Route to Chaos of an Articulated Offshore Loading Platform

2004 ◽  
Author(s):  
A. P. Shashikala
Keyword(s):  
Wind Waves ◽  
Nonlinear Behavior ◽  
Mooring Line ◽  
Response Analysis ◽  
Mooring Lines ◽  
Restoring Force ◽  
Highly Nonlinear ◽  
Waves And Currents ◽  
Time Domain Response ◽  
The Time Domain

Articulated Loading Platforms are compliant structures which undergo excessive displacements due to large hydrodynamic loads produced by wind, waves and currents. Prediction of nonlinear behavior of these complex structures in the nonlinear environment is extremely difficult. The discontinuity in the mooring line stiffness at the equilibrium position due to slackening of the mooring line between tower and tanker introduces nonlinearity in the equation of motion. An attempt to study the effect of the highly nonlinear restoring force and hydrodynamic forces on the system was performed. The time domain response analysis was done on the basis of approximate analytical investigations. The solution of the nonlinear simultaneous equations was performed and the results were interpreted by means of phase plots and poincare mapping. The effect of forcing amplitude on the behavior of the system was studied by varying the frequency ratio. Possible occurrence of sub harmonic and chaotic responses and hence different routes to chaos were also identified. These results can be made use of in the design of mooring lines to avoid structural instabilities in the evolving offshore environment.

Development of a FPSO Motion Simulator

2015 ◽  
Author(s):  
Dexin Zhan ◽  
Worakanok Thanyamanta ◽  
Jason McDonald ◽  
David Molyneux
Keyword(s):  
Control System ◽  
Wind Waves ◽  
Line Tension ◽  
Weather Conditions ◽  
Mooring Line ◽  
Restoring Force ◽  
3 Dimensional ◽  
Motion Simulator ◽  
Heading Control ◽  
Time Domain Response

This paper presents the development of a motion simulator for a moored FPSO, which includes numerical prediction of the FPSO motions in wind, waves and current. It also presents the resulting mooring line tension, 3-dimensional visualization of the FPSO motion, and summary analysis of the resulting motion parameters. The FPSO motion in waves was simulated using an in-house seakeeping code, MOTSIM. A spread mooring line routine, based on catenary theory, was developed and added to MOTSIM to calculate the restoring force of each mooring line. The visualizer (or animator) was developed in-house from open source software, including Ogre, Hydrax and Skyx. It can playback a 3-dimensional view of the simulation (above and below water). The user can view the results in a movie-like format, and change viewing position during the play-back. The user can also run a new simulation from the animator by inputting the required parameters. The program for analyzing the time dependent responses generated by MOTSIM was developed as a stand-alone program using MATLAB. The analyzer can conduct statistical analysis of time-domain response signals. A heading control system and a DP control system were also developed in the simulator and can be activated to help control the FPSO motion if required. A validation of the ship motion prediction and mooring tension was conducted against model experiments using 100-year return period environments with different combinations of wave, wind and current directions. The simulator was developed as a forecasting tool to help operators predict platform performance based of forecast weather conditions.

The Dynamic Response of the Platform Mooring Conditions

2014 ◽  
Vol 501-504 ◽  
pp. 2067-2072
Author(s):  
Xin Xin Wu ◽  
Yue Qin Liu ◽  
Jin Hao Xu ◽  
Chao He Chen
Keyword(s):  
Dynamic Response ◽  
Wind Waves ◽  
Reference Value ◽  
Response Analysis ◽  
Time Curve ◽  
Waves And Currents ◽  
Study Case ◽  
Displacement Force ◽  
Time Domain Response ◽  
Force Time

Using the finite element ANSYS workbench platform Hydrodynamic Diffraction and Hydrodynamic Time Response Module, taking a self-elevating offshore mooring location as a study case for the corresponding value calculated the dynamic response of the platform. Studied in the wind, waves and currents under load, platforms and mooring ropes coupled motion in case of time-domain response analysis, and get mooring ropes mooring and platform displacement force-time curve. Methods discussed in this paper have a certain reference value for offshore platform mooring positioning.

Dynamic Modeling of Ship-to-Ship and Ship-to-Pier Mooring Performance

2018 ◽  
Vol 52 (5) ◽  
pp. 87-93
Author(s):  
Sean Kery
Keyword(s):  
Dynamic Modeling ◽  
Natural Fiber ◽  
Wind Waves ◽  
Mooring Line ◽  
High Modulus ◽  
Mooring Lines ◽  
High Tension ◽  
Constant Tension ◽  
Waves And Currents ◽  
Modulus Polyethylene

AbstractTraditional methods for the design of ship-to-pier moorings in normal and storm conditions and for ship-to-ship moorings under normal conditions are currently based on static calculations. These calculations have served well for many years, first with natural fiber ropes and later with nylon and other low- to medium-modulus synthetics. Key to the success of this simplistic approach is lines that can elongate enough under tension to share the loads between multiple lines. When wire rope mooring lines are introduced, an increased weight catenary and the use of constant tension winches allowed enough compliance for the moorings to load share successfully.Now, we have very lightweight, high-modulus synthetic lines like High Modulus Polyethylene (HMPE), Aramid, and Liquid Crystal Polymer (LCP), where there is almost no stretch and very little weight to form a weight catenary. When used with constant tension winches that allow the mooring load to be shared across multiple lines, these can work well. However, when they are used from bollard to chock to bit with no compliance, they are unable to share the load between multiple lines, and high tension failures occur where a weaker but more compliant mooring line would be fine.This article describes advanced dynamic modeling of ships loaded by wind, waves, and currents in these conditions and the tension sharing between mooring lines of different materials and constructions. The need to share the mooring load between multiple lines is the crux of the issue.

Optimized Mooring Line Simulation Using a Hybrid Method Time Domain Scheme

2014 ◽  
Author(s):  
Niels Hørbye Christiansen ◽  
Per Erlend Torbergsen Voie ◽  
Jan Høgsberg ◽  
Nils Sødahl
Keyword(s):  
Hybrid Method ◽  
Time Domain ◽  
Computation Time ◽  
Mooring Line ◽  
Mooring Lines ◽  
Fem Model ◽  
Input Variables ◽  
The Time Domain ◽  
The Cost ◽  
Short Time

Dynamic analyses of slender marine structures are computationally expensive. Recently it has been shown how a hybrid method which combines FEM models and artificial neural networks (ANN) can be used to reduce the computation time spend on the time domain simulations associated with fatigue analysis of mooring lines by two orders of magnitude. The present study shows how an ANN trained to perform nonlinear dynamic response simulation can be optimized using a method known as optimal brain damage (OBD) and thereby be used to rank the importance of all analysis input. Both the training and the optimization of the ANN are based on one short time domain simulation sequence generated by a FEM model of the structure. This means that it is possible to evaluate the importance of input parameters based on this single simulation only. The method is tested on a numerical model of mooring lines on a floating off-shore installation. It is shown that it is possible to estimate the cost of ignoring one or more input variables in an analysis.

scholarly journals Dynamic Response of Floating Wind Turbine Under Consideration of Dynamic Behavior of Catenary Mooring-Lines

2017 ◽  
Author(s):  
Shuangxi Guo ◽  
Yilun Li ◽  
Min Li ◽  
Weimin Chen ◽  
Yiqin Fu
Keyword(s):  
Dynamic Response ◽  
Wind Turbine ◽  
Nonlinear Dynamic ◽  
Ocean Waves ◽  
Wave Frequency ◽  
Mooring Line ◽  
Mooring Lines ◽  
Restoring Force ◽  
Dynamic Behaviors ◽  
Floating Wind Turbine

Recently, wind turbine has been developed from onshore area to offshore area because of more powerful available wind energy in ocean area and more distant and less harmful noise coming from turbine. As it is approaching toward deeper water depth, the dynamic response of the large floating wind turbine experiencing various environmental loads becomes more challenge. For examples, as the structural size gets larger, the dynamic interaction between the flexible bodies such as blades, tower and catenary mooring-lines become more profound, and the dynamic behaviors such as structural inertia and hydrodynamic force of the mooring-line get more obvious. In this paper, the dynamic response of a 5MW floating wind turbine undergoing different ocean waves is examined by our FEM approach in which the dynamic behaviors of the catenary mooring-line are involved and the integrated system including flexible multi-bodies such as blades, tower, spar platform and catenaries can be considered. Firstly, the nonlinear dynamic model of the integrated wind turbine is developed. Different from the traditional static restoring force, the dynamic restoring force is analyzed based on our 3d curved flexible beam approach where the structural curvature changes with its spatial position and the time in terms of vector equations. And, the modified finite element simulation is used to model a flexible and moving catenary of which the hydrodynamic load depending on the mooring-line’s motion is considered. Then, the nonlinear dynamic governing equations is numerically solved by using Newmark-Beta method. Based on our numerical simulations, the influences of the dynamic behaviors of the catenary mooring-line on its restoring performance are presented. The dynamic responses of the floating wind turbine, e.g. the displacement of the spar and top tower and the dynamic tension of the catenary, undergoing various ocean waves, are examined. The dynamic coupling between different spar motions, i.e. surge and pitch, are discussed too. Our numerical results show: the dynamic behaviors of mooring-line may significantly increase the top tension, particularly, the peak-trough tension gap of snap tension may be more than 9 times larger than the quasi-static result. When the wave frequency is much higher than the system, the dynamic effects of the mooring system will accelerate the decay of transient items of the dynamic response; when the wave frequency and the system frequency are close to each other, the displacement of the spar significantly reduces by around 26%. Under regular wave condition, the coupling between the surge and pitch motions are not obvious; but under extreme condition, pitch motion may get about 20% smaller than that without consideration of the coupling between the surge and pitch motions.

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.

Coupled Surge-Heave-Pitch Dynamic Modeling of Spar-Moonpool-Riser Interaction

2009 ◽  
Author(s):  
Pol D. Spanos ◽  
Vincenzo Nava ◽  
Felice Arena
Keyword(s):  
Nonlinear Differential Equations ◽  
Wave Spectrum ◽  
Nonlinear Behavior ◽  
Analytical Techniques ◽  
Degree Of Freedom ◽  
Equivalent Linearization ◽  
Parameter Study ◽  
Mooring Lines ◽  
Study Results ◽  
Highly Nonlinear

Due to the ongoing rather intense development of deep water gas and oil fields, the technical community has been increasingly focusing its attention to the dynamic behavior of Spar floating structures. Spar dynamics exhibits a highly nonlinear behavior due to the presence of various components such as mooring lines, moonpool, and risers (Spanos et al., 2005). In this regard Gupta et al, 2008a, have studied the reduction of the heave response in a single degree-of-freedom spar model due to the oscillations of water entrapped in the moonpool through the partially closed bottom plates. In this paper a novel coupled six-degree-of-freedom analytical model of a Spar system tensioned by TTR risers is proposed. The model accounts for the interactions among spar hull motions (heave, surge and pitch), the riser motion (heave and surge), and the moonpool. This model involves six coupled nonlinear differential equations comprising nonlinearity terms associated not only with stiffness and damping but also with inertia terms. A dynamic analysis is performed by subjecting the model to JONSWAP ocean wave spectrum compatible extreme forces (corresponding to the 100 yr wave); and to moments applied to the center of gravity computed by means of standard motion program. Both numerical and semi-analytical techniques (equivalent linearization including inertial terms) are used for the determination of the response of the proposed dynamic model both in the time and frequency domains. Some parameter study results are reported, including ones pertaining to the dependence of the spar motion on the open guide plates.

Numerical Modelling of the Mooring Line Failure Induced Performance Changes of a Marine Fish Cage in Irregular Waves and Currents

2019 ◽  
Author(s):  
Hung-Jie Tang ◽  
Ray-Yeng Yang ◽  
Chai-Cheng Huang
Keyword(s):  
Marine Fish ◽  
Low Frequency ◽  
Frequency Region ◽  
Irregular Waves ◽  
Mooring Line ◽  
Lumped Mass ◽  
Lumped Mass Method ◽  
Waves And Currents ◽  
Failure Scenario ◽  
The Time Domain

Abstract This study aims to investigate the performance changes resulted from a mooring line failure of a marine fish cage exposed to irregular waves and current. A numerical model based on the lumped mass method and Morison equation was extended to simulate the mooring line failure scenario. In this study, the failed resulting changes were compared with its normal counterpart in both the time domain and the frequency domain. After one upstream anchor loss, the maximum tension on the remaining anchor has increased significantly, as well as the drift distance of the rearing part (net chamber, floating collar, and tube-sinker) of the fish cage. The resulting changes can also be seen in both the wave-frequency and the low-frequency region in the spectra, including mooring tensions and body motions.

Application of Peak Distribution Method for Response Based Analysis of Mooring Lines Under Tropical Storms

2020 ◽  
Author(s):  
A. Ghasemi ◽  
Y. Drobyshevski ◽  
M. Kimiaei ◽  
M. Efthymiou
Keyword(s):  
Time Domain ◽  
Nonlinear Behavior ◽  
Extreme Value Distribution ◽  
Time Domain Analysis ◽  
System Response ◽  
Response Analysis ◽  
Tropical Storms ◽  
Mooring Lines ◽  
Distribution Method ◽  
Large Variability

Abstract Response based analysis (RBA) is a comprehensive approach for the prediction of extreme responses and design metocean conditions of offshore facilities. For RBA, the structural system needs to be modelled, and its behavior analyzed when subjected to large metocean datasets, usually comprising thousands of different sea states. Due to the dynamic and nonlinear behavior of mooring systems in floating structures, application of conventional time domain analysis for RBA of these systems is a computationally demanding process. Hence, investigation of faster solvers and more efficient methods for the RBA is inevitable. Peak distribution method (PDM), which has recently been introduced and used for response analysis of mooring systems under extreme design conditions, is a possible solution to reduce the computational efforts in RBA by reducing the number of simulations. This study explores the utilization of the PDM for RBA of the mooring system of a turret-moored large FPSO subjected to tropical storms. Large variability of metocean parameters within such storms limits the applicability of intuitive judgement for the selection of governing sea states. The results are compared through both time-domain and frequency-domain simulations and a computationally efficient methodology is proposed. It provides a general robust framework of computing the extreme value distribution of the system response. The proposed methodology can be used for RBA of mooring lines tension under storm conditions comprising large number of sea states.

scholarly journals Mooring Analysis of a Floating OWC Wave Energy Converter

2021 ◽  
Vol 9 (2) ◽  
pp. 228
Author(s):  
Alana Pols ◽  
Eric Gubesch ◽  
Nagi Abdussamie ◽  
Irene Penesis ◽  
Christopher Chin
Keyword(s):  
Incident Wave ◽  
Wave Energy ◽  
Wave Energy Converter ◽  
Numerical Code ◽  
Scale Model ◽  
Mooring Line ◽  
Energy Converter ◽  
Mooring Lines ◽  
Restoring Force ◽  
Acceptable Method

This investigation focuses on the modelling of a floating oscillating water column (FOWC) wave energy converter with a numerical code (ANSYS AQWA) based on potential flow theory. Free-floating motions predicted by the numerical model were validated against experimental data extrapolated from a 1:36 scale model device in regular and irregular sea states. Upon validation, an assessment of the device’s motions when dynamically coupled with a four-line catenary mooring arrangement was conducted at different incident wave angles and sea states ranging from operational to survivable conditions, including the simulation of the failure of a single mooring line. The lack of viscosity in the numerical modelling led to overpredicted motions in the vicinity of the resonant frequencies; however, the addition of an external linear damping coefficient was shown to be an acceptable method of mitigating these discrepancies. The incident wave angle was found to have a limited influence on the magnitudes of heave, pitch, and surge motions. Furthermore, the obtained results indicated that the mooring restoring force is controlled by the forward mooring lines under the tested conditions.

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