基于混合时/频域分析与长期海洋环境追算方法的系泊系统极值与疲劳分析及其可靠度性研究

Fully coupled time-domain analysis, whereby the dynamic motions of the FPSO/mooring are simulated together at every time step, is prohibitively time consuming. Under certain assumptions the fully coupled analysis can be simplified to some extent, e.g. mooring/riser dynamically coupled to the FPSO low frequency motion. However, such an approach simulated in time domain will still be very expensive as the mooring dynamic response needs to be computed at every time step, and moreover the time step has to be very small for the mooring. For the FPSOs, one of the key coupling mechanisms between the floater and the mooring/riser is the damping effects of the latter and its impact on FPSO's low-frequency motion and its associated extreme offset and maximum line tension. In the traditional statically-coupled method, which is still the main design tool, particularly for initial mooring designs, the FPSO low frequency motion is computed by taking into account the mooring/riser stiffness, but the mooring/riser damping contribution has to be empirically estimated. In deepwater the traditional design approach based upon extreme load case analysis may not necessarily produce safe designs. Probabilistic design provides a more rational basis. How much the hydrodynamic coefficients would affect the design outcomes is best quantified on that basis. The proposed work will focus upon the following aspects: o Hybrid time and frequency domain analysis. The low-frequency motion of the FPSO will be computed in the time domain, whilst the wave frequency response of the FPSO/mooring at a given mean offset position will be conducted in the frequency domain. The frequency domain analysis will be based upon a specific linearization approach where the damping to the low frequency FPSO motion from the wave frequency response of the mooring/riser can be accounted for in the form of an increased mean tension. o Simulation over long term hindcast environments. The joint probability distribution of the environmental parameters will be established from the hindcast data. An appropriate severity threshold is first determined and only environments above this level will be analysed. Sensitivity of the overall results to the threshold will be assessed. o Probability distribution of extremes. It is envisaged at this stage that the work will start with simpler models and progressively move to more complex ones. For example, only the wind-generated wave parameters of the sea-state will be considered to start with before incorporating concurrent swells, currents and wind. o Reliability analysis. Based upon the reliability approach, analyse the probabilities of strength and fatigue failures, respectively. In particular, the effects of the hydrodynamic coefficients and their uncertainties on the extreme loading and fatigue life will be investigated.

本项目针对传统系泊系统时域耦合分析计算量较大的问题，建立快速有效的系泊系统的分析方法- - 混合时/频域分析方法。为获得系泊系统长期（在整个生命周期下）更真实的动力响应和极值响应，采用海洋环境追算方法对过去长期海况进行模拟。对系泊系统设计的极值与疲劳两个关键问题进行研究：传统的极值设计方法结果不一定安全，采用概率的方法分析系泊系统的极值；考虑低频成分和波频成分的运动对系泊系统的影响，研究其疲劳寿命分析方法；并分析讨论水动力系数对系泊系统极值分布及疲劳寿命的影响。考虑随机因素对系泊系统极值与疲劳寿命的影响，对系泊系统进行可靠性分析，研究分析水动力系数对系泊系统结构可靠性的影响。上述研究可以建立更合理有效的系泊系统极值与疲劳分析及其可靠性分析方法。揭示水动力系数对系泊系统结构的极值、疲劳寿命及其可靠性的影响，可为系泊系统设计分析的水动力系数的选取奠定理论基础。

1. 项目背景.系统是深水浮式系统的重要组成部分，其设计要考虑到两个问题：一方面要有足够的强度和刚度以保证系泊结构的漂移量在任何情况下都不超出给定的范围；另一方面要保证足够的疲劳强度以保证系泊缆索在复杂交变环境载荷作用下不会疲劳失效。系泊系统的失效将破坏浮式结构的定位，从而可能导致浮式系统其他组成部分（如立管系统等）的连锁失效。从以往实际工程项目中可知，系泊缆索失效的概率较高，从而引起了经济及人员生命的损失。.系泊系统在其生产和服役过程中存在许多不确定因素。其中水动力载荷的计算依赖水动力系数的选取，水动力系数的选取较复杂（主要根据结构形式和流的性质）存在不确定性；传统的设计大多是确定性分析，没有考虑这些随机不确定性因素会影响，这使得原来分析的结果并不安全。在系泊系统的分析和设计中，应该考虑水动力系数参数的不确定性。.2. 主要研究内容.1）.采用并行计算全时域耦合分析法计算浮体-系泊的耦合响应.2）长期波浪条件追算.3）系泊系统极值的概率分布、疲劳分析和可靠性研究.3. 重要结果及关键数据.1） 对于本课题所研究的南海珠江口盆地某海域，其典型海况为：有义波高1m-5m，谱峰周期6s-10s，浪向为60°-100°。.2） 采用POT法能准确预报系泊缆索及FPSO运动响应的极值，然后POT法的准确性受到threshold选值的影响。除POT法外，ACER法也是一种先进的计算方法，且其准确性不受threshold值的影响。推荐采用20分钟的模拟时间外插至3小时，求解3小时的极值，在高效计算的同时也保证了计算的准确性。.3) 在波浪传递主方向上的系泊缆索疲劳损伤最为严重。.4) 法向拖曳力系数对系泊缆索的疲劳寿命影响最大，该结论适用于系泊系统中的所有系泊缆索。而对于极值而言，切向拖曳力系数对系泊缆索的极值影响最大。..4. 科学意义.1） 首次深入讨论了水动力参数对系泊缆索极值及疲劳的影响.2） 提出了采用Kriging 响应面与Monte Carlo相结合的方法计算系泊缆索的极值可靠性及疲劳可靠性。