Showing papers on "Submarine pipeline published in 2017"

Offshore Geotechnical Engineering

Abstract: Introduction The Offshore Environment Offshore Site Investigation Soil Behaviour Piled Foundations Shallow Foundations Anchoring Systems Mobile Drilling Rigs Pipeline and Riser Geotechnics Geohazards

Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola

Abstract: Integrated analysis of high‐quality three‐dimensional (3D) seismic, seabed geochemistry, and satellite‐based surface slick data from the deep‐water Kwanza Basin documents the widespread occurrence of past and present fluid flow associated with dewatering processes and hydrocarbon migration. Seismic scale fluid flow phenomena are defined by seep‐related seafloor features including pockmarks, mud or asphalt volcanoes, gas hydrate pingoes, as well as shallow subsurface features such as palaeo‐pockmarks, direct hydrocarbon indicators (DHIs), pipes and bottom‐simulating reflections (BSRs). BSR‐derived shallow geothermal gradients show elevated temperatures attributed to fluid advection along inclined stratigraphic carrier beds around salt structures in addition to elevated shallow thermal anomalies above highly conductive salt bodies. Seabed evidences of migrated thermogenic hydrocarbons and surface slicks are used to differentiate thermogenic hydrocarbon migration from fluid flow processes such as dewatering and biogenic gas migration. The analysis constrains the fluid plumbing system defined by the three‐dimensional distribution of stratigraphic carriers and seal bypass systems through time. Detailed integration and iterative interpretation have confirmed the presence of mature source rock and effective migration pathways with significant implications for petroleum prospectivity in the post‐salt interval. Integration of seismic, seabed geochemistry and satellite data represents a robust method to document and interpret fluid flow phenomena along continental margins, and highlights the importance of integrated fluid flow studies with regard to petroleum exploration, submarine geohazards, marine ecosystems and climate change.

Bottom-trawling along submarine canyons impacts deep sedimentary regimes.

Abstract: Many studies highlight that fish trawling activities cause seafloor erosion, but the assessment of the remobilization of surface sediments and its relocation is still not well documented. These impacts were examined along the flanks and axes of three headless submarine canyons incised on the Barcelona continental margin, where trawling fleets have been operating for decades. Trawled grounds along canyon flanks presented eroded and highly reworked surface sediments resulting from the passage of heavy trawling gear. Sedimentation rates on the upper canyon axes tripled and quadrupled its natural (i.e. pre-industrialization) values after a substantial increase in total horsepower of the operating trawling fleets between 1960 s and 1970 s. These impacts affected the upper canyon reaches next to fishing grounds, where sediment resuspended by trawling can be transported towards the canyon axes. This study highlights that bottom trawling has the capacity to alter natural sedimentary environments by promoting sediment-starved canyon flanks, and by enhancing sedimentation rates along the contiguous axes, independently of canyons’ morphology. Considering the global mechanisation and offshore expansion of bottom trawling fisheries since the mid-20th century, these sedimentary alterations may occur in many trawled canyons worldwide, with further ecological impacts on the trophic status of these non-resilient benthic communities.

Subsea mining moves closer to shore

Abstract: Mining the deep seabed is fraught with challenges. Untapped mineral potential under the shallow, more accessible continental shelf could add a new dimension to offshore mining and help meet future mineral demand.

Quantitative risk assessment of submarine pipeline instability

Abstract: Ensuring the on-bottom stability of the submarine pipeline is very important for safety concern during the operational stage of submarine pipeline. Due to the action of the external factors, e.g. strong current, wave and soil liquefaction, the submarine pipeline may have lateral displacement, vertical floatation or sinking. Although the submerged weight of submarine pipeline is designed to meet the requirement of on-bottom stability, the loss risk of pipeline on-bottom stability still exists due to the change of ocean environment or seabed. This paper presents a reliability-based assessment methodology for submarine pipeline instability. Firstly, a mechanical model of pipeline on-bottom state is built considering the hydrodynamic loads and pipe-soil interaction, which is the static forces equilibrium equations essentially, and a detailed analysis of loading condition of pipeline on seabed is also conducted. Then, based on the reliability theory, the limit state equations of pipeline instability are developed through converting the forces equilibrium equations. Because the parameters in limit state functions possess random uncertainty, such as wave and current loads, etc., the specific probability distributions are employed to present the random uncertainty of the parameters in limit state functions. The Monte Carlo method is then employed to solve the limit state equations for assessing the pipeline instability probability. The risk level of pipeline instability is judged using the risk ranking in DNV-RP-F107. The case study indicates that the proposed risk assessment methodology for pipeline instability possesses a good application performance. In essence, the assessment results could provide a powerful support for risk management or decision-making of submarine pipeline instability.

Flow-pipe-soil coupling mechanisms and predictions for submarine pipeline instability

Abstract: The stability of a submarine pipeline on the seabed concerns the flow-pipe-soil coupling, with influential factors related to the ocean waves and/or currents, the pipeline and the surrounding soils. A flow-pipe-soil coupling system generally has various instability modes, including the vertical and lateral on-bottom instabilities, the tunnel-erosion of the underlying soil and the subsequent vortex-induced vibrations (VIVs) of free-spanning pipelines. This paper reviews the recent advances of the slip-line field solutions to the bearing capacity, the flow-pipe-soil coupling mechanism and the prediction for the lateral instability, the multi-physical coupling analysis of the tunnel-erosion, and the coupling mechanics between the VIVs and the local scour. It is revealed that the mechanism competition always exists among various instability modes, e.g., the competition between the lateral-instability and the tunnel-erosion. Finally, the prospects and scientific challenges for predicting the instability of a long-distance submarine pipeline are discussed in the context of the deep-water oil and gas exploitations.

Fatigue crack growth investigation on offshore pipelines with three-dimensional interacting cracks

Abstract: Due to high cost of full-scale experimental setup, this study presents a numerical model on fatigue behaviours of offshore pipeline with multiple coplanar cracks under cyclic tensile loadings. The validation on numerical results is made by other researchers' experimental results, and significant parameters affecting fatigue crack growth are studied.

Fuzzy MCDM-based GIS model for subsea oil pipeline route optimization: An integrated approach

Abstract: Proper pipeline route selection is an integral component of a typical oil exploration and transportation project. Improperly selected routes could have severe consequences including pipe failures, oil spillage, and environmental disasters. Consequently, technologies like geographic information systems (GIS) are increasingly being used to facilitate the oil pipeline route selection procedure—especially for onshore routing projects. Surprisingly, not much has been documented on the application of GIS to offshore pipeline routing. With recent discoveries on the merits of offshore oil exploration, it is necessary to extend the analytical capabilities of GIS to the unique offshore domain. However, concerns have been raised regarding the limitations of GIS in accurately prioritizing diverse selection criteria in typical multi-criteria decision-making (MCDM) problems like route selection. Consequently, this paper addresses the offshore/subsea pipeline routing constraint using a hybrid decision support sy...

Tsunamis from submarine landslides

Abstract: Most tsunamis are generated by earthquakes, with secondary, less frequent, mechanisms including subaerial and submarine landslides, volcanic eruptions and (extra-terrestrial) bolide impacts. Different mechanisms generate tsunamis with different magnitudes, travel distances and impacts. Submarine landslides had been mapped and studied for decades but records suggested that only a few had generated tsunamis, and that these were minor. It was not until 1998, when a slump on the seabed offshore of northern Papua New Guinea caused a tsunami wave up to 15 m high that killed over 2200 people, was the significance of submarine landslides in tsunami generation realised. A combination of new (multibeam) seabed mapping technology and the development of improved numerical tsunami models for tsunami generation led to the recognition of the landslide tsunami mechanism of the PNG event. As a result the hazard from submarine landslides in tsunami generation is now recognized and better understood. Extensive mapping of ocean margins reveals that submarine landslides are common. Although many of these probably generated tsunamis, few have been identified, so their hazard remains uncertain. This article describes how the hazard from submarine landslide tsunamis was first recognized, how submarine landslides generate tsunamis, why they were previously discounted as a major hazard, and their potential hazards. An important aspect of the recognition of the tsunami hazard from submarine landslides has been the significance of geology, which has contributed to a subject previously dominated by seismologists.

Submarine landslide and tsunami hazards offshore southern Alaska: Seismic strengthening versus rapid sedimentation

Abstract: The southern Alaskan offshore margin is prone to submarine landslides and tsunami hazards due to seismically active plate boundaries and extreme sedimentation rates from glacially enhanced mountain erosion. We examine the submarine landslide potential with new shear strength measurements acquired by Integrated Ocean Drilling Program Expedition 341 on the continental slope and Surveyor Fan. These data reveal lower than expected sediment strength. Contrary to other active margins where seismic strengthening enhances slope stability, the high-sedimentation margin offshore southern Alaska behaves like a passive margin from a shear strength perspective. We interpret that seismic strengthening occurs but is offset by high sedimentation rates and overpressure. This conclusion is supported by shear strength outside of the fan that follow an active margin trend. More broadly, seismically active margins with wet-based glaciers are susceptible to submarine landslide hazards because of the combination of high sedimentation rates and earthquake shaking.

Tsunami run-up and inundation along the coast of Sabah and Sarawak, Malaysia due to a potential Brunei submarine mass failure.

Abstract: Submarine landslides, also known as submarine mass failures (SMFs), are major natural marine disasters that could critically damage coastal facilities such as nuclear power plants and oil and gas platforms. It is therefore essential to investigate submarine landslides for potential tsunami hazard assessment. Three-dimensional seismic data from offshore Brunei have revealed a giant seabed mass deposited by a previous SMF. The submarine mass extends over 120 km from the continental slope of the Baram Canyon at 200 m water depth to the deep basin floor of the Northwest Borneo Trough. A suite of in-house two-dimensional depth-averaged tsunami simulation model TUNA (Tsunami-tracking Utilities and Application) is developed to assess the vulnerability of coastal communities in Sabah and Sarawak subject to potential SMF tsunami. The submarine slide is modeled as a rigid body moving along a planar slope with the center of mass motion parallel to the planar slope and subject to external forces due to added mass, gravity, and dissipation. The nonlinear shallow water equations are utilized to simulate tsunami propagation from deepwater up to the shallow offshore areas. A wetting-drying algorithm is used when a tsunami wave reaches the shoreline to compute run up of tsunami along the shoreline. Run-up wave height and inundation maps are provided for seven densely populated locations in Sabah and Sarawak to highlight potential risks at each location, subject to two scenarios of slide slopes: 2° and 4°. The first wave may arrive at Kudat as early as 0.4 h after the SMF, giving local communities little time to evacuate. Over a small area, maximum inundated depths reaching 20.3 m at Kudat, 26.1 m at Kota Kinabalu, and 15.5 m at Miri are projected, while the maximum inundation distance of 4.86 km is expected at Miri due to its low-lying coast. In view of the vulnerability of some locations to the SMF tsunami, it is important to develop and implement community resilience program to reduce the potential damage that could be inflicted by SMF tsunamis.

Seabed sediments (grain size) of Nordland VI, offshore north Norway

Abstract: This paper presents a seabed sediments (grain size) map of the Nordland VI area (25,000 km2) off the Lofoten islands, north Norway. The map is based on multibeam echosounder data (bathymetry and backscatter), visual analysis of 215 video transects (each 700-m long), and visual and grain-size analysis of seabed sediment samples from 40 sampling stations acquired by grabs, boxcores and multicores. A total of 14 sediment classes were identified, with sediments varying in grain size from mud to boulders. Seabed types also include bedrock and bioclastic sediments from degrading cold-water coral reefs. The continental shelf is mostly characterised by coarse-grained sediments such as gravelly sand and sandy gravel, especially in till areas. In basins and glacial troughs, finer-grained sediments such as sandy mud and muddy sand dominate. The upper continental slope (300–600-m water depth) is characterised by coarse-grained sediments related to the influence of the strong north-east flowing Norwegian Atlan...

Development of Shore Platforms along the NW Coast of Italy: The Role of Wind Waves

Abstract: Pappalardo, M.; Cappietti, L.; Arozarena Llopis, I.; Chelli, A., and De Fabritiis, L., 2017. Development of shore platforms along the NW coast of Italy: The role of wind waves. This paper investigates whether waves are active morphologic agents capable of shaping the small shore platforms that characterize the rocky coast of NW Italy. Two study areas have been selected along this coastal tract: Calafuria (Livorno) and Lerici-Tellaro (La Spezia), located ca. 120 km apart, the first being shaped in sandstone and the second in dolomite bedrock. Propagation of waves in the nearshore has been simulated by numerical modeling. From wave model results, validated with data from an offshore wave meter buoy, it is inferred that waves break directly on the coast or very close to it at Calafuria, whereas for Lerici-Tellaro shores, waves mostly break up to 150 m seaward of the shore platform. This implies that the amount of energy delivered on the platform is much greater in the first case than in the second c...

Influences of pipe–soil interaction on dynamic behaviour of deepwater S-lay pipeline under random sea states

Abstract: The pipe–soil interactions have an important influence on the dynamic behaviour of offshore pipelines in the touchdown zone (TDZ) during deepwater S-lay. This paper mainly aims to reveal the dynamic behaviour of the pipeline in the TDZ, and its actual dynamic lay effects. A full finite element model for deepwater S-lay systems is primarily developed to simulate the dynamic responses of offshore pipeline from the pipelay vessel via the stinger to the seabed, in which pipelay vessel motions, pipeline hydrodynamics, clashing contacts between the pipeline and the stinger rollers and pipe–seabed interactions are considered particularly. The numerical analysis of an illustrative case for a gas export pipeline being installed to a water depth of 1500 m in the South China Sea is carried out in the time domain using the commercial computer code OrcaFlex. The influences of seabed soil models, soil characteristics, submerged self-weight of the pipe, surface waves, and pipelay vessel motions on the dynamic be...

Geophysical Evidence of Gas Hydrates Associated with Widespread Gas Venting on the Central Nile Deep-Sea Fan, Offshore Egypt

Abstract: Deep-sea fans are favoured settings for the formation of gas hydrates, due to high sedimentation rates and organic matter content that promote the upwelling of methane-rich fluids. Gas hydrates have not been sampled on the Nile fan, but geophysical evidence of their presence is known to the Egyptian hydrocarbon industry. Here we use academic data to document a bottom-simulating reflection (BSR) on the central Nile fan, and examine its relationship to evidence of seabed fluid seepage. We use reprocessed multi-channel seismic profiles acquired from 1973-2002, of varying offset (0.3-4 km) and peak frequency content (10 1-10 2 Hz), together with sonar imagery and subottom profiles (10 3-10 5 Hz). The regional methane hydrate stability zone (RMHSZ) is modeled using a phase boundary for methane hydrate in equilibrium with bottom water of 3.86% salinity (Mediterranean average) and gridded inputs for bathymetry, bottom water temperatures and geothermal gradients. A BSR is observed across the central Nile fan in water depths of 2000-2500 m, as a discontinuous reflection of negative polarity at depths of 220-330 ms below seabed. BSR 'patches' vary in extent and character, but are mainly of low amplitude and, on higher frequency data, may be associated with reduced amplitudes or blanking in the overlying succession. The BSR is observed both in stratified intervals and within interbedded unstratified mass-transport deposits (MTDs, up to 200 ms thick). The depth below seabed of the BSR is comparable to the base of the modeled RMHSZ (converted to travel-time at sediment velocities of 1.6-1.8 km/s). The presence of gas hydrates in the lower part of the RMHSZ is indicated by published resistivity logs from two exploration wells, here located in areas lacking a BSR. We infer that on the central Nile fan gas hydrates are present within a sub-seabed interval up to 250 m thick, that is in places associated with a weak BSR recording basal accumulations of free gas. Previous work shows the central Nile fan is also characterised by widspread phenonema of fluid seepage, expressed at seabed as sonar backscatter anomalies that correspond to authigenic carbonate pavements, developed over timescales of at least 10 3 years, some associated with hydroacoustic flares at their edges recording ongoing gas venting to the water column. The carbonate pavements lie within stratified sediments up to 110 m thick that, in most areas, have been affected by post-depositional deformation of underlying MTDs. Seismic profiles across seepage areas show the stratified sediments to contain pipes linked to faults rooted within the MTDs; the latter are offset in places by more deeply-rooted faults, but the seeps do not correspond to vertical structures rising beneath the RMHSZ. These observations suggest that seabed seeps across the central Nile fan record the long-term rise of gas-rich fluids rich from sources within the hydrate stability zone, in places facilitated by faults rooted in MTDs. Different mechanisms have been suggested to explain gas migration through the stability zone : one is the rise from depth of hot and/or saline fluids in focused flows capable of locally displacing the phase boundary upwards to seabed; an alternative involves the diffuse upwelling of methane-rich fluids over wider areas to drive salt-exclusion at the base of the stability zone, a process proposed to lead to the rise of the 3-phase boundary to seabed within hydrate-choked chimneys. We note that the diffuse upwelling of fluids rich in dissolved methane could also account for the weak and discontinuous BSR, in terms of an upward flux of dissolved methane at rates high enough to limit gas accumulation beneath the stability zone, assuming there to be a minimum in the gas solubility curve. Our calculations of gas solubility show that a minimum is present below the RMHSZ, despite low geothermal gradients on the Nile fan, due to downward-increasing pore water salinities (as recorded at DSDP/ODP sites). We hypothesise the central Nile fan to contain a gas hydrate system driven by the upwelling of pore fluids rich in dissolved methane, at rates that result in: a) a discontinuous BSR beneath gas hydrate deposits; b) salt exclusion at the basal 3-phase boundary which rises to seabed along hydrate-choked vents (chimneys and/or faults). This model of a stability zone containing widely distributed phase-boundary vents is of interest as it implies a system in which temporal fluid fluxes, and perhaps pore pressures, may be regulated by seepage. A gas hydrate system containing vents that connect the basal phase boundary to seabed has been argued to be more sensitive to climate forcing from above. The abundance of seeps on the central Nile fan raises the question whether, by facilitating fluid venting to seabed, it might also be less likely to to result in slope failures? This is of particular interest on the Nile fan as it contains a stratigraphic record of recurrent large-scale failures and has experienced large changes in bottom water temperatures during glacial-interglacial climate cycles that imply basin-wide changes gas hydrate stability. Modeling of the RMGHZ for the last glacial maximum (LGM : -140 m sea level, -4˚C bottom waters) suggests that since deglaciation the stability zone on the Nile fan has reduced in thickness by 160 m (30%) at all water depths below about 800 m. Thus if gas hydrate dissociation during glacial to interglacial transitions can trigger slope failures, the Nile fan is an ideal location for it to take place. The question is whether it has occured, or if transient fluid fluxes were instead relieved by fluid venting? We aim to address the above questions during a forthcoming Franco-Brazilian campaign, focused on understanding the nature and dynamics of the gas vents. The Nile fan contains a gas hydrate system that is associated with widespread gas venting and has experienced large temporal changes, making it an ideal location to explore the linkages between gas hydrate stability, fluid flux and sediment failure over glacial-interglacial timescales.