Incremental wave analysis and its application to performance-based assessment of jacket platforms

In this paper, a relatively novel approach is established to estimate different limit states and accurate behavior of jacket platforms against environmental wave loading. This novel approach which is called Incremental Wave Analysis (IWA) can be an appropriate substitute to current pushover practice. The IWA can take into account the effects of variation in wave height and wave-in-deck loading in the estimating of platforms' behavior. This paper aims to introduce the applications of this approach in deterministic assessment of offshore platforms, comprehensively. The IWA can estimate the collapse-prevention limit state of jacket platforms properly. In addition, an appropriate parameter for ultimate capacity of offshore platforms is introduced which is called Collapse Wave Height (CWH). It is illustrated that the CWH can be utilized as a substitute to RSR parameter in the design and assessment of jacket platforms. Moreover, this novel indicator is able to exceptionally estimate the collapse probability of offshore platforms utilizing a deterministic practice. This is a unique feature that can never be achieved by current pushover method utilizing RSR parameter. The IWA can be carried out using either nonlinear static or dynamic wave analyses. Therefore, in this paper, both Static and Dynamic Incremental Wave Analyses (SIWA and DIWA) will be systematically explained. Before conducting DIWA, some fundamental questions such as importance of dynamic analysis and also proper methodology for conducting dynamic analysis, will be investigated. Comparison between SIWA and DIWA will also bring about worthwhile results which will be discussed thoroughly.     

Pile-soil-structure interaction in pushover analysis of jacket offshore platforms using fiber elements

In this paper, pushover analysis of fixed jacket offshore platforms with the application of “Fiber Elements” which are capable of modeling post-buckling behavior of braces has been conducted and case study on two functional jacket offshore platforms in the Persian Gulf region has been performed. Two-dimensional models of the mentioned platforms are simulated using “DRAIN-3DX” software and the wave force is considered as the lateral load pattern on the structures.In this paper, pushover analysis of jacket offshore platforms is performed by developing numerical models that incorporate different foundation conditions for considering pile-soil-structure interaction. Increasing the displacement of the platforms and recording the relative load factors by incorporating the displacement control method is the basis for performing the analysis. In order to consider the importance of pile-soil interaction, six individual cases are examined which include pile-soil interaction analysis with actual soil in-situ characteristics. Pile head fixed below mud line elevation, pile head hinged below mud line elevation, application of linear pile stubs, application of non-linear pile-stubs & finally a combination of linear and non-linear pile-soil characteristics.In the analyses performed herein, the condition in which pile-soil interaction is considered is set to be the base case. The results from incorporating the non-linear pile stub have the most compatibility with the base case. Application of Linear pile stubs and the combination of linear and non-linear pile has much deviation with the base case. Ultimate strength of the platform in the non-linear pile stub case was very close to the base case.     

Seismic performance of pile-founded fixed jacket platforms with chevron braces

In this study, pile-founded jacket offshore structures with buckling-restrained braces (BRB) and conventional braces were designed using different methodologies such as performance-based (plastic design and displacement-based design) and strength-based (code procedure) methods, and their seismic performances for strength- and ductility-level earthquakes were compared. An analysis model of the jacket considering soil–pile structure interaction (SPSI) was developed by substituting the soil layers with equivalent nonlinear springs having pile–soil interaction characteristics. According to nonlinear static and dynamic analysis results, the application of BRB in chevron-braced jacket structures was found to enhance the seismic behaviour under ductility-level earthquakes. However, under strength-level earthquake, for which model structures mostly remained elastic, the enhancement was marginal. Among the different BRB design methods introduced, displacement-based design method produced more uniform plastic hinge distribution, while other methods provided better results in terms of maximum drift ratios. Based on the relative stiffness index and soil condition introduced in this study, it was found that for system with large ductility, simplified SPSI modelling could be used to reduce the computational cost especially for the preliminary design stage.     

基于BIM和本体的建筑抗震性能评估方法研究

FEMA P-58作为新一代建筑抗震性能评估方法,可给出精细到构件层级的地震损伤后果。然而,评估所需多源异构信息缺乏有效组织与关联,知识难以共享及重用,导致评估效率低下,自动化程度有待提高。文章在对FEMA P-58解读的基础上,提出一套基于建筑信息模型(building information modelling,简称BIM)和本体的建筑抗震性能评估方法。通过构建本体对评估所需信息进行统一组织;预处理工业基础类(industry foundation class,简称IFC)文件从BIM模型中获取建筑基本信息,并识别构件拓扑关系为自动化评估做准备;在BIM建模软件与结构分析软件间进行模型转换提高结构响应分析的效率和质量;借助SWRL(semantic web rule language)和SPARQL(SPARQL protocol and RDF query language)语言分别实现评估逻辑的合理表达以及语义层面的信息查询。该方法适用于建筑在现有评估类型下各性能指标的自动化预测,并具备可扩展性,为将来评估内容增删、更新提供空间。最后,以某一建筑为例进行可行性验证,取得较为满意的成果。     

钢管混凝土组合平台抗冰弹塑性分析

海洋平台是海洋石油开采的主要方式,但是在海中经常会受到海冰破坏,造成巨大的经济损失以及人员伤亡。本文以渤海JZ20-2海洋平台为模型,发展了钢管混凝土导管架海洋组合结构平台体系,利用大型有限元软件ABAQUS模拟海冰对海洋平台作用,通过对海洋平台应力分布情况,对平台进行弹塑性分析,模拟出海洋平台在极端罕遇冰力作用下海洋平台的破坏方式。通过模拟得出钢管混凝土组合结构体系平台可以有效减少导管架海洋平台的应力,钢管混凝土导管架组合结构平台在极端冰力作用下控制振动效果较好且造价较低。     

Remaining fatigue life assessment of aging fixed steel offshore jacket platforms

Actual fatigue life of an existing offshore platform typically deviates from that intended by its designer because aging platforms commonly experience some degradations, mechanical damages, repairs and modifications, and come across demands for the service extension. The current multidisciplinary paper focuses on different aspects of the remaining fatigue life assessment of an aging offshore platform. In the course of a case study, it describes a hindcasting technique for quantitative estimation of the long-term wave climatology, methods for geotechnical/structural modelling of the platform foundation to evaluate the piles fatigue damages accumulated during their driving and in-place service and the outlines for a spectral fatigue modelling and analysis of the fatigue damages to welded structural joints. Potential limitations in the available codes of practice, regarding the fatigue life assessment of existing offshore structures, are highlighted. The reliability of the fatigue life evaluations, possible remedy measures and some fatigue integrity monitoring techniques are discussed.     

Simplified seismic life cycle cost estimation of a steel jacket offshore platform structure

The purpose of the present paper is to develop a simple methodology for seismic life cycle cost (LCC) estimation for a steel jacket offshore platform structure. This methodology accounts for accuracy of LCC modelling as well as simplicity of application. Accuracy is maintained through incorporating the effect of aleatory and epistemic uncertainties in the LCC estimation framework. Simplicity is achieved by using equivalent single-degree-of-freedom (ESDOF) system instead of the full structure and by eliminating full incremental dynamic analysis and fragility analysis. Instead, an approximate fragility curve and a localised incremental dynamic analysis curve are used along with a probabilistic simple closed-form solution for loss estimation. In the design of model structures, different bracing systems are used for the seismic design of the offshore platform, such as conventional and buckling-restrained braces. The proposed LCC methodology is validated through comparison with the results from a more rigorous method. It is found that even though the proposed methodology results in a slightly different solution compared to the reference method, the method can be used as an efficient tool for preliminary LCC evaluation of structures.     

海洋导管架结构的现状研究及应用前景

研究了各种结构形式的特点,并分别对三导管架、四导管架和八导管架结构进行了介绍,根据对导管架节点的连接形式的研究,总结出各种节点的结构优缺点,对导管架节点进行了分类,提出了深海石油平台和近海风力发电平台两种导管架发展前景。     

Sensitivity analysis of jacket-type offshore platforms under extreme waves

Jacket-type offshore platforms play an important role in oil and gas industries in shallow and intermediate water depths such as Persian Gulf region. Such important structures need accurate considerations in analysis, design and assessment procedures. In this paper, nonlinear response of jacket-type platforms against extreme waves is examined utilizing sensitivity analyses. Results of this paper can reduce the number of random variables and consequently the computational effort in reliability analysis of jacket platforms, noticeably. Effects of foundation modeling have been neglected in majority of researches on the response of jacket platforms against wave loads. As nonlinear response of the pile foundation is one of the most important sources of potential nonlinearity in the response of offshore platforms, in this study, a powerful model which is able to consider Pile–Soil–Structure Interaction (PSSI) is employed. Therefore, PSSI parameters as well as other parameters such as uncertainties in the prediction of the wave force on jacket structure and uncertainties in structural model are utilized in sensitivity analyses. In this research, pushover methods as well as an advanced approach named “Incremental Wave Analysis (IWA)” are employed. Consequently, collapse prevention limit state of jacket platforms is investigated through different outcomes of pushover and IWA methods including Reserve Strength Ratio, ultimate capacity, collapse displacement and Collapse Wave Height indicators. In order to consider the effects of correlation between random variables, a robust method of sensitivity analysis named correlation coefficient approach is also employed.     

软黏土中导管架基础水平循环加载离心模型试验

为研究海上风机四腿导管架基础在风、浪等水平循环荷载作用下的受力及变形特性,开展了近海饱和软黏土地基四腿导管架基础水平循环加载离心模型试验。实测获得了水平循环荷载下导管架顶部的位移、基桩顶部的水平位移以及桩身弯矩,并利用实测桩身弯矩推导出桩身变形与桩周土反力。试验结果表明:水平循环荷载作用下导管架顶部的荷载–位移曲线表现出明显的非线性;后排桩的水平位移约为前排桩的80%,且均小于导管架顶部的水平位移,导管架发生了一定角度的倾斜;桩身最大弯矩值出现在泥面下约6D深度处。在此基础上,采用双曲正切型p–y曲线方法拟合试验结果并与API规范作对比,发现API规范p–y曲线的初始刚度和极限土反力均偏小。试验进一步揭示了泥面下5D深度范围内需考虑桩周土反力的弱化现象,且前排桩周土反力的弱化程度明显大于后排桩,在工程设计时应分开考虑下基桩桩周土的强度弱化情况。     

Seismic responses and resilience of novel SMA-based self-centring eccentrically braced frames under near-fault ground motions

In this paper, the seismic responses and resilience of a novel K-type superelastic shape memory alloy (SMA) self-centring (SC) eccentrically braced frame (EBF) are investigated. The simulation models of the SMA-based SC-EBF and a corresponding equal-stiffness traditional EBF counterpart are first established based on some existing tests. Then twenty-four near-fault ground motions are used to examine the seismic responses of both EBFs under design basis earthquake (DBE) and maximum considered earthquake (MCE) levels. Structural fragility and loss analyses are subsequently conducted through incremental dynamic analyses (IDA), and the resilience of the two EBFs are eventually estimated. The resilience assessment basically follows the framework proposed by Federal Emergency and Management Agency (FEMA) with the additional consideration of the maximum residual inter-storey drift ratio (MRIDR). The novel SMA-based SC-EBF shows a much better resilience in the study and represents a promising attractive alternative for future applications.     

API RP2A规范在桩自由站立强度分析中的应用

根据API RP2A第21版中的有关内容和规定,结合某导管架平台桩基设计,介绍了该规范在导管架平台桩的自由站立强度分析中的应用,并提出了一些指导性建议.     

Seismic assessment of masonry arch bridges

In this paper, the seismic performance of existing masonry arch bridges is evaluated by using nonlinear static analysis, as suggested by several modern standards such as UNI ENV 1998-1 2003, OPCM 3274 2004, and FEMA 440 2005. The use of inelastic pushover analysis and response spectrum approaches becomes more difficult when structures other than the framed ones are investigated. This paper delves into the application of this methodology to masonry arch bridges by presenting two particular case studies. The need for experimental tests in order to calibrate the materials and the dynamic properties of the bridge is highlighted, in order to correctly model the most critical regions of the structure. The choice of the control node in the pushover analysis of masonry arch bridges and its influence on seismic safety evaluation is investigated. The ensuing discussion emphasizes important results, such as the unsuitability of the typical top node of the structure for describing the bridge seismic capacity. Finally, the seismic safety of the two bridges under consideration is verified by presenting an in-depth vulnerability analysis.     

海上四桩导管架基础水平受荷离心模型试验

导管架基础广泛应用于海上风力发电和油气开发,水平向风、浪、流、地震等作用是导管架基础发生失效破坏的主要原因。通过离心模型试验针对饱和砂土地基中四桩导管架基础,研究其在沿边长方向和沿对角线方向水平静力作用下各基桩的内力分配、桩周土反力差异和变形特性。导管架基础沿对角线加载时基桩最易被拔出,其下压基桩的桩顶剪力、桩顶负弯矩和桩身最大正弯矩均较上拔基桩大,但二者的桩身水平位移相差不大。对于本文桩间距为5.8倍桩径的导管架基础,由于群桩效应及桩身上拔力降低了桩周土有效应力,沿边长加载时上拔桩在泥面下2.5倍桩径深度范围内的桩周土反力约为下压桩的60%,而沿对角线加载时上拔桩在该深度范围内的桩周土反力仅为下压桩的40%。沿对角线加载时下压桩与上拔桩在桩顶剪力、桩顶最大负弯矩、桩顶轴力及桩身最大正弯矩等参数之间的差别也明显大于沿边长加载情况。与单桩水平加载离心模型试验结果对比发现,同一深度处单桩的桩周土反力介于导管架基础上拔桩与下压桩的桩周土反力之间。     

Typical costs of seismic rehabilitation of existing buildings

This paper presents the methodology used in a FEMA‐funded study to develop a tool for structural engineers and decision makers to use to make estimates for the typical costs for seismic rehabilitation of existing buildings. Copyright © 2007 John Wiley & Sons, Ltd.     

考虑波流联合作用的大直径钢管桩自由站立稳定性分析

随着海洋工程建设水深的不断增加,导管架平台的钢管桩基础呈现出大直径、深贯入的特点。桩基承载力的提高导致了打桩施工难度的增长,不仅要求具有更大锤击能量的打桩设备,而且对打桩全过程的安全控制提出了更高的要求。桩的自由站立稳定性是打桩分析的第一个重要环节。针对置锤荷载的瞬态性特征,以钢管桩自由站立稳定性的静力分析方法为基础,提出了采用动力算法考虑作用于钢管桩上的波浪力和海流力,建立了钢管桩自由站立稳定性分析的动力模拟方法。研究表明,由于API规范法考虑的是荷载长期作用效果,与钢管桩置锤瞬间的瞬态过程不符,用该方法验算桩的自由站立稳定性偏于保守;动力法分析与API规范的计算结果对比表明,规范中的强度折减系数用于验算桩基自由站立稳定性时可以放大1.5倍。     

宝钢马迹山港浅海岩基平台的导管架施工工艺

结合宝钢马迹山港浅海岩基平台导管架施工实例,详细阐述了利用管架整体重量来维持结构稳定,采取整体预制,一次沉放的导管架施工工艺,得出了此工艺可以解决浅海裸露岩基上平台施工难题的结论。     

Seismic risk assessment of steel frames equipped with steel panel wall

This paper aims to conduct risk assessment on steel frame equipped with steel panel wall (SPWF) through probabilistic seismic demand analysis. First, cyclic test on a SPWF specimen with one‐third scale, one bay, and single story is performed, and the typical limit performance levels were determined in accordance with the test results and the stipulated in FEMA 356. Then, finite element models were built for a 12‐story steel frame and two 12‐story SPWF structures with different lateral stiffness ratio. The seismic performance of these models are investigated through nonlinear time–history analyses, and their limits capacities are determined from incremental dynamic analyses. Besides, fragility functions are developed for these models associated with 10%/50 years and 2%/50 years events, as defined in SAC project. Finally, the annual probabilities of each limits and the collapse probabilities in 50 years for the 3 models are calculated and analyzed, and the function associated with the collapse probability and lateral stiffness ratio is developed. The effectiveness of steel panel wall in reducing the seismic risk of the existing steel frame buildings is validated on the basis of the risk analyses.     

Recovery of a 620t jacket from seabed

Recently, a minimum structure fixed template steel tripod jacket was toppled onto the seabed during installation. The jacket weighed 620 t in air and was to be installed at a water depth of 64 m. This study presents the technical, operational and safety considerations in the development of the recovery procedure, which led to the successful retrieval of the jacket and subsequent re-installation. The recovery procedure relied heavily on subsea saturation diving activities and the use of a heavy lift derrick barge. Owing to the delicate situation of the jacket on the seabed with two piles inside two of the jacket legs and one of the piles bent and penetrated into the seabed, and the need to preserve the jacket integrity, the recovery process was a very unique engineering challenge. This recovery of a fully submerged jacket from the seabed was a first, which involved procedures never attempted before.     

Recovery of a 620 t jacket from seabed

Recently, a minimum structure fixed template steel tripod jacket was toppled onto the seabed during installation. The jacket weighed 620 t in air and was to be installed at a water depth of 64 m. This study presents the technical, operational and safety considerations in the development of the recovery procedure, which led to the successful retrieval of the jacket and subsequent re-installation. The recovery procedure relied heavily on subsea saturation diving activities and the use of a heavy lift derrick barge. Owing to the delicate situation of the jacket on the seabed with two piles inside two of the jacket legs and one of the piles bent and penetrated into the seabed, and the need to preserve the jacket integrity, the recovery process was a very unique engineering challenge. This recovery of a fully submerged jacket from the seabed was a first, which involved procedures never attempted before.