深海立管在参激和涡激联合作用下的模型实验及数值模拟

Deepwater risers play an important role in the deep sea oil and gas resources development system. The behavior of risers subject to various excitations must be predicted in order to take strength and fatigue failures into account. This is to ensure the safety of risers in harsh environments during its long service life. Parametric excitation representing the cyclical changes of top tension in the riser caused by floater’s heave motion, as well as vortex-induced vibration (VIV) by ocean current, are the main sources of a riser’s vibration leading to fatigue damage. In deep water, the response of a riser due to VIV may be in high modes and further coupling of structure’s vibration in axial and transverse planes tends to be stronger. Hence, the axial parametric excitation that is time variant makes it more difficult to predict the dynamic response. This research is aimed at exploring the vibration characteristics of a riser subject to both parametric excitation and VIV. A number of model tests will be carried out in the towing basin using homemade but innovative experimental devices. The focus will be on developing parametric excitation devices and on VIV simulation schemes. Meanwhile, the numerical simulations based on the wake oscillator model will be applied to find the empirical parameters and fits. By comparing the response of the riser caused by VIV and the combined excitation (VIV+ parametric excitation), the effects of parametric excitation on VIV in various conditions are identified. The particular interest will be on riser vibration patterns in both in-line and cross flow directions. This research will help to explore the mechanism of riser vibration subject to combined axial and transverse excitations.

深海立管是深海油气资源开发系统的关键装备，由于所处环境恶劣，服役时间长，为确保立管的安全，出于对其强度与疲劳考虑，须对各种激励作用下的振动进行模拟。浮体升沉运动引起的立管顶张力周期性变化导致的参数激励和海流经过立管产生的泄涡是诱发立管振动及其疲劳损伤的主因。随着水深增加，海流可能引发立管的高阶模态及多模态的振动响应，且结构自身轴向和横向存在耦合效应，时变轴向参数激励的存在将使立管动态响应的预测更加困难。本项目以探索参激和涡激联合作用而导致立管的振动特性为研究目标，设计一套可以施加顶端参数激励的立管模型实验装置，在海洋工程水池展开一系列拖曳实验研究。同时，结合基于经验的尾流振子模型进行数值模拟，拟合尾流振子模型中的经验参数，通过比较立管在单纯涡激作用和参激-涡激联合作用下顺流向和横流向的振动响应，分析不同参数组合工况下参数激励对涡激振动的影响，阐明联合激励作用下深海立管动力响应的内在机制。

深海立管在服役期内承受多种荷载作用，求得其振动响应及疲劳寿命是设计中的一个重要环节。在动力学中，上部浮式平台的升沉运动会引起立管顶张力产生周期性的变化，称为参数激励；同时，海流绕过立管时会引发周期性泄涡，产生涡激振动。本项目从试验和数值仿真两方面研究了参激和涡激联合作用下的立管振动特性。在试验方面，进行了长细比为300的立管模型水池拖曳试验。试验中的参激加载装置通过伺服电机驱动滚珠丝杠的方法实现，而海流则通过拖车的水平运动来模拟。通过实测数据比较了涡激单独作用时和参激和涡激联合作用时立管的振动响应，发现当参激频率接近“锁定”频率时会显著增大涡激响应的幅值，并且参激的加入可引发立管控制模态的改变，促使控制模态向低阶迁移。在数值方面，根据Floquet理论，通过应用扩展精细积分方法计算状态转移矩阵，可以精确快速地确定参数激励作用时引发立管响应不稳定的区域；项目还研究了立管在不稳定区的动态响应，将其区分三种不同模式。此外，分别采用轴向和横向非线性耦合模型与传统的欧拉梁模型，对立管受轴向和横向联合激励下的响应进行了对比，发现线性模型由于忽略了横向和轴向的耦合作用，其仿真结果偏小。研究还通过采用改进的尾流振子模型，对参数激励和涡激联合作用下的立管响应进行数值模拟，并与单独激励作用下的响应对比，结果表明在参数激励频率接近泄涡频率时，会增大涡激响应的幅值；而当参数激励幅值较大时，立管的响应被参数共振主导，而流引发的周期性泄涡频率则被扰乱。