Mooring Line Damping Estimation by a Simplified Dynamic Model

2007 ◽  
Author(s):  
Halvor Lie ◽  
Zhen Gao ◽  
Torgeir Moan
Keyword(s):  
Dynamic Model ◽  
Time Domain ◽  
Low Frequency ◽  
Line Tension ◽  
Mooring Line ◽  
Short Term ◽  
Simplified Method ◽  
On Line ◽  
Simulation Results ◽  
Frequency Domain Approach

When predicting slowly varying resonant vessel motions, a realistic estimate of the motion damping is crucial. Mooring line damping, which is mainly induced by the drag force on line, can dominate the total damping of catenary moored systems and methods for predicting mooring line damping are therefore required. Based on a simplified dynamic model of mooring line tension, an approach to estimate the corresponding damping is presented in this paper. Short-term time domain simulations of dynamic line tension are carried out to verify the accuracy of the simplified frequency domain approach. Compared with the simulation results, the practical simplified method proposed herein gives a maximum 30% lower prediction of the damping coefficient of each mooring line and an about 20% smaller estimate of the total line damping and therefore yields conservative estimates of the low frequency vessel motions.

On the Probability Distribution of Mooring Line Tensions in a Directional Environment

2011 ◽  
Author(s):  
Jan Mathisen ◽  
Siril Okkenhaug ◽  
Kjell Larsen
Keyword(s):  
Time Series ◽  
Probability Distributions ◽  
Extreme Value Distribution ◽  
Line Tension ◽  
Extreme Value ◽  
Mooring Line ◽  
Large Set ◽  
Short Term ◽  
Reliability Method ◽  
Line Design

A joint probabilistic model of the metocean environment is assembled, taking account of wind, wave and current and their respective heading angles. Mooring line tensions are computed in the time domain, for a large set of short-term stationary conditions, intended to span the domain of metocean conditions that contribute significantly to the probabilities of high tensions. Weibull probability distributions are fitted to local tension maxima extracted from each time series. Long time series of 30 hours duration are used to reduce statistical uncertainty. Short-term, Gumbel extreme value distributions of line tension are derived from the maxima distributions. A response surface is fitted to the distribution parameters for line tension, to allow interpolation between the metocean conditions that have been explicitly analysed. A second order reliability method is applied to integrate the short-term tension distributions over the probability of the metocean conditions and obtain the annual extreme value distribution of line tension. Results are given for the most heavily loaded mooring line in two mooring systems: a mobile drilling unit and a production platform. The effects of different assumptions concerning the distribution of wave heading angles in simplified analysis for mooring line design are quantified by comparison with the detailed calculations.

Jitter Transmission Analysis and Experimental Research for Frame Structure in the Median and High Frequency Regions

2012 ◽  
Vol 271-272 ◽  
pp. 981-985
Author(s):  
You Yi Wang ◽  
Yang Zhao ◽  
Wen Lai Ma
Keyword(s):  
Dynamic Response ◽  
Frequency Response ◽  
Dynamic Model ◽  
Experimental Research ◽  
High Frequency ◽  
Low Frequency ◽  
Frame Structure ◽  
Wave Method ◽  
Simulation Results ◽  
Theoretical Results

Frame structure is widely used in practical projects. For jitter of the frame structure excited by median and high frequency disturbances, firstly, the dynamic model of thin plate substructure is built by wave method, and then the dynamic model of frame structure is established by combining wave method and substructure technique. At last, the accurate dynamic response was obtained. The simulation of dynamic characteristic is made, and simulation results are compared with FEM results. On this basis, modal experiment and frequency response experiment is done to verify theoretical results. In comparison to FEM, the results by wave method are accurate in low frequency regions, and the results are more accurate in the median and high frequency regions. The experiment proves wave method is correct and effective for jitter transmission analysis of frame structure in the median and high frequency regions.

A Comparison of Methods for the Coupled Analysis of Floating Structures

2007 ◽  
Author(s):  
Ying Min Low ◽  
Robin S. Langley
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
High Frequency ◽  
Geometric Nonlinearity ◽  
Low Frequency ◽  
Alternative Methods ◽  
Coupled Analysis ◽  
Intermediate Water ◽  
Low Frequency Motion ◽  
Frequency Domain Approach

The dynamic analysis of a deepwater floating platform and the associated mooring/riser system should ideally be fully coupled to ensure a reliable response prediction. It is generally held that a time domain analysis is the only means of capturing the various coupling and nonlinear effects accurately. However, in recent work it has been found that for an ultra-deepwater floating system (2000m water depth), the highly efficient frequency domain approach can provide highly accurate response predictions. One reason for this is the accuracy of the drag linearization procedure over both first and second order motions, another reason is the minimal geometric nonlinearity displayed by the mooring lines in deepwater. In this paper, the aim is to develop an efficient analysis method for intermediate water depths, where both mooring/vessel coupling and geometric nonlinearity are of importance. It is found that the standard frequency domain approach is not so accurate for this case and two alternative methods are investigated. In the first, an enhanced frequency domain approach is adopted, in which line nonlinearities are linearized in a systematic way. In the second, a hybrid approach is adopted in which the low frequency motion is solved in the time domain while the high frequency motion is solved in the frequency domain; the two analyses are coupled by the fact that (i) the low frequency motion affects the mooring line geometry for the high frequency motion, and (ii) the high frequency motion affects the drag forces which damp the low frequency motion. The accuracy and efficiency of each of the methods are systematically compared.

The Effect of Turbulence Model on the Response of a Large Floating Wind Turbine

2017 ◽  
Author(s):  
Lene Eliassen ◽  
Erin E. Bachynski
Keyword(s):  
Wind Speed ◽  
Wind Turbine ◽  
Fatigue Damage ◽  
Turbulence Model ◽  
Wind Turbines ◽  
Low Frequency ◽  
Turbulence Models ◽  
Line Tension ◽  
Offshore Wind ◽  
Mooring Line

The wind turbine design standards advise choosing one of two recommended turbulence models for load simulations of offshore wind turbines. The difference in fatigue loads for the two turbulence models is relatively small for bottom-fixed wind turbines, but some floating wind turbines show a higher sensitivity to the chosen turbulence model. In this study, the motions and mooring line fatigue damage of two semi-submersible floating wind turbines are investigated for three different wind speeds: 8 m/s, 14 m/s and 20 m/s, and three different wave states for each wind speed. For both concepts, the CSC 5 MW and the CSC 10 MW, the low-frequency surge response is important for the mooring line tension, and the simulations using the Kaimal turbulence model give the largest variation in tension at the surge eigenfrequency. However, using the Mann turbulence model in the load simulations give a higher response in the range of the blade passing frequency (3P). The CSC 10 MW has a higher aerodynamic thrust relative to the CSC 5 MW, and will therefore have a larger surge response at the lower frequencies than the CSC 5 MW. At the lowest wind speed, where the variation in mooring line tension at surge eigenfrequency is high, the fatigue damage is larger if the Kaimal turbulence model is applied to the load simulations. However, at the highest wind speed, using the Mann turbulence model in the simulations, give a higher mooring line fatigue damage.

scholarly journals A Novel Smartphone App for the Measurement of Ultra–Short-Term and Short-Term Heart Rate Variability: Validity and Reliability Study (Preprint)

2020 ◽  
Author(s):  
Yung-Sheng Chen ◽  
Wan-An Lu ◽  
Jeffrey C Pagaduan ◽  
Cheng-Deng Kuo
Keyword(s):  
Heart Rate ◽  
Time Domain ◽  
Low Frequency ◽  
Smartphone App ◽  
Spearman Correlation ◽  
Validity And Reliability ◽  
Limits Of Agreement ◽  
Short Term ◽  
Low Frequency Power ◽  
Frequency Power

BACKGROUND Smartphone apps for heart rate variability (HRV) measurement have been extensively developed in the last decade. However, ultra–short-term HRV recordings taken by wearable devices have not been examined. OBJECTIVE The aims of this study were the following: (1) to compare the validity and reliability of ultra–short-term and short-term HRV time-domain and frequency-domain variables in a novel smartphone app, Pulse Express Pro (PEP), and (2) to determine the agreement of HRV assessments between an electrocardiogram (ECG) and PEP. METHODS In total, 60 healthy adults were recruited to participate in this study (mean age 22.3 years [SD 3.0 years], mean height 168.4 cm [SD 8.0 cm], mean body weight 64.2 kg [SD 11.5 kg]). A 5-minute resting HRV measurement was recorded via ECG and PEP in a sitting position. Standard deviation of normal R-R interval (SDNN), root mean square of successive R-R interval (RMSSD), proportion of NN50 divided by the total number of RR intervals (pNN50), normalized very-low–frequency power (nVLF), normalized low-frequency power (nLF), and normalized high-frequency power (nHF) were analyzed within 9 time segments of HRV recordings: 0-1 minute, 1-2 minutes, 2-3 minutes, 3-4 minutes, 4-5 minutes, 0-2 minutes, 0-3 minutes, 0-4 minutes, and 0-5 minutes (standard). Standardized differences (ES), intraclass correlation coefficients (ICC), and the Spearman product-moment correlation were used to compare the validity and reliability of each time segment to the standard measurement (0-5 minutes). Limits of agreement were assessed by using Bland-Altman plot analysis. RESULTS Compared to standard measures in both ECG and PEP, pNN50, SDNN, and RMSSD variables showed trivial ES (<0.2) and very large to nearly perfect ICC and Spearman correlation coefficient values in all time segments (>0.8). The nVLF, nLF, and nHF demonstrated a variation of ES (from trivial to small effects, 0.01-0.40), ICC (from moderate to nearly perfect, 0.39-0.96), and Spearman correlation coefficient values (from moderate to nearly perfect, 0.40-0.96). Furthermore, the Bland-Altman plots showed relatively narrow values of mean difference between the ECG and PEP after consecutive 1-minute recordings for SDNN, RMSSD, and pNN50. Acceptable limits of agreement were found after consecutive 3-minute recordings for nLF and nHF. CONCLUSIONS Using the PEP app to facilitate a 1-minute ultra–short-term recording is suggested for time-domain HRV indices (SDNN, RMSSD, and pNN50) to interpret autonomic functions during stabilization. When using frequency-domain HRV indices (nLF and nHF) via the PEP app, a recording of at least 3 minutes is needed for accurate measurement.

The Effect of Mooring Line Pre-Tension on FDPSO’s Motion

2016 ◽  
Author(s):  
Yanlong Sun ◽  
Huilong Ren ◽  
Zhendong Liu ◽  
Liu Yan ◽  
Zepeng Guo
Keyword(s):  
Time Series ◽  
Model Test ◽  
Time Domain ◽  
Prediction Method ◽  
Line Tension ◽  
Mooring Line ◽  
Floating Platform ◽  
Motion Response ◽  
Drilling Unit ◽  
Environment Condition

As a multifunction floating platform, Floating Drilling, Production, Storage and Offloading (FDPSO) combining the well-known Floating Production, Storage and Offloading (FPSO) with a drilling unit. For the environment condition of deep-water oilfield is very severe, the motion response and mooring line tension of FDPSO is a worthy topic of studying. In this study, the numerical time-domain coupled prediction method for the mooring line tension and motion response of FDPSO system is constructed by ANSYS AQWA software. Furthermore, the results of a model test conducted in Harbin Engimeering University are used to investigate the feasibility and validity of the commercial simulation. The effect of mooring line pre-tension on the response of FDPSO is studied by varying the pre-tension of mooring line during the calculation. The time series curve of the mooring line tension and motion response, and the comparison of motion spectrum and mooring line tension spectrum are provided in this article.

Prediction of Mooring Line Tensions for Hurricane Conditions in the Gulf of Mexico

2014 ◽  
Author(s):  
Jan Mathisen ◽  
Torfinn Hørte
Keyword(s):  
Short Interval ◽  
Extreme Value Distribution ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Extreme Value ◽  
Value Distribution ◽  
Mooring Line ◽  
Response Analysis ◽  
Short Term ◽  
Tension Distribution

A probabilistic metocean model for hurricane conditions is briefly described. The model is based on site-specific, hindcast data and defines the time variation of the metocean conditions during the realisation of a hurricane at the site. The annual extreme value distribution of mooring line tension for a large, semi-submersible, mobile drilling unit is computed. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension, conditional on stationary metocean conditions. A large number of different conditions are considered. A response surface is used to interpolate on the short-term distribution parameters in order to describe the tension response during the varying conditions associated with the passage of a hurricane. The hurricane duration is split into a sequence of 15-minute intervals such that the conditions can be assumed stationary during each such short interval. The tension distribution, conditional on the realisation of a hurricane, is accumulated across the sequence of short intervals. The distribution of hurricanes is taken into account to obtain the tension distribution in a random hurricane. Finally, the frequency of hurricanes is taken into account to give the annual extreme distribution of line tension. The characteristic tension computed using 10-year return conditions and the ISO 19901-7 design standard is found to correspond to a return period of 29 years in the test case. The effects of various assumptions in the design analysis are investigated. Sensitivities to simplifications of the metocean model are considered. The effects of uncertainties in the response calculation and in the distribution of peak significant wave height during hurricanes are quantified and included in the response analysis.

Global Performance and Mooring Analysis Using Long Term Analysis Methodology

2016 ◽  
Author(s):  
Mubing Xu ◽  
Anil Sablok ◽  
Oddgeir Dalane
Keyword(s):  
High Speed ◽  
Line Tension ◽  
Mooring Line ◽  
Return Periods ◽  
Short Term ◽  
Analysis Methodology ◽  
Extreme Response ◽  
Global Performance ◽  
Term Analysis

The long term analysis is performed to predict the hull global motion and mooring strength of a Spar platform and the results from long term analysis are compared with the predictions from the short term analysis. The long term motions are also used to investigate the long term riser strength response in Ref. [5]. The results includes the lateral offset, heel angle, heave motion and the mooring line tension at the fairleads. In the short term analysis, the environment events with various return periods and various realizations are considered for each environment load. The Gumbel fitting is used to predict the extreme response. In the long term analysis methodology, the prediction of the global performance and the mooring tension are based on 56-year hindcast wind and wave data with 3-hour intervals. Weibull fitting is used to predict the extreme response for various return periods. The comparison between the long term and the short term predictions indicate that the short term predictions are generally conservative compared to the more accurate, but computationally expensive long term analysis method. The long term methodology is not widely adopted currently due to the computation inefficiency. However, it is expected that this long term methodology could provide a better option in the future with the consideration of its accuracy and the application of high speed computer.

scholarly journals Extending the Functionality of On-line PD Monitoring Equipment for MV Power Cables

2017 ◽  
Author(s):  
Y. Li ◽  
P.A.A.F. Wouters ◽  
P. Wagenaars
Keyword(s):  
Time Domain ◽  
Propagation Velocity ◽  
Characteristic Impedance ◽  
Signal Propagation ◽  
Time Domain Reflectometry ◽  
Monitoring Equipment ◽  
High Frequency Signal ◽  
Medium Voltage ◽  
On Line ◽  
Frequency Domain Approach

<p>An on-line partial discharge (PD) monitoring and location system for medium voltage cable circuits was developed previously. This paper explores ways to extend its range of application. The extension includes PD location method by time domain reflectometry (TDR) when reflections are not easily identifiable. The functionality of the PD monitoring equipment can also be widened by sensing other quantities related to the condition of the cable insulation using the same device. Dynamic cable temperature monitoring can be achieved by recording variation in the high frequency signal propagation velocity along the cable. Water ingress in paper-insulated lead-covered (PILC) cable decreases the cable’s characteristic impedance while it increases the permittivity of the insulation. It can be observed by changes in the reflection pattern from the cable or by a lowered propagation velocity. Instead of recording reflection patterns in time domain, a frequency domain approach based on an impedance scan is investigated to be applied for cables in service.</p>

NorMoor JIP: On Safe Mooring Line Design

2012 ◽  
Author(s):  
Siril Okkenhaug ◽  
Jan Mathisen ◽  
Torfinn Hørte
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Extreme Value ◽  
Mooring Line ◽  
Safety Factors ◽  
Line Design

DNV is currently running a Joint Industry Project, “NorMoor JIP”, on calibration of safety factors for mooring lines together with several oil companies, engineering companies, rig-owners, manufacturers of mooring line components and Norwegian authorities. Our motivation for initiating a study on mooring line safety factors started out with questions raised with regards to the safety level given by the Norwegian regulations. However, this is equally important for other mooring regulations like ISO, API and class-regulations. What we see is that the mooring standards are interpreted and applied in different ways. The reliability level implied by the regulations is not known, and the present safety factors were set when frequency domain analysis was prevalent while time domain analysis is often applied today. DNV carried out the DeepMoor JIP [9] during 1995–2000 using frequency domain analysis and reliability-based calibration. Now, a decade later, the increase in computing capacity makes it feasible to carry out a similar calibration for time-domain analysis of the mooring systems. The objective of the project work is to investigate and compare the characteristic line tension calculated according to design standards with the annual extreme value distribution of the line tension. Further, to calibrate safety factors for mooring line design for the ultimate limit state (ULS) as a function of the target probability of failure. The original proposal for this JIP included calculations for chain and wire rope moorings on a typical drill rig and a turret moored FPSO at three different water depths at Haltenbanken. However, since this JIP has been very well received in the industry, the scope has been extended to include calculations for a production semisubmersible, for fibre rope systems and for Gulf of Mexico environmental conditions. This paper will focus on the reasons for doing this calibration study, and the importance of seeking to agree on unified calculation recipes and requirements. Preliminary results for characteristic tension and annual extreme value distributions of tension for some designs are presented and discussed. The calibration of safety factors will be carried out later in the project when all designs are finalized.

Sign in / Sign up

Export Citation Format

Share Document