Mooring- and Riser-Induced Damping in Fatigue Seastates

2002 ◽  
Author(s):  
D. L. Garrett ◽  
R. B. Gordon ◽  
J. F. Chappell
Keyword(s):  
Coupled Model ◽  
Rational Approach ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Floating Platform ◽  
Mooring Lines ◽  
Equivalent Linear ◽  
Floating System ◽  
Linear Damping ◽  
Stiffness And Damping

Viscous damping due to drag on mooring lines and risers is seastate dependent and significantly affects the motion of a floating platform in deep water, particularly in everyday seastates. This in turn impacts design of the risers, which are typically controlled by fatigue. The dynamic interaction between the platform, mooring and risers cannot be evaluated using conventional uncoupled analysis tools, where each is analyzed separately. Rather, coupled analysis is required to provide a consistent way to model the drag-induced damping from mooring lines and risers. We describe a coupled, frequency domain approach (RAMS – Rational Approach to Marine Systems) for calculating the dynamic response of vessel, mooring and risers. In coupled analysis, the risers and mooring lines are included in the model along with the floater. In this way, damping of the floater motion due to drag on the mooring lines and risers is incorporated directly. It is also valuable to estimate the linear damping factors from the full, coupled analysis results. These damping factors may then, for example, be used in an equivalent linear model of the floating system in which the stiffness and damping effects of the mooring and risers are represented as additions to the floater stiffness and damping matrices. Such a model could be used to efficiently design a subsystem (e.g.; an export riser). We describe a technique to determine the equivalent linear damping factors from the coupled analysis results. This paper also illustrates the use of these methods for a West Africa FPSO. The need for coupled analysis is shown by comparing results from the fully coupled model with those obtained using an uncoupled method in which the mooring line damping is approximated.

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.

Calibration of Mooring Line Damping for Taut-Leg FPSOs Model Testing

2008 ◽  
Author(s):  
Antonio C. Fernandes ◽  
Allan Carre´ de Oliveira ◽  
Fabio P. S. Mineiro ◽  
Joel S. Sales ◽  
Andre´ Luis Rosa ◽  
...  
Keyword(s):  
Numerical Experiments ◽  
Model Testing ◽  
Model Tests ◽  
Reduced Model ◽  
Mooring Line ◽  
Mooring Lines ◽  
Stiffness And Damping

The paper discusses alternatives to represent the MLD (Mooring Line Damping) in models tests with truncated mooring lines. The work has performed both numerical experiments and reduced model tests. The results for stiffness and damping have been compared. This allows further considerations for future designs.

Dual Stiffness Approach for Polyester Mooring Line Analysis in Time Domain: Semisubmersible Case

2013 ◽  
Author(s):  
Arcandra Tahar ◽  
Djoni Sidarta
Keyword(s):  
Traditional Method ◽  
Dynamic Stiffness ◽  
Performance Comparison ◽  
Mooring Line ◽  
Analysis Tool ◽  
Floating Platform ◽  
Mooring Lines ◽  
Coupled Dynamic Analysis ◽  
Stiffness Method ◽  
Motion Characteristics

This paper is a continuation of a series of investigation for the dual stiffness approach for polyester mooring lines. Tahar et. al. (2012) has presented the global performance comparison between the dual stiffness method and the traditional method for the Spar platform. As shown in that study, there are appreciable differences between the former and the later methods especially in lateral motions, which, however, result in little difference in SCR strength response. Is it because the Spar has better motion characteristics than other wet tree floating platforms such as the semisubmersible and FPSO? This paper will investigate the effect of the dual stiffness method and the traditional method to SCR response for a Semisubmersible platform. The fully coupled dynamic analysis tool CHARM3D has been modified to incorporate the dual stiffness approach. Two axial stiffnesses (EA) of polyester line, post installation (static) stiffness and storm (dynamic) stiffness have been convoluted into a dual stiffness to represent the total response of the floating platform in a single run. In the traditional method, the analyses are done twice, one run for each stiffness. Then, the extremes from each run are used as governing values for design. The SCR will be modeled and analyzed using ABAQUS software.

Understanding the Dynamic Coupling Effects in Deep Water Floating Structures Using a Simplified Model

2008 ◽  
Vol 130 (3) ◽  
Author(s):  
Ying Min Low ◽  
Robin S. Langley
Keyword(s):  
Frequency Domain ◽  
Deep Water ◽  
Time Domain ◽  
Low Frequency ◽  
Wave Frequency ◽  
Computational Time ◽  
Coupled Analysis ◽  
Floating Platform ◽  
Coupling Effects ◽  
Mooring Lines

The global dynamic response of a deep water floating production system needs to be predicted with coupled analysis methods to ensure accuracy and reliability. Two types of coupling can be identified: one is between the floating platform and the mooring lines/risers, while the other is between the mean offset, the wave frequency, and the low frequency motions of the system. At present, it is unfeasible to employ fully coupled time domain analysis on a routine basis due to the prohibitive computational time. This has spurred the development of more efficient methods, including frequency domain approaches. A good understanding of the intricate coupling mechanisms is paramount for making appropriate approximations in an efficient method. To this end, a simplified two degree-of-freedom system representing the surge motion of a vessel and the fundamental vibration mode of the lines is studied for physical insight. Within this framework, the frequency domain equations are rigorously formulated, and the nonlinearities in the restoring forces and drag are statistically linearized. The model allows key coupling effects to be understood; among other things, the equations demonstrate how the wave frequency dynamics of the mooring lines are coupled to the low frequency motions of the vessel. Subsequently, the effects of making certain simplifications are investigated through a series of frequency domain analyses, and comparisons are made to simulations in the time domain. The work highlights the effect of some common approximations, and recommendations are made regarding the development of efficient modeling techniques.

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.

An Integrated Methodology for the Design of Mooring Systems and Risers of Floating Production Platforms

2012 ◽  
Author(s):  
Aldo Roberto Cruces Girón ◽  
Fabricio Nogueira Corrêa ◽  
Breno Pinheiro Jacob ◽  
Stael Ferreira Senra
Keyword(s):  
Production Systems ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Design Practice ◽  
Structural Behaviour ◽  
Coupled Models ◽  
Mooring Lines ◽  
Hydrodynamic Behaviour ◽  
Integrated Methodology ◽  
Analysis Tools

Nowadays, coupled analysis tools that allow the simultaneous modelling of the hydrodynamic behaviour of the hull and the structural behaviour of the lines of floating production platforms have been increasingly used. The use of such tools is gradually allowing the introduction of some feedback between the design of risers and mooring systems. In the current practice, that comprises the so-called “hybrid” methodologies, mooring designers have been using these tools to consider the influence of the risers on the platform motions. On the other hand, riser designers can use motions that result from coupled simulations for the analysis of each riser. Such integration is already being implemented in the design practice of Petrobras; however, elsewhere the design of risers and mooring systems may still be performed separately, by different teams, therefore not fully exploiting the benefits that the coupled analysis tools can provide. In this context, this work describes an innovative, fully integrated methodology for the design of mooring systems and risers of floating production systems (FPS). This methodology considers different design stages (from preliminary to advanced), integrating the design activities of mooring lines and risers in a single spiral, allowing gains in efficiency and cost reduction. The initial design stages already include a feedback between riser and mooring analyses. The integrity of the risers can be considered in the mooring design by determining their safe operational zones, and therefore, mooring line pretensions can be modified to improve its structural performance. Then, in advanced stages critical design cases for both mooring and risers systems can be identified and rigorously verified by using fully coupled models. The application of the proposed methodology is illustrated with a case study of a typical FPS, representative of the platforms that have been recently considered for deepwater applications. It should be stressed that the methodology described here does not reflect the current design practice of Petrobras. Presently it is merely a proposal that is being studied and assessed; this work comprises the first draft of the methodology, which will be enhanced and consolidated as the result of current and future studies.

Dual Stiffness Approach for Polyester Mooring Line Analysis in Time Domain

2012 ◽  
Author(s):  
Arcandra Tahar ◽  
Djoni Sidarta ◽  
Alex Ran
Keyword(s):  
Traditional Method ◽  
Dynamic Stiffness ◽  
Performance Comparison ◽  
Mooring Line ◽  
Axial Stiffness ◽  
Analysis Tool ◽  
Floating Platform ◽  
Mooring Lines ◽  
Nonlinear Stress ◽  
Offshore Industry

Polyester mooring lines have been used in the offshore industry since the late ’90s. With increasing oil exploration and production in deeper waters, using polyester lines provides greater benefit than using traditional steel wires and chains. Some advantages of using polyester include a reduction of mooring line weight, a reduction in vessel offset and a reduction in the dynamics of the line tensions. However, unlike steel, polyester lines exhibit axial stiffness characteristics that are nonlinear and vary with time and loading history. Tahar (2001) developed a comprehensive theory and numerical tool to capture this behavior. The formulas allow relatively large elongation and nonlinear stress-strain relationships, as typically observed in polyester fibers. The mooring line dynamics are based on a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system. Since this theory is computationally intensive, the benefits outweigh the costs less than they do for the practical approach recommended by API. Therefore, the fully coupled dynamic analysis tool CHARM3D has been modified to incorporate the API-recommended approach. Two axial stiffnesses (EA), post installation (static) stiffness and storm (dynamic) stiffness, have been convoluted into a dual stiffness to represent the total response of the floating platform in a single run. In the traditional method, the analyses are done twice, one run for each stiffness. Then, the extremes from each run are used as governing values for design. This paper presents the global performance comparison between the dual stiffness method and the traditional method. The effect of motions on SCR strength is also investigated using ABAQUS software.

scholarly journals Alternative Mooring Systems for a Very Large Offshore Wind Turbine Supported by a Semisubmersible Floating Platform

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Jinsong Liu ◽  
Lance Manuel
Keyword(s):  
Wind Turbine ◽  
Integrated System ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
System Response ◽  
Mooring System ◽  
Floating Platform ◽  
Mooring Lines ◽  
Wind Speeds

As offshore wind turbines supported on floating platforms extend to deep waters, the various effects involved in the dynamics, especially those resulting from the influence of moorings, become significant when predicting the overall integrated system response. The combined influence of waves and wind affect motions of the structure and induce tensile forces in mooring lines. The investigation of the system response under misaligned wind-wave conditions and the selection of appropriate mooring systems to minimize the turbine, tower, and mooring system loads is the subject of this study. We estimate the 50-year return response of a semisubmersible platform supporting a 13.2 MW wind turbine as well as mooring line forces when the system is exposed to four different wave headings with various environmental conditions (wind speeds and wave heights). Three different mooring system patterns are presented that include 3 or 6 mooring lines with different interline angles. Performance comparisons of the integrated systems may be used to define an optimal system for the selected large wind turbine.

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.

Nonlinear Response of Coupled Integrated Spar Platform Under Severe Sea States

2012 ◽  
Author(s):  
A. B. M. Saiful Islam ◽  
Mohammed Jameel ◽  
Suhail Ahmad ◽  
Mohd Zamin Jumaat
Keyword(s):  
Deep Water ◽  
Beam Element ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Intermediate Water ◽  
Spar Platform ◽  
Mooring Lines ◽  
Spar Platforms ◽  
Water Conditions ◽  
Wave Heights

The oil and gas industry has moved towards the offshore deep water regions due to depletion of these resources in shallow and intermediate water depths. Conventional fixed jacket type platforms and bottom supported compliant platforms have been found to be inefficient and uneconomical for exploring these resources in deep water regions. In view of deep water conditions, Spar platforms have been seen to be the most economical and suitable alternative offshore platforms. Several operational Spar platforms such as SB-1, Shell’s ESSCO, Brent Spar, Oryx Neptune Spar, Chevron Genesis Spar and Exxon’s Diana Spar in the Gulf of Mexico and North Sea have shown the effectiveness and success of such platforms in deep-ocean. In deep water conditions, the severity of sea states has substantial effects on the spar platform. The mooring lines contribute significant inertia and damping because of their longer lengths, larger sizes, and heavier weights. Precise motion investigation of platforms should consider these actions in deep waters. However, proper dynamics cannot be assessed by the commonly used decoupled quasi-static method that ignores all or part of the interaction effects between the mooring lines and platform. Coupled analysis, which includes the platform and mooring lines in a single model, is the only way to capture the damping from mooring lines in a consistent manner. In the present study, coupled analysis of integrated Spar-mooring system has been performed. Cylindrical spar hull is treated as a rigid beam element and catenary mooring line as hybrid beam element. Nonlinear dynamic responses have been evaluated under several severe sea states of dissimilar wave heights and wave periods. Damping due to mooring lines has been assessed. An automatic Newmark-β time incremental approach has been implemented to conduct the analysis in time domain. Wave induced spar hull motion in surge, heave and pitch direction along with maximum tension in mooring line has been assessed for different wave conditions with and without current in 1018 m water depth. The time histories of spar responses follow substantial alteration for larger wave heights and wave periods. Maximum tensions in mooring line are very sensitive with momentous value for extreme sea loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines.

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