A Contribution on Quasi-Static Mooring Line Damping

1999 ◽  
Vol 122 (2) ◽  
pp. 125-133 ◽  
Author(s):  
Christian Bauduin ◽  
Mamoun Naciri
Keyword(s):  
Finite Element ◽  
Large Fraction ◽  
Low Frequency ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Mooring Cable ◽  
Static Approach ◽  
Numerical Tools ◽  
New Formulation

Since the pioneering work of Huse (1986, “Influence of Mooring Line Damping Upon Rig Motions,” Proc., 18th OTC Conference), it is well known that mooring lines may account for a large fraction of the overall damping present in a moored floating structure. This paper is concerned with the mooring line damping induced by the low-frequency, quasi-static, horizontal motion of the mooring line fairlead. The main advantage of the quasi-static approach is that it is much faster than the more accurate finite element methods, and, secondly, that it does not require any finite element modeling skills. A new formulation is proposed and is compared to the results of Liu et al. (1998, “Improvement on Huse’s Model for Estimating Mooring Cable-Induced Damping,” Proc., 17th OMAE Conference), as well as to time domain results obtained with FLEXRISER. The improvement with respect to the previous quasi-static methods is quite notable and our results are closer to FLEXRISER predictions. Finally, quasi-static results are compared to mooring line damping values measured during model tests for full mooring systems. The agreement between the two is very encouraging and suggests that the simpler quasi-static approach may, in some circumstances, be a valuable substitute for the more complex and time-consuming numerical tools. [S0892-7219(00)00102-3]

Experimental Investigation vs Numerical Simulation of the Dynamic Response of a Moored Floating Structure to Waves

2014 ◽  
Author(s):  
Daniele Dessi ◽  
Sara Siniscalchi Minna
Keyword(s):  
Dynamical Behavior ◽  
Irregular Waves ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Wave Energy Converters ◽  
Time Histories ◽  
Actual Configuration ◽  
Proper Orthogonal ◽  
Dynamic Wave

A combined numerical/theoretical investigation of a moored floating structure response to incoming waves is presented. The floating structure consists of three bodies, equipped with fenders, joined by elastic cables. The system is also moored to the seabed with eight mooring lines. This corresponds to an actual configuration of a floating structure used as a multipurpose platform for hosting wind-turbines, aquaculture farms or wave-energy converters. The dynamic wave response is investigated with numerical simulations in regular and irregular waves, showing a good agreement with experiments in terms of time histories of pitch, heave and surge motions as well as of the mooring line forces. To highlight the dynamical behavior of this complex configuration, the proper orthogonal decomposition is used for extracting the principal modes by which the moored structure oscillates in waves giving further insights about the way waves excites the structure.

Effects of the Mooring Line Configuration on the Dynamics of a Point Absorber

2013 ◽  
Author(s):  
Vincenzo Nava ◽  
Marin Rajic ◽  
Carlos Guedes Soares
Keyword(s):  
Case Studies ◽  
Time Instant ◽  
Floating Body ◽  
Mooring Line ◽  
Mooring Lines ◽  
Point Absorber ◽  
Line Configuration ◽  
Frequency Domains ◽  
Restoring Forces ◽  
Static Approach

The aim of this paper is to study the dynamics of a floating body with characteristics comparable to a point absorber wave energy converter with different mooring systems, in geometrical configuration or in the materials. To this purpose, the dynamics of a moored buoy is investigated. The point absorber is modeled as a spherical buoy in plane two-dimensional motion, and it is studied under the action of irregular unidirectional wind-generated waves, moored to the seabed by means of one, two or three mooring lines. Two different sets of moorings are considered, and typical wires and chains used in offshore technology are considered, leading to a total of 6 case studies. A quasi-static approach is used for modeling the restoring forces needed to keep buoy into station, using an innovative iterative procedure able to predict for each time instant and for each cable the lay down length of the cable, being each mooring line allowed to be taut or slack. Approaches in the time and frequency domains are used to obtain the system responses in intermediate waters, where these facilities are usually installed. Results for all case studies are compared both in terms of statistics of response and tensions on the top of the cable.

Semi-Submersible Floater’s VIM Simulation Method for Mooring Line Safety Assessment

2018 ◽  
Author(s):  
Toshifumi Fujiwara
Keyword(s):  
Safety Assessment ◽  
Simulation Method ◽  
Oscillator Model ◽  
Design Stage ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Wake Oscillator Model ◽  
Wake Oscillator ◽  
The Time Domain

The author proposed the Vortex-induced Motion (VIM) simulation method of a semi-submersible type offshore floating structure using the wake oscillator model based on the potential theory and model test data. This method is easy to use for the time-domain simulation of the VIM amplitude, that is in-line, transverse and yaw motions, of the semi-submersible floater in case of being demented mooring safety assessment of that. The simulation method presented in this paper was modified the single circular floater simulation method with the wake oscillator model for a semi-submersible floater. Some empirical parameters, obtained from the systematic model tests used many semi-submersible floaters, are only decided from external form of the semi-submersible floaters, that is the column / lower hull ratio etc. This simulation method is able to indicate general VIM trend and to be used for the assessment of mooring lines safety in the design stage. Using the VIM amplitude simulation, fatigue damage of mooring lines on one sample semi-submersible floater was investigated as an example.

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 The Quasi-Static Response of Moored Floating Structures Based on Minimization of Mechanical Energy

2021 ◽  
Vol 9 (9) ◽  
pp. 960
Author(s):  
Chun Bao Li ◽  
Mingsheng Chen ◽  
Joonmo Choung
Keyword(s):  
Potential Energy ◽  
Mechanical Energy ◽  
Line Tension ◽  
Line Length ◽  
Theoretical Background ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Floating Structures ◽  
Static Responses

It is essential to design a reasonable mooring line length that ensures quasi-static responses of moored floating structures are within an acceptable level, and that reduces the cost of mooring lines in the overall project. Quasi-static responses include the equilibrium position and the line tension of a moored floating structure (also called the mean value in a dynamic response), etc. The quasi-static responses derived by the classic catenary equation cannot present mooring–seabed interaction and hydrodynamic effects on a mooring line. While a commercial program can predict reasonable quasi-static responses, costly modeling is required. This motivated us to propose a new method for predicting quasi-static responses that minimizes the mechanical energy of the whole system based on basic geometric parameters, and that is easy to implement. In this study, the mechanical energy of moored floating structures is assumed to be the sum of gravitational–buoyancy potential energy, kinetic energy induced by drag forces, and spring potential energy derived by line tension. We introduce fundamental theoretical background for the development of the proposed method. We investigate the effect of quasi-static actions on mooring response, comparing the proposed method’s results with those from the catenary equation and ABAQUS software. The study reveals the shortcomings of the catenary equation in offshore applications. We also compare quasi-static responses derived by the AQWA numerical package with the results calculated from the proposed method for an 8 MW WindFloat 2 type of platform. Good agreement was drawn between the proposed method and AQWA. The proposed method proves more timesaving than AQWA in terms of modeling of mooring lines and floaters, and more accurate than the catenary equation, and can be used effectively in the early design phase of dimension mooring lengths for moored floating structures.

VIM Simulation Method on a Cylindrical Floating Structure

2016 ◽  
Author(s):  
Toshifumi Fujiwara
Keyword(s):  
Fatigue Damage ◽  
Low Frequency ◽  
Safety Evaluation ◽  
Simulation Method ◽  
Offshore Structures ◽  
Dominant Factor ◽  
Floating Structure ◽  
Mooring Lines ◽  
Wake Oscillator ◽  
Low Frequency Motion

A cylindrical floating structure can basically experience Vortex-induced Motion (VIM) in strong current. Since the VIM on the structure with long term low-frequency motion causes fatigue damage of the structure’s mooring lines and risers, precise VIM assessment is needed for the safety evaluation of them. In the standard of the International Organization for Standardization ISO19901-7, ‘Specific requirements on stationkeeping systems for floating offshore structures and mobile offshore units’, for instance, a concrete method of assessing VIM displacement is not represented in the standard document, though the requirement on the VIM demands to do the assessment on the basis of proper ways. Then in this paper, a VIM simulation method on a floating structure with circular cylinder form, that is, for example a Spar, a MPSO (Mono-column type floating Production Storage and Offloading) and so on, is shown using the wake-oscillator model. Transverse VIM is only treated since it is dominant factor on the fatigue damage of the mooring lines and risers. The assessment quality of the simulation method on the transverse VIM of floaters in current is confirmed by model test data.

Behaviour of Mooring Systems for Different Line Pretensions

2014 ◽  
Vol 567 ◽  
pp. 204-209 ◽  
Author(s):  
Montasir Osman Ahmed ◽  
Anurag Yenduri ◽  
V. John Kurian
Keyword(s):  
Experimental Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Preliminary Design ◽  
Mooring Lines ◽  
Matlab Code ◽  
Heading Direction ◽  
Restoring Forces ◽  
Static Approach ◽  
Floating Platforms

Mooring lines are the most commonly used station-keeping systems for floating platforms as they are easy to install and relocate. The mooring lines are usually pre-tensioned so as to use their energy absorption to reduce the platform motions and thereby, to lower the forces in the lines. To decide on the preliminary design of the platforms, it is necessary to investigate the restoring behaviour of the mooring systems for various parameters. In this study, two different mooring configurations with and without mooring line in wave heading direction are considered for determining its behaviour for various pretensions in the lines. A MATLAB code named QSAML has been developed using quasi-static approach to compute the restoring forces of the mooring system. The code is validated with experimental tests and used in this study. It has been observed that with increase in pretension of the mooring line, restoring performance of the mooring system can be improved. The maximum permissible excursions by mooring system in the wave heading direction are found to be more for relatively lower pretension values.

Sensitivity Analysis of an ANN-Based System for Detection of Mooring Line Failure

2020 ◽  
Author(s):  
Djoni E. Sidarta ◽  
Nicolas Tcherniguin ◽  
Philippe Bouchard ◽  
Ho-Joon Lim
Keyword(s):  
Numerical Simulations ◽  
Compressive Load ◽  
Low Frequency ◽  
Mooring Line ◽  
Offshore Platforms ◽  
Ann Model ◽  
Monitoring Equipment ◽  
Mooring Lines ◽  
Inclination Angles ◽  
Noise Tolerant

Abstract Monitoring the integrity of mooring lines on floating offshore platforms is one of the key factors in ensuring safe and productive offshore operations. Sensors, such as inclinometers, compressive load cells or strain sensors, can be used to monitor the inclination angles or tensions on mooring lines. An alternative method using only dry monitoring systems, such as DGPS (Differential Global Positioning System), Gyrocompass and/or IMU (Inertial Measurement / Motion Unit), can also be used to monitor the integrity of mooring lines. This method uses the measured motions and positions of a vessel without any information on the environmental conditions to detect mooring line failure. The detection of mooring line failure is based on detecting shifts in low-frequency periods and mean yaw angles as a function of vessel position, mass and added mass. The proposed method utilizes Artificial Neural Network (ANN) to recognize and classify patterns. The training of an ANN model requires examples/data associated with intact mooring lines and broken mooring line(s). Examples/data of broken mooring line(s) are practically available only from numerical simulations. Therefore, it is important to address these two key topics: (1) Is the real behavior of the floating offshore platform sufficiently aligned with numerical simulations? and (2) The effect of the accuracy of monitoring equipment on the performance of an ANN-based system. The first topic is reviewed briefly with its possible solution including some sensitivity tests, and this paper focuses on addressing the second topic. A system architecture is discussed in this paper along with the accuracy of the monitoring equipment. As an example, an ANN model has been trained to detect a broken mooring line of a spread-moored FPSO. This ANN model has been tested on its performance in dealing with a range of possible errors associated with the monitoring equipment. Furthermore, the tests have been carried out for a combination of variables that are not included in the ANN training, such as: vessel draft (mass), sea state conditions and directions. This paper presents the results of the tests for various variable sensitivities, which cover vessel positions, mean yaw angles and vessel drafts. These are essentially testing the tolerance of a trained ANN model against error or noise in the data. The results show that a trained ANN model can be error/noise tolerant.

Numerical Investigation on Interaction Between a Semi-Submersible Platform and its Mooring System

2015 ◽  
Author(s):  
Yuanchuan Liu ◽  
Yao Peng ◽  
Decheng Wan
Keyword(s):  
Mooring Line ◽  
Mooring System ◽  
Floating Structure ◽  
Mooring Lines ◽  
Flow Solver ◽  
Floating Structures ◽  
Marine Renewable Energy ◽  
Motion Responses ◽  
Increasing Demand ◽  
Energy Engineering

With the increasing demand of floating structures in offshore, coastal and marine renewable energy engineering, the interaction between the mooring system and floating structure becomes more and more important. In this paper, motion responses of a semi-submersible platform with mooring system under regular wave conditions are investigated numerically by a viscous flow solver naoe-FOAM-SJTU based on the open source toolbox OpenFOAM. Influence of the mooring system on the platform motion responses is evaluated in two different ways. Investigations are covered for analysis methods adopted for solving mooring lines and the length of each part of a multi-component mooring line. Several important conclusions are drawn.

scholarly journals Effect of Water Depths on the Hydrodynamic Responses of an FPSO Platform

2018 ◽  
Vol 203 ◽  
pp. 01022
Author(s):  
Matthew Guan ◽  
Montasir Osman Ahmed Ali ◽  
Cheng Yee Ng
Keyword(s):  
Oil And Gas ◽  
Response Spectrum ◽  
Low Frequency ◽  
Mooring Line ◽  
Random Wave ◽  
Statistical Parameters ◽  
Mooring Lines ◽  
Line System ◽  
Wave Condition ◽  
Water Depths

Ship-shaped Floating Production Storage Offloading platforms (FPSO) are commonly used in the production of oil and gas in offshore deepwater regions. The vessel is held in place by mooring lines anchored to the seabed during operation, either in spread or turret mooring arrangement. When designing such systems, water depth is a main factor that needs to be considered. At greater depths, the hydrodynamic properties of mooring lines become important and may not be accurately predicted through traditional experiments or numerical quasi-static models. Numerical simulation using coupled dynamic analysis is thus recommended, as the hull-mooring behaviour is analysed simultaneously, and the damping and added mass properties of the entire mooring line system is taken into account. This paper investigates the motions and mooring line tensions of a turret-moored FPSO at various water depths ranging from 1000 m to 2000 m. The analysis focuses on numerical simulations in the fully coupled dynamic time domain. The study utilizes the commercial software AQWA, with the FPSO model subjected to a unidirectional random wave condition. The hull hydrodynamics is first solved using the 3D radiation/diffraction panel method, and the hull response equation is then coupled with the mooring line equation. The dynamic motions and mooring line tensions results are presented in terms of statistical parameters as well as response spectrum. The results highlight the significance of greater water depths on low frequency responses in surge motions and mooring line tensions, and provides insight on the increasing and decreasing trend of these responses.

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