1. Theoretical Background of High-Pressure Flow Control Components and Manifolds
In science and technology, for many mechanics and physics problems, people have already obtained the basic equations they should follow, either ordinary differential equations or partial differential equations, together with the corresponding definite solution conditions. However, exact solutions by analytical methods can be obtained only for a small number of problems whose equations are relatively simple and whose geometries are quite regular. For most problems, because some features of the equations are nonlinear or because the geometry of the solution region is relatively complex, analytical answers cannot be obtained. There are usually two ways to solve these problems. One is to introduce simplifying assumptions and simplify the equations and geometric boundaries into a manageable form, obtaining the solution under simplified conditions. This method is feasible only in limited cases, because excessive simplification may lead to large errors or even wrong answers. Therefore, over many years people have searched for and developed another solution path: numerical methods. Especially over the past thirty-plus years, with the rapid development and wide application of computers, numerical analysis has become a major tool for solving scientific and technical problems.
The emergence of the finite element method was a major breakthrough in numerical analysis for research. Its basic idea is to discretize a continuous solution region into a finite number of elements connected to one another in a certain way. Because elements can be combined in different connection forms and the elements themselves can have different shapes, complex solution domains can be modeled. Another important feature of the finite element method is that approximate functions assumed inside each element are used piecewise to represent the unknown field function over the whole solution domain. The approximate function in an element is usually expressed by the values of the unknown field function, or its derivatives, at the element nodes and by interpolation functions. In this way, in the finite element analysis of a problem, the values of the unknown field function or its derivatives at each node become new unknowns, converting a continuous infinite-degree-of-freedom problem into a discrete finite-degree-of-freedom problem. Once these unknowns are solved, interpolation functions can be used to calculate approximate values of the field function in each element, obtaining the approximate solution over the entire domain. As the number of elements increases, namely as element size decreases, or as element degrees of freedom and interpolation accuracy increase, the approximation improves continuously. If the elements satisfy convergence requirements, the approximate solution will eventually converge to the exact solution.
Fracture mechanics has developed to the present, and relatively speaking, the linear-elastic part is more mature. Its theory is simple, its foundation is solid, and it has been applied in certain engineering fields. The main criteria include the Griffith criterion, namely the energy release rate criterion, and the Irwin criterion, namely the stress intensity factor K criterion. The main theories of elastic-plastic fracture mechanics are the COD theory, crack opening displacement, proposed in 1960, and the J-integral theory proposed in 1968. These two theories form the body of elastic-plastic fracture mechanics and can be used to analyze the whole process of crack initiation, propagation and instability. As a true discipline, fracture mechanics has developed extremely rapidly. Starting from G. R. Irwin's introduction of the stress intensity factor concept in 1957, it has a history of less than 50 years in this account. It is one of the most active branches of solid mechanics and has had major influence on engineering fields such as metal physics, metallurgy, materials science, aviation, machinery, construction and earthquake engineering. It has shown strong vitality and has been widely used to solve practical engineering problems. There are many successful applications of fracture mechanics at home and abroad. Some countries and departments have even formulated design criteria and acceptance standards based on fracture mechanics. For example, the U.S. B-1 bomber was designed using fracture mechanics. The defect tolerance assessment of production-platform supports in the U.K. North Sea oilfield and the gas pipeline in Alaska are also typical engineering applications of fracture mechanics. The application of fracture mechanics to pressure vessels, especially nuclear vessels, is relatively more mature.
Damage prediction analysis of High-Pressure Manifolds first takes elastic-plastic analysis as its basis. The elastic-plastic analysis method used is the finite element method. Because the geometry of High-Pressure Manifolds is complex and no ready analytical solution can be found, only numerical solutions can be obtained. Based on the numerical solution, damage prediction analysis is then carried out according to the corresponding fracture-mechanics theory.
2. Engineering Significance and Application Value of High-Pressure Flow Control Components and Manifolds
High-Pressure Manifolds are assemblies of high-pressure flow control components. High-Pressure Manifolds are widely used in petrochemical and natural gas pipelines. Because they withstand very high impact pressure and alternating loads internally, wall-thickness wear is considerable. As service years increase, fatigue fracture occurs in High-Pressure Manifolds. In petrochemical production and similar operations, pipelines and manifolds are responsible for transporting various materials. Once an accident occurs, normal production of the installation may be endangered, and severe accidents such as fire, poisoning and explosion may occur. Therefore, research on damage prediction and service-life evaluation of High-Pressure Manifolds is of great concern in engineering. This article studies the issue in depth, starting from strength analysis, and on that basis carries out single-event fracture analysis, fatigue fracture life analysis and crack-growth life analysis of High-Pressure Manifolds. The principles and results of this research can also be extended to damage prediction analysis of materials and parts such as tubing, casing, drill pipe and drill collars in oilfields and petrochemical industries.
The High-Pressure Manifolds currently used on liquid nitrogen pump trucks, fracturing trucks, sand trucks and acidizing-fracturing trucks of the Downhole Fracturing Company of Liaohe Petroleum Exploration Bureau usually have working pressures above 40 MPa and design pressure of 100 MPa. According to the Code for Construction and Acceptance of Industrial Pipeline Engineering, GBJ235-82, industrial pipelines are divided by design pressure into four levels from low to high. When the internal pressure p is greater than 10 MPa, it is considered a high-pressure pipeline, and High-Pressure Manifolds are the fourth level. Because the High-Pressure Manifolds of the downhole fracturing company have a long service cycle and are often subjected to alternating loads, wall thickness is greatly worn and fatigue cracks can easily appear at stress-concentration areas. To ensure safe production and provide a scientific basis for correct use of High-Pressure Manifolds, the research results are applied specifically here and satisfactory results have been obtained.
3. Domestic and International Research Status of High-Pressure Flow Control Components and Manifolds
At present, scholars at home and abroad have carried out many studies on damage, failure and assessment methods for High-Pressure Manifolds and pipelines. However, the pressure loads usually studied are below 40 MPa, and very few studies involve pressure loads of 50 MPa to 100 MPa. From published research results, stress analysis using the finite element method has been limited to the elastic deformation range. In fracture analysis of pipelines, most studies apply linear-elastic fracture criteria and do not consider the influence of the crack-tip plastic zone. Overseas research on pressure pipelines mainly focuses on corrosion fracture of pipelines and safety problems after pipeline failure. For corrosion fracture of natural gas and petroleum pipelines, ANSI/ASME B31G was formulated in 1985. In 1997, Dr. David Batte and Dr. Bin Fu of the British Gas Pipelines Research Centre published papers and further developed this criterion, proposing a more accurate method for crack assessment of corroded pipelines based mainly on failure criteria and numerical analysis. In the early 1980s, extensive international research was carried out on pipeline defect safety assessment technology. Some industrial countries formulated special acceptance standards and defect assessment specifications for pipeline defects. In the late 1980s, an international pipeline integrity research program involving nine countries and regions was organized. Current domestic research is mainly concentrated on defect assessment of pressure vessels, and most research, analysis and experimental work focuses on crack-initiation assessment. However, structural safety analysis must consider the whole process of crack initiation, fracture and instability. Therefore, strength analysis and fatigue-life prediction for High-Pressure Manifolds, especially those with pressure loads above 40 MPa, remain a gap.
4. Research Content of High-Pressure Flow Control Components and Manifolds
High-Pressure Manifolds are used to transport media and ensure that the medium flows smoothly along the designed route. Common high-pressure flow control components include high-pressure straight pipe assemblies, swivel joints, and tee fittings. These components are assembled into high-pressure manifolds so that the transported medium reaches the required location according to design requirements.
The domestic material of straight pipe assemblies, Swivel Joint and tee pipes in High-Pressure Manifolds is 30Cr. The imported material is equivalent to SA372-V-3B steel forgings in the U.S. ASME standard. Detailed data on chemical composition, mechanical properties and fracture parameters of these materials are not available. In this research, chemical composition, mechanical properties and fracture parameters must be tested and measured, entrusted to the Vibration and Strength Testing Center and the Materials Mechanics Laboratory of Dalian University of Technology.
The specific research content is as follows:
Strength analysis of straight pipe assemblies in High-Pressure Flow Control Components: according to specifications, high-pressure parts are divided into 3 in., 2 in. and 1.5 in. sizes. Three representative high-pressure parts are selected from the three types for strength analysis under internal pressure load cases of 50 MPa, 70 MPa and 100 MPa.
Strength analysis of Swivel Joint in High-Pressure Flow Control Components: representative high-pressure parts are also selected from 3 in., 2 in. and 1.5 in. specifications for strength analysis under internal pressure load cases of 50 MPa, 70 MPa and 100 MPa.
Finite element strength analysis of tee fittings in High-Pressure Flow Control Components: one imported and one domestic type are selected for strength analysis under internal pressure load cases of 50 MPa, 70 MPa and 100 MPa.
Finite element strength analysis of straight-pipe ends in High-Pressure Flow Control Components: strength analysis is carried out under internal pressure load cases of 50 MPa, 70 MPa and 100 MPa. Since fracture has occurred at the end, theoretical analysis of the fracture cause is required.
Using linear-elastic fracture mechanics criteria, single-event fracture failure analysis is carried out for straight pipe assemblies, straight-pipe ends, Swivel Joint and tee fittings in High-Pressure Flow Control Components.
Using low-cycle fatigue theory, fatigue fracture analysis is carried out for straight pipe assemblies, straight-pipe ends, Swivel Joint and tee fittings in High-Pressure Flow Control Components.
Using the Paris formula and Miner's linear cumulative damage formula, crack-growth life analysis is carried out for straight pipe assemblies, straight-pipe ends, Swivel Joint and tee fittings in High-Pressure Flow Control Components.
