volume by concentration) flows. Density contrasts are measured using Electrical Resistivity Tomography – a technique initially developed for geophysical characterisation of subsurface reservoirs. Velocity is measured using Ultrasonic Doppler Velocity Profiling and concentration is characterized using an Ultra High Concentration Meter. Secondly, we outline some recently developed geophysical approaches for the quantification of sediment concentration and velocity for real-world flows based on recent work in fjords, estuaries and deep-sea canyons. This includes integrated moored deployments of Acoustic Doppler Current Profilers, Multibeam Sonars, and a novel Chirp array. We outline some limitations and advantages of these methods. Finally, we underline the value and importance of establishing multiple field-scale test sites in a variety of settings, including deep-water, that will enhance the industry's understanding of turbidity current hazards.

Our results demonstrate the importance of near-bed dense layers for turbidity current interaction with seafloor structures. Density contrasts and pressure build up at the base of a flow may lead to uplift, undermining and loss of support, dragging, or pipeline rupture; hence quantification of this layer is crucial for hazard assessment.

Measurements of sediment concentration within turbidity currents are incredibly rare, and yet are a vital input for any numerical model that aims to predict sediment transport by turbidity currents in deep-water settings. Currently it is necessary to infer densities and velocities; however, such inferences are poorly calibrated against experimental or real world data. Our measurements underline the importance of understanding near-bed dense layers.

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More information

Published date: 2015
Venue - Dates: OTC 2015, Houston, United States, 2015-05-04 - 2015-05-07
Organisations: Geology & Geophysics, Marine Geoscience

Identifiers

Local EPrints ID: 387260
URI: http://eprints.soton.ac.uk/id/eprint/387260
ISBN: 978-1-61399-379-8
PURE UUID: 871a3688-2811-4a9f-8c51-9c31ca90b98c

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Date deposited: 11 Feb 2016 16:45
Last modified: 14 Mar 2024 22:46

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Contributors

Author: M.A. Clare
Author: M.J.B. Cartigny
Author: L.J. North
Author: P.J. Talling
Author: M.E. Vardy
Author: J.L. Hizzett
Author: E.J. Sumner
Author: J.E. Hughes Clarke
Author: B. Spinewine

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