Study On Stability Mechanism Of Surface Conductor In Deep Water Well

Surface conductors,served as the holding structure and circulating channel,are the first and most important casing in deepwater drilling.The instability of them has become the major risk in field operation.After surface conductors were jetted into planned position,if any instable condition appears such as sinking and inclination,it would impede setting subsea tree and blow out preventer(BOP)in following process,even result in the invalidated of wellhead.To ensure the stable of surface conductors and reduce the risk of deepwater drilling,developing researches on the mechanism of the stability of surface conductors would be necessary and significant.Based on the water jet theory,this paper firstly analyzed the features of submerged water jet during conductor running process.Then,combined with the failure criterion of cohesive soil,the calculation model of minimum displacement of water jetting to break soil was established.Lastly,for the two conditions of the borehole diameter less than conductor diameter and borehole diameter large than conductor diameter,the analysis models of surface conductor stability based on cavity expansion and contraction theory were established separately.On the basis of geotechnical engineering theory,this paper analyzed the features of the interaction between surface conductors and seabed soils and achieved the curve differential equations.Based on the pile and foundation theory,the calculation method of axial bearing capacity of surface conductors and lateral resistance against conductors in cohesive and cohesionless soil was developed.The function relationship among jetting displacement,drilling bit stick-out values and reducing coefficient of bearing capacity of surface conductors was proposed.Moreover,the calculation model of real time bearing capacity of surface conductors considering the influence of jetting displacement,size of drilling bit and time effect was developed.By field jetting simulation experiment,the stress change rule of surrounding soils during installing surface conductors and the influence of jetting parameters(displacement,drilling bit stick-out value)on bearing capacity and running efficiency of surface conductors were analyzed,and the models of stability and real time bearing capacity of surface conductors were verified.During the process of surface conductor jetting,shaft resistance softening phenomenon would take place in soils around conductors,caused by which the bearing capacity of conductor tend to drop sharply.With the increasing of jetting displacement and drilling bit stick-out,the bearing capacity of surface conductor would decrease in an exponential relationship.The greatest drop occurs when the value of jetting displacement excess 1.2 times of minimum soil broken displacement.If the nozzles of drilling bit stick out from conductor completely,then the influence of stickout value on conductor bearing capacity would be weaken.On the other side,the running speed of surface conductor would rise with the increase of jetting displacement and drilling bit stick-out value in power function relationship.After been set into position,the bearing capacity of surface conductor would recover with time in log function.By finite element analysis method,the model of interaction between surface conductors and seabed soils can be created.The unit of COMBIN39 was utilized to simulate the p-y curve of soil bodies and to realize the elastic-plastic feature of soils during interacting between conductor and soils.Based on the mechanics of materials and virtual work principle,stiffness matrix of surface conductors can be built.With the finite element analysis software ANSYS,sensitivity analysis on surface conductor considering the size surface conductor,wellhead inclination and heights above mudline,riser top tension was processed.According to the results of theoretical models and simulation experiments,considering both of the constraints of capacity broking soils by water jet and surface conductor bearing capacity,the design methods of jetting displacement,drilling bit stickout value,surface conductor jetting depth and standing time were established.These design methods has been applied in several self-support deepwater wells in South China Sea,not only ensured the stability of surface conductors but also lifted the efficiency of conductor jetting.Take a deepwater well among them as an example.The field application of these methods was introduced.