Feasibility of modal expansion for virtual sensing in offshore wind jacket substructures

The present paper investigates the feasibility of modal expansion-based virtual sensing in the context of offshore wind jacket substructures. For this specific application, issues have been reported when expanding wind-driven brace vibrations and wave-driven vibrations in the splash-zone based on a sensor network placed solely above the sea level. These limitations are addressed in this paper by extending the sensor network with sub-sea vibration sensors and a wave radar sensor, which allow for capturing local brace vibration modes and the wave-driven vibration response. The brace expansion is thus improved by including the local brace vibration modes in the expansion basis, while the representation of wave-driven vibrations is improved by including load-dependent Ritz vectors computed based on input from the wave radar sensor. The merit of the proposed extension is explored using a numerical model of an offshore wind turbine supported by a jacket substructure in a simulation setting with different operational and environmental conditions. It is documented that the extended setup provides an improvement in the expansion-based estimation of both wind- and wave-driven vibrations. The former improvement is particularly relevant for operational cases, while the latter is relevant for idling cases. Despite the documented improvements, a systematic reduction in the expansion quality is observed for higher wind speeds in operational cases for both the basic and the extended setup. It is contended that this phenomenon is due to the operational variability of the controller, which violates the fundamental assumption of the structural system being linear and time-invariant.     

Finite element simplified fatigue analysis method for a non-tubular joint of an offshore jacket platform

This paper proposes the finite element simplified fatigue analysis method for fatigue evaluation of the composite non-tubular joint structure of an offshore jacket subjected to wave loads. The skirt pile sleeve of the offshore jacket, which had been in service, was taken as an example of the non-tubular joint structure. SACS software was used for global analysis of multi-directional wave loads for the jacket platform, and ALGOR software was used to build a finite element model, perform finite element analysis, post-process stress results for acquiring the stress range, and perform fatigue evaluation. The analysis results indicate that the extreme stress range is within the allowable stress range and meets the requirements of DNV code. That means the simplified fatigue analysis method is effective and can be used in fatigue design for the non-tubular joint structure of an offshore jacket.     

基于裂纹扩展的海洋平台疲劳可靠性分析

文章基于裂纹疲劳基本理论,针对在役导管架平台出现初始裂纹状态下结构承载力进行分析,考虑结构损伤与腐蚀等因素,建立了典型导管架平台整体结构模型和局部子模型,分析了在该工况下平台整体结构的疲劳强度,并确定了平台结构的疲劳关键部位。采用几何应力外插法计算了热点应力,基于疲劳裂纹扩展的疲劳分析方法,计算了疲劳关键节点的疲劳寿命与疲劳可靠度。该方法能够为海洋平台结构的维护和保养提供一定参考。     

Low-cycle fatigue assessment of offshore mooring chains under service loading

The integrity of mooring chains is essential to the safety of a range of offshore platforms. However, mooring line failures are occurring earlier than their design lives, with a high number of these failures occurring due to fatigue. Early in the fatigue life of the component fatigue initiation processes occur, where the fatigue hotspot is sensitive to the mean load and there is plastic strain accumulation from the multiaxial stress-strain responses of the material, leading to cyclic plastic damage accumulation. The traditional SN approach suggested by mooring standards does not consider these effects, and it is proposed that this lack of consideration under low-cycle fatigue conditions is the reason for the current non-conservative fatigue assessments of mooring chains. This paper aims to develop a fatigue approach based on a critical plane multiaxial fatigue criterion for mooring chains that can consider the damage-induced by the cyclic plasticity and the mean load effect, to investigate the importance of incorporating low-cycle fatigue into the mooring chain life prediction. To develop the critical plane approach, the multiaxial stress-strain states are extracted for the critical plane at the fatigue hotspot from a finite element model of a mooring chain. This is then correlated with a fatigue life prediction provided by conventional fatigue design data. It uses a simulation of an FPSO as a case study to demonstrate the importance of low cycle fatigue, which shows that the mean load effect is significant in reducing the fatigue life for mooring chain applications, while the effect of fatigue damage-induced cyclic plasticity is limited. The fatigue damage accumulation predicted by the critical plane approach is significantly higher than that of the traditional SN approach and should be accounted for in mooring line design.     

AUV navigation around jacket structures I: relative localization based on multi-sensor fusion

Underwater jacket structures or support legs of on-water platforms, such as ports and oil rigs, need to be periodically inspected for maintenance, environmental monitoring, and security reasons. Autonomous underwater vehicles (AUVs) can potentially make these tasks more inexpensive and reliable compared to conventional methods that involve the use of divers and remotely operated vehicles. This paper proposes a robust and practical self-localization method for an underwater vehicle navigating around jacket structures, where the performance of conventional acoustic positioning suffers from multipath degradation. The key idea is to stochastically update the vehicle??s horizontal position and heading relative to the structures using two types of perceptional sensors, sonar and camera, assuming that the configuration of the structure is known. The performance of the method was verified with tank experiments using a jacket mock-up and the AUV Tri-Dog 1.     

近海结构疲劳计算中波浪与结构建模探讨

文章根据作者已提出的基于海况频度分布的统计分类及非线性动力学计算的疲劳评估方法,通过进一步改进波浪能的输入,提出了一种更为有效的计算方法。通过将这一方法运用到一典型导管架结构的疲劳计算,文中首先研究了波浪方向对疲劳破坏的影响,进而分析了波浪谱中若干重要参数及HHT-α时间积分中步长对疲劳计算结果的影响。通过对结构反应的统计研究,发现海况的恶劣程度并不直接影响结构响应非高斯趋势的显著程度。文中提出的方法在减少不确定性的同时又不降低安全标准,有效提高了疲劳计算的精确度,从而可以降低相关近海结构建设和维护成本。文中的计算方法已被成功地应用于北海及墨西哥湾数座近海结构在风和波浪荷载作用下的疲劳评估。     

Stress concentration factors at welds in pipelines and tanks subjected to internal pressure and axial force

In this paper, analytical expressions for stress concentration factors in pipes subjected to internal pressure and axial force are derived for a number of design cases based on classical shell theory. The effect of fabrication tolerances in simple butt welds is assessed. Analyses based on classical mechanics are compared with results from axisymmetric finite element analyses for verification of the presented methodology. Stress concentration factors are presented for circumferential butt welds in pipes welded together from pipes with different thicknesses, welds at buckling arrestors, welds at flanged connections in pipelines, and welds at ring stiffeners on the inside and the outside of the pipes. It also includes stress concentration factors at end closures in pipes for gas storage. Larger pipes are fabricated from plates with a longitudinal weld. This fabrication process introduces out-of-roundness in the pipes. The actual out-of-roundness is a function of internal pressure. An analytical expression for the bending stress in the pipe wall due to this out-of-roundness is presented. The derived stress concentration factors can be used together with a hot spot stress S–N curve for calculation of fatigue damage.     

大型开敞式深水码头导管架结构管节点的疲劳分析

结合大连新港新建30万吨级(兼靠45万t)进口原油码头工程,以导管架结构为研究对象,阐述该结构管节点的疲劳计算方法,并计算开敞式码头导管架结构中管节点的疲劳寿命.结果表明管节点的疲劳寿命满足相应规范的要求,可为实际工程设计提供参考.     

Study on hot spot stress distribution of three-planar tubular Y-joints subjected to in-plane bending moment

The three-planar tubular Y-joint (3Y-joint) is the main part of the fatigue assessment of tripod substructures of offshore wind turbines (OWTs). As typical multiplanar tubular joints, 3Y-joints are affected a lot by multiplanar interaction between braces. Moreover, the locations of hot spot stress (HSS) can vary considerably under different load types. Thus, the distributions of stress concentration factor (SCF) and multiplanar interaction factor (MIF) along weld toe curves are necessary to calculate HSS. Considering these requirements, this study focuses on the 3Y-joint considering the wide application of the tripod substructure of OWT. A finite element (FE) analysis method is introduced and validated. Then, a numerical database is established covering common ranges of parameters used in practice. The SCF and MIF of 3Y-joint under in-plane bending moment are analyzed. Distribution formulas are proposed and proved suitable for calculating HSS in engineering design.     

Finite Element Simplified Fatigue Analysis Method for a Non-tubular Joint of an Offshore Jacket Platform

This paper proposes the finite element simplified fatigue analysis method for fatigue evaluation of the composite non-tubular joint structure of an offshore jacket subjected to wave loads. The skirt pile sleeve of the offshore jacket, which had been in service, was taken as an example of the non-tubular joint structure. SACS software was used for global analysis of multi-directional wave loads for the jacket platform, and ALGOR software was used to build a finite element model, perform finite element analys...     

单点系泊系统将军柱疲劳分析

渤海某FPSO的单点系泊装置固定塔架由导管架、将军柱和上部组块构成,其中将军柱是系泊力的主要承受构件之一,其结构安全至关重要。由于系泊力是典型的交变载荷,作用在结构上会产生疲劳损伤,因此有必要对将军柱进行在位期间的疲劳分析。本文提出一种长期海况下海上固定装置疲劳计算方法,通过AQWA软件建立单点系泊系统的多体耦合水动力模型模型,根据渤海的海况环境资料计算出FPSO运动时域内所受到的的系泊力;基于S-N曲线方法与Miner线性累计损伤理论,通过nCode Designlife疲劳计算软件计算将军柱结构的疲劳寿命和管节点的疲劳损伤;评估结构的疲劳强度,分析易发生疲劳的关键节点位置,并给出增加管节点疲劳寿命的建议及设计方法,为相同类型的海上固定式结构物的结构设计及疲劳分析提供有益的参考及借鉴。     

Efficient optimization design method of jacket structures for offshore wind turbines

With the gradual implementation of offshore wind energy production, the future tendency is to expand into the deeper water. The jacket foundations will take the place of the present monopile foundations when the water depth increases. The foundations account for the majority of the construction cost for offshore wind farms, and the structural optimization of jackets will bring lucrative economic benefits. Structural optimization is a complex iterative process that requires huge computing costs. Therefore, this paper proposes a structural optimization method based on surrogate models to solve this problem effectively and swiftly obtain optimized design schemes of lightweight jackets for offshore wind turbines. The structural responses of jacket wind turbine systems under the equivalent static extreme loads with a recurrence period of 50 years are mainly considered in structural optimization design, and the key optimization variables of jackets are determined by parameter sensitivity analysis. The finite element models of jackets are transformed into surrogate models, and the genetic algorithm is employed to optimize the surrogate models directly. The optimized jackets are additionally verified through coupled dynamic analysis, besides, buckling strength and fatigue life are also checked. And local refined optimizations are carried out for the failure members. According to the optimized design schemes of lightweight jackets for 30 m, 50 m and 70 m water depths, it is demonstrated that the structural optimization design method is adequate and efficient for jackets of wind turbines. Parameter sensitivity analysis can cut the number of optimization variables in half to improve the optimization efficiency. Furthermore, the application of surrogate models can significantly speed up the optimization process by saving about 98.61% of the original time consumed. The optimization design method of the jackets for offshore wind turbines proposed in this paper is suitable for practical engineering, with high precision and efficiency.     

海洋平台结构系统疲劳可靠性分析方法

本文研究了海洋平台结构系统疲劳可靠性评估方法。文中对平台结构简生管节点疲劳可靠性分析模型进行了简单的评述，在该模型的基础上，建立了用S－N曲线模型与断裂力学模型分别计算平台完好管节点及有初始裂纹管节点疲劳寿命的平告结构系统疲劳可靠性分析模型。作为一个算例，文中运用作者开发的程序系统，对一固定式导管架平台进行了疲劳可靠性计算。     

FPSO动态海底电缆保护外皮破损结构强度分析

某动态海底电缆在年度检查时发现电缆锁夹上方电缆保护外皮脱落、露出铠装的情况，影响安全生产。采用OrcaFlex软件建立分析模型，结合浮式生产储卸油装置（Floating Production Storage and Offloading，FPSO）的不同运行工况，对海底电缆的结构状态进行结构强度分析和疲劳分析。经计算，台风条件下的FPSO偏移量是影响海底电缆结构强度的重要因素。因海底电缆保护外皮破损位置的载荷较低，结构强度和疲劳寿命满足要求。根据相关文献计算铠装层的腐蚀速率，海底电缆强度满足要求。     

无底钢套箱围堰设计与施工

钢套箱围堰是为解决水上承台和水上桥墩施工而设计的临时阻水结构,其作用是通过钢套箱围堰和封底混凝土阻水,为水上承台和水上桥墩的施工提供无水的干施工环境。结合洋安大桥工程的实例,重点介绍无底钢套箱的设计、制作、安装和封底混凝土的施工方法,其中钢套箱的分块制作,现场拼装,整体下放工艺因避免使用大型船机设备,大大节约工程成本取得较好的效果,可广泛应用。现场使用结果证明该钢套箱结构设计合理,安装方法得当,定位准确,施工简便可行,取得显著的效果。     

Improving the fatigue life of large offshore foundations

Offshore foundations are subject to fatigue due to wind and wave loads during their lifetime. Post-weld treatment is a group of techniques well known to improve the fatigue life. This study investigates how this is applicable for large structures, i.e. a 2.4 meter jacket T-node. A large scale empirical study was performed and results were obtained from a SN-diagram based on three repetitions and supported by a theoretical background. The as welded samples performed as expected and on the safe side of the category 56 line. Unexpectedly, the burr grinded samples showed no improvement and had an evidently larger variance. Both ultrasonic impact treatment and especially pneumatic impact treatment showed improvements, but did not reach the expected category 125 line. The results were substantiated with strain-gauge measurements for hot spot analysis. The obtained results on fatigue life improvements give rise to apply post weld treatment in an industrial scale for offshore construction. However, it also brings up the discussion of the necessity to redesign structures in order to automate the treatment in a cost efficient manner.     

深水导管架桩腿制造误差与抗屈曲特性研究

文章通过典型算例详细阐述了具有制造误差的深水导管架桩腿屈曲分析方法,将线性扰动加载的特征值届曲预测算法和描述几何非线性屈曲过程的改进的Riks算法应用于导管架桩腿的详细设计校核,对比研究了模态扰动模拟制造误差和直接模拟制造误差模型的分析结果,并对典型桩腿结构在静水外压下的弹塑性屈曲响应特性进行了研究.分析结果表明,随着制造误差的增加,导管架桩腿抗静外压的临界屈曲载荷在下降,而且制造误差大的桩腿的极限承载能力较低.     

波浪非线性对导管架平台疲劳寿命评估的影响研究

基于线性和非线性波浪理论,在渤海海域观测的30种海况下,选取了导管架平台易发生疲劳损伤的几个危险节点作为研究对象,并通过确定性的疲劳分析方法对这些节点进行了疲劳寿命评估。结果表明,在选取的3种阻尼条件下,考虑波浪的非线性因素后,疲劳寿命的评估结果均有所增加,为海工结构的疲劳寿命评估提供了参考意见。     

大型薄膜型LNG运输船结构关键区域分布与控制

以大型薄膜型LNG运输船为例,采用整船直接计算强度评估和疲劳谱分析,确定结构关键区域的分布,在新造船设计和建造阶段对高应力关键区域以及疲劳关键区域进行识别、优化和质量控制,并提出营运阶段的监控要求,以保证船体结构的安全和使用寿命。