Drift Motions of a Floating Barge in Random Multi-Directional Waves

1991 ◽  
Vol 113 (1) ◽  
pp. 37-42 ◽  
Author(s):  
O. Nwogu ◽  
M. Isaacson
Keyword(s):  
Standard Deviation ◽  
Laboratory Tests ◽  
Low Frequency ◽  
Mooring Line ◽  
Floating Structures ◽  
Directional Waves ◽  
Slow Drift ◽  
Wave Directionality ◽  
The Mean ◽  
Directional Sea States

This paper presents results of a numerical and laboratory investigation into the mooring line forces and slow drift oscillations of large floating structures in multidirectional waves. A procedure for computing the spectral density of the second-order forces in random multi-directional waves based on the concept of a bidirectional, bifrequency quadratic transfer function is presented. Laboratory tests were carried out with a floating barge model, restrained horizontally by soft linear springs. The barge was subjected to random multi-directional waves with different degrees of directional spreading. The influence of wave directionality on the mooring line forces and low frequency motions is investigated by comparing results in unidirectional and multi-directional sea states with an identical frequency spectrum. The results indicate a significant reduction of the mean and standard deviation of the surge response, and an increased sway and yaw response. The mooring line forces were affected by wave directionality in a similar manner as the surge response.

Second-Order Wave Loads on a LNG Carrier in Multi-Directional Waves

2008 ◽  
Author(s):  
Mathieu Renaud ◽  
Fla´via Rezende ◽  
Olaf Waals ◽  
Xiao-Bo Chen ◽  
Radboud van Dijk
Keyword(s):  
Low Frequency ◽  
Model Tests ◽  
Second Order ◽  
Wave Loads ◽  
Wave Kinematics ◽  
Directional Waves ◽  
Slow Drift ◽  
Wave Directionality ◽  
Offshore Industry ◽  
Cross Waves

Due to the installation of LNG terminals moored in proximity to the coast, the wave kinematics in shallow water and the consequence on the behavior of those terminals have recently became a major concern of the offshore industry. One key issue is the accurate simulation of the low-frequency motions of LNG carriers, specially the surge, for which the vessel presents low damping, in order to perform the design of the mooring system. The present paper focuses on the effect of wave directionality on second-order slow-drift loads and the related response of the vessel. The paper describes results of model tests in regular cross waves — monochromatic but coming from two directions separated by 90 degrees, as well as bichromatic cross waves. The new “middle field” formulation extended to the case of cross waves, is used to compute the wave drift loads and low-frequency Quadratic Transfer Function (QTF). The results are compared with those from the model tests.

scholarly journals A low-cost acoustic permeameter

2017 ◽  
Vol 6 (1) ◽  
pp. 199-207 ◽  
Author(s):  
Stephen A. Drake ◽  
John S. Selker ◽  
Chad W. Higgins
Keyword(s):  
Standard Deviation ◽  
Relative Error ◽  
Low Cost ◽  
Low Frequency ◽  
Environmental Noise ◽  
Intrinsic Permeability ◽  
Sampling Errors ◽  
Resonance Frequencies ◽  
The Mean ◽  
Frequency Approximation

Abstract. Intrinsic permeability is an important parameter that regulates air exchange through porous media such as snow. Standard methods of measuring snow permeability are inconvenient to perform outdoors, are fraught with sampling errors, and require specialized equipment, while bringing intact samples back to the laboratory is also challenging. To address these issues, we designed, built, and tested a low-cost acoustic permeameter that allows computation of volume-averaged intrinsic permeability for a homogenous medium. In this paper, we validate acoustically derived permeability of homogenous, reticulated foam samples by comparison with results derived using a standard flow-through permeameter. Acoustic permeameter elements were designed for use in snow, but the measurement methods are not snow-specific. The electronic components – consisting of a signal generator, amplifier, speaker, microphone, and oscilloscope – are inexpensive and easily obtainable. The system is suitable for outdoor use when it is not precipitating, but the electrical components require protection from the elements in inclement weather. The permeameter can be operated with a microphone either internally mounted or buried a known depth in the medium. The calibration method depends on choice of microphone positioning. For an externally located microphone, calibration was based on a low-frequency approximation applied at 500 Hz that provided an estimate of both intrinsic permeability and tortuosity. The low-frequency approximation that we used is valid up to 2 kHz, but we chose 500 Hz because data reproducibility was maximized at this frequency. For an internally mounted microphone, calibration was based on attenuation at 50 Hz and returned only intrinsic permeability. We found that 50 Hz corresponded to a wavelength that minimized resonance frequencies in the acoustic tube and was also within the response limitations of the microphone. We used reticulated foam of known permeability (ranging from 2 × 10−7 to 3 × 10−9 m2) and estimated tortuosity of 1.05 to validate both methods. For the externally mounted microphone the mean normalized standard deviation was 6 % for permeability and 2 % for tortuosity. The mean relative error from known measurements was 17 % for permeability and 2 % for tortuosity. For the internally mounted microphone the mean normalized standard deviation for permeability was 10 % and the relative error was also 10 %. Permeability determination for an externally mounted microphone is less sensitive to environmental noise than is the internally mounted microphone and is therefore the recommended method. The approximation using the internally mounted microphone was developed as an alternative for circumstances in which placing the microphone in the medium was not feasible. Environmental noise degrades precision of both methods and is recognizable as increased scatter for replicate data points.

Contribution of the Mooring System to the Low-Frequency Motions of a Semisubmersible in Combined Wave and Current

2009 ◽  
Author(s):  
Amany M. A. Hassan ◽  
Martin J. Downie ◽  
Atilla Incecik ◽  
R. Baarholm ◽  
P. A. Berthelsen ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Damping Ratio ◽  
Low Frequency ◽  
Scale Model ◽  
Mooring Line ◽  
Mooring System ◽  
Current Field ◽  
Mooring Lines ◽  
Mean Values ◽  
Low Frequency Motion

This paper presents the results of an experiment carried out on a semi-submersible model to measure the steady drift force and low frequency surge motions. In the experiments, the influence of mooring systems was also investigated in different combinations of current and sea state. The measurements were carried out with a 1/50 scale model which was moored using horizontal springs and catenary mooring lines. A comparative study of the mean values of steady drift motions and the standard deviation of the low frequency motion amplitudes is presented. In addition, the effect of current on the damping ratio is discussed. It is found that for both horizontal and catenary moorings, the presence of a current increases the damping ratio of the system. For the catenary mooring system, as expected, the presence of mooring lines and their interaction with waves and current increases the damping compared to the damping of the horizontal mooring system. The measured mean values of the surge motions in a wave–current field are compared to the superposed values of those obtained from waves and current separately. For the horizontal mooring, it is found that there is good agreement in moderate sea states, while in higher sea states the measured motion responses are larger. In the wave-current field, the standard deviation of the surge motion amplitudes is found to be less than that obtained in waves alone. This can be explained by the increased magnitude of the damping ratio. Only in the cases of high sea states with the horizontal mooring system, was it found that the standard deviation of the surge motions is slightly larger than those obtained for waves and current separately. This may be explained by the absence of catenary mooring line damping.

scholarly journals A performance assessment of the World Wide Lightning Location Network (WWLLN) via comparison with the Canadian Lightning Detection Network (CLDN)

2010 ◽  
Vol 3 (2) ◽  
pp. 1861-1887 ◽  
Author(s):  
D. Abreu ◽  
D. Chandan ◽  
R. H. Holzworth ◽  
K. Strong
Keyword(s):  
Standard Deviation ◽  
World Wide ◽  
Detection Efficiency ◽  
Low Frequency ◽  
High Peak ◽  
Peak Current ◽  
Lightning Detection ◽  
The World ◽  
The Mean ◽  
The Difference

Abstract. The World Wide Lightning Location Network (WWLLN) uses globally-distributed Very Low Frequency (VLF) receivers in order to observe lightning around the globe. Its objective is to locate as many global strokes as possible, with high temporal and spatial (<10 km) accuracy. Since detection is done in the VLF range, signals from high peak current lightning strokes are able to propagate up to ~104 km before being detected by the WWLLN sensors, allowing for receiving stations to be sparsely spaced. Through a comparison with measurements made by the Canadian Lightning Detection Network (CLDN) between May and August 2008 over a 4° latitude by 4° longitude region centered on Toronto, Canada, this study found that WWLLN detection was most sensitive to high peak current lightning strokes. Events were considered shared between the two networks if they fell within 0.5 ms of each other. Using this criterion, 19 128 WWLLN strokes (analyzed using the Stroke_B algorithm) were shared with CLDN lightning strokes, producing a detection efficiency of 2.8%. The peak current threshold for WWLLN detection is found to be ~20 kA, with the detection efficiency increasing to ~70% at peak currents of ±120 kA. The detection efficiency is seen to have a clear diurnal dependence, with a higher detection efficiency at local midnight than at local noon; this is attributed to the difference in the thickness of the ionospheric D-region between night and day. The mean time difference (WWLLN – CLDN) between shared events was −6.44 μs with a standard deviation of 35 μs, and the mean absolute location accuracy was 7.24 km with a standard deviation of 6.34 km. These results are generally consistent with previous comparison studies of the WWLLN with other regional networks around the world. Additional receiver stations are continuously being added to the network, acting to improve this detection efficiency.

scholarly journals Impact of Treatment Regimen with Moroctocog Alfa (AF-CC) on Bleeding Frequency in Pediatric Aged Subjects with Moderately Severe to Severe Hemophilia A

Blood ◽  
2018 ◽  
Vol 132 (Supplement 1) ◽  
pp. 1205-1205
Author(s):  
Mark P. Smith ◽  
Jeremy Rupon ◽  
Yasser Wali ◽  
Hala Remawi ◽  
Joan Korth-Bradley ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Hemophilia A ◽  
Pediatric Population ◽  
Low Frequency ◽  
Primary Objective ◽  
Severe Hemophilia ◽  
On Demand ◽  
Routine Prophylaxis ◽  
The Mean ◽  
Safety Signals

Abstract Moroctocog alfa (AF-CC) is a B-domain deleted recombinant factor VIII product for the treatment of patients with hemophilia A. It has previously been studied in pediatric aged hemophilia patients, however direct comparisons between treatment regimens have not been evaluated. Importantly, as prophylaxis has been shown to be superior to on-demand treatment, comparisons between different prophylaxis regimens are needed. Reducing the frequency of infusions without sacrificing efficacy may reduce the burden of hemophilia management. This study was an open label, multicenter, randomized crossover study of two routine prophylaxis (RP) regimens of moroctocog alfa (AF-CC) compared to on-demand (OD) therapy in 2 cohorts of children with hemophilia A. The primary objective was to demonstrate that prophylaxis reduces ABR relative to OD therapy. A secondary objective was to compare the efficacy of two different RP regimens. Additional objectives focused on FVIII recovery and safety. Study Design: The study was open to previously treated patients (≥20 exposure days (ED) to any FVIII replacement product) with moderately severe to severe hemophilia A (<2% FVIII) aged 6 months to <16 years. The study enrolled 2 cohorts of subjects who were treated in two segments. The first cohort started with OD treatment for 6 months (segment 1) followed by 1 year of RP of 25 international units (IU)/kg every other day (segment 2). Subjects in the second cohort were randomized to one of 2 RP regimens for 1 year (segment 1) and then crossed over to a second RP regimen for an additional year (segment 2). The RP regimens were high frequency (HF) (25 IU/kg every other day) and low frequency (LF) (45 IU/kg twice weekly). Recovery assessment after administration of 50 IU/kg of moroctocog alfa (AF-CC) was optional. Results: A total of 51 subjects were enrolled in the study: 9 subjects (age 2.4-5.9 yr; median 4.9 yr) in the OD group, 18 subjects (age 1.1-12.7 yr; median 4.7 yr) received LF followed by HF, and 24 subjects (age 1.2 to 9.6 yr; median 4.6 yr) received HF followed by LF. The mean (standard deviation [SD]) ABR for the first cohort (n=9) during the OD segment was 47.0(32.2) and during the RP segment (HF) was 1.5(2.2). In those subjects from cohort 2 that completed both segments (n=38), the mean (standard deviation) ABR for HF was 2.2(4.1) and for LF was 3.3(5.3). Based on a prospectively defined equivalence limit of (-3, 3) bleeds per year, equivalence between these two regimens was demonstrated as the 90% CI for the difference, (0.03, 2.22) fell within this range. The mean (SD) recovery in 6 children aged 3.7 to 5.8 yrs was 1.44(0.61) IU/dL/IU/kg. Three subjects tested positive for FVIII inhibitors, however 2 were considered to be false positives for an overall rate of 2%. There were no new safety signals that emerged during this study. Conclusion: This study met its primary objective of demonstrating that RP reduces the ABR compared to OD. Notably, LF prophylaxis with moroctocog alfa (AF-CC) is as efficacious as a HF prophylaxis regimen. This has important implications as less frequent infusions can improve adherence and the quality of life for patients with hemophilia. Additionally, recovery was as expected in this pediatric population and no new safety signals emerged. Moroctocog alfa (AF-CC) is safe and efficacious for routine prophylaxis in pediatric patients with hemophilia A and a low frequency (twice weekly) prophylaxis regimen is as efficacious as every other day prophylaxis. Disclosures Rupon: Pfizer: Employment. Korth-Bradley:Pfizer Inc.: Employment. Smith:Pfizer: Employment. Rendo:Pfizer: Employment.

Spectral analysis of arterial pressure lability in rats with sinoaortic deafferentation

1995 ◽  
Vol 269 (6) ◽  
pp. R1481-R1488 ◽  
Author(s):  
H. J. Jacob ◽  
A. Ramanthan ◽  
S. G. Pan ◽  
M. J. Brody ◽  
G. A. Myers
Keyword(s):  
Spectral Analysis ◽  
Standard Deviation ◽  
Mean Arterial Pressure ◽  
Arterial Pressure ◽  
Low Frequency ◽  
Spectral Components ◽  
Spectral Profiles ◽  
The Mean ◽  
Normal Fluctuations ◽  
Arterial Pressure Lability

Traditionally, the standard deviation (SD) of the mean arterial pressure (AP) has been used as an index for the AP lability produced by interruption of baroreceptor afferents. Although a useful measure of variance about the mean, the SD does not provide any information about the temporal characteristics of this variability. We employed two different spectral analytic techniques to characterize AP waveforms in rats with sinoaortic deafferentation (SAD) and in sham-operated (Sham) rats to determine if the AP waveform in SAD animals was qualitatively and/or quantitatively different from that of Sham animals. The SAD and Sham animals exhibited qualitatively different spectral profiles, suggesting that lability of AP in SAD animals is not simply an exaggeration of normal fluctuations. In addition, a low-frequency (0.3-0.5 Hz) spectral peak was found in Sham but not SAD animals, suggesting that it is associated with the baroreflex. Finally, we observed in both normal rats and rats without intact baroreceptors that the spectral components of AP are not static but rather vary continuously across time.

scholarly journals A low-cost acoustic permeameter

2016 ◽  
Author(s):  
Stephen A. Drake ◽  
John S. Selker ◽  
Chad W. Higgins
Keyword(s):  
Standard Deviation ◽  
Relative Error ◽  
Low Cost ◽  
Low Frequency ◽  
Environmental Noise ◽  
Intrinsic Permeability ◽  
Sampling Errors ◽  
Resonance Frequencies ◽  
The Mean ◽  
Frequency Approximation

Abstract. Intrinsic permeability is an important parameter that regulates air exchange through porous media such as snow. Standard methods of measuring snow permeability are inconvenient to perform outdoors, fraught with sampling errors and require specialized equipment, while bringing intact samples back to the laboratory is also challenging. To address these issues, we designed, built, and tested a low-cost acoustic permeameter that allows computation of volume-averaged intrinsic permeability for a homogenous medium. Permeameter elements were designed for use in snow but the measurement methods are not snow-specific. The electronic components, consisting of a signal generator, amplifier, speaker, microphone and oscilloscope, are inexpensive and easily obtainable. The system is suitable for outdoor use when it is not precipitating but the electrical components require protection from the elements in inclement weather. The permeameter can be operated with a microphone either internally mounted or buried a known depth in the medium. The calibration method depends on choice of microphone positioning. For an externally located microphone, calibration was based on a low-frequency approximation applied at 500 Hz that provided an estimate of both intrinsic permeability and tortuosity. The low-frequency approximation that we used is valid up to 2 kHz but we chose 500 Hz because data reproducibility was maximized at this frequency. For an internally mounted microphone, calibration was based on attenuation at 50 Hz and returned only intrinsic permeability. We found that 50 Hz corresponded to a wavelength that minimized resonance frequencies in the acoustic tube and was also within the response limitations of the microphone. We used reticulated foam of known permeability (ranging from 2 × 10−7 m2 to 3 × 10−9 m2) and estimated tortuosity of 1.05 to validate both methods. For the externally mounted microphone the mean normalized standard deviation was 6 % for permeability and 2 % for tortuosity. The mean relative error from known measurements was 17 % for permeability and 2 % for tortuosity. For the internally mounted microphone the mean normalized standard deviation for permeability was 10 % and the relative error was also 10 %. Permeability determination for an externally mounted microphone is less sensitive to environmental noise than is the internally mounted microphone and is therefore the recommended method. The approximation using the internally mounted microphone was developed as an alternative for circumstances in which placing the microphone in the medium was not feasible. Environmental noise degrades precision of both methods and is recognizable as increased scatter for replicate data points.

scholarly journals Influence of Mean Tension on Mooring Line Fatigue Life

2020 ◽  
Author(s):  
Erling N. Lone ◽  
Bernt J. Leira ◽  
Thomas Sauder ◽  
Vegard Aksnes ◽  
Øystein Gabrielsen ◽  
...  
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Low Frequency ◽  
Breaking Load ◽  
Fatigue Tests ◽  
Mooring Line ◽  
Load Effect ◽  
Design Curve ◽  
Fatigue Damage Accumulation ◽  
The Mean

Abstract Studies published in recent years have documented a significant mean load effect on fatigue capacity for offshore mooring chain, and show that a reduction of the mean load gives an increase in fatigue life. However, current S-N design curves are based on fatigue tests performed at a mean load of 20% of minimum breaking load (MBL), which is well above the typical mean loads for most mooring systems. This paper investigates the mean loads experienced during fatigue damage accumulation for the mooring system of a typical production semi-submersible, operating in Norwegian Sea conditions. The study is based on numerical, time-domain simulations, using environmental conditions defined from a series of hindcast data. A parameterized S-N design curve suggested by Fernández et al. (2019), incorporating a Smith-Watson-Topper mean stress correction model, is applied for fatigue damage calculation and compared to results for the S-N design curve prescribed by current standards. For the semi-submersible unit considered there is negligible difference in basing the correction on 3-hour mean load compared to the mean load of individual stress cycles, due to small low frequency tension variations. On this basis, a single correction factor is proposed to allow for mean load correction based on results available from a standard fatigue analysis.

scholarly journals Slow Drift Solar Radio Bursts: Harmonic Frequency Ratios, Solar Longitude Dependence, and Frequency Drift Rates

1961 ◽  
Vol 14 (2) ◽  
pp. 234 ◽  
Author(s):  
Marion B Wood
Keyword(s):  
High Frequency ◽  
Second Harmonic ◽  
Low Frequency ◽  
Mean Value ◽  
Radio Bursts ◽  
Harmonic Frequency ◽  
Solar Radio Bursts ◽  
Slow Drift ◽  
The Mean ◽  
Frequency Ratios

Frequency ratios of second harmonic to fundamental bands are determined for 19 slow drift bursts. The ratios measured at the high frequency edges of the bands yield a mean value of 2� 00, whereas the mean value of the low frequency edge ratios is significantly lower than 2.

scholarly journals A performance assessment of the World Wide Lightning Location Network (WWLLN) via comparison with the Canadian Lightning Detection Network (CLDN)

2010 ◽  
Vol 3 (4) ◽  
pp. 1143-1153 ◽  
Author(s):  
D. Abreu ◽  
D. Chandan ◽  
R. H. Holzworth ◽  
K. Strong
Keyword(s):  
Standard Deviation ◽  
World Wide ◽  
Detection Efficiency ◽  
Low Frequency ◽  
High Peak ◽  
Peak Current ◽  
Lightning Detection ◽  
The World ◽  
The Mean ◽  
The Difference

Abstract. The World Wide Lightning Location Network (WWLLN) uses globally-distributed Very Low Frequency (VLF) receivers in order to observe lightning around the globe. Its objective is to locate as many global lightning strokes as possible, with high temporal and spatial (< 10 km) accuracy. Since detection is done in the VLF range, signals from high peak current lightning strokes are able to propagate up to ~104 km before being detected by the WWLLN sensors, allowing for receiving stations to be sparsely spaced. Through a comparison with measurements made by the Canadian Lightning Detection Network (CLDN) between May and August 2008 over a 4° latitude by 4° longitude region centered on Toronto, Canada, this study found that WWLLN detection was most sensitive to high peak current lightning strokes. Events were considered shared between the two networks if they fell within 0.5 ms of each other. Using this criterion, 19 128 WWLLN strokes (analyzed using the Stroke_B algorithm) were shared with CLDN lightning strokes, producing a detection efficiency of 2.8%. The peak current threshold for WWLLN detection is found to be ~20 kA, with its detection efficiency increasing from 11.3% for peak currents greater than 20 kA to 75.8% for peak currents greater than 120 kA. The detection efficiency is seen to have a clear diurnal dependence, with a higher detection efficiency at local midnight than at local noon; this is attributed to the difference in the thickness of the ionospheric D-region between night and day. The mean time difference (WWLLN − CLDN) between shared events was −6.44 μs with a standard deviation of 35 μs, and the mean absolute location accuracy was 7.24 km with a standard deviation of 6.34 km. These results are generally consistent with previous comparison studies of the WWLLN with other regional networks around the world. Additional receiver stations are continuously being added to the network, acting to improve this detection efficiency.

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