FPSO串靠外输系统动力分析

FPSO串靠外输方式因其对环境的适应性好,安全性较高,是FPSO原油外输的主要方式。该文考虑了风、流和系泊缆索的耦合动力,建立了FPSO串靠多浮体系泊系统的时域耦合运动模型,其中浮体时域水动力系数由频域的结果通过快速傅立叶变换转换得到。数值求解时,浮体时域运动方程采用Newmark-β法,系泊缆索采用时域非线性有限元方法。基于自主开发的Fortran计算程序对南海海域一艘张紧式系泊FPSO与其配套的躺底式系泊穿梭油轮及两船间系泊大缆组成的串靠输油系统进行了理论计算,得到了FPSO及穿梭油轮的纵荡运动响应,系泊缆索上端点张力,系泊大缆张力等时间历程。通过谱分析讨论了低频波浪力和波频波浪力对FPSO串靠系泊系统的作用。同时探究了风向和流向对FPSO串靠系统的影响。该串靠多浮体系泊系统耦合动力模型能合理解释物理现象,可用于分析实际作业的串靠外输系泊系统。     

系泊系统动力分析

对于海上定位采用的系泊系统,采用时域有限元法建立了缆索方程的数值模型。以单点系泊和多点系泊的模拟比较为例。对于单点系泊,通过给定上端点正弦运动,对锚泊线进行动力计算,与试验结果进行了比较。对于多点系泊,针对一Spar平台的张紧系泊系统进行了理论计算,并将计算得到的上端点张力时历及统计结果与相应的模型试验结果进行了比较。通过比较可以发现理论计算结果与模型试验结果吻合良好。该数值模型可用于不同水深下平台和系泊系统的耦合计算等。     

系泊系统对海上浮式风机运动响应的影响分析

通过建立一种海上浮式风机运动响应的时域耦合数值计算方法,分析系泊系统对于运动响应的影响。该计算方法由气动力模块、水动力模块、系泊模块和浮体运动求解模块组成。气动力模块采用叶素动量法;水动力模块采用势流理论,通过边界元法进行数值模拟;系泊模块采用悬链线模型,利用Chebyshev多项式拟合计算;浮体运动求解模块采用Runge-Kutta法进行时域计算。根据对OC3-Hywind spar浮式风机进行建模求解,分析了系泊系统对于浮式风机运动响应的影响。     

深海浮式风机在波浪上的运动响应分析

浮式风机系统在作业中受到空气动力、波浪力和系泊系统系泊力的共同作用。该文建立了一种时域耦合分析方法对浮式风机系统进行全耦合运动响应分析,主要包括空气动力模块、水动力模块和系泊系统模块。空气动力模块采用CFD方法模拟风机叶轮受到的风载荷,并通过NARMAX理论建立风载荷模型;水动力模块采用势流理论,通过自由面GREEN函数法计算浮体的频域水动力系数,再将结果转换到时域;系泊系统模块采用锚泊线理论,用三维有限元方法对挠性部件进行分析;系统的运动采用Newmark-β方法求解。在此基础上,针对一深海Spar型浮式风机系统建立耦合模型并进行计算,得出风载荷模型、平台的水动力系数以及在脉动风下浮体运动响应和系泊缆索张力响应。经验证,该耦合模型的计算结果可为风机和系泊系统的设计提供一定的参考依据。     

“世越号”提升阶段双抬浮驳船系泊动力的计算

"世越号"是利用水面双驳船作为基础用安装在驳船上的液压提升系统打捞出水的沉船。在"世越号"被提升离开海底后,三条船的环境载荷都由两条驳船的系泊系统承担。两条驳船平行布置在"世越号"两侧,距离较近,有较强的多体水动力耦合效应。计算三条船的总环境载荷并不是单条驳船环境载荷的简单叠加。在"世越号"打捞工程设计阶段,进行了大量水池模型试验。应用试验结果,建立了含有矫正系数的多体耦合运动计算模型。用矫正的多体耦合运动计算模型算出的船舶运动与水池模型试验结果吻合度很高。应用矫正的多体耦合运动计算模型以及准静态系泊系统计算模型对整个"世越号"打捞系统进行时域计算,得到的系泊缆动态拉力与模型试验结果和现场实测值吻合得比较好,稍有保守。这套系泊力计算方法对于设计出的系泊系统是偏保守的,在设计阶段是比较合理的。     

单点系泊船舶的动态分岔特性研究

研究了潮流作用下单点系泊船舶的动态分岔特性。应用高维微分动力系统Hopf分岔的直接代数判据，得到了潮流作用下单点系泊系统发生Hopf分岔的充要条件，并应用于油轮系泊运动的Hopf分岔研究。以潮流速度和系缆长度为控制参数，在参数平面上给出了Hopf分岔集，应用数值仿真计算检验了理论分析结果，两者相当吻合。由此在参数空间中确定了潮流作用下单点系泊船舶的非线性动力学特性，为系泊系统设计参数的选择提供了理论依据。     

平行靠泊平台水动力响应研究

船舶在海上过驳作业时,多浮体之间往往相互靠泊,形成狭窄水域。在环境载荷的作用下,可能激发流体和浮体的耦合运动,造成靠泊困难。该文针对浅水海域采用坐底平台过驳卸载的方案,评估了平行布置靠泊船之间的水动力干扰,设计了靠泊船只的系泊方案,并在时域中进行了水动力响应分析,得出了靠泊浮体的运动响应以及坐底平台的坐底稳定性和抗滑移性能。结果表明：设计系泊方案能够有效地旁靠两艘浮体,并且结构物之间的相对运动在较小范围内,系泊缆绳的张力值和防撞垫的受力也都在容许范围内。此时,坐底平台承受了较大的水平推力作用,需要较大的压载来增加其湿重,以实现水平抗滑移能力。该工作为浅水海域平台靠泊时的水动力分析和系泊设计提供了相关参考。     

狭窄水域中单点系泊/拖带船舶动力学行为的试验研究

对单点系泊／拖带船舶在狭窄水域中的动力学特性进行了试验研究。找到了船模的四种稳定运动模式；探讨了不同来流速度和系缆长度对于船只的动力学响应和系缆力的影响。试验表明，船模运动呈现出典型的非线性特征，观察到了吸引子的共存、跳跃和Hopf分岔现象，系缆张力的大小与船舶的动力学行为密切相关，振转运动(鱼尾状摆动)发生时的系泊力远大于船只静止时的缆绳张力；振转运动中的突发系缆载荷可能是导致缆绳断裂的重要原因。与宽广水域相比，狭窄水域中的系泊／拖带船只更宜发生振转运动。     

柔性连接多浮体在不规则波中运动的预报方法

本文介绍了一种频、时域杂交法，用于预报柔性连接多浮体在波浪中的运动。其中采用三维频域法适时求解浮体的水动力系数，在时域内求解浮体的运动方程，可同时求解非线性锚链力、连接器受力和护舷受力。利用本方法预报了柔性连接的“口”字形六浮体系统在波浪中的运动，结果与试验结果具有较好的吻合程度。     

浮托驳船系泊定位的数值模拟与模型试验研究

浮托法是一种大型组块安装方法,具有成本低及耗时少等优点,近些年来被广泛地应用于各种海洋平台的安装中.该方法是通过驳船托举平台的上部组块,并在系泊系统的帮助下就位于平台下部结构的正上方,然后通过增加驳船吃水将组块的重量缓慢转移到平台上.该文以一座导管架平台的浮托安装为例,选用一艘T型驳船作为安装船,针对系泊等待状态设计了两种系泊系统,分别从频域和时域两方面进行了数值模拟研究.频域分析借助WAMIT计算程序,得到T型驳船的水动力参数和运动RAO;并基于时频转换理论和准静态锚链力计算方法,使用Fortran自编程序进行时域计算,得到静水下锚泊系统水平刚度和波浪作用下船体运动和锚链拉力.在此基础上开展了相应的模型试验研究,包括静水刚度试验、白噪声波浪试验和不规则波浪试验等,并与数值模拟结果进行对比验证.结果表明:数值计算结果与模型试验结果吻合较好,表明该计算程序有较高的可靠性;通过对两种方案结果的比较,表明方案B较方案A更为安全可靠.     

COUPLING EFFECTS FOR CELL-TRUSS SPAR PLATFORM： COMPARISON OF FREQUENCY- AND TIME- DOMAIN ANALYSES WITH MODEL TESTS

For the floating structures in deepwater, the coupling effects of the mooring lines and risers on the motion responses of the structures become increasingly significant. Viscous damping, inertial mass, current loading and restoring, etc. from these slender structures should be carefully handled to accurately predict the motion responses and line tensions. For the spar platforms, coupling the mooring system and riser with the vessel motion typically results in a reduction in extreme motion responses. This article presents numerical simulations and model tests on a new cell-truss spar platform in the State Key Laboratory of Ocean Engineering in Shanghai Jiaotong University. Results from three calculation methods, including frequency-domain analysis, time-domain semi-coupled and fully-coupled analyses, were compared with the experimental data to find the applicability of different approaches. Proposals for the improvement of numerical calculations and experimental technique were tabled as well.     

COUPLING EFFECTS FOR CELL-TRUSS SPAR PLATFORM: COMPARISON OF FREQUENCY- AND TIME-DOMAIN ANALYSES WITH MODEL TESTS

For the floating structures in deepwater, the coupling effects of the mooring lines and risers on the motion responses of the structures become increasingly significant. Viscous damping, inertial mass, current loading and restoring, etc. from these slender structures should be carefully handled to accurately predict the motion responses and line tensions. For the spar platforms, coupling the mooring system and riser with the vessel motion typically results in a reduction in extreme motion responses. This article presents numerical simulations and model tests on a new cell-truss spar platform in the State Key Laboratory of Ocean Engineering in Shanghai Jiaotong University. Results from three calculation methods, including frequency-domain analysis, time-domain semi-coupled and fully-coupled analyses, were compared with the experimental data to find the applicability of different approaches. Proposals for the improvement of numerical calculations and experimental technique were tabled as well.     

HYDRODYNAMIC BEHAVIOR OF FPSO UNDER VARIUS LOADING IN SURVIVAL STORMS IN SHALLOW WATER

The motion performance and clearance between the hull and seabed of a FPSO (Floating, Production, Storage and Offloading units) in survival storm conditions are closely related to its safety during the operation in shallow water. As an example the behavior of a 160 kDWT FPSO with single point mooring system in shallow water was investigated in this paper. Calculation for the FPSO is made based on the 3-D linear potential flow theory and time-domain numerical simulation method and corresponding model test is carried out in the wave basin at Shanghai Jiaotong University. Both the calculated and experimental results indicate that the heave, roll and pitch motions of FPSO become lazy with the increase of the draft. And the fully loaded FPSO in survival storm touched seabed few times. Therefore, it is concluded that the FPSO should be less loaded than that in the fully loaded condition when the survival storm is coming.     

Time-domain hydrodynamic analysis of pontoon-plate floating breakwater

The hydrodynamic behaviors of a floating breakwater consisting of a rectangular pontoon and horizontal plates are studied theoretically. The fluid motion is idealized as two-dimensional linear potential flow. The motions of the floating breakwater are assumed to be two-dimensional in sway, heave, and roll. The solution to the fluid motion is derived by transforming the governing differential equation into the integral equation on the boundary in time domain with the Green’s function method. The motion equations of the floating breakwater are established and solved with the fourth-order Runge-Kutta method to obtain the displacement and velocity of the breakwater. The mooring forces are computed with the static method. The computational results of the wave transmission coefficient, the motion responses, and the mooring forces of the pontoon-plate floating breakwater are given. It is indicated that the relative width of the pontoon is an important factor influencing the wave transmission coefficient of the floating breakwater. The transmission coefficient decreases obviously as the relative width of the pontoon increases. The horizontal plates help to reduce the wave transmission over the floating breakwater. The motion responses and the mooring forces of the pontoon-plate floating breakwater are less than those of the pontoon floating breakwater. The mooring force at the offshore side is larger than that at the onshore side.     

波流共同作用下单体网格式锚碇网箱水动力特性研究

运用刚体运动学原理和集中质量方法,建立了计算重力式网箱浮架、网衣和锚碇系统受力和运动的数学模型.利用此数学模型获得了波流作用下网箱在网格式锚碇情况下锚碇锚绳受力、浮架运动的计算结果.在此基础上,计算并分析比较了不同设计流速与波况组合作用情况下,单体网箱各类锚绳(锚碇锚绳,连接锚绳和网格锚绳)受力、浮架运动及网衣变形情况.讨论结果表明,在相同工况下,各类锚绳中锚碇锚绳受力最大,连接锚绳受力次之,网格锚绳受力最小.锚碇锚绳、连接锚绳受力随波高、周期、流速的增加有不同程度的增加;网格锚绳受力随波高的增加有一定程度的增加,随流速的增加有较小幅度的减少,与周期变化关系不明显.浮架水平方向运动幅度随波高、周期及流速的增加而有不同幅度的增加;浮架竖直方向运动幅度随波高、周期的增加有不同程度的增加,随流速的增加有较小程度的减小.各种波流工况作用下,浮架倾角随流速的增加有一定程度的减少,随波高的增加有较小程度的增加,与周期变化关系不明显.网衣体积损失率随流速、波高及周期的增加有较小程度的增加.     

THEORETICAL RESEARCH ON HYDRODYNAMICS OF A GEOMETRIC SPAR IN FREQUENCY- AND TIME-DOMAINS

Considering the coupling effects of the vessel and its riser and mooring system, hydrodynamic analyses of a geometric spar were performed both in frequency- and time-domains. Based on the boundary element method, the 3-D panel model of the geometric spar and the related free water surface model were established, and the first-order and second-order difference-frequency wave loads and other hydrodynamic coefficients were calculated. Frequency domain analysis of the motion Response Amplitude Operators （RAO） and Quadratic Transfer Functions （QTF） and time domain analysis of the response series and spectra in an extreme wave condition were conducted for the coupled system with the mooring lines and risers involved. These analyses were further validated by the physical model test results.     

HYDRODYNAMIC INTERACTION BETWEEN FLNG VESSEL AND LNG CARRIER IN SIDE BY SIDE CONFIGURATION

The Floating Liquefied Natural Gas (FLNG) is a new type of floating platform for the exploitation of stranded offshore oil/gas fields. The side by side configuration for the FLNG vessel and the LNG carrier arranged in parallel is one of the possible choices for the LNG offloading. During the offloading operations, the multiple floating bodies would have very complex responses due to their hydrodynamic interactions. In this study, numerical simulations of multiple floating bodies in close proximity in the side by side offloading configuration are carried out with the time domain coupled analysis code SIMO. Hydrodynamic interactions between the floating bodies and the mechanical coupling effects between the floating bodies and their connection systems are included in the coupled analysis model. To clarify the hydrodynamic effects of the two vessels, numerical simulations under the same environmental condition are also conducted without considering the hydrodynamic interactions, for comparison. It is shown that the hydrodynamic interactions play an important role in the low frequency motion responses of the two vessels, but have little effect on the wave frequency motion responses. In addition, the comparison results also show that the hydrodynamic interactions can affect the loads on the connection systems.     

风浪流荷载下基于AQWA的海上Spar型浮式基础稳定性分析

为了解决现阶段可再生能源利用的问题,实现深海风能的有效利用,针对设计出的OC3-Hywind 5MW Spar型浮式平台基础部分在风浪流荷载下的稳定性问题进行研究。利用ANSYS中的水动力模块(AQWA)分别对结构进行频域以及时域耦合运动响应仿真模拟,比较分析了不同工况下浮式结构各个自由度下的运动响应值,以及配重块在系泊不同位置处对结构运动响应值及系泊力的影响情况。结果表明:在不同工况作用下的浮式结构在纵摇自由度方向上的运动响应极值主要受风荷载影响,随着风速增大,响应极值也随之增大,在其他自由度上的响应极值受风、浪荷载影响较小;改变配重块在系泊缆上的悬挂位置对浮式基础在纵摇和首摇自由度方向的运动有着显著影响,并且随着配重块距离锚固点越近,响应值越大,越不利于浮式基础的稳定;未悬挂配重块的系泊力高于悬挂了配重块的系泊力,随着配重块悬挂位置离锚固点越远,系泊力越小,越能保证浮式基础在恶劣海况条件下的稳定性。