深海平台大幅慢漂运动的两次展开求解方法

发布时间：2018-01-15 05:00

本文关键词：深海平台大幅慢漂运动的两次展开求解方法　出处：《大连理工大学》2017年博士论文　论文类型：学位论文

【摘要】：浮式海洋平台(Spar,TLP,FPSO)在海上作业过程中会经受随机波浪、风和水流等荷载的联合作用。特别在不规则波浪下,各个频率的波浪成分相互作用后会产生差频以及和频的高阶波浪成分,其中波浪差频往往会接近海洋平台不同运动模态的自振频率,从而引起结构的大幅低频运动,这种大幅低频运动会减少甚至终止有效钻井作业;而高频项会引起结构高频垂向振动响应,进而引起系泊缆索结构以及采油立管结构的疲劳问题。基于上述背景,本文在势流理论的框架下建立了全时域的波浪与结构物相互作用的两次展开数值模型。本文以二维、三维拉普拉斯方程为基本控制方程建立了二阶时域波浪与结构物相互作用的两次展开二维、三维数值模型。物体的运动响应被分解为低频振荡和高频振荡两部分,其中低频振荡部分的运动响应随着计算的进行实时得到,称之为低频平均位置,随后,物面边界条件和自由水面边界条件在低频平均位置和和静水面进行泰勒展开,物面网格和自由水面网格也随着低频平均位置时时平移,此时采用摄动展开分离级数的方法建立相应的一阶、二阶边值问题,从而保证了这个摄动展开的参数满足的小振幅的基本假设。速度势分解为入射势、散射势以及定常扰动速度势,其中散射势和定常扰动速度势通过格林第二定理建立相应的积分方程求得。随后推导了波浪力的计算公式,建立了物体的运动方程。求解出物体运动响应之后,利用小波变换方法再次展开求出运动响应的低频平均位置,作为下一时刻泰勒展开和摄动展开的平均位置,再进行摄动展开和泰勒级数展开,周而复始,直到计算时间满足要求。从而实现时域内结构物大幅慢漂运动的求解。为了验证两次展开数值模型,本文首先验证了平均位置模拟方法的正确性。通过与其他模拟方法的比较,建议采用基于小波变换的滤波方法来求解平均位置,而且通过计算可以发现,只要选取了恰当的小波尺度,即使使用不同的小波基函数,我们都可以获得任意时间序列的平均位置。然后分别通过计算了二维浮体慢漂运动的运动响应、三维直立圆柱匀速前进运动时受到的波浪力和三维漂浮方箱慢漂运动的运动响应来验证本模型,通过与其他学者解析方法、实验数据以及完全非线性方法的对比,证明了本模型在大幅慢漂运动中的计算结果是正确的。接下来分别采用两次展开方法和基于初始平衡位置的摄动展开方法计算了简单结构物在单向波浪中发生大幅运动时的波浪力和运动响应,两种方法的计算结果相比有较大区别,证明两次展开方法的正确性。在计算单色波浪中圆柱的大幅强迫运动时,此时入射波浪与结构物之间的遭遇频率会时时发生变化,从而导致波浪力的幅值和相位也发生变化,这些变化在两次展开方法得到的计算结果中均有明显体现,而在直接展开方法的计算结果中却无法获得。接着计算双色波与线刚度约束下截断圆柱的耦合作用,比较了两种方法计算产生的运动响应,分别研究了双色波波幅差和角频率对运动响应的影响。发现采用两次展开方法得到的计算结果与采用直接展开方法得到的运动响应有较大区别,并进行了相应的解释,证明了两次展开方法更能反映结构物在大幅慢漂时的运动特征,更有实际意义。本文最后一部分介绍了两次展开方法数值模型在实际海洋平台中的应用,分别计算了双色波与Spar平台的耦合作用以及风和不规则波与半潜平台的耦合作用。计算结果包括了平台受到的波浪力、平台的运动响应、系泊缆索顶端张力以及波面三维等值线图等。通过研究双色波浪与Spar平台的耦合作用,发现由于波浪漂移阻尼的影响,海洋平台大幅慢漂运动时自振频率向低频运动偏移,而这种现象在直接展开方法的结果中无法体现,从而说明采用直接展开方法计算大幅慢漂运动是不准确的;最后通过研究风和不规则波与海洋石油981半潜平台的耦合作用,对海洋平台小幅运动与大幅运动时两种方法的计算结果进行了对比分析,证明了在大幅运动时,两次展开方法得到的结果更加可靠,并据此针对半潜平台在实际海浪中的布置给出了建议。
[Abstract]:Floating platforms (Spar, TLP, FPSO) in the offshore operation process will be subjected to random wave, the combined effects of wind and current loads. Especially in irregular waves, high order wave component of each frequency wave component interaction will produce difference frequency and frequency, the wave frequency difference frequency often close to the ocean platform in different movement modes, thus causing a low frequency motion structure, such a low frequency motion can be reduced or even termination of drilling operations; and a high frequency can cause high frequency vertical vibration response, and cause fatigue problem of mooring cable structure and oil riser structure. Based on the above background, in the framework of this article the potential flow theory was established under the two expansion numerical model of wave diffraction and full time domain interaction. In this paper, two-dimensional, three-dimensional Laplasse equation to establish order two The two launched a two-dimensional time-domain wave structure interaction, a three-dimensional numerical model. The motion response is decomposed into two parts of low frequency oscillation and high frequency oscillation, the low frequency oscillation part of the motion response calculation with real time, called the low average position, then the boundary condition and the free surface boundary condition in the low average position and static water Taylor expansion, surface mesh and free surface grid with low average position from time to time shift, the perturbation method of dynamic expansion separation series to establish the first order corresponding to the two order boundary value problem, so as to ensure the basic assumption of small amplitude parameters to meet the perturbation of the. The velocity potential decomposition for incident potential, scattering potential and stationary disturbance velocity potential, the scattering potential and the constant disturbance velocity potential to establish the corresponding integral Fang Chengqiu Green second theorem Then the calculation formula. The wave force is established, equations of motion objects. After solving the motion response, using wavelet transform method to start again for low frequency average position of motion response, as the next time Taylor expansion and perturbation of the average position, then the perturbation and Taylor series expansion. Again and again, until the calculation time to meet the requirements. So as to realize the time domain structure a slow drift motion is solved. In order to verify the two expansion of the numerical model, this paper verifies the correctness of the simulation method of average position. By comparison with other simulation methods, proposed to solve the average filtering method based on wavelet transform, and by the calculation can be found, as long as the selected wavelet scale is appropriate, even using different wavelet functions, we can get the time series average position Then through the calculation of two-dimensional floating body slow motion response drift, to validate the model response of wave force by 3D uniform forward movement of the vertical cylinder and three-dimensional floating box of slow drift motion, and other scholars through analytic method, comparison of experimental data and non linear method, proved this model in the calculation of a slow drift motion in the result is correct. Then using the two expansion method and based on the initial equilibrium position of the perturbation response dynamic expansion method simple structure calculation of large movements occurred in the one-way wave force and motion, the results of the two methods compared with different correctness proof the two expansion method. In a forced motion in wave calculation of cylindrical monochromatic, encounter frequency between this incident wave and structures will always change, resulting in The amplitude and phase of wave force changes, these changes are in the calculation results obtained in the two expansion method is evident, but not in the results of direct expansion methods. Then calculate the coupling effect of the truncated cylinder bichromatic wave and line stiffness under the constraint of response compares two methods of calculation the effect of exercise, double color difference wave amplitude and angular frequency of motion response are studied. The calculation results of two expansion obtained by direct expansion method and the motion response is quite different, and the corresponding explanation, proved the two expansion method can reflect the structure of the motion feature a slow drift of more practical significance. The last part of this paper introduces the application of the two expansion method in practical numerical model of offshore platform, Spar platform and bichromatic wave coupling were calculated. Cooperation and irregular waves and wind and semi submersible platform coupling effect. The results including the wave force by the platform, the platform motion response and mooring line tension at the top and three-dimensional wavefront contour map. By coupling of bichromatic waves and Spar platform, found that due to the impact of the wave drift damping of the ocean a slow drift motion platform vibration frequency to the low frequency motion deviation, and this phenomenon in direct expansion method can not reflect the results, so that the calculation by direct expansion method a slow drift movement is not accurate; finally, through the research of irregular waves and wind and ocean petroleum 981 semi submersible platform coupling calculation the offshore platform slightly movement and large amplitude motion and the results of the two methods were compared, proved in a campaign, two expansion method get more reliable results, and according to the needle The layout of the semi submersible platform in the actual sea waves is given.

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P751

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