Settlement Prediction And Influence Factors Analysis Of Underwater Caisson Foundation

With increasing shortage of oil and gas resources,China continues to promote the development of offshore oil and gas resources.In the process of the marine oil and gas resources exploitation,subsea production system is one of the most indispensable components.In practical engineering,if the deformation of the underwater caisson foundation used to protect the subsea production system is too large,it could cause the fracture of the manifold,submarine pipeline and umbilical cord cable,which will cause serious engineering accidents.Therefore,it is of great engineering significance to predict the settlement of underwater protecting caisson and analyze the key influencing factors.In this paper,centrifuge model test was conducted to determine the consolidation settlement process of underwater caisson foundation and reflect the actual construction process of soil excavation and foundation placement.Midas GTS NX three-dimensional finite element software and standard theory method were also used to verify the reliability of the prediction results.Additionly,in order to study the key factors affecting the settlement law of the underwater caisson foundation,the seepage/stress coupling in the finite element software ABAQUS was employed to conduct parameter sensitivity analysis.The main work and research results of this paper are as follows:(1)Indoor soil tests were conducted to obtain the parameters of the corresponding remoulded samples because natural soil samples is difficult to be acquired and vulnerable to be disturbed.It can be found that with a increase of soil moisture content,the consolidation coefficient of the soil increases,but soil compression modulus decreases.When the sample moisture content is 30.55%,the deformation parameters of the sample is the most similar to the actual soil samples;(2)Centrifuge model test was carried out to study the settlement law of the underwater caisson foundation.By fitting the centrifuge model test results with Weibull curve and using scale inversion,the final settlement of the underwater caisson foundation in actual engineering was 0.4861 m.(3)Midas GTS NX finite element software was used to verify and analyze the settlement of the underwater caisson foundation.The research results showed that the final settlement of the underwater caisson was 0.4193 m,which was 4.39% different from the theoretical results and 13.74% different from the centrifuge model test results.In the analysis of double-layer soil,when the soft soil is beneath the steel ring wall,the internal stress diffuses along the direction of the depth,while the stress is concentrated when the hard soil is below the steel ring wall.(4)ABAQUS finite element software was used to study the factors influencing the settlement law of the underwater caisson foundation.It can be concluded that the relationship between the permeability coefficient and the time of the consolidation settlement is a linear correlation,and there is also a linear correlation between the soil intensity and the final settlement of the foundation.Additionly,when the steel ring wall height is less than 1.6 times the height of underwater caisson foundation,the final settlement increases with the reduction of the steel ring wall height.In a word,centrifuge model test and finite element softwares were used to carry out the prediction analysis and influence factor analysis of the underwater caisson foundation settlement.The research results has some reference significance for the practical engineering using the caisson to protect the underwater production system.