A systematic approach to offshore engineering for multiple-project developments in geohazardous areas

Evans, Trevor

doi:10.1201/b10132-3

Cited by 17 publications

(15 citation statements)

References 5 publications

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“…Radial faults and polygonal faults are continuous with each other (Gay et al, 2004), reflecting a change in the stress regime near the diapir margins. Block 31 has an irregular morphology, as the sea floor is highly deformed by salt diapirism (Evans, 2010;Hill et al, 2010a;Hill et al, 2010b) and compressional structures, including salt domes and reverse faults . Sediment fluid flow features are also present, such as pock marks (Olu-Le Roy et al, 2007;Savoye et al, 2009;Unterseh, 2013) and cold seeps (Cambon-Bonavita et al, 2009;Olu et al, 2009;Sibuet and Vangriesheim, 2009;Unterseh, 2013;Waren and Bouchet, 2009).…”

Section: Study Sitementioning

confidence: 99%

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Asphalt mounds and associated biota on the Angolan margin

Jones¹,

Walls²,

Clare³

et al. 2014

Deep Sea Research Part I: Oceanographic Research Papers

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Keywords: 6145 0 S 11100 0 E Deep-sea benthic biology Megafauna Hydrocarbon seep Chemosynthetic environment Remotely operated vehicle Seafloor geology Habitat heterogeneity a b s t r a c t Release of hydrocarbons from sediments is important in increasing habitat heterogeneity on deep ocean margins. Heterogeneity arises from variation in abiotic and biotic conditions, including changes in substratum, geochemistry, fluid flow, biological communities and ecological interactions. The seepage of heavy hydrocarbons to the seafloor is less well studied than most other cold seep systems and may lead to the formation of asphalt mounds. These have been described from several regions, particularly the Gulf of Mexico. Here, we describe the structure, potential formation and biology of a large asphalt mound province in Block 31SE Angola. A total of 2254 distinct mound features was identified by side-scan sonar, covering a total area of 3.7 km 2 of seafloor. The asphalt mounds took a number of forms from small (o0.5 m diameter; 13% observations) mounds to large extensive (o50 m diameter) structures. Some of the observed mounds were associated with authigenic carbonate and active seepage (living chemosynthetic fauna present in addition to the asphalt). The asphalt mounds are seabed accumulations of heavy hydrocarbons formed from subsurface migration and fractionation of reservoir hydrocarbons primarily through a network of faults. In Angola these processes are controlled by subsurface movement of salt structures. The asphalt mounds were typically densely covered with epifauna (74.5% of mounds imaged had visible epifauna) although individual mounds varied considerably in epifaunal coverage. Of the 49 non-chemosynthetic megafaunal taxa observed, 19 taxa were only found on hard substrata (including asphalt mounds), 2 fish species inhabited the asphalt mounds preferentially and 27 taxa were apparently normal soft-sediment fauna. Antipatharians (3.672.3% s.e.) and poriferans (2.671.9% s.e.) accounted for the highest mean percentage of the observed cover, with actinarians (0.970.4% s.e.) and alcyonaceans (0.470.2% s.e.) covering smaller proportions of the area. Asphalt mounds represent a common and important habitat on several margin systems globally and should be recognised in future environmental assessment and management of these areas.

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Section: Study Sitementioning

confidence: 99%

“…The geology of the Block 31 area is complex and driven by salt tectonics (Thomas et al, 2011;Evans, 2010;Hill et al, 2010a;Hill et al, 2010b). The entirety of Block 31 is underlain by mobile salt of Aptian age (lower Cretaceous, approximately 130-120 Ma).…”

Section: Study Sitementioning

confidence: 99%

Asphalt mounds and associated biota on the Angolan margin

Jones¹,

Walls²,

Clare³

et al. 2014

Deep Sea Research Part I: Oceanographic Research Papers

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“…Evans, 2011). Major oil companies have recently been faced with important engineering problems related to the presence of free gas in marine sediments (Kortekaas & Peuchen, 2008).…”

Section: Introductionmentioning

confidence: 99%

Mechanical behaviour of gas-charged marine plastic sediments

Sultan¹,

Gennaro²,

Puech³

2012

Géotechnique

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Understanding how free gas modifies the mechanical behaviour of marine plastic sediments remains a challenging issue. Special triaxial tests were carried out on natural sediments recovered from the Gulf of Guinea. Special attention was devoted to the laboratory preparation procedure, involving saturation of natural marine sediment samples with carbonated water and generation of free gas following undrained unloading. Experimental data show that soil destructuration and damage generated by gas expansion and exsolution are at the origin of (a) the increase in the isotropic compressibility and decrease in the preconsolidation pressure, and (b) the decrease in the peak undrained strength and abrupt increase of the pore pressure during undrained shearing. The experimental data also show that flooding of gas bubbles and dissolution of free gas do not imply a complete strength recovery of natural sediments.

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“…AUV Chirp profiler data provide a useful means for imaging the shallow sub-seafloor (less than 50 m below seafloor) and are now routinely acquired as part of deepwater site surveys (Evans, 2011). AUV Chirp profiler data provide a useful means for imaging the shallow sub-seafloor (less than 50 m below seafloor) and are now routinely acquired as part of deepwater site surveys (Evans, 2011).…”

Section: Figure 6: Example Deposits and Facies Types Identified From mentioning

confidence: 99%

Sediment Density Flow Impact Assessment and Mitigation Studies for Deepwater Developments: A Challenge on Multiple Scales

Clare

Thomas

Spinewine³

2013

All Days

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A unified and verifiable approach is outlined for the assessment of submarine sediment density flows (debris flows, high density and dilute turbidity currents) providing diagnostic criteria of their previous occurrence to inform forward-looking analyses. The assessment outlined includes development of: Evolutionary geological model, summarizing temporal, geological and climatic controls on specific flow types, and estimations of recurrence intervals; Terrain model to understand regional to macro-scale spatial controls on flow dynamics, feasible geometries and predicted aspect of flows to structures; Sedimentary facies model to determine flow behaviour and evolution based on sedimentological analysis and interpretation of high resolution seismic data.Crucial factors that influence the nature and magnitude of density flows include: the original initiation event (volume, sediment type and triggering mechanism); interrelationship of slope systems, such as downslope coalescence and densification of flows; degree of confinement caused by the seafloor relief; and the influence of local seafloor gradient. These factors may modify the downslope evolution of a flow from that predicted by conventional models. Examples are shown that may challenge preconceptions of flow dynamics, and outline how geological observations should be integrated to provide robust inputs prior to performing numerical modeling.Case studies are presented from regional to millimetric scale, to summarize key observations that should be used to infer credible flow type, density, direction and magnitude, and are related to recommendations for modeling that may ultimately be translated to a vulnerability assessment. In particular, key metrics for flow characterization that can be derived from sedimentological analysis and interpretation of deposit morphology are discussed, as well as application and limitations of modeling techniques for different flow types.Submarine sediment density flows occur in many environments worldwide that are being exploited for hydrocarbons and traversed by infrastructure associated with telecommunications and energy transport. Apparently subtle variations, on a variety of scales, may affect the nature of flows and, thus the type and magnitude of impact on subsea structures. Without detailed calibration and understanding of systems as a whole, the inputs that are fed in to geomechanical, hydrodynamic and vulnerability models may be misleading or unrepresentative of real-life conditions.

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A systematic approach to offshore engineering for multiple-project developments in geohazardous areas

Cited by 17 publications

References 5 publications

Asphalt mounds and associated biota on the Angolan margin

Asphalt mounds and associated biota on the Angolan margin

Mechanical behaviour of gas-charged marine plastic sediments

Sediment Density Flow Impact Assessment and Mitigation Studies for Deepwater Developments: A Challenge on Multiple Scales

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