Research On Penetration Resistance And Formation Mechanism Of Soil Plug During Suction Anchor Installation

Suction caisson is a new type of anchoring foundation for offshore platforms.Because of its convenient installation,it can be recycled repeatedly and has a short construction period.It has been widely used in offshore wind turbines,offshore oil rigs,floating production storage and offloading units(FPSO)and other marine engineering structures.During the installation of suction caisson in sand,it is ensure that the suction force is within a reasonable range,that is,need to be greater than the penetration resistance during the penetration process,and suction force cannot be applied too large,so as to avoid the piping,resulting in soil instability.In the process of negative penetration,the soil surface heave often occurs in the suction caisson,which is called soil plug,which makes the suction caisson unable to be penetrated completely into the sea bed.Due to the complex current in ocean,the body is often denuded by seawater,which is known as denudation,which eventually leads to instability of the foundation.Domestic and foreign scholars have carried out a large number of researches on this issue from the perspectives of penetrating resistance and seepage effect.In this study,the formation mechanism of soil plug in suction caisson is investigated by means of experiments and analytical methods.Through the 1g model test,the formation process of soil plug during the negative pressure penetration was recorded and measured.In the tests,it was found that during the assisted process of negative pressure slipping,the fine particles would gradually moved along the channel from the bottom to the top of the suction caisson,that is,fine grain migration phenomenon.Happened to the suction force applied to the suction caisson,the soil plug swelled and the porosity increased.The permeability coefficient changed continuously with the increase of penetration depth.Based on the experimental results,the existing theoretical formula has been improved,and the continuous change of permeability coefficient into the required suction formula.Compared with the actual field cases data,it is found that the improved theoretical calculation formula accurately predicted the required suction over those existing formula.Through a further analysis,the main reason for the formation of soil plug is found that the internal permeability of soil changes.Different size suction caissons were fabricated to study the relationship between length-diameter ratio,thickness-diameter ratio and height of soil plug.From the soil plug rate,hydraulic gradient to analyze the formation of soil plug.It is concluded that there is a clear correlation between length-diameter ratio and height of soil plug,while there is no obvious correlation between thickness-diameter ratio and height of soil plug for the thin-walled suction caisson used in the experiment.Through the analysis of the change of soil plug rate and hydraulic gradient,it is verified that the length-diameter ratio of the suction anchor is better than 1.5 when the suction anchor is installed in the dense sand.Through the 1g experimental study and the improved theoretical analysis,it is found that the suction caisson is applied in sand,it can accurately predict the required suction during the penetration process.This can improve the design efficiency and reduce the cost.These finding provide the theoretical basis for the design to use the suction caisson foundation in the South Ocean of China,which will be beneficial for ocean engineering of resource exploration.However,the improved theoretical formula in this study still has some limitations.The required suction for installation in cohesive soil and various complicated environments needs to be further investigated.