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Structural reliability of offshore platforms considering fatigue damage and different failure scenarios
Abstract
Structural systems will normally fail as a consequence of a chain of different components failure es.
In this paper, fatigue reliability of fixed offshore platforms is investigated by analyzing different failure
scenarios. In order to evaluate the occurrence probability of a special scenario, it is divided into a finite
number of sub-scenarios. All combinations of time sequences are generated for a given sequence
of failures, using a specially developed program.
In order to calculate the occurrence probability of each scenario, a massive reliability analysis
should be done for each of corresponding sub-scenarios. A large number of sub-scenarios should be
analyzed, therefore implementing time consuming traditional methods for evaluating fatigue reliability
may be unrealistic, and a simplified and accurate method is required. Herein, an ‘‘Artificial transfer
function’’ is used to calculate the cumulative fatigue degradation in components. The preciseness of the
proposed method is evaluated using a numerical model, and then, it is used to calculate the occurrence
probability of each sub-scenario. Based on the calculated values, probability of occurrence is obtained
for each scenario, and finally, the failure probability of entire system is calculated. The proposed
method can be used in inspection planning and evaluating the life extension of existing offshore
platforms.
... In Mexico, the assessment of marine platforms at the Bay of Campeche follows inspection and maintenance purposes [15, 16 and 13]. The structural reliability of marine platform has been calculated including P-, fatigue and extreme wave effects, [7,10,11]. A previous study, [5], proposed probability distributions of mechanical damage, considering a survey of actual marine platforms damaged in Mexico. ...
... Damage states obtained from the occurrence probability and intensity of these damages, as estimated in previous works, were determined for typical platforms on the Bay of Campeche. The study considered existing denting and out-of-straightness statistics [4] and joints cracking data [10]. The maximum damage intensities were δ/D = 0.16 for denting, ∆/L = 0.02 for out-of straightness and Acrack/Ag = 0.30 for joint cracking. ...
... Substructure longitudinal frame of the studied platform[10] ...
In this paper a formulation to assess the global reliability of marine platforms including mechanical damages is proposed. The considered mechanical damages are: denting, out of straightness, corrosion and cracking in some joints which are mainly caused by ship impacts, dropped objects, corrosion and fatigue. The study is based on a typical marine platform located on the Bay of Campeche, Mexico and the following analyses were performed: 1. Identification of joints prone to fatigue failure, from previous studies. 2. Damage magnitude and location are varied for denting, out of straightness, corrosion and cracking according to previously obtained damage statistics. 3. Global reliability index, against oceanographical hazard, is estimated through an approximated reported expression based on the base shear load demand and remaining capacity of the damaged platform. The formulation may be used to improve the practice and codes for Design, Assessment and Maintenance of Marine Platforms in Mexico.
... The reliability of the panels was found to be sensitive to uncertainties in component thickness and fibre content. The fatigue reliability of fixed offshore platforms was investigated by Gholizad et al. (2012) by analysing different failure scenarios. In order to evaluate the occurrence probability of a special scenario, it was divided into a finite number of sub-scenarios and evaluated separately followed by a comparison of them. ...
... Examples of numerical results are presented for fatigue life of an oil tanker using the proposed method and IACS-CSR. The fatigue reliability of fixed offshore platforms was investigated by Gholizad et al. (2012) by analysing different failure scenarios. In order to evaluate the occurrence of probability of a special scenario, it was divided into a finite number of sub-scenarios and evaluated separately followed by a comparison of them. ...
... There is research to mitigate computational burden of reliability analysis. Gholizad et al. (2012) investigated the fatigue reliability of fixed offshore platforms by analysing different failure scenarios. In order to calculate the occurrence probability of each scenario, a massive reliability analysis should be done for each of the corresponding sub-scenarios using an artificial transfer function instead of implementing time consuming traditional methods. ...
(COMMITTEE MANDATE) Concern for the quasi-static response of ships and offshore structures, as required for safety and serviceability assessments. Attention shall be given to uncertainty of calculation models for use in reliability methods, and to consider both exact and approximate methods for the determination of stresses appropriate for different acceptance criteria.
... Czarnecki and Nowak [12] realized that corrosion at connections can be more important than in beam and column members. This could be because the connection plates and bolts often have different material characteristics with distinct corrosion resistance levels [13]. Connection failures can happen unexpectedly, even if the applied load is smaller than the tensile strength of the steel material [14]. ...
... Buildings 2023, 13, x FOR PEER REVIEW 13 of 17 13. Nonlinear material behavior was defined in ABAQUS FEA. ...
Corrosion damage is a serious problem in steel structures. The cross-sectional loss in the structural members due to corrosion reduces the load-carrying capacity of the members and the stability of their structures. In this study, the main reasons for the collapse of three steel sports infrastructure facilities after moderate snowfall were investigated by conducting field observations and detailed numerical analyses. Finite element models of the structures were developed by considering the effects of different rafter systems and corrosion damage at their columns’ support regions. The load-carrying capacity ratios and stress distributions of the structural members were determined under the effect of the snow load at the time of the collapse. The analysis results were consistent with the damage modes observed during site inspections. The snowfall was not the primary cause of the collapse; however, the section and joint losses due to excessive corrosion, improper erections, and discrepancies between the design project and the as-built project were the main reasons for the collapse.
... This means for OWTs that it must fulfil specific functional requirements and does not become unsafe over, generally, at least 25 years. 13 Several studies 1,3,12,[14][15][16][17][18] have already applied the structural reliability methodology for the safety assessment of offshore structures. However, most reliability methods assume that the system is time-invariant. ...
... 70 This study aims to explore the influence of various mesh sizes on fatigue life prediction. Based on the numerical results, the mesh size used is 15 μm within the pit depth vicinity (crack front) and 1000μm for the rest of the curved plate (15,1000). In this section, two more element size configurations (10, 1000) and (20,1000) are investigated for surface pit 223 μm at different applied loads to study the effect of the mesh size. ...
The structural integrity of offshore wind turbine (OWT) support structures is affected by one of the most severe damage mechanisms known as pitting corrosion-fatigue. In this study, the structural reliability of such structures subjected to pitting corrosion-fatigue is assessed using a damage tolerance modelling approach. A probabilistic model that ascertains the reliability of the structure is presented, incorporating the randomness in cyclic load and corrosive environment as well as uncertainties in shape factor, pit size and aspect ratio. A non-intrusive formulation is proposed consisting of a sequence of steps. First, a stochastic parametric Finite Element Analysis (FEA) is performed using SMART© crack growth and Design Xplorer© facilities within the software package ANSYS. Secondly, the results obtained from the FEA are processed using an Artificial Neural Network (ANN) response surface modelling technique. Finally, the First Order Reliability Method (FORM) is used to calculate the reliability indices of components. The results reveal that for the inherent stochastic conditions, the structure becomes unsafe after the 18th year, before the attainment of the design life of 20 years. The FEA results are in very good agreement with results obtained from analysis steps outlined in design standard BS 7910 and other references designated as 'theoretical analysis methods' in this study. The results predict, for the case study, that the pit growth life is approximately 56% of the total pitting corrosion fatigue life. Sensitivity analysis results show that the aspect ratio of pits at critical size plays a significant role on the reliability of the structure. K E Y W O R D S artificial neural network, non-intrusive formulations, offshore wind structures, pitting corrosion fatigue, reliability index, structural reliability analysis
... Offshore platforms suffer different damages under long-term multiple environmental loads of current, wave, and wind [1]. These platforms are generally welded with numerous circular tube structures. ...
In this research work, the stress concentration factors (SCFs) in tubular X-joints retrofitted with fiber reinforced polymer (FRP) under compression are studied and discussed. After validation of the FE model with several available experimental data, a set of 279 FE models was created to parametrically investigates the influence of the FRP and connection geometry on the SCFs and SCF ratios. In the FE models, the contact between the FRP sheets and steel members (chord, weld, and braces) was modeled. Results showed that the increase of the FRP sheet number leads to a considerable drop in the SCFs. Despite the considerable effect of the FRP on the drop of the SCFs in the X-joints, there is not any study or formula in X-connections with FRP. Finally, the FE results were used to propose two accurate equations for quantifying the SCFs in X-joints with FRP sheets at the saddle and crown points.
... In other words, each member of the platform should be considered as a scenario and, with its failure, the load will be spread to other members, and the probability of failures of each member will change. Equation (1) calculates the failure probability of the entire system [58]. ...
Offshore platforms have had diverse applications in the marine industry, for example, oil
or gas platforms can provide facilities to store the oil and gas before transporting those to refineries. Offshore wind turbines are another well-known use of the offshore platform for generating power. As platforms encounter various strong forces from water and wind currents, the materials used for these structures are mainly steel or concrete. These platforms are classified into different types, according to the depth of water and their applications. In addition, offshore platforms, as artificial reefs may be used for decades at different marine conditions. Consequently, their design and maintenance are very important, otherwise, they can cause irreparable damage to the environment. This paper presents the latest and most significant design and monitoring methods, such as the optimal probabilistic seismic demand model, multi-objective optimization, dynamic response assessment, robust fault-tolerant control, etc., under different environmental and geographical conditions. Moreover, the effective factors on the life and failure of these offshore structures are comprehensively introduced to enhance awareness of them, which can be very helpful to improve the design and construction of more reliable and durable structures.
... Furthermore, the wide-banded fatigue damage of the catenary mooring lines of wind turbines has been evaluated by various artificial neural network models (Li et al., 2018). In addition, the structural fatigue reliability of offshore platforms has been assessed under different failure scenarios (Gholizad et al., 2012), and artificial transfer functions are introduced to save time. 2 The fatigue life reliability of the welded structure of a ship hull has been investigated by the improved Paris law and Monte Carlo simulation (Yan & Huang, 2015). ...
The statistical uncertainties of wave loads have large proportions in the structural fatigue strength analysis of offshore platforms. To investigate the uncertainties of wave loads during predicting fatigue loads, the assessing methods and formulas are proposed involving the wave spectrum, sea wave height, wave loading transfer function, zero-crossing period, possibility of sea state occurrence and wave direction; meanwhile, the model test of the wave loads is carried out in a see-keeping basin for studying the uncertainty of the wave loading transfer function. The corrected formulas of the significant wave height and mean zero-crossing period are statistically regressed by calculating the sea states in the Northwest Pacific and South China Sea. Furthermore, the influence factors of the sea states of difference seasons are analysed, and the distribution law of the wave direction of the wave loads and fatigue life of a semi-submersible platform are investigated. Finally, the fatigue reliability index of typical structures for a semi-submersible platform is assessed on the basis of uncertainties of wave loads and the other uncertainties of structural factors.
... The work of Lupoi et al. [31] focused on the development of T a probabilistic method of seismic assessment which is able to manage the physical complexity of the dam-foundation damage and uncertainties regarding the structural data and external actions. Gholizad et al. [12] have proposed in the field of offshore structures, an assessment method of reliability, which considers different failure scenarios of fatigue structural components. This approach provides more detailed information on the fatigue behaviour of different structure components. ...
The design of concrete elevated water tanks involves several kinds of uncertainties. Traditionally, the design of these structures is based on a deterministic analysis. Partial safety factors prescribed in design codes are applied to take into account these uncertainties and to ensure sufficiently safe design. However, this approach does not allow rational evaluation of the risk related to the structural failure and consequently its reliability. In fact, the partial safety factors can lead to over-designed structures; or to under designed structural components leading to a lack of structural robustness. In this study, a probabilistic approach based on Monte Carlo simulations is used to analyze the reliability of elevated water tanks submitted to hazard seismic loading. This reliability approach, takes into account mainly two parameters. Firstly, the hydraulic charge in the tank container which is a function of time, and secondly, the hazard seismic loading through the Peak Ground Acceleration is considered as a random variable. Fragility curves depending on seismic zones and soil types are obtained by using the probabilistic approach, where they demonstrate the dominant failure modes that can cause the structural failure with respect to different seismic levels, soil types and water height level in the tank container.
... During the use of offshore platforms, their structures suffer from different damages under long-term multiple environmental loads of current, wave, and wind (Gholizad et al., 2012). These platforms are generally welded with numerous circular tube structures. ...
In the present study, the stress concentration factors (SCFs) in circular hollow section X-connections retrofitted with fiber reinforced polymer (FRP) under compressive load are evaluated. After validation of the FE model with several available experimental data, a set of 279 FE models was created to parametrically investigates the efficacy of the FRP and connection geometry on the SCFs and SCF ratios. In the FE models, the contact between the FRP layers and steel members (chord, weld, and braces) was modeled. Results showed that the rise of the FRP sheet number causes a considerable drop in the SCFs. Moreover, the use of FRP causes the propagation of stresses in steel and FRP. Moreover, the rise of the elastic modulus of FRP along the fibers causes the more drop in the SCFs. Despite the notable efficacy of the FRP on the drop of the SCFs in the X-joints, there is not any study or formula in X-connections with FRP. Hence, the FE results were used to propose two precise formulas for quantifying the SCFs in X-connections with FRP layers at the saddle and crown points. Also, the proposed equations were checked according to the UK DoE acceptance standard.
... Applications of the T-D SRA for the design of modern metal structures such as those deployed offshore (i.e., in environments characterized by highly stochastic loads and resistance properties, thus necessitating the need for SRA to account for such uncertainties systematically) include [2,3,44,[47][48][49][89][90][91][92][93][94]. In [95], the fatigue reliability of fixed offshore platforms was investigated by analyzing different failure scenarios. The analysis was divided into a finite number of sub-scenarios in order to evaluate the occurrence probability of a special scenario. ...
This paper presents the state of the art in Structural Reliability Analysis (SRA) methods with a view of identifying key applications of each method and its proposed variations, qualifying characteristics, advantages, and limitations. Due to the increasing complexity and scale of modern offshore jacket structures, it becomes increasingly necessary to propose an accurate and efficient approach for the assessment of uncertainties in their material properties, geometric dimensions, and operating environments. SRA, as a form of uncertainty analysis, has been demonstrated to be a useful tool in the design of structures because it can directly quantify how uncertainty about input parameters can affect structural performance. Herein, attention was focused specifically on the probabilistic fracture mechanics approach because this accounts accurately for fatigue reliability mostly encountered as being dominant in the design of such structures. The well-established analytical/approximate methods such as the First-and Second-Order Reliability Methods (FORM/SORM) are widely used as they offer a good balance between accuracy and efficiency for realistic problems. They are, however, inaccurate in cases of highly non-linear systems. As a result, they have been modified using methods such as conjugate search direction approach, saddle point approximation, subset simulation, evidence theory, etc. in order to improve accuracy. Initially, direct simulations methods such as the Monte Carlo Simulation Method (MCS) with its various variance reduction techniques such as the Importance Sampling (IS), Latin Hypercube Sampling (LHS), etc. are ideal for structures having non-linear limit states but perform poorly for problems that calculate very low probabilities of failure. Overall, each method has its own merits and limitation, with FORM/SORM being the most commonly used, but recently, simulation methods have increasingly been used due to continuous advances in computation powers. Other relevant methods include the Response Surface Methods (RSM) and the Surrogate Models/Meta-models (SM/MM), which are advanced approximation methods and are ideal for structures with implicit limit state functions and high-reliability indices. Combinations of advanced approximation methods and reliability analysis methods are also found in literature as they can be suitable for complex, highly non-linear problems.
... This method is particularly attractive because it reduces significantly the number of simulations by decomposing the N − dimensional integral associated with expected long-term fatigue damage assessment into the sum of N one-dimensional integrals. Gholizad et al. (2012) prepared a work on structural reliability of offshore platforms considering fatigue damage and different failure scenarios. Kim et al. (2018) proposed an innovative method for estimating fatigue performance of risers under vortex-induced vibration. ...
Offshore structures are mainly associated with the oil and gas industry given that globally, nearly one-third of the oil and gas extracted worldwide comes from offshore sources. This situation is likely to continue to rise over the coming decades because of abundant deposits of oil and gas still present in the oceans and society's dependency on hydrocarbon fuels. Regardless, offshore wind shows promise and continues to rise at an exponential rate because it provides means for decarbonization while contributing to economic growth in many countries. As such, wind continuos to be a leading solution against climate change globally. To ensure that a structure will fulfill its function, fatigue design is crucial to ensure an adequate service life because it is responsible for more than 80% of structural failures, most of them catastrophic and without warning. This work aims to evaluate environmental loads and fatigue analysis in a jacket-type platform located in the North Sea. Wave data scatter has been provided, and using Morison's formula, the loads on the structure were used to achieve an applicable normal loading force. Then, a static and a dynamic fatigue analysis for the offshore jacket-type structure under consideration are evaluated and compared.
... Corrosion can be more critical at connections compared with beams and columns for steel structures (Czarnecki & Nowak, 2008). This is because the connection plates and rivets (or bolts) are commonly made of different grades of steel with different corrosion resistance, which leads to galvanic corrosion (Gholizad, Golafshani, & Akrami, 2012). Failure of connections may occur suddenly even if the load effect is smaller than the tensile capacity of the steel (Ni et al., 2010). ...
Corrosion of steel is now a worldwide issue, threatening the safety of steel structures. Literature suggests that little research has been conducted on the prediction of fatigue failures of connections subjected to simultaneous fatigue and corrosion. This paper proposes a new method to predict the failure of connections subjected to fatigue and corrosion. A model is developed to determine the effect of corrosion on S-N curve of corroded steel. The fatigue damage to corroded connections is modelled as a stochastic process which considers the corrosion effects on both stress range and S-N curve. The first-passage probability method is used to determine the time-dependent probability of fatigue failure of connections subjected to fatigue and corrosion. A case study is then provided to demonstrate the application of the proposed methodology. It is found in the paper that not considering the effect of corrosion on S-N curve can lead to 15.45% reduction of the probability of fatigue failure of corroded connections over 150 years, which can be catastrophic. It has also been found, through sensitivity analysis, that corrosion rate can considerably affect the probability of fatigue failure in longer term. The methodology proposed in this paper can help structural engineers and asset managers in making decisions regarding the repair and/or maintenance of connections subjected to the combined effects of corrosion and fatigue.
... In this context, the reliability methods lead to important advances based on the risk assessment and allow to quantify the failure risk of a given structure taking into account the uncertainties and to optimize its design for a considered risk (Breysse 2009;Cremona 2012;JCSS 2001;Lemaire et al. 2009). Thus, one notes a growing interest of the scientific community of civil engineering for the application of probabilistic approaches in the design and analysis of structures' stability, as Lupoi andCallari (2011), Peyras et al. (2012) in the reliability of dams, Gholizad et al. (2012) in the reliability of offshores structures, Monteiro et al. (2016), Nielson and DesRoches (2007) in the probabilistic analysis of RC bridges, and Choi et al. (2006), Wang and Lin (2018) in the reliability of nuclear structures. ...
The deterministic methods used in the design codes of concrete storage tanks on ground are based on the principle of safety coefficients. These deterministic analyses are subject to uncertainties related to the assessment of seismic loading. To analyze the reliability of these structures, one proposes in this study a probabilistic approach by considering two variables, the hydraulic static load inside the tank and the seismic acceleration of the soil. The reliability analysis is conducted using a computer code developed with Matlab© software based on the Monte Carlo simulation method. The failure probability is evaluated at different time of the day for each limit state function considered in this work. Fragility curves are developed representing the failure probability at the different levels of seismic acceleration for different types of soil. The study concludes that there is no soil type effect on the failure risk evaluation, and that the failure mode by sloshing is the most prejudicial failure risk for the reservoir.
... To this end, a hypothesis that assumes equal probability of acoustic emission cracks in the structure joints is made in the sensor layout optimization. Since the tube joint is the most important but the most frequently broken component in the KT jacket offshore platforms, [25][26][27] this paper considers the sensor layout optimization for tube joints. ...
This paper aims to detect the structure damage in KT type jacket offshore platforms using acoustic emission analysis. Experimental investigation has been implemented to analyze the transmission characteristics, attenuation law, and source localization of the acoustic emission signals. The range of energy attenuation coefficient α and the signal amplitude attenuation law were obtained from experimental data. Hence, the layout of acoustic emission sensors was optimized based on the energy attenuation to achieve online damage monitoring for KT-jacket platforms. In order to validate the performance of the optimized sensor layout, another experimental test was conducted on the designed KT-jacket offshore platform to locate the acoustic emission sources. The test results demonstrate that a positioning error of 8 mm or below can be obtained using the optimized sensor layout, and the number of sensors can be reduced by 80% compared with that of the theoretical layout. As a result, the optimized sensor layout enables efficient and effective damage detection for KT-jacket offshore platforms.
... Structural RA of offshore structures, evaluated with respect to fatigue behaviour and considering each single failure scenario, could be computationally intensive and time-consuming. To deal with this, an artificial transfer function (ATF) is used in [79]. The real transfer function, used in the fatigue calculations, is approximated by a twoparameter ATF with a predefined shape, similar to the Pierson-Moskowitz spectrum. ...
Offshore and marine renewable energy applications are governed by a number of uncertainties relevant to the design process and operational management of assets. Risk and reliability analysis methods can allow for systematic assessment of these uncertainties, supporting decisions integrating associated consequences in case of unexpected events. This paper focuses on the review and classification of such methods applied specifically within the offshore wind industry. The quite broad differentiation between qualitative and quantitative methods, as well as some which could belong to both groups depending on the way in which they are used, is further differentiated, based on the most commonly applied theories. Besides the traditional qualitative failure mode, tree, diagrammatic, and hazard analyses, more sophisticated and novel techniques, such as correlation failure mode analysis, threat matrix, or dynamic fault tree analysis, are coming to the fore. Similarly, the well-practised quantitative approaches of an analytical nature, such as the concept of limit states and first or second order reliability methods, and of a stochastic nature, such as Monte Carlo simulation, response surface, or importance sampling methods, are still common practice. Further, Bayesian approaches, reliability-based design optimisation tools, multivariate analyses, fuzzy set theory, and data pooling strategies are finding more and more use within the reliability assessment of offshore and marine renewable energy assets.
... During the utilization of offshore platforms, their structures suffer from various damages under long-term multiple environmental loads of sea wind, wave, and current (Gholizad et al., 2012). Offshore jacket platforms are usually welded with numerous circular tube structures. ...
Among offshore platform structures, multi-planar tubular joints are usually in the most dangerous positions and generate fatigue cracks the earliest. In this study, DX-joints were regarded as the research object, and finite element method was adopted to conduct systematic research on the fatigue properties of multi-planar tubular joint welds. Axial, in-plane bending moment, and out-of-plane bending moment loads were applied to explore the stress distribution along the weld path and the hotspot location under different load conditions. Results indicated that the hotspot stress (HSS) location of DX-joints changed with the variation in the combined stress ratio. Through HSS analysis, cracks were prefabricated at the hotspot location of DX-joints to analyze the change rule of the stress intensity factor (SIF). The influence of different loads, crack lengths, and crack depths on SIF was studied to estimate the growth trend of the weld crack and its dominance. The findings can provide theoretical guidance for fatigue damage and safety assessment of offshore platforms.
Current study presents a state-of-the-art structural spatiotemporal multimodal risk assessment methodology, that is particularly well-suited for multimodal structural dynamics, either recorded physically or numerically simulated over a representative timelapse. Offshore Jacket-type offshore platform, operating in Bohai Bay waters had been selected for the method’s verification. This investigation had shown that, in the presence of in-situ environmental stressors, it is possible to appropriately estimate dynamic structural system’s failure and damage risks. High dimensionality of engineering dynamic structural systems, alomg with nonlinear nonstationary inter-correlations between critical structural components, often present challenges for contemporary reliability methods, mostly limited to univariate and bivariate systems. Operating Jacket platform, subjected to in situ wave loads, had been chosen for this case study to benchmark advocated risk evaluation methodology. Number of hotspot stresses had been selected to represent multivariate structural system. Advocated multimodal method had been proven to be suitable for robust assessment of operational failure/damage risks, as well as for accurate structural life projection. Novel non-parametric deconvolution extrapolation scheme had been employed, providing enhanced numerical stability. Given increased safety concerns within offshore, naval, marine engineering, advocated multimodal reliability methodology may be utilized for safer and economically more viable structural design.
Current study presents state of the art approach to risk and reliability assessment for multivariate nonlinear dynamic systems. Specifically, a novel hypersurface Gaidai risk evaluation methodology has been presented for evaluating offshore structural risks. The advocated approach is particularly suitable for offshore engineering multidimensional dynamic systems, possessing a large number of critical components, that have either been physically observed/measured, or numerically modelled over a representative timelapse. Advocated Gaidai hypersurface structural risks evaluation methodology is applicable for a wide range of engineering and industrial systems. This study demonstrates that given in situ environmental conditions, it is well possible to assess accurately the risks of system failures, damages or natural hazards, caused by excessive structural dynamics. Multivariate dynamic systems, possessing nonlinear cross-correlations between critical system’s components often present design challenges, when utilizing classic structural risks evaluation approaches, as those are mostly only univariate or bivariate. Dynamic offshore Jacket hot spot stresses have been utilized as an example in this reliability investigation. Modelling system's excessive dynamics being often challenging because of the non-stationarity of the system and the intricacy of fluid-structural dynamic interactions, arising from in situ wave-induced loads, influencing Jacket’s structural dynamics. Nonlinearities greatly affect structural dynamics, e.g., 2nd, 3rd, and higher order effects within fluid-structural interactions. The methodology presented in this work offers a straightforward, effective, yet precise means of assessing the risks of failure or hazard for multivariate, non-stationary, non-linear dynamic offshore systems. For bivariate cases, a verification note has been added
Alternating current field measurement (ACFM) has been proven to be a promising technique for sizing cracks on underwater structures. Precision and visualization are two big challenges for measuring underwater structure cracks, which is constrained by signal attenuation, interference factor and lags of signal processing. In this paper, an optimum visual ACFM (VACFM) system based on time-constant method (TCM) is proposed for visualization and accurate measurement of underwater structure cracks. Based on optimized results by finite element model (FEM), a novel probe and signal processing system are developed and sealed. Results obtained from crack inspection experiment with TCM show that the VACFM system is able to size the lengths and visually display the profiles of underwater structure cracks. The measuring errors of the crack length increase as the scanning speed increases; it is less than 10% at scanning speed of 3.81 mm/s, and smaller to longer cracks.
The fatigue-induced sequential failure of a structure having structural redundancy requires system-level analysis to
account for stress redistribution. System reliability-based design optimization (SRBDO) for preventing fatigue-initiated
structural failure is numerically costly owing to the inclusion of probabilistic constraints. This study incorporates the Branch�and-Bound method employing system reliability Bounds (termed the B3
method), a failure-path structural system reliability
analysis approach, with a metaheuristic optimization algorithm, namely grey wolf optimization (GWO), to obtain the optimal
design of structures under fatigue-induced system failure. To further improve the efficiency of this new optimization framework,
an additional bounding rule is proposed in the context of SRBDO against fatigue using the B3
method. To demonstrate the
proposed method, it is applied to complex problems, a multilayer Daniels system and a three-dimensional tripod jacket structure.
The system failure probability of the optimal design is confirmed to be below the target threshold and verified using Monte
Carlo simulation. At earlier stages of the optimization, a smaller number of limit-state function evaluation is required, which
increases the efficiency. In addition, the proposed method can allocate limited materials throughout the structure optimally so
that the optimally-designed structure has a relatively large number of failure paths with similar failure probability.
Two new techniques for risk and reliability analysis of multidimensional nonlinear dynamic systems are presented in this study, combined into one novel Gaidai reliability method. First, a novel reliability method for assessing offshore structural risk and reliability is proposed. This method is particularly helpful for offshore engineering multidimensional dynamic structures with numerous components that are either physically measured over a meaningful time period, or numerically modelled. Second, novel deconvolution technique for extreme values prediction is described. This technique is ideally suited for use in a variety of engineering applications. This work shows that under actual climatic circumstances, it is also possible to appropriately estimate the risks of system failures and dangers brought on by severe structural responses. Managing multi-dimensional dynamic systems and their cross-correlation across many system components is not necessarily a practical advantage for traditional reliability methodologies dealing with system time series. This reliability study utilized dynamic offshore Jacket stresses as an example. Due to system non-stationarity and complexity of fluid-structural interactions resulting from wave stresses acting on Jacket support structure, modelling such events is highly difficult. Extreme motions significantly increase the contribution of nonlinearities, resulting in impacts of second, third, and higher order fluid-structural interaction. Methods put forward in this study provides a simple, efficient, and accurate way to evaluate failure/hazard risks for multi-dimensional nonlinear dynamic systems.
The design process and asset management of floating offshore wind turbines are subject to many uncertainties. Risk and reliability analysis methods can be utilized to analyze these uncertainties in a systematic manner. Thus, in this chapter, reliability methods that are applied in the offshore and marine renewable energy industries are classified and analyzed in terms of their suitability for the offshore wind industry as well as their strengths and drawbacks, and current trends and alternative approaches for overcoming the remaining limitations are elaborated on. After a general overview and categorization of the main reliability assessment techniques, a focused description and classification of qualitative and quantitative approaches for offshore wind turbine systems is provided. This analysis is underlain by an in-depth literature review that is systematically conducted for the most recent research studies from the last decade, with ‘offshore’ and ‘reliability’ as the main keywords. While the focus lies on offshore wind turbines, for which, however, the information density is still quite low, other literature and application examples from related offshore industries are taken into account as well. More than 100 articles are examined in total, with additional knowledge gleaned from conferences and industry experience. Finally, the challenges for common reliability assessment techniques when being applied to offshore wind turbines are elaborated, addressing how and to what extent the detailed methods are currently capable of dealing with these challenges, what limitations remain, and which approaches may evolve further.
Safety management is a highly crucial process implemented in offshore structures for meeting safety criteria by eliminating potential risks. Accordingly, safety indicators are utilized to monitor the systems and assess the safety performance by meeting SMARTER criteria. Partly, the historical development of the reliability and risk assessment methods are extensively discussed along with their advantages and disadvantages. Therefore, this paper focuses on the review of safety management processes and their elements including safety indicators as well as discusses the process of optimizing maintenance strategies using reliability and risk assessment approaches for offshore structures. The paper’s results showed that utilizing SMARTER criteria for monitoring and evaluating the safety performance of offshore structures produced the most adequate safety system. For instance, the safety performance evaluation framework displayed functionality for benchmarking and constant enhancement of the Malaysian offshore oil and gas platforms through conducting a comparison between 14 safety factors using a scoring system in order to fully determine the mechanisms and interactions of the factors with the safety performance. Lastly, safety performance is established as a reference for the improvement of safety practices in offshore structures where the risk indicator system can provide a scientific benchmark for risk mitigation and investment decisions.
This paper investigates from a structural reliability assessment (SRA) perspective the fatigue reliability using the S-N curve approach compared with the fracture mechanics (FM) approach for a typical welded offshore wind turbine (OWT) jacket support structure. A non-intrusive formulation was developed for an OWT jacket support structure in 50 m deep water, consisting of a sequence of steps. First, stochastic parametric 3D (three-dimensional) Finite Element Analysis (FEA) simulations are performed, taking into account stochastic variables such as wind loads, wave loads and soil properties using facilities within the software package ANSYS. Secondly, the FEA results are post-processed using an Artificial Neural Network (ANN) response surface modelling technique deriving the performance functions expressed in terms of stochastic variables. Finally, the First Order Reliability Method (FORM) is applied in calculating the reliability index values of components. The developed framework was applied to elucidate the fatigue damage process, including the small to long crack transition among other stages, for structural steels used for OWT jacket applications. The FM formulation investigated includes a crack growth formulation based on the bilinear crack growth law, considering both segments of the crack growth law as non-correlated and correlated in calculating the reliability index (RI). Sensitivity analysis results showed a strong dependence of the structure's reliability levels on the uncertainties of the crack growth law constants measured in terms of coefficient of variation (COV). Also investigated, was the reliability of the structure reassessed and updated in the presence of assumed structural health monitoring/ condition monitoring (SHM/CM) data. The results from the case study revealed that fracture reliability is highly sensitive to the initial crack size. It is recommended to apply the S-N curve method at the design stage while the FM approach applied towards the end of the design life as the structure approaches failure.
Global wind energy has developed rapidly in recent years, and the offshore wind turbines (OWTs) have been applied to much more applications. Because the support structure of an OWT is prone to damage, it is crucial to strengthen the structural reliability to prevent unexpected failures and reduce operation costs by performing fault diagnosis, remaining useful life prediction, and condition-based maintenance. Through real-time monitoring on the operating status of the OWT support structures, the operation measurements can be obtained to detect initial structural cracks and predict the crack propagation to assess potential risks and perform just-in-time maintenance; as a result, the turbine downtime and maintenance costs can be significantly reduced and the wind turbine service life can be prolonged. This paper introduces the latest progress on the reliability analysis of the OWT support structures. First of all, the types of the support structures are reviewed; then, the reliability research progress on the support structure is presented and the existing challenges are discussed. In light of the digital twin (DT) technology, a solution to existing challenging issues is proposed by introducing the DT into the OWT support structures. This new DT framework will enable real-time monitoring, fault diagnosis and operation optimization of the OWT support structures, which may provide a useful application prospect in the reliability analysis of the OWT support structures in the future.
This study investigates strategies for solving the system reliability of large three-dimensional jacket structures. These structural systems normally fail as a result of a series of different components failures. The failure characteristics are investigated under various environmental conditions and direction combinations. The β-unzipping technique is adopted to determine critical failure components, and the entire system is simplified as a series-parallel system to approximately evaluate the structural system reliability. However, this approach needs excessive computational effort for searching failure components and failure paths. Based on a trained artificial neural network (ANN), which can be used to approximate the implicit limit-state function of a complicated structure, a new alternative procedure is proposed to improve the efficiency of the system reliability analysis method. The failure probability is calculated through Monte Carlo simulation (MCS) with Latin hypercube sampling (LHS). The features and applicability of the above procedure are discussed and compared using an example jacket platform located in Chengdao Oilfield, Bohai Sea, China. This study provides a reference for the evaluation of the system reliability of jacket structures.
Hydrocarbon leaks have a major accident potential and it could give significant damages to human, property and environment.To prevent these risks from the leak in design aspects, installation of ESD system is representative. Because the ESD system should be operated properly at any time, It needs high reliability and much cost. To make ESD system with high reliability and reasonable cost, it is a need to find specific design method.In this study, we proposed the multi-objective design optimization method and performed the optimization of the ESD system for 1st separation system to satisfy high reliability and cost-effective.‘NSGA-II (Non-dominated Sorting Genetic Algorithm-II)’ was applied and two objective functions of ‘Reliability’ and ‘Cost’ of system were defined. Six design variables were set to related variables for system configuration. To verify the result of the optimization, the results of existing design and optimum design were compared in aspects of reliability and cost. With the optimization method proposed from this study, it was possible to derive the reliable and economical design of the ESD system.
http://ltu.diva-portal.org/smash/get/diva2:1021777/FULLTEXT02.pdf
The subject of this thesis is evaluation of possible life extension of existing offshore jacket structures. This thesis is a contribution to the further development of the theoretical background for the procedures and standards in life extension of offshore installations. The relevant standards for life extension of existing offshore jacket structures are reviewed, with focus on ultimate limit state analysis and fatigue analysis. In general, it can be said that the existing standards and procedures recommend assessment of existing structures for life extension based on: 1) Linear analysis and component checks, 2) Non-linear system strength analysis and component checks, and 3) Structural reliability analysis for the ultimate limit state check and: 1) SN fatigue analysis, 2) Fracture mechanics crack growth analysis, and 3) Structural reliability analysis for the fatigue limit state.
This thesis proposes that a risk evaluation of an ageing structure is needed as a part of the assessment. Such a risk evaluation should include identification of hazards and failure
modes, and possibly include an identification of the preventive measures (barriers) for
reducing the likelihood of these hazards and failure modes. A review of hazards and failure
modes for an ageing offshore jacket structure are presented, and preventive actions to limit the hazards have been investigated using a barrier analysis approach. It is concluded that a review of hazards and failure modes should be included as a part of an assessment for life extension, as installation specific hazards and failure modes may be present.
Further work on system strength parameters for an offshore jacket structure is also included.
It is chosen to represent the system strength with the reserve strength ratio indicator, and a reasonable criterion for this parameter is established. Similarly, it is chosen to represent the damage strength with the damaged strength ratio and a recommendation with respect to this parameter is also given. The robustness towards wave-in-deck loading is represented by introducing a reserve freeboard ratio. A criterion for this reserve freeboard ratio has also been suggested. The combined hazard of wave-in-deck loading and the system strength is found to be very important and combined criteria is found necessary.
The use of probabilistic methods in assessment for life extension is also evaluated. In this
thesis it is focused on a predictive Bayesian approach for probabilistic assessment. Within this approach the assigned probabilities are a measure of the uncertainty about the structure, and not an element of the structure itself. Decision methods based on the probabilistic analysis should take into account this understanding of the probability. A coherent methodology for decision making based on the predictive Bayesian approach is found, and presented in this thesis. The focus is by this shifted towards minimising the risks with the model used for the probabilistic analysis, instead of meeting a criterion for failure probability.
Finally, the degradation of an offshore jacket structure has been simulated through the life of the structure. Development in the failure probability has been studied with respect to
different inspection and repair scenarios. Effects like subsidence and load redistribution after a component has failed are accounted for. From a purely structural point of view, life
extension seems to be possible for an offshore jacket structure providing that sufficient
inspection and repair is performed, the structure has sufficient strength and the freeboard is sufficient. Possible hazards like corrosion and pile related failures have not been included in this study, and the conclusion is based on these limitations.
Marine Structural Design, Second Edition, is a wide-ranging, practical guide to marine structural analysis and design, describing in detail the application of modern structural engineering principles to marine and offshore structures. Organized in five parts, the book covers basic structural design principles, strength, fatigue and fracture, and reliability and risk assessment, providing all the knowledge needed for limit-state design and re-assessment of existing structures. Updates to this edition include new chapters on structural health monitoring and risk-based decision-making, arctic marine structural development, and the addition of new LNG ship topics, including composite materials and structures, uncertainty analysis, and green ship concepts. Provides the structural design principles, background theory, and know-how needed for marine and offshore structural design by analysis. Covers strength, fatigue and fracture, reliability, and risk assessment together in one resource, emphasizing practical considerations and applications. Updates to this edition include new chapters on structural health monitoring and risk-based decision making, and new content on arctic marine structural design.
This paper describes the application of probabilistic techniques to a detailed fatigue analysis of offshore structures. The structural properties are characterized by stress transfer functions in the properties are characterized by stress transfer functions in the frequency domain. Long-term statistics follow by accumulation and fatigue damage is computed using Miner's law. Spectral analysis is used to evaluate short-term stress statistics for each sea state in the wave climate.
Introduction
General Description of Fatigue in Offshore Structures
Failure caused by fatigue in welded structures is related to the variable loads experienced by the structure during its life. Fatigue under random loading conditions, as experienced offshore, is a complex subject. The present state of the art for welded steel structures generally accepts the following as trueWelded connections are most susceptible to the development of fatigue cracks.Small, sharp defects inevitably exist in welds and can cause cracks.Fatigue in offshore structures is predominantly a matter of crack propagation as a result of these defectsIn as-welded connections, stress of yield-strength magnitude in tension exist because of residual stresses.Therefore, stress fluctuations range from tension yield downward and this range of the fluctuation alone is the governing parameter.Consequently, fully tensile stress cycles and partially or wholly compressive stress cycles are equally partially or wholly compressive stress cycles are equally damaging in welded structures.The fatigue strength of welded connections is independent of the yield strength of presently used structural steels.Crack propagation and consequent fatigue damage in an offshore environment will continue at some rate, no matter how small the stress range - that is, no endurance limit is found such as in constant-amplitude, cyclic loading above 2 x 10(6) cycles.Shear stresses may be neglected in fatigue life calculations.
Stress fluctuations normally result predominantly from wave loads. These wave-induced stresses are of variable magnitude and occur in random order. The true time history of the local stresses almost invariably is simplified because it can be described adequately in statistical terms by a reasonable number of stress blocks. Each stress block is composed of a number of cycles of constant stress. Thus, the sequence of variations in the true stress history is lost. The cumulative effect of all stress blocks representing the stress history is estimated by Miner's rule of linear accumulation of damage:
nb ni Fd = -----, ni i=1
where Fd = damage ratio
and
nb = number of stress blocks considered
A fatigue failure is assumed to occur when the damage ratio, Fd, reaches unity. In fatigue analysis, this criterion is considered the definition of a local fatigue failure. This does not necessarily imply an actual failure nor a partial or complete collapse of the structure.
JPT
P. 657
This paper provides insights and interpretations regarding the use of API RP2A-WSD 22nd Edition. In 2010, OTC paper #20837 discussed modifications beingmade to and reasons behind the changes to API 2A-WSD in conjunction with othernew API standards, 2MET, 2EQ, 2SIM and 2GEO and the elimination of APIBulletins 2INT-MET, 2INT-DG and 2INT-EX. In this paper, emphasis is placed onuse of the 22nd Edition in combination with the other new standards.
General
API RP 2A-WSD 22nd Edition was balloted in 2011 and approved by 96% of thevotes cast. There were over 339 individual comments (the vast majorityeditorial) with some 21 still in final resolution by API TG13 at the time thispaper was written. This has resulted in the API RP 2A-WSD 22nd Edition finaltable of contents to include the following (with reference to the 21st EditionSection in parentheses):Section 1: ScopeSection 2: Normative ReferencesSection 3: Terms, Definitions and AcronymsSection 4: Planning (21st Edition Section 1)Section 5: Design Criteria and Procedures (21st Edition Section 2)Section 6: Structural Steel Design (21st Edition Section 3)Section 7: Strength of Tubular Joints (21st Edition Section 4)Section 8: Fatigue (21st Edition Section 5)Section 9: Foundation Design (21st Edition Section 6)Section 10: Other Structural Components and Systems (21st Edition Section7)Section 11: Material (21st Edition Section 8)Section 12: Drawings and Specifications (21st Edition Section 9)Section 13: Welding (21st Edition Section 10)Section 14: Fabrication (21st Edition Section 11)Section 15: Installation (21st Edition Section 12)Section 16: Inspection (21st Edition Section 13)Section 17: Accidental Loading (21st Edition Section 18)Section 18: Reuse (21st Edition Section 15)Section 19: Minimum and Special Structures (21st Edition Section 16)Annex A (Informative): API 2A-WSD 21st Edition vs. 22nd EditionCross-referenceAnnex B (Informative): CommentaryBibliography
Changes from OTC paper #20837 and specific topics of importance in using thenew API Standards are discussed below.
A method is developed for calculating the probability of progressive collapse due to fatigue for structural systems in which members are connected in parallel and for parallel member systems connected in series. Formulation of the method is “exact” and is based on the assumptions of equal member load sharing and identically distributed fatigue lives. Numerical integration is used to evaluate the probabilities that also include the' effect of statistical correlation between the fatigue lives of different elements. An expression to quantify the degree of correlation is developed. The method is applied to systems of up to eight parallel members, with each member consisting of an arbitrary number of joints of equal fatigue life connected in series. The resulting probabilities are compared to a previously published progressive collapse study, which used a Monte Carlo simulation technique. Insight is provided concerning the importance of correlation effects in systems fatigue analysis. The method can be useful in fatigue design criteria development, since it relates the reliability of an individual element to the reliability of the complete system comprised of the elements. Because the method is “exact,” it can be used as a good test case for future work in the development of system reliability methods for more general applications.
The paper describes a methodology for estimation of reliability of fixed offshore structures with respect to fatigue and extreme stress. The failure criteria for fatigue are formulated using fracture mechanics principle and that for extreme stress using API Code. Advanced Second Moment method is used to find the reliability index. The total life of the structure is divided into a set of stationary sea-states, occurring during storms and described by directional power spectrum. The method has been illustrated through application to a typical jacket platform. Usefulness of the methodology for planning in-service inspections has been highlighted.
The performance of friction dampers to mitigate the wave-induced vibrations in jacket-type offshore platforms has been investigated in this study. Due to the random nature of ocean waves, a full stochastic analysis method has been used to evaluate the response of the structures equipped with these devices. A stochastic linearization technique has also been used to take the nonlinear behavior of these hysteretic dampers into account. At last, the developed mathematical formulation has been applied to evaluate the response of realistic models, and to find out the optimal values for the adjustable parameters of the friction dampers to dissipate the wave induced vibrations of the platforms.
An exact general closed-form approach to estimation of fatigue reliability of parallel systems, which represent a set of failure paths of structural systems, is presented. The parallel system considered here is completely general and comprises components that are of different strength and share unequal loads. The system failure is regarded as the sequential failure of its members. A general expression for the notional probability of fatigue failure of this system is developed using Wirsching's model of fatigue failure of the system nodes and by relating the compressed times to failure of nonfailed nodes following failure of some of the nodes in the system to their nominal times to failure corresponding to the state when all system members are intact. The general expression is first validated by comparison to a known parallel system in which system members are of equal strength and share equal loads, and then employed to estimate the failure probability of another system in which the system components are of unequal strength.
General relationships between individual joint reliability and overall system reliability in a redundant structure with no repair program are presented. Using a fatigue reliability model, a Monte Carlo analysis provided a distribution of time to failure for various degrees of redundancy. Random variables in the model account for uncertainty in the process of determining fatigue strengths of the joints. The distribution parameters for these random variables are representative of welded tubular joints in offshore platforms. Figures are shown which relate the target safety index of a joint to the reliability against various definitions of “functional failure” of the structure. Also included is an example of how the probability of a second failure occurring within a specified time interval after an initial failure can be predicted. This can provide insight in determining the necessary frequency of inspections.
The contents of this book are: Overview of Offshore Steel Structures; Loads on Ocean Structures; Fracture Mechanics As a Tool in Fatigue Analysis; Basic Fatigue Properties of Welded Joints; Significance of Defects; Improving the Fatigue Strength of Welded Joints; Effects of Marine Environment and Cathodic Protection on Fatigue of Structural Steels Fatigue of Tubular Joints; Unstable Fracture; Fatigue Life Calculations; and Fatigue in Building Codes Background and Applications.
Tension leg platform (TLP), a deep water oil exploration offshore compliant system, is vulnerable to considerable fatigue damage over its design life period due to the dynamic excitations caused by the oscillating waves and wind. In the presence of random loading like, that produced by the wind and waves, reliability against fatigue and fracture failure becomes important. The reliability assessment against fatigue failure considers the uncertainties associated with the parameters and procedures employed for the fatigue damage estimation. This fatigue estimate requires a dynamic response analysis under various environmental loadings. In the present study a non-linear dynamic analysis of the platform has been carried out for response calculations. The response histories so obtained have been employed for the study of fatigue reliability analysis of TLP tethers under long crested random sea and associated wind. Fatigue damage estimation of tether joints is made using Palmgren–Miner's rule (S–N curve approach) and fracture mechanics approaches. The stress ranges employed are described by Rayleigh distribution. Non-linear limit state functions based on above two approaches are derived in terms of random variables. First order reliability method (FORM) and Monte Carlo simulation technique have been employed for reliability estimation. The influence of various random variables on overall probability of failure has been studied through sensitivity analysis.
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