Research On Main Control Factors And Control Methods For Bearing Capacity Of Offshore Drilling Conductor

Offshore drilling conductors are essential conduits that connect offshore drilling platforms to the shallow seabed soil.Their primary function is to provide a closed loop channel for the circulation of drilling fluid,while also bearing the load of wellhead equipment during the offshore drilling process.Additionally,they provide support for the wellhead and Christmas tree during the oil and gas production process,ensuring that the drilling and production strings remain stable and do not sink.The riser’s bearing capacity mainly comes from the friction between the pipe column and the shallow seabed soil.Therefore,the depth of penetration,structural dimensions,and characteristics of the seabed soil are crucial factors that affect the ultimate bearing capacity of the riser.Typically,drilling and hammering methods are used to install the drilling conductor.API specifications provide calculation methods for the ultimate bearing capacity of conductors under two installation methods in shallow water operations.However,the ultimate bearing capacity model for the drilling method of the drilling conductor did not take into account the influence of critical factors such as waiting time,height of cement sheath return,and length of the secondary surface casing.In the ultimate bearing capacity model of the conductor installed by hammering method,the influence of main control factors such as group pile action,time effect,and composite pipe column length was not considered,resulting in complex accidents such as sinking or hammer refusal during operations.Therefore,there is an urgent need to conduct research on the impact of main control factors on the ultimate bearing capacity of the conductor,improve the calculation accuracy of the ultimate bearing capacity model,and ensure the safety of offshore operations.In response to the limitations of the existing calculation models for the ultimate bearing capacity of the drilling method’s conductor,a simulation experiment on the influence of the main control factors of the drilling method on the ultimate bearing capacity of the conductor was conducted in this paper.The simulation experiment revealed the influence of the main control factors such as the cement sheath waiting time,cement sheath diameter,and cement sheath return height on the ultimate bearing capacity during the cementing operation of the conductor.Research suggests that the waiting time for the cement sheath to solidify can significantly affect the bonding strength between cement and seabed soil,thereby affecting the ultimate bearing capacity of conductor;The diameter and return height of the cement sheath can affect the contact area between the pipe column and the seabed soil,thereby affecting the ultimate bearing capacity.The experimental research results show that the bonding strength between the cement sheath and the seabed soil shows a linear increasing trend with the increase of the waiting time.When the waiting time reaches 48 hours,the bonding strength can reach 60% of the original soil strength;When the waiting time exceeds 48 hours,the growth trend of bonding strength will gradually slow down.The ultimate bearing capacity of the conductor increases linearly with the increase of the return height of the cement sheath,and decreases linearly with the decrease of the return height of the cement sheath.To address the issue of insufficient consideration of main control factors in the ultimate bearing capacity model of the hammering method of drilling conductor,we conducted a simulation experiment to study the influence of main control factors,such as the waiting time for hammering construction and the length of the secondary surface casing,on the ultimate bearing capacity of the conductor.The experimental research results indicate that:(1)when the waiting time is less than 48 hours,the ultimate bearing capacity of the conductor installed by hammering method increases with the increase of waiting time;when the waiting time is greater than 48 hours but less than168 hours,the increasing trend of ultimate bearing capacity slows down;when the waiting time reaches 168 hours,the ultimate bearing capacity increases to its maximum value.(2)In the experiment on the bearing capacity of a composite pipe column composed of a conductor and a surface casing,it was found that the bearing capacity of the composite pipe column increased sharply at the lower end of the conductor,indicating that the composite pipe column bears more resistance at the variable cross-section.Based on theoretical analysis and experimental research results,we established a calculation model for the ultimate bearing capacity of the composite string of conductor and surface casing.The variation trend of the penetration and hammering number of drilling conductor at different positions(edge pile,corner pile,center pile)was analyzed through a simulation experiment of conductor hammering.The variation law of the ultimate bearing capacity of conductor under group pile conditions was studied,and the order of the ultimate bearing capacity of conductor at different positions from large to small was obtained: center pile,edge pile,corner pile,and single pile.It was found that the ultimate bearing capacity of the center pile was 2.469 times that of a single pile,indicating that the pile group effect can cause changes in the ultimate bearing capacity of the conductor.By testing and analyzing the ultimate bearing capacity of a large number of active offshore drilling conductors,it was found that the combined load of the wellhead and Christmas tree of each well did not exceed 50 tons,while the ultimate bearing capacity of the composite pipe string structure formed by the conductor and surface casing of each well exceeded 200 tons.Therefore,there is a significant margin of bearing capacity for the actual offshore conductor composite string.It is currently a hot issue in the industry whether the bearing capacity of the jacket production platform can be fully utilized.This article proposes a load transfer structure that can disperse and transfer part of the load in the wellhead area of the jacket platform to the composite conductor string in each well,thereby reducing the pile leg bearing capacity of the jacket platform.Taking the NB35-2 platform as an example,the ultimate bearing capacity of the composite conductor string was analyzed,and the calculation results showed that the surplus of the ultimate bearing capacity of the composite conductor string could reach520 tons.A load transfer efficiency calculation model was established based on the structural characteristics of the target platform,and a connection scheme between the jacket platform and the composite conductor string was provided.Through finite element numerical simulation,it is concluded that under the load transfer scheme proposed in this paper,the composite conductor string can bear at least 15% of the load on the jacket platform.The research results indicate that the load transfer method for the excess bearing capacity of drilling conductor can provide a scientific basis for optimizing the size of jacket platform pile legs and designing pile driving depths.