自升式平台水平偏移的数值模拟方法研究
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摘要
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,Ubcaused by bending of leg,Urcaused by rotation of spudcans and Udcaused by horizontal displacement of spducans.Udis 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 with3 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(Bis 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,Ud,and Ubis 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 Urand 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 mto 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.
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,Ubcaused by bending of leg,Urcaused by rotation of spudcans and Udcaused by horizontal displacement of spducans.Udis 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 with3 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(Bis 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,Ud,and Ubis 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 Urand 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 mto 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.
引文
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[13]张其一,栾茂田.复合加载情况下双层地基极限承载力研究[J].岩土力学,2009,30(4):1131-1136.Zhang Q Y,Luan M T.Study on ultimate bearing capacity of two-layered subsoil under combined loading[J].Rock and Soil Mechanics,2009,30(4):1131-1136.
[14] Morison J R,Johnson J W,Schaaf S A.The force exerted by surface waves on piles[J].Journal of Petroleum Technology,1950,2(5):149-154.
[2] Zhang P Y,Ding H Y.Bearing capacity of the bucket spudcan foundation for offshore jack-up drilling platforms[J].Petroleum Exploration and Development,2011,38(2):237-242.
[3]李大勇,冯凌云,张雨坤,等.饱和细砂中裙式吸力基础水平单调加载模型试验—承载力及变形分析[J].岩土工程学报,2013,35(11):2030-2037.Li D Y,Feng L Y,Zhang Y K,et al.Model tests on lateral bearing capacity and deformation of skirted suction caissons in saturated fine sand under horizontal monotonic loading[J].Chinese Journal of Geotechinical Engineering,2013,35(11):2030-2037.
[4]中国船级社.海上移动平台入级与建造规范[S].北京:人民交通出版社,2005.China Classification Society.Rules for the Classification and Construction of Offshore Mobile Platforms[S].Bejing:China Communications Press,2005.
[5]谢娜娜,马廷霞,刘国昊.自升式海洋平台桩腿桩靴有限元分析[J].石油矿场机械,2013,42(11):32-38.Xie N N,Ma Y X,Liu G H.Finite element analysis of jack-up offshore platform’s leg and sabot[J].Oil Field Equipment,2013,42(11):32-38.
[6]彭熙民.港海一号自升式钻井平台结构静力分析[J].中国海洋平台,2001,16(4):122-127.Peng X M.The structure analysis of jack-up drilling platform‘GanghaiNo.1’[J].China Offshore Platform,2001,16(4):122-127.
[7] Hambl E C,Imm GR,Stahl B.Jack-up performance and foundation fixity under developing storm conditions[C].//OTC,Houston,Texas:[s.n.],1990:369-380.
[8] Hambl Y E C,Nicholson B A.Jack-up synamic stability under extreme storm conditions[C].//Proceeding of 23rd OTC,Houston,Texas:[s.n.],1991:261-272.
[9]王庚荪,孔令伟,杨家岭,等.水平载荷作用下土体与桶形基础的相互作用[J].工程力学,2004,21(2):107-113.Wang G S,Kong L W,Yang J L,et al.Interaction of soil mass and bucket foundation under horizontal loads[J].Engineering Mechanics,2004,21(2):107-113.
[10]高喜峰,刘润,谭振东,等.基于桩土相互作用的桩体自沉及屈曲稳定性研究[J].中国造船,2012,53(2):113-121.Gao X F,Liu R,Tan Z D,et al.Study on pile self-penetration and buckling stability considering interaction between pile and subsoil[J].Shipbuilding of China,2012,53(2):113-121.
[11] Zheng J B,Hossain,M S,Wang D.3Dlarge deformation FE analysis of spudcan foundations on layered clays using CEL approach[C].//Constitutive Modeling of Geomaterials:Advances and New Applications.Heidelberg:Springer,2013,803-810.
[12] Tian Y,Cassidy M J,Randolph M F,et al.A Simple Implementation of RITSS and Its Application in large deformation analysis[J].Computers and Geotechnics,2014,56:160-167.
[13]张其一,栾茂田.复合加载情况下双层地基极限承载力研究[J].岩土力学,2009,30(4):1131-1136.Zhang Q Y,Luan M T.Study on ultimate bearing capacity of two-layered subsoil under combined loading[J].Rock and Soil Mechanics,2009,30(4):1131-1136.
[14] Morison J R,Johnson J W,Schaaf S A.The force exerted by surface waves on piles[J].Journal of Petroleum Technology,1950,2(5):149-154.