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.

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 to the Evaluation of VIV Analyses Using Wake Oscillator and Vortex Tracking Models

2008 ◽  
Author(s):  
Daniel Lyrio Carneiro ◽  
Gilberto Bruno Ellwanger ◽  
Nelson Szilard Galgoul
Keyword(s):  
Degrees Of Freedom ◽  
Oil And Gas ◽  
Lateral Displacement ◽  
Three Dimensional ◽  
Low Frequency ◽  
Offshore Structures ◽  
Gas Production ◽  
Current Profile ◽  
Mooring Lines ◽  
Wake Oscillator

Fatigue due to vortex-induced vibrations (VIV) is one of the major uncertainties today in the design of slender offshore structures, such as risers, pipelines, umbilicals, tendons and mooring lines, required for oil and gas production in deep waters. The absence of reliable tools for quantitative analyses of this phenomenon is a technological barrier, which is being faced by several research groups. This paper presents valuable VIV results achieved by the author, using “wake oscillator” and “vortex tracking” models, when researching for his M. Sc. dissertation. Time domain analyses were performed using a commercial software. First results describe the response of the evaluated models for two degrees-of-freedom rigid cylinders by tracing lateral displacement versus flow velocity curves. These curves are plotted over others previously published in recognized recommended practices, articles and theses. Afterwards, VIV analyses results for a steel catenary 10-inch diameter riser in three-dimensional current profiles were compared to measured values. The considered riser, installed in 910m water depth offshore Brazil, is possibly the only monitored SCR with no VIV suppression devices in the world today. The results were considered satisfactory, despite some discrepancies: the model which appeared to be one of the most attractive for the rigid cylinder case, failed to predict VIV in the SCR under an irregular current profile, for example. Vortex tracking models presented excessive low frequency response in the SCR analyses. Authors believe that this response is unrealistic, and these frequencies can be dissipated by using a more adequate damping model than that employed by the utilized program.

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.

Frequency-Domain Calculations of Moored Vessel Motion Including Low Frequency Effect

2008 ◽  
Author(s):  
C. Le Cunff ◽  
Sam Ryu ◽  
Jean-Michel Heurtier ◽  
Arun S. Duggal
Keyword(s):  
Frequency Domain ◽  
Drag Force ◽  
Low Frequency ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Second Order ◽  
Wave Frequency ◽  
Diffraction Radiation ◽  
Floating Structure ◽  
Low Frequency Motion

Frequency-domain analysis can be used to evaluate the motions of the FPSO with its mooring and riser. The main assumption of the frequency-domain analysis is that the coupling is essentially linear. Calculations are performed taking into account first order wave loads on the floating structure. Added mass and radiation damping terms are frequency dependent, and can be easily considered in this formulation. The major non-linearity comes from the drag force both on lines and the floating structure. Linearization of the non-linear drag force acting on the lines is applied. The calculations can be extended to derive the low frequency motion of the floating structure. Second order low frequency quadratic transfer function is computed with a diffraction/radiation method. Given a wave spectrum, the second order force spectrum can then be derived. At the same time frequency-domain analysis is used to derive the low frequency motion and wave frequency motion of the floating system. As an example case, an FPSO is employed. Comparison is performed with time domain simulation to show the robustness of the frequency-domain analysis. Some calculations are also performed with either low frequency terms only or wave frequency terms only in order to check the effect of modeling low and wave frequency terms, separately. In the case study it is found that the low frequency motion is reduced by the wave frequency motion while the wave frequency motion is not affected by the low frequency motion.

Motion Responses of a Catenary-Taut-Hybrid Moored Single Module of a Semisubmersible Very Large Floating Structure in Multisloped Seabed

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Yiting Wang ◽  
Xuefeng Wang ◽  
Shengwen Xu ◽  
Lei Wang
Keyword(s):  
Shallow Water ◽  
Time Domain ◽  
Large Scale ◽  
Bottom Topography ◽  
Three Dimensional ◽  
Low Frequency ◽  
Mooring System ◽  
Floating Structure ◽  
Mooring Lines ◽  
Motion Responses

Motion responses of moored very large floating structures (VLFSs) in coastal regions are remarkably influenced by shallow water, seabed topography, and mooring system, which were given particular focus in this paper. A three-dimensional (3D) numerical model of a moored semisubmersible single module (SMOD) was described, and time domain simulated and experimentally validated. A catenary-taut-hybrid mooring system was adopted considering coastal space limitations. Large-scale catenary mooring lines were deployed on the deep water side, while taut chains were used on the shore side to decrease the anchor radius. Although the mooring system may induce a stiffness difference between the two sides, the effectiveness of the mooring system was demonstrated by time-domain simulation and model tests. The moored semisubmersible SMOD in shallow water exhibits significant low frequency characteristics. Water depth, asymmetric stiffness, and bottom topography effects were investigated by a series of sensitivity studies. The results show that these factors play an important role in motion responses of the moored SMOD, which can further conduce to better understandings on the hydrodynamic of the semisubmersible-type VLFSs.

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]

USING FIELD DATA FOR THE EVALUATION OF THE IMPACT OF ULTRA-LOW FREQUENCY MOTIONS ON THE FATIGUE OF MOORING LINES

2019 ◽  
Author(s):  
Guilherme Borzacchiello ◽  
Carl Albrecht ◽  
Fabricio N Correa ◽  
Breno Jacob ◽  
Guilherme da Silva Leal
Keyword(s):  
Field Data ◽  
Low Frequency ◽  
Mooring Lines ◽  
The Impact

scholarly journals Effect of Roughness of Mussels on Cylinder Forces from a Realistic Shape Modelling

2021 ◽  
Vol 9 (6) ◽  
pp. 598
Author(s):  
Antoine Marty ◽  
Franck Schoefs ◽  
Thomas Soulard ◽  
Christian Berhault ◽  
Jean-Valery Facq ◽  
...  
Keyword(s):  
Marine Species ◽  
Offshore Structures ◽  
Offshore Wind ◽  
Mooring Lines ◽  
Shape Modelling ◽  
Offshore Wind Turbines ◽  
Specific Effects ◽  
Hydrodynamic Loading ◽  
Floating Offshore Wind Turbines ◽  
Realistic Shape

After a few weeks, underwater components of offshore structures are colonized by marine species and after few years this marine growth can be significant. It has been shown that it affects the hydrodynamic loading of cylinder components such as legs and braces for jackets, risers and mooring lines for floating units. Over a decade, the development of Floating Offshore Wind Turbines highlighted specific effects due to the smaller size of their components. The effect of the roughness of hard marine growth on cylinders with smaller diameter increased and the shape should be representative of a real pattern. This paper first describes the two realistic shapes of a mature colonization by mussels and then presents the tests of these roughnesses in a hydrodynamic tank where three conditions are analyzed: current, wave and current with wave. Results are compared to the literature with a similar roughness and other shapes. The results highlight the fact that, for these realistic roughnesses, the behavior of the rough cylinders is mainly governed by the flow and not by their motions.

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.

Sign in / Sign up

Export Citation Format

Share Document