The thermal regime around buried submarine high voltage cables

The expansion of offshore renewable energy infrastructure and the need for trans-continental shelf power transmission require the use of submarine High Voltage (HV) cables. These cables have maximum operating surface temperatures of up to 70°C and are typically buried 1–2 m beneath the seabed, within the wide range of substrates found on the continental shelf. However, the heat flow pattern and potential effects on the sedimentary environments around such anomalously high heat sources in the near surface sediments are poorly understood. We present temperature measurements from a 2D laboratory experiment representing a buried submarine HV cable, and identify the thermal regimes generated within typical unconsolidated shelf sediments—coarse silt, fine sand and very coarse sand. We used a large (2 × 2.5 m) tank filled with water-saturated spherical glass beads (ballotini) and instrumented with a buried heat source and 120 thermocouples, to measure the time-dependent 2D temperature distributions. The observed and corresponding Finite Element Method (FEM) simulations of the steady state heat flow regimes, and normalised radial temperature distributions were assessed. Our results show that the heat transfer and thus temperature fields generated from submarine HV cables buried within a range of sediments are highly variable. Coarse silts are shown to be purely conductive, producing temperature increases of >10°C up to 40 cm from the source of 60°C above ambient; fine sands demonstrate a transition from conductive to convective heat transfer between c. 20°C and 36°C above ambient, with >10°C heat increases occurring over a metre from the source of 55°C above ambient; and very coarse sands exhibit dominantly convective heat transfer even at very low (c. 7°C) operating temperatures and reaching temperatures of up to 18°C above ambient at a metre from the source at surface temperatures of only 18°C. These findings are important for the surrounding near surface environments experiencing such high temperatures and may have significant implications for chemical and physical processes operating at the grain and sub-grain scale; biological activity at both micro-faunal and macro-faunal levels; and indeed the operational performance of the cables themselves, as convective heat transport would increase cable current ratings, something neglected in existing standards

Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland)

Alkali basaltic diatremes such as Elie Ness (Fife, Scotland) expose a range of volcanic lithofacies that points to a complex, multi-stage emplacement history. Here, basanites contain phenocrysts including pyrope garnet and sub-calcic augites from depths of ~60km. Volcanic rocks from all units, pyroclastic and hypabyssal, are characterised by rare earth element (REE) patterns that show continuous enrichment from heavy REE (HREE) to light REE (LREE), and high Zr/Y that are consistent with retention of garnet in the mantle source during melting of peridotite in a garnet lherzolite facies. Erupted garnets are euhedral and unresorbed, signifying rapid ascent through the lithosphere. The magmas also transported abundant pyroxenitic clasts, cognate with the basanite host, from shallower depths (~35–40km). These clasts exhibit wide variation in texture, mode and mineralogy, consistent with growth from a range of compositionally diverse melts. Further, clinopyroxene phenocrysts from both the hypabyssal and pyroclastic units exhibit a very wide compositional range, indicative of polybaric fractionation and magma mixing. This is attributed to stalling of earlier magmas in the lower crust — principally from ~22 to 28km — as indicated by pyroxene thermobarometry. Many clinopyroxenes display chemical zoning profiles, occasionally with mantles and rims of higher magnesium number (Mg#) suggesting the magmas were mobilised by juvenile basanite magma. The tuffs also contain alkali feldspar megacrysts together with Fe-clinopyroxene, zircon and related salic xenoliths, of the ‘anorthoclasite suite’ — inferred to have crystallised at upper mantle to lower crustal depths from salic magma in advance of the mafic host magmas. Despite evidence for entrainment of heterogeneous crystal mushes, the rapidly ascending melts experienced negligible crustal contamination. The complex association of phenocrysts, megacrysts and autoliths at Elie Ness indicates thorough mixing in a dynamic system immediately prior to explosive diatreme-forming eruptions.Clough and Mykura Fund of the Geological Society of Edinburgh; Timothy Jefferson Fund of the Geological Society of Londo

Segregation of particles in a tapered fluidized bed

Tapered fluidized beds are widely used in industrial operations to fluidize a wide range of particle sizes, and are thought to induce relatively strong particle mixing. Like their straight-sided counterparts, tapered fluidized beds are often considered as a homogeneous emulsion phase through which bubbles propagate. However, it has been shown that gas flow through the dense-phase of tapered beds is heterogeneous, generating a central fluidized core and unfluidized peripheral regions. When polydisperse particle mixtures are fluidized in a tapered bed, the structure becomes much more complex once the particles reach the minimum fluidization velocity (Umf), and a variety of segregation structures are generated. The aim of this study is to investigate how the two different types of structure (i.e. flow structure owing to the tapered shape of the bed and the segregation structures) interact and affect each other. Experiments were performed in a tapered (? = 15°) planar bed using bidisperse mixtures of ballotini. The growth and extent of flow and segregation structures were measured, as well as the fabrics observed under different conditions. Under most conditions, the structure of the tapered bed is unaffected by the bed composition and segregation structures that form. An exception is at flow-rates just in excess of Umf when vertical columns of particles form, completely displacing larger-scale flow structures.The time scale for particle turnover in the central fluidized region is much shorter than that of particles captured within the peripheral regions. However, at sufficiently high gas flow-rates, uniform mixing can take place across the entire width of the bed

Supplementary files for Gernon et al. (2023) Kimberlite ascent by rift-driven disruption of cratonic mantle keels

Supplementary files for Gernon et al. (2023) Kimberlite ascent by rift-driven disruption of cratonic mantle keels This directory contains datasets, GIS files, Uninet model files and R scripts used to generate the figures in Gernon et al. (2023) Kimberlite ascent by rift-driven disruption of cratonic mantle keels. </span

Eruptive history of an alkali basaltic diatreme from Elie Ness, Fife, Scotland

The Elie Ness diatreme (Fife, Scotland) is an ideal place to study the internal architecture and emplacement processes of diatremes. Elie Ness is one of approximately 100 alkali basaltic diatremes and intrusions in the East Fife area, emplaced during Upper Carboniferous to Early Permian times into an extensive rift system in the northern Variscan foreland. Within the diatreme, seven lithofacies and three lithofacies associations (LFAs 1-3) are recognised. Field, petrographic and geochemical studies demonstrate that the diatreme experienced a protracted history of eruption and infill, initially driven by volatile expansion and later by magma-water interaction. Massive lapilli tuffs of LFA 1 contain abundant highly vesicular juvenile scoria and magma-coated clasts, which are best explained by a magmatic origin for the early explosive eruptions. On a large-scale, the tuffs are well mixed and locally exhibit small-scale degassing structures attributed to fluidisation processes occurring within the diatreme fill. The occurrence of abundant volcaniclastic autoliths and megablocks within LFA 1 can be explained by subsidence of volcaniclastic strata from the maar crater and upper diatreme during emplacement. Pyroclastic density current deposits of LFA 2 form a series of continuous sheets across the diatreme, some of which may have originated from phreatomagmatic explosions in a neighbouring vent. We attribute the overall bedding pattern to a combination of primary volcanic processes and post-depositional folding related to movement along an adjacent fault. Minor steeply inclined breccias and tuffs of LFA 3 cross-cut the LFA 2 succession and are interpreted as late-stage volcaniclastic dykes and conduits, signalling the final phase of eruptive activity at Elie Ness. The study offers new insights into the volcanic evolution of diatremes fed by low viscosity, alkali-rich magmas

Variations of olivine abundance and grain size in the Snap Lake kimberlite intrusion, Northwest Territories, Canada: A possible proxy for diamonds

The Snap Lake hypabyssal kimberlite is a 15 degrees inclined intrusion that intrudes Archaean rocks of the Slave craton. Mapping has identified two main lithofacies within the intrusion, olivine-rich kimberlite (ORK) and olivine-poor kimberlite (OPK). Extensive bulk sampling has demonstrated that diamonds vary in both abundance and size across the intrusion. Since fresh forsteritic olivine and diamond possess similar densities they can be expected to be hydraulically equivalent. This study investigates the local scale variation of olivine grain size and content using image analysis, and whether olivine can be used as a proxy for diamond concentration and size. Direct measurement of diamonds is made difficult by extremely low concentrations of around 0.4 ppm and by the inability of the sampling methodology to sample selectively. Olivines from the two kimberlite varieties possess distinctly different crystal size distributions and this, together with contrasting groundmass textures, mineralogy and phlogopite compositions, indicates that the OPK and ORK represent two distinct batches of kimberlite magma. The general lack of sorting of olivine grains and the presence of decussate phlogopite textures in the ORK indicate that this magma crystallized rapidly, but was probably preceded by the OPK which had a more complex crystallization history as revealed by complex zoning patterns of its phlogopite grains. A methodology for comparing olivine and diamond size distributions has been developed, and a strong correlation between diamond and olivine size distributions is demonstrated. The correlation is used to demonstrate that diamond size distributions can be predicted from the proportions of OPK and ORK that comprise each bulk sample. It can be concluded that the two textural varieties of kimberlite possess distinctly different olivine (and diamond?) populations and that olivines are a good proxy for understanding diamond distributions at a local scale in the Snap Lake intrusion. (C) 2009 Elsevier B.V. All rights reserve

Effect of Sediment Properties on the Thermal Performance of Submarine HV Cables

The thermal performance of high-voltage submarine cables is controlled by the effectiveness of heat transfer through the sediments in which they are buried. This paper assesses the influence of sediment parameters on this heat transfer using finite-element simulations. Particular attention is paid to the role of convective heat transfer, with a method being developed to define installation scenarios where this may become significant. This paper shows that the permeability is the dominant factor and can have a significant impact on cable temperatures, even though it is typically neglected in conventional models

Pyroclastic flow deposits from a kimberlite eruption : the Orapa South Crater, Botswana.

The Orapa Diamond Mine (Republic of Botswana) exposes a bi-lobate kimberlite pipe that erupted during the Late-Cretaceous epoch (~ 93 Ma) through Archaean basement and volcano-sedimentary rocks of the Karoo Supergroup. Geological mapping of the crater zone of the South Pipe has revealed a 15-25 m thick in-situ kimberlite pyroclastic flow deposit. The pyroclastic deposit fills in the crater and completely drapes lower units, indicating that the parent flow originated from an adjacent kimberlite pipe. The unit comprises a basal coarse lithic concentration layer exhibiting imbricated clasts, which grades upwards into massive poorly sorted lapilli tuff. The tuff contains abundant sub-vertical degassing structures defined by lithic enrichment and depletion in fine-grained material. Degassing structures commonly emanate from blocks in the basal layer. The presence of degassing structures and a coarse basal layer distinguishes this deposit from pipe-filling massive volcaniclastic kimberlite, which is typically homogeneous in terms of texture and clast size over distances on the order of 100s of metres. Studies of the thermal remanent magnetism in basalt clasts from the deposit, together with serpentine-diopside assemblages, indicate that it was emplaced at elevated temperatures on the order of 200-440 °C, consistent with deposition from a pyroclastic flow. The lithofacies characteristics can be explained by the interaction of the pyroclastic flow with the complex topography of a pre-existing crater. © 2009 Elsevier B.V. All rights reserved