Onboard monitoring of fatigue damage rates in the hull girder

Most new advanced ships have extensive data collection systems to be used for continuous monitoring of engine and hull performance, for voyage performance evaluation etc. Such systems could be expanded to include also procedures for stress monitoring and for decision support, where the most critical wave-induced ship extreme responses and fatigue damage accumulation can be estimated for hypothetical changes in ship course and speed in the automatically estimated wave environment.The aim of this paper is to outline a calculation procedure for fatigue damage rate prediction in hull girders taking into account whipping stresses. It is conceptually shown how such a method, which integrates onboard estimation of sea states, can be used to deduce decision support with respect to the accumulated fatigue damage in the hull girder.The paper firstly presents a set of measured full-scale wave-induced stress ranges in a container ship, where the associated fatigue damage rates calculated from a combination of the rain-flow counting method and the Palmgren-Miner damage rule are compared with damage predictions obtained from a computationally much faster frequency fatigue analysis using a spectral method. This analysis verifies the applied multi-modal spectral analysis procedure for fatigue estimation for cases where hull girder flexibility plays a role.To obtain an automated prediction method for the fatigue damage rates it is in the second part of the paper shown how a combination of the full-scale onboard acceleration and stress measurements can be used to calculate sea state parameters. These calculated environmental data are verified by a comparison to hindcast data.In the third part of the paper the full-scale fatigue stress ranges are compared to results from an analytical design oriented calculation procedure for flexible ship hulls in short-term estimated sea states.Altogether, it is conceptually shown that by a combination of the onboard estimated sea state parameters with the described analytical fatigue damage prediction procedure a method can be established for real-time onboard decision support which includes estimates of fatigue damage rates.     

Time domain analysis procedures for fatigue assessment of a semi-submersible wind turbine

Long term time domain analysis of the nominal stress for fatigue assessment of the tower and platform members of a three-column semi-submersible was performed by fully coupled time domain analyses in Simo-Riflex-AeroDyn. By combining the nominal stress ranges with stress concentration factors, hot spot stresses for fatigue damage calculation can be obtained. The aim of the study was to investigate the necessary simulation duration, number of random realisations and bin sizes for the discretisation of the joint wind and wave distribution. A total of 2316 3-h time domain simulations, were performed.In mild sea states with wind speeds between 7 and 9 m/s, the tower and pontoon experienced high fatigue damage due to resonance in the first bending frequency of the tower from the tower wake blade passing frequency (3P).Important fatigue effects seemed to be captured by 1 h simulations, and the sensitivity to number of random realisations was low when running simulations of more than 1 h. Fatigue damage for the tower base converged faster with simulation duration and number of random realisations than it did for the platform members.Bin sizes of 2 m/s for wind, 1 s for wave periods and 1 m for wave heights seemed to give acceptable estimates of total fatigue damage. It is, however, important that wind speeds that give coinciding 3P and tower resonance are included and that wave periods that give the largest pitch motion are included in the analysis.     

Experimental and numerical investigation of fatigue damage due to wave-induced vibrations in a containership in head seas

Fatigue cracks have been known to occur in welded ships for several decades. For large ocean-going ships wave-induced vibrations can, depending on trade and design, cause up to 50% of the fatigue damage. The vibrations may be due to springing and whipping effects. In this paper, we address the fatigue damage caused by wave-induced vibrations in a containership of newer design trading in the North Atlantic. The fatigue damage was obtained both experimentally and numerically. The experimental results were found from tests performed with a flexible model of the ship, while the numerical predictions were done using nonlinear hydroelastic strip theory. The measurements showed that the wave-induced vibrations contributed approximately 40% of the total fatigue damage. The numerical method predicted the wave frequency damage well, but was found to overestimate the total fatigue damage by 50%. This was mainly due to an overprediction of the wave-induced vibrations. The discrepancy is partly related to three-dimensional (3D) effects which are not included in the two-dimensional (2D) slamming calculation, and partly to an overprediction of the springing contribution. Moreover, the numerical method does not account for the steady wave due to forward speed. By using a simplified approach we show that high-frequency damage can be significantly reduced by including the steady wave for the relevant vessel, implying better agreement with the experimental results.     

基于谱分析法的FPSO剩余疲劳寿命研究

疲劳破坏是船舶与海洋工程结构破坏的主要模式之一。多年来,船舶结构的疲劳断裂问题一直是造船界广泛关注的问题[1]。对于由大型油船改装而成的FPSO而言,预测并延长其服役寿命是很关键的。本文通过谱分析法对船体疲劳损伤度进行计算,分别对油船和FPSO阶段进行计算从而得到FPSO剩余疲劳寿命。通过建立3D有限元模型,采用热点应力方法来确定评估处应力传递函数,分别计算各个短期海况损伤度并通过线性叠加来计算总的损伤度以及剩余疲劳寿命。根据疲劳评估结果,更加高效地实施船体结构的检测及维修。     

Simplified buckling-and-contact-based expansion method for assessment of crashworthiness of double-hulled ship structure

Ship-to-ship collision events can have severe consequences such as loss of life and environmental degradation. For this reason, modern ship designs are required to incorporate a double-hulled structure to prevent inner-hull damage from such events. Using the experimental or numerical method to analyze the crashworthiness of double-hulled ship structures entails much effort, for which reason neither method is easy to adopt at the early design stage. In this paper, an existing simplified method called Ito's method is improved by a new buckling-and-contact-based expansion method. This method can be applied to double-hulled-structure or outer-hull-local-rupture failure mode. The perpendicular bow-to-side collision scenario is assumed for a conservative estimation of damage to a double-hulled structure. The method was verified in the present study by numerical ship collision simulations of several cases. The results for the buckling-and-contact-based expansion method and numerical simulation were similar for a blunt shape of striking body but different for a sharp shape.     

Environmental lumping for efficient fatigue assessment of large-diameter monopile wind turbines

Fatigue damage is one of the governing factors for the design of offshore wind turbines. However, the full fatigue assessment is a time-consuming task. During the design process, the site-specific environmental parameters are usually condensed by a lumping process to reduce the computational effort. Preservation of fatigue damage during lumping requires an accurate consideration of the met-ocean climate and the dynamic response of the structure. Two lumping methods (time-domain and frequency-domain) have been evaluated for a monopile-based 10 MW offshore wind turbine, both based on damage-equivalent contour lines. Fatigue damage from lumped load cases was compared to full long-term fatigue assessment. The lumping methods had an accuracy of 94–98% for the total long-term fatigue damage and 90% for individual wind speed classes, for aligned wind and waves. Fatigue damage was preserved with the same accuracy levels for the whole support structure. A significant reduction of computational time (93%) was achieved compared to a full long-term fatigue assessment. For the cases with 30° and 60° wind-wave misalignment, there was a mean underestimation of approximately 10%. Variations in penetration depth did not affect the selection of the lumped sea-state parameters. This work presents a straightforward method for the selection of damage-equivalent lumped load cases, which can adequately preserve long-term fatigue damage throughout the support structure, providing considerable reduction of computational effort.     

循环载荷下考虑累积塑性影响的船体板CTOD理论及数值模拟研究

裂纹尖端张开位移(CTOD)是研究大范围屈服的低周疲劳破坏的重要参数之一,其值可反映结构材料抵抗低周疲劳裂纹形成和扩展的能力,是评估结构材料韧性的重要参量以及分析低周疲劳破坏引起裂纹扩展的主要控制参量。文章基于弹塑性断裂力学理论,从循环J积分着手,以裂纹尖端累积塑性应变为重要参量,建立循环载荷下船体板CTOD理论模型,并在有限元模拟中分析了应力比、应力幅等相关因素影响。将本模型结果与有限元计算结果进行了比较,发现结果吻合良好。结果表明：在考虑累积塑性影响下,该模型能较好地反映在循环载荷下船体板CTOD的变化规律,同时也为正确评估循环载荷下船体板低周疲劳破坏与累积塑性破坏两种破坏模式耦合作用的总体断裂破坏提供了途径。     

Probabilistic fatigue analysis of marine structures using the univariate dimension-reduction method

Probabilistic fatigue analysis of offshore structures requires the numerical simulation of a huge number of loading cases to compute the long-term multi-dimensional integral associated to the fatigue damage assessment. This paper proposes the implementation of the univariate dimension-reduction method developed by Rahman and Xu [1] in order to compute the long-term fatigue damage more efficiently. This method is particularly attractive because it reduces significantly the number of simulations by decomposing the N-dimensional integral associated to expected long-term fatigue damage assessment into the sum of N one-dimensional integrals. In addition, this paper compares the univariate-dimension reduction method with the brute force direct integration methodology and other methods based on Taylor expansions, such as perturbation approach and asymptotic expansion method discussed by Low and Cheung [2]. Two comprehensive examples are included to show the effectiveness of the method. At first, the performance of the univariate dimension-reduction method is evaluated by assessing the fatigue damage of a theoretical structure represented by a single stress response amplitude operator (RAO). Then, in order to show a case of practical application, the fatigue damage is evaluated for a Steel Lazy Wave Riser (SLWR) connected to an FPSO in a water depth of 2200 m.     

MTMD to increase fatigue life for OWT jacket structures using Powell's method

The Powell's method was developed to determine the optimal stiffness and damping of multi-tuned mass dampers (MTMD) in offshore wind turbine (OWT) support structures under fatigue loads. Numerical examples indicated that the Powell's method results are always better than those using MTMD formulations. With the exception of the blade passing (3P) frequency, it was found in this work that a positive integer (n) multiple of the 3P frequency will also result in a large wind-induced vibration, which can be excited by the frequency of the first structural vertical rotation mode and will cause significant fatigue damage. The first translation mode TMD installed at the tower top is efficient to increase fatigue life at the tower and brace connections, but it cannot reduce fatigue damage at the column and brace connections below the platform. The second translation mode TMD can reduce fatigue damage resulting from large wave loads and thus increase the fatigue life of the braces and columns. The mode-3 TMD with a reduction in the 3(3P) vertical rotation can effectively increase the fatigue life of the braces and columns. Thus, the appropriate use of these TMDs can be effective for the fatigue problem of OWT support structures.     

Numerical simulation of fatigue crack propagation under superimposed stress histories containing different frequency components with several mean stress conditions

Fatigue crack propagation behavior under superimposed stress histories containing different frequency components with several mean stress conditions was investigated. Numerical simulation of fatigue crack propagation based on an advanced fracture mechanics approach using the RPG (Re-tensile Plastic zone Generating) stress criterion for fatigue crack propagation was improved to extract the effective part from the applied stress history for fatigue crack propagation. The parameter, which is based on the plastic hysteresis energy consumed in the vicinity of a crack tip, was applied and implemented into the numerical simulation code of fatigue crack propagation. Fatigue crack propagation tests under various superimposed stress conditions with several mean stress conditions were performed and compared with the fatigue crack propagation histories obtained from the improved numerical simulations. These comparisons show the validity of the proposed procedure for extracting the effective stress history from the superimposed stress histories with different frequency components and mean stresses. Additionally, practical fatigue strength evaluations based on the linear cumulative fatigue damage parameter were conducted to investigate the tendency of the fatigue damage value under these stress conditions.     

海上风机在三种风载荷作用下的疲劳分析方法研究

文章提出一种基于等效疲劳载荷的快速有效的结构优化设计方法,首先通过bladed模拟得到时域下的风载荷,然后通过雨流计数法则和等效损伤理论得到相应的疲劳载荷谱和等效疲劳载荷,接着以导管架式海上风机为例,利用AN-SYS对其进行三维建模,选取三种典型管节点和两种非管节点,基于热点应力法计算了其在三种风疲劳载荷作用下的疲劳损伤,通过比较三种载荷作用下的疲劳损伤结果,验证了等效疲劳载荷的可靠性.接着又计算了各等效疲劳载荷分量单独作用下的海上风机焊接节点的疲劳损伤,得出各疲劳载荷分量对疲劳总损伤的贡献,可以为设计者提供更好的载荷设计依据.相比于传统的时域疲劳分析方法和疲劳载荷谱方法,等效疲劳载荷方法更加方便有效.     

Fatigue damage prediction of ship rudders under vortex-induced vibration using orthonormal modal FSI analysis

Recently, the fatigue failure of ship rudders owing to vortex-induced vibration has increased as commercial ships become faster and larger. However, previous methods are inappropriate for fatigue failure prevention owing to the lack of fluid–structure interaction considerations. This study aims to develop a fatigue damage prediction method that can be applied at the design stage to prevent fatigue failure of ship rudders under vortex-induced vibration. The developed prediction method employed the fluid–structure interaction (FSI) method to properly consider the fluid–structure interaction and implemented orthonormal mode shapes to reflect the complex geometry and boundary conditions of the ship rudders. For validation, vortex-induced vibration of the hydrofoil model was obtained using the developed method, and the prediction results matched well with the experimental results. Then, the fatigue damage of the ship rudder model under vortex-induced vibration was predicted using the developed method, and their characteristics are discussed. The stress distribution obtained using the developed method matched well with the geometrical characteristics of the ship rudders. The potential for fatigue failure due to the resonance of vortex-induced vibration was expected by comparing the stress distributions for various flow velocities to the S–N curves provided by the DNV classification.     

Improvement in capacity factor and fatigue assessment of VLMOS for wind power generation based on navigation simulation

The very large mobile offshore structure (VLMOS) for wind power generation is one example of challenging future technologies to create a new backbone of energy resource of our country. The National Institute for Environmental Studies Japan has been studying this concept and found that increasing the capacity factor and reducing the weight of the structure are keys to make this concept successful. We investigated the navigation logic to improve the capacity factor and performed a fatigue assessment to ensure the structural safety of light weight design. Taking the knowledge on the seasonal weather condition in the Pacific Ocean into consideration, improved strategy of the VLMOS navigation for maximizing the generating power is proposed. Navigation simulations show that it is easy to achieve more than 40% capacity factor and to escape from high seas. Using wind and wave data obtained from the navigation simulations, an assessment of the fatigue damage has been performed. It is stated that the structure has enough fatigue strength for a 100 years of operation even if wind turbines get more than 40% of the capacity factor.     

塔式立管涡激疲劳损伤敏感参数研究

针对西非海域的塔式立管,考虑沿水深变化的不同流速截面,分析塔式立管在不同参数下的涡激振动响应及疲劳损伤沿管分布,讨论立管顶部张力、管内流体密度、立管外径、立管壁厚和SHEAR7能量阈值等参数的变化对疲劳损伤的影响.结果显示,由于制造和安装误差等因素导致这些参数的改变,对结构的涡激振动疲劳损伤影响显著,应该在工程设计中引起注意.     

基于累积递增塑性变形的船体裂纹板断裂韧性研究

裂纹尖端张开位移（CTOD）是评估结构材料韧性以及分析低周疲劳破坏引起的裂纹扩展的重要参量。文章结合Dugdale模型,以裂纹尖端累积塑性应变为控制参量,提出了一个循环载荷下含裂纹船体板的CTOD计算模型;利用有限元法模拟了裂纹尖端累积塑性应变、平均应力、裂纹长度等相关因素影响;结合最小二乘法拟合出了基于累积塑性应变、平均应力比以及裂纹长度的两阶多项式。文中基于累积塑性应变的CTOD计算模型为正确评估循环载荷下船体板的累积塑性破坏提供了一种新途径。     

基于应力集中系数的SWATH船疲劳节点筛选方法研究

小水线面双体船的疲劳强度问题十分突出,疲劳强度校核节点众多且目前还没有一个明确的疲劳节点筛选原则。针对这些问题,研究基于典型节点的热点应力集中系数有限元计算和名义应力分析,对小水线面双体船全船结构中可能发生疲劳强度失效的节点进行疲劳寿命估算,并根据估算结果对需要进行热点应力分析的疲劳节点进行筛选。计算结果表明,该方法在处理大量结构节点的疲劳寿命估算问题中,计算效率较高,能够反映全船的节点疲劳强度特性,且估算结果较为可靠。该方法不仅适用于小水线面双体船,对于其他类型的船舶,该方法同样具有适用性。     

Efficient long-term fatigue analysis of deepwater risers in the time domain including wave directionality

Accurate fatigue assessment is a challenging and crucial aspect of riser design. The prediction of the long-term fatigue damage must account for numerous sea states of different wave heights, periods, and directions. Each sea state entails a dynamic analysis, often performed in the time domain owing to the significant nonlinearities. Because of the short-term uncertainties from irregular waves, the simulation duration must be sufficiently long for results to converge. To alleviate the hefty computational cost of long-term fatigue analysis, researchers have proposed efficient methods, but these are not without drawbacks; in particular, wave directionality is commonly neglected. This paper presents an efficient method for long-term fatigue analysis based on time domain simulation, considering wave directionality among other things. The proposed method is based on an enhanced version of control variates to reduce the variance in Monte Carlo simulations (MCS). The control function is constructed by training artificial neural network (ANN) models using existing MCS data. Here, a customized scheme is developed to allow for the situation that the training data and ANN prediction cases have different wave directions. The proposed method is unbiased and provides an error estimate. Simulations are performed on a floating system, and the proposed method is found to improve the efficiency of MCS significantly. Different scenarios such as fixed and random wave directions are compared, confirming that wave directionality is critical and should be included in a long-term fatigue assessment.     

Fatigue damage analysis of the deepwater riser from VIV using pseudo-excitation method

The fatigue damage of a long deepwater riser undergoing in-line and cross-flow vortex-induced vibration (VIV) in deepwater is numerically studied using pseudo-excitation method (PEM) in present paper. For evaluating the fluid–structure interaction problem of vortex-induced vibration of deepwater riser at high Reynolds number, the strip theory is employed in this paper, and the discrete vortex method (DVM) is used to calculate the VIV of each strip to obtain the load spectrum as the pseudo-excitation, while the finite volume method (FVM) is employed to evaluate the structure dynamics of a deepwater riser. The VIV is considered as a stationary random process. The response of riser to vortex induced excitation is calculated using pseudo-excitation method. The DVM model and pseudo-excitation method are both validated by comparing their numerical results with experiments. The fatigue damage of one deepwater riser is evaluated based on the Palmgren–Miner Rule.     

A reliability approach to fatigue crack propagation analysis of ship structures in polar regions

Fatigue is a common failure mode in ship structures. For structures with an initial crack, the fatigue crack propagation behavior needs to be considered. The purpose of this study is to establish a procedure for analysis of fatigue crack propagation of ship structures in combination with reliability methods. The stress intensity factor (SIF) and geometry correction factor are calculated by means of finite element analysis. Validation for the SIF calculation is achieved by comparing the computed results with those based on related solutions. Since fatigue damage usually occurs in weld areas, the effect of such components on the fatigue crack propagation behavior was also considered in this work. The Paris law in combination with the Monte Carlo technique are employed for the fatigue crack propagation analysis in this study. Reliability updating based on inspection for cracks is also carried out. A computer program was developed for the purpose of fatigue crack propagation analysis within the framework of reliability methods. An application example of fatigue crack propagation in relation to the hull of the icebreaker Xuelong 2 is presented. The sensitivity of the procedure to key analysis parameters (sample size, initial crack size) is also considered. Finally, the effect of low temperatures on the computed results is also analyzed.     

Fatigue analysis of the bow structure of FPSO

The bow structure of FPSO moored by the single mooting system is rather complicated. There are many potential hot spots in connection parts of structures between the mooting support frame and the forecastle. Mooting forces, which are induced by wave excitation and transferred by the YOKE and the mooting support frame, may cause fatigue damage to the bow structure. Different from direct wave-induced-forces, the mooting force consists of wave frequency force (WF) and 2nd draft low frequency force (LF) , which are represented by two sets of short-term distribution respectively. Based on two sets of short-term distribution of mooting forces obtained by the model test, the fatigue damage of the bow structure of FPSO is analyzed, with emphasis on two points. One is the procedure and position selection for fatigue check, and the other is the application of new formulae for the calculation of accumulative fatigue damage caused by two sets of short-term distribution of hot spot stress range. From the results distinguished features of fatigue damage to the FPSOs bow structure can be observed.