Structural Connection Fatigue Evaluation Methodology Using Time Domain Approach

Sagar Samaria, Bob Zhang, S. Tallavajhula, J. Kyoung
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Abstract

There is an ever-increasing demand for life extension of existing floating platforms worldwide. To adequately support these life extension projects there is a need to predict fatigue life of floating structures more accurately using a time domain approach. However, structural fatigue damage calculations using time domain response analysis can be very time consuming and challenging. An efficient and effective structural analysis methodology is developed to calculate accumulated fatigue damage for structural connections in a floating offshore platform using a response-based time domain routine. The methodology discussed in this paper can be applied to estimate fatigue life for hull critical connections in floaters such as Spars, TLPs or Semis as well as local structural supports such as mooring foundations and riser foundations. It also provides the option to generate stress histograms that can be utilized for Fracture Mechanics Evaluation (FME) of welds in structural connections. To calculate the accumulated fatigue damage at desired locations on a floating platform, the time domain analysis employs a Stress Intensification Factor (SIF) which correlates global loads with local stresses. In cases where a crack initiation is observed on a structural connection, fracture mechanics is used to evaluate the structural integrity of the weld. The FME requires fatigue stress range histograms as one of the input parameters. The stress ranges and cycles that are calculated and used to compute the fatigue damage using this methodology can be converted to stress range histograms which can then be used in the FME. The standard method to compute fatigue damage for a structural connection is by using an S-N fatigue approach under the assumption of linear cumulative damage (Palmgren-Miner rule). The methodology discussed in this paper uses a rainflow counting algorithm to effectively calculate the stress range and cycles which are then utilized for computing the fatigue damage. This methodology can be applied to green field projects involving a new design or for life of field studies of an existing platform requiring life extensions. It is particularly beneficial for brownfield projects where more accurate re-evaluation of fatigue life is needed. It can also provide Clients with reliable Engineering Criticality Assessments (ECA) and enable them to plan in-service inspections and repair work. As an application, a typical truss connection for a Spar platform is used to evaluate structural fatigue damage and generate the stress range histogram for FME. Also, a comparative study is performed for a typical truss connection fatigue damage result between the traditional approach used and the method discussed in this paper.
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基于时域法的结构连接疲劳评价方法
世界范围内对现有浮式平台寿命延长的需求日益增长。为了充分支持这些延长寿命的项目,需要使用时域方法更准确地预测浮式结构的疲劳寿命。然而,使用时域响应分析进行结构疲劳损伤计算非常耗时且具有挑战性。提出了一种基于响应的时域程序计算海上浮式平台结构连接处累积疲劳损伤的有效结构分析方法。本文所讨论的方法可用于估算Spars、tlp或semi等浮子的船体关键连接以及系泊基础和隔水管基础等局部结构支撑的疲劳寿命。它还提供了生成应力直方图的选项,可用于结构连接焊缝的断裂力学评估(FME)。为了计算浮式平台上期望位置的累积疲劳损伤,时域分析采用了应力增强因子(SIF),该因子将整体载荷与局部应力联系起来。如果在结构连接处观察到裂纹起裂,则使用断裂力学来评估焊缝的结构完整性。FME需要疲劳应力范围直方图作为输入参数之一。使用该方法计算并用于计算疲劳损伤的应力范围和循环可以转换为应力范围直方图,然后可以在FME中使用。计算结构连接疲劳损伤的标准方法是在线性累积损伤假设下采用S-N疲劳法(Palmgren-Miner规则)。本文讨论的方法采用雨流计数算法有效地计算应力范围和循环,然后用于计算疲劳损伤。这种方法可以应用于涉及新设计的新油田项目,也可以应用于需要延长现有平台寿命的现场研究。对于需要更准确地重新评估疲劳寿命的棕地项目尤其有益。它还可以为客户提供可靠的工程关键性评估(ECA),并使他们能够计划在役检查和维修工作。以某平台典型桁架连接为例,对结构疲劳损伤进行了评估,并生成了FME应力范围直方图。并对典型桁架连接的疲劳损伤结果进行了对比研究。
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