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Application of 400g unidirectional

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Application of 400g unidirectional prepreg to anticorrosion for oil pipeline in Dubai


In 2015, Dubai Petroleum Pipe Line Administration found mechanical damage in somewhere of oil pipeline in routine inspection, where the outside diameter of pipeline was 559mm and normal wall thickness was 10mm. On the topmost of surface of pipeline existed recession due to mechanical damage. The recession was 15mm deep, and external diameter was about 200mm. The measurement result of pipeline wall thickness at the recession showed that the wall thickness of pipeline at the recession thinned by about 2mm. 

Application of carbon-fibre composite reinforcement technology to pipeline reinforcement is provided with following advantages: 
①Solderless without flame operation, able to restore in running status of being under pressure;
②Simple and rapid construction and short operation time;  ③The characteristic of elastic modulus of carbon fibre being very close to that of steel makes the composite able to bear as much pipeline pressure as possible, reduce stress level of defected pipeline, and limit dilatational strain of pipeline; 
④With high tensile strength, the carbon fibre can realize extremely high security when applied to pipeline restoration; besides carbon-fibre composite has excellent creep resistance with strength basically unchanged with increase of service time;

⑤The reinforcement layer of carbon-fibre composite has small thickness, which can facilitate subsequent antiseptic treatment; 
⑥Carbon fibre reinforcement has flexible winding and laying modes, hence able to reinforce defects (including high welding seam reinforcement and severe misalignment)of circumferential weld and spiral welding seam, and restore irregular pipelines such as elbow, T-joint and reducers, etc.  
⑦Can be applied to restoration and reinforcement of defects such as erosion, mechanical damage and crack, etc. or applied to pressure lifting and reinforcement for the whole section of pipeline, showing the field of application is extensive.  

Reinforcement scheme 

The research result by related departments is to emergently repair the pipeline using carbon-fibre composite 400g unidirectional prepreg reinforcement technology. The repair scheme is to first filling resin to fill and restore the recession, then wind high strength carbon fibre 400g unidirectional prepreg around the pipeline. 
 After establishing reinforcement scheme, used finite element analysis software to calculate and verify this scheme.    First calculated the stress distribution on pipe wall when flawless pipeline bears internal pressure. With pipeline in good condition, when it bears internal pressure of 3MPa, the loop stress in the pipeline should be 83.85MPa. Established finite element model to analyze this scheme. In the finite element model, simulatively calculated recession. To facilitate establishing model, took only one eighth of pipeline to establish model. 
   In finite element model, took 207GPa for elastic modulus of steel material, 0.3 for Poisson ratio, and 450MPa for yield strength. Set the running pressure in pipeline as 3MPa, number of reinforcement layer for carbon-fibre composite as 6. According to above conditions, obtained the distribution nephogram for hoop stress in non-reinforced pipeline and reinforced pipeline.

The calculation result showed that severe stress concentration existed at the edge of recession in non-reinforced pipeline. The maximal hoop stress reached 322MPa, far exceeding the hoop stress for pipeline wall when flawless pipeline bore pressure of 3MPa.  And this stress was close to yield strength of steel material. When the internal pressure fluctuated in the pipeline, endurance failure was deadly easy to occur at stress raisers under action of fatigue load.  If it is not reinforced, the wall thickness at recession may continue to reduce, rendering the stress concentration at pipeline recession more severe. 
  After reinforced with carbon-fibre composite, the pipeline's loop stress reduced significantly, with the hoop stress in the most severe region being 164MPa. This stress was lower than yield strength of steel material, and pipeline distortion was still within elastic range. After reinforced, the pipeline's stress concentration will be largely alleviated, and the pipeline will be in safe range of stress. 

The construction flow after the reinforcement scheme is established is: 
①Pretreat the external surface of pipeline to remove erosion resistant coating;
②Use electric descaling tool to burnish the surface of pipeline, to reach descaling requirements of St3 level; 
③Use cleaning agent to clean the surface of pipeline and dry it intensively; 
④Smear filling resin at recession, repair till the surface of defected position is leveled off; 
⑤After the filling resin is solidified preliminarily, wind high strength carbon-fibre composite and guarantee the composite covers the defected position, wind 6 layers; 
⑥Make antiseptic treatment for the reinforced area, then backfill. 
After restoring above defected pipeline by reinforcement, the pipeline's normal running pressure was restored with satisfactory effect.

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