Danish North Sea Research Articles

Illite distribution and morphology explaining basinal variations in reservoir properties of Upper Jurassic sandstones, Danish North Sea

Abstract Growth of fibrous illite may destroy an otherwise promising sandstone reservoir, even sandstones characterized by microquartz coatings. Therefore detailed petrographical and diagenetic investigations were performed in order to understand the factors that control illite precipitation and other factors affecting the reservoir properties in the Upper Jurassic Heno Formation. The Heno Formation is encountered in large parts of the Danish Central Graben, where its lower part – the Gert Member – was mainly deposited in back-barrier environments and its upper part – the Ravn Member – was deposited in a shoreface environment. In total, 215 core samples from 15 wells were investigated petrographically by optical and scanning electron microscopy. This was supplemented by plug porosity and permeability measurements and X-ray diffraction of bulk and clay fraction. The investigations show, among other things, that microquartz coatings may help to preserve porosity and permeability, whereas the permeability is reduced when microquartz occurs combined with fibrous illite. Illite precipitation is largely governed by the amounts of K-feldspar present during burial. Consequently, the initial abundance of K-feldspar (in the sediment source area) and its alteration during and immediately after deposition, in particular during subaerial exposure (at sequence boundaries), has a major influence on the final reservoir properties of the sandstones. The risk of illite precipitation can largely be predicted from the available sediment source material, depositional environment and burial depth.

Organofacies composition of Upper Jurassic – lowermost Cretaceous source rocks, Danish Central Graben, and insight into the correlation to oils in the Valdemar Field

Abstract The organic-rich marine shales of the Upper Jurassic - lowermost Cretaceous Farsund Formation are the main hydrocarbon source rocks for the Cretaceous chalk reservoir of the Valdemar Field (Danish North Sea). Geochemical analyses of oil extracts from core samples across the greater Valdemar Field show substantial heterogeneity of the reservoir oils, suggesting contribution from kitchen areas with source rocks of variable organofacies and thermal maturity. In order to better understand the source rock facies that charged the field, a refined organofacies characterization of the Upper Jurassic - lowermost Cretaceous marine shale source rock succession (Farsund Formation) in the Danish Central Graben was established. This organofacies characterization is based on integration of petrographic and geochemical data and demonstrates large variations in both the regional and local distribution of the organofacies at the sequence level in the shales within the Danish Central Graben. In addition, the organofacies characterization shows that the respective sequences in the succession may be represented by several organofacies at the same time. Part of the explanation to the heterogeneity in the oil types accumulated within the Valdemar Field may lie in the presence of different organofacies within the uppermost sequences (Vol-3 to Ryaz-1) in the three kitchen areas. Each organofacies presumable generates different types of oils and the variation in the relative proportions of the different organofacies likely determines the composition of the generated oils. One dimensional (1-D) maturity modelling at three pseudo-well locations, each representing a potential Farsund Formation kitchen, shows that all kitchens are in the main oil window and that mainly oil was expelled from the Volg-3 to Ryaz-1 sequences. Modelling indicates that (i) average values of TOC, HI, thickness and the kinetic model are the main controls on generation and expulsion of hydrocarbons; and (ii) by increasing these parameters, expulsion could also occur from the lower part of the succession.

SOURCE ROCK QUALITY VARIATIONS OF UPPER JURASSIC – LOWERMOST CRETACEOUS MARINE SHALES AND THEIR RELATIONSHIP TO OILS IN THE VALDEMAR FIELD, DANISH NORTH SEA

The main source rocks for the hydrocarbons at the Valdemar field (Danish North Sea) are the Upper Jurassic – lowermost Cretaceous organic‐rich marine shales of the Farsund Formation. However, geochemical analyses of retained petroleum in reservoir cores show variations in oil type and maturity which indicate a complex charging history. This paper reviews the organofacies and source rock quality variations in 55 samples of the Farsund Formation from the North Jens‐1 well (Valdemar field) within a sequence stratigraphic framework in order to discuss the source of the hydrocarbons. Petrographic and geochemical data, including biomarker analyses, were integrated in order to characterize the kerogen composition, original source rock potential and depositional environment of the Farsund Formation.The thermal maturity, source rock quality and kerogen quality all vary at the sequence level, and in general change upwards from early mature, primarily gas‐prone Type II kerogen in the Kimmeridgian Kimm‐2 and Kimm‐3 sequences to immature, highly oil‐prone sapropelic Type II kerogen in the Volg‐4 and Ryaz‐1 sequences (Volgian, Ryazanian). The kerogen has a maceral composition dominated by fluorescing amorphous organic matter (AOM) and liptodetrinite, with variable but generally minor amounts of terrigenous organic matter. The stratigraphic distribution of organic matter is similar to that in regional observations from the Danish Central Graben but minor differences occur, especially in the amount of fluorescing AOM in the Kimmeridgian sequences. The decrease in terrigenous input (vitrinite) upwards through the marine shale succession likely reflects a marine transgression of the Danish Central Graben area during Late Jurassic time. The source potential of the Upper Jurassic – lowermost Cretaceous shales in the North Jens‐1 well is generally lower than that observed regionally, including an absence of relatively organic‐rich, oil‐prone intervals in the older part of the succession which have been demonstrated to occur elsewhere in the Danish Central Graben. However, in agreement with the regional trend, back‐calculated source rock data and calculated Ultimate Expulsion Potentials show that the uppermost Volgian (Volg‐4) and Ryazanian (Ryaz‐1) sequences are the most oil‐prone intervals. The Ryaz‐1 sequence represents a condensed section formed during a period characterised by low sedimentation rates and high preservation of algal organic matter.Biomarker compositions from source rock extracts from the North Jens‐1 well cannot be directly correlated to Valdemar reservoir oils, suggesting that the mature organofacies which charged the oils are not represented in the samples from North Jens‐1.

Seaweed resources of the Baltic Sea, Kattegat and German and Danish North Sea coasts

AbstractDue to low salinity and lack of hard substrata, the Baltic Sea and Kattegat area and German and Danish North Sea coasts are characterized by a relatively low diversity of seaweeds. At the same time the areas are severely eutrophicated, which has caused extensive shifts in macroalgal communities toward opportunistic species. Unattached seaweed communities dominated byFurcellaria lumbricalis, which have been a resource for hydrocolloid production since the 1940s, have been severely reduced due to eutrophication and unsustainable harvesting and are nowadays only exploited commercially in Estonia. On the other hand, the biomass of opportunistic seaweeds of various red, green and brown algal genera has increased. They cause ecological problems, are a nuisance on many tourist beaches and constitute at the same time a potential bioresource that is so far only exploited to a limited extent for production of energy and fertilizer. Commercial seaweed cultivation is largely focused onSaccharina latissimaand still very limited, but is currently being expanded as a compensation measure for sea-based fish aquaculture. Also land-based seaweed cultivation is primarily employed for recycling of nutrients in tank animal aquaculture, but in most cases so far only on an experimental scale.

Characterization of heteroatom distributions in the polar fraction of North Sea oils using high-resolution mass spectrometry

Resins are a sub-fraction of crude oil consisting of heteroatom-containing polar compounds. These species are of key importance in oil recovery, oil fluid thermodynamic properties, refining processes and for the environmental impact of petroleum production. Due to the inherent complexity of crude oil, little is known of the absolute molecular structure of these compounds. Here we show how a combination of separation science and mass spectrometry reveal the compositional space of resins isolated from Danish North Sea crudes. By employing spectral stitching mass spectrometry, we resolve over 15,000 unique peaks, an increase of 50% compared to broadband acquisition. The results indicate that the dominant component of North Sea resins are basic nitrogencontaining compounds with small to medium sized aromatic cores. Using chemometric data treatment it becomes evident that each field displays unique profiles, and that these fractions can be used for oil-oil and oil-source correlations. Further, a more detailed knowledge of their composition will allow us to gain a better understanding of the interactions between resins, hydrocarbons and reservoir rock material. Improved understanding of rock-fluid and fluid-fluid interactions is key for development and understanding of improved recovery mechanisms for tight chalk reservoirs.

An integrated workflow for fracture characterization in chalk reservoirs, applied to the Kraka Field

Abstract Oil and gas production of tight chalk reservoirs frequently rely on the presence of natural fractures, which increases the effective permeability of the reservoirs. Fracture characterization is therefore imperative in optimizing production schemes and obtaining economically viable recovery factors. Subsurface fracture characterization is often deemed challenging as the available data is typically of varying age and quality, and represents different scales. We have developed an integrated workflow for fracture characterization in chalk to address these challenges. The workflow is based on data from borehole images, cores and seismic. These data are typically available for most chalk (and hydrocarbon) fields. The interpreted borehole image dataset contains over 17 000 manual dip picks, ensuring a statistically viable base. A total of 150 m of core is available from 3 wells. The applied 3D seismic cube covers an 8 × 5 km hydrocarbon chalk field in the Danish North Sea. In this workflow, the scale-gap between the data sets is bridged by the introduction of two ant-tracked attribute volumes, which display structural trends below the resolution of amplitude seismic. Further insight into the intricacy of subsurface fracture systems is obtained from fracture density logs, which provide an opportunity to study spatial distribution of fractures as well as a qualitative measure of fracture clustering. Cumulative density distribution plots and calculation of the variation coefficient of fracture spacing provide a more quantitative analysis of the fracture distribution. The workflow, presented here in a step-by-step manner, is a general approach applied to data from the Kraka Field of the Danish North Sea. In the Kraka Field, the usage of this integrated approach shows that the fracture pattern in this region is more complex than previously suggested; probably controlled by the regional maximum horizontal stress and salt movements.

Buried late Quaternary channel systems in the Danish North Sea – Genesis and geological evolution

The Danish North Sea has been subject to extensive seismic surveying (both 2D and 3D) during the past four decades. The 3D seismic data from the area has mainly been used for hydrocarbon exploration, but in the last decade, it has also been utilized for investigating the Quaternary sediments and especially buried incised valleys. The erosional and depositional processes involved in forming such valleys are important in paleolandscape reconstruction, but are often difficult to determine, since sediment deposited by flowing water, displays ambiguous geometric structures. In this study, we combine the analysis of infill geometries, from high-resolution 2D seismic data, with plan-view morphology analysis, from 3D seismic data, and lithological data from borehole reports, in order to constrain the origin of buried channels in the Danish North Sea. This data integration makes differentiations of a subaerial vs. subglacial origin more robust. The study focusses on analyzing a proglacial to fluvial channel system, which was initiated as a subglacial tunnel valley system during the Last Glacial Maximum, and subsequently evolved as a fluvial system after deglaciation. The channel system was active in a marsh environment during a period of subaerial exposure in the late Weichselian to early Holocene. During the Holocene transgression, the system was gradually flooded leaving estuarine tidal deposits in the underfilled channels. Once the area was completely flooded, the channel system was buried under marine sands.

Feasibility study for carbon capture utilization and storage (CCUS) in the Danish North Sea

An innovative addition to the conventional oil and gas extraction by introducing enhanced oil recovery (EOR) techniques in the Danish North Sea is something which has been sought for long time. Combining the offshore EOR technology with the carbon capture and storage (CCS) is considered to be a possibly attractive solution as the associated benefit is twofold: (1) increased oil recovery through miscible displacement via CO2 pushing the hydrocarbons out of the reservoir and (2) the disposal of a greenhouse gas such as the CO2. A feasibility study for implementing a carbon capture, storage and utilization (CCUS) project is presented in the scope of this study, in a North Sea Chalk Field which is the oldest and one of the largest oilfields in the Danish sector, both for the technical and the economic merit, by means of using a commercial reservoir simulation package. The capital (CAPEX) and operational (OPEX) expenditures associated with such a long term development project is estimated. Results suggest that around 100 million barrels of additional oil reserves can be unlocked via CO2 injection, while 40 million tones of CO2 is being trapped in the reservoir.

Bicarbonate flooding of homogeneous and heterogeneous cores from a carbonaceous petroleum reservoir

Two extensive core flooding experiments along with supplementary tests were conducted to investigate the effect of bicarbonate as a potential determining ion for improved oil recovery from Danish North Sea reservoir chalk samples. Different sequences of brines were injected to see the impact on tertiary oil recovery. The core flooding experiments were conducted using two kinds of core plugs, homogeneous and heterogeneous (fractured). As injection of bicarbonate solutions leads to increase in pressure drop across the homogeneous core plug during injection, it was inferred that permeability reduction may be caused by fines production. The same flooding scenario was repeated for a fractured core plug. The difference in recovery from homogeneous and heterogeneous core plugs was observed: a significantly higher additional recovery was obtained for the heterogeneous core plug. This effect cannot be explained just by changing the chemical interactions between oil and rock surface. Adding bicarbonate to the injection brine changed the equilibrium and caused the sulfate ion in seawater to precipitate. Mixing injection brine with seawater from secondary injection caused the fines migration in the porous space, while high pH of bicarbonate solutions causes emulsification. In the accompanying crude oil/bicarbonate brine contact tests, formation of dynamically stable emulsions was observed. It was hypothesized that the fines migration and emulsification are causes for the microscopic flow diversion to the oil saturated regions and additional recovery from the heterogeneous rocks.

Discrete-element modelling of hanging wall deformation along the D-1 fault system, Danish North Sea, with implications for gas chimney interpretations

This study uses 3D seismic mapping, NNCC attribute analysis and 2D SDEM modelling of soft-sediment deformation to analyse hanging wall deformation related to faulting across a listric fault detaching along the top of a major salt roller. The computational model experiments used the overall fault and salt-structure evolution as boundary conditions to predict the distribution of faults within the hanging wall. The modelling successfully reproduced geometries similar to the syn- and antithetic faults that are visible on seismics, but it also showed the presence of well-defined vertical deformation zones, both in the hanging wall and the foot-wall of the listric fault. In the model, salt movement controls the formation of the deformation zones. The similarity of NNCC-inferred vertical chimneys and the modelled deformation zones suggests that the subtle vertical chimneys identified in the 3D seismic data might actually be related to vertical to sub-vertical deformation zones caused by the presence and movement of the salt structure.

Meandering river deposits in sediment cores, the Middle Jurassic Alma Field, Southern Danish Central Graben

Fluvial deposits are amongst the most important terrestrial hydrocarbon reservoirs, but the complex nature of these deposits is challenging in subsurface reservoir characterisation. This study is the first detailed facies analysis of the meandering river deposits of the Middle Jurassic Alma Field situated in the southern Danish North Sea. The fluvial sandstones and their associated deposits are described and interpreted based on studies from two core sites (Alma-1X and Alma-2X). The facies analysis of the cores demonstrates the presence of three meandering river facies associations: Channel deposits, channel margin deposits and floodplain deposits. The channel deposits comprise channel thalweg and point bar sediments, the channel margin deposits include crevasse channel and crevasse splay sediments, while the floodplain deposits comprise overbank and backswamp sediments. The point bar deposits are composed of fine- to medium-grained sandstones but can contain intervals of finer grained sediments, particularly in their upper parts where they can grade into muddy sandstones or true heterolithic deposits. Preserved sand body thicknesses (channel thalweg and point bar deposits) in both Alma cores have a mean value of 2.6 m and a maximum value of 4.35 m (Alma-1X) and 6.55 m (Alma-2X). Using maximum values of channel deposit thicknesses, and assuming the preservation conditions are met, the width of the largest ancient channel belt in Alma-1X would be between 90 m and 200 m or around 900 m, depending on whether the fluvial system is mud-rich or sand-rich. The same method applied to Alma-2X gives a width of the largest channel belt between 130 m and 330 m or around 1300 m. Fluvial sediments of the Middle Jurassic Scalby Formation (north-east England) were deposited in a sandy meandering river with sedimentary characteristics corresponding to those observed in the Alma cores. Outcrop analogue investigations of this formation were carried out to examine the architecture of the fluvial facies in a two-dimensional section with emphasis on channel thalweg and point bar deposits. Combined evidence from core analysis and outcrop analogue studies suggests that the fluvial deposits in the Alma Field represent a mixed-load meandering river system with sandy point bars. The meandering river system developed on a coastal plain with overbank fines and organic-rich backswamp deposits. The mud-rich or heterolithic deposits in the upper part of the point bar facies intervals are noteworthy and could indicate markedly fluctuating discharge in a mixed-load river.

Diagenetic impact on reservoir sandstones of the Heno Formation in the Ravn-3 well, Danish Central Graben

Oil and gas production from siliciclastic reservoirs has hitherto been in the Danish Central Graben mostly from Palaeogene and Middle Jurassic sandstone. The Ravn field was the first Upper Jurassic field to start operation. The reservoir is composed of sandstone of the Heno Formation. Production takes place at a depth of 4000 m, which makes Ravn the deepest producing field in the Danish North Sea. The Heno Formation mainly consists of marine shoreface deposits, where foreshore, middle and lower shoreface sandstones constitute the primary reservoir. The results of this study of the diagenetic impact on the mineralogical composition, porosity and permeability are presented here. Microcrystalline quartz has preserved porosity in the sandstone, whereas illite, quartz overgrowth and carbonate cement have reduced both porosity and permeability.

Temporal organic facies variations of Upper Jurassic - lowermost Cretaceous source rocks in the Danish Central Graben, North Sea

The Upper Jurassic – lowermost Cretaceous marine mudstones of the Farsund Formation constitute the principal source rocks in the Danish North Sea sector with the uppermost Bo Member considered most prolific. However, the areal distribution of the Bo Member is limited and since it is thermally immature over large areas it is unlikely that this interval has generated sufficient quantities of petroleum to fill the reservoirs in the Danish sector. In search for additional source rocks potential we have investigated the lower part of the Farsund Formation using a sequence stratigraphic framework. Organic geochemical screening and petrographic analyses of core and cuttings samples have been performed to define the source potential and map the regional distribution of organic-rich sequences in the Upper Jurassic succession. The study outlines variations of both maturity, source rock quality, and kerogen quality at sequence level with an upward improvement from poor gas-prone Type II kerogen in the Kimm-1 and Kimm-3 sequences to highly oil-prone Type II kerogen in the Volg-1, Volg-4 and Ryaz-1 sequences. The kerogen is dominated by fluorescent AOM and liptinite and structural alginite in form of Leiosphaerida- and Tasmanites-type telalginites throughout the entire succession, with vertical- and regional variations in the amount of larger Tasmanites. The study shows that the shales in Volg-1 and -3 sequences are organic-rich and oil prone with TOC > 4 wt% and HI > 340 mg HC/g TOC. Particularly the Volg-1 sequence, being the thickest and laterally most widespread forms a significant supplementary source for oil in the Danish sector.

Environmental drivers of harbour porpoise fine-scale movements

Quantifying intraspecific variation in movement behaviour of marine predators and the underlying environmental drivers is important to inform conservation management of protected species. Here, we provide the first empirical data on fine-scale movements of free-ranging harbour porpoises (Phocoena phocoena) in their natural habitat. Data were obtained from six individuals, tagged in two areas of the Danish North Sea, that were equipped with Global Positioning System (GPS) and dive recorder units (V-tags). We used multi-model inference and model averaging to evaluate the relative importance of various static and dynamic environmental conditions on the movement characteristics: speed, turning angle, dive duration, dive depth, dive wiggliness (a proxy for prey chasing behaviour), and post-dive duration. Despite substantial individual differences in horizontal and vertical movement patterns, we found that all the tracked porpoises responded similar to variation in environmental conditions and displayed movements that indicate a higher likelihood of foraging behaviour in shallower and more saline waters. Our study contributes to the identification of important feeding areas for porpoises and can be used to improve existing movement-based simulation models that aim to assess the impact of anthropogenic disturbance on harbour porpoise populations.

Oil production monitoring and optimization from produced water analytics; a case study from the Halfdan chalk oil field, Danish North Sea

Produced water analysis is a direct source of information to the subsurface processes active in an oil field. The information is, however, complex and requires a multidisciplinary approach and access to multiple data types and sources to successfully unlock and decode the processes. We apply data analytics on a combined data set of water chemistry and oil and gas production data measured in the production stream from five wells in the Halfdan field. The field is produced applying extensive water injection to ensure the most efficient water sweep of the reservoir. Relationships between daily production data and water chemistry are examined with Principal Component Analysis (PCA), and systematics with respect to predictability of daily changes in the oil production from water chemistry are examined with partial least square (PLS) regression models. For each well, the water chemistry provides a high degree of predictability with respect to daily oil cut in the production stream. The results have potential for application within prediction of sweep efficiency, by-passed oil and for prediction of water break-through. Full potential, however, depend on successful implementation of water chemistry-oil production analytics into other data domains such as seismic (4D) data and well work-over data.

The rescue of seismic field data from exploration activities in the Danish North Sea

In 2009, the Danish Energy Agency informed the Geological Survey of Denmark and Greenland (GEUS) that a new legal act (Lov om fordring/Forældelsesloven) would become effective in 2011. The new act introduced a limitation period of three years on the government’s right to request data from licensees’ oil and gas activities in Denmark (www.retsinformation.dk 2007), and it became the catalyst for a major change in the standard procedure for licensees’ submission of seismic data related to exploration activities to GEUS. A consequence of the new legal act was that the Danish authorities would have to request data from the licensee (following the Consolidated Act of the Use of the Subsoil) from the licensee no later than three years after their generation or publication; otherwise the authorities would have no legal right to the data. It was emphasised by the Danish Energy Agency that GEUS would have to submit all requests for outstanding data to the licensees no later than 1 January 2011.

Structural characterization of marine nano-quartz in chalk and flint from North Sea Tertiary chalk reservoirs for oil and gas

A new type of quartz, a nano-quartz consisting of spherical particles, is assumed to have formed by crystallization in the sea during sedimentation of the chalk in the North Sea and to have remained largely intact during burial diagenesis. The presence and nature of this nano-quartz have not been identified until recently, despite the fact that this quartz is the type present in the Upper Cretaceous-Danian chalk reservoir for oil and gas in the North Sea, both in the flint and as dispersed particles. In the present work detailed structural analysis of the nano-quartz has been carried out by X-ray diffraction, solid-state 29 Si and 27 Al MAS NMR, thermal analysis including water release, IR-absorption spectroscopy, and elemental analysis supplemented with analysis of oxygen isotope composition. It is found that IV Al substitutes for Si and that VI Al probably is bonded to hydroxyls on the particle surface of the nano-quartz structures. The charge compensation of tetrahedral Al 3+ , in addition to its conventional way of compensation by formation of the local structural arrangement [AlO 4 /M + ]° defects (M + = H + , Na, K, Li), can occur at the expense of the OH − group coordinating one of the four tetrahedral Si 4+ nearest to the Al 3+ tetrahedron. The most significant feature of the North Sea nano-quartz deduced in the present investigation is the presence of [4H] Si defects, also known as hydrogarnet defects. This defect is present in up to 5% of the tetrahedral sites, whereas Al 3+ occupies less than 1% of the tetrahedral sites. Two types of distribution of the [4H] Si defects were determined. In one of them the [4H] Si defects aggregates parallel to the (0001) plane to form platelets as cracks with hydroxylated surfaces on both sides. The second type of [4H] Si defect occurs in the form of isolated tetrahedral vacant sites.The formation of the aggregated [4H] Si platelets lying in the (0001) plane mostly increases the c parameters of the structure, whereas the isolated [4H] Si defects and K+Na impurities contribute to increasing the a parameters. The remarkable correlation of the positional distribution of the samples revealed from the relationships between a and c parameters and between amount of OH − groups responsible for formation of [4H] Si defects and a and c parameters can be considered as evidence for the validity of the structural formulas and, in general, of the main structural features of the studied samples. The unusually high content of [4H] Si defects in the nano-quartz samples may be related to their formation by precipitation in waters of the Danish North Sea.

Vaporization of Crude Oil by Supercritical CO2 at Different Temperatures and Pressures: Example from Gorm Field in the Danish North Sea

The effect of temperature on supercritical carbon dioxide (SC-CO2) extraction of crude oil from the Gorm oil field of the North Sea was investigated at temperatures of 40–70 °C and pressures of 20–60 MPa. MALDI-TOF, GC-MS, and NMR analyses were conducted in order to characterize the oil. Generally, the total recovery and vapor recovery increased, while the liquid recovery decreased, when the temperature was increased from 40 to 70 °C. At lower temperatures (40 and 50 °C), the liquid recovery increased from a minimum of 10% at 30 MPa to a maximum of 60% at 60 MPa, whereas the vapor recovery was 16% on average over the entire pressure range. In contrast, at higher temperatures (60 and 70 °C), the liquid recovery was 20% on average at all pressures, while the vapor recovery increased from 20% at 30 MPa to 58% at 60 MPa on average. This sharp increase of vapor recovery at higher pressures signified retrograde vaporization. This observed phenomenon suggested that the extraction of some types of oil from high-pressure reservoirs could be achieved by vaporization using SC-CO2.

Butanol fermentation of the brown seaweed Laminaria digitata by Clostridium beijerinckii DSM-6422

Seaweed represents an abundant, renewable, and fast-growing biomass resource for 3rd generation biofuel production. This study reports an efficient butanol fermentation process carried out by Clostridium beijerinckii DSM-6422 using enzymatic hydrolysate of the sugar-rich brown seaweed Laminaria digitata harvested from the coast of the Danish North Sea as substrate. The highest butanol yield (0.42g/g-consumed-substrates) compared to literature was achieved, with a significantly higher butanol:acetone-butanol-ethanol (ABE) molar ratio (0.85) than typical (0.6). This demonstrates the possibility of using the seaweed L. digitata as a potential biomass for butanol production. For the first time, consumption of alginate components was observed by C. beijerinckii DSM-6422. The efficient utilization of sugars and lactic acid further highlighted the potential of using this strain for future development of large-scale cost-effective butanol production based on (ensiled) seaweed.

Sensitivity of Risk-Based Maintenance Planning of Offshore Wind Turbine Farms

Inspection and maintenance expenses cover a considerable part of the cost of energy from offshore wind turbines. Risk-based maintenance planning approaches are a powerful tool to optimize maintenance and inspection actions and decrease the total maintenance expenses. Risk-based planning is based on many input parameters, which are in reality often not completely known. This paper will assess the cost impact of this incomplete knowledge based on a case study following risk-based maintenance planning. The sensitivity study focuses on weather forecast uncertainties, incomplete knowledge about the needed repair time on the site as well as uncertainties about the operational range of the boat and helicopter used to access the broken wind turbine. The cost saving potential is estimated by running Crude Monte Carlo simulations. Furthermore, corrective and preventive (scheduled and condition-based) maintenance strategies are implemented. The considered case study focuses on a wind farm consisting of ten 6 MW turbines placed 30 km off the Danish North Sea coast. The results show that the weather forecast is the uncertainty source dominating the maintenance expenses increase when considering risk-based decision-making uncertainties. The overall maintenance expenses increased by 70% to 140% when considering uncertainties directly related with risk-based maintenance planning.