T-N Curves and Fracture Mechanics Based Mooring Fatigue Analysis for a Semi-Submersible

2017 ◽  
Author(s):  
Xutian Xue ◽  
Nian-Zhong Chen
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Failure ◽  
Line Tension ◽  
Fatigue Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Bottom Line ◽  
Mooring Lines ◽  
Element Analysis ◽  
Chain Link

This paper is to perform T-N curves and fracture mechanics based fatigue analysis for mooring lines of a semi-submersible installed in Gulf of Mexico (GoM). The wave frequency (WF) and the low frequency (LF) load processes are regarded as two random processes and the load combination of the two processes is considered. Frequency-domain analysis is then conducted for calculating the tension ranges along hybrid-material mooring lines induced by motions of WF, LF and the combined WF and LF. The narrow-banded spectral method is used for calculating the mooring line tension and the fatigue damage of mooring lines is estimated by T-N curves and fracture mechanics based approaches. The fracture mechanics based analysis is combined with a finite element analysis to predict crack propagation at different locations of a studless chain link. It was found that the crown section of a mooring chain is the most critical location subjected to fatigue failure in a studless chain link. The most critical points prone to fatigue failure of the catenary and taut mooring systems designed for the semi-submersible are at the bottom end of top line and the top end of bottom line, respectively. Also, fatigue lives of mooring lines in the catenary mooring system are generally longer than those of the taut mooring system. In addition, a comparison between fatigue lives of mooring lines predicted by T-N curves and fracture mechanics based approaches shows that the results estimated by both approaches are generally comparable.

Fracture Mechanics Based Mooring Fatigue Analysis for a Semi-Submersible Subjected to Triple Narrow-Band Loading Processes

2019 ◽  
Author(s):  
Xutian Xue ◽  
Nian-Zhong Chen ◽  
Yongchang Pu
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Fatigue Analysis ◽  
Second Order ◽  
Combination Method ◽  
Mooring System ◽  
Initial Surface ◽  
Loop Current ◽  
Mooring Lines ◽  
First Order

Abstract In the general offshore operating locations, mooring systems are normally considered to be primarily affected by environmental loadings induced by waves, wind and current. WF motion induced by first-order waves, LF motion induced by second-order waves and wind would make the most contribution to the fatigue damage of mooring lines. However, it was reported recently that in the Gulf of Mexico (GoM), the fatigue life of mooring lines can be significantly affected by the vortex induced motion (VIM) induced by loop current. The aim of this presented paper is to address the influence of VIM on fatigue life of the mooring system operating at the central of the GoM through performing a fracture mechanics (FM) based fatigue analysis for an offshore mooring system. A frequency-domain mooring analysis for the semi-submersible is conducted where WF motion induced by first-order waves, LF motion induced by second-order waves and wind, and VIM induced by loop current are taken into account. WF motion, LF motion and VIM are treated as three independent loading processes. A wide-band loading combination method is then used for predicting the loading processes acting on the mooring system combining WF motion, LF motion and VIM. A fracture mechanics based analysis is performed to examine the fatigue life of mooring system, in which initial surface cracks in previous existence are assumed to grow from the surfaces of mooring chain links connecting to the fairleads. The stress intensity factor ranges to estimate the crack growth in the FM based analysis are obtained from a finite element (FE) analysis.

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.

Time Variant Reliability of Mooring System Considering Corrosion Deterioration

2005 ◽  
Author(s):  
Zhen Gao ◽  
Torgeir Moan ◽  
Svein E. Heggelund
Keyword(s):  
Reliability Analysis ◽  
Failure Probability ◽  
Breaking Strength ◽  
Mooring Line ◽  
Mooring System ◽  
Uniform Corrosion ◽  
Element Analysis ◽  
Weakest Link ◽  
Chain Link ◽  
Chain Links

This paper deals with time variant overload reliability analysis of a mooring system due to corrosion deterioration. A probabilistic model for uniform corrosion is adopted to predict the strength degradation. A simplified method and nonlinear finite element analysis are used to calculate the breaking strength of the chain link and comparison is made. The strength of one mooring line is modeled by a weakest link system. The effect of correlation in corrosion models for different chain links in one mooring line and the effect of higher corrosion rate in the splash zone are discussed. The annual failure probability of the most loaded mooring line of a semi-submersible is calculated. The first and second order motions of the semi-submersible and the corresponding line tensions are found by a simplified analysis. The time variant reliability analysis is performed by approximating the degraded strength by a piece-wise constant model. The annual failure probability is obtained for different years. It is found that the annual failure probability increases significantly as the chain is corroded.

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.

Snap Load Induced by Slack-Taut Process in a Taut Mooring Line

2019 ◽  
Author(s):  
Dongsheng Qiao ◽  
Wei Tang ◽  
Yunfei Suo ◽  
Jun Yan ◽  
Yugang Li ◽  
...  
Keyword(s):  
Line Tension ◽  
Mooring Line ◽  
Mooring System ◽  
Analysis Model ◽  
Dynamic Tension ◽  
Element Analysis ◽  
Floating Structures ◽  
Dynamic Finite Element Analysis ◽  
Calculation Results ◽  
Frequency Components

Abstract The large amplitude motion of floating structures could cause slack-taut transformation in the taut mooring system, which may result in snap load. The dynamic finite element analysis model is established to simulate the slack-taut process through adding a series of sinusoidal excitation with different amplitudes and frequencies to the upper end of a taut mooring line. During the slack-taut process, the minimum dynamic tension could be close to zero, and the maximum dynamic tension could come up to several times of pretension. The change laws of dynamic tension during the slack-taut process are compared and summarized. The calculation results show that the phenomenon of slack-taut could occur when the amplitude and frequency of excitation reach some certain value. The mooring line tension spectra show that the doubling and higher frequency components appear in addition to the frequency of excitation. The results could provide a reference for further investigating on the mechanism of snap load and the design of mooring system.

A Fundamental Study on Motion Characteristics of the Large-Scale Floating Coal Transshipment Station by Elastic Mooring Lines

2020 ◽  
Author(s):  
Hiroaki Eto ◽  
Ryo Sekiguchi ◽  
Hitomi Kashima ◽  
Tomoki Ikoma ◽  
Yasuhiro Aida ◽  
...  
Keyword(s):  
Large Scale ◽  
Line Tension ◽  
Asia Pacific ◽  
Floating Body ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Fundamental Study ◽  
Bulk Carriers ◽  
Motion Characteristics

Abstract This paper describes the motion characteristics and cargo handling efficiency of the Large-Scale Floating Coal Transshipment Station (LFTS). Indonesia is the main country supplying coal in the Asia-Pacific region, it is important to ensure stable coal supply to Japan. Because the topography of the seabed near East Kalimantan Island, Indonesia’s main coal production area, is shallow, it is difficult for bulk carriers to reach the coast. And then, Large-scale Floating Transposition Station for Loading Coal (hereafter LFTS) was proposed, which will be used as a transposition station between small coal barge coming down the river and bulk carriers stay offshore. By installing LFTS, improvement of coal transport efficiency is expected. As a previous study, the motion characteristics of LFTS using a catenary chain in its mooring system were grasped. However, LFTS can carry up to 500,000 tons of coal, and the draft of LFTS tends to change greatly depending on the coal loading conditions. Besides, the tidal difference in the sea area where the LFTS is installed is about 2 m, and the mounting position of the mooring system on the LFTS side moves up and down by about 10 m at maximum due to changes in the draft and tidal differences. For this reason, when the mounting position of the mooring system is in the lowest state, the mooring line is loosened and the horizontal force is reduced, and it is considered that sufficient restraining force is not exhibited. And, when the mounting position of the mooring system becomes high, the mooring line tension increases and the mooring line may break in some cases. Therefore, in this study, an attempt was made to use an elastic mooring line as a mooring system for LFTS. An elastic mooring line is a mooring line that incorporates a highly stretchable material between mooring lines that connect anchors and floating body. Even if the mooring line attached to the LFTS moves up and down, an appropriate tension always acts on the elastic mooring line, and it can be expected to suppress the oscillation of the floating body and prevent the mooring line from breaking due to excessive tension. However, elastic mooring lines are mainly used for mooring small structures such as piers and aquaculture facilities, but there are no examples where these mooring lines have been applied to structures over 500m like LFTS. Therefore, elastic mooring lines are adopted for the mooring system of LFTS, systematically calculated according to various setting conditions of elastic mooring lines, and it is grasped whether elastic mooring lines can be applied to LFTS, and the motion characteristics of LFTS moored by elastic mooring lines was also grasped.

A Novel Hybrid Method of Pre-Tension Adjustment of Mooring Lines in the Model Test

2020 ◽  
Author(s):  
Junfeng Du ◽  
Hongchao Wang ◽  
Anteng Chang ◽  
Mingyuan Sun ◽  
Junrong Wang ◽  
...  
Keyword(s):  
Model Test ◽  
Line Tension ◽  
Line Length ◽  
Mooring Line ◽  
Cyclic Process ◽  
Mooring System ◽  
Ideal Model ◽  
Mooring Lines ◽  
Target Value ◽  
The Ideal

Abstract Mooring lines are the most commonly used station-keeping systems for floating platforms, and the pre-tension of the mooring lines has significant effect on the dynamics of floater/mooring system. However, during the model test, which is the most important method for the research on the floater/mooring system, the pre-tension of the designed mooring lines in the basin are always significantly different with the target values of the ideal model due to the error of the manufacture of the mooring lines and positon locating of the anchor point and so on. As a result, the adjustment of the pretension of the mooring lines based on the ideal model is an important task during the model test. The pre-tension is always adjusted by increasing or decreasing mooring line length. While all the mooring lines are connected with the floater and coupled with each other, pre-tension adjustment is an unwarranted and cyclic process which often wastes a lot of time. This paper presents a novel methodology for regulating the mooring line pre-tension based on matrix calculation. The increment matrix of the mooring line tension can be obtained by increasing or decreasing the length of each mooring line in the numerical model. Assuming that the increment the mooring line tension is linear to the change of the mooring line length, the adjustment amount of each mooring line length can be calculated from the difference of the real pre-tension and the target pre-tension. However, the mooring line tension is always nonlinear to the mooring line length. Calculating the increment matrix of the mooring line tension and adjustment of the mooring line length is also a cyclic process until the error of real pre-tension of the model in the basin and target value can meet the accuracy requirement. This method is used in a model of a mooring system with 12 mooring lines which are connected with a semi-submersible platform. The test results show that the pre-tension of mooring lines can be adjusted to the target value quickly based on the method proposed in this paper.

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.

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.

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.

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