自升式平台水平偏移的数值模拟方法研究

Research on Numerical Simulation of Horizontal Offset for Jack-ups

自升式平台的水平偏移会增加作业难度,严重时降低平台的坐底稳定性。基于平台主体发生水平偏移U的原因,将水平偏移分成三部分:桩腿弯曲引起的水平偏移Ub,桩靴转动引起的水平偏移Ur以及桩靴平移引起的水平偏移Ud。利用有限元软件ABAQUS建立圆柱腿自升式平台-地基耦合模型,考虑桩土之间法向挤压、切向摩擦的接触关系,利用Coulomb定律计算摩擦力。利用子程序DLOAD实现波浪力的加载,波浪采用Airy波模型。针对桩周无回填土体、桩周土体未完全固结以及固结完成三个阶段,探究桩靴直径、回填土强度、以及埋置深度对U,Ud和Ub的影响。并与中国船级社《海上移动平台入级与建造规范》(2005)中推荐的方法进行对比。最后给出了相关的结论。

The horizontal offset of the jack-ups will increase the difficulty of the operation,and reduce the stability of the platform.Due to the causes of the horizontal offset of jack-up platforms,the total horizontal offset is divided into three parts,Ub caused by bending of leg,Ur caused by rotation of spudcans and U dcaused by horizontal displacement of spducans.Ud is comparatively small,which can be neglected.The finite element model of cylindrical-leg platform coupling with foundation is established using ABAQUS considering real interaction obeying Coulomb’s law.In the numerical model,the leg cylindrical leg is discreted with 4-node reduced integral shell elements,and the hull and spudcan are simplified as rigid bodies.The coupling constraint method is used to guarantee the consistence of degrees at the junction between the leg and hull,and that between the leg and spudcan.The hard clay soil with a undrained shearing strength(Su)of 50 kPa is modeled as perfect elasto-plastic material with Mohr-Coulomb constitutive model,and the Young’s modulus is taken as 500 Su,and the Poisson’s ratioν=0.49.The soil mass is discreted with 3 dimensional 8-node reduced integralcontinnum elements.Finner meshes with the minimum size of 0.1 m are accepted by the soil mass around the spudcan.The updated-lagrangian method considering the geometric nonlinearity is adopted.The horizontal and vertical boundaries are 10 B(B is the spudcan diameter)away from the spudcan to avoid the boundary effect.User subroutine DLOAD combing Airy wave model is used to apply wave load on legs.The influence of spudcan diameter,strength of back-fill and embeded depth on U,U d,and Ub is investigated for three stages:(1)no back-fill around spudcans,(2)consolidation of back-fill has not finished,and(3)consolidation of back-fill has finished.The results are compared with that of models based on the recommendation by CCS’s standard.Conclusions are as follows:(1)The increasing of the spudcan’s diameter and the strength of back-fill will decrease the offset’s components Ur and Ub,and reduce the offset of the platform under the wave load.In cases where the spudcan’s embedded depth is 6 m,the displacement decreases apparently when the spducan’s diameter is enlarged from 6 m to 8 m.The back-fill of the surrounding soil can reduce the displacement of the platform with the spducan’s diameter of 6 m.(2)Under the condition of normal operation,the offset from this model is comparatively larger than than that from the hinge model recommended by the“Rules for the Classification and Construction of Offshore Mobile Platform”(2005),except for somes cases where the large spudcan is shallowly embedded.(3)The spudcan and the leg rotates under the wave load,the drive the surrounding soil to move.The rotation center moves upwards and inside of the platform.The rotation of soil begins to disappearand turn to the extensive horizontal displacement when the embedded depth reaches 12 m.