Interaction Between Mooring Line Damping and Response Frequency as a Result of Stiffness Alteration in Surge

2006 ◽  
Author(s):  
Lars Johanning ◽  
George H. Smith ◽  
Julian Wolfram
Keyword(s):  
Limit State ◽  
Extraction Process ◽  
Tidal Range ◽  
Mooring Line ◽  
System Response ◽  
Mooring Lines ◽  
Wave Basin ◽  
A Chain ◽  
Conversion Efficiencies ◽  
Line Design

The design and operation of a chain mooring for a wave energy converter (WEC) is considered. Experimental measurements of a mooring line were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The laboratory procedures were designed to resemble tests undertaken earlier in the year at ‘full’ scale in 24 m water depth. This paper describes and compares these measurements and relates the results to earlier work on mooring lines by Webster [1]. Measurements of both the damping and response frequencies of the mooring are described. Although the present results support partly the conclusions of the earlier work, care must be taken in how these are applied when one is considering mooring line design for WECs. It is concluded that there are significant differences for a WEC for both operational and limit state design in comparison with a more conventional offshore system such as an FPSO or CALM. Although the primary requirement is still one of station-keeping two further considerations may be of great importance. Firstly if a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere with one another and this will have great significance for the loading experienced by the line. This can be exacerbated by variations in tidal range which will have a larger effect in comparison with a conventional deepwater mooring. A second factor may arise if the mooring system response is critical to the WEC energy extraction process. If the mooring becomes part of the ‘tuned’ system then changes in the mooring properties of damping and natural frequency could seriously affect energy conversion efficiencies.

Effects of Mooring Line Design Parameters on the Line Dynamics and Fatigue Response of Subsea Mooring Lines

2018 ◽  
Author(s):  
Hamid Sedghi ◽  
Mehrdad Kimiaei
Keyword(s):  
Dynamic Analysis ◽  
Fatigue Damage ◽  
Dynamic Responses ◽  
Statistical Characteristics ◽  
Design Parameters ◽  
Mooring Line ◽  
Engineering Practice ◽  
Mooring Lines ◽  
Fatigue Design ◽  
Line Design

Dynamic characteristics of mooring lines play an important role in overall structural response and fatigue design of mooring systems. Full dynamic analysis including line dynamics is a vital part of fatigue design process although in time domain it needs excessive computational efforts. For fatigue analysis of mooring lines where hundreds of different environmental loads have to be checked, alternative analysis approach such as quasi-dynamic analysis with implicit inclusion of the line dynamic effects are used widely in engineering practice. This paper presents the results of series of case studies on the effects of various mooring line design parameters on the line dynamics as well as the mooring line dynamic fatigue response. Various mooring line composition types (all chain and chain-polyester-chain) used in different mooring configurations (catenary, semi-taut and taut) with variable range of mooring line pretensions connected to a floater in shallow and deep water depths are studied. Ratios of fatigue damage results between dynamic and quasi-dynamic results as well as the relation between fatigue damage and statistical characteristics of the line dynamic responses for different line configurations and load cases are investigated in detail.

scholarly journals Alternative Mooring Systems for a Very Large Offshore Wind Turbine Supported by a Semisubmersible Floating Platform

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Jinsong Liu ◽  
Lance Manuel
Keyword(s):  
Wind Turbine ◽  
Integrated System ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
System Response ◽  
Mooring System ◽  
Floating Platform ◽  
Mooring Lines ◽  
Wind Speeds

As offshore wind turbines supported on floating platforms extend to deep waters, the various effects involved in the dynamics, especially those resulting from the influence of moorings, become significant when predicting the overall integrated system response. The combined influence of waves and wind affect motions of the structure and induce tensile forces in mooring lines. The investigation of the system response under misaligned wind-wave conditions and the selection of appropriate mooring systems to minimize the turbine, tower, and mooring system loads is the subject of this study. We estimate the 50-year return response of a semisubmersible platform supporting a 13.2 MW wind turbine as well as mooring line forces when the system is exposed to four different wave headings with various environmental conditions (wind speeds and wave heights). Three different mooring system patterns are presented that include 3 or 6 mooring lines with different interline angles. Performance comparisons of the integrated systems may be used to define an optimal system for the selected large wind turbine.

CFD-Based Numerical Wave Basin for Global Performance Analysis

2016 ◽  
Author(s):  
Guangyu Wu ◽  
Jang Whan Kim ◽  
Hyunchul Jang ◽  
Aldric Baquet
Keyword(s):  
Performance Analysis ◽  
Surface Flow ◽  
Design Tool ◽  
Offshore Platform ◽  
Mooring Line ◽  
Mooring Lines ◽  
Wave Modeling ◽  
Wave Basin ◽  
Global Performance ◽  
Numerical Wave

Several recent benchmark studies have demonstrated that Computational Fluid Dynamics (CFD) is capable of capturing both nonlinear and viscous effects in offshore marine hydrodynamics and predicting well certain wave- and current-induced offshore platform motion. In order to apply CFD for practical global performance analysis of a complete hull-mooring-riser coupled floating system, we develop an advanced numerical wave basin that combines CFD, nonlinear irregular wave modeling, and finite-element mooring modeling. Specifically, CFD is used to simulate the violent free-surface flow with hull motions; nonlinear wave modeling is applied to generate a realistic wavefield and provide initial and far-field conditions to CFD for efficient long-duration simulation; and mooring modeling is two-way coupled with CFD to account for dynamic mooring response and its effects on hull motion. In this study, to demonstrate the capability of such tool, the global performance of a semi-submersible with 4 mooring lines in a 3-hour extreme sea state is simulated for both head and quartering sea. The simulation results are compared to model test data of hull motion, mooring line tension, and relative wave elevation around the hull for validation. It is shown with spectrum and statistics that the simulations predict well the platform’s global performance in all frequency ranges, including low frequency where the mooring lines have the greatest influence on the motion response. Compared to the predictions from a conventional global performance design tool that is based on diffraction analysis and empirical coefficients, the CFD results show significant improvements. The encouraging results from this study indicate that a CFD-based numerical wave basin, although still computationally expensive, is technically ready to be a complementary tool to physical wave basin for offshore platform global performance design.

scholarly journals Dynamics of The FKT System with Different Mooring Lines

2019 ◽  
Vol 26 (1) ◽  
pp. 20-29 ◽  
Author(s):  
Jo-Ti Wu ◽  
Jiahn-Horng Chen ◽  
Ching-Yeh Hsin ◽  
Forng-Chen Chiu
Keyword(s):  
Kuroshio Current ◽  
Normal Operation ◽  
Mooring Line ◽  
Joint Research ◽  
Mooring Lines ◽  
Neutral Buoyancy ◽  
Hydrokinetic Energy ◽  
Small Depth ◽  
Wave Effects ◽  
Line Design

Abstract To harness the endless hydrokinetic energy of the Kuroshio current, the joint research team of the National Taiwan University and the National Taiwan Ocean University has developed a floating Kuroshio turbine (FKT) system in Taiwan. In normal operation, the system floats at a certain small depth from the ocean surface to reduce the wave effects and take advantage of faster current speeds. In the present study, the effect of the mooring line on the system dynamics is investigated computationally. Two different auxiliary mooring line designs and, for each design, three different common mooring lines (polyester ropes of neutral buoyancy, iron chains, and 6×19 wires ropes with wire core) are examined. The study makes use of several commercial and in-house packages, integrated to find various coefficients. It is found that the mooring line, the auxiliary mooring line design, and the gravity centre can have a significant effect on system fluctuations in normal operation if the combination of these factors is not properly matched.

An Experimental Study of Mooring Line Damping in Shallow Water

2018 ◽  
Author(s):  
Wen-Yang Hsu ◽  
Tzu-Ching Chuang ◽  
Ray-Yeng Yang ◽  
Wei-Ting Hsu ◽  
Krish P. Thiagarajan
Keyword(s):  
Shallow Water ◽  
Dynamic Test ◽  
Processing Technique ◽  
Simple Approach ◽  
Image Processing Technique ◽  
Mooring Line ◽  
Line Geometry ◽  
Static Test ◽  
Wave Basin ◽  
A Chain

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used and the tension force at the fairlead was measured by tension gauges. Moreover, the mooring line geometry and velocity were computed from image processing technique. Series of horizontal motions at fairlead were driven from a programmable wavemaker. Regular surges with different frequency and pretensions were tested in this system to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, mooring line keeps a typical catenary shape and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed and it cyclically goes through the stage of semi-taut and fully taut. We successfully reproduced a snap event in the laboratory scale and the resulting mooring line damping can considerably increase in this manner. The mechanism of snap load is investigated by the measured tension, surge and mooring line velocities.

Experimental and Numerical Study on Large Truncation of Deepwater Mooring Line

2009 ◽  
Author(s):  
Yihua Su ◽  
Jianmin Yang ◽  
Longfei Xiao ◽  
Gang Chen
Keyword(s):  
Dynamic Response ◽  
Water Depth ◽  
Numerical Study ◽  
Unit Length ◽  
Low Frequency ◽  
Model Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Wave Basin

Modeling the deepwater mooring system in present available basin using standard Froude scaling at an acceptable scale presents new challenges. A prospective method is to truncate the full-depth mooring lines and find an equivalent truncated mooring system that can reproduce both static and dynamic response of the full-depth mooring system, but large truncation arise if the water depth where the deepwater platform located is very deep or the available water depth of the basin is shallow. A Cell-Truss Spar operated in 1500m water depth is calibrated in a wave basin with 4m water depth. Large truncation arises even though a small model scale 1:100 is chosen. A series of truncated mooring lines are designed and investigated through numerical simulations, single line model tests and coupled wave basin model tests. It is found that dynamic response of the truncated mooring line can be enlarged by using larger diameter and mass per unit length in air. Although the truncated mooring line with clump presents a “taut” shape, its dynamic characteristics is dominated by the geometry stiffness and it underestimates dynamic response of the full-depth mooring line, even induces high-frequency dynamic response. There are still two obstacles in realizing dynamic similarity for the largely truncated mooring system: lower mean value of the top tension of upstream mooring lines, and smaller low-frequency mooring-induced damping.

An Experimental Study of Mooring Line Damping and Snap Load in Shallow Water

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Wen-Yang Hsu ◽  
Tzu-Ching Chuang ◽  
Ray-Yeng Yang ◽  
Wei-Ting Hsu ◽  
Krish P. Thiagarajan
Keyword(s):  
Shallow Water ◽  
Dynamic Test ◽  
Mooring Line ◽  
Line Geometry ◽  
Static Test ◽  
Image Processing Techniques ◽  
Wave Basin ◽  
Touchdown Point ◽  
A Chain ◽  
Processing Techniques

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water, where snap loading may occur more frequently. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used, and the tension force at the fairlead was measured by tension gages. Moreover, the line geometry, touchdown point speed, and mooring line velocity were derived from image processing techniques. Surge motions at fairlead were driven from a programmable wavemaker. Regular surge motions with different frequencies and pretensions were tested in this system in order to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, the mooring line keeps a typical catenary shape, and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed, and it cyclically goes through the stage of semitaut and fully taut. We successfully reproduced a snap event in the laboratory scale, and the resulting mooring line damping can considerably increase in this manner. Two criteria for snap event were examined, and both of them were verified by the experiments.

A Reliability-Based Comparative Study for Mooring Lines Design Criteria

2006 ◽  
Author(s):  
Alberto Omar Vazquez-Hernandez ◽  
Gilberto Bruno Ellwanger ◽  
Lui´s Volnei Sudati Sagrilo
Keyword(s):  
Return Period ◽  
Design Procedure ◽  
Mooring Line ◽  
Design Criteria ◽  
Mooring Lines ◽  
Load Effect ◽  
Extreme Response ◽  
Line Design ◽  
Water Depths

The characteristic load effect for the design of mooring systems can be defined by means of three procedures: 1) an extreme sea state with a given return period, 2) a set of sea states on a contour line associated to a return period or 3) extreme response (tension) statistics for a long-term period. This work presents the result of a reliability-based partial safety factor calibration study for a LRFD mooring line design criteria considering the three approaches mentioned above. The calibration exercise is applied to three FPSOs considering North Sea environmental conditions and different water depths: 200m, 800m and 3000m. The mooring systems investigated take into account lines made up of chains and polyester ropes. It is shown that the design procedure based on the long-term response, among all water depths investigated, is the one that presents less scattered reliability indices around the target level.

scholarly journals Water Depth Variation Influence on the Mooring Line Design for FOWT within Shallow Water Region

2021 ◽  
Vol 9 (4) ◽  
pp. 409
Author(s):  
Wei-Hua Huang ◽  
Ray-Yeng Yang
Keyword(s):  
Wind Turbine ◽  
Water Depth ◽  
Return Period ◽  
Limit State ◽  
The Other ◽  
Ultimate Limit State ◽  
Mooring Lines ◽  
Floating Offshore Wind Turbines ◽  
Line Design ◽  
Water Depths

The objective of this paper was to present the modeling and optimization of mooring lines for floating offshore wind turbines (FOWT) located in various water depths from 50 m to 100 m in Taiwan western offshore areas. A semi-submersible floating wind turbine system is considered based on Offshore Code Comparison Collaborative Continuation (OC4) DeepCwind platform with the National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine. The mooring lines proposed consist of a catenary mooring with studless chains. Three nominal sizes of the mooring chain links are taken into account with diameters of 95 mm, 115 mm and 135 mm. According to this configuration, a total of five mooring designs for different water depths (i.e., 50 m, 60 m, 70 m, 80 m, 100 m) are analyzed according to the rules and regulations of the two certification institutions, Det Norske Veritas (DNV) and American Petroleum Institute (API). Considering ultimate limit state (ULS), fatigue limit state (FLS) and maximum operating sea state (MOSS) based on a typhoon with a 50-year return period and current with a 10-year return period, 25-year design life, as well as 1-year return period, respectively, long-term predictions of breaking strength, fatigue and stability are performed. The software OrcaFlex version 10.3 d is used to simulate and design the mooring lines. The obtained results show that the shallow mooring design of 50 m water depth case presents the heaviest chains among the other water depths, increasing their mooring costs. On the other hand, the 100 m water design has much longer mooring lines, making this parameter the cost driving one. Thus, the minimum mooring cost range is from 60 m to 80 m water depth.

scholarly journals Parametric Study of a Taut Compliant Mooring System for a FOWT Compared to a Catenary Mooring

2020 ◽  
Vol 8 (6) ◽  
pp. 431 ◽  
Author(s):  
Magnus Thorsen Bach-Gansmo ◽  
Stian Kielland Garvik ◽  
Jonas Bjerg Thomsen ◽  
Morten Thøtt Andersen
Keyword(s):  
Parametric Study ◽  
Environmental Benefits ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Wave Energy Converters ◽  
Cost Of Energy ◽  
Wave Basin ◽  
Line Angle ◽  
Energy Converters

The catenary mooring system is a well recognized station keeping method. However, there could be economical and environmental benefits of reducing the footprint. In the last decades, more focus has been given to synthetic mooring lines and different mooring layouts to optimize the levelized cost of energy (LCOE) for offshore renevable energy converters such as wave energy converters. Therefore, this work presents a parametric study of two important parameters, namely the mooring line angle and line pretension, for a taut mooring configuration focusing on the dynamic response when applied to the TetraSpar floating foundation compared to a catenary mooring system. The work is based on experimental results conducted in the wave basin at Aalborg University (AAU) and compared to analytical stiffness calculations. In addition, a numerical model was tuned based on the main dynamics to achieve the tension response. The results showed satisfying dynamic behavior where the angle and pretension mainly influenced the surge and yaw natural periods. The motion response showed similar behavior between the chosen parameters, and larger pitch amplitudes were found compared to the catenary system.

Sign in / Sign up

Export Citation Format

Share Document