﻿ 基于雨流计数法的锚泊线疲劳强度评估
 舰船科学技术  2016, Vol. 38 Issue (12): 59-64 PDF

Evaluation on fatigue strength of mooring line based on rain-flow count method
LIU Juan, SUN Zheng, LIU Jing-xi
College of Naval Architecture and Ocean Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Abstract: The mooring system of FPSO composed of multiple mooring lines. It is critical to evaluate the fatigue strength of the mooring line to guarantee the safety of the structure. In this paper improved rain-flow count method, T-N curve and Miner linear fatigue cumulative damage theory were used to evaluate the fatigue performance of mooring line. From the analyses of the fatigue damage of mooring line, we can get the information of the disadvantageous position, which provide important basis for the maintenance of FPSO mooring system.
Key words: rain-flow count method     mooring line     fatigue strength     stress-time curve
0 引 言

1 疲劳强度评估步骤及原理 1.1 疲劳强度评估步骤

 图 1 锚泊线疲劳损伤评估过程 Fig. 1 The evaluation process of fatigue strength
1.2 雨流计数法程序的实现方法

1）峰谷值点的提取

 $({P_i}-{P_{i-1}})({P_i}-{P_{i + 1}}) ＜ {\rm{0}}{\text{，}}$ (1)

2）循环数的提取

① 升式。如果abbdca，则记录一个全循环bcb′，如图 2 所示。得到的幅值为Sa=|b-c|/2，平均值为 ${S_p}{\rm{ = }}\left| {b + c} \right|/2$

 图 2 波形 1 Fig. 2 Up waveform

② 降式。如果abbdca，则记录一个全循环bcb′，如图 3 所示。得到的幅值为Sa=|b-c|/2，平均值为 ${S_p}{\rm{ = }}\left| {b + c} \right|/2$

 图 3 波形 2 Fig. 3 Down waveform

1.3 锚泊线的 T-N 曲线

Miner 理论中使用的 S-N 曲线[10] 是表示一定循环特征下标准试件的疲劳强度与疲劳寿命之间关系的曲线，通常它是由大量实验结果来确定的。在锚泊线的疲劳分析中一般使用的是 T-N 曲线，即只考虑张力作用产生的疲劳，而不考虑弯曲、扭转等别的作用因素，其计算公式为：

 $N{R^M} = K{\text{。}}$ (2)

1.4 Miner 线性累积损伤理论

 $D = \sum\limits_{i = 1}^{i = n} {{D_i}} {\text{，}}$ (3)

 $L = \frac{{\rm{1}}}{D}({\text{年}}){\text{，}}$ (4)

 ${D_i}{\rm{ = }}\frac{{{n_i}}}{K}ER{\rm{[}}_i^M{\rm{]}}{\text{。}}$ (5)

2 锚泊线疲劳损伤评估算例 2.1 算例分析

 图 4 FPSO 系泊系统锚泊线布置 Fig. 4 The arrangement of mooring lines

2.2 计算结果 2.2.1 锚泊线张力时间历程及疲劳载荷谱

 图 5 缆线 L2 张力响应时历曲线 Fig. 5 The tension response of Mooring lineL2

 图 6 典型海况下的疲劳载荷谱 Fig. 6 The fatigue load spectrum of typical ocean condition

2.2.2 长期海况下的累积疲劳损伤计算

3 结 语

1）通过计算 64 个短期海况下 FPSO 系泊系统的疲劳损伤并累积起来，可以发现锚泊线的疲劳损伤主要发生在钢链部分，钢链的疲劳损伤远大于钢缆的疲劳损伤。在 1 年的长期海况条件下，钢链的疲劳损伤约为钢缆疲劳损伤的 1 643 倍。这表明钢缆的疲劳性能要显著优于钢链的疲劳性能。在实际工程应用中，钢链的疲劳情况需要引起特别注意。在它的日常维护中，应重点注意钢链的维护保养。

2）在计算每个短期海况条件下的疲劳损伤时可以发现，较小的张力范围引起的疲劳损伤几乎可以忽略不计。在累积计算长期海况条件下的疲劳损伤时可以发现，若干波高Hs 较小的短期海况所引起疲劳损伤也可以忽略不计。

3）在不考虑各短期海况分布概率的影响，钢链与钢缆在各个短期海况条件下的疲劳损伤如图 7 所示。可以发现，在同一短期海况条件下，钢链的疲劳损伤也要远大于钢缆的疲劳损伤。在跨零周期不变Tz 的情况下，随着波高Hs 的增大，FPSO 的运动变得剧烈会导致钢链或者钢缆的疲劳损伤的增大。

 图 7 疲劳损伤随海况变化图 Fig. 7 The fatigue damage in different ocean condition

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