Parametric Study of Mooring Lines on a Small Buoy

2003 ◽  
Author(s):  
Quanming Miao ◽  
Jinzhu Xia
Keyword(s):  
Mooring Line ◽  
Diffraction Radiation ◽  
Hydrodynamic Coefficients ◽  
Analysis Method ◽  
Mooring Lines ◽  
Slow Drift ◽  
Radiation Theory ◽  
Exciting Forces ◽  
Wave Exciting Forces ◽  
Drift Forces

A shallow-draft cylindrical buoy and mooring lines comprise an integrated dynamic system responding to environmental loading due to wind, current and waves in a complex way. In this paper, a time-domain decoupled buoy motion analysis method will be applied to study numerically the effect of the length, pre-tension and coordinates of attached points of the mooring lines on the motions of the buoy and the loads of the mooring lines. The wind and the current speeds are assumed to be constant and the wind and current forces are estimated from empirical formulations. The hydrodynamic coefficients, wave exciting forces and slow drift forces of the buoy are obtained from the 3D diffraction-radiation theory. The numerical results show how the length of the mooring line influences the maximum mooring loads at severe sea. The results also quantitatively show how the pre-tension and the coordinates of attached point of the mooring lines affect the motions and loads of the moored system.

The Motions of a Ship on a Sloped Seabed

2006 ◽  
Author(s):  
Bas Buchner
Keyword(s):  
Water Depth ◽  
Diffraction Theory ◽  
Added Mass ◽  
Interference Effects ◽  
Large Size ◽  
Correct Calculation ◽  
Exciting Forces ◽  
Wave Exciting Forces ◽  
Local Water ◽  
Drift Forces

In standard diffraction theory it is assumed that the water depth is constant and that the seabed is infinitely large. To account for a local varying bathymetry in shallow water (as it can occur for offshore LNG terminals) it is sometimes considered to introduce a second fixed body on the seabed representing this bathymetry in diffraction theory. Based on the results presented in this paper it can be concluded that this is (without special measures) not possible. The refraction and interference effects are too strong and affect the wave exciting forces on the LNG carrier in an incorrect way. A large size of the second body and smoother edges of this body do not improve the situation. However, a second body in diffraction theory, when chosen properly with respect to size and shape, can contribute to the correct calculation of the added mass and damping of vessels on sloped seabeds as this varies with the local water depth over the length of the vessel. This will clearly affect the motion response of the vessel. This can be seen for instance in the pitch-heave coupling. This will influence the motions of the ship in waves, as well as the resulting drift forces and related mooring loads.

Study on Weight Control Methods of Single Point Mooring System of FPSO in Deepwater

2015 ◽  
Author(s):  
Yuan Hongtao ◽  
Zeng Ji ◽  
Chen Gang ◽  
Mo Jian ◽  
Zhao Nan
Keyword(s):  
Weight Control ◽  
Linear Time ◽  
Single Point ◽  
Line Tension ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Control Methods ◽  
Analysis Method ◽  
Mooring Lines

This paper applies 3D potential theory and non-linear time domain coupled analysis method to analyze motion response of FPSO and dynamic response of mooring line of single mooring system. In addition, respectively to calculate mooring line tension of tension type and composite mooring line type and added buoy in mooring line. There the paper analyze different mooring lines to affect on the weight of single point mooring system of deepwater FPSO. Which expects to provide a theoretical basis for single point mooring system design and weight control.

Performance Evaluations of Taut-Wire Mooring Systems for Deepwater Semi-Submersible Platform

2011 ◽  
Author(s):  
P. Chen ◽  
S. Chai ◽  
J. Ma
Keyword(s):  
Line Tension ◽  
Dynamic Loads ◽  
Mooring Line ◽  
Hydrodynamic Coefficients ◽  
Mooring Lines ◽  
Coupled Motion ◽  
Motion Responses ◽  
Parametric Studies ◽  
Motion Performance ◽  
Dip Angle

In order to investigate the effect of taut-wire mooring system on the motion performance of semi-submersible platforms, parametric studies of coupled motion responses are conducted using a time domain analysis in this study. The nonlinear dynamic characteristics of mooring lines and the interactions of platform and mooring lines are investigated. The parametric studies consist of investigating the effects of the hydrodynamic coefficients CA and CD of mooring line, tension dip angle, mooring line pretension, different taut-mooring arrangements and total number of mooring lines on the motion performance of a semi-submersible platform in water depth of 1500 meters, which is subjected to a 100 year return significant wave height of 13.3 meters, a peak period of 15.5 seconds, a current speed of 1.97 meters per second and wind speed of 55 meters per second. The wind and current both act in the same direction as the ocean waves in this study in order to estimate the maximum mooring line loads. The environmental load direction is varied from 0° to 90° at the interval of 15 degrees. Seven directions are calculated in total. The research results show that the different parameters, such as the hydrodynamic coefficients of the mooring line, tension dip angle, pre-tension, arrangement angle of mooring lines and total number of mooring lines, have different effects on the coupled motion responses. In particular, the arrangement angles of mooring lines have significant effect on motion responses and dynamic loads of mooring lines. The motion performance of semi-submersible platform and mooring line dynamic loads can be controlled effectively when these parameters are selected reasonably throughout parametric studies carefully designed and conducted.

Reliability Analysis of Mooring Lines for Deep Water Floating Systems

2012 ◽  
Author(s):  
K. Gurumurthy ◽  
Suhail Ahmad ◽  
A. S. Chitrapu
Keyword(s):  
Dynamic Analysis ◽  
Reliability Analysis ◽  
Deep Water ◽  
Reliability Assessment ◽  
Mooring Line ◽  
Analysis Method ◽  
Mooring Lines ◽  
Line Load ◽  
Coupled Dynamic Analysis ◽  
Floating Systems

Reliability analysis of mooring lines requires an accurate prediction of extreme responses for large number of sea states even for a short-term based approach. In deep water, the interactions between the floater motions and the large number of risers and mooring lines become significant and must be considered for accurate prediction of floater motions as well as line dynamics. Time-domain coupled dynamic analysis procedures have been shown to give more accurate results but at a higher computational expense. Therefore, efficient computational tools are required for reliability analysis of mooring lines for deep water floating systems. Enhanced decoupled dynamic analysis method, in which the floater motions are computed by coupled analysis considering a coarse finite element model of the mooring line, is an efficient method and provides results comparable in accuracy with the fully coupled dynamic analysis procedures. This paper presents the application of enhanced de-coupled dynamic analysis method for reliability assessment of mooring lines for deep water floating systems. For reliability analysis of mooring lines, the methodology presented in Ding et al. [5] is adopted. Reliability analysis of a critically loaded mooring line for a deep water classical spar floater under extreme environmental loads is performed using environmental contour approach. Mooring line tension time histories under various storm conditions are calculated using enhanced de-coupled dynamic analysis. The uncertainty in the predicted maximum mooring line load due to different storm events, variability in met-ocean conditions and numerical models is considered. Probability of failure and the corresponding reliability index of the mooring line are calculated. The impact of variability in predicted mooring line load, line capacities and factors of safety on mooring line reliability are studied. It is seen that enhanced de-coupled dynamic analysis, which predicts the mooring line loads as accurately as coupled dynamic analysis with lesser CPU time, can be used more efficiently for reliability assessment of mooring lines for deep water floating systems.

Effect of Slow-Drift Loads on Nonlinear Dynamics of Spread Mooring Systems

1998 ◽  
Vol 120 (4) ◽  
pp. 201-211 ◽  
Author(s):  
M. M. Bernitsas ◽  
B.-K. Kim
Keyword(s):  
Nonlinear Dynamics ◽  
Horizontal Plane ◽  
Parametric Design ◽  
Slow Motion ◽  
Third Order ◽  
Mooring Lines ◽  
Slow Drift ◽  
Restoring Forces ◽  
The Mathematical Model ◽  
Drift Forces

Spread mooring systems (SMS) may experience large-amplitude oscillations in the horizontal plane due to slow-drift loads. In the literature, this phenomenon is attributed to resonance. In this paper, it is shown that this conclusion is only partially correct. This phenomenon is investigated using nonlinear stability and bifurcation analyses which reveal an enhanced picture of the nonlinear dynamics of SMS. Catastrophe sets are developed in a parametric design space to define regions of qualitatively different system dynamics for autonomous SMS, including mean drift forces. Limited time simulations are performed to verify the qualitative conclusions drawn on the nonlinear dynamics of SMS using catastrophe sets. Slowly varying drift forces are studied as an additional excitation on the autonomous SMS and simulations reveal that slow drift may cause resonance or bifurcations with stabilizing or destabilizing morphogeneses. The mathematical model of SMS is based on the slow-motion maneuvering equations in the horizontal plane (surge, sway, yaw), including hydrodynamic forces with terms up to third-order, nonlinear restoring forces from mooring lines, and environmental loads due to current, wind, and wave-drift.

Sensitivity of a Floating Bridge Global Responses to Different Wave Drift Force Models

2021 ◽  
Author(s):  
Carlos Eduardo Silva de Souza ◽  
Nuno Fonseca ◽  
Marit Irene Kvittem
Keyword(s):  
Mooring Line ◽  
Sea State ◽  
Mooring Lines ◽  
Wave Drift ◽  
Floating Bridge ◽  
Promising Solution ◽  
Bridge Girder ◽  
Wave Drift Forces ◽  
Wave Incidence ◽  
Drift Forces

Abstract Floating bridges are a promising solution for replacing ferries in the crossing of Norwegian fjords. Their design involves the adoption of accurate, but at the same time efficient models for the loads the structure is subjected to. Wave drift forces at the bridge’s pontoon may excite the bridge’s lower horizontal modes, with consequences to the loads on the bridge and mooring lines. Newman’s approximation is normally adopted to calculate the wave drift forces in such applications. A common simplification is to assume that the pontoons are fixed in the calculation of wave drift coefficients, while it is known that wave frequency motions may significantly influence drift loads. This paper evaluates the consequences of this simplification, in comparison to coefficients obtained considering the pontoons’ motions. First, the effect of the bridge deflection, due to mean drift, on the pontoon’s motions, is evaluated. It is found that this effect is negligible. Then, the RAOs are used in the calculation of wave drift coefficients, showing very different results than those obtained with fixed pontoons. Time-domain simulations are then performed with wave drift coefficients calculated with both approaches, with focus on the bridge girder moments and mooring line tensions. It is shown that using wave drift coefficients obtained with fixed pontoon is a non-conservative simplification, depending on sea state and wave incidence direction.

Wave Drift Forces and Responses in Current

2013 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Jan Roger Hoff ◽  
Elin Marita Hermundstad ◽  
Rolf Baarholm
Keyword(s):  
Basic Element ◽  
Irregular Waves ◽  
Mooring Line ◽  
Vertical Column ◽  
Wave Drift ◽  
Slow Drift ◽  
Initial Comparison ◽  
Mooring Forces ◽  
Wave Drift Forces ◽  
Drift Forces

The influence from a current on wave drift forces and resulting slowly varying vessel responses can be quite significant. In this paper the effect is reviewed and further investigated. Several works have been published on this complex topic during the last 20–25 years, while it is only to a little extent taken consistently into account in standard industry tools. Simplified methods are often used, if any, and /or empirical correction from model test data. Thus there is a need to improve standard tools in this respect. The effects on slowly varying vessel motions and resulting extreme mooring line loads are demonstrated through time series sequences from selected, previous model tests with FPSO’s and semisubmersibles in steep irregular waves. Wave-current interaction effects that can be larger than the effects from current and wind alone are identified. It is also confirmed from these examples that extreme mooring forces usually occur due to extreme slow-drift motions. An overview description is given of a new, general numerical potential theory code for industry use, MULDIF-2, where wave-current-structure interaction is consistently included as a basic element in the formulation. Main items in the approach are addressed and referred to previous works in the literature. Results from an initial comparison against previous results on drift forces on a vertical column are given, and a good agreement is found. Further verification and validation work is in progress.

scholarly journals Transient Responses Evaluation of FPSO with Different Failure Scenarios of Mooring Lines

2021 ◽  
Vol 9 (2) ◽  
pp. 103
Author(s):  
Dongsheng Qiao ◽  
Binbin Li ◽  
Jun Yan ◽  
Yu Qin ◽  
Haizhi Liang ◽  
...  
Keyword(s):  
Service Condition ◽  
Mooring Line ◽  
Mooring System ◽  
Transient Effects ◽  
Floating Platform ◽  
Transient Responses ◽  
Mooring Lines ◽  
Performance Deterioration ◽  
Steady State Responses ◽  
Influence Process

During the long-term service condition, the mooring line of the deep-water floating platform may fail due to various reasons, such as overloading caused by an accidental condition or performance deterioration. Therefore, the safety performance under the transient responses process should be evaluated in advance, during the design phase. A series of time-domain numerical simulations for evaluating the performance changes of a Floating Production Storage and Offloading (FPSO) with different broken modes of mooring lines was carried out. The broken conditions include the single mooring line or two mooring lines failure under ipsilateral, opposite, and adjacent sides. The resulting transient and following steady-state responses of the vessel and the mooring line tensions were analyzed, and the corresponding influence mechanism was investigated. The accidental failure of a single or two mooring lines changes the watch circle of the vessel and the tension redistribution of the remaining mooring lines. The results indicated that the failure of mooring lines mainly influences the responses of sway, surge, and yaw, and the change rule is closely related to the stiffness and symmetry of the mooring system. The simulation results could give a profound understanding of the transient-effects influence process of mooring line failure, and the suggestions are given to account for the transient effects in the design of the mooring system.

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.

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.

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