Study On Long-term Settlement And Stiffness Development Of Spudcan Foundation Under Consolidation

Jack-up rigs now have been extensively used in offshore oil and gas exploration and increasingly in temporary production and maintenance work,which are generally installed in shallow to moderate water depth up to 150 m.The jack-up platform usually has to be in service for several months to several years after installation.During longterm work,the excess pore pressure under the spudcan would continue to dissipate,causing the consolidation and hardening of the soil.Because of consolidation,spudcan settlement would happen.At the same time,due to the hardening of the soil,the fixity of the spudcan would be improved,which can change the internal-force of the whole structure.Therefore,the long-term settlement and stiffness development of spudcan is an issue of concern for the offshore jack-up industry.At present,the research on the long-term behavior of spudcan are mostly focused on the increase of bearing capacity,but few on the long-term settlement and stiffness.Small strain analysis method is mostly used in the relevant finite element study,which cannot consider the whole process of spudcan installation to the working stage.This paper reports a study dedicated to this specific subject.The spudcan installation and consolidation under operational load are continually modelled using a dual-stage Eulerian–Lagrangian analysis technique.The feasibility and reliability of the numerical calculation were first verified by comparison with centrifuge experiments.In the study of settlement,parametric studies are undertaken for spudcans under various soil,loading,embedment and geometrical conditions,so as to identify appropriate forms of dimensionless settlement and time which can unify the settlement time histories of different scenarios.Particular attention is paid to the development of the dimensionless time,which can reflect the drainage effect of the seabed and hence the influence of embedment depth.The unified settlement development curve may be helpful for operators to assess the consolidation degree,project the final settlement and even backcalculate the operative coefficient of consolidation.In the study of settlement,the discussion focuses on the long-term development of spudcan rotational stiffness.In the parameter study,firstly,according to the SNAME method,the value range of the rotational stiffness before and after consolidation after installation are determined.Then,according to the results,the appropriate dimensionless form of stiffness and time is determined to unify the time history of stiffness development.Finally,a calculation method is provided for engineering units to predict the development of long-term rotational stiffness of spudcan.