Numerical Simulation Research On Effects Of Mooring Activities On Underwater Caisson

With the increase of demand for offshore oil and gas resources,the influence of offshore mooring activities on oil and gas exploitation engineering has been paid more and more attention.Underwater caisson can be used to protect subsea oil and gas production systems from mooring activities.However,the allowable displacement of the straddle pipe used to connect different caisson monomers is limited,which puts higher requirements on the overall settlement or inclination of the caisson.In this paper,based on the CEL method,the influence of anchor dropping and anchor dragging on the underwater caisson was simulated by ABAQUS,and the influence of key factors,such as undrained shear strength s_u and anchor velocity,was discussed.Based on the analysis of single influence factor on the law of anchor dropping and anchor dragging on the underwater caisson,the quantitative calculation expressions are given.The main contents are as follows:Firstly,the influence of s_u on the caisson during anchor dropping is simulated.In general,the change of the caisson displacement with the increase of s_u can be divided into three stages:when s_u=1?3 k Pa,the displacement of the caisson decreases rapidly.In this stage,the flow pattern of the soil around the caisson is dominated by the overall upper left flow;when s_u=6?15k Pa,the caisson displacement increases with the increase of s_u.The reason is that compared with the previous stage,with the increase of s_u,the flow pattern of the soil changes from the upper right flow to the upper left dominated;when s_u=15?25 k Pa,the displacement of the caisson is about 3.5 cm,which does not change significantly with s_u.Secondly,the influence of the penetration velocity and the distance between the anchor drop point and the caisson(dropping distance)on the displacement of the caisson during the anchor dropping is simulated.The results show that when the penetration velocity is less than12 m/s,the maximum difference of the vertical displacement of the caisson is less than 0.11%.In addition,the displacement of caisson decreases with the increase of drop distance.According to the numerical results at different drop distances,the calculation expressions of the safe drop distance at different penetration velocities are given.Thirdly,based on the analysis of the calculation data of the influence of different factors on the caisson during the anchor dropping,two kinds of prediction methods are presented:The subsection prediction method is a method of combining the three-stage relationship of the caisson displacement under different drop distances,and using the subsection fitting to give the calculation expression of the caisson displacement;the simplified prediction method is a method which ignores the lower convex points in the caisson displacement curve and gets the expression by fitting the remaining points.The subsection prediction method is only applicable to the conditions with a drop distance of 0.5?2 times the anchor length.The simplified prediction method is applicable to a wider range,but the predicted value of the lower convex part is about 26%larger than the calculated result,that is,the predicted result is conservative.Fourthly,the influence of anchor dragging on the caisson is simulated.The results show that both the angle and displacement of the caisson are far beyond the allowable upper limit in the engineering,which will cause fatal influence on the structure.In this paper,combined with the maximum anchor holding force that the ship anchor can provide under actual conditions,the actual displacement of the caisson is determined according to the time history curve of the anchor pulling force and the displacement of caisson.Based on the analysis of the calculation results of the influence of s_u,anchor weight,anchor dragging velocity and dragging distance on the caisson,the actual displacement variation law of the caisson under different influencing factors is summarized,and the setting principle of the range of anchor forbidden area is given in combination with the sailing distance of the ship.In summary,this article focuses on the influence of s_u and drop distance on the displacement of the caisson in the process of anchor dropping,and proposes two underwater caisson displacement prediction methods.The simplified displacement prediction method considers comprehensive factors and is simple to calculate,and is more recommended;in addition,the influence of s_u on the displacement of the caisson during anchor dragging is discussed,and measures to set an anchor-forbidden area to reduce the damage of the caisson are proposed.