Time-Domain Coupled Dynamic Analysis of Mooring Systems in Extreme Sea Condition

2018 ◽  
Author(s):  
Xuliang Han ◽  
ShiSheng Wang ◽  
Bin Xie ◽  
Wenhui Xie ◽  
Weiwei Zhou
Keyword(s):  
Dynamic Analysis ◽  
Body Surface ◽  
Time Domain ◽  
Mooring Line ◽  
Mooring System ◽  
Dynamic Coupling ◽  
Spar Platform ◽  
Coupled Dynamic Analysis ◽  
Motion Responses ◽  
The Time Domain

In order to predict the coupled motion and external wave load for the design of deepwater floating structure system, based on the three-dimensional time-domain potential flow theory, this paper present the indirect time-domain dynamic coupling method and the body nonlinear dynamic coupling method. The perturbation expansion theory is adopted to evaluate hydrodynamic on the fixed mean wetted body surface for the former method. The transient free surface Green function has been extended and applied to calculate the nonlinear hydrodynamic on the instantaneous wetted exact body surface for the latter method. The finite element model is employed to solve dynamic response of mooring line. Then asynchronous coupled method is adopted to achieve the coupled dynamic analysis of platform and mooring lines. The time-domain motion responses and spectrum analysis of Spar platform are verified and compared with the traditional indirect time-domain coupling dynamic method when the mooring system is completed. Also the time-domain motion responses and statistical characteristic of Spar platform are investigated with one mooring line broken in extreme sea condition. Some conclusions are obtained, that is, dynamic coupling effects are significant and transient position hydrodynamic calculation of platform has a great influence on the low frequency motion. The results also show that the influence on the global performance of mooring system is different when the broken line is in different place. A remarkable influence occurs when the broken mooring line is in the head-wave direction.

Coupled Dynamic Analysis of a Moored Spar Platform in Time Domain

2010 ◽  
Author(s):  
M. D. Yang ◽  
B. Teng
Keyword(s):  
Boundary Condition ◽  
Free Surface ◽  
Dynamic Analysis ◽  
Time Domain ◽  
Surface Boundary ◽  
Spar Platform ◽  
Mooring Lines ◽  
Coupled Dynamic Analysis ◽  
Free Surface Boundary Condition ◽  
Surface Boundary Condition

A time-domain simulation method is developed for the coupled dynamic analysis of a spar platform with mooring lines. For the hydrodynamic loads, a time domain second order method is developed. In this approach, Taylor series expansions are applied to the body surface boundary condition and the free surface boundary condition, and Stokes perturbation procedure is then used to establish corresponding boundary value problems with time-independent boundaries. A higher order boundary element method is developed to calculate the velocity potential of the resulting flow field at each time step. The free-surface boundary condition is satisfied to the second order by 4th order Adams-Bashforth-Moultn method. An artificial damping layer is adopted on the free surface to avoid the wave reflection. For the mooring-line dynamics, a geometrically nonlinear finite element method using isoparametric cable element based on the total Lagrangian formulation is developed. In the coupled dynamic analysis, the motion equation for the hull and dynamic equations for mooring lines are solved simultaneously using Newmark method. Numerical results including motions and tensions in the mooring lines are presented.

Dynamic Response Analysis of a Triangular Tension Leg Platform

2015 ◽  
Author(s):  
Qiang Guo ◽  
Gang Ma ◽  
Liping Sun ◽  
Hongwei Wang ◽  
Na Cui
Keyword(s):  
Time Domain ◽  
Three Dimensional ◽  
Response Analysis ◽  
Mooring System ◽  
Preliminary Design ◽  
Tension Leg Platform ◽  
Operation Conditions ◽  
Motion Responses ◽  
Tendon Tension ◽  
The Time Domain

The tension leg platform is widely used in the world. In this paper, a newly developed tension leg platform is evaluated under the environment loads of the South China Sea. The focus is on the coupling response of the platform hull and tendons. The three dimensional potential theory is used to analyze the new developed tension leg platform and its mooring system in the time domain. The new developed TLP is in a triangular-shape with three group tension legs. Every group consists of five tendons; the mooring system has been optimized after preliminary design. Coupling analysis in time domain has been conducted to evaluate its motion and tendon tension under different environmental loads. The results demonstrate the great improvement in the motion responses of this new developed TLP. The coupled motion responses of this platform with tendon lines system in extreme environmental conditions have also been evaluated in order to evaluate the safety in operation conditions.

Impact of Bathymetry on the Mooring Design of an Offshore Floating Unit

2016 ◽  
Author(s):  
Vivek Jaiswal ◽  
Srinivas Vishnubhotla ◽  
Sean Cole ◽  
Robert B. Gordon ◽  
Partha Sharma
Keyword(s):  
Time Domain ◽  
Previous Method ◽  
Mooring Line ◽  
Mooring System ◽  
Analysis Program ◽  
Design Assessment ◽  
True Representation ◽  
The Time Domain ◽  
Constant Slope ◽  
The Impact

Moored offshore floating units may operate in regions where the bathymetry changes significantly over the mooring spread. Traditional mooring analysis methods make the assumption that the seabed slope is constant along the azimuth direction of each mooring line. This assumption, while reasonable for a seabed with nearly constant slope, can lead to significant errors with respect to the line tensions and vessel offsets in the mooring design assessment when the seabed slope is variable. This paper demonstrates the impact of bathymetry changes on the mooring design with the help of numerical analysis examples. The floating vessel considered is a semi-submersible moored by an eight point all chain catenary mooring system. Two methods of analysis are compared. In the first method, a true representation of the seabed surface that accounts for all variations in the bathymetry is used. In the second method, the anchor depths and the seabed slopes at the anchor locations estimated in the previous method are used, however, with the assumption of constant seabed slopes along the line azimuth directions. The dynamic analysis program Orcaflex is used for performing the numerical analyses in the time domain for both the methods. Differences in the performance of the mooring system are demonstrated by comparing the static and the dynamic line tensions as well as the vessel offsets in different environmental conditions. The paper also discusses how maximum offsets and line tensions are estimated.

Investigation on the Use of Different Approaches to Mooring Analysis and Appropriate Safety Factors

2012 ◽  
Author(s):  
Sojan Vasudevan ◽  
Paul Westlake
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Safety Factors ◽  
Dynamic Time ◽  
The Time Domain

This paper presents the results of the analyses of a twelve line catenary mooring system using a quasi-static method in the frequency domain, and uncoupled and coupled dynamic methods in the time domain. The latter is found to produce significantly higher tensions. The reasons for these differences are investigated. The minimum line tension safety factors required by design codes do not distinguish between uncoupled and coupled dynamic analyses and some codes use the same factors even for quasi-static analyses. Consequently, the present mooring system passes the acceptance criteria based on quasistatic frequency domain and uncoupled dynamic time domain analyses but does not meet the same criteria when a coupled dynamic time domain analysis is employed. It is understood that because the coupled time domain analysis determines the vessel motions using all forces the accuracy of mooring line tension estimation will be improved over other methods. Hence the application of less conservative safety factors is proposed.

Progressive Mooring-Line Failure of a Deepwater MODU in Hurricane Conditions

2009 ◽  
Author(s):  
Zhi Zhang ◽  
M. H. Kim ◽  
E. G. Ward
Keyword(s):  
Time Domain ◽  
Analysis Data ◽  
Field Analysis ◽  
Mooring Line ◽  
Coupled Dynamic Analysis ◽  
Hurricane Ivan ◽  
Analysis Computer ◽  
Drilling Unit ◽  
The Time Domain ◽  
Analysis Computer Program

During 2004–2005, three consecutive category-5 storms hit the central region of Gulf of Mexico (GOM) and damaged numerous drilling and production platforms. Since then, a number of forensic studies have been conducted to better understand the failure causes and mechanism. In this study, a representative deepwater semi-submersible Mobile Offshore Drilling Unit (MODU) that suffered a mooring failure and went adrift during the hurricane Ivan was selected for numerical forensic study. The platform responses and progressive mooring-line failure are simulated in the time domain. A time-domain vessel-mooring coupled dynamic analysis computer program is used to simulate the sequence of progressive mooring-line failure of the MODU in the hindcast Ivan environment, in which the wind, wave, and currents were non-collinear. The case of non-collinear environment is also compared with that of collinear environment. The numerical prediction and simulated results are compared with the available field-analysis data.

scholarly journals Time and Frequency Domain Dynamic Analysis of Offshore Mooring

2021 ◽  
Vol 9 (7) ◽  
pp. 781
Author(s):  
Shi He ◽  
Aijun Wang
Keyword(s):  
Dynamic Analysis ◽  
Frequency Domain ◽  
Time Domain ◽  
Linearization Method ◽  
Euler Method ◽  
Single Component ◽  
Hydrodynamic Drag ◽  
Lumped Mass ◽  
Mooring Lines ◽  
The Time Domain

The numerical procedures for dynamic analysis of mooring lines in the time domain and frequency domain were developed in this work. The lumped mass method was used to model the mooring lines. In the time domain dynamic analysis, the modified Euler method was used to solve the motion equation of mooring lines. The dynamic analyses of mooring lines under horizontal, vertical, and combined harmonic excitations were carried out. The cases of single-component and multicomponent mooring lines under these excitations were studied, respectively. The case considering the seabed contact was also included. The program was validated by comparing with the results from commercial software, Orcaflex. For the frequency domain dynamic analysis, an improved frame invariant stochastic linearization method was applied to the nonlinear hydrodynamic drag term. The cases of single-component and multicomponent mooring lines were studied. The comparison of results shows that frequency domain results agree well with nonlinear time domain results.

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.

Experimental Validation of a Dynamic Simulation

1990 ◽  
Author(s):  
K. Harold Yae ◽  
Su-Tai Chern ◽  
Howyoung Hwang
Keyword(s):  
Dynamic Analysis ◽  
Dynamic Simulation ◽  
Time Domain ◽  
Experimental Validation ◽  
Initial Conditions ◽  
Hydraulic Cylinder ◽  
Force Output ◽  
Inverse Dynamic ◽  
Inverse Dynamic Analysis ◽  
The Time Domain

Abstract Using forward and inverse dynamic analysis, the dynamic simulation of a backhoe has been compared with experiments. In the experiment, recorded were the configuration and force histories; that is, velocity and position, and force output from the hydraulic cylinder-all were measured in the time domain. When the experimental force history is used as driving force in the simulation, forward dynamic analysis produces a corresponding motion history. And when the experimental motion history is used as if a prescribed trajectory, inverse dynamic analysis generates a corresponding force history. Therefore, these two sets of motion and force histories — one set from experiment, and the other from the simulation that is driven forward and backward with the experimental data — are compared in the time domain. The comparisons are discussed in regard to the effects of variations in initial conditions, friction, and viscous damping.

On the Dynamic Analysis of Roller Chain Drives: Part II — Case Study

1992 ◽  
Author(s):  
Nicholas M. Veikos ◽  
Ferdinand Freudenstein
Keyword(s):  
Dynamic Analysis ◽  
Time Domain ◽  
Equations Of Motion ◽  
Axial Stiffness ◽  
Roller Chain ◽  
Computer Aided ◽  
Computational Aspects ◽  
The Time Domain ◽  
Chain Drives

Abstract Part I of this paper (5) summarized the previous work and has described the theoretical and computational aspects of a computer-aided procedure which has been developed by the authors for the dynamic analysis of roller chain drives. Lagrange’s equations of motion have been derived by assuming the roller chain to behave as a series of masses lumped at the roller centers and connected by bars of constant axial stiffness. The equations of motion are solved in the time domain until steady state conditions are achieved.

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