A Study On Key Monitoring Techniques For The Internal Localized Corrosion Of Marine Pipelines

The marine transportation pipelines become an important part of the oil and gas exploitation system in the ocean.Since corrosive agents are included in the pipe fluids during the oil and gas exploitation,the pipeline internal corrosion comes to be a significant issue which brings a lot of trouble for ocean engineering.As most of the marine pipelines are long-distance,the complex and variation of the pipeline internal environment such as the flow pattern,fluid temperature,sand concentration and the corrosive components can lead to different localized corrosion cases.When there is a dramatic temperature differences between the pipe inner wall and the outer wall,the condensation of the hot vapor on the top of the pipeline can cause top of line corrosion(TLC).In a low flow rate pipeline section,the accumulation of the corrosion product and the sand particles at the bottom of the pipe may induce under deposit corrosion(UDC).In contrast,when the flow rate in the pipe is high,the fluids having a high velocity can possibly lead to erosion corrosion.At present,it is still a hard issue to on-line monitor these pipeline localized corrosion issues.Therefore,under the funding of Key Projects in the National Science&Technology Pillar Program during the Twelfth Five-year Plan Period of China:Corrosion Monitoring and Design of Cathodic Protection System for LW3-1 gas field,a corrosion monitoring system based on the ring pair electrical resistance sensor(RPERS)was developed and used to study the pipeline localized corrosion in simulated sea floor pipelines.The main work in this thesis can be concluded as:(1)The RPERS system used for the inner localized corrosion monitoring of the sea floor pipelines was proposed.A sliced ring was cut from the targeted pipe and divided into six segments.In combination of special designed measurement circuit,the sensor system can realize the localized corrosion monitoring of the whole pipe circle without destroy its integrity.In comparison with traditional ER probes,RPERS possesses a higher measurement accuracy and wider application range.Even when there is a significant temperature difference between the pipe inner wall and outer wall,RPERS still can precisely probe the corrosion depths.(2)In conjunction with electrochemical measurements and RPERS,the corrosion caused by the accumulation of mineral deposit on the steel surface and its inhibition effect by using inhibitor EDTMPS(Ethylene Diamine Tetra Methylene Phosphonic Acid Sodium)was studied.The test results indicate that the oxygen difference cell induced by the deposit is the main factor for the propagation of UDC.EDTMPS performs an effective inhibition effect for the corrosion of bare steel and UDC in static electrolyte.However,obvious failure of EDTMPS for UDC was probed by RPERS in the test pipeline and it dramatically enhanced the corrosion at the deposit-covered area.(3)In order to solve the measurement error of the localized corrosion rate in the coupled multi-electrode sensor(MES),a new method which using the combination of both ER method and MES(ER-MES)was proposed.ER-MES successfully realized the synchronous measurement of the galvanic current and the localized corrosion depth.Through using ER-MES system to study the localized corrosion behavior of deposit covered pipeline steel along the oxygen pumping direction,it is found that the deposit covered area behaved as anodes and the diffusion of the corrosion product beneath the deposit can significantly influence the corrosion process.(4)The flow accelerated corrosion(FAC)and erosion corrosion were studied using wire beam electrode(WBE)to understand their initiation and propagation processes.It is found that FAC was induced by the transportation of the analyte from the initial pits to the downstream which finally lead to a“Flow mark”corrosion morphology.However,erosion corrosion was firstly induced by the impingement of the sand particle at the initial anodic sites on the steel surface which lead a crater formation.With the accumulation of analyte in the crater,the crater will propagate into a stable pit and lead to new crater form around it due to the sequence sand impingements.