Deep-water Systems Research Articles

Submarine channel evolution linked to rising salt domes, Gulf of Mexico, USA

An examination of halokinetics and channel evolution together in a deepwater system provides an opportunity to investigate how submarine channel morphology is locally affected by rising salt domes. The study area is located in the northern Gulf of Mexico (GOM), directly off the Louisiana continental slope in a prominent salt dome region. The influence of salt growth on submarine channel evolution is relatively understudied, particularly in the GOM. Utilizing high-resolution 3D seismic and well data and seismic geomorphology techniques, a long-lived (~3 Myr) Plio-Pleistocene submarine channel system has been investigated to show a relationship between variable phases of salt motion and planform morphology of preserved submarine channels.Our data suggest that local salt motion acts as a driver for submarine channel evolution. During the late Pliocene, when salt moved upward at a relatively fast rate, channels show distinct entrenchment with narrow channel belts and overall less sinuosity. When salt motion slowed down at the beginning of the Pleistocene, channels aggraded rapidly with preserved levees, and moved toward an equilibrium state with the expansion of channel belt widths. As our results indicate, the rate of salt diapirism exerted a first-order control on channel location and morphology and distribution of reservoir-prone units. This study cautions against readily invoking allogenic factors (e.g., sea level and climate) in explaining changes in submarine channel behavior and associated fan sedimentation, particularly in regions with salt tectonics.

The role of mass-transport deposits and turbidites in shaping modern lacustrine deepwater channels

Subaquatic canyons are an important pathway for sediment transport into oceanic and lacustrine basins. Understanding the mechanisms governing their geomorphological evolution is a key to predict the sediment distribution patterns through these sediment conduits as well as to implement geo-hazard assessments. Submerged channels developed in large lacustrine basins offer a small-scale natural laboratory to understand the sedimentological processes operating in submarine channels. For this reason, a multidisciplinary research initiative -including time-lapse, high-resolution bathymetric surveys, innovative coring using submersibles, in situ geotechnical tests, and geophysical and sedimentological analyses-was applied to unravel the factors controlling the geomorphological evolution of the Rhone delta channels in Lake Geneva during the last decades. The morphology of the lacustrine Rhone Delta consists of a freshwater delta system deeply incised by nine canyons (C1C9). Geotechnical measurements in proximal areas and sediment cores retrieved in the distal fans at the end of each canyon revealed complex sediment dynamics. No turbidity current events have occurred in the easternmost canyons (C1C4) during the last decades while the western canyons sediment record (C5C9) indicated repeated flushing events during the 20th century. The main “active” canyon C8 has been dominated by turbidite activity on the canyon floor with frequent overspill events along the levees. A large 6.2 × 106 m3 Mass-Transport Deposit (MTD) that resembles a debrite in its upper section was found in the distal area of the active channel. The MTD was dated at 1998–2000 CE and most likely originated from proximal delta areas affected by frequent slope failures of the steep channel walls. In situ geotechnical tests on the modern proximal channel floor showed an unconsolidated soft top-layer that might have served as a low-friction surface favouring the MTD long run-out distance to the distal part of the channel. The MTD has had a major effect morphological evolution of the distal channel by filling the existing conduit, indirectly promoting the formation of a new channel. The role of MTD emplacement in subaquatic channels has important implications for hydrocarbon exploration as they control channel avulsion processes and the location of sand-prone deposits. This study gives a detailed insight on poorly investigated short-term sedimentological dynamics that affect the long-term evolution of turbidite systems and channel migration processes. This detailed model of a river-dominated deep-lacustrine depositional system can be used as an analog for similar modern and ancient deep-water systems.

Submarine fans: A critical retrospective (1950–2015)

When we look back the contributions on submarine fans during the past 65 years (1950–2015), the empirical data on 21 modern submarine fans and 10 ancient deep-water systems, published by the results of the First COMFAN (Committee on FANs) Meeting (Bouma et al., 1985a), have remained the single most significant compilation of data on submarine fans. The 1970s were the “heyday” of submarine fan models. In the 21st century, the general focus has shifted from submarine fans to submarine mass movements, internal waves and tides, and contourites. The purpose of this review is to illustrate the complexity of issues surrounding the origin and classification of submarine fans. The principal elements of submarine fans, composed of canyons, channels, and lobes, are discussed using nine modern case studies from the Mediterranean Sea, the Equatorial Atlantic, the Gulf of Mexico, the North Pacific, the NE Indian Ocean (Bay of Bengal), and the East Sea (Korea). The Annot Sandstone (Eocene–Oligocene), exposed at Peira-Cava area, SE France, which served as the type locality for the “Bouma Sequence”, was reexamined. The field details are documented in questioning the validity of the model, which was the basis for the turbidite-fan link. The 29 fan-related models that are of conceptual significance, developed during the period 1970–2015, are discussed using modern and ancient systems. They are: (1) the classic submarine fan model with attached lobes, (2) the detached-lobe model, (3) the channel-levee complex without lobes, (4) the delta-fed ramp model, (5) the gully-lobe model, (6) the suprafan lobe model, (7) the depositional lobe model, (8) the fan lobe model, (9) the ponded lobe model, (10) the nine models based on grain size and sediment source, (11) the four fan models based on tectonic settings, (12) the Jackfork debrite model, (13) the basin-floor fan model, (14) supercritical and subcritical fans, and (15) the three types of fan reservoirs. Each model is unique, and the long-standing belief that submarine fans are composed of turbidites, in particular, of gravelly and sandy high-density turbidites, is a myth. This is because there are no empirical data to validate the existence of gravelly and sandy high-density turbidity currents in the modern marine environments. Also, there are no experimental documentation of true turbidity currents that can transport gravels and coarse sands in turbulent suspension. Mass-transport processes, which include slides, slumps, and debris flows (but not turbidity currenrs), are the most viable mechanisms for transporting gravels and sands into the deep sea. The prevailing notion that submarine fans develop during periods of sea-level lowstands is also a myth. The geologic reality is that frequent short-term events that last for only a few minutes to several hours or days (e.g., earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.) are more important in controlling deposition of deep-water sands than sporadic long-term events that last for thousands to millions of years (e.g., lowstand systems tract). Submarine fans are still in a stage of muddled turbidite paradigm because the concept of high-density turbidity currents is incommensurable.

Characterization of deep-marine channel-levee complex architecture with palynofacies: An outcrop example from the Rosario Formation, Baja California, Mexico

Identification of specific elements of a deep-marine system currently relies on detailed sedimentological studies, which can be problematic with sub-surface targets. Here we propose identification of specific architectural elements with palynofacies, hypothesizing that organic matter will not be uniformly spread in turbidite successions. An integrated sedimentological and palynological study was conducted on outcrops of an Upper Cretaceous – Paleocene, slope channel-levee complex, of the Rosario Fm., Baja California. System architecture is well constrained by previous work, allowing certainty in sample placement. Architecture reflects a lateral progression from channel axis to distal elements, via overbank terrace; internal levees within the channel-belt; channel-bounding external levee, with an inner and outer component, grading into hemipelagites. Two hundred samples, placed in sedimentary logs, were collected from mudstones across the system. 10 g of sediment per sample was processed for analysis of three hundred pieces of organic matter. Samples display a range of both allochthonous terrestrial and relatively autochthonous marine matter. Results show a decrease in sorting of matter away from the channel axis, where dense humic materials are dominant. Lighter particles, e.g. plant cuticle, were retained in suspension at lower energy, being preferentially deposited in channel distal settings. Exploratory ordination analysis was used to explore these trends. The primary mechanism inferred for this distribution is hydrodynamic sorting, as the competence of turbidity currents to transport particles reduces with distance from the sediment conduit. Variation in palynofacies observed in the architectural elements allows a classification scheme to be developed, enabling recognition of depositional sub-environments within deepwater systems. This scheme can now be applied to subsurface samples to assist characterization of subsurface deepwater channel-levee complex architecture, understanding of which is vital for hydrocarbon exploration.

Markov chains and entropy tests in genetic-based lithofacies analysis of deep-water clastic depositional systems

AbstractThe aim of this study was to examine the relationship between structural elements and the so-called genetic lithofacies in a clastic deep-water depositional system. Process-sedimentology has recently been gaining importance in the characterization of these systems. This way the recognized facies attributes can be associated with the depositional processes establishing the genetic lithofacies. In this paper this approach was presented through a case study of a Tertiary deep-water sequence of the Pannonian-basin.Of course it was necessary to interpret the stratigraphy of the sequences in terms of “general” sedimentology, focusing on the structural elements. For this purpose, well-logs and standard deep-water models were applied.The cyclicity of sedimentary sequences can be easily revealed by using Markov chains. Though Markov chain analysis has broad application in mainly fluvial depositional environments, its utilization is uncommon in deep-water systems. In this context genetic lithofacies was determined and analysed by embedded Markov chains. The randomness in the presence of a lithofacies within a cycle was estimated by entropy tests (entropy after depositional, before depositional, for the whole system). Subsequently the relationships between lithofacies were revealed and a depositional model (i.e. modal cycle) was produced with 90% confidence level of stationarity. The non-randomness of the latter was tested by chi-square test.The consequences coming from the comparison of “general” sequences (composed of architectural elements), the genetic-based sequences (showing the distributions of the genetic lithofacies) and the lithofacies relationships were discussed in details. This way main depositional channel has the best, channelized lobes have good potential hydrocarbon reservoir attributes, with symmetric alternation of persistent fine-grained sandstone (Facies D) and muddy fine-grained sandstone with traction structures (Facies F)

Palinspastic restoration of an exhumed deepwater system: A workflow to improve paleogeographic reconstructions

The Permian Laingsburg depocenter, Karoo Basin, South Africa, is the focus of sedimentological and stratigraphic research as an exhumed analog for offshore hydrocarbon reservoirs in deepwater basins. Thin-skinned thrust tectonics during the Permo-Triassic Cape Orogeny result in postdepositional deformation of the Permian basin fill. Regional-scale cross sections reveal two structural domains: a southern domain in the Laingsburg depocenter comprising 8–11-km wavelength north-verging fault-propagation folding, driven by buried low-angle ([Formula: see text]) reverse faults that coalesce at depth as part of a megadetachment below the Lower Paleozoic Cape Supergroup; and a northern domain to the north of the Laingsburg depocenter of short-wavelength, low-amplitude, asymmetrical folding facilitated by a detachment within the Permian Ecca Group. Five detailed structural cross sections permit the palinspastic restoration, and a calculation of the amount of shortening, across a [Formula: see text] area in the Laingsburg depocenter. Average shortening across the study area is −16.9% (5.8 km), and it decreases south to north. Shortening estimates from the Upper Ecca Group increase from 4.3 km near Matjiesfontein in the west to 10.4 km near Prince Albert in the east. Three-dimensional restorations of stratigraphic surfaces are consistent with these figures (−17%), and they allow paleogeographic and isopach thickness maps to be resorted to their configuration at the time of deposition. Structural restoration can be routinely used in outcrop studies to improve the accuracy of dimensions (e.g., volumetrics) and reconstructions (e.g., sediment dispersal patterns) derived from ancient sedimentary systems. The workflow presented here will add value to exhumed basin analogs by presenting prekinematic configurations at the frontal margins of fold-thrust belts.

Short length-scale variability of hybrid event beds and its applied significance

Hybrid event beds (HEBs) are a type of deep-water sediment gravity flow deposit that generally comprise a basal clean sandstone overlain by a variety of muddier and less-permeable sandy facies. They are thought to be emplaced by combinations of turbidity currents, transitional flows and debris flows, all as part of the same transport event. To date, a number of studies have highlighted the common presence of HEBs mainly in the outer and marginal parts of deep-water systems where they replace beds composed dominantly of clean sand up-dip and/or axially over scales of km to 10 s km. In addition to these broad patterns, important yet poorly understood short-length facies changes (over metres to 100 s m) occur, modifying the overall texture and reservoir characteristics at or beneath typical spacing of production wells. The nature and origin of the short length-scale transitions is here addressed in four well-exposed HEB-prone outcrops: the Cretaceous-Paleocene Gottero Sandstone and the Miocene Cilento Flysch, both in Italy, the Carboniferous Mam Tor Sandstone in northern England, and the Carboniferous basal Ross Sandstone Formation, Western Ireland. A series of detailed correlation panels show marked lateral variations in internal bed make-up for most of the hybrid event beds studied. This variability typically involves lateral changes in the proportions of the cleaner basal sandstone and the overlying muddy sandstone division that occur without substantial change in the overall event bed thickness. The variability is inferred to reflect the complex fingering between the up-dip sandstone-dominated part of the event bed and the down-dip linked debrite due to internal erosion (ploughing) of the debrite into the basal clean sand. Where the upper part of the bed is dominated by large mudstone rafts, these may have foundered into liquefied sand and been injected and partly fragmented by the sand intrusions. The variable thickness and continuity of the basal clean sandstones have important implications for reservoir characterisation; significant variability in bed character at interwell scale can be anticipated. Rugose contacts between the intra-bed facies divisions may impact on drainage and sweep efficiency during hydrocarbon production.

Methods for Estimation of Multiplier Effect of Investments in Development of Infrastructure of Inland Water Transport in the Russian Federation in the Frameworks of Federal Target Programs

The offered method for estimation of influence of investments in development of infrastructure of inland waterways provided by Transport strategy of the Russian Federation for the period up to 2030 and Federal target program “The development of transportation system of the Russian Federation (2010-2020s)” on growth of economy of Russia allows estimating the integral multiplier effect. In the study there are methodological approaches and estimations of multiplier effect of investments in realization of number of major investment projects for liquidation of limiting sites on Single Deep-Water System (SDWS) of European part of Russia with use of transport and production multipliers. The offered method provides detalization of economic effect when forecasting and control for realization of Federal target programs on water transport. The mechanism of estimation of branch effect offered by authors is detailed in sources of gaining economic growth in a branch (the direct and indirect branch effect) and allied branches that helps to get the notion of objectives and forecasts of their development for the period of program validity and subsequent period of objects’ operation. There is also a graphic structural diagram that provides considering the structure of multiplier effect and following the interrelation of indicators—the estimation elements. The offered algorithm of estimation of single multiplier effect elements provides discerning the sequence and the way of estimation of main effectiveness indicators of Federal target programs. In the paper there are also the results of estimation by offered diagram with use of software on the data of Federal target program “Development of transportation system of the Russian Federation (2010-2020s)” and Transport strategy of the Russian Federation for the period up to 2030 designed by the authors. Using the results of estimations we have also performed the quantitative estimation of multiplier effect of investments in liquidation of limiting sites on SDWS that makes up 1295 bln. rbs.

Constraining the sedimentology and stratigraphy of submarine intraslope lobe deposits using exhumed examples from the Karoo Basin, South Africa

Intraslope lobe deposits provide a process record of the infill of accommodation on submarine slopes and their recognition enables the accurate reconstruction of the stratigraphic evolution of submarine slope systems. Extensive exposures of discrete sand-prone packages in Units D/E and E, Fort Brown Formation, Karoo Basin, South Africa, permit analysis of the sedimentology and stacking patterns of three intraslope lobe complexes and their palaeogeographic reconstruction via bed-scale analysis and physical correlation of key stratal surfaces. The sand-prone packages comprise tabular, aggradationally to slightly compensationally stacked lobe deposits with constituent facies associations that can be attributed to lobe axis, lobe off-axis, lobe-fringe and distal lobe-fringe environments. Locally, intraslope lobe deposits are incised by low aspect ratio channels that mark basinward progradation of the deepwater system. The origin of accommodation on the slope for lobe deposition is interpreted to be due to differential compaction or healing of scars from mass wasting processes. The stacking patterns and sedimentary facies arrangement identified in this study are distinct from those of more commonly recognized basin-floor lobe deposits, thereby enabling the establishment of recognition criteria for intraslope lobe deposits in other less well exposed and studied fine-grained systems. Compared to basin floor lobes, intraslope lobes are smaller in volume, influenced by higher degrees of confinement, and tend to show aggradational stacking patterns.

Sedimentology, Stratigraphic Architecture, and Depositional Context of Submarine Frontal-Lobe Complexes

Abstract Frontal lobes develop during discrete periods of progradation in deep-water systems, and commonly form on the lower slope to base of slope. In reflection seismic datasets, they are identified as high-amplitude reflectors that are cut as the feeder channel lengthens. Here, an exhumed sand-prone succession (> 80% sandstone) from Sub-unit C3 of the Permian Fort Brown Formation, Laingsburg depocenter, Karoo Basin, South Africa, is interpreted as a frontal-lobe complex, constrained by its sedimentology, geometry, and stratigraphic context. Sub-unit C3 crops out as a series of sand-prone wedges. Individual beds can be followed for up to 700 m as they thin, fine, and downlap onto the underlying mudstone. The downlap pattern, absence of major erosion surfaces or truncation, and constant thickness of underlying units indicates that the wedges are non-erosional depositional bodies. Their low aspect ratio and mounded geometry contrasts markedly with architecture of terminal lobes on the basin floor. Furthermore their sedimentology is dominated by decimeter-scale sinusoidal stoss-side preserved bedforms with a range of low-angle to high-angle climbing-ripple laminated fine-grained sandstones. This indicates that the flows deposited their load rapidly close to, and downstream from, an abrupt decrease in confinement. The sedimentology, stratigraphy, cross-sectional geometry, and weakly confined setting of a sand-prone system from the Giza Field, Nile Delta, is considered a close subsurface analogue, and their shared characteristics are used to establish diagnostic criteria for the identification and prediction of frontal-lobe deposits. In addition, deposits with similar facies characteristics have been found at the bases of large external levee deposits in the Fort Brown Formation (Unit D). This could support models in which frontal lobes form an initial depositional template above which external levees build, which provides further insight into the initiation and evolution of submarine channels.

Coupled dynamics of deepwater structures – issues and challenges

ABSTRACTOffshore systems consisting of floaters linked to line structures such as risers and mooring lines are favoured for deepwater applications. The dynamic coupling analysis of a floater together with the line structures has been the subject of several studies in recent years. Powerful numerical tools have been developed implementing full-fledged time-domain approaches as well as more efficient frequency-domain approaches for the analysis of deepwater systems. The present article provides a brief overview of coupled analysis techniques employing both time-domain and frequency-domain approaches, focusing on the basic modelling issues and challenges. It is worth formulating compact but sufficiently accurate dynamic models of line structures. The consequence of truncation errors of numerical integration schemes in long-term time-domain simulations needs to be studied. There is a need to standardise the statistical linearisation approach applied to dynamic response which spreads over a very wide frequency range. Verification of the nonlinear drag force model in the Morison equation through basic computational fluid dynamics studies may be undertaken.

Constraining sub-seismic deep-water stratal elements with electrofacies analysis; A case study from the Upper Cretaceous of the Måløy Slope, offshore Norway

Electrofacies represent rock facies identified from wireline-log measurements, and allow extrapolation of petrophysical characteristics away from stratigraphic intervals that are calibrated to core. This approach has been employed to reduce uncertainty in the identification of the sub-seismic depositional elements in the late Cenomanian–Coniacian succession of the northern Måløy Slope, offshore Norway. Core logging permits identification of eleven distinct sedimentary facies that are grouped into four facies associations: FA A-turbidite sandstones, FA B-heterolithic siltstones and sandstones, FA C-debrites and FA D-slide and slump deposits. Each facies association is defined by a distinct combination of petrophysical characteristics, including porosity, density, gamma-ray, sonic and resistivity. Using neural network analysis, electrofacies are calibrated with sedimentary facies, thereby allowing us to map their thickness and stacking patterns within the studied deep-water succession. We demonstrate that this approach is particularly useful where the presence of glauconite makes the distinction between sandstone- from shale-rich units difficult using gamma-ray logs alone. Our results indicate that the succession of interest is dominated by debris flows and slide and slump deposits, which are commonly poorly imaged on seismic reflection datasets in the northern North Sea. The methodology presented here can aid the correlation of deep-water stratal elements at production and exploration scales in stratigraphic successions that have undergone similar burial histories.Furthermore, this method may help in the identification of mass flow deposits that are present in Upper Cretaceous deep-water systems of the North Sea.

Failure Analysis of Fiber Optic Communication System in Deep-Water Remotely Operated Vehicle ROSUB 6000

AbstractSingle-mode fiber optic systems can play vital roles in cabled deep-water vehicle operations at greater depths (>3,000 m). One kind of single-mode fiber optic system, the ROSUB 6000, is used in a deep-water work-class remotely operated vehicle (ROV). Fiber optic link failure of ROV telemetry and sound navigation and ranging were noticed at a water depth of 3,050 m during the ROSUB 6000 system sea trials. A failure analysis of the fiber optic communication system was carried out with the link data logged during different phases of the deep-sea trials. The results from the failure analysis carried out during deep-sea trials showed an increase in the fiber optic link loss from a depth of 900 m onwards. Further analysis of the fiber optic link loss in the laboratory involved pressure and low-temperature testing of all the subsea components in the ROV telemetry link. From the laboratory pressure test results, it was concluded that pressure was not the root cause of the fiber optic link failure. On further analysis, a complete fiber optic link failure was noticed during the low-temperature testing of the subsea components. Furthermore, the low-temperature testing of the individual subsea components revealed that the fiber optic rotary joint (FORJ) insertion loss increased rapidly at low temperatures. This FORJ insertion loss led to complete failure of the fiber optic links in the ROV. The degradation of index-matching fluid in the FORJ was identified to be the root cause of fiber link failure.

Can an Anthropocene Series be defined and recognized?

Abstract We consider the Anthropocene as a physical, chronostratigraphic unit across terrestrial and marine sedimentary facies, from both a present and a far future perspective, provisionally using an approximately 1950 CE base that approximates with the ‘Great Acceleration’, worldwide sedimentary incorporation of A-bomb-derived radionuclides and light nitrogen isotopes linked to the growth in fertilizer use, and other markers. More or less effective recognition of such a unit today (with annual/decadal resolution) is facies-dependent and variably compromised by the disturbance of stratigraphic superposition that commonly occurs at geologically brief temporal scales, and that particularly affects soils, deep marine deposits and the pre-1950 parts of current urban areas. The Anthropocene, thus, more than any other geological time unit, is locally affected by such blurring of its chronostratigraphic boundary with Holocene strata. Nevertheless, clearly separable representatives of an Anthropocene Series may be found in lakes, land ice, certain river/delta systems, in the widespread dredged parts of shallow-marine systems on continental shelves and slopes, and in those parts of deep-water systems where human-rafted debris is common. From a far future perspective, the boundary is likely to appear geologically instantaneous and stratigraphically significant.

A prodeltaic system controlled by hyperpycnal flows and storm waves: reinterpretation of the Punta Negra Formation (Lower-Middle Devonian, Argentine Precordillera)

The Punta Negra Formation (FPN) constitutes a Lower-Middle Devonian sandstone unit located in the Argentine Precordillera. Based on detailed facies and ichnological analyses, this unit, which had been previously interpreted as a deep-water system, has been reinterpreted in this study as a prodelta system, in which the distribution and sedimentation of the clastic deposits were controlled by hyperpycnal flows associated with storms. Five lithofacies attest the deposition by unidirectional hyperpycnal flows combined with storm-generated oscillatory flows. Fossil traces testify variable energetic conditions of the depositional system with the colonization of opportunistic organisms immediately after the deposition of high energy combined flows and the colonization of more specialized organisms during the long low-energy periods. Three architectural elements, deposited from coastline to offshore, have been recognized: 1) Channelized sandstone, composed of the thickest and coarsest sandstone beds and interpreted as the proximal portion deposited close to the mouth of the distributaries of the delta system; 2) Tabular sandstone, which are composed of combined and oscillatory flow deposits with lesser thickness, grain size and spacing of the undulated bed forms; 3) Tabular sandstone interbedded to mudstone, which represents the most distal architectural element and is composed of thin sandstone and thicker mudstone strata, deposited by low-energy combined flows.

Seismic modeling in the analysis of deep-water sandstone termination styles

Innovative seismic forward modeling is used to illustrate the sensitivity within seismic data, and its application in the interpretation of onlap and pinch-out of terminating deep-water sandstones, two critical components in deep-water exploration and production. Sandstone quality, net-to-gross estimates, volume calculations, vertical connectivity, and stratigraphic trapping are all dependent on the sandstone extent and their seismic characteristics in these settings. However, seismic resolution is commonly insufficient to resolve the critical reservoir parameters. Seismic modeling of termination styles based on integrated outcrop and subsurface properties allows for depth- and resolution-focused predictive models to be built for improved subsurface analysis. This technique is currently underused as a method to better understand the sensitivity of seismic data to the target lithologies and their geometries. The Gres d'Annot Formation is a well-studied sand-prone deep-water system of Paleogene age, deposited in a bathymetrically complex setting. Six end-member termination styles are discussed, including three sand-prone styles—simple onlap (O s), draping onlap (O d), and bed thickening (O t)—and three heterolithic styles—advancing pinch-out (P a), convergent pinch-out (P c), and convergent thickening and pinch-out (P ct). Local thickening close to the system margins is common in both sand-prone and heterolithic terminating strata and plays an important function in the appropriate distribution of sandstone. The outcrops are interpreted as potential (process) analogs for the complex sandstone distribution and termination patterns observed in plays like the Paleogene of the Gulf of Mexico and the Jurassic of the northern North Sea.

Analysis of deepwater offshore structures: Issues and challenges

A brief overview of analysis of deepwater offshore structures with reference to modeling issues is presented. Floaters with moorings and risers exhibit nonlinear response under environmental loads. The modeling of such structures poses many challenges because of the flexibility of the riser/mooring system, fluid–structure interaction and random nature of wave loads. Tensioned beam finite elements can be used for structural modeling of line elements. Computational fluid dynamic modeling may be used to firm up hydrodynamic coefficients required in the Morison equation for the estimation of fluid loads on structural elements, as well as to evolve simplified models to represent vortex-induced motions. While wave-body interaction of large-volume structures may be modeled using computational fluid dynamic tools, the use of numerical models employing the fully nonlinear potential flow theory is worth exploring. Simpler analytical models for use in design analysis may be developed by employing coupled models based on nonlinear structural finite element methods and computational fluid dynamic tools. Simpler methods of evaluating stress-range statistics required for fatigue damage estimation need to be developed for the case of nonlinear stochastic response of deepwater systems.

Taper design and non-linear analysis of bend stiffeners at the riser–vessel interface

The connection of a flexible riser to the floating production system is identified as a critical area with respect to extreme deformation and fatigue damage. Bend stiffeners are designed to provide a gradual stiffness transition at the riser–vessel interface, protecting the riser against overbending and from the accumulation of fatigue damage. Thus, they are of vital importance to deep-water oil and gas production systems. A methodology for the bend-stiffener taper design is firstly outlined in this paper. The exact expression of the angle at the tip of the bend stiffener required for the design is derived. Then, a mathematical formulation and the numerical solution procedure for the geometrical and material non-linear analysis of the riser-stiffener system are developed. The governing differential equations are obtained from geometrical compatibility, equilibrium of forces and moments, and non-linear material constitutive relations. An analytical approach to predict the bending moment versus curvature relation for each cross-section is presented assuming that the bend-stiffener material is non-linear symmetric, while for asymmetric materials the constitutive relation is calculated numerically. A numerical solution procedure is implemented in MATLAB code to solve the boundary value problem. Furthermore, the flexible pipe structure exhibits an approximately bi-linear hysteretic bending moment against curvature relation. The effect of the bending stiffness bi-linear behaviour on the bend-stiffener response is evaluated for the non-linear material case. A case study is carried out and the results show the potential influence of the material non-linearity on the stiffener response and the need to incorporate the effect of non-linear bending of the pipe in bend-stiffener analyses.

High-Resolution 3D Laser Imaging Advances to Subcentimeter Precision

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 23006, ’SS MTS: Subsea Monitoring - High-Resolution 3D Laser Imaging for Inspection, Maintenance, Repair, and Operations,’ by C. Embry, M. Hardy, and B. Nickerson, 3D at Depth; N. Manning, CDL; D. Goodyear, UTEC Survey; and D. Richardson, SPE, and J. Pappas, SPE, RPSEA, prepared for the 2012 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. 3D laser imaging is a powerful data-collection system that provides 3D information for a specific region of interest. Developing the technology to provide high-definition subsea laser imaging enables the deepwater industry to use the current state of the art in 3D metrology and related best practices developed for the terrestrial market. Terrestrial laser scanners commonly produce centimeter spatial and range accuracy at the several-hundred-meter range. Because of the absorption of water, realizable deepwater systems are limited to tens-of-meters range, depending on the target and water conditions. Here, subcentimeter accuracy from greater than 6 m is demonstrated for an underwater laser system. Introduction Sonar systems have been the main tools for measurement and survey for many years and have become very sophisticated. Many companies have developed high-resolution multibeam sonar systems that can produce angular resolutions as small as 0.2°. However, the resolution of these systems is fundamentally limited by the underlying physics; a wave-length of light is more than 108 times smaller than ultrasonic wavelengths and can achieve angular resolutions of less than 100 Μrad (0.006°). Therefore, light-based sensors inherently have higher spatial resolutions than sonar systems. The underwater 3D laser sensor presented here has the fundamental advantage of providing subcentimeter precision from greater than 7 m, a capability that, to the authors’ knowledge, has not been demonstrated previously. The Sensor The optical head [laser imaging unit (LIU)] consists of the transmitter subsystem (laser and accompanying optics), the receiver subsystem (detector and accompanying optics), and the scanner. These subsystems can be optimized for different operational scenarios and environ-mental conditions. For this prototype system, we used a diode-pumped, passively Q-switched, Nd:YAG laser with an external nonlinear crystal to perform the frequency doubling to 532-nm laser light. The receiver is a high-speed silicon photo diode, and the scanning subsystem is a two-axis galvanometer scanner to allow programmable scanning in two orthogonal directions. Along with the LIU are multiple electronic control systems. These include the control subsystem for controlling the LIU and monitoring-system health; the central processing unit and bus for general system control and operation; the data subsystem for data collection, transformation, and processing; the data-storage system for housing data locally; the power and environmental monitoring system to convert the host-vehicle power to the various voltages required by the sensor; and the input/output (communication) subsystem. All of these systems are housed within the subsea sensor (Fig. 1). The prototype sensor is 8.2 in. in diameter and 28 in. long and weighs 76.1 lbm in air.

DUCTILE STRUCTURES IN GRAVITATIONAL DECOLLEMENT STRUCTURE SYSTEM AT PASSIVE CONTINENTAL MARGIN OF WEST AFRICAN

This article deals with the ductile structures in the deep-water gravitational decollement structure system at the Passive Continental Margin of West African.It is found that ductile structures are widely distributed along the whole passive continental margin,and the ductile salt rocks are particularly developed in the southwest and northwest within the transitional structural layer(J-K).In the Niger delta at the middle part of the passive margin of West Africa,ductile bed is mainly consisting of mudstone of Tertiary.According to the structural characteristics of the gravitational detachment structure system,the ductile structures could be divided into three types: the gravitational decollement extensional structures at the upper part characterized by normal fault and ductile;the gravitational decollement diapir structures at the middle part;and the gravitational decollement thrust structures at the lower part.According to the time of deformation of the ductile structure,the ductile structures could be grouped into two types of epigenetic and syngenetic structures.Deformation of ductile beds is mainly affected by the basement tilting and deposition rate of the overlying strata.The control of structures over oil and gas accumulation decreases in an order of diapir,fold,thrust and ductile welding structure.