Research On The Design Of Test Piles For Underwater Vertical Tree Sea Trial

With the rapid development of China's economy,land oil and gas resources can no longer meet the needs of China's production.The exploration and production scope of oil and gas resources is gradually achieving a strategic shift from land and shallow-water to deepwater.Subsea Christmas tree is the key equipment for deep-water oil and gas development,and its development technology has been monopolized by western countries for a long time.This situation seriously restricts China's independent development of offshore oil and gas resources.In order to break this situation,our country has gradually begun to research on the subsea Christmas tree system,and has completed the model machine fabricate of Subsea christmas tree.Subsea Christmas tree has poor working conditions and complex structural functions,so a series of experiments is needed to verify whether their functions meet production needs.The sea test is an important test method to test Subsea Christmas tree,which can better simulate the working performance of the Christmas tree under actual environmental conditions.At present,the domestic subsea Christmas tree is in the technical research,lacking relevant sea trial standards and sea trial equipment.The vertical Christmas tree detective pole is the key equipment in the underwater trial operation of the vertical subsea production tree.It is mainly used to simulate the actual underwater wellhead function and needs to be fixed on the seabed.In this paper,the detective pole of vertical Christmas tree and its underwater fixing are studied.The main work is as follows:Firstly,on the basis of the underwater vertical subsea Christmas tree structure and the sea trial plan and the sea trial process,the functional requirements of the detective pole sea test were clarified,the design of the detective pole structure was completed,and the installation test process was analyzed.Secondly,according to the downward stage of Christmas tree and the characteristics of marine environment,the relationship between the environmental load and the depth of lowering is analyzed when the Christmas tree is installed without guide,and according to the mechanical analysis of the guiding installation of the Christmas tree,a mechanical model of the wire rope guiding installation is established.Using MATLAB to obtain the relationship between the lateral displacement of the Christmas tree and depth of drop and the force of the wire rope on the test pile under different sea test depths,and calculate the minimum fixed force required for the test pile.Then,based on the elastic theory,the wall thickness of the wellhead is preliminarily designed,the results show that the minimum wall thickness of the wellhead is 96.1mm.Based on the finite element method,the structural strength of the wellhead under hydrostatic pressure test and actual working conditions is analyzed,the results show that the wellhead meets the strength requirements during the test work.At the same time,the Mooney-Rivin theory and finite element method are used to analyze the sealing performance of the plug under the pre-tight sealing state and hydrostatic test state.The results show that the plug sealing shape meets the test requirements in hydrostatic test process.Finally,the underwater fixing method of the test pile is analyzed,and the failure form of the cylindrical foundation under the overturning,sliding and foundation instability is established.Taking the 100m sea depth as an example,the cylinder diameter ratio of the cylindrical foundation under the ultimate load is calculated,and the influence law of the cylinder number,the cylinder spacing and the aspect ratio on the limit state of the cylinder foundation is analyzed;the ultimate bearing capacity finite element model and the uplift bearing model are used to analyze the influence of the length-to-diameter ratio of the cylinder foundation,the friction coefficient of the soil,cohesive force and the elastic modulus on the bearing capacity and pull-out resistance of the cylinder foundation.The minimum construction negative pressure and buckling stability of the cylindrical foundation with different aspect ratios is analyzed.