Evaluating the Long-Term Fatigue Response of Mooring Lines Using an Asymptotic Approximation

2012 ◽  
Author(s):  
Ying Min Low ◽  
Sai Hung Cheung
Keyword(s):  
Asymptotic Approximation ◽  
Classical Method ◽  
Joint Probability ◽  
Approximate Solutions ◽  
Mooring Line ◽  
Perturbation Approach ◽  
Mooring Lines ◽  
Approximate Methods ◽  
Direct Numerical Integration

A major challenge in the design of a mooring line or riser is the evaluation of the fatigue damage accumulated over the lifetime of the structure. The long-term environmental condition is usually represented by a scatter diagram, or a joint probability density of the significant waveheight and a characteristic wave period. Since it is computationally impractical to consider a large number of sea states, a common practice is to condense the sea states into a small number of blocks, but this procedure inevitably introduces significant errors owing to the coarse discretization. In view of the need for efficient but accurate approaches, this paper investigates the application of an asymptotic approximation, which is an established technique for estimating probability integrals, but it has so far not been applied to the fatigue design of moorings and risers. In addition, a classical method known as the perturbation approach is examined. The above approximate methods are implemented on an FPSO floating system, and direct numerical integration is carried out to ascertain the accuracy of the approximate solutions.

scholarly journals Fatigue Analysis of Deepwater Hybrid Mooring Line Under Corrosion Effect

2014 ◽  
Vol 21 (3) ◽  
pp. 68-76 ◽  
Author(s):  
Dongsheng Qiao ◽  
Jun Yan ◽  
Jinping Ou
Keyword(s):  
Fatigue Damage ◽  
Oil And Gas ◽  
Line Tension ◽  
Chain Structure ◽  
Mooring Line ◽  
Mooring System ◽  
Load Spectrum ◽  
Sea State ◽  
Mooring Lines

Abstract In the deepwater exploitation of oil and gas, replacing the polyester rope by a wire in the chain-wire-chain mooring line is proved to be fairly economic, but this may provoke some corresponding problems. Te aim of this paper is to compare the fatigue damage of two mooring system types, taking into account corrosion effects. Using a semi-submersible platform as the research object, two types of mooring systems of the similar static restoring stiffness were employed. Te mooring lines had the chain-wire-chain and chain-polyester-chain structure, respectively. Firstly, the numerical simulation model between the semi-submersible platform and its mooring system was built. Te time series of mooring line tension generated by each short-term sea state of South China Sea S4 area were calculated. Secondly, the rain flow counting method was employed to obtain the fatigue load spectrum. Thirdly, the Miner linear cumulative law model was used to compare the fatigue damage of the two mooring system types in long-term sea state. Finally, the corrosion effects from zero to twenty years were considered, and the comparison between the fatigue damage of the two mooring system types was recalculated.

A Reliability-Based Comparative Study for Mooring Lines Design Criteria

2006 ◽  
Author(s):  
Alberto Omar Vazquez-Hernandez ◽  
Gilberto Bruno Ellwanger ◽  
Lui´s Volnei Sudati Sagrilo
Keyword(s):  
Return Period ◽  
Design Procedure ◽  
Mooring Line ◽  
Design Criteria ◽  
Mooring Lines ◽  
Load Effect ◽  
Extreme Response ◽  
Line Design ◽  
Water Depths

The characteristic load effect for the design of mooring systems can be defined by means of three procedures: 1) an extreme sea state with a given return period, 2) a set of sea states on a contour line associated to a return period or 3) extreme response (tension) statistics for a long-term period. This work presents the result of a reliability-based partial safety factor calibration study for a LRFD mooring line design criteria considering the three approaches mentioned above. The calibration exercise is applied to three FPSOs considering North Sea environmental conditions and different water depths: 200m, 800m and 3000m. The mooring systems investigated take into account lines made up of chains and polyester ropes. It is shown that the design procedure based on the long-term response, among all water depths investigated, is the one that presents less scattered reliability indices around the target level.

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.

Radiant Heat Transfer From Isothermal Dispersions With Isotropic Scattering

1967 ◽  
Vol 89 (4) ◽  
pp. 300-308 ◽  
Author(s):  
R. H. Edwards ◽  
R. P. Bobco
Keyword(s):  
Heat Transfer ◽  
Approximate Solutions ◽  
Radiant Heat ◽  
Second Order ◽  
Isotropic Scattering ◽  
Radiant Heat Transfer ◽  
Approximate Methods ◽  
First Order ◽  
Order Solution ◽  
Radiant Transfer

Two approximate methods are presented for making radiant heat-transfer computations from gray, isothermal dispersions which absorb, emit, and scatter isotropically. The integrodifferential equation of radiant transfer is solved using moment techniques to obtain a first-order solution. A second-order solution is found by iteration. The approximate solutions are compared to exact solutions found in the literature of astrophysics for the case of a plane-parallel geometry. The exact and approximate solutions are both expressed in terms of directional and hemispherical emissivities at a boundary. The comparison for a slab, which is neither optically thin nor thick (τ = 1), indicates that the second-order solution is accurate to within 10 percent for both directional and hemispherical properties. These results suggest that relatively simple techniques may be used to make design computations for more complex geometries and boundary conditions.

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.

Validation of Mooring Simulations (for Mooring Integrity Assessment) With In-Service Tension Measurements

2021 ◽  
Author(s):  
Willemijn Pauw ◽  
Remco Hageman ◽  
Joris van den Berg ◽  
Pieter Aalberts ◽  
Hironori Yamaji ◽  
...  
Keyword(s):  
Numerical Models ◽  
Weather Conditions ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Data Set ◽  
Integrity Assessment ◽  
Motion Data ◽  
Load Cells ◽  
Line Loads

Abstract Integrity of mooring system is of high importance in the offshore industry. In-service assessment of loads in the mooring lines is however very challenging. Direct monitoring of mooring line loads through load cells or inclinometers requires subsea installation work and continuous data transmission. Other solutions based on GPS and motion monitoring have been presented as solutions to overcome these limitations [1]. Monitoring solutions based on GPS and motion data provide good practical benefits, because monitoring can be conducted from accessible area. The procedure relies on accurate numerical models to model the relation between global motions and response of the mooring system. In this paper, validation of this monitoring approach for a single unit will be presented. The unit under consideration is a turret-moored unit operating in Australia. In-service measurements of motions, GPS and line tensions are available. A numerical time-domain model of the mooring system was created. This model was used to simulate mooring line tensions due to measured FPSO motions. Using the measured unit response avoids the uncertainty resulting from a prediction of the hydrodynamic response. Measurements from load cells in various mooring lines are available. These measurements were compared against the results obtained from the simulations for validation of the approach. Three different periods, comprising a total of five weeks of data, were examined in more detail. Two periods are mild weather conditions with different dominant wave directions. The third period features heavy weather conditions. In this paper, the data set and numerical model are presented. A comparison between the measured and numerically calculated mooring line forces will be presented. Differences between the calculated and measured forces are examined. This validation study has shown that in-service monitoring of mooring line loads through GPS and motion data provides a new opportunity for mooring integrity assessment with reduced monitoring system complexity.

scholarly journals Dynamic Response of Floating Wind Turbine Under Consideration of Dynamic Behavior of Catenary Mooring-Lines

2017 ◽  
Author(s):  
Shuangxi Guo ◽  
Yilun Li ◽  
Min Li ◽  
Weimin Chen ◽  
Yiqin Fu
Keyword(s):  
Dynamic Response ◽  
Wind Turbine ◽  
Nonlinear Dynamic ◽  
Ocean Waves ◽  
Wave Frequency ◽  
Mooring Line ◽  
Mooring Lines ◽  
Restoring Force ◽  
Dynamic Behaviors ◽  
Floating Wind Turbine

Recently, wind turbine has been developed from onshore area to offshore area because of more powerful available wind energy in ocean area and more distant and less harmful noise coming from turbine. As it is approaching toward deeper water depth, the dynamic response of the large floating wind turbine experiencing various environmental loads becomes more challenge. For examples, as the structural size gets larger, the dynamic interaction between the flexible bodies such as blades, tower and catenary mooring-lines become more profound, and the dynamic behaviors such as structural inertia and hydrodynamic force of the mooring-line get more obvious. In this paper, the dynamic response of a 5MW floating wind turbine undergoing different ocean waves is examined by our FEM approach in which the dynamic behaviors of the catenary mooring-line are involved and the integrated system including flexible multi-bodies such as blades, tower, spar platform and catenaries can be considered. Firstly, the nonlinear dynamic model of the integrated wind turbine is developed. Different from the traditional static restoring force, the dynamic restoring force is analyzed based on our 3d curved flexible beam approach where the structural curvature changes with its spatial position and the time in terms of vector equations. And, the modified finite element simulation is used to model a flexible and moving catenary of which the hydrodynamic load depending on the mooring-line’s motion is considered. Then, the nonlinear dynamic governing equations is numerically solved by using Newmark-Beta method. Based on our numerical simulations, the influences of the dynamic behaviors of the catenary mooring-line on its restoring performance are presented. The dynamic responses of the floating wind turbine, e.g. the displacement of the spar and top tower and the dynamic tension of the catenary, undergoing various ocean waves, are examined. The dynamic coupling between different spar motions, i.e. surge and pitch, are discussed too. Our numerical results show: the dynamic behaviors of mooring-line may significantly increase the top tension, particularly, the peak-trough tension gap of snap tension may be more than 9 times larger than the quasi-static result. When the wave frequency is much higher than the system, the dynamic effects of the mooring system will accelerate the decay of transient items of the dynamic response; when the wave frequency and the system frequency are close to each other, the displacement of the spar significantly reduces by around 26%. Under regular wave condition, the coupling between the surge and pitch motions are not obvious; but under extreme condition, pitch motion may get about 20% smaller than that without consideration of the coupling between the surge and pitch motions.

Effects of the Mooring Line Configuration on the Dynamics of a Point Absorber

2013 ◽  
Author(s):  
Vincenzo Nava ◽  
Marin Rajic ◽  
Carlos Guedes Soares
Keyword(s):  
Case Studies ◽  
Time Instant ◽  
Floating Body ◽  
Mooring Line ◽  
Mooring Lines ◽  
Point Absorber ◽  
Line Configuration ◽  
Frequency Domains ◽  
Restoring Forces ◽  
Static Approach

The aim of this paper is to study the dynamics of a floating body with characteristics comparable to a point absorber wave energy converter with different mooring systems, in geometrical configuration or in the materials. To this purpose, the dynamics of a moored buoy is investigated. The point absorber is modeled as a spherical buoy in plane two-dimensional motion, and it is studied under the action of irregular unidirectional wind-generated waves, moored to the seabed by means of one, two or three mooring lines. Two different sets of moorings are considered, and typical wires and chains used in offshore technology are considered, leading to a total of 6 case studies. A quasi-static approach is used for modeling the restoring forces needed to keep buoy into station, using an innovative iterative procedure able to predict for each time instant and for each cable the lay down length of the cable, being each mooring line allowed to be taut or slack. Approaches in the time and frequency domains are used to obtain the system responses in intermediate waters, where these facilities are usually installed. Results for all case studies are compared both in terms of statistics of response and tensions on the top of the cable.

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