Friction damper for vibration control in offshore steel jacket platforms

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.     

Earthquake response spectrum analysis of offshore platforms

Response spectrum techniques for multi-component seismic analysis of steel template offshore platforms are evaluated. The dynamic characteristics of the structures taking into account the added masses of water and piled support conditions are modelled from practice. Comparison is made between various modal summation rules SRSS, API, CQC and different directional summation rules SRSS, SUM, MCQC. The problem of a residual mode and its effect on support design is also investigated.     

Time-dependent seismic fragility curves for aging jacket-type offshore platforms subjected to earthquake ground motions

This paper assesses the seismic vulnerability of a recently designed jacket-type offshore platform through development of fragility curves and presents the formulation of time-dependent seismic fragility curves to capture the effects of ageing and deterioration on the seismic vulnerability of the representative platform. Among the various ageing processes, the corrosion deterioration of the structure is taken into account by applying a time-dependent model of corrosion deterioration of the tubular elements in the splash zone. Incremental dynamic analysis is performed on the initial uncorroded and corroded platforms considering soil–pile–structure interaction. A full probabilistic analysis is then performed to develop time-dependent seismic fragility curves for the immediate occupancy and collapse prevention limit states. Finally, a time variant cubic model is proposed to develop fragility curves at any point in time during platform’s service life, without need to perform a complete time-consuming fragility analyses. Results show substantial increase in probability of failure of the platform throughout its lifetime due to corrosion deterioration. The results also indicate that the time-dependent fragility curves can be used in offshore platforms to assess the seismic vulnerability of the structure and demonstrate the influence of various factors affecting the seismic vulnerability of the platform during its lifetime.     

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.     

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.     

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.     

Remaining fatigue life of steel railway bridges under enhanced axle loads

In this study, various fatigue damage models proposed by researchers have been briefly discussed and found that the models are problem specific and their efficacy needs to be checked for high cyclic fatigue cases such as in railway bridges. Towards this, field studies were conducted to obtain the strain responses from a steel bridge during the passage of scheduled trains and test train formation with enhanced axle loading. Instrumentation was carried out at critical locations to obtain the responses from the girder. Three different scenarios have been considered to avoid the influence of noise. Further, numerical simulation of the bridge subjected to train loading at different speeds was carried out using ANSYS to obtain synthetic data of strain response from the validated finite element model. Analysis was carried out for normal as well as for futuristic speed of the trains. Responses obtained from field measurements as well as from numerical investigations were used to calculate the damage indices. Based on the damage indices, remaining fatigue life of the bridge was evaluated. The present study can be helpful in assessing the health condition of the railway bridges and to check the suitability of further increase in axle load or speed of trains.     

Maintenance and decommissioning real options models for life-cycle cost-benefit analysis of offshore platforms

Maintenance and decommissioning real options (RO) models are developed for life-cycle cost-benefit (LCCB) analysis of offshore platforms. Uncertainties about hydrocarbon prices, maintenance costs, environmental loading, structural capacity and damage due to deterioration are taken into consideration in the RO modelling. Expressions are derived for expected costs and benefits in terms of the availability function of the platform, which depends on the hazard and restoring functions, and on the annual probability of failure of the structure. Results show that the use of the net present value (NPV) approach can significantly underestimate the LCCB. The RO formulation developed is shown to provide a proper framework to account for the value of the managerial flexibility to consider different options along the service lifetime of the structure, such as maintenance and decommissioning, and to make decisions adapting to future information.     

Assessment of offshore platforms under extreme waves by probabilistic incremental wave analysis

In this study, a novel probabilistic framework named Probabilistic Incremental Wave Analysis (PIWA) is established in order to assess the performance of jacket offshore platforms under extreme waves. The PIWA can take into account the uncertainties in three main elements consisting of sea state parameters, structural response and collapse capacity. The main advantage of the PIWA approach is reflected in decoupling of the wave hazard and structural analyses via an intermediate variable known as the wave height intensity measure. Despite the fact that most of the uncertainties associated with structural response are concentrated in wave hazard, this will enable the PIWA to estimate the probability of failure accurately. Moreover, both static and dynamic wave analyses can be utilized in the PIWA procedure. In this approach, multiple incremental wave analyses are employed to estimate the distribution of structural demand for a wide range of wave height intensities. Subsequently, the mean annual frequency of exceeding a structural limit state is calculated for which this research addresses two different methodologies including demand-based and wave height-based approaches. Furthermore, a new probabilistic-based Reserve Strength Ratio (RSR) is proposed and the probability of exceeding various levels of RSR is provided. To reduce the large number of simulations and hence improving the computational effort in the PIWA procedure, a combination of Latin Hypercube Sampling and Simulated Annealing optimization technique is utilized as an efficient sampling scheme. The PIWA procedure is employed in probabilistic assessment of an existing jacket offshore platform located in the Persian Gulf as well.     

考虑钢材锈蚀的桥梁焊接节点概率疲劳寿命评估

由于循环外荷载和腐蚀性环境的共同作用,近海桥梁结构易受疲劳破坏.本文提出了一种考虑钢材锈蚀的桥梁结构改进疲劳可靠度评估模型,并基于青马大桥长期结构健康监测数据对其典型焊接节点的概率疲劳寿命进行了评估.通过建立有效截面衰减模型和材料疲劳性能退化函数,利用Miner损伤累积准则的连续概率函数表达式,提出了考虑钢材锈蚀的疲劳可靠度解析模型.同时考虑交通载荷(包括汽车载荷和火车载荷)和台风影响,通过统计分析长期应变监测数据建立了标准日应力谱.采用有限混合分布方法及基于遗传算法的混合参数估计算法,得到了标准日应力谱多模态有限混合分布函数.比较分析了考虑和未考虑钢材锈蚀的焊接节点疲劳寿命失效概率和可靠度指标,研究结果表明钢材锈蚀因素对桥梁焊接节点的疲劳可靠度评估结果有较大影响.     

钢箱梁焊接细节基于长期监测数据的疲劳可靠性评估:疲劳可靠度指标

提出基于长期监测数据的钢箱梁焊接细节疲劳可靠度的评估方法,并以润扬大桥悬索桥和斜拉桥钢箱梁顶板-U型肋焊接细节为对象开展应用研究。首先,建立疲劳损伤极限状态方程并讨论方程中各个参数的概率分布特性,在此基础上,采用最优化方法进行疲劳可靠度的求解,得到疲劳可靠度指标随服役时间增长的变化规律和日循环次数Nd的随机性对可靠度的影响。最后重点研究疲劳荷载效应增长对可靠度的影响,并据此提出对大桥日常运营管理的建议。结果表明:①将Nd作为随机变量处理是偏安全的,且Nd的随机性对斜拉桥焊接细节可靠度影响要较悬索桥更为显著;②悬索桥焊接细节的疲劳可靠度明显低于斜拉桥;③考虑疲劳荷载效应的增长会显著减小两桥焊接细节的疲劳可靠度;④为了得到钢箱梁焊接细节在服役期内的疲劳可靠度的准确评估,对其进行长期的应变监测是必要的。     

基于三参数威布尔分布模型的铸钢及对接焊缝腐蚀疲劳寿命评估方法

在海洋腐蚀环境和往复荷载耦合作用下,结构会产生腐蚀疲劳问题。为了更准确地拟合铸钢母材及对接焊缝的腐蚀疲劳应力 寿命（S-N）曲线,采用基于三参数威布尔分布对曲线进行修正,得到不同失效概率下的S-N 曲线表达式。采用概率权重矩法(PWM)求出威布尔参数的两个通解,其拟合结果接近,相对误差不超过5%。与Stromeyer模型S-N曲线拟合结果进行对比,发现基于三参数威布尔模型S-N 曲线具备更好的弯曲形状且拟合精度更高。对于G20Mn5QT铸钢母材,其拟合结果与疲劳试验值的误差平均值为13.8%;对于对接焊缝,其拟合结果与疲劳试验值的误差平均值为11.8%。     

Response of flexible structures in random seas

A general stochastic formulation which addresses the relative motion effects of flexible structures in a random seaway is presented. The formulation utilizes a least-squares cubic polynomial approximation to the nonlinear drag force contribution. This influences the process of developing the stochastic equations of motion in two ways. First, by introducing higher order moments in the covariance form of the equations. Through the use of Gaussian closure these moments are replaced by equivalent expressions involving only second order moments. Secondly, the cubic drag force approximation results in the appearance of a three-fold convolution of the velocity spectrum in the final form of the spectral equations. Although, an iterative solution technique is required to solve the resulting spectral equations, this formulation provides the engineer with a means to directly evaluate the structural displacement spectrum at selected elevations. Once the response spectra are known extremal response behaviour can be predicted. A single-degree-of-freedom model of a monotower platform and a multi-degree-of-freedom model of a marine riser are used to illustrate the influence of the various approximations on the response predictions. Issues addressed include the impact on the viscous damping in the structural model as a result of first and third order approximations to the viscous drag force and the need to include higher modes for very flexible structures.     

Integrating reliability and structural health monitoring in the fatigue assessment of concrete bridge decks

The overall objective of this article is to demonstrate how the concepts of reliability and structural health monitoring (SHM) can be integrated to create bridge assessment and decision systems. The steel-free concrete bridge deck system was chosen as a specific case study providing tangible focus for the research. The bridge assessment model therefore focused on fatigue cracking issues associated with wheel loads due to heavy truck traffic. The bridge assessment model required five components: a vehicle load model, a fatigue damage accumulation model, a residual strength model, a reliability model and an SHM decision model. Each of these components is discussed within the article with specific reference to previous research on steel-free bridge deck systems. The proposed model is used to develop a decision threshold for monitoring the oldest steel-free bridge deck in service.