| The permanent jacket platforms are the most common facilities for offshore gas and petroleum exploitation,the foundation type of which is usually the steel pipe pile.Mobile jack-up rigs are employed for offshore drilling and work-over.Because the cantilever beam of a jack-up rig has a limited length,the jack-up rig has to operate in close to a permanent jacket platform.The clearance(edge to edge)between the spudcan and a pile foundation of the jacket platform is usually within one spudcan diameter,which has an obvious influence on adjacent piles.However,no specific calculation methods or control measures for resolving this problem are provided in current guidelines,and it has become a hot research issue in recent years.This paper presents a series of small-scale model tests and centrifuge model tests.With the combination of numerical simulation and theoretical analysis method,the paper systematically investigates the deformation and bearing capacity of adjacent platform piles by the spudcan penetration and extraction,the analysis approach is proposed for evaluating the deformation and bearing capacity of adjacent pile foundations.The main contents and achievements are as follows:(1)Three large deformation finite element(LDFE)methods,CEL,ALE and MSM,are performed to simulate the spudcan penetration process in centrifuge model tests,which is to analyze the soil failure mechanism and back-flow during penetration.Focusing the integration scheme,computational efficiency and user-application for three LDFE methods,the advantages and disadvantages of each method are summarized.The penetration resistance in model tests is predicted by the LDFE method,and the current guideline method is also discussed in detail.(2)The LDFE methods are adopted to calculate the soil horizontal displacement during spudcan penetration and extraction,the range of disturbed soil by spudcan in clay or sand is evaluated.The model tests are conducted to monitor the distribution of lateral earth pressure during spudcan penetration and extraction,and decreased strength in the surrounding soil is also measured after spudcan extraction.(3)A series of centrifuge model tests are conducted to investigate the deformation and bearing capacity of adjacent pile foundations by the spudcan penetration and extraction.The test results reveal the lateral behaviour of adjacent piles due to the spudcan penetration and its influencing factors.Meanwhile,the maximum horizontal deformation of adjacent piles and corresponding position are also investigated.Then a two-stage analytical method and a three-dimensional CEL method are proposed to evaluate the deformation of adjacent piles.Based on the centrifuge model test results,the two proposed methods are proven to be reasonable.(4)The small-scale and centrifugal model tests reveal the effect of spudcan penetration and extraction on the vertical bearing capacity of adjacent pile foundations.Developing the user subroutine USDFLD,the decreased strength distribution of disturbed soil is introduced in the finite element analysis.Then a numerical method considering the effect of spudcan penetration and extraction is proposed to evaluate the vertical bearing capacity of adjacent piles.(5)The results show the reduction in the lateral bearing capacity of adjacent piles and obvious decrease in the lateral soil reaction.The proposed p-y curve models for the reference test in clay or sand are presented.Based on these proposed p-y curves,the modified p-y curve method considering the effect of spudcan penetration and extraction is presented by introducing a strength attenuation coefficient,aiming to evaluate the lateral bearing capacity of adjacent piles.Finally,the modified p-y curve method is compared with the centrifuge model test results,and this proposed approach is proven to be reasonable.The above research results provide an engineering analysis approach for offshore pile foundation design and stability checking due to spudcan penetration and extraction. |
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