Study On Mechanics Problems In Underwater Repair Of Subsea Pipeline

In pace with rapid development of the global economy,there is an increasing need for energy.Countries are beginning to focus on the more potential marine resources,and consider the development of marine oil and gas resources and related technologies as an important strategic goal of national economic development.As the main way of oil and gas transmission,the safe operation of submarine pipelines is the guarantee of the normal operation of the system.Relevant statistics show that the domestic submarine pipelines’ frequency of leakage or damage is 0.2% per year per 1,000 kilometer.In order to deal with subsea pipeline accidents quickly,it is necessary to develop underwater repair technology and repair equipment.At present,the domestic underwater repair technology of subsea pipeline is backward,repair equipment is seriously dependent on imports,and relevant industry enterprises are working for the nationalization of underwater repair equipment and technology.The localization of underwater repair equipment and technology relies on the solution of relevant problems.Some investigation shows that the key problems of mechanics in underwater repair of subsea pipelines include: connection with elliptical defect pipeline,sealing in underwater repair,straightened misalignment of pipelines and fastening of pipelines in complex environment.In view of these four problems,based on installment load,structural strength,sealing reliability,the method of straightened misalignment and the tightness property of pipelines,this article carried on the following research contents:1.Mechanical properties research for connection of elliptic pipeline.The interference installation load prediction and the structural safety evaluation of the submarine elliptical pipeline and the mechanical connector in the connection progress are a difficult problem.The paper simplifies pipeline with elliptical defect into the elliptical pipeline,and establishes the nonlinear FEM of the elliptic pipeline and connector based on ANSYS,and combines the classical theory of composed thick-wall cylinder to validation.Interference contact friction between the elliptical pipeline and the connector is simulate by FEM,and contact stress is established based on the equivalent method.Moreover,the empirical installation load formula of the submarine pipeline repair connection with elliptic defect is proposed with the consideration of friction.The relationship between Von mises stress of submarine pipeline and the parameters,such as ellipticity and interference fit,is simulated and analyzed in the process of repair and installation,and a method of safety assessment of submarine pipeline with elliptic defect based on strength is proposed.2.Sealing material compare and sealing reliability research.Sealing as a key problem of underwater repair,its performance is related to the success of repair.This paper summarizes common sealing forms and sealing materials for underwater mechanical repair,reviews available sealing criteria and related sealing performance parameters for laying foundation of the sealing reliability analysis.The 2D axisymmetric model of the mechanical connector Oring seal structure is established by the finite element software ANSYS,the sealing pressure and the sealing area on the contact surface are analyzed,and evaluates sealing reliability based on the Buchter’s criterion.3.Study of the pipeline connection method under the pre-stress condition.The submarine pipeline is so much in the pre-stress state,therefore the two ends of pipeline will misalign after cutting,which has proposed the new challenge to connection.In view of the problem of straightened pipeline misalignment,this paper establishes the theoretical model of based on cantilever beam model and Winkler elastic foundation beam,Considering the relevant assumptions,designs a set of misalignment schemes based on the theoretical model,including the position of the load,the control load or the displacement and the limit working condition.Finally,the FEM is used to analyze the cases and verifies the feasibility of the scheme.4.Analysis for connection stability under complex environmental loads.It is necessary to consider whether repaired connecting pipeline can withstand the complex environmental loads such as wave,current,internal pressure,temperature and free-span during service.Aiming at the threaded teeth fastening structure in connectors,the ideal super high-strength fastening mechanics model is simplified,and the empirical theoretical relationship between axial fastening force and structural dimension,material parameters is given.The elastic-plastic fastening model of threaded teeth fastening structure and pipeline was established by the finite element software ABAQUS,the consistency verification was carried out in theoretical,and the stress analysis and fastening force factors were analyzed by FEM.According to DNV(Det Norske veritas)specification,this article simplifies the complex environmental loads such as internal pressure,temperature and suspension,and puts forward a method of tightness property evaluation for engineering application based on comparison load and fastening force.