基于小波变换的海洋平台损伤检测研究

当结构物产生损伤时,结构的动力特性会发生变化,从而引起结构振动响应产生相应的变化。小波变换是奇异性信号的良好检测工具,可以用于识别结构损伤的发生。建立了某海洋平台有限元模型,得到了随机海浪作用下不同损伤工况的结构振动响应。基于小波变换损伤检测,针对不同类型杆件损伤、不同位置杆件损伤、不同程度的噪声污染以及不同方向振动响应信号情况下,分别研究了海洋平台结构的损伤检测问题,得到了一些有用的结论。     

海洋平台结构损伤识别与定位新方法及试验验证

基于小波变换的时频局域化特性和BP神经网络的非线性映射特性,结合两者优点提出了基于小波包分析和神经网络方法的海洋平台三步法损伤定位方法。对海洋平台结构加速度响应信号进行小波包分析,提取小波包特征向量,将小波包结点能量变化量指标作为BP网络的输入向量,逐步确定损伤位置。设计一典型导管架式海洋平台试验模型,分别进行岸上脉冲激励及水池中波浪激励下平台结构损伤识别与定位模型试验,对该方法的可行性和适用性进行了验证。     

大型船舶碰撞引起的海洋导管架平台结构损伤分析

主要对某一海洋导管架平台结构受到大吨位起重铺管船的碰撞进行了详细的非线性数值动力模拟。采用非线性弹簧来模拟受损构件的凹陷特性，计算分析了船舶侧向撞击导管架平台结构在不同的碰撞接触时间下的构件损伤情况，与实际结构损伤的检测结果相比较，反演计算分析了船舶以不同的速度与平台发生碰撞，得到了不同的碰撞接触时间和最大撞击力，以及导管架平台结构主要节点的应力、位移时间历程曲线，可以确定受到撞击后导管架平台结构构件和管节点的损伤程度，对提出合理可行的修理加固方案提供了理论依据。     

随时间变化的海洋平台结构系统可靠性分析

海洋平台在服役期间由于受到各种损伤,其系统可靠性将随时间而变化。本文分析了几种典型损伤——疲劳损伤、腐蚀损伤对海洋平台结构系统可靠性的影响,在考虑其对结构构件强度和承受载荷的影响情况下分析随时间而变化的海洋平台结构的系统可靠性。首先得到考虑时间因素的结构构件的强度和载荷函数,采用分枝界限法寻找结构的主要失效路径,最后以Ｄｉｔｌｅｖｓｅｎ界限法求取结构的系统失效概率。算例说明,在考虑时间因素的条件下,结构的系统可靠性将会迅速下降。按以上思路编制了空间刚架在构件强度随时间变化条件下的通用可靠性分析程序,算例表明程序是合理有效的。     

隔水管卡簧式快速接头强度分析

隔水管是井口平台的重要组成构件。许多早期建设的井口平台隔水管采用卡簧式快速接头进行机械连接,卡簧作为主要连接构件,其完好程度对结构的强度有重要影响。本文针对卡簧式快速接头,基于ANSYS建立了快速接头有限元实体模型,找到了合理模拟卡簧与公母接头接触、施加端部作用力及模拟损伤破坏等问题的方法,进行了快速接头的强度分析;并在此基础上分析了卡簧完好、损伤25%、损伤50%、完全损坏情况下的接头应力变化,比较了卡簧损伤位置对接头强度的影响。计算结果表明,卡簧损伤50%时卡簧对接头几乎没有约束作用;卡簧的约束作用主要是由受拉一侧卡簧提供。     

结构损伤初步定位的简便方法

提出将基于动态特性的检测方法与局部物理检测手段相结合的探伤思路。利用动力学方法进行实时监测及损伤区域的粗略定位,再由物理探测方法实现损伤程度和位置的精确判断,从而降低对动态检测方法的精度要求。此外,文中尝试直接由结构的动力响应信号构建能量指标识别结构损伤的方法,不需要进行模态参数识别,算法简单,有望应用于海洋平台、高层建筑等可简化为串联多自由度体系的结构的实时监测和损伤初步定位。     

考虑腐蚀和冲刷的海洋平台损伤识别

以某导管架平台为模型,对分别存在腐蚀和桩基冲刷影响的结构进行模态分析,表明在腐蚀和桩基冲刷的影响下海洋平台的模态参数有明显的时变效应。基于频率变化和神经网络算法的损伤识别方法有较好的记忆能力和泛化能力,可准确识别结构损伤程度。忽略腐蚀和桩基冲刷的影响,基于神经网络算法的损伤识别结果产生较大误差,难以对损伤程度实现有效识别。考虑腐蚀和桩基冲刷影响,提出基于神经网络算法的海洋平台损伤识别方法的改进方案。数值研究表明,改进方法识别误差很小,可对损伤程度实现有效识别。     

复杂载荷作用下立管触地段损伤管道的动力响应

触地段(Touchdown zone, TDZ)是在役钢悬链线立管(Steel catenary riser, SCR)的关键部位,在复杂载荷作用下,极易形成损伤缺陷,其载荷寿命的评估是深海结构工程中的一个关键问题。本文以大型有限元软件ABAQUS为平台,运用损伤管道实体单元与土弹簧阻尼单元相互作用的模型模拟触地段损伤海底管道在复杂载荷作用下的动力响应,数值计算考虑了管-土相互作用过程中的材料非线性、几何非线性以及接触非线性。讨论了单一环向体积损伤位于触地段管道的不同位置时,触地段损伤管道在不同载荷作用下的动力特性及特征点的动力响应。结果表明,管道所受内外压力以及管道提升端的竖向位移载荷会影响结构的自振频率;体积损伤部位的动力响应较完好部位更剧烈;体积损伤的位置和动力载荷频率对管道动力放大系数的影响很大;当动力载荷的激励频率越接近结构基频时,损伤管道的动力响应及动力放大系数越大。     

基于模型修正的海洋平台结构损伤识别试验研究

损伤识别是海洋平台结构健康监测的核心问题,对于保障结构的安全服役具有重要的意义。文中将交叉模型交叉模态(CMCM)有限元模型修正方法应用到了导管架式海洋平台结构的损伤识别中。针对实测模态含噪声条件下该方法遇到的"病态"求解难题,引入了截断奇异值分解正则化方法,提出了基于正则化模型修正的损伤识别过程。为验证该过程的有效性,制做了某导管架平台的钢质缩尺简化模型,并在两个典型构件处设置了损伤;利用冲击激励对结构进行了振动测试;根据模态识别结果,采用正则化模型修正过程对各损伤工况进行了损伤识别。结果表明,利用有限的低阶含噪声模态,借助正则化求解技术,该模型修正方法可有效地识别导管架平台结构的损伤。     

具有裂纹损伤桩腿的海洋石油平台有限元分析

采用有限元分析方法和断裂力学方法,将含裂纹构件的裂纹以线弹簧处理,建立了带有裂纹损伤桩腿的等效梁和管单元的单元刚度矩阵的计算方法。对检验方法的正确性,使用该方法对几种损伤构件进行了强度计算,并与有限元细化网格计算结果和实验结果进行比较,最后将等效单元计算程序接入大型有限元程序ＳｕｐｅｒＳＡＰ,对受损后的平台整体结构进行了应力计算和强度分析。     

A Study on Crack Detection with Modal Parameters of A Jacket Platform

Crack detection procedures by different modal parameters are analyzed for identitying a crack and its location and magnitude in a jacket plafform. The first ten natural frequencies and modal shapes of the jacket models are obtained by numerical experiments based on NASTRAN Code. A crack at different locations and of different magnitudes is imposed in the model at the underwater beams. Then, the modal evaluation parameters are calculated numerically, to illustrate the evaluation of modal parameter criteria used in jacket crack detection. The sensitivities of different modal parameters to different cracks are analyzed. A new technique is presented for predicting the approximate location of a breakage in the absence of the data of an intact model. This method can be used to detect a crack in underwater menbers by use of incomplete mode shapes of the top members of the jacket.     

Damage Localization of Offshore Platforms Under Ambient Excitation

In this paper Nondestructive Damage Detection (NDD) for offshore platforms is investigated under operational conditions. As is known, there is no easy way to measure ambient excitation, so damage detection methods based on ambient excitation have become very vital for the Structural Health Monitoring (SHM) of offshore platforms. The modal parameters (natural frequencies, damping ratios and mode shapes) are identified from structural response data with the Natural Excitation Technique (NExT) in conjunction with the Eigensystem Realization Algorithm (ERA) . A new method of damage detection is presented, which utilizes the invariance property of element modal strain energy. This method is to assign element modal strain energy to two parts, and defines two damage detection indicators. One is compression modal strain energy change ratio (CMSECR); the other is flexural modal strain energy change ratio (FMSECR). The present modal strain energy is obtained by incomplete modal shape and structural stiffness matr     

Damage detection in offshore platform structures from limited modal data

Development of efficient global damage detection techniques for offshore structures is of great importance. The present paper applies the iterative modal strain energy (IMSE) method to locate and quantify the damage for three dimensional frame structures, when limited, spatially incomplete modal data are available. One theoretical development is a new procedure to directly incorporate the spatially-incomplete mode shapes in the algorithm, where slave degrees of freedom iterative updating procedure (SDUP) is developed for evaluating the damage indicator and damage severity. The merits of this new method are that both the modal frequencies and spatially incomplete mode shapes can be used. Also, the modal frequencies do not need to pair the mode shapes one by one. To demonstrate the feasibility and effectiveness of the developed algorithm, numerical studies are conducted for a 3D offshore platform based on data generated from finite element models. Numerical results demonstrate that the present method is effective for the damage assessment of a 3D frame structures when limited, spatially incomplete modal data are available. After considering more practical factors, it is expected to be more applicable in structural damage assessment.     

Modal Parameters Identification of A Real Offshore Platform From the Response Excited by Natural Ice Loading

This paper investigates the possibility of utilizing response from natural ice loading for modal parameter identification of real offshore platforms. The test platform is the JZ20-2 MUQ jacket platform located in the Liaodong Bay, China. A field experiment is carried out in winter season, as the platform is excited by floating ices. The feasibility is demonstrated by the acceleration response of two different segments. By the SSI-data method, the modal frequencies and damping ratios of four structural modes can be successfully identified from both segments.The estimated information from both segments is almost identical, which demonstrates that the modal identification is trustworthy. Furthermore, by taking the Jacket platform as a benchmark, the numerical performance of five popular time-domain EMA methods is systematically compared from different viewpoints. The comparisons are categorized as:(1) stochastic methods versus deterministic methods;(2) high-order methods versus low-order methods;(3) data-driven versus covariance-driven stochastic subspace identification methods.     

A New Probability of Detection Model for Updating Crack Distribution of Offshore Structures

1 .IntroductionNondestructiveinspection (NDI)isveryimportantforensuringthereliabilityofoffshorestructuresintheirservicelives (Lauraetal.,1 996 ) .Itiswellknownthatdetectionofflawsinvolvesconsider ablestatisticaluncertainties.Asaresult,theprobabilityofdetection (POD)forallflawsofagivensizehasbeenusedintheliteraturetodefinethecapabilityofaparticularNDItechniqueinagivenen vironment.SincethedataofPODusuallyscatterlargely ,itisdifficulttodeterminewhichmodelfitstheavailabledatabest.Thismodelun…     

Using incomplete modal data for damage detection in offshore jacket structures

The development of robust techniques for early damage detection for offshore structures is crucial to avoid the possible catastrophe caused by structural failures. This article applies the cross-model cross-mode (CMCM) method for damage detection that is capable of identifying the damage to individual members of offshore jacket platforms, when limited, spatially incomplete modal data is available. Basically, the CMCM method is classified as a direct, physical property adjustment model updating method. Implementing this method requires only a few modes measured from the damaged structure. In dealing with spatial incompleteness, this paper investigates both model reduction and modal expansion techniques. Specifically, either Guyan (static condensation) or SEREP (system equivalent reduction expansion process) transformation matrix, between the master and slave degrees-of-freedom, is employed in the model reduction or modal expansion process. One theoretical development is an iterative procedure to compute the transformation matrix associated with the (unknown) damaged structure. Numerical studies are conducted for a jacket platform with multiple damaged members based on synthetic data generated from finite-element models. The results suggest that (i) Guyan scheme always outperforms SEREP, (ii) model reduction is always better than modal expansion, and (iii) the CMCM method in conjunction with iterative Guyan reduction approach yields the best damage location and severity estimate.     

On natural frequencies and modal shapes in two-dimensional asymmetric and symmetric moonpools in finite water depth

In this paper theoretical models are proposed for computing the natural frequencies and modal shapes of two-dimensional asymmetric and symmetric moonpools in the finite water depth. The boundary value problem is solved by using a domain decomposition approach. On the outer vertical boundary bounded by the beam of the two bodies, linearized velocity potential is assumed to be nil. Eigenvalue problem is formulated by matching the velocity potential and fluid flux on the common boundaries to obtain the natural frequencies and modal shapes of the free surface elevation. In the symmetric moonpool cases, so-called single mode approximations (SMA) have been derived and can be adopted for rapid estimation of the natural frequencies for both piston and sloshing modes. The present results have been extensively compared with the solutions using the two-dimensional infinite water depth model developed by Molin [1], the numerical solutions and experimental data by Faltinsen et al. [2]. It is found that the solutions have been improved from the infinite water depth model. It is demonstrated that the proposed models can well predict the resonance frequencies and modal shapes for the two-dimensional asymmetric and symmetric moonpools.     

浅海导管架平台结构损伤诊断试验研究

基于模型试验研究了浅海导管架平台的结构损伤诊断,采用标量型ARMA模型识别出结构的三阶模态参数,分别采用遗传算法和模态应变能法对试验模型不同模拟损伤工况进行了损伤诊断。基于遗传算法,提出了采用不同自由度一阶模态参数的损伤诊断方法。研究表明,由于海洋平台的前三阶模态分别为不同自由度的一阶模态,采用两阶或三阶模态识别损伤时,损伤杆件影响较小的自由度方向的模态参数对结构的损伤诊断将产生不利的影响,成为识别噪声。因此,可分别采用不同自由度的一阶模态进行结构损伤诊断。     

Comparative study of broadband damage localization methods applied to test data

This application-oriented paper presents comparison of various broadband frequency based structural damage localization indices using experimental data from a full-scale structure known as the I-40 Bridge. First, three ‘damage-sensitive’ response parameters determined in the time and/or spectral domain, modal domain and wavelet domain are presented for damage localization in the context of a ‘non-model-based’ damage identification approach. Secondly, experimental modal data (namely natural frequencies, mode shapes and modal damping) obtained from this full-scale bridge subjected to various damage conditions is employed for assessment of the robustness of these methods on ‘real-world’ applications. Consequently, the results obtained are compared with those obtained from existing resonance frequency based damage identification methods. The results obtained demonstrate the improved capability of the broadband methods to localize damage in a full-scale structure despite sparse modal information and limited measurement grid points.     

圆板结构周向表面裂纹识别的振动功率流方法

从结构噪声的观点,利用振动功率流方法对圆板在集中力作用下的周向表面裂纹进行了诊断研究。周向表面裂纹模拟为转动弹簧,利用断裂力学的有关理论得到其转动刚度。在高频情况下,研究了圆板在中心受集中载荷作用下的弯曲波运动以及输入的振动功率流,分析了振动功率流与破损位置及其特征尺寸的关系,从而可进行有效的破损诊断。