The Anchor-Last Deployment Problem for Inextensible Mooring Lines

1975 ◽  
Vol 97 (3) ◽  
pp. 1046-1052 ◽  
Author(s):  
Robert C. Rupe ◽  
Robert W. Thresher
Keyword(s):  
Numerical Model ◽  
Equations Of Motion ◽  
Simultaneous Equations ◽  
Mooring Line ◽  
Integration Scheme ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Concentrated Masses ◽  
Predictor Corrector ◽  
Lagrange’S Method

A lumped mass numerical model was developed which predicts the dynamic response of an inextensible mooring line during anchor-last deployment. The mooring line was modeled as a series of concentrated masses connected by massless inextensible links. A set of angles was used for displacement coordinates, and Lagrange’s Method was used to derive the equations of motion. The resulting formulation exhibited inertia coupling, which, for the predictor-corrector integration scheme used, required the solution of a set of linear simultaneous equations to determine the acceleration of each lumped mass. For the selected cases studied the results show that the maximum tension in the cable during deployment will not exceed twice the weight of the cable and anchor in water.

A Possible Failure Mode for Leeward Mooring Lines on a Floating Storage Unit

2013 ◽  
Author(s):  
Vegard Aksnes ◽  
Terje Nybø ◽  
Halvor Lie
Keyword(s):  
Numerical Model ◽  
Failure Mechanism ◽  
Steel Wire ◽  
Original System ◽  
Mooring Line ◽  
Storage Unit ◽  
Mooring Lines ◽  
Wire Segment ◽  
Bending Radius ◽  
The Wire

The floating storage unit Navion Saga at the Volve field in the North Sea suffered from two mooring line breaks in steel wire ropes in 2011. Investigations of the broken ropes indicated that a possible failure mechanism could be high stresses near the wire socket induced by large bending moments in leeward mooring lines. The scope of the current study has been to make a numerical model capable of capturing such behaviour of the steel wire rope and to check if the minimum bending radius could be as low as the rope’s specified minimum bending radius. The numerical model has revealed a possible failure mechanism. The connecting link plate between the upper chain segment and the upper wire segment lies initially on the seabed. When lifted off the seabed, the link plate and the wire socket will fall to the seabed at a higher speed than the upper wire segment. A transverse wave in mooring line plane propagating towards fair-lead is generated when the wire socket hits the seabed. The wave leads to large curvature in the wire near the socket. Sensitivity studies of the governing parameters have been performed to assess the uncertainties of the numerical model. A modified system is presented and it is shown that the phenomenon which is likely to have caused failure in the original system will not occur for the modified one.

scholarly journals Numerical Study on Hydrodynamic Responses of Floating Rope Enclosure in Waves and Currents

2020 ◽  
Vol 8 (2) ◽  
pp. 82
Author(s):  
Hui Yang ◽  
Yun-Peng Zhao ◽  
Chun-Wei Bi ◽  
Yong Cui
Keyword(s):  
Numerical Study ◽  
Line Tension ◽  
Wave Period ◽  
Windward Side ◽  
Mooring Line ◽  
Fish Stock ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Design Factors ◽  
Volume Retention

Enclosure aquaculture is a healthy and ecological aquaculture pattern developed in recent years to relieve the pressure due to the wild fish stock decline and water pollution. The object of this paper was a floating rope enclosure, which mainly consisted of floaters, mooring lines, sinkers and a net. In order to optimize mooring design factors, the hydrodynamic responses of the floating rope enclosure with different mooring systems in combined wave-current were investigated by experimental and numerical methods. Physical model experiments with a model scale of 1:50 were performed to investigate the hydrodynamic characteristics of a floating rope enclosure with 12 mooring lines. Based on the lumped mass method, the numerical model was established to investigate the effects of mooring design factors on the mooring line tension, force acting on the bottom, and the volume retention of the floating rope enclosure. Through the analysis of numerical and experimental results, it was found that the maximum mooring line tension of the floating rope enclosure occurs on both sides of the windward. Increasing the number of mooring lines on the windward side is helpful to reduce the maximum mooring line tension. Waves and current both have an influence on the mooring line tension; in contrast, currents have a more obvious effect on the mooring line tension than waves. However, the influence of the wave period on the maximum mooring line tension is small. The force endured by the bottom of the floating rope enclosure also changes periodically with the wave period. Yet, the maximum force endured by the bottom of floating rope enclosure occurred at the windward and leeward of the structure. The volume retention of the floating rope enclosure increased with the increasing amount of mooring lines.

Three-Dimensional Dynamic Analysis Method of Multi-Component Mooring Lines

2019 ◽  
Author(s):  
Yuda Apri Hermawan ◽  
Yoshitaka Furukawa
Keyword(s):  
Dynamic Analysis ◽  
Numerical Method ◽  
Three Dimensional ◽  
Experimental Results ◽  
Mooring Line ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Dynamic Behaviors ◽  
Component Object ◽  
Deep Depth

Abstract Complicated mooring system well-known as a multi-component mooring line is highly required owing to the deep depth of water and severe sea conditions. Since the dynamic behaviors of such mooring line are quite complex, proper numerical method is indispensable to predict the dynamic behaviors of a multi-component mooring line efficiently and precisely. In this paper, a numerical method improving the lumped mass method is proposed to introduce the three-dimensional dynamic analysis of multi-component mooring line with the motion of an anchor and clump weights. The mooring line is regarded as a multi-component object which has nonuniform segment line characteristics. In this method, lumped mass technique is developed to represent the three-dimensional dynamic behavior of each segment individually, allowing the motion of bottom-end segment as well as the anchor. Then, the motion of the end-segment is regarded as the motion of the upper-end of lower segment. Meanwhile, calculation method of initial condition for dynamic calculation is developed by adopting the basic principle of multi-component mooring line catenary equations. The results of time histories representing the three-dimensional dynamic analysis of mooring line are obtained and compared with other numerical and experimental results presented in published papers. The results show good agreement with both numerical and experimental results.

scholarly journals Experimental Study on Development of Mooring Simulator for Multi Floating Cranes

2021 ◽  
Vol 9 (3) ◽  
pp. 344
Author(s):  
Junhyeok Bae ◽  
Juhwan Cha ◽  
Min-Guk Seo ◽  
Kangsu Lee ◽  
Jaeyong Lee ◽  
...  
Keyword(s):  
Model Test ◽  
Equations Of Motion ◽  
Mooring Line ◽  
Analysis Program ◽  
The Finite Element Method ◽  
Mooring Lines ◽  
Calculation Results ◽  
Floating Crane ◽  
Pile Model ◽  
Equations Of Motions

In this study, the coupled motion of a mooring system and multifloating cranes were analyzed. For the motion analysis, the combined equations of motions of the mooring line and multifloating cranes were introduced. The multibody equations for floating cranes were derived from the equations of motion. The finite element method (FEM) was used to derive equations to solve the stretchable catenary problem of the mooring line. To verify the function of mooring simulator, calculation results were compared with commercial mooring software. To validate the analysis results, we conducted an experimental test for offshore operation using two floating crane models scaled to 1:40. Two floating crane models and a pile model were established for operation of uprighting flare towers. During the model test, the motion of the floating cranes and tensions of the mooring lines were measured. Through the model test, the accuracy of the mooring analysis program developed in this study was verified. Therefore, if this mooring analysis program is used, it will be possible to perform a mooring analysis simulation at the same time as a maritime work simulation.

scholarly journals Numerical Study of a Moored Structure in Moving Broken Ice Driven by Current and Wave

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Biao Su ◽  
Karl Gunnar Aarsæther ◽  
David Kristiansen
Keyword(s):  
Numerical Model ◽  
Numerical Study ◽  
Torsional Stiffness ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Broken Ice ◽  
Ice Field ◽  
Drifting Ice

This paper presents a numerical model intended to simulate the mooring force and the dynamic response of a moored structure in drifting ice. The mooring lines were explicitly modeled by using a generic cable model with a set of constraint equations providing desired structural properties such as the axial, bending, and torsional stiffness. The six degrees-of-freedom (DOF) rigid body motions of the structure were simulated by considering its interactions with the mooring lines and the drifting ice. In this simulation, a fragmented ice field of broken ice pieces could be considered under the effects of current and wave. The ice–ice and ice–structure interaction forces were calculated based on a viscoelastic-plastic rheological model. The hydrodynamic forces acting on the floating structure, mooring line, and drifting ice were simplified and calculated appropriately. The present study, in general, demonstrates the potential of developing an integrated numerical model for the coupled analysis of a moored structure in a broken ice field with current and wave.

Decay of Vibrations Along Deepwater Mooring Lines

2011 ◽  
Author(s):  
Alex Argyros ◽  
Robin S. Langley ◽  
R. V. Ahilan
Keyword(s):  
Numerical Solutions ◽  
Equations Of Motion ◽  
Rapid Assessment ◽  
Mooring Line ◽  
Universal Curve ◽  
Mooring Lines ◽  
Hydrodynamic Damping ◽  
Ongoing Work ◽  
Vessel Response ◽  
Truncation Techniques

Model tests for global design verification of deepwater floating structures cannot be made at reasonable scales. An overview of recent research efforts to tackle this challenge is given first, introducing the concept of line truncation techniques. In such a method the upper sections of each line are modelled in detail, capturing the wave action zone and all coupling effects with the vessel. These terminate to an approximate analytical model, that aims to simulate the remainder of the line. The rationale for this is that in deep water the transverse elastic waves of a line are likely to decay before they are reflected at the seabed. The focus of this paper is the verification of this rationale and the ongoing work, which is considering ways to produce a truncation model. Transverse dynamics of a mooring line are modelled using the equations of motion of an inextensible taut string, submerged in still water, one end fixed at the bottom the other assumed to follow the vessel response, which can be harmonic or random. Nonlinear hydrodynamic damping is included; bending and VIV effects are neglected. A dimensional analysis, supported by exact benchmark numerical solutions, has shown that it is possible to produce a universal curve for the decay of transverse vibrations along the line, which is suitable for any kind of line with any top motion. This has a significant engineering benefit, allowing for a rapid assessment of line dynamics — it is very useful in deciding whether a truncated line model is appropriate, and if so, at which point truncation might be applied. Initial efforts in developing a truncated model show that a linearized numerical solution in the frequency domain matches very closely the exact benchmark.

A Novel Power Series Method for the Analysis of an Offshore Mooring Line

2021 ◽  
Author(s):  
Kabutakapua Kakanda ◽  
Zhaolong Han ◽  
Bao Yan ◽  
Narakorn Srinil ◽  
Dai Zhou
Keyword(s):  
Finite Element ◽  
Power Series ◽  
Initial Conditions ◽  
Dynamic Strain ◽  
Mooring Line ◽  
Series Method ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Bivariate Polynomials ◽  
Power Series Method

Abstract The mechanics of offshore mooring lines are described by a set of nonlinear equations of motion which have typically been solved through a numerical finite element or finite difference method (FEM or FDM), and through the lumped mass method (LMM). The mooring line nonlinearities are associated with the distributed drag forces depending on the relative velocities of the environmental flow and the structure, as well as the axial dynamic strain-displacement relationship given by the geometric compatibility condition of the flexible mooring line. In this study, a semi analytical-numerical novel approach based on the power series method (PSM) is presented and applied to the analysis of offshore mooring lines for renewable energy and oil and gas applications. This PSM enables the construction of analytical solutions for ordinary and partial differential equations (ODEs and PDEs) by using series of polynomials whose coefficients are determined, depending on initial and boundary conditions. We introduce the mooring spatial response as a vector in the Lagrangian coordinate, whose components are infinite bivariate polynomials. For case studies, a two-dimensional mooring line with fixed-fixed ends and subject to nonlinear drag, buoyancy and gravity forces is considered. The introduced boundary and initial conditions enable the analysis of an equilibrium or steady-state of a catenary-like mooring line configuration with variable slenderness and flexibility. Polynomials’ coefficients computation is performed with the aid of a MATLAB package. Numerical results of mooring line configurations and resultant tensions are presented for deep-water applications, and compared with those obtained from a semi-analytical and finite element model. The PSM applied to the mooring line in the present study is efficient and more computationally robust than traditional numerical methods. The PSM can be directly applied to the dynamic analysis of mooring lines.

Hydrodynamic Behaviors of Aquaculture Net Cages After the Successive Mooring Lines Failure

2021 ◽  
Author(s):  
Hung-Jie Tang ◽  
Ray-Yeng Yang
Keyword(s):  
Mooring Line ◽  
Mooring System ◽  
Domino Effect ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Upstream Side ◽  
Anchor Points ◽  
The Time Domain ◽  
Net Cages ◽  
Hydrodynamic Behaviors

Abstract This paper aims to study the successive mooring line failure (also known as the domino effect) and the collision between floating collars for aquaculture net cages subjected to currents. The numerical model of this study is developed based on the Morison equation and the lumped-mass scheme in the time domain. This model is then applied to see if the domino effect of moorings will happen after releasing the anchor point #1 on the upstream side. In this study, we adopt four different current speeds (0.5, 1.0, 1.5, 2.0 m/s) and three different safety factors (SF, 1.0, 1.5, 2.0) settings to calculate the number of mooring failures, and to see whether it will cause floating collars collision. The results show that in the case of the SF is 2.0, the domino effect will not be triggered, and the floating collar collision will not occur. When the SF is 1.5, and the current speed is up to 1.0 m/s or higher, only the two anchor points on the upstream side will fail and no collision will occur. However, if the SF is not considered (that is, 1.0), the domino effect will occur under all the four current speeds, and the floating collar collision will all occur. Therefore, we suggest that in order to avoid the domino effect of the mooring system of aquaculture net cages from currents, the SF of the mooring system design must be at least 2 times.

scholarly journals Dynamic Analysis of Embedded Chains in Mooring Line for Fish Cage System

2018 ◽  
Vol 25 (4) ◽  
pp. 83-97
Author(s):  
Hui-Min Hou ◽  
Guo-Hai Dong ◽  
Tiao-Jian Xu ◽  
Yun-Peng Zhao ◽  
Chun-Wei Bi
Keyword(s):  
Numerical Model ◽  
Dynamic Response ◽  
Single Point ◽  
Mooring Line ◽  
Mooring System ◽  
Lumped Mass ◽  
Model Case ◽  
Cage System ◽  
Net Cage ◽  
Offshore Aquaculture

Abstract Investigation of the embedded chains in soil starts to play an important role in understanding the structural performance of mooring system, when the embedded anchors will be employed to sustain large loads with the gradually growth of installation depth of offshore aquaculture farm. The aim of this study is to investigate the dynamic response of mooring line considering the influence of embedded chains in clay soil for net cage system. Lumped-mass method is used to establish the numerical model for evaluating the performance of mooring line with embedded chains. To validate the numerical model, comparisons of numerical results with the analytical formulas and the experimental data are conducted. A good agreement of the profile and the tension response is obtained. Then, the effect of embedded chains on the static and dynamic response of mooring line is evaluated, and the dynamic behavior of mooring system considering embedded chains for net cage system is investigated. The results indicate that the soil resistance on embedded chains should be included to predict the mooring line development and the load on the embedded anchors in the numerical simulations. An appropriate safety factor should be included if employing the simplified model Case C at the initial design phase. And the effect of embedded chains on the holding capacity of embedded anchors in single-point mooring system for single net cage cannot be negligible during the design and operation phases. Consequently, it is profound to take into account the interaction of embedded chains and soil for accurately predicting the reliability of mooring system for fish cage.

scholarly journals Experimental Validation and Comparison of Numerical Models for the Mooring System of a Floating Wave Energy Converter

2020 ◽  
Vol 8 (8) ◽  
pp. 565 ◽  
Author(s):  
Bruno Paduano ◽  
Giuseppe Giorgi ◽  
Rui P. F. Gomes ◽  
Edoardo Pasta ◽  
João C. C. Henriques ◽  
...  
Keyword(s):  
Wave Energy ◽  
Numerical Models ◽  
Mooring Line ◽  
Small Scale ◽  
Mooring System ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Wave Energy Converters ◽  
Efficiency Cost ◽  
Numerical Instabilities

The mooring system of floating wave energy converters (WECs) has a crucial impact on power generation efficiency, cost of delivered energy, proper operation, reliability and survivability. An effective design, addressing such competing objectives, requires appropriate mathematical models to predict mooring loads and dynamic response. However, conversely to traditional offshore engineering applications, experience in modelling mooring systems for WECs is limited, due to their unique requirement of maximising the motion while minimising loads and costs. Even though modelling approaches and software are available for this application, guidelines and critical comparison are still scarce. This paper proposes a discussion and validation of three mooring-line models: one quasi-static approach (developed in-house) and two dynamic lumped-mass approaches (the open source MoorDyn and the commercial OrcaFlex). The case study is a 1:32-scale prototype of a floating oscillating water column WEC tested in a wave tank, with three mooring lines, each one comprising of a riser and a clump weight. Validation, performed by imposing fairlead displacements and comparing resulting tensions, shows good agreement. The small scale may induce numerical instabilities and uncertainties in the parameter estimation. Finally, likely due to internal resonance of this particular mooring system, high-frequency content in the mooring tension is found, albeit absent in the kinematics of the floater.

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