Validation of COSMIC-2-Derived Ionospheric Peak Parameters Using Measurements of Ionosondes

Although numerous validations for the ionospheric peak parameters values (IPPVs) obtained from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) have been conducted using ionosonde measurements as a reference, comprehensive evaluations of the quality of the COSMIC-2 data are still undesirable, especially under geomagnetic storm conditions. In this study, the IPPVs measured by ionosondes (Ramey, Boa Vista, Sao Luis, Jicamarca, Cachoeira Paulista, and Santa Maria) during the period October 1, 2019 to August 31, 2021, are used to evaluate the quality of COSMIC-2 data over low-latitude regions of the Americas. The results show that the NmF2 (hmF2) from COSMIC-2 agrees well with the ionosonde measurements, and the correlation coefficients for the two sets of data at the above six stations are 0.93 (0.84), 0.91 (0.85), 0.91 (0.88), 0.88 (0.79), 0.96 (0.83), and 0.96 (0.87), respectively. The data quality of COSMIC-2 derived NmF2 is largely dependent on geomagnetic latitude. It was also found that NmF2 derived from COSMIC-2 tends to be underestimated over the stations in Boa Vista and Cachoeira Paulista, which are close to the crests of the equatorial ionization anomaly (EIA), whilst that of the other stations is slightly overestimated. A comparison between COSMIC-measured and ionosonde-derived hmF2 indicates that the former is systematically higher than the latter. In addition, the differences in the two NmF2 datasets derived from COSMIC-2 and ionosonde measurements at night are generally smaller than those of daytime, when the EIA is well developed, and vice versa for hmF2, whose RMSE is slightly smaller during daytime (with the exception of Ramey). Furthermore, NmF2 obtained from COSMIC-2 is shown to perform best in summer at Ramey, Boa Vista, Sao Luis, and Santa Maria, best in winter at Jicamarca and Cachoeira Paulista. Finally, the COSMIC-2 electron densities capture the ionospheric dynamic enhancements under a moderate geomagnetic storm condition very well.

Influence of an Integral Heave Plate on the Dynamic Response of Floating Offshore Wind Turbine Under Operational and Storm Conditions

The hydrodynamic performance of the floating foundation for offshore wind turbines is essential to its stability and energy harvesting. A semi-submersible platform with an integral heave plate is proposed in order to reduce the vertical motion responses. In this study, we compare the heave, pitch, and roll free decay motions of the new platform with a WindFloat-type platform based on Reynolds-Averaged Navier-Stokes simulations. The differences of the linear and quadratic damping properties between these platforms are revealed. Then, a FAST (Fatigue, Aerodynamics, Structures, and Turbulence) model with the consideration of fluid viscosity effects is set up to investigate the performance of the new platform under storm and operational conditions. The time-domain responses, motion spectra, and the mooring-tension statistics of these two platforms are evaluated. It is found that the integral heave plate can increase the viscous hydrodynamic damping, significantly decrease the heave and pitch motion responses, and increase the safety of the mooring cables, especially for the storm condition.

Numerical study on ocean response to storm in Arctic Ocean

<p>A one-column ocean model was used to study the ocean response to storm in Arctic Ocean. We design a number of idealized experiments by different surface forcing and initial condition. The results show the phenomena of mixed-layer extension during the storm events. The intensification of surface current reflects the momentum flux injected from wind and ice movement. Furthermore, by changing the surface heat and freshwater fluxes, the dependences of mixed-layer variation on surface forcing are discussed. Finally, the numerical tests on different initial conditions show how the pre-storm condition affects the ocean response to storm. The results therefore reveal the dependence of mixed-layer extension on the initial stratification.</p>

Moderate geomagnetic storm condition, WAAS Alerts and real GPS positioning quality

The most significant part of the Wade Area Augmentation System (WAAS) integrity data consists of the User Differential Range Error (UDRE) and the Grid Ionospheric Vertical Error (GIVE). WAAS solutions are not completely appropriate to determine the GIVE term within the entire wade area coverage zone taking in account real irregular structure of the ionosphere. It leads to the larger confidence bounding terms and lower expected positioning availability in comparison to the reality under geomagnetic storm conditions and system outages. Thus a question arises: is the basic WAAS concept appropriate to provide the same efficiency of the integrity monitoring for both “global differential correction (i.e. clock, ephemeris etc)” and “local differential correction (i.e. ionoshrere, troposhpere and multipath)”? The aim of this paper is to compare official WAAS integrity monitoring reports and real positioning quality in US coverage zone (CONUS) and Canada area under geomagnetic storm conditions and system outages. In this research we are interested in comparison between real GPS positioning quality based on one-frequency C/A ranging mode and HAL (VAL) values which correspond to the LP, LPV and LPV200 requirements. Significant mismatch of the information between WAAS integrity data and real positioning quality was unfolded as a result of this comparison under geomagnetic storms and system outages on February, 2011 and June 22, 2015. Based on this result we think that in order to achieve high confidence of WAAS positioning availability alerts real ionospheric measurements within the wide area coverage zone must be involved instead of the WAAS GIVE values. The better way to realize this idea is to combine WAAS solutions to derive “global differential corrections” and LAAS solutions to derive “local differential corrections”.

Snap Load Criteria for Mooring Lines of a Floating Offshore Wind Turbine

There are several challenges facing the design of mooring system of floating offshore wind turbines (FOWTs), including installation costs, stability of lightweight minimalistic platforms, and shallow water depths (50–300m). For station keeping of FOWTs, a proper mooring system is required in order to maintain the translational motions in surge and sway and the rotational motions in yaw of the platform within an adequate range. A combination of light pre-tension, shallow water depth and large platform motions in response to a survival storm condition can result in snap-type impact events on mooring lines, thus increasing the line tension dramatically. In this paper, we present a new snap load criterion applicable to a catenary mooring system and compare it with Det Norske Veritas’ criterion for marine operations. As a case study, we examine the extreme tension on a catenary mooring system of a semi-submersible FOWT exposed to a 100-year storm condition. The software OrcaFlex was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic loads for FOWT analyses. Snap-type impact events were observed in the numerical simulations and were characterized by two criteria. Tension maxima were fitted using composite Weibull distributions (CWDs) and comparisons of probability exceedance were made for the two different snap load criteria.

Cantilever Force Distribution on Jack-Up Rig

Cantilever is the most important structure of the jack-up rig. Jack-up usually has three working conditions: drilling condition, storm condition, preloading condition. The fulcrums of the cantilever in the drilling condition are less than the other two. So the distribution of the cantilever force in the storm and preloading conditions becomes extremely complicated with the increase of fulcrums. In order to facilitate the evaluation of jack-up rig strength, a simplified analytical method and a finite element method (FEM) are presented to distribute the cantilever force on a jack-up rig during the storm and preloading conditions. For the simplified analytical method, the fulcrum force is obtained from the distance between the fulcrum and the position of the cantilever resultant force. For the FEM, the cantilever is assumed to be perfect rigid body without any strain. The fulcrum force is calculated according to the balance of the fulcrum forces. Both results of fulcrum force are analyzed and compared. It is shown that they are in good agreement. Thus both methods can be applied in distributing the cantilever force to evaluate the strength of jack-up rig.