Influence of Nonlinear Mooring Stiffness on Hydrodynamic Performance of Floating Bodies

2009 ◽  
Author(s):  
Huilong Ren ◽  
Jian Zhang ◽  
Guoqing Feng ◽  
Hui Li ◽  
Chenfeng Li
Keyword(s):  
Equilibrium Position ◽  
Equations Of Motion ◽  
Offshore Structures ◽  
Hydrodynamic Performance ◽  
Coupled Analysis ◽  
Mooring System ◽  
Main Function ◽  
Ocean Engineering ◽  
Mooring Lines ◽  
Floating Structures

Coupled dynamic analysis between floating marine structures and flexible members such as mooring lines and risers, is a challenging work in the ocean engineering field. Coupled analysis on mooring-buoy interactions has been paid more and more concern for recent years. For floating offshore structures at sea, the motions driven by environmental loads are inevitable. The movement of mooring lines occurs due to the excitation on the top by floating structures. Meanwhile the lines restrict the buoy’s motion by forces acting on the fareleads. Positioning is the main function of mooring system, its orientation effects can’t be ignored for floating structures such as semi-submersible, FPS, and TLP, especially when the buoy’s equilibrium position shifting to another place. Similar as hydrostatic restoring forces, mooring force related with the buoy’s displacement can be transformed into mooring stiffness and can be added in the differential equations of motion, which is calculated at its equilibrium point. For linear hydrodynamic analysis in frequency domain, any physical quantity should be linear or be linearized, however mooring stiffness is nonlinear in essence, so the tangent or differential stiffness is used. Steel chains are widely used in catenary mooring system. An explicit formulation of catenary mooring stiffness is derived in this article, which consists of coupled relations between horizontal and vertical mooring forces. The effects of changing stiffness due to the shift of equilibrium position on the buoy’s hydrodynamic performance are investigated.

An enhanced semi-coupled methodology for the analysis and design of floating production systems

2020 ◽  
pp. 1-33
Author(s):  
Aldo Roberto Cruces Giron ◽  
William Steven Mendez Rodriguez ◽  
Fabrício Nogueira Correa ◽  
Breno P Jacob
Keyword(s):  
Equilibrium Position ◽  
Production Systems ◽  
Equations Of Motion ◽  
Coupled Analysis ◽  
Mooring Lines ◽  
Analysis And Design ◽  
Stiffness Matrices ◽  
Linear Effects ◽  
The Mean ◽  
Nonlinear Static

Abstract This work presents an enhanced hybrid methodology for the analysis and design of floating production systems (FPS). The semi-coupled (S-C) procedure exploits advantages of coupled and uncoupled models, incorporated into a three-stage sequence of analyses that can be fully automated within a single analysis program, presenting striking reductions of computational costs. The procedure begins by determining, through a full nonlinear static coupled analysis, the mean equilibrium position of the FPS with its mooring lines and risers. Then, it automatically evaluates equivalent 6-DOF stiffness matrices and force vectors representing the whole array of lines. Finally, these matrices/vectors are transferred to the dynamic analysis, solving the global 6-DOF equations of motion restarted from the static equilibrium position. This way, the S-C methodology represents all non-linear effects associated to the lines and consider their influence on the dynamic behavior of the hull. However, in some situations it could still overestimate dynamic amplitudes of LF motions, and/or underestimate amplitudes of line tensions. Thus, to improve the overall accuracy, enhanced procedures are incorporated to better represent damping and inertial contribution of the lines. Results of case studies confirm that this methodology provides results adequate for preliminary or intermediary design stages.

Research on the Arrangement of the Mooring Lines of the Thruster Assisted Mooring System

2013 ◽  
Author(s):  
Gang Zou ◽  
Lei Wang ◽  
Feng Zhang
Keyword(s):  
Mooring Line ◽  
Mooring System ◽  
Main Function ◽  
Time Domain Simulation ◽  
Mooring Lines ◽  
Dynamic Positioning System ◽  
Optimum Arrangement ◽  
Line Angle ◽  
Mooring Forces ◽  
Offshore Industry

As the offshore industry is developing into deeper and deeper water, station keeping technics are becoming more and more important to the industry. Based on the dynamic positioning system, the thruster assisted mooring system (TAMS) is developed, which consisted of mooring lines and thrusters. The main function of the TAMS is to hold a structure against wind wave and current loads with its thruster and cables, which is mainly evaluated by the holding capacity of the system. The arrangement of the mooring lines (location of anchor or the mooring line angle relative to platform) will directly affect the TAMS holding capacity because of the influence of the directions of the mooring forces. So finding out an optimum arrangement of the mooring lines is essential since the performance of the TAMS depends greatly on the arrangement of the mooring lines. The TAMS of a semi-submersible platform, which is studied in this paper, consisted of eight mooring lines. By fixing the layout of the thrusters and changing the location of each mooring line for every case, the performances of the TAMS are analyzed. The platform motions, mooring line tensions and power consumptions are compared to obtain the optimum arrangement of mooring lines, and thus a thruster assisted mooring system with a better performance can be achieved. Time domain simulation is carried out in this paper to obtain the results.

Experimental Study on the Effects of Mooring System on Air Gap Response of Semi-Submersible Platform

2020 ◽  
Author(s):  
Xu Li ◽  
Longfei Xiao ◽  
Handi Wei ◽  
Mingyue Liu
Keyword(s):  
Model Tests ◽  
Air Gap ◽  
Mooring System ◽  
Complex Task ◽  
Ocean Engineering ◽  
Free Surface Elevation ◽  
Mooring Lines ◽  
Scaled Model ◽  
Air Gap Responses ◽  
Floating Platforms

Abstract The air gap response is crucial for the safe design and operation of large-volume floating platforms such as semi-submersible and tension leg platforms. It is a complex task to perform numerical simulation on the air gap response considering the wave free surface elevation and the motions of the floating vessel. Therefore, the prediction of air gap response still relies heavily on model tests. This paper attempts to investigate the effects of the mooring system, especially the effects of the length of mooring lines, on the air gap response of semi-submersible platform based on model tests results. The scaled model of the semi-submersible platform is supported by a symmetric mooring system composed of 8 mooring lines. A set of model tests with different length of mooring lines was performed in the State Key Laboratory of Ocean Engineering basin at Shanghai Jiao Tong University, and the air gap responses of 15 locations were measured using wave probes. The results indicate that the mooring system plays an important role in the air gap response of semi-submersible platform.

A New Nonlinear Coupled Analysis Tool for Floating Structures

2012 ◽  
Author(s):  
Zhiling Li ◽  
Carlos Llorente ◽  
Cheng-Yo Chen ◽  
Chang Ho Kang ◽  
Edmund Muehlner ◽  
...  
Keyword(s):  
Performance Analysis ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Analysis Tool ◽  
Wave Load ◽  
Mooring Lines ◽  
Dynamic Interactions ◽  
Floating Structures ◽  
Coupled Method ◽  
Global Performance

For the global performance analysis of a floater, the traditional semi-coupled method models mooring lines/risers as nonlinear massless springs and ignores 1) the inertial effects from mooring lines/risers, 2) the current and wave load effects on mooring lines/risers, and 3) the dynamic interaction between mooring lines/risers and the floater. However, these effects are deemed critical for deepwater and ultra deepwater floating structures as they may have a significant impact on the floaters’ motions and mooring line/riser tensions. This paper presents the development and verification of a time-domain nonlinear coupled analysis tool, MLTSIM-ROD, which is an integration of a recently developed 3D rod dynamic program, ROD3D, with the well-calibrated floater global performance analysis program, MULTISIM (Ref [9]). The ROD3D was developed based on a nonlinear finite element method and merged with MULTISIM by matching the forces and displacements of mooring lines/risers with the floater at their connections. MLTSIM-ROD can thus predict the floater’s large displacement/rotation motions and mooring line/riser tensions including all the coupled effects between the floater and mooring lines/risers. In this paper, global performance predictions for a SPAR in the Gulf of Mexico in deepwater were carried out using MLTSIM-ROD. The results were then verified with those from other coupled analysis programs. The paper also presents the results of motions and mooring line/riser tensions of the SPAR using both the coupled and semi-coupled methods. The results from the coupled and semi-coupled analyses indicate that the floater’s motions and mooring line/riser tensions could be significantly influenced by the dynamic interactions between the floater and mooring lines/risers. Hence, the coupled method needs to be considered for deepwater floating structures.

Prediction of Hydrodynamic Damping of Moored Offshore Structures Using CFD

2019 ◽  
Author(s):  
Changqing Jiang ◽  
Ould el Moctar ◽  
Thomas E. Schellin
Keyword(s):  
Offshore Structures ◽  
Least Square ◽  
Navier Stokes ◽  
Coupled Analysis ◽  
Total System ◽  
Mooring System ◽  
Restoring Force ◽  
Hydrodynamic Damping ◽  
Restoring Forces ◽  
Decay Tests

Abstract Usually, mooring system restoring forces acting on floating offshore structures are obtained from a quasi-static mooring model alone or from a coupled analysis based on potential flow solvers that do not always consider nonlinear mooring-induced phenomena or fluid-structure interactions and the associated viscous damping effects. By assuming that only the mooring system influences the restoring force characteristics, the contribution of mooring-induced damping to total system damping is neglected. This paper presents a technique to predict hydrodynamic damping of moored structures based on coupling the dynamic mooring model with a Reynolds-averaged Navier-Stokes (RANS) equations solver. We obtained hydrodynamic damping coefficients using a least-square algorithm to fit the time trace of decay tests. We analyzed a moored offshore buoy and validated our predictions against experimental measurements. The mooring system consisted of three catenary chains. The analyzed response comprised the decaying oscillating buoy motions, the natural periods, and the associated linear and quadratic damping characteristics. Predicted motions, natural periods, and hydrodynamic damping generally well agreed to comparable experimental data.

Coupled Analysis of Floating Structures With a New Mooring System

2011 ◽  
Author(s):  
Zhiming Yuan ◽  
Chunyan Ji ◽  
Minglu Chen ◽  
Yun Zhang
Keyword(s):  
Water Depth ◽  
Deep Water ◽  
Coupled Analysis ◽  
Mooring System ◽  
Full Time ◽  
Analysis Method ◽  
Floating Structures ◽  
Motion Performance ◽  
Improved Performance ◽  
Hydrocarbon Reserves

As the exploitation of hydrocarbon reserves moves towards deeper waters, the floating structures are becoming more and more popular, and the catenary and taut mooring systems are two widespread mooring systems which are used for these floating structures. However, both of them have their inherent drawbacks. The aim of the present work is to develop and validate a new mooring system which will overcome these shortcomings. To this end, the motion performance of a semi-submersible platform is simulated by employing a full time domain coupled analysis method. It is shown that the new mooring system yields very good motion performance when benchmarked against the taut mooring system, and the reasons for this improved performance are discussed. Also, the new mooring system is compatible with the characteristic of catenary mooring system, which eliminates the requirement of anti-uplift capacity of the anchors. The second aim of this paper is to explore the proper water depth in employing this new mooring system. For this purpose, several typical water depths are simulated. It is found that the new mooring system works well both in deep water and ultra-deep water. But, as the water depth becomes deeper, the advantages of the new mooring system are reduced.

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.

Numerical Simulation of a Spar Interacting With a Polyester Mooring System

2004 ◽  
Author(s):  
Minsuk Kim ◽  
Yu Ding ◽  
Jun Zhang
Keyword(s):  
Energy Dissipation ◽  
Cyclic Loads ◽  
Mooring System ◽  
Floating Structure ◽  
Mooring Lines ◽  
Floating Structures ◽  
System A ◽  
Large Elongation ◽  
The Mean ◽  
Offset Curve

A numerical scheme, known as CABLE3D, originally developed for the simulation of dynamics of steel mooring lines is extended to allow for special properties pertaining to polyester ropes, such as relatively large elongation under tension, dependence of the modulus on loads, and energy dissipation under cyclic loads. The modified CABLE3D is then integrated into a numerical package, known as COUPLE6D, for computing the interaction between a floating structure and its polyester mooring system. A classical SPAR is chosen in this study as a representative of deepwater floating structures. By considering large elongation in polyester ropes, the static offset curve of a polyester mooring system is softer than that calculated under the assumption of small elongation. The effects of the mean loads on the modulus of polyester ropes are much greater than those of the dynamic loads. The energy dissipation in polyester ropes under cyclic loading does not play a significant role in the responses of the SPAR and tensions in a polyester mooring system. The above observations though made based on a classical SPAR may still have important implications to other floating structures moored by a polyester mooring system.

Numerical Investigation on Interaction Between a Semi-Submersible Platform and its Mooring System

2015 ◽  
Author(s):  
Yuanchuan Liu ◽  
Yao Peng ◽  
Decheng Wan
Keyword(s):  
Mooring Line ◽  
Mooring System ◽  
Floating Structure ◽  
Mooring Lines ◽  
Flow Solver ◽  
Floating Structures ◽  
Marine Renewable Energy ◽  
Motion Responses ◽  
Increasing Demand ◽  
Energy Engineering

With the increasing demand of floating structures in offshore, coastal and marine renewable energy engineering, the interaction between the mooring system and floating structure becomes more and more important. In this paper, motion responses of a semi-submersible platform with mooring system under regular wave conditions are investigated numerically by a viscous flow solver naoe-FOAM-SJTU based on the open source toolbox OpenFOAM. Influence of the mooring system on the platform motion responses is evaluated in two different ways. Investigations are covered for analysis methods adopted for solving mooring lines and the length of each part of a multi-component mooring line. Several important conclusions are drawn.

Design Through Simulation: Finite Element Capabilities for Ocean Engineering

2008 ◽  
Author(s):  
Dean M. Steinke ◽  
Ryan S. Nicoll ◽  
Bradley J. Buckham
Keyword(s):  
Finite Element ◽  
Design Optimization ◽  
Early Stage ◽  
Offshore Structures ◽  
Cable Model ◽  
Ocean Engineering ◽  
Remotely Operated Vehicle ◽  
Mooring Lines ◽  
Nonlinear Simulation ◽  
Early Stage Design

Design optimization and testing of marine technology and offshore structures, such as risers, moorings, or manned and unmanned submersibles is a challenge. This is due to many factors including weather, costly ship time, and the need for experienced off-shore personnel. Nevertheless, early stage design optimization is critical to a project’s success. There is a need for simulation facilities that can capture the complexity and the non-linear dynamics of large mechanical and structural systems, and provide accurate assessment of design variations. This article outlines the development of a nonlinear simulation tool for modeling mechanical systems and structures in the ocean. The framework design and simulation set-up procedures are discussed. The main components of the simulator, a nonlinear finite element cable model and a rigid body model, are discussed. Next, this paper shows how these fundamental models are used to simulate risers, remotely operated vehicle (ROV) umbilicals, and mooring lines. In addition, a module that produces the effects of vortex induced vibration (VIV) based on recent developments on the wake oscillator model is presented. Payin and payout simulations of a ROV tether are also presented to demonstrate the use of the variable-length capabilities of the cable model. Lastly, this paper discusses how ROV instrumentation can be simulated, permitting the design and refinement of instrumentation processing algorithms, such as a Kalman filter, or controllers.

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