Study On CO₂ Internal Corrosion And Residual Strength Of Multiphase Offshore Pipeline

With the increasing application of offshore multiphase transportation technology in the exploitation of offshore oil and gas fields, multiphase internal corrosion containing CO₂, especially slug flow internal corrosion has become a serious potential safety hazard affecting subsea pipelines. Because of the complication of the medium composition transported in oil pipeline and the complexity of slug flow, new task is proposed for multiphase flow internal corrosion containing CO₂ and residual strength analysis evaluation of corroded pipeline. This paper aims at two problems which are multiphase flow CO₂ corrosion and corrosion pipeline residual strength. Using numerical simulation, experimental research and theoretical analysis method respectively, influence factor and corrosion mechanism are analyzed in multiphase flow internal corrosion containing CO₂. Axial uniform corrosion and pitting pipeline residual evaluation method is established. Interaction law and computing method of interaction coefficient between biaxial corrosion and double pitting are proposed in corrosion pipeline. With the above research methods and results, effective technical support is provided to eliminate subsea multiphase flow transportation internal corrosion security hidden danger. Research contents and research results are as follows:Applying transient multiphase flow simulation software OLGA and the numerical simulation method, combining the multiphase flow pipeline simulation functions in OLGA, selecting different CO₂ corrosion rate prediction models, the effect of different pipeline operation conditions, routing conditions and corrosive medium conditions on the oil water mixed pipeline containing CO₂ and wet gas pipeline containing CO₂ is qualitatively analyzed. The operation conditions include CO₂ pressure, outlet pressure, inlet temperature, capacity, moisture content. The routing conditions are pipeline angle and the height of the riser. The corrosive medium conditions include Cl^- concentration, HCO₃^- concentration and corrosion inhibitors concentration.The result demonstrates that, the corrosion rate change rule of wet gas pipeline and oil and gas mixed pipeline is various on account of different two-phase flow conditions. In wet gas pipeline, the slug that may occur can control the pipeline corrosive rate through influencing liquid holdup, wall shear rate and wall wettability. While in the oil water mixed pipeline the liquid holdup, wall shear rate and wall wettability show little change because it is full pipe flow.Combining the features of loop-line test, autoclave experiment and electrochemical experiment, the high-pressure rectilinear oscillation experiment facility is developed. Referring to the quantitative analysis result of numerical stimulation, applying experimental research method and adopting weight-loss test, linear polarization technique and inductance as corrosion rate testing methods, the influences of pipeline operation pressure, CO₂ pressure, pipeline temperature, gas-liquid ratio, oil-water ratio, corrosive inhibitor concentration, Clconcentration, HCO₃^- concentration on the corrosion rate in wet gas pipeline containing CO₂ are quantitatively analyzed. The results show that the material uniform corrosion rate and localized corrosion rate reach the maximum between 60oC-80 oC instead of 60 oC in former papers. The relationship of CO₂ pressure and CO₂ corrosion rate is exponent sign. Gas-oil ratio influences CO₂ corrosion rate by effecting wall shear rate and wall wettability. Content of crude oil affects CO₂ corrosion rate by changing wall wettability. As the concentration of HCO₃^- increases, the uniform corrosion rate increases first and then decreases. There is a critical value of corrosive inhibitor concentration. When the concentration exceeds the critical value, the corrosion rate is small and increasing the concentration has little impact on the uniform corrosion rate. When the concentration is lower than the critical value, the corrosion rate is higher.According to emendatory pipeline corrosion evaluation specifications B31G、BS7910、DNV-RP-F101 and PCORRC, residual intensity of high uniaxial corrosion and pitting pipeline is analyzed. It shows that the current pipeline corrosion evaluation specifications are conservative.Utilizing finite element software ANSYS, nonlinear numerical simulation is carried out to analyze the effect of factors such as axial corrosion length, depth, width, morphology and location in uniaxial corrosion, pitting diameter and depth in pitting corrosion, on the pipeline failure stress. Based on the results, residual strength calculation method of uniaxial corrosion pipeline and single pitting pipeline is developed according to the stress concentration theory. It turns out that the new method can predict the residual strength of X60 high uniaxial corrosion pipeline and single pitting pipeline with little errors and lower conservation, when compared with the current calculation method proposed by the specifications.Utilizing ANSYS and nonlinear numerical simulation, this paper quantitatively analyzes the influence of biaxial corrosion axial spacing, biaxial corrosion ring spacing, the same axial corrosion depth, the same axial corrosion length, biaxial corrosion depth, biaxial corrosion length and cross corrosion, on the residual strength of biaxial corrosion. This paper also analyzes the influence of double pitting axial spacing, double pitting ring spacing, same double pitting diameter, same double pitting depth, different double pitting diameters and different double pitting depth on the residual strength of double pitting pipelines. The interaction laws of biaxial corrosion and double pitting are put forward respectively. And the interaction coefficient calculation methods of biaxial corrosion and double pitting are established respectively.