The efficient maintenance of offshore structural integrity using reliability analysis

Efficient maintenance of offshore structures is one of the priority development areas of the offshore industry world-wide. The maintenance of the installations in a hostile environment such as the North Sea poses a particular problem. Owing to the large number of wave-induced stress cycles experienced by the structures, the integrity is subject to progressive degradation under fatigue crack growth. If cracks are discovered in service, the operators of the structures will need to decide on the appropriate course of action. The priorities for actions will also need to be determined for cracks of various sizes found in different locations of the structures. In the past, the above decisions have depended very much on engineering judgement. However, the latest developments in reliability fracture mechanics (RFM) analysis have for the first time provided an objective criterion for this type of decision making. Moreover, this criterion is consistent with the established structural design criteria based on reliability analysis of strength—load interaction. Therefore, it has become a real possibility to incorporate structural integrity considerations by design. The reliability-fracture-mechanics-based fatigue analysis includes studies on the uncertainties of load history, materials crack growth behaviour, fracture mechanics modelling, non-destructive inspection and other environmental factors. This paper will review the development in this methodology. In order to integrate the above analysis into the overall design process, it is useful to carry out reliability analysis for the complete structural system. The current state of the art of structural system reliability analysis is therefore briefly reviewed. The necessary development in order to accommodate fatigue reliability degradation is then discussed.     

General principles of the use of safety factors in design and assessment

Any structure or component can be made to fail if it is subjected to loadings in excess of its strength. Structural integrity is achieved by ensuring that there is an adequate safety margin or reserve factor between strength and loading effects. The basic principles of ‘allowable stress’ and ‘limit state’ design methods to avoid failure in structural and pressure vessel components are summarised. The use of risk as a means of defining adequate safety is introduced where risk is defined as the product of probability of failure multiplied by consequences of failure. The concept of acceptable ‘target’ levels of risk is discussed. The use of structural reliability theory to determine estimates of probability of failure and the use of the reliability index β are described. The need to consider the effects of uncertainties in loading information, calculation of stresses, input data and material properties is emphasised. The way in which the effect of different levels of uncertainty can be dealt with by use of partial safety factors in limit state design is explained. The need to consider all potential modes of failure, including the unexpected, is emphasised and an outline given of safety factor treatments for crack tip dependent and time dependent modes. The relationship between safety factors appropriate for the design stage and for assessment of structural integrity at a later stage is considered. The effects of redundancy and system behaviour on appropriate levels of safety factors are discussed.     

Fatigue reliability assessment of offshore structures

Substantial laboratory and field experience has indicated that, owing to the large number of wave stress cyles experienced by offshore steel structures, fatigue cracking should be the main consideration of structural reliability assessment. This paper presents the latest implementation of probabilistic fracture mechanics modelling for fatigue reliability analysis of the most common offshore structural component, the welded tubular joints. Coupled with the recent findings in inspection reliability, effective maintenance and integrity monitoring policies can be formulated. Examples of many practical situations have been analysed to illustrate the applications of the methodology.     

Leak-before-break analysis of a pipe containing circumferential defects

Assessment of aged offshore structures based on current and historical data is a critical issue for the life extension of offshore structures. The leak-before-break (LBB) concept was first developed and applied in the nuclear power industry to ensure the safety of the pressurized components in nuclear power plants. This concept can also be applied as a robust and cost effective tool for structural integrity management of offshore structures that are either partly submerged (jackets, semi-submersible, ships), or contain fluids (pipes, pressure vessels). The core idea of LBB consists of guaranteeing that enough time is available between the moment a crack breaks through the hull or wall, causing a detectable leakage, and the moment when the crack becomes unstable, causing a structural failure. This paper presents a case study of applying the LBB concept for a tether string that is part of the mooring system of an offshore structure. The tether string is a steel pipe containing circumferential defects. The critical through-thickness crack size was calculated with ductile tearing being taken into account. It has been found that the choice of failure criterion has a significant impact on the determination of the critical through-thickness crack size, which might have influence on the verification of leak-before-break. The effect of residual stress on leak-before-break has also been studied.     

复合材料连接结构健康监测技术研究进展

连接结构是大型复合材料结构的关键环节,对保证复合材料结构的完整性具有重要作用。由于复合材料连接结构存在复杂的非线性耦合因素,使得复合材料连接结构的强度和破坏模式分析十分困难,因此,必须对复合材料连接结构的健康状态进行监测、诊断、评价和预测,通过在线监测获得的信息实时掌握结构的健康状况与对外界载荷的响应,并在此基础上对未来可能发生的缺陷和故障进行预报,以便能在合适时间段内采取措施,以保证复合材料结构的安全服役并取得最大的经济效益。以飞行器复合材料连接结构为背景,首先简要分析了复合材料胶接连接、机械连接和混合连接形式的损伤和失效模式,然后重点介绍了基于波传播法、阻抗法、智能涂层监测法、真空比较监测法、光纤传感监测法和混合集成监测法的复合材料连接结构健康监测(SHM)技术的研究进展,最后讨论了飞行器复合材料连接结构健康监测技术的发展趋势和面临的挑战。      

Remote nondestructive evaluation technique using infrared thermography for fatigue cracks in steel bridges

Long‐standing infrastructure is subject to structural deterioration. In this respect, steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair. However, the inaccessibility of such structures makes inspection time consuming and labour intensive. Therefore, there is an urgent need for developing high‐performance nondestructive evaluation (NDE) methods to assist in effective maintenance of such structures. Recently, use of infrared cameras in nondestructive testing has been attracting increasing interest, as they provide highly efficient remote and wide area measurements. This paper first reviews the current situation of nondestructive inspection techniques used for fatigue crack detection in steel bridges, and then presents remote NDE techniques using infrared thermography developed by the author for fatigue crack detection and structural integrity assessments. Furthermore, results of applying fatigue crack evaluation to a steel bridge using the newly developed NDE techniques are presented.     

Recent development in the fast corrosion fatigue analysis of offshore structures subject to random wave loading

In many areas of engineering, such as the offshore industry, welded steel joints are widely used as standard structural components. These joints are usually subject to long-term random wave loading and are therefore susceptible to fatigue damage. In many cases, the complex service loading is described by stress (or strain) power spectra, each representing a stationary sea state. These power spectra are obtained from hydrodynamic analysis or in situ monitoring. These will then be used in design calculations, feasibility studies or in-service assessment of fatigue damage on the structures.

Usually, the power spectra will have to be realized into real-time histories and then counted before fatigue analysis can be carried out. On many occasions where a large number of design options or joints need to be analysed, this process becomes very time consuming and expensive. The situation is further complicated by the calculations involving corrosion fatigue for joints in sea water.

The paper will start with a brief presentation of the fatigue analysis procedures for offshore welded joints and several existing models that were derived to bypass the laborious load history analysis mentioned above. More effort, however, will be concentrated on presenting the development of a new model. This model not only provides a more consistent and accurate prediction, but has also been adopted successfully for corrosion fatigue analysis.     

Predicting the fatigue life of offshore structures by the single-moment spectral method

The dependence of fatigue damage accumulation on power spectral density (psd) is investigated for Gaussian random processes representing stresses in offshore structures. This involves extensive computer simulation of representative stress time histories and related analyses to predict the time of fatigue failure. The recently introduced single-moment (SM) spectral method is shown to be very attractive for predicting fatigue failure. The SM method predicts the damage accumulation rate based only on a single calculation from the psd curve (a moment integral). The accuracy of the SM method and of other spectral methods is studied by comparing results with ones calculated from the much more expensive alternative of using simulated stress time histories, rainflow analysis, and Palmgren-Miner calculations.The SM method is shown to be even simpler than most spectral methods, and all spectral methods are much simpler than any method involving simulation of stress time histories. The accuracy of the SM method is shown to be quite good for a wide variety of situations. Furthermore, there are situations in which the SM method is significantly more accurate than other simple spectral methods, including the very commonly used Rayleigh approximation.It appears that the SM method may provide a valuable tool for predicting the fatigue life of offshore structures subject to complicated time histories of loading, as is required both in the design of new structures and in the reassessment of aging structures. In order to allow a proper balance between fatigue life and initial cost, there is a need for a fatigue prediction method which is both accurate and easy to apply, and the SM method appears to be adequate on both bases.     

Ensuring structural damage tolerance of russian aircraft

This paper considers most common problems related with structural integrity of civil aircraft in Russia taking into account the development of regulatory requirements, prevention of multiple site fatigue damages, improvements of crack resistance of structural materials, optimizations of aircraft type structures, development of methods for residual strength analyses of stiffened structures as well as for crack growth rates under random service loading spectra, experimental results for crack resistance degradation, methods to prevent structural failure for long operated aircraft due to corrosion.     

Fatigue strength of fillet‐welded joints at subzero temperatures

Ships and offshore structures may be operated in areas with seasonal freezing temperatures and extreme environmental conditions. While current standards state that attention should be given to the validity of fatigue design curves at subzero temperatures, studies on fatigue strength of structural steel at subzero temperatures are scarce. This study addresses the issue by analysing the fatigue strength of welded steel joints under subzero temperatures. Although critical weld details in large welded structures are mostly fillet‐welded joints, most published data are based on fatigue crack growth rate specimens cut out of butt‐welded joints. This study analyses fillet‐welded specimens at ?20°C and ?50°C against controls at room temperature. Significantly higher fatigue strength was measured in comparison to estimates based on international standards and data from design codes even at temperatures far below the allowed service temperature based on fracture toughness results.     

Effect of Stiffening Configurations on Fracture of Aluminium Structures

Abstract:  Traditionally, marine structures have been designed to resist yielding, buckling and fatigue, but not fracture. This is because existing data regarding fracture on large-scale models are limited. Consequently, adequate methods and procedures to design vessels to resist fracture have not been developed, although the shipbuilding industry is seeking to achieve advanced and more efficient concepts and designs for vessels with improved safety and performance using optimised structural design. The rapidly increasing application of lightweight materials and thin-walled structures in several industries requires fundamental understanding of mechanisms and mechanics of fracture that govern stiffened panels. Therefore, a comprehensive tool consisting of application of advanced fracture models, material calibration, and validation through component testing is provided that will increase the survivability envelope of new vessels. This paper presents the effect of stiffening configurations on fracture of aluminium structures by studying the structural response of various stiffened plates which are compared with unstiffened plates represented by small-scale compact tension (CT) specimens. It is shown that mapping of crack patterns in stiffened plates is feasible and can enable ship designers to evaluate critical areas within a structure with respect to crack initiation, propagation and optimum material usage.     

Simultaneous monitoring of acoustic emission and ultrasonic attenuation during fatigue of 7075 aluminium

Although ultrasonic attenuation monitoring has been demonstrated to be capable of providing a quantitative assessment of integrity during fatigue, much less success has been achieved in developing the corresponding capability using acoustic emission monitoring. A thorough review of the previous work employing either technique for the examination of aluminium during fatigue is presented. In addition, results from simultaneous monitoring of ultrasonic attenuation and acoustic emission during the fatigue of 7075 aluminium demonstrate the feasibility for utilizing acoustic emission as a means of continually surveying the structural integrity of a piece. It is evident from the results that, due to the passive nature of the acoustic emission monitoring, sensor location relative to failure location is not critical, thus allowing surveillance of structures with irregular geometries. However, the utilization of high frequency sound in ultrasonic attenuation monitoring makes this technique for inspecting known problem areas extremely useful in the presence of intense environmental noise. The overall findings indicate the capability of both techniques for assessing structural integrity during fatigue while providing complementary practical advantages.     

海洋平台结构振动控制综述

振动将会影响平台工作人员的身心健康，导致结构疲劳和破坏，降低平台的实用性和生存性，给生产生活带来极大威胁。对结构振动控制的研究和应用现状作了回顾，总结了国内外海洋平台振动控制的研究进展，并对今后的研究提出了建议和展望。     

Cost and safety optimization of structural design specifications

The design of buildings, bridges, offshore platforms and other civil infrastructure systems is controlled by specifications whose purpose is to provide the engineering principles and procedures required for evaluating the safety of structural systems. The calibration of these codes and specifications is a continuous process necessary to maintain a safe national and global infrastructure system while keeping abreast of new developments in engineering principles, and data on new materials, and applied loads. The common approach to specification calibration is to use probabilistic tools to deal with the random behavior of materials and to account for the uncertainties associated with determining environmental and other load effects. This paper presents a procedure to calibrate load factors for a structural design specification based on cost and safety optimization. The procedure is illustrated by determining load factors that may be applicable for incorporation in a bridge design specification. Traditional code calibration procedures require a set of pre-determined safety levels that should be used as target values that each load combination case should satisfy. The procedure in this paper deduces the failure cost implied in present designs, and provides consistent safety levels for all load combination cases. For greater accuracy, load effects showing variance in time have been modeled by separating them into two random variables; time dependent r.v. (wind speed, vehicular loads, etc.) and time independent r.v. (modeling uncertainties). The total expected lifetime cost is used in the optimization to account for both initial construction cost and future equivalent failure costs.     

The effect of trimming on strain gauge accuracy

During a programme of fatigue testing welded tubular joints, similar to those in use in North Sea offshore structures, it was necessary to measure the strain as close to the weld toe as possible and also to detect crack initiation. To this end general purpose miniature foil strain gauges were used. The backings of these gauges were trimmed prior to installation to enable placing the measuring grid as close as possible to the weld toe. The investigation described was carried out to assess the accuracy of these strain gauge installations.     

The effect of fatigue pre‐cracking forces on fracture toughness

Testing procedures for the determination of the fracture toughness of a material by monotonic loading of fatigue pre‐cracked specimens are well established in standards such as BS 7448, BS EN ISO 15653, ISO 12135, ASTM E1820 and ASTM E1921. However, a review of these standards indicates a wide range of permitted fatigue pre‐cracking forces, whilst the underlying assumption in each standard is that the pre‐cracking conditions do not affect the fracture toughness determined. In order to establish the influence of different fatigue pre‐cracking forces on the fracture toughness, tests were carried out on specimens from an API 5L X70 pipeline steel. Single‐edge notch bend specimens of Bx2B geometry were notched through thickness and tested at temperatures of +20 °C, ?80 °C and ?140 °C to show the fracture behaviour in different regions of the fracture toughness ductile‐to‐brittle transition curve. Fatigue pre‐cracking was conducted on a high‐frequency resonance fatigue test machine over a range of pre‐cracking forces permissible within the various standards and beyond. The results showed that an excessively high pre‐cracking force can result in a significant overestimation of the value of fracture toughness for material exhibiting brittle behaviour, whilst very low fatigue pre‐cracking forces appeared to result in an increase in scatter of fracture toughness. A review of standards indicated that there was a possibility to misinterpret the intention of the ISO 12135 standard and potentially use excessively high pre‐cracking forces. Suggested clarifications to this standard have therefore been proposed to avoid the risk of overestimating fracture toughness.     

Strength Analysis of Structural Elements of Hydrogen Power-Generating Equipment

The strength of materials and structural elements operating in contact with high-pressure gaseous hydrogen is studied as applied to the problems of hydrogen power-generating industry. The requirements to the design of structural elements operating in hydrogen are formulated. We propose a model of influence of high-pressure gaseous hydrogen on the strength and plasticity characteristics of structural materials and apply this model to the development of the corresponding procedures of strength analysis and evaluation of the cyclic fatigue life and durability of structural elements of hydrogen power-generating equipment taking into account the dislocation and diffusion mechanisms of hydrogen transport in the metal. The data on the properties of materials in hydrogen, the choice of the proper safety and durability margins, and the evaluation of admissible stresses in the structures are presented. The analysis of strength, cyclic fatigue life, and the number of cycles to failure are illustrated by special examples.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 89–94, November–December, 2004.     

A stop-hole method for marine and offshore structures

Stop holes have been widely used as temporary repair means in marine and offshore engineering to extend the service life of cracked structural components that cannot be immediately replaced. However, there is a lack of guidance with respect to the appropriate size of a stop hole that is usually determined based on the judgment of the attending surveyor/inspector. In the present paper, a method is proposed for determining the appropriate size of a stop hole for cracked marine and offshore structures. In addition to the traditional concerns over the size and nature of the crack, the proposed method incorporates high-cycle and low-cycle fatigue analyses into remain service life prediction taking into account both long-term and short term wave-induced loading. The effects of the return period of the potential severest sea condition, the crack length, and the environmental severity factor on the remaining service life of a stop hole are investigated.     

海冰与直立结构相互作用的离散单元数值模拟

在冰区油气开发中,海冰会导致海洋结构的强烈振动,并对结构累积疲劳损伤、上部油气管线和设备,以及操作人员健康构成很大的威胁。在海冰与直立海洋结构的相互作用中,冰荷载的幅值和频率一直是海冰工程研究的重要内容。该文针对海冰与直立海洋结构作用中的破碎过程,建立了海冰的离散单元模型。它将海冰离散为若干个规则排列且具有粘接-破碎功能的颗粒单元,并通过海冰单轴压缩试验对单元间的粘接强度进行确定;在此基础上对直立海洋平台结构的作用过程进行了数值计算,获得了不同桩径下的冰荷载和结构冰振响应,为冰区平台结构设计和现役海洋结构的疲劳寿命评估提供了参考依据。     

Fatigue issues in aircraft maintenance and repairs

Many design considerations are involved in ensuring structural integrity of Boeing jet transports, which have common design features validated by extensive analyses, tests, and service performance. Designing for continued structural integrity in the presence of damage such as fatigue or corrosion is an evolutionary process. Performance demands, increasing structural complexity, and aging fleet reassessments have required development of standards suitable for application by large teams of engineers. This presentation is focused on such methods with special emphasis on practical fatigue reliability considerations. Durability evaluations are based on quantitative structural fatigue ratings which incorporate reliability considerations for test data reduction and fleet performance predictions. Fatigue damage detection assessments are based on detection reliability estimates coupled to damage growth and residual strength evaluations. Data are presented to airline operators on detection check forms which permit efficient maintenance planning to achieve required fatigue damage detection reliability levels.