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| Numerical analysis on ultimate bearing capacity of spudcan of jack-up platform in two-layered clay |
| REN Li-hui, JIAO Yong-shu, QI De-xuan |
| School of Mechanical Engineering, Hebei University of Technology, Tianjin 300130, China |
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Abstract The prediction of ultimate bearing capacity (UBC) on submarine layered foundations is a key point for the installation of offshore jack-up drilling platform.At present,the estimation of UBC of a layered foundation is mainly based on the empirical formulas for which the application scopes and limiting conditions are not clear.In order to evaluate the UBC of spudcan of jack-up on hard-soft clay foundations,the instalation of spudcan was modeled as a quasi-static process of gradully penetrating on a half-infinate elastic-plastic foundation.The soil was considered as an isotropic,elastic-plastic,Mohr-Coloumb material.The spudcan-soil interface was defined as a master-slave interface obeying Coloumb friction law.Non-linear numerical analysis on UBC of spudcan of jack-up platform on hard-soft clay foundations was carried out with finite element method (FEM) by using ABAQUS.The impacts of the strength ratio of overlying to underlying strata and the relative thickness of overlying clay on the UBC were analysed.The UBC data from FEM and the results calculated from the empirical formulas were compared.The results showed that when the relative thickness of overlying clay was fixed,the lower the strength ratio of over-lying to underlying strata S ut/Sub was,the larger the UBC of double-layered foundations would be.When H/B=1.0,the Brown and Meyerhof formula could be used to calculate the UBC only when Sut/Sub > 3 0..The scope of application of the method of projection area was 4.0 < Sut/Sub < 6.0.If the strength ratio was fixed,the larger the relative thickness of overlying clay was,the smaller influence of underlying soft soil on the UBC of double-layered foundations would be. When Sut/Sub > 3.0.,the Brown and Meyerhof formula and the method of projection area could be utilized to predict the UBC of double-layered foundations only when H/B < 1.0.The higher the strength ratio for overlying to underlying strata and the smaller the relative thickness of overlying stratum were,the greater the possibility for punch-through would be.These conclusions have certain instructional significance to engineering design of installation of spudcan.
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Received: 29 December 2016
Published: 28 August 2017
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双层黏土中自升式平台桩靴极限承载力数值分析
海底层状地基极限承载力的预测是海上自升式钻井平台插桩作业设计的关键。目前，成层土地基极限承载能力的确定主要依靠经验公式，但其适用范围和限制条件并不明确。为了研究桩靴在硬软双层黏土地基中的极限承载力，将自升式平台的插桩过程视为桩靴在半无限弹塑性地基上逐渐贯入的准静态过程，将土体视为满足摩尔库伦屈服准则的各向同性弹塑性体，桩土之间的接触定义为符合库伦摩擦定律的主从接触面接触，在ABAQUS平台上利用非线性有限元法对自升式钻井平台桩靴在硬软双层黏土地基中的极限承载力进行数值计算，分析上、下土层强度比和上层土相对厚度比对极限承载力的影响，并与现有经验公式的结果进行对比。结果表明，Brown&Meyerhof公式和投影面积法只适用于硬软差别明显的双层土地基。当上层土相对厚度比H/B一定时，上、下层土强度比Sut/Sub越小，双层土地基的极限承载力越大。当H/B=1.0时，只有在Sut/Sub > 3.0的情况下才可用Brown&Meyerhof公式计算其极限承载力，而投影面积法适用于4.0 < Sut/Sub < 6.0的情况。当上、下层土强度比一定时，上层土相对厚度比越大，下部软土层对双层地基极限承载力的影响越小。在Sut/Sub=3.0时，只有在H/B < 1.0的情况下，Brown&Meyerhof公式和投影面积法计算的双层土地基的极限承载力才是合理的。上、下层土强度比越大，上层土相对厚度比越小，发生穿刺的可能性就越大。所得结论对于插桩作业的设计和施工具有一定的工程指导意义
关键词：
自升式平台,
桩靴,
双层黏土地基,
有限元法,
极限承载力,
穿刺
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