Frequency Domain Fatigue Analysis for a Unbonded Flexible Riser: Damage Induced by Dynamic Bending

Jiabei Yuan, Yucheng Hou, Z. Tan
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Abstract

The service life of flexible risers is a vital design parameter in offshore field development. The standard approach to calculate fatigue life is the nonlinear time-domain analysis. The approach uses time history of riser responses in local structure assessment to get the fatigue damage of tensile layers. Another approach is the linearized frequency-domain analysis. Instead of using time history of stress and rainflow counting technique, the approach uses stress spectrum and empirical mathematical terms to estimate the fatigue damage. The frequency domain approach is significantly faster. However, due to the whole system being linearized, the latter usually produces different results and is considered to be less accurate than the time domain approach. To address this issue, Baker Hughes previously developed an approach which uses the frequency domain technique as base solution and calibration factors from limited time domain cases. The approach is limited to tensile wires at the end fitting entrance where tension and tensile stress is directly linked. In this paper, a similar approach is proposed to be applied for tensile fatigue at all regions, whose tensile stress are induced by a combination of tension, pressure, bending and friction between layers. Since tensile stress is not directly related to any single riser response, the stress spectrum is predicted by using a transfer function. With the predicted stress spectrum, the fatigue damage of each case is calculated with Dirlik’s method and SN curves. The paper summarizes the development of the hybrid frequency domain approach. The fatigue damage of risers from several projects are acquired with both time domain and frequency domain approaches. The approach is significantly faster than traditional time domain approach and produces conservative results. Furthermore, discussions are made on options to improve the approach and reduce the conservatism in the frequency domain fatigue analysis.
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无粘结柔性隔水管频域疲劳分析:动态弯曲损伤
柔性立管的使用寿命是海上油田开发中一个重要的设计参数。计算疲劳寿命的标准方法是非线性时域分析。该方法利用隔水管响应时程进行局部结构评估,得到拉伸层的疲劳损伤情况。另一种方法是线性化频域分析。该方法采用应力谱和经验数学术语来估计疲劳损伤,而不是使用应力时程和雨流计数技术。频域方法明显更快。然而,由于整个系统是线性化的,后者通常会产生不同的结果,并且被认为不如时域方法准确。为了解决这个问题,贝克休斯之前开发了一种方法,该方法使用频域技术作为基本解决方案,并从有限的时域情况中进行校准。该方法仅限于在张力和拉应力直接相关的末端接头入口处的拉伸丝。本文提出了一种类似的方法来应用于所有区域的拉伸疲劳,这些区域的拉伸应力是由张力、压力、弯曲和层间摩擦共同引起的。由于拉应力与任何单个隔水管响应没有直接关系,因此应力谱可以通过使用传递函数来预测。在预测应力谱的基础上,采用Dirlik法和SN曲线计算了各壳体的疲劳损伤。本文综述了混合频域方法的研究进展。采用时域和频域两种方法对几个工程的隔水管进行了疲劳损伤分析。该方法比传统的时域方法速度快,且结果保守。此外,还讨论了在频域疲劳分析中改进方法和降低保守性的选择。
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