Fatigue Reliability-Based Inspection Planning Methodology for Corroded Mooring Chain Links

2021 ◽  
Author(s):  
Filipe A. Rezende ◽  
Marina L. Simão ◽  
Ricardo S. Gomes ◽  
Paulo M. Videiro ◽  
Luis V. S. Sagrilo
Keyword(s):  
Corrosion Rate ◽  
Limit State ◽  
Engineering Practice ◽  
Rate Model ◽  
Fatigue Reliability ◽  
Inspection Planning ◽  
Chain Segment ◽  
Mooring Lines ◽  
Operational Life ◽  
Chain Links

Abstract Mooring systems are responsible to limit the offsets of oil and gas floating production systems due to environmental actions. Therefore, they are extremely important for the overall safety of the floating unit and of the risers system connected to it. Mooring lines are subjected to pre-tension and environmental loads effects and must be designed to comply with the ultimate, accidental and fatigue limit state criteria. In general, floating units mooring lines are composed by a top chain segment, a rope segment, and a bottom chain segment. Most of the accounted failures in mooring lines are related to fatigue damage in the chain links of the top or bottom segments. Material degradation due to corrosion effects plays an important role in this failure mechanism. Engineering practice usually recommends that a constant corrosion rate model is to be adopted in the design. Periodic inspections are to be carried out in the mooring lines in order to assess the corrosion effects and other issues during the operational life of the floating system. However, corrosion is a complex phenomenon, which behavior is difficult to predict. Adopting a deterministic corrosion rate can lead to non-realistic results, compromising the system safety and the inspection planning. Therefore, to consider the problem uncertainties and comply with standards guidance, this work proposes a time-dependent fatigue reliability-based method to update the inspection planning using the results of previous inspections. The method is applied in a case study of a corroded chain segment of a mooring line, assuming a continuously-updated corrosion rate model and the S-N curve fatigue approach. Results show that the proposed method, based on solid safety assumptions, is a feasible and more reasonable way to define inspection dates, avoiding the mooring system to operate in unacceptable levels of risk.

A Robust and Efficient Computational Method for Fatigue Reliability Update Using Inspected Data

2009 ◽  
Author(s):  
Y.-T. Wu ◽  
A. P. Ku ◽  
C. M. Serratella
Keyword(s):  
Importance Sampling ◽  
Error Control ◽  
Limit State ◽  
Computational Method ◽  
Fatigue Reliability ◽  
Inspection Planning ◽  
Limit States ◽  
New Approach ◽  
Stiffened Panels ◽  
Reliability Methods

This paper presents a new methodology for reliability-based inspection planning focusing on robust and accurate computational strategies for fatigue-reliability updating using inspection results. The core of the proposed strategy is a conditioned sampling-based method, implemented by a Fast Probability Analyzer (FPA) software where efficiency is achieved by using the importance sampling principal. For a single component or limit state, FPA first generates Markov-Chain Monte Carlo (MCMC) samples in the failure domain, then applies an adaptive stratified importance sampling (ASIS) method to compute probability of failure (PoF) with error control. Once the MCMC samples have been created, solving a reliability updating problem is fairly straightforward and computationally robust relative to the conventional system reliability methods that rely on linearization of the limit states. The new approach is demonstrated using examples including stiffened panels of a ship-shaped vessel where reliability is updated using inspection results from 100 panel connections.

Fatigue Reliability of Catenary Mooring Lines Under Corrosion Effect

2008 ◽  
Author(s):  
Julien Lardier ◽  
Torgeir Moan ◽  
Zhen Gao
Keyword(s):  
Crack Propagation ◽  
Crack Growth ◽  
Fatigue Cracking ◽  
Sensitivity Study ◽  
Local Effect ◽  
Mooring Line ◽  
Fatigue Reliability ◽  
Mooring Lines ◽  
Chain Links

Experiences show that mooring line failures have occurred due to crack propagation in chain links. Moreover, the crack propagation is accelerated by corrosion. To maintain the safety of floating structures in a long-term period of, say 20–30 years, it is therefore crucial that these chain links are properly designed as well as inspected and repaired during operation. The aim of this paper is to establish a reliability-based formulation for assessing mooring chains under the deterioration due to combined fatigue cracking and corrosion wastage. The crack growth in chain links has been modelled using the 2-dimensional Paris-Erdogan equation, which is validated by comparison with the SN data on a reliability basis. The corrosion is treated by considering the diameter reduction and the fatigue crack growth rate, using three zones of corrosion, namely the splash, catenary and bottom zones. A fatigue reliability case study of studless chain links is carried out considering various system models and proper correlation between links. The material correlation is modelled by splitting variables for the global and local effect. Accumulated failure probabilities of one year up to twenty years have been calculated to illustrate the effect of corrosion on fatigue reliability. A sensitivity study has been carried out on the initial crack size, the crack aspect ratio and the material correlation between links, when applying the fracture mechanics approach. Based on the case study, it has been found that the splash zone has an important effect on the failure probability of the entire mooring line, especially under corrosion of long time.

Value of information of in situ inspections of mooring lines

2021 ◽  
pp. 1748006X2098740
Author(s):  
Jorge Mendoza ◽  
Jacopo Paglia ◽  
Jo Eidsvik ◽  
Jochen Köhler
Keyword(s):  
Pitting Corrosion ◽  
Value Of Information ◽  
Structural Reliability ◽  
Structural Safety ◽  
Statistical Dependence ◽  
Mooring System ◽  
Mooring Lines ◽  
Corrosion Condition ◽  
Chain Links

Mooring systems that are used to secure position keeping of floating offshore oil and gas facilities are subject to deterioration processes, such as pitting corrosion and fatigue crack growth. Past investigations show that pitting corrosion has a significant effect on reducing the fatigue resistance of mooring chain links. In situ inspections are essential to monitor the development of the corrosion condition of the components of mooring systems and ensure sufficient structural safety. Unfortunately, offshore inspection campaigns require large financial commitments. As a consequence, inspecting all structural components is unfeasible. This article proposes to use value of information analysis to rank identified inspection alternatives. A Bayesian Network is proposed to model the statistical dependence of the corrosion deterioration among chain links at different locations of the mooring system. This is used to efficiently update the estimation of the corrosion condition of the complete mooring system given evidence from local observations and to reassess the structural reliability of the system. A case study is presented to illustrate the application of the framework.

Performance-Based Seismic Design

2012 ◽  
pp. 23-50 ◽  
Author(s):  
Oscar Möller ◽  
Marcelo Rubinstein ◽  
Fabián Savino ◽  
Ricardo O. Foschi
Keyword(s):  
Neural Networks ◽  
Structural Analysis ◽  
Earthquake Engineering ◽  
Optimization Algorithm ◽  
Limit State ◽  
Design Parameters ◽  
Total Cost ◽  
Operational Life ◽  
Performance Based Seismic Design ◽  
Dynamic Structural Analysis

An approach is presented to structural optimization for performance-based design in earthquake engineering. The objective is the minimization of the total cost, including repairing damage produced by future earthquakes, and satisfying minimum target reliabilities in three performance levels (operational, life safety, and collapse). The different aspects of the method are considered: a nonlinear dynamic structural analysis to obtain responses for a set of earthquake records, representing these responses with neural networks, formulating limit-state functions in terms of deformations and damage, calculating achieved reliabilities to verify constraint violations, and the development of a gradient-free optimization algorithm. Two examples illustrate the methodology: 1) a reinforced concrete portal for which the design parameters are member dimensions and steel reinforcement ratios, and 2) optimization of the mass at the cap of a pile, to meet target reliabilities for two levels of cap displacement. The objective of this latter example is to illustrate model effects on optimization, using two different hysteresis approaches.

Reliability Analysis of Fatigue Crack Growth with JC Method Based on Scientific Materials

2011 ◽  
Vol 63-64 ◽  
pp. 882-885 ◽  
Author(s):  
Xiao Li Zou
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Normal Distribution ◽  
Crack Growth ◽  
Reliability Index ◽  
Limit State ◽  
State Equation ◽  
Fatigue Reliability ◽  
Crack Propagation Process ◽  
Log Normal

Since the fatigue crack propagation process from initial size till final fracture is affected by lots of random factors, it is difficult to evaluate the fatigue reliability. Based on reliability theory, the first order second moment method ( JC method) is adopted to analyze and compute the fatigue reliability. To account for the uncertainties of material resistance, the parameters in the deterministic fatigue crack growth rate equation and material fracture toughness are taken as random variables with Normal distribution or Log-Normal distribution. Consequently, the limit state equation of fatigue crack growth is derived. The fatigue reliability index at any moment is calculated iteratively through JC method. As a computation example, the curve of fatigue crack growth reliability index with time is presented.

Research on the Optimization of the Offset and the Power Consumption for a DP Assisted Mooring System

2013 ◽  
Author(s):  
Hongchao Wang ◽  
Lei Wang ◽  
Hanlin Liu
Keyword(s):  
Power Consumption ◽  
The Other ◽  
Engineering Practice ◽  
Dynamic Positioning ◽  
Positioning System ◽  
Mooring System ◽  
Mooring Lines ◽  
Positioning Accuracy ◽  
Dynamic Positioning System ◽  
Two Measures

DP assisted mooring system is a new positioning system including mooring system and dynamic positioning system. In this paper, two measures are proposed to reduce the offset and the power consumption of a generic semi-submersible platform which is equipped with a DP assisted mooring system during operation. One is to tighten the windward mooring lines and slacken the leeward mooring lines, the other is to reduce or cancel the forbidden angles of the thrusters adjacent to the failed thrusters. Given that the two measures can both reduce the offset and the power consumption of the semi-submersible unit, it is suggested to combine the two measures together to obtain better positioning accuracy. This paper is helpful to engineering practice.

Interaction Between Mooring Line Damping and Response Frequency as a Result of Stiffness Alteration in Surge

2006 ◽  
Author(s):  
Lars Johanning ◽  
George H. Smith ◽  
Julian Wolfram
Keyword(s):  
Limit State ◽  
Extraction Process ◽  
Tidal Range ◽  
Mooring Line ◽  
System Response ◽  
Mooring Lines ◽  
Wave Basin ◽  
A Chain ◽  
Conversion Efficiencies ◽  
Line Design

The design and operation of a chain mooring for a wave energy converter (WEC) is considered. Experimental measurements of a mooring line were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The laboratory procedures were designed to resemble tests undertaken earlier in the year at ‘full’ scale in 24 m water depth. This paper describes and compares these measurements and relates the results to earlier work on mooring lines by Webster [1]. Measurements of both the damping and response frequencies of the mooring are described. Although the present results support partly the conclusions of the earlier work, care must be taken in how these are applied when one is considering mooring line design for WECs. It is concluded that there are significant differences for a WEC for both operational and limit state design in comparison with a more conventional offshore system such as an FPSO or CALM. Although the primary requirement is still one of station-keeping two further considerations may be of great importance. Firstly if a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere with one another and this will have great significance for the loading experienced by the line. This can be exacerbated by variations in tidal range which will have a larger effect in comparison with a conventional deepwater mooring. A second factor may arise if the mooring system response is critical to the WEC energy extraction process. If the mooring becomes part of the ‘tuned’ system then changes in the mooring properties of damping and natural frequency could seriously affect energy conversion efficiencies.

Temporal Variability in Corrosion Modeling and Reliability Updating

2006 ◽  
Vol 129 (4) ◽  
pp. 265-272 ◽  
Author(s):  
Daniel Straub ◽  
Michael Havbro Faber
Keyword(s):  
Corrosion Rate ◽  
Temporal Variability ◽  
Reliability Assessment ◽  
Corrosion Process ◽  
Quantitative Modeling ◽  
Co2 Corrosion ◽  
Inspection Planning ◽  
Corrosion Loss ◽  
Temporal Models ◽  
Time Invariant

Many parameters influencing corrosion degradation are time variant and the corrosion process is thus subject to temporal variability, the real characteristics of which are commonly neglected in reliability assessment. After a short overview on the quantitative modeling of corrosion loss, a comparison is made between different temporal models of corrosion degradation and the consequences of applying an inappropriate model are investigated. The effect of temporal variability is then investigated in detail and illustrated in an example considering CO2 corrosion in pipelines. It is demonstrated how the time-variant corrosion process can be consistently represented by time-invariant random variables, using equivalent values of the corrosion rate. Finally, the influence of temporal variability on reliability updating following inspections is investigated and it is shown how this effect can be accounted for in inspection planning.

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