轴向载荷波动下海上测试管柱动力响应与安全系数分析

Analysis on dynamic response and safety coefficient of offshore testing string under axial load fluctuation

为研究海上测试管柱在作业过程中轴向载荷波动对管柱强度和变形的影响,针对泥线上管柱的作业特点,对适应于简化条件下的泥线上管柱的轴向力计算方法及振动模型进行了推导;基于建立的管柱有限元模型及其固有频率分析结果,应用Workbench进行了不同频率、不同水深、不同波动幅度轴向力下测试管柱的动力响应分析及对应的安全系数计算。研究结果表明:轴向力三角形波动一定周期后,响应参数趋于稳定,波动幅度对管柱的响应结果影响不大;同幅度载荷变化下,正弦波动时的最大应力、最大变形响应数值较三角形波动时要小;随着水深的增加,最大变形、最大应力的数值均明显减小,二者响应规律基本一致;在基频不同倍数的波动下,波动频率对响应频率、稳定前的时间占比、幅度等影响明显;测试管柱在轴向波动载荷作用下的强度均满足材料的使用安全系数,但正弦波动时管柱的变形与应力始终随轴向力的变化而变化,正弦波动下的管柱易出现周期性疲劳破坏。

In order to study the influence of axial load fluctuation on the strength and deformation of offshore testing string during the operation process, the calculation method of axial force and vibration model of the testing string above the mud line, which were adapted to the simplified conditions, were deduced according to the operation characteristics of the string a- bove the mud line. Based on the established finite element model of the testing string and the analysis results of its natural frequency, the dynamic response analysis and the corresponding safety coefficient calculation of the testing string under the axial force with different frequencies, water depths and fluctuation amplitudes were carried out by using Workbench. The results showed that the response parameters tended to be stable after a certain period of axial force with the triangular fluctuation, and the fluctuation amplitude had little effect on the response results of string. Under the same amplitude of load change, the response values of the maximum stress and the maximum deformation under the sine fluctuation were smaller than those under the triangular fluctuation. With the increase of water depth, both the maximum deformation and the maximum stress decreased obviously with the basically same response laws. Under the fluctuation with different times of natural frequency, the fluctuation frequency had an obvious effect on the response frequency, the proportion of time before the stabilization, the amplitude and others. The strength of the testing string under the fluctuating axial load could meet the usage safetycoefficient of the material, however, the deformation and stress of the string always changed with the change of axial force under the sine fluctuation, so the cyclic fatigue failure of string under the sine fluctuation was easy to occur.