Study On The Cause And Behavior Of Corrosion Inside And Outside The Submarine Double-layer Pipeline

The stop production of oil field and large-area pollution of sea area was caused by submarine pipeline damage.Corrosion is an important cause of submarine damage.The impact of corrosion on the overall performance of the pipeline should be assessed,regardless of the time at which the submarine pipeline is in service.The B offshore platform to SPM submarine pipeline is the double insulated pipe.There were 732 reductions on the outer wall of the inner tube in first internal dectection.There were 4618 reductions on the outer wall of the inner tube in second internal dectection.We mainly studied the causes and behaviors of corrosion of the inner tube of the B offshore platform to SPM submarine pipeline.It has provided theoretical basis for the safe operation and the prevention of corrosion failure accident of the submarine pipeline.In this paper,the mechanical properties and metallurgical structure of the welding joint were tested and analyzed by the material property experiment of welding joints.By weight-loss test,electrochemical test and scanning Kelvin Probe test studid the corrosion law of the influence factors of the inner tube and the corrosion mechanism of galvanic corrosion of welding joint of the outer tube.Rdsidual strength assessment and residual life prediction of inner tube could be used to judge whether the inner tube was satisfied or not.The main contents and achievements of the paper include the following aspects:(1)The corrosion of the inner tube was determined by the external detection of the inner tube,corrosion product composition,the seawater corrosion mechanism of carbon steel,the appearance and shape of the insulation layer.The reason of corrosion was that the seawater was immersed from the welding joints of outer tube,contacted with the outer wall of the inner tube.(2)The X-ray test of welding joints results showed that it did not meet the standard requirements.The tensile test results showed that the average tensile strength of 1?3 welding joints was less than 490MPa,which did not meet the API Spec 5L standard,and the 5?7 welding joints meeted the standard requirements.The average impact energy was greater than 50J,which meeted the engineering acceptance criteria.The microhardness of weld seam was higher than that of base metal and heat affected zone.The high strength of the weld seam led to the decrease of ductility.Seawater has increased the stress corrosion of the welding joints,but it has little influence.(3)The weight-loss results of welding joints showed that the corrosion rate of weld seam was higher than the base material and heat affected zone in outer tube.By means of electrochemical corrosion experiment,the corrosion current density of the weld seam was maximum,and the diameter of the capacitive arc was minimum.The galvanic corrosion experiments showed that the couple current of weld seam-heat affected zone was the most.The SKP results showed that there was the galvanic corrosion effect of welding joint and base metal.The weld seam corrosion was accelerated as the anode,and the base material was protected as the cathode.(4)The weight-loss results of inner tube showed that the corrosion rate decreased with the increase of immersion time,the decrease magnitude of corrosion rate gradually reduced.The electrochemical test results showed that the influence of different factors on the corrosion of the inner tube was different.By means of electrochemical corrosion experiment,the corrosion law and corrosion mechanism of X65 steel with temperature,Cl-and SO42-were obtained.The SKP results showen that the surface potential of X65 electrode increased with the increase of immersion time,and the local corrosion exacerbated.(5)The evaluation results of the residual strength of the pipeline could meet the requirements of the service.The results of the three standards were basically accurate and reliable.The forecast remaining life results of the first internal dectection was 5.47 years.After the second internal dectection,the remaining life results of the pipeline were about 0 years,the pipeline has been unable to continue service.It is suggested to replace the welding joint and the serious corrosion of the inner tube.