Ostgaard N.,University of Bergen |
Laundal K.M.,Teknova |
Juusola L.,University of Bergen |
Asnes A.,Bergen Oilfield Services |
And 3 more authors.
Geophysical Research Letters | Year: 2011
The auroral substorm display in the conjugate hemispheres offers a unique tool to understand how the Earth's plasma and magnetic environment respond to changes in the solar wind and the interplanetary magnetic field (IMF). Earlier studies have demonstrated that substorm onset locations in the two hemispheres are systematically displaced due to the orientation of the IMF, but it is still a controversy which IMF parameter is most important. We have analysed more than 6600 substorms identified from global auroral images by Polar UVI from years 1996-2000 plus 2007 and IMAGE FUV from years 2000-2005. We find very strong statistical support for earlier conjugate auroral imaging observations, according to which the IMF clock angle, thetac, organizes the average substorm onset locations in both hemispheres. The IMF thec control is a manifestation of dayside/lobe reconnection geometry and magnetic tension on open field lines before tail reconnection resulting in closed field lines with asymmetric footpoints for all thec angles. By organizing the average substorm locations by the IMF By component only we also find statistical significance. The relation is not linear, as reported earlier, but reveals saturation effects that can be explained by the non-uniform penetration of IMF By into the closed magnetosphere. Copyright 2011 by the American Geophysical Union.
Halvorsen S.A.,Teknova |
Mjaavatten A.,Yara International |
ECMOR 2010 - 12th European Conference on the Mathematics of Oil Recovery | Year: 2010
Statoil has developed a model with short computational time to predict the rate dependent gas/oil ratio (GOR) from a horizontal well. The oil flow towards the wellbore is based on a one-dimensional model by Konieczek. The model performs remarkably well for medium time production optimization (weeks, months), while the predictions during the first days after a large change in the production can be poor. An improved one-dimensional model for the flow towards the wellbore is proposed, where the oil flow is treated as a superposition of three terms: 1) Radial flow towards the wellbore and towards a mirror well. 2) Flow to correct for modified boundary conditions due to the radial flows. 3) Flow due to height variations of the gas/oil contact (GOC). The new model takes care of the current short term and near-well deficiencies: Effect of 2D flow close to the wellbore, gas breakthrough due to viscous gas fingering, and horizontal/vertical anisotropy. Based on analysis and preliminary testing the new model should have equally good medium and long term capabilities and considerably improved short term and near-well behaviour, compared to the present implementation.
Ostgaard N.,University of Bergen |
Geophysical Monograph Series | Year: 2012
In this chapter, we first give a short introduction to auroral conjugate studies since the 1960s. We do not intend to give a review of the earlier findings because such papers already exist, but we will describe in some detail new results from the last couple of years based on global imaging in the UV wavelengths by the IMAGE and Polar satellites. We review the results from a paper showing how the interplanetary magnetic field (IMF) clock angle, θc, or IMF By can be used to organize the substorm locations in the conjugate hemispheres and how they give slightly different information. Then, we report a study where complete asymmetric aurorae were observed. Based on the results from these two papers, we discuss the interhemispheric currents that can be associated with these observations. We present three candidates for producing such currents that are related to IMF orientation, By and Bx, and tilt angle. The three candidates are (1) the penetration of IMF θc or By into the closed magnetosphere, (2) the differences in solar wind dynamo efficiency due to IMF Bx and tilt angle, and (3) the conductivity differences due to tilt angle. We claim that all these candidates are consistent with our recent observations. © 2012. American Geophysical Union. All Rights Reserved.
Reistad J.P.,University of Bergen |
Reistad J.P.,Birkeland Center for Space Science |
Ostgaard N.,University of Bergen |
Ostgaard N.,Birkeland Center for Space Science |
And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013
We have investigated a data set of 19 h of simultaneous global conjugate auroral imaging from space. The data set consists of 10 sequences with durations from 1 to 5 h during active geomagnetic conditions (average AE ∼ 400 nT). We have identified 15 features (including two presented earlier) of auroral forms that appear mainly in one hemisphere, and we define this as non-conjugate aurora. Three generator mechanisms has been suggested for producing interhemispheric currents and non-conjugate aurora: (1) Hemispherical differences in solar wind dynamo efficiency due to interplanetary magnetic field (IMF) Bx and dipole tilt angle leading to asymmetric region 1 currents in the two polar hemispheres, (2) interhemispheric currents induced by the penetration of IMF By into the closed nightside magnetosphere, and (3) hemispheric differences in ionospheric conductivity controlled by the dipole tilt angle inducing interhemispheric currents on closed field-lines. We want to find out if our observations are consistent with these mechanisms. Our analysis shows that five features were consistent with the IMF By penetration mechanism, seven features consistent with the solar wind dynamo mechanism, three features consistent with the conductivity mechanism, and two features could not be explained by any of the three suggested mechanisms. Because two features were consistent with two different mechanisms, the numbers add up to 17 although the total number of features is 15. The analysis also shows the expected correlation between the magnitude of the longitudinal shift of conjugate points, ΔMLT, and the occurrence of non-conjugate aurora consistent with the By mechanism. Key Points Our observations are consistent with the three suggested generator mechanisms Observed By asymmetries occurs during DeltaMLT > 1 hour SW dynamo observations consistent with statistical studies related to IMF Bx ©2013. American Geophysical Union. All Rights Reserved.
Pierobon L.,Technical University of Denmark |
Kandepu R.,Teknova |
Haglind F.,Technical University of Denmark
ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) | Year: 2012
With increasing incentives for reducing the CO2 emissions offshore, optimization of energy usage on offshore platforms has become a focus area. Most of offshore oil and gas platforms use gas turbines to support the electrical demand on the platform. It is common to operate a gas turbine mostly under part-load conditions most of the time in order to accommodate any short term peak loads. Gas turbines with flexibility with respect to fuel type, resulting in low turbine inlet and exhaust gas temperatures, are often employed. The typical gas turbine efficiency for an offshore application might vary in the range 20-30%. There are several technologies available for onshore gas turbines (and low/medium heat sources) to convert the waste heat into electricity. For offshore applications it is not economical and practical to have a steam bottoming cycle to increase the efficiency of electricity production, due to low gas turbine outlet temperature, space and weight restrictions and the need for make-up water. A more promising option for use offshore is organic Rankine cycles (ORC). Moreover, several oil and gas platforms are equipped with waste heat recovery units to recover a part of the thermal energy in the gas turbine off-gas using heat exchangers, and the recovered thermal energy acts as heat source for some of the heat loads on the platform. The amount of the recovered thermal energy depends on the heat loads and thus the full potential of waste heat recovery units may not be utilized. In present paper, a review of the technologies available for waste heat recovery offshore is made. Further, the challenges of implementing these technologies on offshore platforms are discussed from a practical point of view. Performance estimations are made for a number of combined cycles consisting of a gas turbine typically used offshore and organic Rankine cycles employing different working fluids; an optimal media is then suggested based on efficiency, weight and space considerations. The paper concludes with suggestions for further research within the field of waste heat recovery for offshore applications. Copyright © 2012 by ASME.
Strater H.,Carl von Ossietzky University |
Knabe S.,Carl von Ossietzky University |
Meyer T.J.J.,Teknova |
Bauer G.H.,Carl von Ossietzky University
Progress in Photovoltaics: Research and Applications | Year: 2013
Fluorescence collectors (FSCs) offer the possibility to collect and concentrate direct and ambient light and are, thereby, an option to decrease the cost of high-efficiency solar modules. So far, scattering, absorption and reabsorption of the incoming and fluorescence light cannot be discriminated completely in experiments. Thus, the contribution to the discrepancy of experimentally achieved efficiencies from the theoretical one has not been identified. To understand the propagation of the fluorescence light by illumination at a certain distance from the edge of the FSC, we have developed a two-dimensional analytical model based on geometrical optics that reproduces the experimental results. Under certain conditions for each distance from the edge regimes of detection angles exist for which no fluorescence light of the edge occurs. Copyright © 2011 John Wiley & Sons, Ltd.
Society of Petroleum Engineers - SPE Digital Energy Conference and Exhibition 2013 | Year: 2013
During the last decade, wired pipes telemetry has emerged as a prevailing alternative to mud pulse telemetry. The current data rate of wired pipes is 57,000 bts and it is expected that this will increase by 20 orders of magnitude to reach 1 Mbts. Such an exponential increase of the amount of data poses serious challenges when it comes to decision making in drilling operations. Ironically enough, an overflood of data, can provide a poverty of attention . Appropriate methods for handling the explosion of data in drilling operations should be developed based on the synergy of interdisciplinary science ranging from artificial intelligence to physics. To take full advantage of wired pipe, such methods should be in place in order to ensure an optimal usage of the information for reducing the non-productive time and ensuring safe operations. In fact, we can resume the ultimate aim of a drilling operation in simple words: drilling fast and safe. In this position paper, we delineate a set of modules that are a part of a comprehensive decision support system which interface the wired data measurements so that to automatize many of the manual operations in drilling. Ongoing work is being performed to tackle each of the modules in details. Copyright 2013, Society of Petroleum Engineers.