Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo process involving the magnetorotational instability (MRI). High-resolution simulations exhibit a tendency towards statistical self-organization of MRI dynamo turbulence into large-scale cyclic dynamics. Understanding the physical origin of these structures, and whether they can be sustained and transport angular momentum efficiently in astrophysical conditions, represents a significant theoretical challenge. The discovery of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has notably served to highlight the role of turbulent magnetic diffusion in the seeming decay of the dynamics at low magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a common regime in accretion disks. The connection between these simple structures and the statistical organization reported in turbulent simulations remained elusive, though. Here, we report the numerical discovery in moderate aspect ratio Keplerian shearing boxes of new periodic, incompressible, three-dimensional nonlinear MRI dynamo solutions with a larger dynamical complexity reminiscent of such simulations. These "chimera" cycles are characterized by multiple MRI-unstable dynamical stages, but their basic physical principles of self-sustainment are nevertheless identical to those of simpler cycles found in azimuthally elongated boxes. In particular, we find that they are not sustained at low Pm either due to subcritical turbulent magnetic diffusion. These solutions offer a new perspective into the transition from laminar to turbulent instability-driven dynamos, and may prove useful to devise improved statistical models of turbulent accretion disk dynamos.Comment: 12 pages, 8 figures, submitted to A&

Numerical study on splash zone crossing with GRP protection cover.

For oil and gas fields moving further offshore, it has been more common to use a subsea production system or a combination of rig and subsea structures. These subsea structures must be overtrawlable if they are not marked with a buoy or vessel. One solution for overtrawlable subsea structures is to use a protection cover. This study focuses on GRP pipeline protection covers, which are highly weather-sensitive due to their low weight and large surface area. Installation of subsea protective covers typically involves overboard, lowering through the splash zone, and lowering to the seabed. This study will focus on the lowering through the splash zone, which is usually considered one of the most critical phases of a deployment operation. The hydrodynamic property for the cover is calculated based on DNV-RP-H103 simplified methods [1]. The vessel’s properties are collected with a diffraction analysis program that calculates the responses and loading for wet bodies using potential flow theory. Hence, body results are generated for: damping, added mass, displacement RAOs, and load RAOs. In addition, the calculated pressure in the fluid will yield the sea state RAOs for the fluid pressure. The vessel was then implemented into the numerical time-domain program with the cover. For the numerical methods, two sensitivity studies are conducted to evaluate the number of wave seeds and vessel motion to be applied to assess sea states. First, the numbers of seeds selected were based on the extreme values for the sling tension. Then, analyzing the extreme values with a statistical method and the cumulative average for minimum and maximum tension. The extracted results indicated that 30 seeds were enough to yield reliable predictions of the sea states. The same procedure was applied for the vessel motion as for the number of wave seeds. In addition, the simulation demo file was closely investigated to observe any differences between the motions. From the extracted extreme values, the difference between the motion where slightly more conservative for the coupled motion. However, the uncoupled vessel motion was selected to assess the allowable sea states. The uncoupled motion would better represent the real-life, where for the coupled vessel it was observed large yaw motions for longer wave periods In the assessment of the sea states, three simulation cases were selected to investigate the hydrodynamic forces during the splash zone crossing. Firstly, the different lifting angles of the cover made it possible to adjust the waterplane area. Secondly, evaluate the vessel position for wave directions of 165 and 180 degrees. Lastly, use the vessel to disrupt the incoming waves generating a shielding effect for the cover. Concerning the operational criteria, the extreme values were fitted to a Gumbel probability paper to assess the allowable sea states. The results indicated that the shielding effect on the cover would yield the best applicable sea states for all tested methods. For the different tested lifting angles of the cover. The cover with the lowest waterplane area resulted in the highest sea states due to lower hydrodynamic forces on the cover. Lastly, for the two tested wave directions. The vessel for head waves yielded slightly better allowable sea states. The increased roll motion for the wave direction at 165 degrees resulted in larger slamming loads and lower sling tensions for the cover

Transmission and Reflection of Bose-Einstein Condensates Incident on a Gaussian Potential Barrier

We investigate how Bose-Einstein condensates, whose initial state is either irrotational or contains a single vortex, scatter off a one-dimensional Gaussian potential barrier. We find that for low atom densities the vortex structure within the condensate is maintained during scattering, whereas at medium and high densities, multiple additional vortices can be created by the scattering process, resulting in complex dynamics and disruption of the atom cloud. This disruption originates from two different mechanisms associated respectively with the initial rotation of the atom cloud and the interference between the incident and reflected matter waves. We investigate how the reflection probability depends on the vorticity of the initial state and on the incident velocity of the Bose-Einstein condensate. To interpret our results, we derive a general analytical expression for the reflection coefficient of a rotating Bose-Einstein condensate that scatters off a spatially-varying one-dimensional potential.Comment: 9 pages, 9 figure

Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows

The nature of dynamo action in shear flows prone to magnetohydrodynamic instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to the three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a large-scale axisymmetric toroidal magnetic field and non-axisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this large scale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard mean-field dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of time-dependent coherent large-scale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understand the transitional and statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review

Numerical study on deployment of subsea template using coupled and uncoupled model

This study compares deployment of a subsea template simulated as a coupled model and as an uncoupled model in the time domain simulation software Orcaflex. Defining vessel motion as prescribed simplifies the model and will therefore also decrease the simulation time. Models with predefined vessel motions are called uncoupled models. Vessel motion in a coupled model is a continuously calculated reaction to the forces acting on the vessel. Some software might struggle to run coupled models. The deployment simulations are narrowed down to focus on the incident where the template crosses the splash zone when lifted with an offshore construction vessel. Noticeable differences between the allowable sea state results are observed from the two different simulation methods. Running the time domain simulation as an uncoupled model gives lower allowable sea states than the results from the coupled time domain simulation model.publishedVersio

Comparative study on two deployment methods for large subsea spools

The demand for subsea spool deployment is increasing with the expansion of offshore projects. For a project to install multiple spools, different deployment methods can be used. The choice of method may influence the safety and the total cost of the project. Thus, it is important to evaluate different deployment methods in the planning phase. This study addresses weather window analysis of two deployment methods for large subsea spools. The purpose is to compare the efficiency of the two methods in terms of total installation time for projects with different numbers of spools. Numerical modeling and time-domain simulations of the critical activities are carried out. The simulations together with the operational criteria provide the allowable sea states, which are the key input for weather window analysis. Hindcast data from a site in the Barents Sea are used for weather window analysis. The total installation time is compared for various months, different total numbers of spools and transportation durations. The influence of the possible increase of the allowable sea states for the critical activity on the total installation time is also evaluated. Through the comparative studies, recommendations to select the proper deployment method for different situations are provided.publishedVersio