Identification of linear and non-linear multi-modal VIV responses for flexible deepwater risers
Identification of linear and non-linear multi-modal VIV responses for flexible deepwater risers
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发表时间：
2005
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通讯作者：
Xiangqun Li
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作者：
Xiangqun Li
近海能源勘探一直在向越来越深的水域发展。对于浮式海上钻井结构，隔水管系统是一个关键要素。涡激振动（VIV）是深水立管开发的主要问题，因为涡激振动是立管疲劳损坏的主要原因。对于深水平台，水流是引起涡激振动响应的主要因素。由于水深增加，深水平台具有长而灵活的结构，因此它们有可能受到非常高的振动模式，即多模态涡激振动。涡激振动响应的频率、振幅和模态通常是深水隔水管设计的重点，因为它们与隔水管材料特性一起沿着直接决定隔水管的疲劳寿命。近年来，立管涡激振动响应的研究取得了很大的进展，但仍存在许多不确定性，特别是对于具有多模态涡激振动响应的立管。例如，对于多模态涡激振动响应，频率锁定现象和模态共振仍然没有完全理解;在文献中找不到具有多模态涡激振动的柔性立管在立管长度上的振动形状和横截面中的运动轨迹。多模态涡激响应中包含的频率和振型分量在顺向和横流方向上都没有在以前的工作中发表。--本研究旨在提高对水流中多模态涡激振动的认识。研究内容包括：（1）多模态涡激振动响应的频率特性，如频率-流速关系和频率锁定现象;（2）多模态涡激振动响应的振幅特性，如振幅-流速关系、振幅范围和振幅共振; iii）多模态涡激振动响应的频谱特性，如主频率，多模态涡激振动响应的模态特性，如模态分布、主模态和模态与流速的关系; v）静水中柔性立管的模态系统参数，包括模态质量、模态阻尼、模态刚度和非线性阻尼;以及vi）模态参数与VIV响应之间的相关性。--在回顾了有关涡激振动研究的最新文献后，提出了一种用于本研究的实验方法。在原型立管的基础上，根据立管的质量、弯曲刚度和频率比的相似性，设计了一个长度畸变的立管模型。设计并进行了两个模型隔水管试验。第一个是振动台激振试验，旨在研究模态系统参数。利用振动台对隔水管进行激励，测量隔水管的响应。基于振动台/立管系统的简化控制方程，从频率响应函数估计模态系统参数。模态分析用于估计线性模态系统参数，Bendat的技术用于估计柔性立管的非线性阻尼。--另一个模型立管试验是电流激励试验。本试验旨在研究水流中的涡激振动响应。均匀流是由拖曳小车产生的。采用16对加速度计测量立管上16个位置的涡激振动响应。谱分析和模态分析是分析测量数据的两种主要工具。--发现横向和纵向多模态涡激振动响应的频率遵循柔性立管的Strouhal频率。测得的Strouhal数约为0.12。在某些模态固有频率下，顺流和横流方向都出现了频率锁定现象。柔性立管在顺流和横流方向上的振动幅值在0.3 ~ 0.9D（D为立管直径）之间波动，不存在随流速增加而增大的趋势。共振不强，因为振动能量由几个模式共享。柔性立管的涡激振动响应包含许多模态分量，但有一个或两个模态占主导地位。--还发现，柔性梁具有各种固有频率，对应于各种振型。模态固有频率取决于张力。估计的附加质量系数Ca取决于柔性立管的张力和振型，范围从1.0到3.7，并且估计的阻尼系数Cd具有相对较大的分散性，范围从0.5到2.5。模态参数估计值与涡激响应之间存在一定程度的相关性。
Offshore energy exploration has been moving into ever increasing water depths. For floating offshore drilling structures, the riser system is a crucial element. Vortex-induced vibration (VIV) is a major concern for deepwater riser developments, as vortex-induced vibration is a major cause of riser fatigue damage. For deepwater risers, current is the dominant factor causing VIV responses. Due to the increased water depth, deep-water risers have long and flexible structures, so that they have the potential to be subject to very high modes of vibration, i.e. multi-modal VIV. The frequencies, amplitudes and modes of VIV responses are usually the focus of deepwater riser design, as they, along with riser material properties, directly determine the riser fatigue life. In recent years, much effort has been devoted into the investigation of riser VIV response, but there are still many uncertainties, especially for the risers with multi-mode VIV responses in currents. For example, frequency lock-in phenomena and modal resonances are still not fully understood for multi-modal VIV responses; the vibration shapes over riser length and the motion trajectories in the cross-sectional plane for a flexible riser with multimodal VIV cannot be found in the literature. The frequency and mode components contained in the multi-modal VIV responses in both in-line and cross-flow directions have not been published in previous work. -- This research aims to improve the understanding of multi-modal VIV in currents. The research objectives include i) frequency characteristics for multi-modal VIV responses, such as frequency versus current velocity and frequency lock-in phenomenon; ii) amplitude characteristics for multi-modal VIV responses, such as amplitude versus current velocity, amplitude range and amplitude resonance; iii) spectral characteristics for multi-modal VIV responses, such as dominant frequencies, power spectrum versus current velocity and power spectrum versus location on the riser; iv) modal characteristics for multi-modal VIV responses, such as modal distribution, dominant mode and mode versus current velocity; v) modal system parameters for a flexible riser in calm water, including modal mass, modal damping, modal stiffness and non-linear damping; and vi) the correlation between the modal parameters and the VIV responses. -- After a review of the state-of-the-art literature involving VIV investigation, an experimental method was proposed for this research. Based on a prototype riser, a length distorted model riser was designed with similarity of the mass, the bending stiffness and the frequency ratio. Two model riser tests were designed and conducted. The first one was a shaker-excitation test, which was designed to investigate the modal system parameters. A shaker was used to generate an excitation to the riser, and the riser responses were measured. The modal system parameters were estimated from the frequency response functions based on a simplified governing equation for the shaker/riser system. Modal analysis was used to estimate the linear modal system parameters, and Bendat's technique was used to estimate the non-linear damping for the flexible riser. -- Another model riser test was a current-excitation test. This test was designed to investigate the VIV responses in currents. The uniform currents were generated by towing carriage. Sixteen pairs of accelerometers were used to measure the VIV responses at sixteen locations on the riser. Spectral analysis and modal analysis are two major tools to analyze the measured data. -- It was found that the frequencies of the multi-modal VIV responses in both the crossflow and in-line directions follow the Strouhal frequencies for a flexible riser. The measured Strouhal number was about 0.12. The frequency lock-in phenomena occurred in both the in-line and cross-flow directions at some modal natural frequencies. The vibration amplitudes fluctuate from 0.3 to 0.9D (D denotes the riser diameter) in both the in-line and cross-flow directions for the flexible riser tested, and no increasing trend existed as the current velocity increases. The resonances are not strong as the vibration energy is shared by several modes. The VIV responses for a flexible riser contained a number of modal components, but one or two were the dominant modes. -- It was also found that flexible risers have a variety of natural frequencies, which correspond to a variety of mode shapes. The modal natural frequencies depend on the tension. The estimated added mass coefficients Ca depend on the tensions and mode shapes for a flexible riser, ranging from 1.0 to 3.7, and the estimated damping coefficients Cd have a relatively big scatter, ranging from 0.5 to 2.5. There is a degree of correlation between the estimated modal parameters and the VIV responses.